UniversidadeVigo. Departamento de química física. Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods

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4 UniversidadeVigo Departamento de química física Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods Memoria presentada por Sergio Gómez Graña para optar al Grado de Doctor por la Universidad de Vigo con Mención Internacional Vigo, Junio de 2013

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6 D. Luis Manuel Liz Marzán, Catedrático del Departamento de Química Física de la Universidad de Vigo y D. Andrés Guerrero Martínez, investigador Ramón y Cajal del departamento de Química Física de la Universidad Complutense de Madrid Certifican Que Sergio Gómez Graña, licenciado en Química, ha realizado en el Departamento de Química Física de la Universidade de Vigo, y bajo su dirección, el trabajo de investigación, descrito en la presente memoria, que lleva por título Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods, con el fin de optar al grado de Doctor por la Universidad de Vigo con Mención Internacional. Vigo, Junio 2013 Fdo.: Luis Manuel Liz Marzán Fdo.: Andrés Guerrero Martínez

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8 Para todos los que la han hecho posible, Every day may not be good, but there's something good in every day No todos los días son buenos, pero hay algo bueno cada día

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10 Outline Thesis scope Chapter 1: General introduction Anisotropic Metal Nanoparticles Optical Properties of Metal Nanorods Wet-Chemistry Synthesis of Metal Nanorods Techniques for Structural Characterization of Metal Nanorods Structural Characterization of Metal Nanorods Self-Assembly of Metal Nanorods Chapter 2: Surfactant (Bi)Layers on Gold Nanorods Introduction Experimental section and methods Results and discussion Conclusions Chapter 3: Nanoparticles: Stabilizing {100} facets Introduction Experimental details Results and discussion Conclusions

11 Chapter 4: Self-assembly of Nanorods Mediated by Gemini Surfactants for Highly Efficient SERS-Active Supercrystals Introduction Experimental section Results and discussion Conclusions Chapter 5: Chiral Assemblies of Gold Nanorods Introduction Experimental section and methods Results and discussion Conclusions Appendix I Appendix II Appendix III Appendix IV General conclusions Resumen

12 List of publications Agradecimientos

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14 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 13 Thesis scope The concepts that seeded nanotechnology were first discussed in 1959 when Richard Feynman raised the question: Why cannot we write the entire 24 volumes of the Encyclopaedia Britannica on the head of a pin? Nanotechnology is the development of research and technology at atomic, molecular or macromolecular level, in a scale of approximately nm, to obtain new materials that will lay the foundations for a whole set of technological developments that have in common the use and manipulation of objects with at least one dimension in the nanometre size range. Within this context, the wet-chemical synthesis, which is a field at the crossroads of conventional inorganic cluster chemistry and classical colloid chemistry, becomes a natural starting point for the bottom-up fabrication of nanotechnology-based materials. In particular, nanoparticles offer the advantage of combining the three-dimensional control of particle size and shape with that of monolayer surface chemistry in metals, rendering them ideal building blocks for nanoarchitecture design. Humans have been using nanoparticles in the development of macroscopic materials for centuries without knowledge of their microscopic structure. One of these oldest objects is the Lycurgus Cup, form the fourth century AC, which contains nanocrystals of coinage metals that were precipitated within the glass of a cage cup as colloids forming a silver-gold alloy. When the cup is viewed in reflected light the minute metallic particles are just coarse enough to scatter some green light, whereas in transmitted light they absorb the blue end of the visible spectrum more effectively, resulting in red transmission. The first documented scientific investigation into the chemical synthesis of colloidal metal nanoparticles was reported by Michael Faraday in 1857, who prepared gold colloids by reducing an aqueous solution of

15 14 Thesis scope chloroauric acid with white phosphorus in carbon disulphide, obtaining ruby fluids of dispersed gold spheres with nanometre dimensions. Thereafter, the scientific development of metal nanoparticles has been tremendous, especially during the past five decades, driven in part by the improvements in colloidal synthesis of nanocrystals and the development of novel electron microscopy techniques that provided an accurate description of the structure of matter. An area of nanotechnology where optical properties (absorption, emission or scattering) of metal nanoparticles have been fully exploited is nanoplasmonics, in which the excitation of localized surface plasmon resonances on nanostructures of coinage metals has been used to obtain devices with specific structural and photonic properties. Although spherical shaped nanoparticles have been the most widely investigated systems exhibiting plasmonic properties, due to the relative easiness of preparation, these isotropic nanocrystals lack a geometrical preference toward assembly and organization, thus limiting their potential applications to form systems with tailored optical properties and directionality. In contrast, one of the most appealing advantages of anisotropic metal nanoparticles is their versatility toward directional organization and site-specific chemical functionalization. Within this context, metal nanorods stand out as highly efficient plasmonic systems that achieve intense and tuneable optical activity at the visible-near-ir spectral region. However, it has not been until the late twentieth century, when the first reports on the synthesis, characterization and optical properties of anisotropic nanocrystals began to be published, which shows the difficulties encountered in the control of nanoparticle shape through wet-chemistry methods. The research described in the thesis is focused on the development of new strategies for synthesis and self-assembly of metal nanorods, which could serve as a platform for building up macroscopic systems with plasmonic applications, thus requiring a deep understanding of the colloidal and crystalline state of the anisotropic nanoparticles. These investigations are framed in the context of

16 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 15 working in the Colloid Chemistry Group of Vigo University, where the synthesis and functionalization of metal nanoparticles have been widely developed, thus allowing control over nanoparticle composition, size and shape. Characterizations of the prepared nanorod colloids were carried out by means of UV-vis spectroscopy and electron microscopy techniques, and also in the framework of existing collaborations, small angle scattering techniques (Xrays and neutrons) and high angle annular dark field scanning transmission Electron Microscopy have been used for the same purpose. The thesis has been structured in five chapters. Chapter 1 collects a general introduction where issues like the reasons behind the current interest on anisotropic metal nanoparticles, and more specifically metal nanorods, are introduced. Chapter 2 is focused on the colloidal characterization of the indissoluble entity formed by gold nanorods and surfactant stabilizers, using transmission microscopy and small angle X-ray scattering techniques. From these investigations, an accepted concept but not directly characterized before, as the presence of a surfactant bilayer around the nanoparticles with a defined morphology (thickness, density, interdigitation, and patterning) is determined. Moreover, we introduce the unique self-assembly process of gold nanorods driven by the high amphiphilic character of gemini surfactants. The synthesis and the growth mechanism of silver nanoparticles in aqueous media through the seed-mediated process on pre-synthesized gold nanoparticles are shown in chapter 3. We have chosen gold nanoparticles as seeds due to their well defined crystalline structure and morphology. From the crystalline structure point of view the seeds can be classified as single crystalline seeds (such as gold octahedrons and gold nanorods) and pentatwinned seeds (such as pentatwinned gold nanorods). We have studied the silver overgrowth through a wet chemical approach and we provide a full structural characterization of the resulting core-shell nanoparticles. The results obtained clearly demonstrate that the crystalline structures of the seed as well

17 16 Thesis scope as the preferential stabilization of certain facets are the key factors that promote the silver deposition in an isotropic or anisotropic manner. Another main objective of the thesis is the study of the self-assembly process of metallic nanorods forming plasmonic supercrystals. In chapter 4, the gemini surfactant directed self-assembly of monodisperse gold and silver nanorods into standing superlattices is studied by simple drop casting. For this purpose, several physico-chemical aspects such as the concentration of surfactant, the humidity and the temperature of the drying process are investigated in order to obtain a long-range assembly of gold and silver nanorods with a close-packed arrangement within the supercrystal. Moreover, the application of these assemblies for surface enhanced Raman scattering (SERS) detection is described, showing that the nearly perfect threedimensional organization of gold and, especially, silver nanorods render these systems excellent for SERS substrates with uniform electric field enhancement, leading to reproducibly high enhancement factors. Chapter 5 discusses a new type of assembly of nanoparticles which provides us with a unique class of plasmonic metamaterial consisting of gold nanorods organized in three-dimensional chiral structures and yielding a record circular dichroism anisotropy factor for metal nanoparticles across visible and near-infrared wavelengths. The fabrication process is easily upscaled, as it involves the self-assembly of gold nanorods on a fibre backbone with chiral morphology. The measurements are fully supported by theoretical modeling based on coupled dipoles, unravelling the key role of gold nanorods in the chiroptical response. Finally, general conclusions are summarised after the five chapters. A general synopsis, in Spanish, of this dissertation was included at the end of the thesis due to requirements established by the regulation of the Universidade de Vigo.

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20 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 19 Chapter 1: General introduction 1.1 Anisotropic Metal Nanoparticles A material property is isotropic when it does not depend on how the sample is turned and it is anisotropic when it does depend on the orientation of the sample with respect to some external frame. Figure 1-1 shows the difference between the isotropic or aniosotropic growth of a material. Such anisotropy of a property is due to the structural arrangement of the material in one (1D), two (2D) or three (3D) dimensional oriented structural building blocks. A leading thread of the materials science is related to the structure/property relations. Most often, this concern is with the level of a certain property (or its strength ) and its connection to the scale of the microstructure. The structural elements responsible for anisotropy are not so much scale as morphology; for example, not the size of the grains in a polycrystalline aggregate, but their shape and, in particular, their orientation. The totality of crystalline orientations is called the texture of the material sample. The anisotropy of properties in a polycrystalline aggregate depends both on the anisotropy of the single-crystal property and on the texture of the polycrystal. The anisotropy of a property is generally restricted by certain symmetry considerations which in part follow from the symmetry elements of the underlying material structure (but also depend on the kind of property being considered). Symmetry considerations are in fact of paramount concern in the treatment of the directionality of material properties. The words structure and properties, sample and material, texture and anisotropy, and anisotropy and symmetry, all have very specific meanings in materials science (and sometimes slightly different meanings in geology and crystallography, for example). [1]

21 20 Chapter 1: General introduction Figure 1-1: Schematic view of isotropic growth, which is equal in all the directions and anisotropic growth, in which the directional growth is different depending on the direction, conferring to the material vectorial properties. A fundamental goal of materials science is the design and synthesis of materials with tailored shape and size. This goal underpins the growing interest in the fabrication of intelligent and complex materials such as artificial bones, teeth and cartilage, nanowires for electronic circuitry, shape memory alloys, and plasmon based sensors.[ 2 ] In particular, there has been tremendous progress over the past decade in the synthesis of gold and silver nanocrystals of various sizes and shapes and with good yield and monodispersity. [3],[4],[5],[6] It is perhaps salient to point out that noble metal nanoparticles have been synthesized for over 2000 years in a whole range of media including glasses, salt matrices, polymers, the gas phase and in water - invariably these methods have resulted in the formation of spheres. It is only the past decade that significant progress towards non-spherical nanoparticle synthesis has been achieved. Fine metal particles with tailored nanometre-scale dimensions are of great interest due to their unusual properties. Fundamentally, the mean free path of an electron in a metal at room temperature is nm, and one would predict that as the metallic particle shrinks to this dimension, unusual effects

22 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 21 should be observed. Indeed, gold nanospheres of diameter 100 nm or smaller appear red (not gold colour) when suspended in transparent media (Figure 1-2). [6],[7],[8] Figure 1-2: Photographs showing the difference in colour between bulk gold and gold nanoparticles. The optical properties of metal nanoparticles are tuneable throughout the visible and near-infrared regions of the spectrum, as a function of nanoparticle size, shape, aggregation state and local environment. [4],[5],[9] Interest in the shape-controlled synthesis of metal nanostructures began to take hold in the early 1990 s, when Masuda et al. [10] and Martin [11] developed techniques to prepare anisotropic gold nanorods (AuNRs) by electrochemical reduction into nanoporous aluminium oxide membranes. These methods produced relatively monodisperse structures, but due to the low yield and somewhat large diameter (>100nm), the optical response from these AuNRs was, at the time, difficult to discern and largely dominated by multipolar plasmon resonance modes. Wang and co-workers later demonstrated the synthesis of much smaller AuNRs (ca. 10nm in diameter) by electrochemical oxidation of a gold plate electrode in the presence of cationic, quaternary ammonium surfactants (cetyltrimethylammonium bromide (CTAB), and tetraoctylammonium bromide (TOAB)) and under ultrasonication. [12] The resulting AuNRs solutions exhibited plasmon resonance modes for their short (transverse) and long (longitudinal) axis polarizations, verifying for the first time with AuNRs the optical theory

23 22 Chapter 1: General introduction proposed by Gans in 1912 for the scattering and absorption of spheroidal plasmonic nanoparticles. [13] Murphy et al. [14] and Nikoobakht and El-Sayed [15] later developed a colloidal growth method to produce monodisperse AuNRs in high yield based on seeded growth. In this method, small (ca. 1.5 nm diameter) single-crystal seed particles, produced from the reduction of chloroauric acid by borohydride in the presence of CTAB, are aliquot into Au + growth solution prepared from the mild reduction of chloroauric acid by ascorbate and the addition of silver nitrate (AgNO3) and CTAB. Using this method, AuNRs ca nm in diameter and up to 300 nm in length can be obtained in relatively high yield, allowing for their subsequent use in a number of biomedical applications. [16] AuNR aspect ratio can be controlled by the seed/gold salt ratio or by the relative concentration of additive impurity ions. For some time, the precise mechanism and purported reproducibility of AuNRs growth has remained a hotly debated topic, confounded by the fact that some AuNRs preparations contained additive impurity ions such as silver and others did not. [9] Proposed contributions include underpotential deposition, halide adsorption, surface packing density, and alloy formation among others. Electron microscopy indicated that the AuNRs grow along the [001] direction with less stable crystalline facets along the sides of the rods and more stable crystalline facets at their tips. [17],[18],[19] There are many examples that demonstrate the enormous potential of metal nanorods in different fields such as surface enhanced Raman scattering (SERS), plasmonics, biosensing and biomedical applications. SERS permits identifying the nature of molecules in extremely low concentrations using gold or silver nanoparticles as enhancing substrates, even reaching the single molecule detection limit. [20] On the other hand, recent progress in nanooptics has paved the route toward the development of highly sensitive optical transducers using the localized surface plasmon resonance (LSPR) of metal

24 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 23 nanoparticles. [21] In fact, AuNRs have been recently used to prepare new materials with improved optical circular dichroism, which interest stems from a wide range of applications in biology and physics, including the structural determination of proteins and DNA and the pursuit of negative refraction. [22] Diagnostics and detection is another important field where metal nanoparticles can contribute very positively. Their use as labels in diagnostics and detection is due to a unique combination of chemical and physical properties that allow biological molecules to be detected at low concentrations. For example, methods that allow target molecules to be detected with the unaided eye have been developed, on the basis of specific target molecules inducing a visible colour change. [23] Other detection methods are based on interactions between gold nanoparticles and molecules located in close proximity to their surface. These include methods in which light emission from such molecules is enhanced (surface enhanced spectroscopies) or quenched (fluorescence), and methods in which the accumulation of specific target molecules induce subtle changes in the extinction spectra of gold nanoparticles that can be monitored in real time with inexpensive equipment. 1.2 Optical Properties of Metal Nanorods One of the most interesting and powerful properties of metal nanostructures is the LSPR. A plasmon is a collective oscillation of the free electrons in a metal. At the surface of a metal, plasmons take the form of surface plasmon polaritons, also simply called surface plasmons (Figure 1-3). Surface plasmons are optically excited, and light can be coupled into standing or propagating surface plasmon modes through a grating or a defect in the metal surface. Because it is the oscillating electric field of the incoming plane wave that excites surface plasmons, light with a high angle of incidence couples most efficiently. When a surface plasmon is confined to a particle of a size

25 24 Chapter 1: General introduction comparable to the wavelength of light, that is, a nanoparticle, the particle s free electrons participate in a collective oscillation. [2] This process can be divided into two types of interactions: scattering, in which the incoming light is reradiated at the same wavelength in all directions, and absorption, in which the energy is transferred into vibrations of the lattice (i.e., phonons), typically observed as heat. Together, these processes are referred to as extinction. [24] Figure 1-3: Ilustrations of a) surface plasmons and b) a localized surface plasmon resonances. Reproduced from reference [2]. The LSPR has two important effects. First, electric fields near the particle s surface are greatly enhanced, this enhancement being greatest at the surface and rapidly falling off with distance. Second, the particle s optical extinction has a maximum at the plasmon resonant frequency, which occurs at visible wavelengths for noble metal nanoparticles. The specifics of the LSPR response of metal nanostructures depend on a number of variables, particularly the size, shape, and morphology of the nanostructure, as well as the dielectric environment. [25],[26],[27] Consequently, controlling the morphology of gold nanostructures is a powerful route to control the LSPR response. Even small changes in aspect ratio or corner sharpness can have a large impact. [26] The resonance condition for the coherent electronic motion with the electric field is unique, which is observed experimentally as a single plasmon band in visible spectroscopy (Figure 1-4). However, additional high-order plasmon excitation modes can occur when the small particle condition is no longer fulfilled. [26] In the case of an ellipsoidal particle, the anisotropy in shape

26 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 25 provokes the plasmon band to be splitted into two modes, either along (longitudinal) or across (transverse) the particle (Figure 1-4). Figure 1-4: Localized surface plasmon resonance bands in the vis-nir spectra of gold nanospheres (A) or nanorods (B) Spherical Metal Nanoparticles. Mie theory The ability of a metal nanoparticle to support an LSPRs is dependent on its dielectric function, which includes a real part ( ) and an imaginary part ( ), both of which vary with excitation wavelength (λ). The dielectric function of a material reflects the unique interaction between its electrons and light. The finiteness of the speed of light has important consequences that affect the optical response of metal nanoparticles. First of all, the electromagnetic field cannot penetrate beyond a certain depth inside the metal, the so-called skin depth, which is of the order of 15 nm in the vis-nir. [28] But more importantly, LSPR red shifts take place as the particle size increases, and retardation effects start to play a significant role when the diameter of the particle becomes a sizeable fraction of the mode wavelength, m, in the surrounding medium. [2829] These effects are clearly observed in spherical metal nanoparticles, for which, in the early 20th century, Gustav Mie developed an analytical solution to Maxwell s equations that describes the scattering and absorption of light by

27 26 Chapter 1: General introduction spherical particles. [30] The Mie s solution can be largely simplified in the case of small particles, kr 1, where only the first, dipole term is significant and thus: ( ) (1.1) where is the extinction cross section, R is the radius, and is the relative dielectric constant of the medium surrounding the nanosphere. This equation shows that the interaction between a metal nanoparticle and light depends strongly on its dielectric properties ( and ). Although other factors are also important, from an engineering perspective the material properties of the plasmonic structure are the key, as the environment and other parameters (like excitation wavelength) are often fixed. When the denominator of the bracketed expression in equation 1.1 approaches zero will become extremely large and the optical absorption and scattering at this particular frequency would also be exceedingly strong. This is known as a resonance condition. To achieve this, must be: (1.2) which is not possible for standard dielectrics and non-metals that typically have values between 1 and 50. [26] This explains the dependence of the LSPR extinction peak on the surrounding dielectric environment. For example, for a solution of gold nanospheres in water ( 1.7 nm), the expected wavelength where is about 520 nm, according to the real dielectric function for gold. Also, indeed, the experimentally observed absorption spectrum of a gold colloid has a strong peak at that wavelength. As this example illustrates, the sensitivity to originates from the slope of the real part of the dielectric function in the observed wavelength range. Figure 1-5 plots and for Ag, Au, and Si, showing that Si has large positive values for, similar to other nonmetallic materials. Equation 1.1 also indicates that should be close to zero to support a strong resonance, a condition that can only be satisfied by some of the

28 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 27 metals. 31 In general, no LSPR sufficiently strong for plasmonic applications can be formed without a negative, and large values mean a lossier or weaker plasmon. Silver suffers lower losses than gold, providing silver with a higher plasmonic efficiency; however, gold is often chosen for experiments as it is easier to work with chemically and less prone to oxidation. Figure 1-5: Plot of the (A) real,, and (B) imaginary,, components of the dielectric function of Ag, Au, and Si as a function of wavelength. Reproduced from reference [38]. In a small metal particle the dipole created by the electric field of the light wave sets up a surface polarization charge, which effectively acts as a restoring force for the free electrons. The net result is that, when the resonance condition (equation 1.2) is fulfilled, the long wavelength absorption by the bulk metal is condensed into a single LSPR as shown in Figure Small Ellipsoidal Metal Nanoparticles. Gans Theory Mie theory as formulated above is strictly applicable only to spherical particles. In 1912, Richard Gans generalized Mie s result to spheroidal particles of any aspect ratio in the small particle approximation. [13] He predicted that, as a consequence of the surface curvature of the ellipsoid, which determines the restoring force or depolarization field, the surface plasmon mode would split into two different modes, transverse and longitudinal (along the short and

29 28 Chapter 1: General introduction major axes, respectively). Therefore, the response was quantified as a function of the ellipsoid aspect ratio. Electron microscopy reveals that the shape of most nanorods is closer to cylinders or sphero-capped cylinders than to ellipsoids. [19] However, an analytical solution for such shapes is not possible. Solutions have been found for the case of an infinite cylinder and for oblate and prolate ellipsoids. Gans found that the absorption cross section for a prolate spheroid of dimensions a, b and c along the x, y and z axes (j = 1,2,3), respectively, and where a>b=c, is: ( ) ( ) (1.3) where V is the volume of the ellipsoid and Pj, the depolarization factor along the j axis, which accounts for the restoring force of the electrons along that axis to their initial situation: [ ( ) ] (1.4) (1.5) Here is the ellipticity of the ellipsoid, which is intrinsically related to its aspect ratio through ( ) (1.6) Thus, Gans s theory predicts a strong dependence of the optical properties of elongated particles with their aspect ratio. Because of the difficulty in preparing non-spherical samples, little experimental work to support Gans formulation existed until the 1950s. In the 1960s, Stookey and Araujo stretched glasses containing small silver spheres. [32] The small particles were aligned in the molten glass to form necklaces, which exhibited red-shifted absorption spectra. More recently, it has become possible to chemically prepare small gold

30 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 29 or silver nanorods (AgNRs). In explaining the optical properties of these small rods, it has been common to treat them as ellipsoids, which allows the Gans formula to be applied. [33] Gans s equation predicts how the plasmon mode peak position varies with aspect ratio for small ellipsoids embedded in the same medium. Thus, small changes in aspect ratio lead to drastic changes in transmitted colours as seen in the samples in Figure 1-6. The fact that the plasmon band appears to be drastically red-shifted from the positions predicted by the Gans model, led El- Sayed and co-workers to propose that the water layers around the AuNRs were polarized and had a substantially higher refractive index than water. [34] From the micrographs, one explanation is that cylindrical particles exhibit longitudinal surface plasmon bands red-shifted from those of similar sized ellipsoids. Figure 1-6: The colour of AuNRs and the respective micrographs. The colour changes take place for very small changes in mean aspect ratio (from 2.1 to 4.2). Reproduced from reference [5] Numerical Methods The deviation of AuNR morphologies from the spheroidal geometry was one of the reasons behind the development of new theoretical approaches that

31 30 Chapter 1: General introduction allow us to understand the optical response of such complex nanocrystals. Unfortunately, the complexity of the electromagnetic field in the presence of arbitrarily-shaped nanoparticles is such that Maxwell s equations must be solved using numerical methods, which is not a straightforward task. In this context, methods such as the boundary element method (BEM), discrete dipole approximation (DDA), finite difference in the time domain (FDTD) or T-matrix have proven the highest versatility and efficiency, and thus have become the most popular procedures. In the BEM, the electromagnetic field scattered by a nanoparticle is expressed in terms of boundary charges and currents, which upon imposing the customary boundary conditions for the continuity of the parallel components of the electric and magnetic fields, leads to a system of surface-integral equations. This system is solved by discretizing the integrals using a set of N representative points distributed at the boundaries, so that it turns into a set of linear equations that are solved numerically by standard linear algebra techniques. [35] The finite-difference time-domain (FDTD) method is arguably the simplest, both conceptually and in terms of implementation, of the full-wave techniques used to solve problems in electromagnetics. However, as with all numerical methods, it does have its share of artefacts and the accuracy is contingent upon the implementation. The FDTD method can solve complicated problems, but it is generally computationally expensive (solutions may require a large amount of memory and computation time). The FDTD method employs finite differences as approximations to both the spatial and temporal derivatives that appear in Maxwell s equations. The discrete dipole approximation (DDA) is a general method to compute scattering and absorption of electromagnetic waves by particles of arbitrary geometry and composition. Initially the DDA was proposed by Purcell and Pennypacker, who replaced the scattered by a set of point dipoles. These

32 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 31 dipoles interact with each other and the incident field, giving rise to a system of linear equations, which is solved to obtain dipole polarizations. All the measured scattering quantities can be obtained from these polarizations. [36] All three methods discussed in this section can be applied to arbitrarily complex geometries and frequency-dependent dielectric functions, though FDTD requires the permittivity to be expressed as a sum over a limited number of Lorentzians. DDA and FDTD rely on simple volume parameterizations, which can be advantageous when very complex geometries are considered. BEM and DDA apply to separated particles with no need to parameterize the medium between them, whereas FDTD can become prohibitively time consuming in this case. BEM can be advantageous when sharp corners and narrow gaps between metal surfaces are considered. 1.3 Wet-Chemistry Synthesis of Metal Nanorods Noble metals prefer to form quasi-spherical over anisotropic particles because they crystallize in the face-centred cubic (fcc) lattice, and typically exhibit surface energies that result in largely spherical (e.g., truncated octahedral) Wulff shapes. Nevertheless, in recent years strong progress in shape-selective syntheses of metal nanoparticles has been achieved by kinetic growth of nanostructures and by surface-specific adsorption of polymers or ions. [37],[38] Gold and silver nanorods have been prepared by reduction of gold and silver salts in hard templates, for example, single-walled carbon nanotubes. [39] The most established method for the preparation of rod-like gold and silver nanoparticles is the reduction of silver or gold salts in aqueous solutions of CTAB in the presence of seed particles. [14],[15],[40] The reduction of the metal salts can be achieved by the use of chemical reducing agents as well as electrochemically or photochemically. The aspect ratio of the rods can be

33 32 Chapter 1: General introduction controlled by the concentration ratio between metal salt and seed particles. [15],[14],[40] The crystalline properties of seed particles (single or twinned crystalline structures) govern the morphology of the final products. Applications exploiting the surface plasmon resonance, SERS, or optical activity effects of metal nanoparticles require stability of the particle shape over a longer period of time because all these properties are strongly governed by the particle shape. Therefore, it is indispensable to investigate not only various approaches to synthesize anisotropic nanostructures but also the stability of the particles against shape-changing effects in general, as Xia et al. pointed out in a recent review. [41] For that purposes, the main seed-mediated synthesis that have been used for the preparation of metal nanorods during the present thesis are: (i) synthesis of pentatwinned nanorods, (ii) synthesis of single-crystal nanorods, and (iii) synthesis of core-shell nanorods Synthesis of Pentatwinned Nanorods Another synthetic approach that has become increasingly popular in recent years is the use of preformed nanocrystals as seeds for further growth. Unlike the methods discussed so far, the nucleation and growth steps are separated in this type of synthesis, allowing for a greater control over the final morphology. [41] This method is highly versatile and can be used to manipulate the size, aspect ratio, and shape of the resulting nanostructures. [14],[38],[42],[ If the metal being deposited has the same crystal structure and lattice constant as the seed, the crystal structure of the seed will be transferred to the product via epitaxial overgrowth. Despite this, the final shape of the nanostructure can deviate from that of the initial seed as the crystal habit is also governed by the growth rates of different crystallographic facets, making this a versatile and interesting system. There are two main categories of seed-mediated syntheses: homogeneous and heterogeneous growth. If the seed crystal is composed of the same metal as the atoms being deposited onto the surface, then this is a homoepitaxial process, for example, the growth of silver on small silver seeds.

34 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 33 This approach has been exploited by several groups to achieve more precise control over the size and aspect ratio of metal nanostructures. By controlling the growth conditions in aqueous surfactant solutions it was possible to synthesize AuNRs with tuneable aspect ratio. It was found that addition of AgNO3 influences not only the yield and aspect ratio control of the AuNRs but also the mechanism for rod formation and correspondingly their crystal structure. [5] Moreover, citrate-capped gold nanoparticles, prepared through reduction of HAuCl4 with borohydride ions have traditionally been chosen as seeds for AuNR growth. An important issue has been the origins of the twinning planes observed in such rods. A detailed TEM analysis revealed that the gold seeds are multitwinned particles, with diameters of 4 5 nm. This method was first reported by Murphy s group. [14] Synthesis was performed by the addition of citrate-capped small gold nanospheres to a growth solution obtained by the reduction of gold ions with ascorbic acid in the presence of CTAB. The addition of small seed particles into the Au + solution resulted in complete reduction to metallic gold, which is catalysed by the surface of the seeds and leads to the gradual change in shape from quasi-spherical to rod-like crystal. It was determined that addition of less seed generally led to higher aspect ratio rods. The aspect ratio could be precisely controlled through a careful variation of the amount of seed added to the growth solution; it was observed that a decrease in the reaction rate favours nanorod formation over isotropic growth, thereby increasing the yield of nanorods. Interestingly, the rods prepared in this way (multitwinned particle seeds, no Ag + ) exhibited a pentagonal cross section and a five-fold twinning can be easily distinguishing (Figure 1-7).

35 34 Chapter 1: General introduction Figure 1-7: Morphology dependence of gold nanoparticles grown from either singlecrystal d) or multiply-twinned e) seeds, in the presence a)-c) and absence f)-h) of silver nitrate. Reproduced from reference [9]. Seeded growth is a viable method for controlling the aspect ratio of AgNRs (Figure 1-8). [14],[40],[42] Silver decahedrons can serve as seeds in the synthesis of AgNRs with a pentagonal cross section. In this case, the decahedrons grew along the fivefold twinning axis into faceted, pentagonal nanorods through a kinetically controlled pathway. The aspect ratio of the pentagonal nanorods could be controlled in the range of 1-12 through silver deposition in an aqueous solution at 95 ºC with citrate as the reducing agent, and the aspect ratio could be pushed even higher than 40 if an additional purification step was included between different growth stages. [42] The diameter of these nanorods was found to match that of the seeds, while the lengths could be varied from 50 nm to 2 μm depending on the amount of silver precursor added into the solution. Other seed-mediated growth methods for

36 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 35 generating AgNRs and nanowires have also been reported. By using a micellar solution composed of CTAB, rounded AgNRs with a diameter of nm and an aspect ratio of , as well as nanowires with a diameter of nm with lengths in the range of 1-4 μm, could be formed. [40] Owing to the poor control over the synthesis of AgNRs in aqueous media, these kind of nanoparticles have been synthetized through the seeded mediated synthesis used AuNRs as seeds in order to gain mayor control over the final nanoparticles. [50] Figure 1-8: Polyol method for synthesizing Ag nanostructures. In (A) the reduction of silver ions by ethylene glycol leads to the formation of nuclei. Nuclei can contains multiply twinned boundary defects, single twinned boundary defects, or are singlecrystalline with no boundary defects. These seeds are then grown into different nanostructures like (B) spheres, (C) cubes, (D) truncated cubes, (E) right bipryamids, (F) bars, (G) spheroids, (H) triangular plates, and (I) wires. Reproduced from reference [38].

37 36 Chapter 1: General introduction Synthesis of Single-Crystal Nanorods Contrary to the results obtained with citrate-capped gold nanoparticle seeds, when CTAB is used during borohydride reduction of HAuCl4, singlecrystal seeds are obtained, which are also smaller in size. Significant improvement of this method was achieved in 2003 by El-Sayed et al., who were able to minimize the formation of spheroidal particles and produce the rod-like morphology in high yield. [15] Authors made two modifications to this method: replacing sodium citrate with a stronger CTAB stabilizer in the seed formation process and utilizing silver ions to control the aspect ratio of AuNRs. When CTAB-capped Au seeds were used (in the presence of AgNO3) single-crystal AuNRs in unprecedented high yields are obtained. However, only after the high resolution transmission electron microscopy (HRTEM) study by Liu and Guyot-Sionnest, [43] was it understood that the seeds were single-crystals with diameters around 1.5 nm (Figure 1-7). Because of the extremely high yield of nanorods obtained from single-crystal seeds, this has become by far the most popular synthetic route. This work also demonstrated that a twin plane is not a pre-requisite for rod growth. The protocol includes two steps: i) synthesis of seed solution by the reduction of chloroauric acid in the presence of CTAB with sodium borohydride and ii) the addition of the seed solution to the Au + stock solution in the presence of CTAB. Silver nitrate is introduced to the gold solution before seed addition to facilitate the rod formation and to tune the aspect ratio as well. This method produces high yield AuNRs (99%) with aspect ratios from 1.5 to 4.5 and avoids repetitive centrifugations for sphere separation. The aspect ratio of the nanorods can also be controlled consistently by keeping the silver nitrate concentration constant and varying the amount of seed. [5] Liu and Guyot-Sionnest found that by controlling the ph of the reaction between 2 and 4, the reaction time is increased (from 1 h up to 2 h) and longer nanorods can be obtained. [43] Recently, Murray and co-workers demonstrated tuning of LSPR through addition of aromatic salicylate additives as well as hydrochloric

38 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 37 acid, which was explained through an effect on the micellar structure of CTAB during rod growth. [44],[45] This method can produce higher aspect ratio AuNRs with fewer spherical impurities. Many variations on this general scheme have been published. One strategy is to perform a one-pot synthesis of nanorods by adding sodium borohydride directly to the nanorod growth solution instead of forming a separate seed solution. [46],[47],[48] Although these methods are generally termed seedless, the addition of sodium borohydride still forms seed gold nanoparticles in situ. While the main benefit of these procedures is the simplicity of preparation, one problem that we have encountered is that the quality of even fresh borohydride can be variable and, given the technique s sensitivity to borohydride concentration and lack of seeds in the procedure, reproducibility of this method may be fairly low Synthesis of Core-Shell Nanorods In heteroepitaxial seed-mediated growth, the seeds and added atoms are chemically different. In this method, alloy, core-shell, and other complex nanostructures can evolve from the nucleation and growth of foreign seeds. Similarly to homoepitaxial methods, the growth can be forced into unique patterns by varying the types of reductant, precursor, and capping agent. [49],[50] As a prerequisite for heteroepitaxial growth, there must be a close match in lattice constant between the seed and the deposited metal (for example, gold and silver have a lattice mismatch of only 0.25%). Large differences in lattice constants can lead to non-epitaxial growth and unexpected structures with very different geometries from the original seeds. [49],[50] AuNRs of ca. 20 nm in diameter were coated with silver in an aqueous system containing AgNO3, ascorbic acid, PVP, and CTAB. Silver deposition is favored on the AuNRs faces than the tips, being covered the entire AuNR. [50],[51]

39 38 Chapter 1: General introduction 1.4 Techniques for Structural Characterization of Metal Nanorods The structural characterization of the different samples studied in this thesis has been carried out mainly by means of electron microscopy and small angle scattering techniques. On one hand, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) are based on the interaction between an accelerated electron beam and the matter on which it is irradiated. As a result of this interaction, a number of electron beam-matter interaction processes occur. The different types of electron microscopy techniques employed differ in the type of collected signal to generate the image or to carry out the analysis. On the other hand, the use of scattering techniques (light, neutrons and X-rays) to probe the structure of materials at the mesoscopic (1nm to 1 m) scale is very popular. This is due to different properties, an important one being that these techniques are non-invasive. A second advantage is that they yield physical quantities averaged over the whole sample. [52] The scattering at small-angles originate from the spatial fluctuations of the electron density (for X-ray scattering, SAXS), or neutrons (for neutron scattering, SANS) within the material. [53] Electron Microscopy A) Scanning Electron Microscopy SEM is a powerful tool for examining and interpreting microstructures of materials, and is widely used in the field of materials science. The principle of SEM is based on the interaction of an incident electron beam and the solid specimen. [54] The signals that derive from electron-sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of the materials making up the sample. In most applications, data are collected over a selected

40 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 39 area of the surface of the sample, and a two dimensional image is generated that displays spatial variations in these properties. The SEM is also capable of performing analyses of selected point locations on the sample; this approach is particularly useful in qualitative or semi-quantitative determination of chemical compositions, crystalline structure, and crystalline orientations. Figure 1-9: Scheme of a scanning electron microscope with its different components. Due to the manner in which the image is created, SEM images have a characteristic three-dimensional appearance and are useful for judging the surface structure of the sample. The filament (electron gun) used in a scanning electron microscope generates a beam of electrons in a vacuum created inside

41 40 Chapter 1: General introduction the chamber where the samples are kept for analysis. That beam is collimated by electromagnetic condenser lenses, focused by an objective lens, and scanned across the surface of the sample by electromagnetic detection coils (as is shown in Figure 1-9). The primary imaging method is by collecting secondary electrons that are released by the sample. The secondary electrons are detected by a scintillation material that produces flashes of light from the electrons. The light flashes are then detected and amplified by photomultiplier tube. By correlating the sample scan position with the resulting signal, a black and white image can be formed that is absolutely similar to what would be seen through an optical microscope, thanks to the illumination and shadowing of the surface topography by electrons. Secondary electrons are most valuable for showing morphology and topography on samples (Figure 1-9). B) Transmision Electron Microscopy Figure 1-10: Scheme of a transmission electron microscope with its different components

42 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 41 TEM is one of the most important tools of nanotechnology for imaging nanomaterials with sub-nanometre resolution. The most important application of TEM is the atomic-resolution real-space imaging of nanoparticles. By forming a nanometre size electron probe, TEM is unique in identifying and quantifying the chemical and electronic structure of individual nanoparticles. [55] TEM has become indispensable for characterization of nanocrystalline materials, particularly when particle shape is important. Additionally, it is very powerful for revealing atom distributions on nanocrystal surfaces even when they are passivated with polymers. Today s TEM is a versatile tool that provides not only atomic-resolution lattice images but also chemical information at a spatial resolution of 1 nm or better, allowing direct identification of the chemistry of a single nanocrystal. [55] With a finely focused electron probe, the structural characteristics of a single nanoparticle can be fully characterized. In this technique a thin specimen is imaged by an electron beam, which is irradiated through the sample at uniform current density. [56] The typical acceleration voltage in an operational transmission electron microscope is between 80 and 200KV (Figure 1-10). The electrons are emitted from a thermionic (tungsten of lanthanum hexaboride filaments) or field emission (tungsten filament) electron guns. The illumination aperture and the area of specimen illuminated are controlled by a set of condenser lenses, the function of the objective lens is either image of diffraction pattern formation of the specimen Small angle techniques Small-angle scattering was discovered in the late 1930s by Guinier during X-ray diffraction experiments on metal alloys. [57] The basic formalism of smallangle scattering is similar for light, neutrons and X-rays. [58] The important difference is in the interaction of the radiation with the scattering medium. The scattering of light originates form refractive index variations while neutrons are scattered by atomic nuclei. As a result, these scattering techniques are very

43 42 Chapter 1: General introduction complementary. [53] When a coherent beam (either light, neutrons or X-rays) of flux illuminates a sample with a certain volume and thickness, a given fraction of the flux is elastically scattered in the direction of the coherent beam within a solid angle 52] Figure 1-11 depicts the scattering geometry of a typical SAXS experimental setup. A highly collimated and monochromatic X-ray beam of wave length ( ) impinges on a sample and the scattered intensity in the forward direction is recorder by a two dimensional detector. The transmitted primary beam is fully absorbed by the beam stop placed in front of the detector and the entire flight path before and after the sample is in vacuum to avoid absorption and scattering by air. In the experiment, the number of photons scattered as a function of the scattering angle ( ) is measured. For a given sample, the amount of recorded photons varies with the number of incident photons per second per unit area (photon flux) and the sample to detector distance. Therefore, the quantity that can be compared in different experiments is the number of photons scattered into unit solid angle normalized to the incident photon flux. The scattering at small angles is fully elastic because of the high energy of the radiation as compared to typical excitations in the sample. [59] Therefore, the magnitudes of the incident (ki) and scattered (ks) wave vectors are equal,, and the refractive index is close to unity. The momentum transfer or scattering vector, and its magnitude, ( ) This quantity indicates the typical length scales probed by the underlying scattering experiment and it has a unit of reciprocal length. Two main sources of neutrons are used for SANS experiments, one is steady-state reactors and the other is spallation source. In the first case, neutrons are continuously produced by fission processes. In the second case, a pulsed neutron beam (typically with 25 or 50 Hz frequency) is generated by the

44 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 43 collision of high-energy protons which chop off heavy atoms. The time-of-flight method is used on the instruments to analyse the neutrons arriving on the detector. Consequently, the geometry and handling of SANS experiments depends on the kind of source. Figure 1-11: Schematic layout of a SAXS setup depicting the incident, scattered and transmitted X-ray beams, the 2-D detector, and the definition of the scattering vector (q). Represented from reference [59]. The spectrometer D22 (ILL, France) was used to develop part of the work described in this thesis. Line D22 will be described as example for steady-state instrument, and a scheme is shown in Figure [60] Figure 1-12: Schematic representation the steady-state instrument D22 at the Institut Laue Langevin. Represented from reference [60].

45 44 Chapter 1: General introduction 1.5 Structural Characterization of Metal Nanorods A thorough understanding of the atomic structure of nanocrystals and the formation of facets at their surface is required to optimize their properties. For gold nanocrystals, it is known that the catalytic and optical properties can be tuned in a reproducible manner by controlling their morphology. [9] The classical description of metal nanorods growth is based on the formation of AuNRs single-crystals with a <001> longitudinal growth direction. [17] It is widely accepted that the physical properties of nanostructures depend on the type of surface facets. [61],[62] For AuNRs, the surface facets have a major influence on crucial effects such as reactivity and ligand adsorption and there has been controversy regarding facet indexing. [18],[63] Wang et al. carried out a detailed high resolution transmission electron microscopy (HRTEM) study to elucidate the crystalline structure of AuNRs, which seemed to indicate formation of nanocrystals with an octagonal cross-section, bound by alternate {100} and {110} facets that would converge at the tips in the form of {110} and {111} facets, respectively. [17] Although this analysis was carried out on nanocrystals synthesized electrochemically, it was assumed that it holds for those obtained by chemical seeded growth. In fact, Liu and Guyot-Sionnest corroborated these observations in 2005 on AuNRs synthesised by a seeded growth method. [43] Such a description would imply that the actual areas of the two types of lateral facets are not equal but the angles between them are all equivalent. However, the proposed assignment is not the only possible interpretation of their analysis because the study was carried out on AuNRs lying flat on the TEM grid, so that not all the different orientations of the nanocrystals could be probed.

46 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 45 Figure 1-13: HRTEM image of a standing AuNR in the [001] zone axis; the measured angles between lateral facets are indicated; black arrows point to <110>-type directions and red arrows to <100>-type directions. Note that small bevels can be seen, mainly at <110>-type corners. Represented from reference [18]. A HRTEM study of standing AuNRs synthetized in gemini surfactants was also performed by Carbo-Argibay et al. [18] The key to determining the crystalline structure of AuNRs is the capability of analysing them standing perpendicularly to the substrate so that the different orientations and crystallographic directions, in the cross-section, can be properly distinguished. When standing AuNRs were imaged by HRTEM, their octagonal cross-section was readily identified, as shown in the example selected for Figure 1-13 and in many others analysed particles. It is clear from that the <100> and <110> directions are always in coincidence with corners meaning that the lateral facets are {250}-type. A representative model of the crystalline structure proposed by Wang et al. [17] and the novel crystalline structure proposed by Carbó-Argibay and coworkers [18] are represented in Figure 1-14.

47 46 Chapter 1: General introduction Figure 1-14: Representation of the two models shown in cross-section (1) and in 3D view (2). A) Model proposed by Wang; the section is enclosed by alternating {100} and {110} facets with different areas; the angles between the facets are all 135º. B) Our model; all facets are of {250}-type and with the same area, and have alternate angles of 136.4º and 133.6º between them. Note that the <100> and <110> directions in the new model are pointing to the edges between facets, contrary to Wang s model, in which they are pointing to the centre of the facets. Represented from reference [18]. Katz-Boon et al. also reported a meticulous characterization of singlecrystal AuNRs. [63] They identified the apexes of the octagonal cross-section as <110> and <100> and the lateral facets of the rods as the direction that bisects the angle formed by these two directions (45º), that is, the <5 12 0> direction. If we compare to an octagon made of {250} facets, the eight angles can also be considered close to the angles made by an octagon of {5 12 0}. Such a difference is within the error range of the technique and therefore one can think that in the actual structure of single-crystal AuNRs the directions of the lateral edges are

48 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 47 <100> and <110>, while the lateral facets present high Miller indexes, that can be either {5 12 0}, or {250} or both, taking into consideration that a crystalline surface is not something rigid, but dynamic, and the structure can easily change between two types of facets as similar as {5 12 0} and {250}. As alternative to conventional HRTEM, surface morphologies of nanocrystals can be characterized in three dimensions using electron tomography. [64],[65] AuNRs, obtained by seed-mediated growth in aqueous solution, assisted by surfactants (CTAB [15] or gemini surfactants [80] ) and Ag + ions were investigated. The results are presented in Figure 1-15-a and b, revealing a faceted morphology for both types of AuNR, in agreement with previous reports. [17],[63],[18] These observations are complemented by intensity profiles acquired from high-resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images. HAADF-STEM yields a signal that is proportional to the thickness of the specimen and to the atomic mass of the atoms in the sample. [66],[67]

49 48 Chapter 1: General introduction Figure 1-15: Comparison between AuNRs grown with CTAB and gemini surfactants. a,b) 3D visualizations of tomographic reconstructions of both types of AuNR. c e) 3D reconstruction of AuNRs. a) Three orthogonal slices through the reconstruction of a AuNR grown with CTAB. It can be seen that {110} and {100} facets compose the morphology of the rod. The tip is rounded, with clear terraces at the {101} planes. b) Three corresponding slices through a reconstruction of a AuNR synthesized with the gemini surfactant. For this rod a more rounded morphology of the cross-section is observed including {520} facets. The facets composing the tip of the AuNR are comparable to the AuNR grown with CTAB. f,g) The intensity profile acquired from the first projection f) corresponds to a model where the morphology is composed of {110} and {100} facets. d,g,h) The intensity profile acquired from the projection of the rod grown with the gemini surfactant. Represented from reference [19]. 1.6 Self-Assembly of Metal Nanorods Self-assembly refers to the process by which nanoparticles or other discrete components spontaneously organize due to direct specific interactions and/or indirectly, through their environment. [68] Self-assembly is typically

50 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 49 associated with thermodynamic equilibrium, the organized structures being characterized by a minimum in the system s free energy, though this definition is perhaps too broad. [69] Essential in self-assembly is that the building blocks organize into ordered, macroscopic structures, either through direct interactions (e.g., by interparticle forces), or indirectly using a template or an external field. The thermodynamic forces that drive self-assembly may need to be modulated, either by rational use of chemistry or templating, or directed by means of external fields. Directed self-assembly still employs the basic principles of selfassembly by carefully choosing and constructing the building blocks, yet facilitates the process by modulating the thermodynamic forces without going into advanced and intricate techniques. This may also give rise to novel ordered non-equilibrium structures, free from the constraints of entropy maximization, and hence these systems can reside in a state of local equilibrium within the global free energy with low entropy states often characterized by complex spatial or coherent spatiotemporal organization. [70] A common example of selfassembly is the precious opal, which is basically a self-assembly of silica particles 150 to 300 nm in diameter. These spherical particles adopt a closepacked hexagonal order in crystalline phase. The size distributions and packing order of these particles determine the colour and quality of the precious opal. [71] Nowadays, monodisperse nanoparticles such as gold, silver, cobalt, etc. are readily synthesized, and can be reproducibly made in large quantities with a variety of shapes and sizes in a controlled way. [9],[41],[72] Similarly, the assemblies of the very same nanoparticles isotropic as well as anisotropic are beginning to show, in turn, collective properties which set them apart from their disordered dispersions and bulk samples. [4] Self assembly has emerged as a powerful technique for controlling the structure and properties of ensembles of inorganic nanoparticles. Indeed, nanoparticle arrays (two- or threedimensional (2D or 3D)), have been shown to present properties different from their bulk dispersions, and many applications are envisaged for such superstructures, from photonics [73] to nanoelectronics. [74] Thus, research

51 50 Chapter 1: General introduction activities in the field of nanoscience are shifting from the synthesis of individual nanoparticles to the preparation of nanoparticle ensembles, the characterization of the properties of nanostructures, and the realization of their applications. [70],[75] Interest in ensembles versus individual nanoparticles is driven by the ability to realize and exploit the collective properties of nanoparticles in clusters containing several nanoparticles of large-scale 2D or 3D lattices. In some cases, the coupling of electronic, magnetic or optical properties of individual nanoparticles in their clusters lead to new, interesting and potentially useful properties of nanoparticle ensembles. The generation of nanostructures containing from several to thousands of anisotropic nanoparticles paved the way for applications that exploit directional (vectorial) properties of nanoparticle arrays. For example, assembly of nanorods enables the realization of properties attributed to the directionality and the double size confinement (along the length and the diameter of nanorods), which find applications in metamaterials, [76] in materials for information storage, [77] and in sensing devices. [78] Significant progress has been gained through various assembly techniques which will be broadly separated into four categories: binding of rods to a surface, evaporation-induced assembly, polymer-composite driven assembly, and solution-phase assembly. The process of drying a solution containing nanoparticles can have a particularly large effect on their self-assembly. Several different forces are applicable as the process of drying occurs. Nanoparticle concentration is increased greatly during solvent evaporation and may be further concentrated at the edge of a drying drop through convective forces, thereby increasing interparticle interactions. Large capillary forces at a water drying front can also be utilized to direct and control self-assembly of nanostructures. [75] Drying of an aqueous solution of rods can lead to interesting selforganization even without any additives. There is a large tendency for inorganic nanorods to assemble in a side-to-side fashion at the appropriate

52 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 51 concentration of CTAB in solution. The propensity of rods to assemble in this fashion often leads to phase separation of rods from spherical impurities during the drying process. [79] Lower aspect-ratio CTAB-capped rods can be further assembled into large-scale smectic- or nematic- like assemblies on length scales of several microns, through slow-drying of a concentrated rod solution. [20] By utilizing a slow drying process and the coffee-ring effect, rods are able to assemble into larger and more defect-free crystalline superstructures as seen in Figure The monodispersity of rods used during crystallization affects the quality of the final assembled structure and thus has been more effectively achieved using highly uniform rods developed in our lab through a growth procedure in a cationic gemini surfactant which produces highly monodisperse AuNRs, which were found to form close packed structures. AuNRs can be standing or laying down, and can be in a nematic or hexagonally close-packed phase throughout the entire sample area. [80] CTAB-capped-rods, in particular, appear to have a high propensity for forming such structures if there is a sufficiently high concentration of free CTAB in the drying solution. It is possible that depletion-mediated effects may play an important role in driving the assembly similar to what has been observed in solution since the CTAB concentration has been found to be an important variable. [81] This type of assembly is not limited only to surfactant-capped rods. For instance, hexa(ethylene glycol) undecanethiol functionalized gold rods were also slowly evaporated to form close-packed nanorod superstructure similar to those that can form with CTAB-capped rods. [81] It was determined that both rod and capping agent concentrations affected the assembly. [37] However, moving to a lithographically patterned substrate can lead to much more efficient self-assembly. [82],[83] Such technique utilizes capillary forces at the edge of drying droplet [83] or at an evaporative boundary of a confined solution. [82] The template is patterned to contain assembly sites into which

53 52 Chapter 1: General introduction nanorods can fit, which is achieved by thermal scanning-probe lithography [83] or by electron-beam lithography. [82] Figure 1-16: SEM micrographs of AuNRs synthetized in 16-EO 1-16 surfactant, on a silicon wafer obtained at high concentration (10-6 M). a) Partial view of the ring formed by dorp casting. b,c) Top view of the ordered nanocrystal superlattice at different magnifications. The inset in (c) is the fast Fourier transform of the image. d) Side view of an island in which 14 layers of standing AuNRs can be distinguished. Represented from reference [80] As is the case for surface deposition, incorporation of AuNRs into polymers could be critical for their real-world applications. Composites of poly(vinyl alcohol) (PVA) and nanorods coated with PVA were described by van der Zande. [84] The AuNRs were shown to be uniformly and randomly dispersed in the matrix. However, mechanical stretching of the PVA films led to a high degree of nanorod alignment along the stretching direction. Nearcomplete alignment was obtained after stretching the PVA film to about 5 times its original length. This led to the polarization of the optical response of the film; specifically, the longitudinal absorbance shrank to zero when light was

54 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 53 polarized 90 to the alignment axes. A similar result was demonstrated by Perez-Juste and co-workers (Figure 1-17). [85] Figure 1-17: a), ab) Experimental UV-vis-NIR spectra of streched PVA films containing AuNRs with two different aspect ratios -2.2 (a) and 2.9 (b)- for varying polarization angles. c), d) Simulated spectra of AuNRs in a matrix with a refractive index of 1.5. Index, is a photograph of PVA films containing AuNRs aligned parallel and perpendicular to the electric field of polarized incoming light. Represented from reference [85]. Similarly, noble-metal nanoparticles with localized surface-plasmon resonances have been recently used to prepare new materials with improved optical circular dichroism. [86] This interest stems from a wide range of applications in biology and physics, including the structural determination of proteins and DNA [87] and the pursuit of negative refraction. [88] Guerrero- Martinez et al. have published a novel class of metamaterial consisting on AuNRs organized in a 3D chiral structures and yielding a record circular dichroism anisotropy factor for metal nanoparticles (>0.02) across visible and near-infrared (Vis NIR) wavelengths ( nm). [89]

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62 CHAPTER 2 Surfactant (Bi)Layers on Gold Nanorods Gold nanorods in aqueous solution are generally surrounded by surfactants or capping agents. This is crucial for anisotropic growth during the synthesis and for their final stability in solution. When CTAB is used, a bilayer has been evidenced from analytical methods even though no direct morphological characterization of the precise thickness and compactness has been reported. The type of surfactant layer is also relevant to understand the marked difference in further self-assembly properties of gold nanorods as experienced using 16-EO1-16 surfactant instead of CTAB. To obtain a direct measure of the thickness of the surfactant layer on gold nanorods synthesized by the seeded growth method, we have coupled TEM, SAXS and SANS experiments for the two different cases, CTAB and 16-EO1-16. Despite the strong residual signal from micelles in excess, it can be concluded that the thickness is imposed by the chain length of the surfactant and it corresponds to a bilayer with partial interdigitation.

63 62 Surfactant (Bi)Layers on Gold Nanorods 2.1 Introduction Surfactants are commonly used during the synthesis, stabilization, and crystallization of various types of colloidal nanoparticles including metallic ones. Due to their high versatility, surfactants play a key role in different steps of metal nanoparticles formation such as solubilisation of the initial reactants, evolution toward the final nanocrystal shape, and nanoparticle stabilization in different solvents. [1] In the particular case of gold nanorods (AuNRs), the surfactant seems to be one of the structure-directing agents inducing anisotropic growth in aqueous media through binding to specific nanocrystal facets. [2] In the seeded growth method in aqueous solution, the most popular synthesis of AuNRs, a large amount of cetyltrimethylammonium bromide (CTAB) is present, some of it remaining attached to the surfaces at the end of the reaction. [3],[4] When properly dried, these anisotropic nano-objects can selfassemble into 3D superlattices with a parallel orientation with respect to the macroscopic substrate. [5] Moreover, it has been recently shown that when CTAB is replaced by a gemini surfactant ((Oligooxa)alkanediyl-, bis(dimethylhexadecylammonium bromide surfactant (16-EO1-16)), [6] more monodisperse AuNRs are obtained with a fascinating capability for 3D selfassembly into standing AuNRs superlattices with significantly larger areas. [7] These strong differences between the two cases renew the initial interest to characterize the surface layers of the amphiphilic on AuNRs in aqueous solutions. Indeed, the surfactant surface layer is thought to be the source of the colloidal stabilization, and it is known that 16-EO1-16 surfactants have higher surface activity, with enhanced tendency to form aggregates in solution than their monomeric equivalent. [8],[9] Characterization of the CTAB surfactant layer on AuNRs has been reported using TGA, FTIR, and zeta potential. [10],[11] The methods used were of analytical nature and thus based on counting the amounts of each species from a relative composition point of view. The extracted values were in good

64 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 63 agreement with the presence of a surfactant bilayer on the surface of the AuNRs. However, there has been no direct measurement of the morphological characteristics (thickness, density, interdigitation level, and patterning) of such CTAB bilayer. To date, equivalent analytical measurements for 16-EO1-16 surfactant on AuNRs are not even available. Direct imaging of surfactant layers adsorbed on solid macroscopic surfaces could be carried out using atomic force microscopy (AFM). For CTAB on mica substrates, this technique revealed that the nanostructure of the layer varies with time. [12] For a concentration twice the critical micelle concentration (CMC), the layer is initially made of parallel cylindrical micelles of thickness 42 Å with a repetition distance of 70 Å. After 24 h of equilibrium, a flat layer is finally observed. The observed thickness is above that formerly determined by the surface force apparatus (SFA). [13],[14] In this technique, a real surface average is obtained in very well-defined geometrical conditions. Moreover, in the SFA technique, the measured thickness could unambiguously be attributed to a repulsive bilayer of CTAB molecules. The value found in that case was 33 ± 1 Å. For symmetric 16-EO1-16 surfactants it has been found that the patterning very much depends on the chain length and specific spacers so that longer spacers lead to strong patterning while short ones even lead to flat bilayers on mica. [15] All these experiments have the advantage to be performed in situ with the presence of solvent. Their main drawback is the macroscopic dimension of the surfaces, even if AFM probes only a very small part of it. An excellent way to probe in situ the structure of nanoobjects is small angle scattering of either X-rays or neutrons (denoted as SAXS and SANS, respectively). [16],[17] These techniques are particularly sensitive to the shape of the objects in a size ranging from 1 nm to a few hundreds of nanometres. [18] When the objects are made of two different materials, as it is the case in AuNRs stabilized by surfactants, neutrons further offer the capability to change the relative weight of each material in the scattering signal. This contrast variation technique is very popular in biology and soft matter where X-rays can hardly

65 64 Surfactant (Bi)Layers on Gold Nanorods distinguish between functional groups. [19] For aqueous solutions, this technique has been used to characterize the thickness of the surfactant bilayer stabilizing aqueous magnetic fluids or to measure the structure of non-ionic surfactant layers adsorbed on spherical monodisperse silica. [20],[21],[22],[23] In this last example, the average thickness together with the inner structure of the surfactant layer was accessible showing that a silica bead was decorated with micelles whose shape depends on the silica core size. Concerning gold nanoparticles dispersed in aqueous phase, few examples can be found about the SANS characterization of the surrounding surfactant layer. [24],[25] In this work, small angle scattering was used to gain a better characterization of the surfactant layer remaining on AuNRs after seeded growth synthesis for both the classical CTAB and a 16-EO1-16 surfactant counterpart, and to reveal the possible differences between them. This was a challenging task due to the typically very low concentration of nanocrystals in solution, their potential nanoparticle polydispersity in size and shape, together with the inherent presence of free micelles in solution. Therefore, to obtain firmly experimentally grounded conclusions we coupled both SAXS, to determine the polydispersity, and SANS to obtain the morphological characteristics of the surfactant layer. 2.2 Experimental section and methods Synthesis and sample preparation for scattering analysis: Gemini surfactant: Bis (hexadecyl dimethyl ammonium) dietyl ether bromide surfactant (16-EO1-16), and Bis (tetradecyl dimethyl ammonium) dietyl ether bromide surfactant (14-EO1-14), were synthesized according to procedures described in the literature. [26] (See Appendix I Figure I-1) Seed solution: CTAB solution (4.7 ml, 0.1 M) was mixed with 25 L of 0.05 M HAuCl L of sodium borohydride was added quickly and with

66 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 65 vigorous stirring. The resulting solution of CTAB-stabilized gold seeds (2-3 nm) was kept at 30 ºC. AuNRs synthesis in CTAB: AuNRs were prepared by seeded growth. [4],[27] The seeds were grown in the presence of CTAB (0.1 M), HAuCl4 (0.5 mm), AgNO3 (0.06 mm) and ascorbic acid (0.75 mm) as a mild reducing agent. The growth solution was kept at 30 ºC during 2 hours in a thermostatic bath. The nanocrystals were purified by centrifugation (3500 rpm, r = 4.5 cm, 20 min), decreasing the content of spheres obtained after synthesis. AuNRs synthesis in gemini surfactant: Analogously to the previous synthesis, AuNRs were prepared by seeded growth at 30 ºC through reduction of HAuCl4 with ascorbic acid on CTAB-stabilized gold seeds in the presence of 16-EO1-16 (0.05 M), HCl (ph 2 3), and AgNO3 (0.12 mm). [7] After synthesis no further purification of AuNRs was performed. Same synthesis was carried out for 14-EO1-14 just replacing the 16-EO1-16 for 14-EO1-14 keeping the concentrations of the other reactants equal. After synthesis no further purification of AuNRs was performed. Sample preparation for scattering analysis (purification and solvent exchange versus stability of AuNRs): The use of scattering techniques requires obtaining stable dispersions of nanoparticles with a minimum of additional components in the solution (gold nanoparticles with different morphologies and surfactant micelles). As a first stage, the best possible separation of spherical particles from the AuNRs is required. For CTAB, one centrifugation step was used to minimize the spheres content as described above. Regarding 16-EO1-16, the different attempts of shape purification were not effective and samples had to be used as such. A second requirement is to reduce the excess of free surfactant (CTAB or 16-EO1-16) derived from AuNRs synthesis. However, the surfactant molecules in the layer around the nanoparticles are in dynamic equilibrium with free surfactant molecules in the solution. It is known that aggregation of AuNRs can be induced when the purification process is pushed too far, usually

67 66 Surfactant (Bi)Layers on Gold Nanorods decreasing the concentration of surfactant below the CMC. [28] In the present case, this problem also occurred during the solvent exchange step and the attempts to replace the supernatant after centrifugation by pure water induced aggregation in few minutes. As a consequence, the following protocol was followed: after the synthesis and one centrifugation step when necessary (as described above), the AuNRs solutions were centrifuged three times (8000 rpm, r=4.5 cm, 40 min) to remove excess reactants. The final concentration of surfactants was kept at the CMC of CTAB (1.0 mm) and 16-EO1-16 (0.5 mm) to avoid undesired agglomeration of particles. Analogously for SANS experiments, the procedure was repeated three times more, using solutions of pure D2O or a mixture (H2O/D2O) at the contrast matching condition with surfactants at their respective CMCs (1 mm for CTAB and 0.05 mm for 16-EO1-16). During all these sample preparations, the absorbance at 400 nm was used to quantify the total concentration of gold atoms in the solutions, [29] and the stability of the AuNRs was checked by following both the transversal and longitudinal plasmon bands in the corresponding extinction spectra. Two gold concentrations (2.5 mm and 6.0 mm) were chosen for this study. Preparation and characterization of Gold nanorods supercrystals Taking advantage of the exceptional self-assembly properties of gemini surfactants, AuNRs were synthetized using the gemini surfactants 14- EO1-14 and 16-EO1-16, following the protocol described in the experimental section of this chapter. To carry out the study of self-assembly of AuNRs, the nanoparticles were concentrated 100 times (up to 50 mm on Au concentration), keeping constant the surfactant concentration (1.0 mm). To analyse the selfassembly process by Scanning Electron Microscopy (SEM), one drop of 10 L of the AuNRs solution was deposited by simple drop casting on an indium tin oxide (ITO) slide, and also on kapton film to proceed to study the self-assembly by SAXS in transmission. Kapton was chosen to use because of the low absorption in the q range which is our interest.

68 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 67 Size and shape characterization: After the synthesis and washing steps, TEM samples were prepared from the obtained solutions by drying one droplet on a carbon copper TEM grid. The grids were analyzed in a JEOL JEM 1010 TEM operating at an acceleration voltage of 100kV. The analysis was performed on 750 particles to describe the distribution in size and shape of the gold nanoparticles. The SAXS experiments were performed in a home built camera at the LIONS laboratory. The SAXS camera is equipped with a copper rotating anode, collimating optics and a MAR 2D detector. Samples were analyzed in borosilicate capillary with 1.5 mm path length. After radial averaging, the intensity was calibrated using the classical procedures. The SANS experiments were performed at the ILL on D22 beam line. Three configurations were used at =0.8 nm to ensure a large final q-range from 2.31x10-3 to 5.37x10-2 Å -1. Typical acquisitions in 1 mm path length cells were: 4000 s to 6000 s for low q-range, 2200 s to 3600 s for medium q-range and 600 s to 900 s for large q-range depending on the composition of the solvent (D2O or contrast match). Absolute intensities were obtained after radial averaging using the classical procedure developed at ILL. [30] Analysis of the scattering diagrams: The SAXS and SANS diagrams were fitted using the particular distribution of cylinders as revealed by the TEM images. [31] Indeed, the distribution of width and length around their mean (R0, L0) are not decoupled as usually assumed. [32] Using a representation of diameters versus lengths (L,D) of the individual nanoparticles, it was unraveled that the cloud of nanoparticles is distributed along two major axis (u,v), rotated with an angle as compared to (L,D): (2.1) (2.2)

69 68 Surfactant (Bi)Layers on Gold Nanorods In the present cases, the distribution in size and shape were experimentally obtained from TEM analysis. The distributions were fitted with a Gaussian shape in u and v (Eq. 3): ( ) (2.3) In summary, the experimental TEM cloud is described by five parameters R0, L0, u, v,. Using these distributions, the SAXS and SANS scattering intensities can be calculated. Regarding the SAXS diagrams, the contrast in scattering length density (proportional to electron density) is very strong between the gold core and water well above the contrast between water and the surfactant layer. [18],[33] Accordingly, three levels of scattering length density are enough to describe the system. Thus, the fits were performed using a summation of the intensities over this (u,v) type of distribution (Eq. 4): ( ) ( ) ( ) ( ) (2.4) where N, being the number density of objects, (1.21x10 12 cm -2 ) the scattering length density contrast between gold and water, [33] f(u,v) the normalized distribution over u and v, and V and Pcyl, the volume and form factor of a cylinder of dimension L and R, respectively. ( ) ( ) (2.5) with ( ) { ( ) (( ) ) } ( ) In most cases, the direct use of the distribution parameters deduced from TEM yields perfectly satisfying adjustment of the SAXS diagrams. However, in some occasions, slight adjustments were required. All size parameters obtained by TEM and SAXS analysis are listed in Table 1.

70 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 69 For the SANS experiments, the scattering length densities (SLD) were calculated using the NIST CNR scattering length density calculator. [34] One gets -0.3 x cm -2 for the alkyl chain layer and 4.51 x cm -2 for the gold core. The latter can be matched by a mixture of 75% D2O (6.38 x cm -2 ) and 25%vH2O (-0.56 x cm -2 ). [18],[30] The SLD profile takes into account the surfactant polar head layer but not its solvation due to its small size compared to the core radius and the alkyl chain thickness. Accordingly, no single contrast is dominating the two others and three levels of scattering length density are mandatory to properly describe the system. For D2O solvent or D2O/H2O mixture, a model for core-shell nanorods was used to calculate the intensity scattered by nanoparticles surrounded by a surfactant layer. For core-shell AuNRs with a constant layer thickness t and a polydispersity in u and v, the scattering intensity is given by (Eq. 6): ( ) ( ) (2.6) with ( ) ( ) ( ) ( ) ( ) ( ) (2.7) The resolution of the instrument has been taken into account in the final determination of the scattered intensity I(q), using the classical procedure described in reference 17. Based on the f(u,v) determination by TEM or slight adjustments of it to recover the SAXS diagram, the calculated SANS diagrams were fitted to the experimental ones using only the thickness t as a parameter (see Table 1). The concentration of AuNRs is imposed by the value found by SAXS fitting. Using the same set of parameters to reproduce TEM, SAXS, SANS in D2O and SANS at the contrast match is a strong validation to characterize the AuNRs and particularly the surfactant layer stabilizing them.

71 70 Surfactant (Bi)Layers on Gold Nanorods 2.3 Results and discussion The first step was checking the signal coming from the CTAB micelles in D2O in the absence of nanoparticles. As is shown in Figure 2-1, the residual signal coming from the free micelles cannot be neglected as compared to the total scattered intensity, especially at large q. After purification and solvent exchange of AuNRs, the SANS patterns of a D2O solution of AuNRs ([Au] = 2.5 mm) stabilized by CTAB in equilibrium with an excess of CTAB ( CMC) is shown in Figure 2-1. Therefore, the intensity from free micelles must be subtracted to correctly identify the signal from the AuNRs and their surrounding surfactant shell. The experimental SANS signal from residual CTAB micelles ([CTAB] = CMC) was subtracted from the SANS signal from AuNRs solution stabilized by CTAB (as shown in Figure 2-1). Subtraction does not affect the positions of the minima but enhances the amplitude of the oscillation. This scattered intensity is quite low (below 10 2 cm 1 ) in the interesting q range ( Å 1 ) for the selected gold concentration ([Au] = 2.5 mm). This value corresponds to a AuNR solid volume fraction equal to 2.6 x 10-5, a very low value three orders of magnitude lower than the silica solid volume fraction used in the recent study on surfactant-coated silica. [22],[23] Moreover, considering an average radius of 56 Å for the gold core of AuNRs, the volume fraction of adsorbed bilayer (assuming a thickness of 34 Å) would be 4.0 x 10-5, leading to a surfactant concentration 0.1 times the CMC for CTAB. Accordingly, the concentration of gold was increased to 6 mm to raise the signal-to-noise ratio by a factor of two. Above 6.0 mm, the aggregation between AuNRs was too important to allow correct measurements. The signal from residual micelles remains a strong limitation to characterize the organic shell around AuNRs because the surfactant layer is not static as described in the literature for CTAB, [35] and thus, it is not possible to eliminate completely the unbound CTAB without aggregation of the AuNRs. Therefore, the method used here ensures the stability of the AuNRs with a control on the concentration of

72 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 71 the excess surfactant. However, as shown in Figure 2-1, subtraction is not reliable beyond q = 0.1 Å -1. Figure 2-1: SANS pattern of ( ) AuNRs solution in D2O ([Au] = 2.5 mm, [CTAB] CMC) and ( ) CTAB micelles ([CTAB] CMC). ( ) SANS pattern of AuNRs solution after subtraction of the signal from residual CTAB micelles. (Insets) UV-vis spectra of the AuNRs solution (top right) and schematic view of AuNRs in equilibrium with micelles and monomers (bottom left) are shown. The general behavior is very similar for the AuNRs stabilized with the 16- EO1-16 surfactant (oligooxa)alkanediyl-, -bis(dimethylhexadecylammonium bromide (16-EO1-16), where an excess of surfactant was also necessary to ensure stability. Nevertheless, for 16-EO1-16 the tendency toward aggregation was

73 72 Surfactant (Bi)Layers on Gold Nanorods found to be higher as shown in Figure 2-2, due to the higher amphiphilic character that 16-EO1-16 surfactants display in water. [36],[37] At 6.0 mm, an increase of the intensity in the low q-range region was observed in the SANS patterns of AuNRs stabilized by 16-EO1-16 in D2O, contrary to the CTAB case. This behavior is in good agreement with the specific self-assembly properties of AuNRs stabilized with 16-EO1-16 surfactants recently evidenced. [7] For further analysis and shape fitting, the q 4 regime due to the envelope of the selfassembled AuNRs was subtracted following a classical procedure. [38] Figure 2-2: SANS patterns of AuNRs solution in D2O ([Au] = 6.0 mm in D2O), stabilized with 16-EO1-16 (Δ) or CTAB (O). (Inset) SANS patterns of AuNRs solution ([Au] = 2.5 mm) at the contrast match (H2O/D2O, 25/75) stabilized by 16-EO1-16 (Δ) or CTAB (O).

74 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 73 Figure 2-3: SAXS and SANS scattering length densities for a AuNR stabilized by CTAB layers. Characterization of the Bilayer by Coupling SAXS and SANS SAXS and SANS patterns for AuNRs stabilized by 16-EO1-16 in D2O ([Au] = 6.0 mm and [16-EO1-16] = 0.5 mm) are compared in Figure 2-4. In the SANS pattern, one observes a shift of the oscillation toward a lower q value, an indication that neutrons are seeing thicker objects than X-rays. The marked difference in the q dependence of the two signals is direct proof of a system with more than two levels of scattering. These levels correspond to the gold core, the surfactant layer, and the solvent. For SAXS patterns, the X-rays are interacting with the electrons of the material, giving us a distribution of the shape and size of the nanoparticles. As a consequence, the surfactant and water have close values of electronic densities as compared to the gold core, thus, the surfactant shell does not show a strong scattering in the SAXS diagram. In contrast, the large difference of scattering lengths for neutrons between hydrogen and deuterium makes SANS the best way to characterize the

75 74 Surfactant (Bi)Layers on Gold Nanorods surfactant layer. The profiles of scattering length density for the AuNRs stabilized by surfactant in D2O are given in Figure 2-3. As is shown in Figure 2-3, the SAXS will give the shape and size of AuNR and subtracting this value in the SANS data we will obtain the value of the surfactant bilayer. The gold core nanoparticles can even be matched by a D2O/H2O (75/25) mixture. Surfactant (concentration) [Au], mm (UV) R 0 (Å) L 0(Å) u (for TEM) v v (SANS) T, (Å) CTAB (6 mm) (10) EO 1-16 (6 mm) (10) CTAB (2.5 mm) EO 1-16 (2.5 mm) Table 2-1: Parameters issued from TEM AuNR distribution (R 0, L 0, u, v, ) with the (L, D) representation and used to calculate the theoretical SAXS intensity (the fitted values, when done, in place of TEM measured ones are in italic). These parameters are used to fit SANS patterns with the thickness t of the organic layer as the only one variable. The analysis can be pushed beyond the initial conclusion that a layer different from pure gold is present on the surface of the AuNRs. A quantitative fitting of the scattering data was performed using equations 1.6. First, the SAXS theoretical intensity was calculated with the parameters issued from the TEM nanoparticle distribution in length and diameter (L, D) representation. It matches the experimental SAXS patterns as shown in Figure 2-4 for AuNRs stabilized by 16-EO1-16 and in Figure 2-5 for AuNRs stabilized by CTAB. It is remarkable that the obtained nanoparticles were not described by a Gaussian distribution in L and D but in u and v defined by the anisotropic branch in the (L, D) representation (shown in the inset of Figure 2-4 for 16-EO1-16 and Figure 2-5 for CTAB). The parameters are summarized in Table 2-1, and one can note

76 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 75 that 16-EO1-16 leads to shorter AuNRs than CTAB, with a similar diameter and polydispersity in v. In a second stage, the same (L, D) distributions were used to calculate the SANS intensity using the thickness t and scattering length density ρ2 of the surfactant layer to adjust the calculated curves on the experimental ones. The scattering length density of an alkyl chain is usually taken around cm 2. [18],[39] Figure 2-4: SAXS and SANS patterns of AuNRs stabilized by 16-EO1-16 ([Au]=6.0 mm and [16-EO1-16] = 0.5 mm): ( ) SAXS; (Δ) SANS for D2O solution; lines correspond to the theoretical curves obtained with the parameters given in Table 1. (Inset right) Experimental TEM distribution used to calculate the SAXS and the SANS intensity for AuNRs covered by an organic layer with a thickness of 30 Å; (left) TEM image of the sample in D2O.

77 76 Surfactant (Bi)Layers on Gold Nanorods Figure 2-5: SAXS and SANS patterns of AuNRs stabilized by CTAB ([Au] = 6.0 mm and [CTAB] = 1.0 mm): ( ) SAXS; ( ) SANS for D 2O solution; (x) SANS for the same solution at the contrast match of gold (D 2O/H 2O (75/25)); lines correspond to the theoretical curves obtained with the parameters given in Table 2-1. (Inset right) Experimental TEM distribution used to calculate the SAXS and SANS intensity for AuNRs covered by an organic layer with a thickness of 34 Å; (left) TEM image of the sample in D 2O. The SAXS pattern and two SANS patterns (D2O and the contrast match condition) could be fitted using a common set of morphological parameters. They are shown in Figure 2-5 for the AuNRs stabilized by CTAB. The fit of three different curves with a common set of parameters strongly supports the morphological conclusions that can be extracted. The best fits were obtained for a thickness of 34 Å. This value is in agreement with the study of Kékicheff et al., [40] in which a 33 Å thickness for a CTAB bilayer on mica immersed in aqueous CTAB micelle solution has been reported using the surface force

78 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 77 apparatus. [41] This is also in agreement with the results from Sau et al. [11] by TEM on dried AuNRs stabilized by CTAB, where the smallest interparticle distance was found to be 34 Å, suggesting that only two layers of CTAB are present between the obtained dried AuNRs. Also, 3D superstructures of AuNRs in layers, with the same orientation and with a hexagonal arrangement, leading to micrometer-sized self-assembled areas were obtained. A minimum distance between AuNRs surfaces can be estimated at approximately 3 nm, a value that is in agreement with TEM. Taking into account geometrical considerations for the 16-EO1-16 surfactant, in which the length of one of the cationic alkyl chains is around 1.7 nm when completely extended, we propose a simple model of surfactant nanoparticle interaction in which a 16-EO1-16 surfactant bilayer is connected to two AuNRs. [22] Figure 2-6: SAXS and SANS patterns of AuNRs stabilized by 16-EO 1-16 ([Au] = 2.5 mm and [16-EO 1-16] = 0.5 mm): ( ) SAXS; ( ) SANS for D 2O solution; ( ) SANS for

79 78 Surfactant (Bi)Layers on Gold Nanorods the same solution at the contrast match of gold (D 2O/H 2O (75/25)); lines correspond to the theoretical curves obtained with the parameters given in Table 1. In inset: (right) experimental TEM distribution used to calculate the SAXS and the SANS intensity for AuNRs covered by an organic layer with a thickness of 34 Å; (left) TEM image of the sample in D 2O. Similar results were obtained for AuNRs stabilized by 16-EO1-16 (Figure 2-6) with a thickness of Å for the two different gold concentrations. One can note that the agreement with a flat bilayer goes in that case up to q = 0.15 Å 1 due to a lower CMC contribution. Figure 2-7: Effect of the thickness t on the SANS scattered intensity for similar (R 0, L 0, u, v, 0) and constant number of gold nanoparticles for AuNRs stabilized by CTAB ([Au] = 6.0 mm, [CTAB] = CMC): (t = 40 Å (black dotted line), t = 34 Å (red line), t = 30 Å (green line), t = 20 Å (brown line)). The inset shows the I q 2.9 versus q representation.

80 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 79 We stress that the calculated scattering intensity is very sensitive to the thickness t as demonstrated in Figure 2-7 (CTAB case) where four different thicknesses were tested (40, 34, 30, and 20 Å) for a given concentration of gold nanoparticles using the scattering length density (ρ2) of CTAB for the organic layer. The value of 34 Å yields the best fit, and even if 30 Å is within the experimental accuracy, the two other extreme values (20 and 40 Å) can be discarded (for both the position of the minima and the values of the intensity). Definitely a monolayer with a small fraction of extra charge (a start of double layer), mandatory to ensure colloidal stability, can be eliminated. Regarding the scattering length density of the organic layer ρ2 the model shows that it is not possible to recover the experimental intensity by increasing ρ2 above cm 2 (i.e., increasing the hydration of the organic layer above 20%). As shown in Figure 2-8, for a given concentration in AuNRs and a constant amount of adsorbed surfactant a larger thickness (corresponding to a larger ρ2) is incompatible with the experimental scattered intensity in absolute units. Hence, applied to the present cases, the sensitivity analysis gives the following. For both CTAB and 16-EO1-16 surfactants, the fitting is sensitive to the thickness of the layer and we can definitely conclude that the thickness is neither 40 nor 20 Å, the best value is laying in the window Å. The comparable values for 16-EO1-16 and CTAB surfactants layers reflect the equal length of their alkyl chains. Indeed, this surfactant feature imposes the overall layer thickness, more than the hydrophilic headgroup type does. However, in both cases, the measured layer thickness is less than a full compact double layer of surfactant. Indeed, a completely extended alkyl chain would be around 20 Å leading to a bilayer of 40 Å. Taking into account that these two values were firmly discarded, we propose a model of surfactant nanoparticle interaction in which CTAB or 16-EO1-16 builds up a bilayer by interdigitation of the alkyl tails by strong hydrophobic interactions. The obtained bilayer stabilizes the AuNRs in solution. This is a first demonstration that the bilayer is fully expanded at the surface of the gold nanoparticles, since the dimension of

81 80 Surfactant (Bi)Layers on Gold Nanorods the surfactant layer was not accessible with the previous techniques used. Finally, beyond the overall thickness of the surfactant layer it is also found that the type of surfactant head strongly influences the stability, as shown by the enhanced tendency of the 16-EO1-16 stabilizing layer to induce attractive interactions between the AuNRs when the concentration is increased. Figure 2-8: Effect of the scattered length density of the organic layer 2, for similar (R 0, L 0, u, v, 0) and constant number of gold nanoparticles for AuNRs stabilized by CTAB ([Au] = 6.0 mm, [CTAB] = CMC): ( 2 = cm -2, t = 34 Å) (red line), ( 2 = cm - 2, t = 40Å) (green line), ( 2 = cm -2, t = 55Å) (brown line)). The inset shows the I q 2.9 versus q representation.

82 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 81 Structure of Gold Nanorod Supercrystals determined by SAXS When a highly concentrated AuNR colloid solution (~10-6 M) stabilized with 14-EO1-14 and 16-EO1-16 was drop casted on an ITO slide, the deposition of AuNRs typically resulted in the formation of coffee ring-like deposits (Figure 2-9-a and -c). As clearly shown in Figure 2-9-b and -d, AuNRs self-assemble into standing multilayers, with extraordinary long-range order. Figure 2-9-b and -d shows representative SEM images of a representative multilayer 3D array with hexagonal arrangement. As expected from such an arrangement, each NR monolayer is shifted by half the inter-nr distance with respect to the adjacent layers. Figure 2-9: SEM micrographs of AuNRs on an ITO slide obtained at high concentration (10-6 M). a) and b) correspond to AuNRs synthetized in 16-EO 1-16, being a) a partial view of the ring formed upon casting and b) the top view of the ordered nanocrystal superlattice. c) and d) correspond to AuNRs synthetized in 14-EO 1-14, being c) a partial view of the ring formed upon casting and d) the top view of the

83 82 Surfactant (Bi)Layers on Gold Nanorods ordered nanocrystal superlattice. The inset is a high magnification view of an island of standing AuNRs in which some layers can be distinguished. The crystalline arrangement of such multilayer supercrystals was investigated by means of small angle X-ray scattering (SAXS). In particular, we measured three different samples and the SAXS data were compared to theoretical scattering analyses, specifically design for the expected structure. The calculation follows the scheme presented in appendix I, for lattices made of one monolayer of 30 X 30 AuNRs organized in a 2D hexagonal packing. Hence, the AuNRs superlattice has a finite extension and the envelope presents a form factor. As we considered for the calculation superlattices made of monodispersed AuNRs, the form factor oscillation is propagating far in the scattering spectra and produces the high frequency oscillations in the scattered intensity reported in the Figure Figure 2-10: SAXS diagrams for the assemblies of AuNRs synthetized in 16-EO 1-16 [a) and b)] and 14-EO 1-14 [c) and d)]. a) and c) are the experimental SAXS diagrams

84 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 83 obtained for a AuNRs supercrystal, and b) and d) have incorporated a theoretical SAXS diagram obtained by the program described in Appendix I. Figure 2-11: a) corresponds to AuNRs with R=6.5 nm and the 4 th Bragg peak is present. b) corresponds to AuNRs with R=7.2 nm and the 4 th Bragg is missing in both the experimental scattering diagram and the theoretical curve. The theoretical plots were obtained for rods of length L = 100 nm but the length of the AuNRs does not have too much of influence on the signal since the interaction peaks stem from a parallel ordering of the rods. All the theoretical diagrams were calculated for a center-center distance of a = 22 nm. Now, to demonstrate the influence of the size of the AuNRs (at a fixed center-

85 84 Surfactant (Bi)Layers on Gold Nanorods center distance) three different radii of the AuNRs (6.5 nm, 7.17 nm and 8.2 nm) were used and compared to the experimental data. The experimental scattering diagrams were compared to the theoretical ones. To match theoretical positions of the peaks in q, the q-range of the experiment was multiplied by an ad hoc factor. Thus for the sample synthesized with 16-EO1-16 the q-range was multiplied by The best fit of the Bragg series was obtained for a radius of 6.5 nm. In particular, this allows canceling the 5 th peak of the theoretical structure factor for a hexagonal lattice. Indeed, in that case Figure 2-11 shows the second minimum of the form factor of the AuNRs (related to the diameter of the rods) at the same position as the fifth Bragg peak. Taking the same parameters as for the standing layer, a similar calculation was performed with an average over the whole possible orientations of the superlattice. Thus, in that case, they were not considered as perfectly standing but were allowed to take all possible orientations. A fit with the same Bragg peaks and the same disappearance of the 5 th peak was obtained. Hence, the Bragg peaks are too weakly defined in the experimental diagram and it is not possible to determine with the present data if the AuNRs are all standing perpendicular to the substrate or if they form a 3D hexagonal layer. Thus we could not confirm by SAXS that the AuNRs were all standing. However, interesting data can be extracted from the spectra. Indeed, applying the contraction to the fitting parameters leads to real values of R = 6.24 nm and a center to center distance of a = 21.1 nm in the hexagonal lattice. In that case, the face to face separation is df-f = 8.64 nm. df-f is calculated using equation 2.8. (2.8) This obtained value is larger than the thickness of two bilayers of 16-EO1-16 which is 6.4 nm. This result was recovered for the two other cases as well. In the case of the sample synthesized with 14-EO1-14, this is the 4 th peak of the

86 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 85 theoretical scattering which is absent in the experimental scattering diagram. The q-range did not need to be contracted and the absolute distances of the fit are a = 22 nm, R = 7.17 nm and df-f = 7.66 nm. Finally, a second sample was synthesized with 16-EO1-16 but with larger AuNRs. Again, the 4 th peak is absent. After correction of the contraction factor, the absolute distances are a = 27. 7nm, R = 9.03 nm and df-f = 9.65nm. The distance is again a bit above two bilayers of 16-EO1-16, but close to what was found for the former case of 16- EO1-16 sample. 2 R a L Stabilizing layer: mono or bilayer? d f-f =a-2 R Figure 2-12: geometrical definitions for a hexagonal close packing of AuNRs. The larger distances found in the SAXS experiments as compared to the ones in SEM, may arise from an effect of the drying. Indeed, during the evaporation required for SEM experiments, the capillary forces drive the AuNRs more closely together. This can induce the release of the extra layer in the interstitial gap left between the surface of the AuNRs, which are not in contact (see Figure 2-12).

87 86 Surfactant (Bi)Layers on Gold Nanorods 2.4 Conclusions Complementary measurements of TEM, SAXS and SANS have allowed us to extract a fine description of the structure of surfactant- stabilized AuNRs. TEM and SAXS were used to characterize the gold cores. The unique feature of neutrons with contrast variation combined with the very high flux of the D22 spectrometer have permitted us to extract the signal of the surface layer from that of free micelles and to determine the thickness of both CTAB and 16-EO1-16 surfactant layers. A bilayer thickness of 32 ± 2 Å with an almost nominative alkyl chain density (80 100%) was observed for the two cases within experimental accuracy. [42] These results confirm the presence of a surfactant bilayer on the surface of AuNRs, a result which was widely accepted but opens the question of a potential interdigitation of the alkyl chain as the obtained thickness is smaller than twice the extended alkyl chain length. However, some questions remain open. No internal patterning of the bilayer could be detected due to the overall weakness of the signal arising from the intrinsic low concentration of this type of nanoparticle suspension and a strong contribution from free micelles. More details could have been obtained by increasing the AuNRs concentration by at least one order of magnitude; unfortunately, the corresponding suspensions were not sufficiently stable with time. Accordingly, measurement of a different amount of adsorption on the ends of the AuNRs as compared to the lateral facets, which is often invoked as the source of the anisotropic growth and chemical reactivity of AuNRs, is still to be demonstrated by direct structural techniques. The effect of the gemini surfactant bilayers on the self-assembly of the AuNRs is still under study, however, as preliminary results SAXS transmission studies of the assemblies of AuNRs can be reproduced by the model developed for a 2D hexagonal close packing. These results are in good agreement with the observations published recently by Guerrero-Martinez and coworkers, [7] where

88 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 87 they confirm by SEM the perfect hexagonal geometry of the arrangements of AuNRs under drop casting conditions. References [1] Y. Xia, Y. Xiong, B. Lim, S. E. Skrabalak, Angew. Chem. Int. Ed. 2009, 48, 60. [2] C. J. Murphy, L. B. Thompson, A. M. Alkilany, P. N. Sisco, S. P. Boulos, S. T. Sivapalan, J. A. Yang, D. J. Chernak, J. Huang, J. Phys. Chem. Let. 2010, 1, [3] N. R. Jana, L. Gearheart, C. J. Murphy, Chem. Mater. 2001, 13, [4] B. Nikoobakht, M. A. El-Sayed, Chem. Mater. 2003, 15, [5] T. Ming, X. Kou, H. Chen, T. Wang, H.-L. Tam, K.-W. Cheah, J.-Y. Chen, J. Wang, Angew. Chem. Int. Ed. 2008, 47, [6] R. Zana, Y. Talmon, Nature 1993, 362, 228. [7] A. Guerrero-Martínez, J. Pérez-Juste, E. Carbó-Argibay, G. Tardajos, L. M. Liz- Marzán, Angew. Chem. Int. Ed. 2009, 48, [8] M. Borse, V. Sharma, V. K. Aswal, P. S. Goyal, S. Devi, J. Colloid Interf. Sci. 2005, 284, 282. [9] R. Zana, J. Colloid Interf. Sci. 2002, 248, 203. [10] B. Nikoobakht, M. A. El-Sayed, Langmuir 2001, 17, [11] T. K. Sau, C. J. Murphy, Langmuir 2005, 21, [12] W. A. Ducker, E. J. Wanless, Langmuir 1999, 15, 160. [13] P. Kékicheff, H. K. Christenson, B. W. Ninham, Colloids Surf. 1989, 40, 31. [14] P. Richetti, P. Kékicheff, Phys. Rev. Lett. 1992, 68, [15] S. Manne, T. E. Schäffer, Q. Huo, P. K. Hansma, D. E. Morse, G. D. Stucky, I. A. Aksay, Langmuir 1997, 13, [16] T. Narayanan, Lecture Notes Phys. 2009, 776, 133. [17] I. Grillo, Soft Matter characterization, [18] P. Lindner, T. Zemb, Neutrons, X-rays and light: Scattering Methods Applied to soft Condensed Matter, [19] P. Schurtenberger, Neutron, X-rays and light: Scattering Methods Applied to soft Condensed Matter, [20] L. Shen, E. Laibinis, T. A. Hatton, J. Magn. Mater. 1999, 194, 37.

89 88 Surfactant (Bi)Layers on Gold Nanorods [21] R. Patel, R. V. Upadhyay, V. K. Aswal, J. V. Joshi, P. S. Goyal, J. Magn. Mater. 2011, 323, 849. [22] D. M. Lugo, J. Oberdisse, M. Karg, R. Schweins, G. H. Findenegg, Soft Matter 2009, 5, [23] D. M. Lugo, J. Oberdisse, A. Lapp, G. H. Findenegg, J. Phys. Chem. B 2010, 114, [24] H. Jia, I. Grillo, S. S. Titmuss, Langmuir 2010, 26, [25] K. Rahme, J. Oberdisse, R. Schweins, C. Gaillard, J. D. Marty, C. Mingotaud, F. Gauffre, Chem. Phys. Chem. 2008, 9, [26] A. Guerrero-Martínez, G. González-Gaitano, M. H. Viñas, G. Tardajos, J. Phys. Chem. B 2006, 110, [27] J. Pérez-Juste, I. Pastoriza-Santos, L. M. Liz-Marzán, P. Mulvaney, Coorind. Chem. Rev. 2005, 249, [28] M. Sethi, G. Joung, M. R. Knecht, Langmuir 2009, 25, 317. [29] P. K. Jain, K. S. Lee, I. H. El-Sayed, M. A. El-Sayed, J. Phys. Chem. B 2006, 110, [30] [31] F. Hubert, F. Testard, G. Rizza, O. Spalla, Langmuir 2010, 26, [32] A. Henkel, O. Schubert, A. Plech, C. Sönnichsen, J. Phys. Chem. C 2009, 113, [33] B. Abécassis, F. Testard, O. Spalla, B. P, Nano Lett. 2007, 7, [34] [35] H. Takahashi, Y. Niidome, T. Niidome, K. Kaneko, H. Kawasaki, S. Yamada, Langmuir 2006, 22, 2. [36] R. Oda, I. Huc, M. Schmutz, S. J. Canday, F. MacKintosh, Nature 1999, 399, 566. [37] F. M. Menger, J. S. Keiper, Angew. Chem., Int. Ed. 2000, 39, [38] O. Spalla, S. Lyonnard, F. Testard, J. Appl. Crystallogr. 2003, 36, 338. [39] V. K. Aswal, P. S. Goyal, S. De, S. Bhattacharya, H. Amenitsch, S. Bernstorff, Chem. Phys. Let 2000, 329, 336. [40] P. Kékicheff, H. K. Christenson, B. W. Ninham, Colloids Surf. 1989, 40, 31. [41] P. Richetti, P. Kékicheff, Phys. Rev. Lett. 1992, 68, 1951.

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92 Nanoparticles: Stabilizing {100} facets Seed-mediated synthesis is the most popular methodology to control the size and shape of colloidal metal nanoparticles. The final nanocrystal shape will be defined by the crystalline structure of the initial seed as well as by the presence of ligands and capping agents that would help to stabilize certain crystallographic facets. In this chapter, we describe the synthesis and the growth mechanism of core-shell gold-silver nanoparticles in aqueous solutions, through the seed-mediated silver deposition on pre-synthesized gold nanoparticles, which display a well-defined crystalline structure and morphology. We studied the silver overgrowth process through a full structural characterization of the resulting core-shell nanocrystals. The results obtained demonstrate that the crystalline structures of the gold seed as well as the preferential stabilization of certain facets are the key factors behind silver deposition in isotropic or anisotropic fashion.

93 92 Nanoparticles: Stabilizing {100} facets 3.1 Introduction A vast number of literature reports have been devoted to the seeded growth of metal nanoparticles, in which starting from a metal nanoparticle seed it is possible to control the final particle shape and size. [1],[2],[3] The final particle shape is thought to be mainly dictated by the crystalline structure of the seed as well as by the presence of ligands and stabilizers that would help to stabilize certain crystallographic facets. The seeded growth method can be applied in aqueous media, usually mediated by the presence of surfactants, [4] or in organic solvents such N,N-dimethylformamide (DMF) or polyol processes systems. [5] The detailed analysis of the growth process and the crystalline structure of the particles allow establishing the corresponding growth mechanism in each case. For instance, Mirkin and co-workers proposed a general method to explain the shape evolution of small spherical and single crystalline gold nanoparticles in aqueous media and in the presence of surfactants. [6] More precisely they studied the influence of halide ions, either in the absence or in the presence of silver ions, on the gold overgrowth; proposing different types of growth mechanism: (i) a kinetically controlled mechanism in the absence of silver ions, (ii) a silver underpotential (Ag UPD) deposition mechanism based on the interaction of silver with the surface of gold particles, or (iii) an Ag UPD mechanism influenced by the concentration and type of halide ions. In most cases the overgrown particles are single crystalline and they could present higher or lower energy surface facets depending on the growth mechanism. The mechanistic considerations made by Mirkin and co-workers cannot be extrapolated in a straightforward manner to the seeded growth in organic solvents, mainly because the reaction conditions differ from those in aqueous media. For instance, higher temperatures are required and the solvent is actually acting as the reducing agent too. Polyol and DMF are the most widely used organic solvents for the shape-controlled synthesis of metal nanoparticles from preformed seeds and employing poly(n-vinylpyrrolidone) (PVP) as

94 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 93 stabilizer (which also has been shown to display a certain reducing ability) and shape directing agent. [5] We have recently reported the ability of N,Ndimethylformamide (DMF) as solvent and reducing agent in the seeded growth of preformed gold nanoparticles in the presence of PVP. [ 5],[7] The crystallinity of the initial seeds together with the selectively interaction of PVP with the different crystallographic facets have been identified as the main factors behind the final particle morphology. The results indicated that the growth mechanism is governed by the stabilization of facets in the order {111}>{110}>{100} which is not in fully agreement with the general sequence of surface energies for Au fcc planes. [8] Taking this into account the overgrowth of single crystalline or pentatwinned gold seeds is expected to end up in the formation of (single crystal) octahedrons and decahedrons respectively. [9] Different polyols have been employed by several groups for the shapecontrolled synthesis of silver and gold metal nanoparticles from preformed seeds. For instance, Song et al. showed that octahedral gold seeds can be readily grown, either retaining the original shape or transforming into cubes in 1,5- pentanediol by the addition of silvers ions. The growth mechanism proposed by these authors is based on the stabilization or slower growth of the {111} or {100} facets respectively, mediated by PVP adsorption and Ag UPD. [10] Surprisingly, if decahedral gold seeds are employed, overgrowth with either gold or silver produces pentatwinned gold or heterometal Ag-Au-Ag nanorods and nanowires. Therefore, the growth implies symmetry breaking and the preferential stabilization of the {100} facets in both metals. [11],[12] The Ag UPD mechanism that stabilizes and suppress the growth of the {100} facets has been claimed as the main factor in the growth mechanism. Xia and co-workers have employed ethylene glycol to study the shape evolution of different metal particles through the seed-mediated growth. [1] For instance, they showed that single crystalline silver seeds could be grown into octahedrons (enclosed by {111} facets) or cubes (enclosed by {100} facets) when either sodium citrate or PVP were used as capping agents, respectively.[ 13] Recently, the same group

95 94 Nanoparticles: Stabilizing {100} facets demonstrated that the surfaces energies of both facets can be reversed by controlling the coverage density of PVP molecules on the surface of the silver nanoparticles. [14] In general, we can consider that the seed-mediated synthesis is currently the most popular for the shape-controlled synthesis of nanoparticles. In nonaqueous solvents this process has been widely explored for silver, gold and other metals allowing in each case to propose a possible growth mechanism. [13] Nevertheless, in aqueous media the method has been developed to generate mainly gold nanoparticles and, [3] although different synthetic approaches have been recently proposed for silver, [15],[16],[17],[18] the shape-controlled synthesis of silver nanoparticles still needs to be explored more deeply. In this work we study the synthesis and the growth mechanism of coreshell gold-silver nanoparticles in aqueous solution through the seed-mediated growth on pre-synthesized gold nanoparticles. Gold nanoparticles have chosen as seeds gold nanoparticles because of their well-defined crystalline structure and morphology. From the crystalline structure point of view the seeds are classified as single crystalline, such as gold octahedrons and gold nanorods (AuNRs), and pentawinned, such as pentawined AuNRs. We studied the silver overgrowth through a wet chemical approach and we provide a full structural characterization of the resulting core-shell nanoparticles. The results clearly demonstrate that the crystalline structures of the seed as well as the preferential stabilization of certain facets are the key factors that promote the silver deposition in an isotropic or an isotropic manner for different seed morphologies. 3.2 Experimental details Chemicals: cetyltrimethylammonium bromide (CTAB), benzyldimethylhexadecylammonium chloride (BDAC), HAuCl4 3H2O, NaBH4, ascorbic acid (A.A.), silver nitrate, HCl (37%), trisodium citrate and

96 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 95 butenoic acid were purchased from Sigma-Aldrich. All chemicals were used as received. Milli-Q grade water was used as solvent. Gold nanoparticle synthesis. Low aspect ratio gold nanorods: AuNRs were prepared by the Ag+-mediated seeded growth method. [19] Seed solutions were made by mixing a CTAB solution (4.7 ml, 0.1 M) with 25 μl of 0.05 M HAuCl4, we kept this solution at 30 ºC for 5 minutes and then 300 μl of sodium borohydride was added quickly and with a vigorous stirring. The resulting solution was kept at 30 ºC. An aliquot of seed solution (24 μl) was added to a growth solution (10 ml) containing CTAB (0.1 M), HAuCl4 (0.5 mm), ascorbic acid (0.8 mm) and silver nitrate (0.08 mm). The reaction beaker was kept at 30 ºC overnight. AuNRs were washed by centrifugation (8500 rpm, 25 min) twice, the supernatant was discarded and the precipitate was redispersed in a 10 mm BDAC solution. High aspect ratio gold nanorods: Longer AuNRs were also prepared by seeded growth. [19] An aliquot (24 μl) of the same seed solution used for shorter gold nanorods, was added to a growth solution (10 ml) containing CTAB (0.1 M), HAuCl4 (0.5 mm), ascorbic acid (0.8 mm), silver nitrate (0.12 mm), and HCl (18.6 mm). The reaction flask was kept at 300C overnight. AuNRs were washed by centrifugation (8000 rpm, 25 min) twice, the supernatant was discarded and the precipitate was redispersed in a solution of BDAC 10mM. Pentatwinned gold nanorods: Pentatwinned AuNRs were prepared following a previously reported seeded growth method. [20],[21] The seed solution (gold spheres of ca. 3.5 nm diameter) was prepared as follows: 20 ml of an aqueous solution containing mm HAuCl4 and 0.25 mm trisodium citrate was prepared in a flask. Then, 0.3 ml of freshly prepared 0.01 M NaBH4 solution was added to the solution under vigorous stirring. After 30 s, stirring

97 96 Nanoparticles: Stabilizing {100} facets was slowed down and the colloidal dispersion was kept between 40 and 45 ºC for 15 min to ensure removal of excess NaBH4. In a second step the seeds were grown to 5.5 nm as follows: 5 ml of growth solution consisting of 1.25 x 10-4 M HAuCl4 and 0.04 M CTAB at ºC was mixed under stirring with ml of 0.1 M ascorbic acid. Subsequently, 1.67 ml of the 3.5 nm Au-citrate seed solution was quickly added while stirring. As a result, CTAB capped Au nanoparticles with an average diameter of ca. 5.5 nm were obtained. In the last step, pentatwinned AuNRs were grown. Briefly, to 250 ml of a growth solution ([HAuCl4] = mm and [CTAB] = 8 mm) at 20 ºC, ml of 0.1 M ascorbic acid was added. After homogenization, 750 μl of 5.5 nm Au-CTAB seed solution was added and allowed to react for several hours. As a result, a mixture of spheres, plates and rods was obtained. To separate the gold nanorods, the method reported by Jana 22]] was employed by centrifuging a total volume of 250 ml at 6500 rpm for 10 min. The supernatant was discarded and the precipitate was redispersed in CTAB 0.1 M. The samples were concentrated again by centrifugation (10 min at 6500 rpm), and the concentrated sample (4 ml) was first heated at 50 ºC for 5 min and then cooled down. After cooling, the precipitate was collected and redispersed in 9 ml of water. Gold Octahedrons: Gold octahedrons were prepared by the seeded growth method from AuNRs. The growth solution was prepared mixing 50 ml BDAC (10 mm) with 500 μl HAuCl4 (0.05 M) and 221 μl butenoic acid was added to reduce the Au+3 to Au+. The solution was kept at 30 ºC until color disappeared (20 min) and 825 μl of washed AuNRs was added as seeds, the flask was stored at 30 ºC for 2 hours. Gold octahedrons were washed by centrifugation (3500 rpm, 20 min), the supernatant was discarded and the precipitate was redispersed in a solution of BDAC 10 mm. Silver overgrowth. Silver coating was carried out following a modification of the procedure previously reported by Vaia and co-workers. [18] Regardless of the morphology

98 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 97 of the gold nanoparticles used as seeds, a growth solution (5 ml) was prepared by mixing, under stirring, 10 mm BDAC, 1 mm of AgNO3, 4 mm of ascorbic acid and 0.25 mm of Au 0 (as the corresponding gold nanoparticles). After the last addition, the temperature was increased up to ºC and kept for 3 hours. Finally, the obtained solution was centrifuged (6000 rpm 20 min) and redispersed in water. Characterization techniques Optical characterization was carried out by UV/VIS spectroscopy with a HP-Agilent 8453 or a Cary 5000 spectrophotometer. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM 1010 transmission electron microscope operating at an acceleration voltage of 100 kv. Scanning electron microscopy (SEM) images have been obtained using a JEOL JSM-6700F FEG scanning electron microscope operating at an acceleration voltage of 5.0 kv for secondary-electron imaging (SEI). Tilt series for 3D tomography were acquired using a FEI Tecnai G2 microscope, operated at 200 kv. A single tilt tomography holder (Fishione model 2020) was used for acquisition, and the alignment and reconstruction were carried out using the FEI Inspect3D software. High angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images were acquired using a double aberration corrected Titan microscope operating at 300 kv in STEM mode. 3.3 Results and discussion As pointed out in the introduction in the present work we wanted to study the silver deposition on pre-formed gold seeds with a well-defined crystalline structure, in order to establish the key parameters that govern the growth mechanism and eventually the shape and crystalline structure of the core-shell particle. Figure 3-1-a, -d and -g shows representative transmission

99 98 Nanoparticles: Stabilizing {100} facets electron microscopy (TEM) images of the three different gold seeds employed, as well as the corresponding counterparts after the silver deposition. For the sake of clarify, a bright field TEM image of the core-shell nanoparticles have been also included were the preferential deposition of silver can be easily discerned. Figure 3-1: Representative TEM images of the different gold nanoparticles used as seeds: a) gold octahedrons, d) AuNRs and g) pentatwinned AuNRs), scale bars represent 100 nm), the corresponding Au@Ag counterparts after silver deposition: b) gold octahedrons coated with silver, e) AuNRs coated with silver and h) silver overgrowth on pentatwinned AuNRs, scale bars represent 100 nm) and representative HAADF-STEM images of the core-shell particles where the preferential deposition of silver can be easily elucidated ( c), f) and i)). For a better understanding of the growth mechanism we decided to study separately the silver deposition for the gold seeds with different morphology and crystallography. A detailed characterization of the synthesized core-shell nanoparticles by means of high resolution transmission microscopy (HRTEM)

100 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 99 and electrom tomography. Such an analysis allows us to establish the corresponding morphology, crystalline structure and index of the different facets upon silver growth. This information, together with well-defined structure of the gold seed helps us to propose a mechanism for silver deposition. Silver deposition on single crystalline gold seeds Two types of gold single crystalline gold seeds were selected, namely gold octahedral, which are highly isotropic and enclosed by eight {111} facets, and AuNRs, which display high anisotropy and are enclosed by eight {520} lateral facets along a (100) zone axis. [23] We present next a detailed structural characterization of the core-shell nanoparticles resulting from silver coating in both cases. Figure 3-1-a (and Figure II-1 in the appendix II) shows a representative TEM image of the gold octahedra used as seeds with an average side length of ca. 60 nm. Although the TEM projections of the particles seem to indicate a hexagonal cross section, this is a consequence of the octahedral shape, which was confirmed by scanning electron microscopy (SEM), see Figure II- 1 in the appendix II. [ref] Silver deposition was performed at relatively low surfactant concentration (10mM BDAC), which determines that the silver precursor is AgBr as recently proposed by Park and coworkers, [18] being ascorbic acid the reducing agent. The process is carried out at 60ºC to promote a controlled silver reduction. After silver growth, TEM suggest that the particles present a square cross section, with an average side length of 102 nm (see Figure 3-1-b). Figure II- 2 in the appendix II shows representative SEM images in which the cubic morphology of the grown nanoparticles can be clearly elucidated. To understand how the deposition of the silver shell proceeded from the gold core, we carried out high resolution STEM (HRSTEM) imaging on selected samples to investigate the morphology and crystalline structure (see Figure 3-2- a). Since the signal in STEM mode depends on the atomic number of the

101 100 Nanoparticles: Stabilizing {100} facets elements, it is clear that the brighter inner region corresponds to the Au octahedron (square projection in Figure 3-2-a) and the less bright outer area corresponds to the silver shell. The corresponding fast Fourier transform (FFT) of the image reveals the absence of splitting in the diffraction spots, thereby confirming that silver has grown on the gold see in an epitaxial fashion (Figure 3-2-b). Electron tomography was also used to obtain a three dimensional volume rendering reconstruction of the particles as shown in Figure 3-2-a. The 3D image shows that the gold octahedron is surrounded by a silver cube and the six vertices of the inner octahedron are pointing at the six faces of the outer cube (see Figure 3-2-c). The FFT of the particle in the [100] zone axis (see Figure 3-2-b) shows that the six faces of the silver cub are of the [100] type, as expected. Rotating the same particle 45 o along the [001] axis (Figure 3-2-c, -e), allowed us to index also the facets of the gold octahedron. As expected, the FFT of the HRSTEM image shows that the octahedron is surrounded by {111} facets. Similar results were obtained by analyzing the selected area electron diffraction pattern (SAEDP) of the bright field TEM (BFTEM) images of the same particle oriented along the [001] zone axis (see Figure II-3 in the appendix II). A similar analysis was performed in the case of AuNRs coated with silver (see Figure 3-1-e), where analogous results were obtained for nanoparticles with low and high aspect ratio. AuNRs seeds were synthetized through the seeded growth process from single crystalline gold seeds, assistant by silver ions. [19] The crystalline structure of AuNRs has been recently revisited, revealing an octagonal cross section with high index {520} lateral facets. [23] Figure 3-1-e shows that upon silver coating the 2D projection of the particles shows a rectangular shape, but a closer look discloses the presence of particles standing perpendicular to the TEM grid, clearly revealing a square cross section as previously reported. [18] Additionally, Figure 3-1-f shows a low resolution representative high angle annular dark field scanning transmission electron microscopy (HAADF STEM) image where the AuNRs cores and the silver shell can easily be distinguished. A representative TEM image of a particle lying on

102 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 101 the grid with its corresponding SAEDP is shown in Figure II-4 (Appendix II). The diffraction pattern indicates that the nanorod is oriented along a [100] zone axis and the absence of splitting indicates again the epitaxial growth of Ag on the AuNR. Therefore, we conclude that the external morphology of the particle is indeed enclosed by six {100} facets. Figure 3-2: Upper panel; a) HRSTEM image of a Au@Ag particle, b) The corresponding FFT of the particle of image a, showing that the outer facets of the cube are {100} planes. The absence of splitting on the diffraction spots reveals epitaxial growth of silver on the gold and c) Volume rendering of the reconstructed particle of image 1a, at the same orientation with the particle in the STEM 2D projection, confirming the arrangement of the Au octahedron in the silver cube. Lower panel; d) HRSTEM image of the particle 2a, rotated 45º along the [001] zone axis, as indicated. e) The corresponding FFT of the particle 2d, showing that the facets of the octahedron are {111} planes. f) Volume rendering of the reconstructed particle of image 2d, at the same orientation with the particle in the STEM 2D projection, confirming the arrangement of the Au octahedron in the silver cube. Analysis of the standing AuNRs allowed us to confirm the crystalline structure of the lateral facets of both the AuNR core and the external Ag shell. Figure 3-3-a1 and Figure 3-3-a2 show a high resolution HAADF-STEM projection of the cross section of a standing Au@Ag nanorod (Au@AgNR) and tilted 45 degrees to demonstrate the rod shape of the particle. The

103 102 Nanoparticles: Stabilizing {100} facets corresponding Fast Fourier transform patters allow us to confirm that the crystalline facets of the external Ag shell are of the {100} type (see Figure 3-3-b). Regarding the inner AuNR, although the octagonal cross section is not perfectly defined it is clear from the HRTEM image that the corners or smaller faces are in coincidence with the [100] and [110] directions, while the side facets are mainly composed of {520} facets, in accordance with previous results (see Figure 3-3-A and -B). [23],[24] Figure 3-3: Upper panel: (a1) High resolution HAADF-STEM projection of an nanorod standing perpendicular to the C support and tilted 45 degrees (a2). Middle

104 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 103 panel: b) Fourier transform of projection a1 revealing that the external facets of the Ag shell are of type {100}. c) 3D rendering of the tomographic reconstruction of the same nanorod, shown from the same direction as the STEM projection. Lower panel: A) High resolution STEM projection of the standing AuNR core and B) the corresponding isosurface rendering of the tomographic reconstruction. This detailed projection indicates that the side facets of the nanorod are mainly composed of {520} facets. Additional TEM analysis shows that once the silver coating has induced the transformation of the original octagonal cross section AuNR into a Au@AgNR with the shape of a rectangular prism with six 100 facets, silver deposition is more or less uniform in all facets leading to a gradual decrease in the aspect ratio, eventually leading to transformation of the prism into a cube. Figure II-5 in the appendix II shows an example in which the initial AuNR were coated with increasing amounts of silver leading to a decrease of the overall aspect ratio of the particles decreases from 2.3 to 1.9. Silver deposition on penta-twinned AuNR seeds To analyse the influence of the crystalline structure of the seed on the silver deposition, pentatwinned AuNRs were selected because they have a welldefined crystalline structure, which is different from that of the single crystal AuNR while maintaining their anisotropy (see Figure 3-1-g). Pentatwinned AuNRs were synthesized using the seed mediated growth method through reduction of HAuCl4 with ascorbic acid, in the presence of preformed pentatwinned seeds and CTAB. [18] The seed structure has been shown to determine the final twinned crystalline structure of the grown AuNRs. [25] Nowadays, it is accepted that pentatwinned gold nanorods are enclosed by five {100} lateral facets and ten {111} tip facets. [26],[25] Coating of pentatwinned AuNRS was performed under similar experimental conditions as for the single crystalline gold seeds and a representative HAADF-STEM image of the grown NRs is shown in Figure 3-4- a. The contrast between Au and Ag allows as to see that in this case the deposition preferentially occurred at the tips of the nanorods, thus increasing their aspect ratio, while the initial AuNR remained at the center at the center of

105 104 Nanoparticles: Stabilizing {100} facets the nanorod. The aspect ratio of the resulting core-shell nanorod can thus be controlled by varying the ratio between silver nitrate and AuNR seeds in the growth process (see Figure II-6). A B C D E Figure 3-4: A) Representative HAADF-STEM image of Au@Ag core-shell pentawinned nanorods. B)Low resolution STEM image of a standing Au@Ag nanorod

106 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 105 and C) after 18º tilting. D) HRTEM image of a pentatwinned AuNR and the corresponding FFT patterns at both sides of the nanorod, which indicate that the left side corresponds to an <111> zone axis whereas the right side corresponds to an <110> zone axis. E) Scheme of the pentatwinned nanorod cross section oriented in the <110> and <111> zone axis. Further analysis was carried out to disclose the structure of the Au@AgNRs. Figure 3-4-b shows a STEM image of the cross section of a standing Au@AgNR where one can nicely distinguished the inner and brighter pentagon that reflects the pentagonal cross section of the AuNR core. Additionally, a darker, concentric pentagonal corona can be also appreciated that corresponds to the silver shell, confirmed by tilttilng the sample in the TEM, which reveals an elongated shape (Figure 3-4-a). Although the Ag shell seems to be deposited in an epitaxial manner, unfortunately, due to the thickness of the core-shell nanorod did not allow us to obtain a HRTEM image. Further evidence of epitaxial growth was provided the absence of splitting in the spots of the FFT patterns of two selected areas located at opposite sides of a core-shell nanorod (see Figure 3-4-d). The FFT also indicates that this pentatwinned Au@AgNR is was oriented in both the <110> and <111> zone axes and therefore only at the right side the {100} facet is actually parallel to the electron beam. Thus, the Ag deposition process seems to involve growth on the {111} facets and stabilization of the {100} lateral facets. Contrary to what was found to take place in the single crystalline seeds, the growth of pentatwinned seeds results in an increase of the aspect ratio and therefore, increased anisotropy. Growth mechanism In order to establish a growth mechanism we have to take into account a number of points that determine the final particle morphology. First of all, the experimental conditions were found to be essential for the homogeneous growth of the particles; particularly the precursor silver complex and the reduction conditions. Park et al. have shown that at relatively low surfactant

107 106 Nanoparticles: Stabilizing {100} facets concentrations, the addition of silver nitrate lead to the formation of small AgBr nanoparticles that act as precursor for Ag reduction. [18] Additionally, the reduction rate should be also careful controlled, since a fast reduction leads to highly faceted and irregular nanoparticles, but a slow reduction allows a more uniform deposition. The reducing power of ascorbic acid can be tuned by the solution ph or by temperature. Increasing the ph to values close to the pka of ascorbic acid increases considerably its reducing power and therefore subtle differences in ph increases the reduction rate, leading to irregular growth and eventually to nucleation. [18] The reaction temperature has also been reported to affect the redox potential of ascorbic acid but in a smaller extent scale, [27] i.e. increasing the temperature solution from 30ºC to 60º C it is possible to finely control the silver reduction process. The surfactant counter ion, i. e. chloride versus bromide, is also important because it will affect the silver precursor as well as the binding affinity for gold/silver surfaces. At relatively low surfactant concentrations (10mM CTAB or BDAC) a relationship [surfactant]:[agno3] ratio of 10 will produce the formation of Ag-Br or Ag-Cl complexes that, due to solubility, will evolve faster for bromide. Therefore, the aging of [Surfactant]:[AgNO3] produces more AgX clusters that, depending on the reduction conditions, could lead to particle nucleation. [18] In our particular experimental conditions we have found that the reproducibility is greater when we employed BDAC rather than using CTAB most probably because the AgCl clusters precursor develop less that AgBr, avoiding nucleation and promoting homogenous growth. Regardless of the crystallinity of the gold seed particles, as well as the index of the facets that enclose them, we have shown that with the silver coating the particles evolve into a morphology comprised by {100} facets. Taking all these considerations into account we propose a growth mechanism in which the final nanoparticle shape and therefore their crystallinity is based on a combination of kinetic control and surface

108 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 107 stabilization by the halide/surfactant adsorption. The kinetic control implies the slow reduction of the silver precursor which is essential to allow differences in the surface energy of the different facets in combination with halide/surfactant adsorption. 3.4 Conclusions We have demonstrated so far that under our experimental conditions the slow reduction of silver ions, in the presence of BDAC as stabilizer, on presynthesized gold nanoparticles with well-defined crystalline structures leads to the preferential growth of the {100} facet which is not in full agreement with the general sequence of surface energies for the different crystallographic fcc planes. But the surface energies of the different facets can be affected by the adsorption of different chemical species such as halide ions or/and surfactants. This effect would explain why single crystalline gold nanoparticles such as octahedrons and nanorods will evolve into single crystalline cubes enclosed by six {100} facets, and pentatwinned gold nanorods will evolve into core-shell nanorods with increased aspect ratio since the seed particles are enclosed by five {100} lateral facets. References [1] Y. Xia, Y. Xiong, B. Lim, S. E. Skrabalak, Angew. Chem. Int. Ed. 2009, 48, 60. [2] C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. Gao, L. Gou, S. E. Hunyadi, T. Li, J. Phys. Chem. B 2005, 109, [3] M. Grzelczak, J. Perez-Juste, P. Mulvaney, L. M. Liz-Marzan, Chem. Soc. Rev. 2008, 37, [4] Xiao, J.; Qi, L. Nanoscale 2011, 3, [5] Pastoriza-Santos, I.; Liz-Marzán, L. M. Adv. Funct. Mater. 2009, 19, 679 [6] Langille, M. R.; Personick, M. L.; Zhang, J.; Mirkin, C. A.Defining J. Am. Chem. Soc. 2012, 134,

109 108 Nanoparticles: Stabilizing {100} facets [7] Sánchez-Iglesias, A.; Pastoriza-Santos, I.; Pérez-Juste, J.; Rodríguez-González, B.; García de Abajo, F. G.; Liz-Marzán, L. M. Adv. Mater. 2006, 18, 2529 [8] Carbó-Argibay, E.; Rodríguez-González, B.; Pacifico, J.; Pastoriza-Santos, I.; Pérez- Juste, J.; Liz-Marzán, L. M. Angew. Chem. Int. Ed. 2007, 46, 8983 [9] Carbó-Argibay, E.; Rodríguez-González, B.; Pérez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzán, L. M. Nanoscale 2010, 2, 2377 [10] Seo, D.; Yoo, C. I.; Park, J. C.; Park, S. M.; Ryu, S.; Song, H. Angew. Chem. Int. Ed , 763. [11] Seo, D.; Park, J. H.; Jung, J.; Park, S. M.; Ryu, S.; Kwak, J.; Song, H. J. Phys. Chem. C 2009, 113, [12] Jung, J.; Seo, D.; Park, G.; Ryu, S.; Song, H. J. Phys. Chem. C 2010, 114, [13] Xia, X.; Zeng, J.; Zhang, Q.; Moran, C. M.; Xia, Y. J. Phys. Chem. C 2012, 116, [14] Xia, X.; Zeng, J.; Otejen, L. K.; Li, Q.; Xia, Y. J. Amer. Chem. Soc. 2012, 134, 1793 [15] Xia X.; Xia, Y. Nano Lett. 2012, 12, 6038 [16] Gong, J.; Zhou, F.; Li, Z., Tang, Z. Langmuir 2012, 28, 8959 [17] M. F. Cardinal, B. Rodriguez-Gonzalez, R. A. Alvarez-Puebla, J. Perez-Juste, L. M. Liz-Marzan, J. Phys. Chem. C 2010, 114, [18] Park, K.; Drummy, L. F.; Vaia, R. A. J. Mater. Chem., 2011, 21, [19] B. Nikoobakht, M. A. El-Sayed, Chem. Mater. 2003, 15, [20] rez- ste,. Liz-Marz n, L. M. Carnie, S. Chan, D.. C. M lvaney,. Adv. Funct. Mater. 2004, 14, 571. [21] E. Carbo-Argibay, B. Rodriguez-Gonzalez, I. Pastoriza-Santos, J. Perez-Juste, L. M. Liz-Marzan, Nanoscale 2010, 2, [22] N. R. Jana, Chem. Commun. 2003, 0, [23] E. Carbó-Argibay, B. Rodríguez-González, S. Gómez-Graña, A. Guerrero- Martínez, I. Pastoriza-Santos, J. Pérez-Juste, L. M. Liz-Marzán, Angew. Chem. Int. Ed. 2010, 49, [24] B. Goris, S. Bals, W. Van den Broek, E. Carbó-Argibay, S. Gómez-Graña, L. M. Liz- Marzán, G. Van Tendeloo, Nat. Mater. 2012, 11, 930. [25] C. J. Johnson, E. Dujardin, S. A. Davis, C. J. Murphy, S. Mann, J. Mater. Chem. 2002, 12, [26] Liu, P. Guyot-Sionnest, J. Phys. Chem. B 2005, 109, [27] N. R. Jana, L. Gearheart, C. J. Murphy, Chem. Mater. 2001, 13, 2313.

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112 CHAPTER 4 Self-assembly of Au@Ag Nanorods Mediated by Gemini Surfactants for Highly Efficient SERS-Active Supercrystals The spontaneous fabrication of self-assembled 3D highly ordered aggregates of standing supercrystals of core-shell gold-silver nanorods stabilized by gemini surfactants, which operate as plasmonic antennae enhancement of electrical field, is presented. The nearly perfect threedimensional organization of core-shell gold-silver nanorods render these systems excellent surface enhanced Raman scattering spectroscopy substrates with high optical activity, large homogenous sensing areas and the potential to maximize the SERS signal with respect to their gold nanorod supercrystal counterparts.

113 112 Self-assembly of Nanorods Mediated by Gemini Surfactants for Highly Efficient SERS-Active Supercrystals 4.1 Introduction Research on efficient substrates for surface-enhanced Raman scattering (SERS) spectroscopy has recently focused on three-dimensional (3D) plasmonic supercrystals, [1] with the aim of obtaining crystalline superlattices of metal nanoparticles with intense and controlled antenna effects, which result in huge electric field concentration. [2] The self-assembly of colloidal metal nanoparticles based on the interactions between molecules located at the nanocrystal surfaces has proven the most effective approach toward controlling nanoparticle spacing in 3D substrates. [3] In this context, anisotropic plasmonic nanoparticles such as gold nanorods (AuNRs) have been used to prepare SERS supercrystal substrates with uniform and high plasmonic antenna enhancements of the electric field. [4] Notwithstanding, the efficiency of the fabrication of selfassembled supercrystals, as optical sensors with the potential to maximize SERS signals can be improved by just using silver instead of gold and/or by refining the substrate production. [5] Aiming at the high self-assembly efficiency of biomolecules such as DNA or proteins, synthetic bio-inspired capping agents have gained popularity as surface templates in the formation of self-assembled colloidal supercrystals over micrometre length scales. [6] Among them, gemini surfactants comprising two hydrophobic tails and two hydrophilic head-groups linked by a molecular spacer, [7] have proven excellent stabilizers and binders in nanoparticle synthesis and self-assembly, [8] since they may mimic the bilayer formation of phospholipids in biomembranes. [9] In fact, we have recently investigated the structural role of gemini surfactants on the controlled synthesis of monodisperse AuNRs that self-assemble into ordered standing 2D and 3D superlattices by simple drop casting. [10] Considering the higher plasmonic efficiency of silver nanoparticles, [11] and encouraged by the need of optimization of the self-assembly process that may allow the design of largescale and highly optically active supercrystals for SERS detection, we report on

114 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 113 a straightforward approach to obtain long-range plasmonic supercrystals made of standing core-shell gold-silver nanorods (Au@AgNRs), stabilized with gemini surfactants, by controlling the drop casting conditions. Synthesis of Nanorods 4.2 Experimental section Gemini surfactant: The gemini surfactant bis(hexadecyl dimethylammonium)diethyl ether bromide (16-EO1-16) (see Appendix I Figure I-1), was synthesized according to procedures previously described in the literature. [12] Gold nanorods (AuNRs): AuNRs were prepared by the seeded growth method. [13] The seed solution was made mixing a CTAB solution (4.7 ml, 0.1 M) with 25 L of 0.05M HAuCl4; this solution was kept at 30 ºC for 5 min and afterwards 300 L of sodium borohydride (0.01 M) was added quickly under vigorous stirring. The resulting solution was kept at 30 ºC. An aliquot of seed solution (24 μl) was added to a growth solution (10 ml) containing CTAB (0.1 M), HAuCl4 (0.5 mm), ascorbic acid (0.8 mm), silver nitrate (0.12 mm), and HCl (18.6 mm). The reaction beaker was kept at 30 ºC overnight. AuNRs were washed by centrifugation twice (8000 rpm, 25 min), the supernatant was discarded and the precipitate was redispersed in 10 ml of gemini surfactant 16- EO1-16 solution (50 mm). Four hundred particles were measured on TEM images to determine the aspect ratio (5.2 ± 0.5), and the average length (68 ± 4 nm) and width (14 ± 1 nm) of the AuNRs (see Appendix III Figure III-2). The errors of aspect ratio and dimensions represent standard deviations. Silver overgrowth on gold nanorods (Au@AgNRs): Silver coating was made following a modification of a procedure previously reported by Vaia et al. [14] A growth solution (5 ml) was prepared mixing under stirring 5 mm of 16- EO1-16, 1 mm of AgNO3, 0.25 mm of Au 0 as seed AuNRs, and 4 mm of ascorbic

115 114 Self-assembly of Nanorods Mediated by Gemini Surfactants for Highly Efficient SERS-Active Supercrystals acid. After the last addition, the temperature was raised up to ºC and maintained for 3 hours. The solution was then centrifuged (6000 rpm, 20 min) and redispersed in water. Four hundred Au@AgNRs particles were measured on TEM images to determine the aspect ratio (2.1 ± 0.3), and the average length (71 ± 3 nm) and width (35 ± 2 nm) of the Au@AgNRs (see Appendix III Figure III-2). The errors of aspect ratio and dimensions represent standard deviations. Characterization of Nanorods Transmission electron microscopy (TEM) images were obtained with a JEOL JEM 1010 transmission electron microscope operating at an acceleration voltage of 100 kv. Optical characterization was carried out by UV/Vis/NIR spectroscopy with a Cary 5000 spectrophotometer using 10 mm path length quartz cuvettes for aqueous NRs solutions. Supercrystals Preparation The preparation of supercrystals of Au@AgNRs and AuNRs in coffee ring deposits were carried on ITO-coated glass slides using the previously described colloidal solutions of monodisperse nanorods in water. To control the extent of the self-assembly, previously optimized conditions such as the concentration of 16-EO1-16 above the critical micelle concentration (1.0 mm), and the nanoparticle concentration (10 6 M) were kept constant. [10] Droplets (10 L) were evaporated under controlled humidity (35, 60 and 90%) and temperature conditions (20, 30 and 40 ºC) at a Envirotronics LH4003 Laboratory Test Chamber, taking the drying stage after several hours. Supercrystals Characterization Scanning electron microscopy (SEM) images of Au@AgNRs and AuNRs supercrystals have been obtained using a JEOL JSM-6700F FEG scanning electron microscope operating at an acceleration voltage of 5.0 kv for secondary-electron imaging (SEI). Optical characterization was carried out by

116 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 115 UV/Vis/NIR spectroscopy with a Cary 5000 spectrophotometer using 1 mm thick ITO-coated glass slides for NR assemblies. Characterization of SERS Enhancement Efficiency In order to characterize the Au@AgNRs and AuNRs supercrystals films, benzenethiol (BT) was adsorbed from the gas phase over the whole surface of the films by casting a drop of BT (0.1 M in ethanol) on a Petri dish where the film was also contained. Surfaces were then mapped in a Renishaw Invia System using the Renishaw StreamLine accessory with a 50 objective. Mapping areas of μm 2, with a step size of 1 µm (40 40 spectra each) were recorded upon excitation with a 633 nm (HeNe) laser line. Acquisition times were set to 200 ms with power at the sample of 1 mw. 4.3 Results and discussion Au@AgNRs (length = 71 ± 3 nm; width = 34 ± 2 nm) were prepared from monodisperse AuNR seeds (length = 68 ± 5 nm; width = 13 ± 2 nm) following a synthetic protocol for the controlled reduction of silver ions onto AuNRs, adapted from the work by Vaia and co-workers. [14] The mixture of conventional surfactants originally employed during the growth was replaced by the gemini surfactant bis(hexadecyl dimethylammonium)diethyl ether bromide (16-EO1-16). [12] The success of the synthesis is exemplified in Figure 4-1, where the UV/Vis absorbance spectra of the AuNR seeds and the corresponding final Au@AgNR colloids in aqueous solution are displayed together with representative TEM images. Silver deposition led to an increase of the intensity and blue-shift of both the longitudinal and transverse localized surfaceplasmon resonance (LSPR) bands of the AuNRs, at 815 and 520 nm, respectively (Figure 4-1), which is in agreement with previous reports on the effect of combining the plasmonic response of gold and silver nanostructures. [11] Additionally, other high-energy modes arise upon reduction of silver onto AuNRs. The optical properties of Au@Ag nanocuboids have been recently

117 116 Self-assembly of Nanorods Mediated by Gemini Surfactants for Highly Efficient SERS-Active Supercrystals reported by several authors, [15],[16] who provided a full description of their plasmonic responses. The spectrum of the Au@AgNRs consistently presents the LSPR bands associated with a longitudinal LSPR (647 nm), a transverse LSPR (453 nm), and two higher order hybrid modes (397 and 341 nm). TEM analysis shows that after silver coating, the nanorod aspect ratio decreases significantly from 5.2 to 2.1 (see Appendix III), which reflects a favoured growth of silver on high index lateral facets. [17],[18] TEM clearly shows that the final Au@AgNRs present a core-shell morphology with an external rectangular prism morpholgy (Figure 4-1). Figure 4-1: UV/Vis absorbance spectra of the AuNR seeds (black) and the final Au@AgNRs (red) synthesized with 16-EO 1-16 surfactant. The insets show representative TEM micrographs of the corresponding AuNRs and Au@AgNRs (upper left inset is the cross section of one Au@AgNR). Optimization of the preparation of 3D supercrystals from Au@AgNRs was carried out by drop casting 10 L volumes of the colloidal solution ([16-EO1-16] = 10-3 M, [Au@AgNRs] = M) at controlled temperature (20, 30 and 40 ºC)

118 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 117 and humidity (35, 60, and 90%) conditions on indium tin oxide (ITO)-covered glass substrates (see Appendix III). A direct consequence of the interfacial aggregation properties of 16-EO1-16 at interfaces can be gathered from the scanning electron microscopy (SEM) micrographs in Figure 4-2. Spontaneous formation of self-assembled superlattices of Au@AgNRs was readily observed at optimized drop casting conditions (20 ºC and 90% of humidity). In good agreement with previous studies of AuNRs stabilized with gemini surfactants, [10] the deposition of nanoparticles typically resulted in the formation of coffee ring deposits (~40-70 m wide and ~5 mm external diameter, Figure 4-2-a and -b and a significant amount of inner coffee ring deposits. [19] As shown in Figure 4-2-b, three different adjacent and concentric ring regions were observed at the coffee ring pattern. Whereas disordered nanoparticle assemblies are observed at the outer edge (1) (Figure 4-2-c), with ring widths lower than 10 m, Au@AgNRs self-assemble into standing supercrystals in the intermediate ring region (2) (Figure 4-2--d), where layers of nanocrystals span ~40 m across with an extraordinary long-range order. Figure 4-2--e also shows a representative micrograph of the inner edge (3) of the coffee ring deposit (ring widths < 15 m), in which a multilayer 3D array of lying Au@AgNRs can be observed. From a closer view of Figure 4-2--d and -e, it can be stated that Au@AgNRs self-assemble with a simple tetragonal arrangement (a = b = 38 nm, c = 75 nm) and an interparticle distance of 3.5 ± 1 nm (see the fast Fourier transform of the SEM micrograph in Figure 4-2-d in the Appendix III), [20] in which each bilayer of side-by-side nanorods is ordered without shifts with respect to the adjacent layers in the c direction of the arrangement. This result differs from the typical hexagonal close-packed arrangement obtained in the case of AuNRs, [21] in which each nanorod monolayer is shifted by half the interparticle distance with respect to the adjacent layers. [10] We attribute this difference to the rectangular prism structure of the core-shell particles, [14],[15] in which the characteristic curvature of AuNRs [22] is lost during the silver deposition.

119 118 Self-assembly of Nanorods Mediated by Gemini Surfactants for Highly Efficient SERS-Active Supercrystals Figure 4-2: SEM micrographs of on ITO, obtained at optimized drop casting conditions ([16-EO 1-16] = 10-3 M, [Au@AgNR] = 10-6 M, relative humidity = 90% and T = 20 ºC). a) Partial view of the drop pattern formed upon casting. b) Magnification of the coffee ring deposits in (a), where 3 concentric ring regions are observed. c) Enlarged view of region (1) showing disordered assembled Au@AgNRs (ring width < 10 m). d) Partial view of region (2) that shows the existence of standing 3D supercrystals (ring width ~ 40 m). e) Region (3) in which lying 3D supercrystals are observed (ring widths < 15 m). The dimensions of the ordered region (2) formed by standing Au@AgNR supercrystals within the nanoparticle ring deposits are summarized in Table 4-1. In contrast to the non-significant changes observed at low relative humidities (35% and 60%), long-range extensions of the 3D standing supercrystals are obtained at high relative humidity (90%). Additionally, the width of region (2) decreases when increasing deposition temperature form 20 to 40 ºC, indicating that slow water evaporation is beneficial for the extension of the supercrystal (see Appendix III). These results are in good agreement with the formation of an intermediate smectic-b liquid-crystalline phase of nanoparticles upon controlled and slow solvent evaporation. [23] Figure 4-4 illustrates the UV-Vis- NIR absorbance spectra of the Au@AgNRs colloidal solution together with the

120 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 119 nanoparticle coffee ring deposits obtained by drop casting onto ITO-covered glass substrates at 20º C, and relative humidities of 90% and 35%. As expected, broadening and red shift of all plasmon modes occur, as a consequence of plasmon coupling between the close-packed Au@AgNRs at the coffee ring deposits. [23] The spectrum of the Au@AgNR deposits upon excitation by nonpolarized UV-Vis-NIR irradiation perpendicularly oriented to the substrates, displays a less intense lower energy mode (~900 nm), whereas the transverse LSPR (~582 nm) and the two high energy hybrid modes (~420 and 348 nm) increase in intensity. Therefore, the collective plasmon responses indicate that Au@AgNRs are mainly self-assembled within standing supercrystals at the coffee ring deposits, especially in the case of 90% humidity where a higher intensity of the transverse LSPR is observed (Figure 4-4), in good agreement with the spanning dimensions obtained for region (2) at different humidities (Table 4-1). Figure 4-3: a) Schematic diagram of different stages during drying, showing the initial random distribution of AuNRs in the isotropic solution, self-assembly during drying, formation of smectic-b liquid crystalline phases, and final generation of 3D superlattices of standing NRs. b) Representation of a gemini-nr monolayer, in which bilayers of gemini surfactant bind two adjacent NRs by strong van der Waals hydrophobic interactions of the alkyl chains (blue region), and intense electrostatic

121 120 Self-assembly of Nanorods Mediated by Gemini Surfactants for Highly Efficient SERS-Active Supercrystals interactions at the NR surface owing to both gemini binding sites (red spheres). Reproduced from reference [10]. The binding effects of gemini surfactants resulting from the formation of thousands of van der Waals and electrostatic bonds at the NR bilayer NR interfaces combine to lend overall stability to the 2D and 3D arrays. The fact that small side-by-side aggregates are formed at low particle concentration [10] suggests that the 3D assemblies obtained from more highly concentrated solutions develop during the evaporation process by merging of bilayers formed in solution (Figure 4-3-a). [10] These 3D superstructures can result from the formation of smectic-b liquid-crystalline phases in which all the NRs are oriented in layers with the same orientation and with a cubic compact arrangement, leading to micrometer-sized self-assembled areas upon solvent evaporation. This model considers at a molecular level two areas of interaction that will be responsible for the long-range self-assembly (Figure 4-3-b): 1) a region of intense van der Waals hydrophobic interactions arising from the interpenetration of the gemini surfactant alkyl chains, and 2) intense electrostatic interactions at the NR surface level because of the presence of two (charged) surfactant binding sites. Figure 4-4: Normalized UV/Vis/NIR absorbance spectra of the Au@AgNRs colloid solution (black) and the corresponding coffee ring deposits obtained by drop casting at

122 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods ºC, and relative humidities of 90% (red) and 35% (blue) onto ITO-covered glass substrates. Table 4-1: Average widths (± 2 m) of the supercrystal intermediate layer (2) of Au@AgNRs drop casted on ITO at different environmental conditions (temperature and relative humidity). Temperature (ºC) Relative Humidity (%) m 17 m 40 m m 16 m 30 m m 15 m 25 m Figure 4-5: SEM micrographs of AuNRs on ITO, obtained at optimized drop casting conditions ([16-EO 1-16] = 10-3 M, [AuNR] = 10-5 M, relative humidity = 90% and T = 20 ºC). a) Magnification of the coffee ring deposits. b) Enlarged view of region (2) showing standing 3D supercrystals (ring width ~ 30 m). Characterization of the SERS enhancing efficiency of the Au@AgNR supercrystals was carried out using benzenethiol (BT), a well-known SERS probe with high affinity for noble metals. BT was retained on the supecrystals surface in gas phase. For the sake of comparison, another colloidal crystal was prepared at the same optimized concentrations and drop casting conditions, with the AuNRs used as seeds during the silver overgrowth process (Figure 4-5-a). After controlled drying, hexagonal close-packed arrangements of AuNRs with an average interparticle distance of 3.0 ± 1 nm was obtained (see the fast

123 122 Self-assembly of Nanorods Mediated by Gemini Surfactants for Highly Efficient SERS-Active Supercrystals Fourier transform of the SEM micrograph in Figure 4-5-b which is in the Appendix III). Although thiol groups have a high affinity for both silver and gold, the stability constant is considerably higher for gold (pk= 25) than for silver (pk= 12). [24] However, due to the similar crystallographic structure of both metals, similar size of the constituent nanoparticle building blocks and retention method from the gas phase (up to total surface coverage), we can assume a similar number of molecules on both surfaces. Direct comparison is thus possible. Although the characteristic SERS spectrum of BT was clearly identified in both supercrystals, those composed by Au@AgNRs yielded 4 fold more intensity (Figure 4-6-b). This is consistent with the fact that silver is a much more efficient optical enhancer, and for similar particle size and shape, it yields enhancement factors (EFs) 2 or 3 orders of magnitude larger than those of gold. [11] In our case, silver is a small coating on gold nanorods and accordingly, the extra enhancement is smaller than that expected for pure silver rods. [11] To test for the homogeneity of the intensity of the signal, a key parameter for the development of quantitative applications, SERS mapping was carried out over an extended area (30 x 135 m, step size 1 m). Notably three different regions, (1) to (3) in Figure 4-6-a, are observed in good agreement with SEM and UV/Vis/NIR results (Figure 4-2 and Figure 4-4). Region (1) is characterized by the formation of conventional but randomly distributed tip to tip and/or side to side hotspots, [25] while regions (2) and (3) show supercrystals either standing or lying, respectively. When comparing the SERS intensity it becomes clear that randomly distributed nanorods cannot compete with the highly organized superstructures (Figure 4-6-c). Standing supercrystals offers EF about two orders of magnitude larger than that of the random hot spots. In the case of the lying crystals, the EF is about 20 fold larger. This last observation also indicates the importance of the geometric orientation of the anisotropic constituents in the supercrystal. Although both are ordered 3D structures, the standing crystals give rise to 4 fold more intensity than the lying structures. This last observation also indicates the importance of the geometric orientation of the anisotropic

124 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 123 constituents in the supercrystal. Although both are ordered 3D structures, the standing crystals give rise to 4 fold more intensity than the lying structures. This observation cannot be simply explained in light of the formation of a dense and ordered collection of hot spots but as the electric field transfer on the supercrystal. [4] Figure 4-6: a) Drop profile and SERS mapping of BT (1072 cm -1, ring breathing) evaporated on Au@AgNRs and Au@NRs coffee ring deposits (optimized drop casting conditions on ITO: 20 ºC and 90% humidity). Excitation laser line = 633 nm, power at the sample 1mW, acquisition time 200 ms, m, step size 1 m 2 and 4050 spectra. b) SERS spectra of BT on region (2) of Au@AgNRs and Au@NRs supercrystals. c) Comparisons between BT SERS intensities (1072 cm -1 ) on Au@AgNRs and Au@NRs supercrystals at the different ring regions (1), (2) and (3).

125 124 Self-assembly of Nanorods Mediated by Gemini Surfactants for Highly Efficient SERS-Active Supercrystals 4.4 Conclusions The optimization of the self-assembly of in standing supercrystals at the coffee ring deposits under drop casting has been used to fabricate SERS substrates with long-range sensing areas ( m 2 ), uniform electric field enhancement and high intensity hot spots. Au@AgNRs SERS-active supercrystals have been demonstrated to overperform their AuNRs counterparts as SERS substrates. We thus anticipate the use of these novel highly optically active sensors as a new type of SERS substrate that will offer new possibilities of ultrasensitive screening of analytical targets relevant to medical and environmental science. References [1] H. Ko, S. Singamaneni, V. V. Tsukruk, Small 2008, 4, [2] G. W. Bryant, F.. Garcıa de Abajo,. Aizp r a, Nano Lett. 2008, 8, 631. [3] M. R. Jones, R. J. Macfarlane, B. Lee, J. Zhang, K. L. Young, A. J. Senesi, C. A. Mirkin, Nat. Mater. 2010, 9, 913. [4] R. A. Alvarez-Puebla, A. Agarwal, P. Manna, B. P. Khanal, P. Aldeanueva-Potel, E. Carbó-Argibay, N. Pazos-Pérez, L. Vigderman, E. R. Zubarev, N. A. Kotov, L. M. Liz- Marzán, Proc. Natl. Acad. Sci. U.S.A. 2011, 108, [5] A. Guerrero-Martínez, M. Grzelczak, L. M. Liz-Marzán, ACS Nano 2012, 6, [6] W. Cheng, M. J. Campolongo, J. J. Cha, S. J. Tan, C. C. Umbach, D. A. Muller, D. Luo, Nat. Mater. 2009, 8, 519. [7] F. M. Menger, J. S. Keiper, Angew. Chem. Int. Ed. 2000, 39, [8] M. S. Bakshi, Langmuir 2009, 25, [9] M. Muñoz-Úbeda, S. K. Misra, A. L. Barrán-Berdón, C. Aicart-Ramos, M. B. Sierra, J. Biswas, P. Kondaiah, E. Junquera, S. Bhattacharya, E. Aicart, J. Am. Chem. Soc. 2011, 133, [10] A. Guerrero-Martínez, J. Pérez-Juste, E. Carbó-Argibay, G. Tardajos, L. M. Liz- Marzán, Angew. Chem. Int. Ed. 2009, 48, 9484.

126 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 125 [11] M. Fernanda Cardinal, B. Rodríguez-González, R. A. Alvarez-Puebla, J. Pérez-Juste, L. M. Liz-Marzán, J. Phys. Chem. C 2010, 114, [12] G. Tardajos,. Montoro, M. H. i as, M. A. ala o, A. s. G errero-mart nez, J. Phys. Chem. B 2008, 112, [13] B. Nikoobakht, M. A. El-Sayed, Chem. Mater. 2003, 15, [14] K. Park, L. F. Drummy, R. A. Vaia, J. Mater. Chem. 2011, 21, [15] M. B. Cortie, F. Liu, M. D. Arnold, Y. Niidome, Langmuir 2012, 28, [16] R. Jiang, H. Chen, L. Shao, Q. Li, J. Wang, Adv. Mater. 2012, 24, OP200. [17] Y. Yang, W. Wang, X. Li, W. Chen, N. Fan, C. Zou, X. Chen, X. Xu, L. Zhang, S. Huang, Chem. Mater. 2012, 25, 34. [18] B. Goris, S. Bals, W. Van den Broek, E. Carbó-Argibay, S. Gómez- Graña, L.M. Liz- Marzán, Gustaaf Van Tendeloo, Nature Mater. 2012, 11, 930. [19] T. A. H. Nguyen, M. A. Hampton, A. V. Nguyen, J. Phys. Chem. C 2013, 117, [20] S. Gómez-Graña, F. Hubert, F. Testard, A. Guerrero-Martínez, I. Grillo, L. M. Liz- Marzán, O. Spalla, Langmuir 2011, 28, [21] Y. Xie, Y. Jia, Y. Liang, S. Guo, Y. Ji, X. Wu, Z. Chen, Q. Liu, Chem. Commun. 2012, 48, [22] B. Goris, S. Bals, W. Van den Broek, E. Carbó-Argibay, S. Gómez-Graña, L. M. Liz- Marzán, G. Van Tendeloo, Nat. Mater. 2012, 11, 930. [23] C. Hamon, M. Postic, E. Mazari, T. Bizien, C. Dupuis, P. Even-Hernandez, A. Jimenez, L. Courbin, C. Gosse, F. Artzner, V. Marchi-Artzner, ACS Nano 2012, 6, [24] R. M. Smith, A. E. Martell, in: Critical Stability Constants. Vol. 4: Inorganic complexes, Plenum Press, New York, USA [25] Jiang, K. Bosnick, M. Maillard, L. Brus, J. Phys. Chem. B 2003, 107, 9964.

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128 CHAPTER 5 Chiral Assemblies of Gold Nanorods Localized surface-plasmon resonances in noble-metal nanoparticles have been recently used in new materials to improve optical circular dichroism. This interest stems from a wide range of applications in biology and physics, including the structural determination of proteins and DNA, and the pursuit of negative refraction. Surface plasmon-mediated circular dichroism has been explored using small spherical metal particles as building blocks, which resulted in moderate signals over a narrow spectral range. We present in this chapter a novel class of metamaterial comprising gold nanorods organized in three-dimensional chiral structures and yielding a record circular dichroism anisotropy factor (> 2%) at visible and near-infrared wavelengths ( nm). The fabrication process can be easily upscaled by self assembly of the gold nanorods on a chiral fibre backbone. Our measurements are fully supported by theoretical modelling based on coupled dipoles, unravelling the role of gold nanorods in the reported chiroptical response.

129 128 Chiral Assemblies of Gold Nanorods 5.1 Introduction Optical activity from plasmonic nanoparticles is a rapidly emerging field at the frontier of nanophotonics and conventional spectroscopy. [1],[2] This interest stems from the multidisciplinary nature of possible applications in biology, [3] chemistry, [4],[5] and optics of novel metamaterials. [6],[7] The pursuit of plasmonic optical activity is justified by the potential cooperation between two key components. First, studies of optical activity, circular dichroism (CD) in particular, in a chiral sample can reveal a wealth of detailed structural information. [4],[8] Second, plasmonic nanoparticles confer the host material unique and exquisitely tuneable optical properties in the UV-vis-NIR range. [9],[10] Nanoparticles of coinage metals can act as nanoantennas for visible light, whereby the electromagnetic field undergoes a resonant interaction with the conduction electrons of the particle, known as localized surface plasmon resonance (LSPR). [11],[12] The optical cross section characterizing the strength of the interaction between the sample and incident light in scattering and absorption can be enhanced several fold with respect to the physical area of the particle. [13] The excitation of LSPRs is also accompanied by focusing of the electromagnetic field in sub-wavelength mode volumes near the particle boundary, [14],[15] a property that finds many applications in surface-enhanced spectroscopies. [16] A direct consequence of these electromagnetic properties is the strong sensitivity of plasmonic nanoantennas to their immediate environment and, in particular, the interaction between adjacent particles [17] that leads to a hybridization of the LSPR modes supported by the individual particles. [18],[19] In the context of optical activity, the precise arrangement of nanoparticles within a chiral structure is crucial. [20],[21] Theoretical studies, as well as initial experimental demonstrations, have shown the potential of noble-metal nanoparticles, [22] nanorods in particular, to produce surface-plasmon-mediated CD (SP-CD) signals of outstanding

130 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 129 intensity. [21],[23] The interest of these new materials, with improved optical circular dichroism, [24] stems from a wide range of applications in biology and physics, including the structural determination of proteins and DNA [25] and the pursuit of negative refraction. [26] SP-CD in solution has been explored to date using small spherical metal particles, invariably resulting in moderate signals over a narrow spectral range. [5],[8],[9] We describe in this chapter a novel class of metamaterial consisting of gold nanorods (AuNRs) organized in three-dimensional (3D) chiral structures and yielding a record circular dichroism anisotropy factor for metal nanoparticles (>0.02) across visible and near-infrared (Vis NIR) wavelengths ( nm). Known as plasmonic nanoantennas, [27] these particles are characterized by the resonant collective interaction of their conduction electrons with light in the form of both scattering and absorption, with resonance frequencies that can be tuned across the Vis NIR spectrum by simply changing the aspect ratio of the nanocrystals. [28] Moreover, the LSPR of AuNRs is very sensitive to the presence and relative orientation of neighbouring particles. [17] These two properties combined make AuNRs promising building blocks for intense and tuneable SP-CD. We have designed a new nanocomposite using a self-assembly strategy [29] with AuNRs adsorbed onto a scaffold of supramolecular fibres with helical morphology through specific non-covalent interactions. The fabrication process can be easily upscaled, as it involves the self-assembly of AuNRs on a fibre backbone with chiral morphology. Our measurements are fully supported by theoretical modelling based on coupled dipoles, unravelling the key role of AuNRs in the chiroptical response. 5.2 Experimental section and methods Experimental details Tetrachloroauric acid (HAuCl4 3H2O), silver nitrate (AgNO3), sodium borohydride (NaBH4), ascorbic acid, concentrated HCl, NH4OH (32%), and

131 130 Chiral Assemblies of Gold Nanorods cetyltrimethylammonium bromide (CTAB) were purchased from Aldrich. Poly(vinylpyrrolidone) (PVP, Mw. 10,000) was purchased from Fluka. All chemicals were used as received. Pure Milli-Q grade water was used in all preparations. Characterization Zeta potential was determined through electrophoretic mobility measurements, using a Malvern Zetasizer 2000 instrument. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM 1010 transmission electron microscope operating at an acceleration voltage of 100 kv. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM- 6700F FEG scanning electron microscope operating at an acceleration voltage of 3.0 kv for secondary-electron imaging (SEI). Optical characterisation was carried out by UV-Vis-NIR spectroscopy with a Cary 5000 spectrophotometer, using a 0.1 cm path length quartz cuvette. Circular dichroism spectra were recorded on a Jasco J-815 spectrometer, using a 0.1 cm path length quartz cuvette. Synthesis, polyelectrolyte coating and characterisation of gold nanospheres and AuNRs Citrate-stabilised Au nanospheres were synthesised as reported by Turkevich et al. [30] A sodium citrate solution (100 ml, 2.0 mm) was heated under vigorous stirring. Upon boiling, HAuCl4 (1 ml, 25 mm) was injected. The resulting particles were coated with negatively charged citrate ions. AuNRs were prepared by seeded growth [31] on CTAB-stabilised Au nanoparticle seeds (2-3 nm) at 27 ºC through reduction of HAuCl4 with ascorbic acid in the presence of CTAB (0.1M), HCl (ph 2 3), and AgNO3 (0.12 mm). Then, both gold nanospheres and AuNRs solutions (5 ml) were centrifuged twice (8000 rpm, 20 min) to remove excess reactants and diluted to obtain a final concentration of CTAB close to 1 mm. Such stabilised gold

132 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 131 nanoparticles were mixed with a PVP aqueous solution (5 ml, 4 g/l) and stirred overnight. [32] The mixtures were centrifuged twice at 4500 rpm, and redispersed in ethanol (2 ml) to remove excess water and PVP. The concentrations of gold nanospheres and AuNRs at this stage were M and M, respectively. Zeta potential values of +20 mv and +10 mv were measured for AuNR@CTAB and AuNR@PVP, respectively. Zeta potential values of 25 mv and +10 mv were measured for the gold nanospheres stabilised with citrate and PVP, respectively. The diameter of gold nanospheres after polyelectrolyte coating was determined from TEM images of 1997 particles, with a mean value of 16.7 nm ± 0.1 nm (Figure IV-1 of Appendix IV) gold nanoparticles were measured by TEM (Figure IV-1 of Appendix IV) to determine the average length of the AuNRs (44.5 ± 0.1 nm), width (16.6 ± 0.1 nm) and aspect ratio (2.72 ± 0.01), where the uncertainty represents the standard error of the sample mean. The UV-vis extinction spectrum of the gold nanospheres in ethanol registers a maximum at ~520 nm as shown in the experimental spectra of Figure IV-2 in Appendix IV. The UV-Vis spectrum of AuNRs in ethanol is displayed in Figure IV-2 in Appendix IV, showing transverse and longitudinal LSPR peaks at ~520 nm and ~720 nm, respectively. Preparation and characterisation of the nanocomposite Enantiomeric fibre synthesis Both enantiomers of anthraquinone-based oxalamide fibre precursors were synthesised following a previously reported procedure. [33] These compounds have been used as excellent gelators in aromatic solvents and alcohols. R and S oxalamide enantiomers self-organise into right- P and left- M handed twisted fibres, respectively, of widths in the nanometre range and lengths of several microns (Figure IV-3 of the Appendix IV). Self-assembly occurs via a cooperative and unidirectional hydrogenbonding pattern established between oxalamide units and stacking interaction between the anthraquinone rings.

133 132 Chiral Assemblies of Gold Nanorods Enantiomeric fibre precursor 1 (5 mg) was mixed with DMF (500 μl) in a closed test tube and the mixture was stirred in a water bath at 95 ºC until the solution recovered total transparency. Subsequently, ethanol (3 ml) was added, and the mixture was stirred in a water bath at 95 ºC to yield a clear solution. Finally, water (8 ml) was added and the mixture was stirred at room temperature for 2 h. The stability of the fluid dispersion of fibres (Figure IV-3-e of the Appendix IV) was checked with UV-vis and CD measurements until stabilisation of the spectra. Nanocomposite preparation Aqueous P or M fibre dispersions (typically ~250 μl) were mixed with varying volumes of AuNRs in ethanol ( 10 μl) in a 0.1 cm path length, quartz cuvette. These samples were stirred until complete stabilisation of the SP-CD signal (figure IV-4 of the Appendix IV shows the CD measurements of the P and M nanocomposites). Circular dichroism measurements can often suffer from linear dichroism and birefringence artefacts originating from a certain optical anisotropy of the sample. [34] Here, the fluid dispersion of fibres should prevent a preferential alignment of fibres (and nanoparticles attached to them) along one particular direction of the cell; the micron-sized metal-fibre composites are freely floating in solution and therefore assume a random orientation with respect to the incident light beam (Figure IV-3-e of the Appendix IV). We further verified the absence of preferential alignment of the fibres in the cuvette by performing two control experiments. First, CD measurements on the nanocomposites were carried out in 1 1 cm cuvettes, with and without stirring (clockwise and anticlockwise). The resulting spectra did not show any significant differences. Second, CD spectra were measured before and after 5.3 Results and discussion AuNRs with an average length of 45 nm and average width of 17 nm were prepared by a seeding growth method, [31] and subsequently coated with the

134 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 133 amphiphilic polymer poly(vinylpyrrolidone) (PVP) in ethanol. [32] Fibres having a helical morphology were obtained by adding water to a DMF/ethanol solution of anthraquinone-based oxalamide 1 (Figure 5-1-a), [33] forming a fluid dispersion. In Figure 5-1-a and b, we present scanning electron microscopy (SEM) images of twisted fibres with right- P and left-handedness M, corresponding to R-1 and S-1, respectively, with widths in the hundrednanometre range and lengths of several micrometres. Figure 5-1: Representative electron micrographs of the anthraquinone-based oxalamide fibres with chiral morphology and fibre-aunr nanocomposites. a)

135 134 Chiral Assemblies of Gold Nanorods Chemical structure of the anthraquinone-based oxalamide 1, showing the asymmetric carbon atom. b, c) Scanning electron microscopy (SEM) images of the P and M fibres, respectively, after solvent evaporation. d) SEM micrograph of the P bulk nanocomposite. e) Transmission electron microscopy (TEM) image of the M nanocomposite showing twisted fibres with adsorbed AuNRs. For the preparation of the nanocomposites, a dispersion of AuNRs was added to either the P or M fibre dispersion, leading to spontaneous assembly of nanoparticles onto the fibre surface (as explained in the experimental section of this chapter, Figure 5-1-d and -e). AuNRs are preferentially aligned along the longitudinal direction of the fibres through non-covalent interactions. The UV/Vis absorbance spectra of the nanocomposites in solution reveal the characteristic resonance bands associated with the transverse (ca. 520 nm) and longitudinal (ca. 720 nm) LSPR modes of AuNRs (Figure 5-2-b). Furthermore, the UV/Vis spectra also feature the background contribution from the supramolecular fibres (at ca. 400 nm). The CD spectra of the nanocomposites consistently present a strong bisignated Cotton effect [34] at the position of the longitudinal LSPR (Figure 5-2- a). Both enantiomeric nanocomposites show mirror-image CD responses, allowing us to disregard artefacts originating from linear dichroism and/or linear birefringence (see the Appendix III), [35] which may occur in samples with macroscopic anisotropy. [34 ] For the sake of comparison, we also prepared P and M nanocomposites with gold nanospheres [30] (average diameter 15 nm) and measured their UV/Vis and CD spectra (Figure 5-1-a and -b). No optical activity was observed at the LSPR wavelength (ca. 520 nm) for gold nanospheres. This result suggested that plasmon-induced CD mechanisms arising from the interaction between fibres and nanoparticles are not significant under our experimental conditions. [36] We propose that the observed SP-CD signal originates from a 3D chiral arrangement of the AuNRs. Working with this hypothesis, we now set out to explain our results with a coupled-dipole model, [37],[38] in which each particle is described as an electric dipole. In this framework, Govorov and coworkers

136 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 135 recently predicted the possibility of inducing SP-CD in helical arrangements of gold nanospheres. [21] Nevertheless, they concluded that small variations of geometry or composition in the system can greatly diminish the CD signal (Figure 5-3-a). Accordingly, a sample with a finite amount of disorder is expected to show only negligible SP-CD. This was confirmed by our experimental findings for gold nanospheres (Figure 5-2-a). Figure 5-2: a) Experimental CD and b) UV/Vis spectra in fluid suspensions (0.1 cm path length). Solid lines show the results for AuNRs (length 45 nm, width 17 nm);

137 136 Chiral Assemblies of Gold Nanorods dashed lines show the results for gold nanospheres (average diameter 15 nm). CD spectra are shown for both P (red) and M nanocomposites (blue). Both enantiomeric nanocomposites show identical UV/Vis response (only the P nanocomposite spectrum is shown). The concentration of AuNRs and nanospheres in the nanocomposite solutions were 1.7x10-9 mol L -1 and 6.3x10-9 mol L -1, respectively. In contrast, we predict a robust SP-CD for a chiral structure bearing elongated particles that are oriented along a helix (Figure 5-3-b). Notably, a chiral arrangement of nanoparticles requires at least four nanospheres but it can be obtained with only two AuNRs. Furthermore, the intensity of the normalized SP-CD rapidly increases with the number of AuNRs in the assembly. Surprisingly, even a slight departure from sphericity (aspect ratio 1:1.1) is sufficient to greatly improve the robustness of the CD lineshape with respect to the number of the particles present in the system. Figure 5-3: Simulated CD spectra using a coupled-dipole model. Nanoparticles immersed in a homogeneous medium of refractive index 1.33 are arranged following a helical curve with 12 nm radius and 15 nm pitch. The nanoparticles are placed every 90 degrees of gyration on the helix. The simulated spectra are scaled by the number of particles N (N=2 7). a) Spheres with 5 nm radius, reproducing the results of Govorov and co-workers. [21] b) Prolate ellipsoids of semi-axes 4.5 nm x 5 nm (aspect ratio 1:1.1) with the major axis tangential to the helix. A model for our real system that mimics the nanocomposite morphology is shown in Figure 5-4-a. AuNRs immersed in a homogeneous medium were positioned on a 100 nm radius tube, and each nanoparticle was oriented

138 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 137 following a helix with a 3 micrometre pitch. To limit the number of free variables in the model, we considered the simplest configuration of two AuNRs (Figure 5-4-b), thereby restricting the model to one-to-one interactions between particles. CD spectra for a monodisperse sample were simulated by varying the relative position of the two particles independently along z and. Summation of all spectra provides the overall CD spectrum (Figure 5-4-c), which predicts a bisignated Cotton effect with a more intense low-energy wing and a zerocrossing point slightly blue-shifted from the longitudinal LSPR. The bisignated lineshape, which is well reproduced with this minimal two-rod model, is strongly reminiscent of the characteristic response of molecules with coupling between two identical chromophores (extinction coupling theory). [39] This analogy is not fortuitous, as both models are based on dipole dipole interactions, albeit at a different scale. The major difference between experimental and modelling results was found in the width of the CD features. Polydispersity was therefore introduced in the model using the distribution of particle sizes obtained from TEM measurements (see Appendix III). This resulted in an inhomogeneous broadening of the LSPR and the SP-CD (Figure 5-4-c and -d). With this addition our model faithfully reproduced the experimental CD spectra (Figure 5-4e and -f). The discrepancy in absolute SP- CD intensity can be explained by different factors. The model ignores interactions between more than two particles. Additionally, interactions between particles in direct contact cannot be considered within the coupleddipole approximation. [40],[38]

139 138 Chiral Assemblies of Gold Nanorods Figure 5-4: Representation of the nanocomposite and corresponding calculated CD spectra. a) General sketch of the system. AuNRs are randomly positioned onto the surface of a cylinder of radius R=100 nm. The orientation of each particle is defined so that its long axis is tangential to a helical curve of 3 m pitch. b) Same as in (a), but with only two particles. The parameters describing the relative position of the two AuNRs are the distance z along the cylinder axis and the rotation angle around the cylinder. c) Modelled CD and d) extinction spectra e using the coupled-dipole model described in the Supporting Information. The calculations are presented for monodisperse (red) and polydisperse (black) particle size distributions. In (d), the dashed curves represent the average extinction spectra without electromagnetic coupling between dipoles (experimentally, this corresponds to a dilute solution of nanoparticles), showing no significant differences in the LSPR. e) Experimental CD and f) extinction spectra of the P nanocomposite reproduced from Figure 5-2 The optical activity of chiral systems is often measured through the anisotropy factor (g-factor) [Equation (5.1)]: [4] (5.1) where and are the molar circular dichroism and molar extinction, respectively. Figure 5-5-a shows the concomitant increase in the g-factor with the concentration of AuNRs in the nanocomposite, reaching a maximum value of At a low concentration of particles, TEM images show partial coverage of AuNRs adsorbed on the surface of the fibres, corresponding to relatively large interparticle distances and, consequently, to weak electromagnetic coupling and moderate SP-CD. Upon increasing the concentration of AuNRs,

140 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 139 the coverage extends until complete saturation of the fibre surface, yielding the most intense CD and the maximum g-factor. Figure 5-5: Anisotropy factor (g-factor) of the nanocomposite and comparison with other chiral nanoparticle systems. a) Evolution of the g-factor with the concentration of AuNRs, maintaining constant the concentration of P fibres in the nanocomposite. The g-factor reaches a maximum value of upon saturation of the fibre surface with particles (TEM insets). b) Representative values of chiroptical metal nanoparticles in fluid media with strong anisotropy factors and the corresponding typical spectral ranges. As a comparison with other relevant chiroptical metal nanoparticles in solution, Figure 5-5-b shows representative values of the highest reported g- factors and their corresponding spectral ranges. In the UV region, the g-factor of organic molecules can be enhanced by small metal nanospheres up to [36] Additionally, small metal nanospheres with SP-CD only register values below [5],[41],[42],[43] In the present study, we obtained outstanding SP-CDs from AuNRs in fluid suspensions with g-factors as high as covering a wide Vis NIR range. [44] This value is comparable with the highest g-factors reported for molecules such as polyaromatic compounds (0.05), [45] alleno acetylenic macrocycles (0.01), [46] and protein complexes (0.06). [47] The potential of localized plasmons in generating strong SP-CD signal is expected to be most fully realized for nanostructures with sizes in the hundred-nanometre range (such as

141 140 Chiral Assemblies of Gold Nanorods a dimer of AuNRs presented herein), where the dimensions are commensurate with the intrinsic helicity of incident light. 5.4 Conclusions The original combination of AuNRs with a 3D chiral structure proposed herein paves the way for a new realm of applications for circular dichroism. We experimentally demonstrated unprecedented levels of anisotropy factor in the visible near infrared region using a versatile and generic self-assembly strategy. We thus anticipate the use of such plasmonic nanoantennas as powerful chirality probes upon attachment to proteins or DNA for in situ structure determination, [48],[49] by selecting the appropriate dimensions, morphology, and functionalization of the metal nanoparticles. Other particularly promising applications for this new class of chiroptical metamaterials include non-linear optics, [50] negative refraction, [27] and surfaceenhanced Raman optical activity. [51] We are currently focusing our effort on the non-trivial task of preparing new organic precursors to obtain chiral fibres in which the helix morphology is better defined, and parameters such as diameter and pitch are controlled, with the aim of obtaining metal nanoparticle composites with further enhanced optical activity. [23] References [1] C. Gautier, T. Bürgi, ChemPhysChem 2009, 10, 483. [2] C. Noguez, I. L. Garzón, Chem. Soc. Rev. 2009, 38, 757. [3] D. B. Amabilino, Chirality at the nanoscale, Wiley-VCH, Weinheim, [4] N. Berova, L. D. Bari, G. Pescitelli, Chem. Soc. Rev. 2007, 36, 914. [5] W. Chen, A. Bian, A. Agarwal, L. Liu, H. Shen, L. Wang, C. Xu, N. A. Kotov, Nano Lett. 2009, 9, 2153.

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145

146 Appendix I

147 146 Appendix I N + Br O N + Br Figure I- 1: Chemical structure of the gemini surfactant bis(hexadecyl dimethylammonium)diethyl ether bromide (16-EO 1-16). N + Br O N + Br Figure I- 2: Chemical structure of the gemini surfactant bis(tetradecyl dimethylammonium)diethyl ether bromide (14-EO 1-14).

148 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 147 SAXS and SANS patterns for AuNRs in aqueous solution with a total gold concentration of 2.5 mm Figure I- 3: ( ) SAXS and ( ) SANS patterns of AuNRs stabilized by 16-EO1-16 in D 2O ([Au] = 2.5 mm, [16-EO 1-16] = 0.5 mm). In inset: (up-right) SAXS and SANS scattering length densities for a AuNR stabilized by CTAB layers; (bottom-left) ( ) SAXS and ( ) SANS patterns of gold nanorods stabilized by CTAB in D 2O ([Au] = 2.5 mm, [CTAB] = 1 mm).

149 148 Appendix I Calculation of the scattering by a multilayer of gold nanorods The N rods ( ) are monodispersed and have a radius R and a length L ( ). They all present the same angle with the direction u normal to the substrate. For a standing, is zero. For a flat layer, it is 90. On the other hand, q is making an angle with the normal u. In transmission and small angle, is 90. v and w are the orthogonal axis in the plane of the substrate. The projection of L on the ( v, w ) plane is collinear with v The inplane scattering vector q q. u u is making an angle with v. When one considers a unique assembly of finite extension deposited onto the substrate: I I 1 I 2 with, I N f ( q)cos( q. r and I 1 i i) 1 2 N f ( q)sin( q. r 2 i i) 1 2 (I.1) and, f q) 2sin J ( q R) ( q H ) q R 1 i( c (I.2) and, q L q. L and, q q q L L Being the scattering amplitude of the oriented rods in the direction q. At this level, the scattering is anisotropic with spots reflected the lattice seen under the angle and contains the effects of finite size and convolution

150 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 149 between oriented shape and structure with possibly some peaks disappearing in the form factor minima. When one considers a collection of identical lattices of N oriented rods, the lattices being disoriented only in the plane ( v, w ), the scattered signal becomes isotropic and can be obtained by averaging over at a constant and : I I 1 I (I.3) 2 which can be simplified due to the symmetry brought by the average to: I 2 I I (I.4) with: and ( ( ) ( )) (I.5a) N 2 1 fi( q)cos( q. r i ) 1 I (I.5b) If now, one considers many of these assemblies disoriented in space, the scattering corresponds to a 3D average of the terms I1 and I2: I (I.6) I 1 I 2 with: I N 2 1 fi( q)cos( q. r i ) 1 (7a) and, I N 2 2 fi( q)sin( q. r i ) 1 (7b) The 3D average is different from the 2D average: I I.

151 150 Appendix I Hexagonal packing The N rods are included in a parallelepiped with and are positioned on a 2D hexagonal lattice in the plane ( v, w ) and lamellar in the u direction. Using 3D orthogonal axis with u being a normal to one assembly surface, the positions of the N rods are defined as: r. i r. u u r. v v r w w u u v v w w i and one gets: i i i i i q. r q( u cos( ) v sin( )cos( ) w sin( )sin( )) i i i i. Taking I1 as defined by eq1, and considering that a 2D hexagonal lattice is identical to tow cubic ( a, b 3a) lattices shifted from half a cell unit, one can rewrite: N 1 cos( q. r ) i k l m cos( q. r p, klm ) k l m cos( q. r i, klm ) a 3 a 4 (I.8) With r p klm and, ( kh; lb; ma) r i, klm ( kh;( l 1/ 2) b;( m 1/ 2) a ) Calculation for the first cubic lattice, contribution a 3 in equation I.8 Developing the cos terms in equation I.8, one can arrive to:

152 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 151 a l 3 k m k l m cos( q. r p, klm ) cos( qlb sin( )cos( ) qmasin( )sin( )) sin(( q( khcos( )) l m 1 1 k 1 cos( q( khcos( )) sin( qlb sin( )cos( ) qmasin( )sin( )) (I.9) Using the classical results: K S K, h cos( ) cos( q( khcos( )) 1 K sin( qh cos( )) 2 K 1 cos( qh cos( )) sin( qh cos( ) / 2) 2 (I.10a) and K T K, h cos( ) sin( q( khcos( )) 1 K sin( qh cos( )) 2 K 1 sin( qh cos( )) sin( qh cos( ) / 2) 2 (I.10b) one arrives to: a S 3 k l m cos( q. r p, klm ) K, qhcos( ) S L, qbsin( )cos( ) S M, qasin( )sin( ) S K, qhcos( ) T L, qbsin( )cos( ) T M, qasin( )sin( ) T K, qhcos( ) T L, qbsin( )cos( ) S M, qasin( )sin( ) T K, qhcos( ) S L, qbsin( )cos( ) T M, qasin( )sin( ) (I.11) For / 2 (small angle scattering in transmission) it simplifies to: L, qb cos( ) S M, qa sin( ) T L, qb cos( ) T M, qa sin( ) S a 3 K (I.12)

153 152 Appendix I Calculation for the second cubic lattice, contribution a 4 in equation I.8 Using S i L b L, b sin( ) cos( ) cos( lb sin( ) cos( )) 2 b cos sin( ) cos( ) S L, b sin( ) cos( ) 2 b sin sin( ) cos( ) T 2 1 L, b sin( ) cos( ) (I.13a) and T i L b L, b sin( ) cos( ) sin( lb sin( ) cos( )) 2 b cos sin( ) cos( ) T 2 b sin sin( ) cos( ) S L, b sin( ) cos( ) 2 1 L, b sin( ) cos( ) (I.13b) one gets finally: a S 4 K L M cos( q. r i, klm ) K, qhcos( ) Si L, qbsin( )cos( ) Si M, qasin( )sin( ) S K, qhcos( ) Ti L, qbsin( )cos( ) Ti M, qasin( )sin( ) T K, qhcos( ) Ti L, qbsin( )cos( ) Si M, qasin( )sin( ) T K, qhcos( ) S L, qbsin( )cos( ) T M, qasin( )sin( ) i i (I.14) which again for / 2 simplifies to: a 4 KT i L, qb cos( ) S i M, qa sin( ) L, qb cos( ) T M, qa sin( ) KS i i (I.15) and finally the scattered intensity of a given orientation is given by: I f ( q)( a a ) (I.16) Then the 2D or 3D averages are performed numerically:

154 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 153 I I 2 I 1 2 I 1 (I.17)

155

156 Appendix II

157 156 Appendix II Figure II- 1: SEM image of the octahedrons used as seeds for the silver overgrowth. The 8 facets {111} are easily distinguish.

158 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 157 Figure II- 2: Silver coating of gold octahedrons a) TEM image where is discerning the gold octahedral core of the final cubic shape b) and c) SEM images of silver coating octahedrons in which are clearly appreciable the cubic structure of the new nanoparticles.

159 158 Appendix II Figure II- 3: a) TEM projection and b) acquired diffraction pattern of an octahedron on a carbon support. The diffraction pattern indicates that the cube is viewed along a [100] zone axis. The absence of splitting indicates the epitaxial growth of Ag on the gold octahedra.

160 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 159 Figure II- 4: A) TEM projection and acquired selected area electron diffraction pattern of an on a carbon support (scale bar 50 nm). B) The diffraction pattern indicates that the nanorod is viewed along a [100] zone axis and the absence of splitting indicates the epitaxial growth of Ag on the AuNR.

161 160 Appendix II Figure II- 5: Representative TEM images of the different silver growths on AuNRs (scale bars represent 20 nm). a) AuNRs used as seeds for the silver overgrowth (A.R. =2. 9) b) first silver overgrowth of AuNRs, (A.R. =1.7) c) second silver overgrowth using the previous one as seeds, (A.R. =1.4) d) third silver overgrowth using the nanoparticles of c) as seeds (A.R. =1.2). It is observing that the silver deposition is preferentially on the side faces of AuNRs than the tips, decreasing the aspect ratio of the nanoparticles.

162 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 161 Figure II- 6: a) and b) TEM images of pentatwinned AuNRs coated with silver at different concentrations of gold seeds, b) image corresponds with a less concentration of pentatwinned AuNRs which infer higher aspect ratio of the final nanoparticles. c) SEM image of the pentatiwinned Au@AgNRs of image a) after a concentration step and d) HAADF image of the nanoparticles represented in b)

163

164 Appendix III

165 164 Appendix III Figure III- 1: Representative TEM images and size distributions of AuNRs and a, b) TEM images of AuNRs and respectively. c, d) Long axis (68 ± 4 nm) and short axis (13 ± 1 nm) distribution histograms of AuNRs, respectively. e, f) Long axis (71 ± 3 nm) and short axis (34 ± 2 nm) distribution histograms of Au@Ag NRs, respectively.

166 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 165 Figure III- 2: SEM micrographs of on ITO-coated glass, obtained at optimized drop casting conditions ([16-EO 1-16] = 10-3 M, [Au@AgNR] = 10-5 M, relative humidity = 35% and T = 20 ºC). a) Partial view of the drop pattern formed upon casting. b) Magnification of the coffee ring deposits in (a), where 3 concentric ring regions are observed. c) Enlarged view of region (1) showing disordered assembled Au@AgNRs (ring width < 2 m). d) Partial view of region (2) that shows the existence of standing 3D supercrystals (ring width ~ 14 m). e) Region (3) in which lying 3D supercrystals are observed (ring widths ~ 16 m).

167 166 Appendix III Figure III- 3: Fast Fourier transform of the SEM micrograph in Figure 2d, which confirms perfect tetragonal arrangement, with an average distance between neighboring metal particles of 3.5 ± 1 nm in Au@AgNRs supercrystals.

168 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 167 Figure III- 4: Fast Fourier transform of the SEM micrograph in Figure 4b, which confirms perfect hexagonal geometry, with an average distance between neighboring metal particles of 3.0 ± 1 nm in AuNRs supercrystals.

169 168 Appendix III Figure III- 5: Normalized UV/Vis/NIR absorbance spectra of the AuNRs colloid solution (black) and the corresponding coffee ring deposits obtained by drop casting at 20 ºC, and relative humidities of 90% (red) and 35% (blue) onto ITO-covered glass substrates.

170

171

172 Appendix IV

173 172 Appendix IV Figure IV-1: Representative TEM images and size distributions of gold nanospheres and AuNRs after PVP coating. a, b) TEM images of gold nanospheres and AuNRs. c) Size distribution histogram of gold nanospheres (16.7nm ± 0.1nm). d, e) Long axis (44.5nm ± 0.1nm) and short axis (16.6nm ± 0.1nm) distribution histograms of AuNRs, respectively.

174 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 173 Figure IV- 2: UV-vis spectra of PVP coated gold nanospheres (dashed line) and AuNRs (solid line) in ethanol. The concentration of gold nanospheres and AuNRs were M and M, respectively.

175 174 Appendix IV Figure IV- 3: SEM and sample images. a) (P) fibres, the arrow indicates the sense of gyration. b) Nanocomposite with AuNRs attached. c, d) Nanocomposite with gold nanospheres. e) Photograph of the quartz cuvette containing the fluid dispersion of fibres.

176 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 175 Figure IV- 4: Circular dichroism of the (M) (dashed lines) and (P) (solid lines) nanocomposites. Different concentrations of AuNRs were used to test the reproducibility of the homoenantiomeric nanocomposite preparations.

177 176 Appendix IV Coupled-dipole model The coupled-dipole approximation [1],[2],[3],[4] is commonly used to simulate circular dichroism, and in particular the interaction between isolated elements of a system such as the different branches of a large molecule, [1],[2],[5] the lattice of atoms in a carbon nanotube, [6] the chiral arrangement of atoms in small individual clusters, [7],[8] or more recently the interaction between small gold nanospheres. [9] This modeling technique was first introduced by DeVoe [1],[2] to study the optical properties of molecular aggregates in a solution, and later revisited [3],[4],[10] to include more accurate retardation effects. In the context of plasmonics, the coupled-dipole approximation has found application to describe the scattering properties of individual [11] and collections of plasmonic nanoparticles. [12] We seek a minimal model to unravel the role of gold nanoparticles in our experimental observations of circular dichroism. Given the complexity of the real nanocomposite sample and its lack of well-defined morphology, a number of simplifying assumptions have to be made. Our main hypothesis is that the observed circular dichroism is a result of the chiral arrangement of AuNRs in space. Thus we chose to treat the surrounding medium (the solvent and the fibres) as a homogeneous medium of refractive index 1.4. From the perspective of electromagnetic theory, this means that the propagation of light waves in the system can only be affected by the interaction with the nanoparticles, and ignores the possibility of molecular CD induction between the fibres and the particles, [13],[14],[15] and also of direct interaction between light and fibres. This approximation is further supported by the fact that the fibres alone do not produce a significant degree of CD in this spectral region. Our second major simplifying assumption is that the gold nanoparticles are sufficiently small and weakly interacting so that each nanoparticle can be described as an electric dipole. These two requirements are largely verified in our experimental conditions; we use particles much smaller than the

178 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 177 wavelength, and we find that the UV-vis spectrum is almost unaffected by the concentration of AuNRs (Figure IV- 4). Circular dichroism is calculated from the difference in extinction for lefthanded (LH) and right-handed (RH) circularly polarised light, averaged over the full range of possible incident light directions. The CD cross-section is obtained by rotating the incident field over all incident directions; the orientation-average is evaluated by a numerical integration with a Gauss- Legendre quadrature scheme. Realistic model For the monodisperse situation, the two particles are identical with semiaxis dimensions. To account for the polydispersity of the particle sizes, we extracted from the TEM measurements of the dispersion of aspect ratios in the population of AuNRs by fitting a Gaussian density to the distribution of aspect ratios (excluding the small population of ~10% nanospheres). Because the longitudinal LSPR position is more sensitive to the aspect ratio of the particles than to their volumes, we kept the volume fixed (6545 nm 3, the mean value from TEM measurements) and varied independently the aspect ratio of the two particles with a Gaussian density of mean value 3.1 and standard deviation The mean aspect ratio (3.1) was adjusted in order to match the LSPR position of the experimental data. [16] We conclude this section with a number of critical remarks with respect to the approximations used in this model. Within the coupled-dipole framework, we have neglected the effect of disparity in the fibre morphology; the fibres probed in our experiments have a wide range of diameters, a nonhomogeneous twist, and are generally not straight. Further, not all the nanoparticles are ideally oriented along a helix, and the influence of larger groups than dimers may be more important than the simple intensity scaling we inferred from Figure 6-2. These approximations are not intrinsic limitations of the coupled dipole model; in fact they can be largely overcome simply by

179 178 Appendix IV using a (much) larger space of parameters in the model that we chose to restrict for simplicity. Another set of approximations concerns the limitations of the coupleddipole model. It is well-known that the coupled-dipole approximation breaks down when the interparticle separation is below a certain distance, typically of the order of the particle size. To properly acknowledge the contribution of closely-packed clusters of particles (as shown in the TEM inset of Figure 5-4) will require a more accurate numerical technique that includes higher-order multipoles in the solution of the scattering problem. Similarly, we restricted our study to ellipsoidal particles of moderate volume and aspect ratio; the rigorous treatment of arbitrarily shaped particles would require a more general numerical technique. Lastly, we note that these models all assume a homogeneous surrounding medium. In the experimental conditions, the particles are however in contact with the supramolecular fibres, which have a different refractive index than water, and in particular the signature of absorption and circular dichroism originating from the supramolecular fibres indicates that they will be accompanied by dispersion of the dielectric function, and circular birefringence respectively. Effect of the concentration of gold NRs anisotropy factor The chiral efficiency of the nanocomposite was tested through the analysis of the CD and UV-vis responses in the plasmonic region at different concentrations of AuNRs. The concentration of fibres was maintained low and constant in the aqueous solution to avoid saturation effects in the SP-CD signal and to complete the adsorption process of AuNRs on the fibre surface. In this preparation we repeated the nanocomposite synthesis with a lower concentration of fibres (2/5). shows the effect of the concentration of AuNRs

180 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 179 on the CD and UV-vis spectra, and the corresponding g-factor spectra (g is defined as the ratio of circular dichroism over molar extinction, ). The sample with highest concentration of AuNRs is presented in Figure IV- 6 Error! No se encuentra el origen de la referencia.. The particles cover almost uniformly the fibres, and nearest neighbouring AuNRs are in close proximity. Figure IV-5: Circular dichroism and UV-vis measurements, and the corresponding g- factor of the (P) nanocomposite, at different concentrations of AuNRs. The concentration of particles is increased for each spectrum from 0 M to M.

181 180 Appendix IV Figure IV-6: TEM images of the nanocomposite with high NRs concentration ( M) at different magnifications

182 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 181 References [1] DeVoe, H. Optical Properties of Molecular Aggregates. II. Classical Theory of the Refraction, Absorption, and Optical Activity of Solutions and Crystals. J. Chem. Phys. 43 (1965). [2] DeVoe, H. Optical Properties of Molecular Aggregates. I. Classical Model of Electronic Absorption and Refraction. J. Chem. Phys. 41, (1964). [3] Draine, B. & Flatau, P. Discrete-dipole approximation for scattering calculations. J.Opt. Soc. Am. A 11, (1994). [4] Yurkin, M. & Hoekstra, A. The discrete dipole approximation: An overview and recent developments. J. Quant. Spectrosc. Ra. 106, (2007). IX Conference on Electromagnetic and Light Scattering by Non-Spherical Particles. [5] Kim, M.-h., Ulibarri, L., Kelle, D. & Maestre, M. F. The psi-type circular dichroism of large molecular aggregates. III. Calculations. J. Chem. Phys. 84, (1986). [6] Noguez, C. Optical circular dichroism of single-wall carbon nanotubes. Phys. Rev. B 73, 1 14 (2006). [7] Román-Velázquez, C. E., Noguez, C. & Garzón, I. L. Circular Dichroism Simulated Spectra of Chiral Gold Nanoclusters: A Dipole Approximation. J. Phys. Chem. B 107, (2003). [8] Noguez, C. & Garzón, I. L. Optically active metal nanoparticles. Chem. Soc. Rev. 38, (2009). [9] Fan, Z. & Govorov, A. O. Plasmonic Circular Dichroism of Chiral Metal Nanoparticle Assemblies. Nano Lett. 10, (2010). [10] Purcell, E. & Pennypacker, C. Scattering and absorption of light by nonspherical dielectric grains. Astrophys. J. 186, (1973). [11] Schatz, G. C. & Duyne, R. P. V. Discrete dipole approximation for calculatingextinction and Raman intensities for small particles with arbitrary shapes. J. Chem. Phys. 103, (1995). [12] Schatz, G. C., Jensen, L., Kelly, K. L. & Lazarides, A. A. Electrodynamics of Noble Metal Nanoparticles and Nanoparticle Clusters. J. Clust. Sci. 10, (1999). [13] Shemer, G. et al. Chirality of silver nanoparticles synthesized on DNA. J. Am. Chem. Soc. 128, (2006). [14] George, J. & Thomas, K. G. Surface plasmon coupled circular dichroism of Au nanoparticles on peptide nanotubes. J. Am. Chem. Soc. 132, (2010). [15] Govorov, A. O., Fan, Z., Hernandez, P., Slocik, J. M. & Naik, R. R. Theory of circular dichroism of nanomaterials comprising chiral molecules and nanocrystals:

183 182 Appendix IV plasmon enhancement, dipole interactions, and dielectric effects. Nano Lett. 10, (2010). [16] Pérez Juste, J., Pastoriza Santos, I., Liz Marzán, L. & Mulvaney, P. Gold nanorods: Synthesis, characterization and applications. Coordin. Chem. Rev. 249, (2005).

184 183 Appendix III

185

186 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 185 General conclusions Even though specific conclusions have been displayed at the end of each chapter, the most relevant, global conclusions of the research reported in this thesis are shorted in what follows. Complementary measurements of TEM, SAXS and SANS have allowed us to extract a fine description of the surfactant bilayer that stabilizes gold nanorods. A bilayer thickness of 32 ± 2 Å was observed for both conventional and gemini surfactants within the experimental accuracy. Taking advantage of the excellent self-assembly properties of gemini surfactants, SAXS studies of the supercrystals of AuNRs can be reproduced by the model developed for a 2D hexagonal close packing We have demonstrated that under our experimental conditions the slow reduction of silver ions, in the presence of BDAC as stabilizer, on presynthesized gold nanoparticles with well-defined crystalline structures leads to the preferential growth of the {100} facet. This effect would explain why single crystalline gold nanoparticles will evolve into single crystalline Au@Ag cubes enclosed by six {100} facets, and pentatwinned gold nanorods will evolve into core-shell nanorods with increased aspect ratio since the seed particles are enclosed by five {100} lateral facets. The optimization of the self-assembly of gold-silver core-shell nanorods in standing supercrystals at the coffee ring deposits under drop casting has been used to fabricate SERS substrates with long-range sensing areas, uniform electric field enhancement and high intensity hot spots. Silver nanorods SERS-active supercrystals have been demonstrated to overperform their gold counterparts as SERS substrates.

187 186 General conclusions The original arrangement of gold nanorods with a 3D chiral structure proposed herein place us on the way for a new territory of applications for circular dichroism. We experimentally demonstrated unprecedented levels of anisotropy factor in the visible near infrared region using a versatile and generic self-assembly strategy. As a general conclusion, we can state that Colloid Chemistry and more precisely the seed-mediated method as a bottom-up approach in Nanotechnology is a highly flexible tool to tune the morphology of plasmonic metal nanoparticles. This thesis has thus contributed to the development of methods to produce gold nanoparticles and silver-gold core-shell nanoparticles with novel assemblies strategies and interesting optical response, which is expected to be relevant to applications based on nanoplasmonics, such as ultrasensitive detection by surface enhancement Raman scattering (SERS) or like plasmonic nanoantennas as powerful chirality probes upon attachment to proteins or DNA for in situ structure determination.

188

189

190 Resumen En este resumen se pretende ofrecer una visión global del trabajo que se ha descrito a lo largo de los capítulos que conforman la presente disertación. Los nanovarillas de oro y plata han sido el objeto de esta tesis doctoral y sobre ellos van a girar los diferentes estudios que hemos realizado. Primeramente se ha caracterizado la bicapa de surfactante que envuelve a las nanopartículas metálicas en disolución. Se ha llevado a cabo el crecimiento de plata sobre los nanovarillas de oro, observándose una clara preferencia en la deposición de la plata en unas caras frente a otras. Se ha estudiado el auto-ensamblaje de los nanovarillas de oro cubiertos con plata y como se produce un incremento la señal SERS frente a los nanovarillas de oro. Finalmente se procedió al uso de los nanovarillas de oro en fibras quirales que poseen señal de dicroísmo circular para aumentar esta, y que los nanovarillas de oro sean como nanoantenas.

191 190 Resumen Objetivo La investigación descrita en la tesis se centra en el desarrollo de nuevas estrategias para la síntesis y el autoensamblaje de nanovarillas metálicos, que podrían servir de plataformas para la construcción de sistemas macroscópicos con aplicaciones plasmónicas, lo que requiere un conocimiento profundo del estado coloidal y cristalino de las nanopartículas anisotrópicas. Estas investigaciones se enmarcan en el contexto del trabajo del Grupo de Química Coloidal de la Universidad de Vigo, donde la síntesis y funcionalización de las nanopartículas metálicas han sido ampliamente desarrolladas, lo que permite el control de la composición, el tamaño y la forma de las nanopartículas. Las caracterizaciones de los nanovarillas se lleva a cabo mediante espectroscopia UV-vis, técnicas de microscopía electrónica, y también en el marco de las colaboraciones existentes, técnicas de dispersión a pequeños ángulos (rayos X y neutrones) y microscopía electrónica de transmsión a grandes ángulos y campo oscuro anular, se han utilizado para el mismo propósito. La tesis se ha estructurado en cinco capítulos. El capítulo 1 recoge una introducción general en la se introducen las razones por las que interesa el estudio de las nanopartículas metálicas anisotrópicas, y más concretamente, en nuestro caso los nanovarillas metálicos. El capítulo 2 está centrado en la caracterización de un tipo de estabilizante de los nanovarillas de oro (surfactantes de cabeza catiónica), para ello se han usado técnicas de microscopía electrónica de transmisión y dispersión a pequeños ángulos de neutrones y rayos X. A partir de estas investigaciones se determina la presencia de una bicapa de surfactante alrededor de las nanopartículas. Esto es un avance ya que esta bicapa no estaba caracterizada de manera directa, todas las caracterizaciones anteriores eran por medio de técnicas indirectas. Por otra parte, se introduce el proceso de auto-ensamblaje de los nanovarillas de oro que es impulsado por el alto carácter anfifílico que poseen los surfactantes tipo gemelos.

192 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 191 La síntesis y el mecanismo de crecimiento de nanopartículas de plata en medio acuoso a través del proceso de crecimiento de semillas que son nanopartículas de oro pre-sintetizadas se describe en el capítulo 3. Hemos elegido como semillas las nanopartículas de oro debido a que su estructura cristalina y su morfología están bien definidas. Desde el punto de vista de la estructura cristalina de las semillas se puede clasificar como semillas monocristalinas (tales como octaedros de oro y nanovarillas de oro) y semillas policristalinas (tales como los nanocindros de oro policristalinos). Se ha estudiado el crecimiento de plata mediante una síntesis en medio acuoso, ofreciendo una completa caracterización estructural de las nanopartículas obtenidas que serán del tipo núcleo-corteza. Los resultados obtenidos demuestran claramente que las estructuras cristalinas de las semillas, así como, la estabilización preferente de ciertas caras cristalinas son los factores clave que promueven la deposición de la plata de una manera isotrópica o anisotrópica. Otro objetivo principal de la tesis es el estudio del proceso de autoensamblaje de los nanovarillas metálicos para formar supercristales plasmónicos. En el capítulo 4, el surfactante tipo gemini es usado para obtener el auto-ensamblaje de forma vertical de nanovarillas metálicos. Para este fin, varios aspectos físico químicos, tales como la concentración de surfactante, la humedad y la temperatura del proceso de secado se han investigado con el fin de obtener un supercristal de tamaño micrométrico de nanovarillas metálicos auto-ensamblados de forma vertical. Además, se describe la aplicación de estos ensamblajes para dispersión Raman aumentada por superficie (SERS), mostrando que la casi perfecta organización tridimensional de los nanovarillas metálicos hace que estos sistemas sean excelentes para sustratos para SERS con un aumento uniforme del campo eléctrico. En el capítulo 5 se discute un nuevo tipo de ensamblaje de nanopartículas que nos proporciona una clase única de metamaterial plasmónico consistente en nanovarillas de oro organizados en tres dimensiones gracias a estructuras quirales, produciendo un aumento en el dicroísmo circular. El proceso de

193 192 Resumen fabricación es fácil de escalar, ya que implica el auto-ensamblaje de nanovarillas de oro sobre una fibra con morfología quiral. Las mediciones son totalmente compatibles con modelos teóricos tomando como base el modelo de acoplamiento de dipolos. Introducción General Richard Feynman puso la semilla de la nanotecnología con la pregunta: " Por qué no podemos escribir los 24 volúmenes de la Enciclopedia Británica en la cabeza de un alfiler?". La nanotecnología es la ciencia que se centra en el estudio a nivel atómico, molecular o macromolecular, en la escala de nm aproximadamente, con la finalidad de obtener nuevos materiales que constituyan la base para un conjunto de desarrollos tecnológicos que tienen en común el uso y manipulación de objetos, con al menos una dimensión en el intervalo de tamaños nanométrico. Dentro de este contexto, la síntesis en medio acuoso, que es un campo entre la química inorgánica convencional y la química coloidal, se convierte en un punto de partida para la fabricación tipo bottomup de los nuevos materiales basados en la nanotecnología. En particular, las nanopartículas metálicas ofrecen la ventaja de combinar el control tridimensional del tamaño y la forma de la nanopartícula, haciéndolas ideales para el diseño de nanobloques organizados. Los seres humanos durante siglos han estado usando nanopartículas en el desarrollo de materiales macroscópicos sin conocimiento de su estructura microscópica. Uno de los más antiguos objetos en lo que esto sucede es la Copa de Licurgo, realizada en el siglo IV A.C. y la cual contiene nanocristales de metales los cuales han precipitado como coloides y son una aleación de plata y oro. [1] Cuando la copa se ve en la luz reflejada las partículas metálicas son lo suficientemente gruesas para dispersar luz verde, mientras que en la luz transmitida las nanoparticulas absorben en el azul del espectro visible con, lo que resulta en la transmisión del color rojo. La primera investigación científica documentada en la síntesis química de las nanopartículas metálicas fue

194 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 193 realizada por Michael Faraday en 1857, quien preparó coloides de oro mediante la reducción de una disolución acuosa de ácido cloroaúrico con fósforo blanco en disulfuro de carbono, obteniendo fluidos de rubí, una dispersión de esferas de oro de dimensiones nanométricas. [2] A partir de entonces, el desarrollo científico de las nanopartículas metálicas ha sido enorme, sobre todo durante las últimas cinco décadas, impulsado en gran parte por las mejoras en la síntesis de nanocristales coloidales y el desarrollo de nuevas técnicas de microscopía electrónica que proporcionaron una descripción más precisa de su estructura. Un área de la nanotecnología, donde las propiedades ópticas (absorción, emisión o dispersión) de las nanopartículas metálicas se han estudiado es la nanoplasmónica, en la que, la excitación de resonancias plasmónicas localizadas en la superficie han sido usadas en nanoestructuras metálicas para obtener dispositivos con propiedades estructurales y fotónicas específicas. Aunque las nanopartículas esféricas han sido los sistemas más investigados debido a sus propiedades plasmónicas y a la relativa facilidad de preparación, estos nanocristales isotrópicos no tienen una preferencia en la organización, limitando así sus aplicaciones potenciales para formar sistemas de medida con propiedades ópticas y la direccionales. En contraste, una de las ventajas más atractivas de las nanopartículas metálicas anisotrópicas es su versatilidad hacia la organización direccional y específica, destacándose los nanovarillas metálicos como sistemas plasmónicos altamente eficientes y que logran una intensa actividad óptica en el dominio espectral del visible-infrarrojo cercano. Sin embargo, no ha sido hasta finales del siglo XX, cuando los primeros artículos sobre la síntesis, caracterización y propiedades ópticas de nanocristales anisotrópicos comenzaron a ser publicados, lo que muestra las dificultades encontradas en el control de la forma de las nanopartículas a través de la síntesis en medio acuoso.

195 194 Resumen Bicapa de surfactante sobre los nanovarillas de oronanovarilla Los surfactantes son comúnmente usados durante la síntesis de nanopartículas metálicas siendo fundamentales en la estabilización y cristalización de las mismas. Debido a su alta versatilidad, los surfactantes juegan un papel clave en diferentes etapas de la formación de las nanopartículas metálicas, tales como la solubilización de los reactivos iniciales, la evolución hacia la forma final del nanocristal, y la estabilización de las nanopartículas en diferentes disolventes. [3] En el método de crecimiento mediante semillas y que se produce en disolución acuosa, la cual es la síntesis más popular para los nanovarillas de oro, una gran cantidad de bromuro de cetiltrimetilamonio (CTAB) está presente, parte de él permanece unido a la superficie de la nanopartícula al final de la reacción. [4] Por otra parte, se ha demostrado que cuando el CTAB es reemplazado por un agente surfactante del tipo gemini (dimethyl hexadecyl ammonium bromide (16-EO1-16)), [5] se obtienen nanovarillas de oro más monodispersos y con una capacidad para auto-ensamblarse de forma vertical en 3 dimensiones. [6] Estas diferencias hace que la caracterización de la capa superficial que rodea a los nanovarillas de oro tenga especial relevancia para conocer las propiedades que estos puedan tener. Dicha caracterización de la capa de surfactante (CTAB) en nanovarillas de oro se ha realizado mediante TGA, FTIR, y potencial zeta. [7], [8] Los métodos utilizados son de naturaleza analítica por lo que nos darán las cantidades de casa especie desde un punto de vista de su composición y no nos proporciona una medida directa de las características morfológicas como pueden ser grosor, densidad, nivel de empaquetamiento

196 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 195 Un método excelente para medir in situ la estructura de nanoobjetos son las técnicas de dispersión a ángulos pequeños (rayos X o neutrones) (denotado como SAXS y SANS, respectivamente). [9],[10] Primeramente se han realizado imágenes de microscopía electrónica de transmisión para estar seguros de que poseíamos nanovarillas de oro (como se muestra en la Ilustración R- 1), además de saber su diámetro, anchura y su polidispersidad tanto en diámetro como en la anchura. El siguiente paso fue efectuar medidas de SAXS y SANS a una disolución de agua con el surfactante a analizar (CTAB y surfactante del tipo gemini (16-EO1-16)) a la concentración micelar crítica (CMC) la cual será la misma concentración en la que mediremos las nanopartículas. Esto se realizará para luego poder hacer una correcta sustracción de las señales de las micelas a dichas concentraciones. Ilustración R- 1: Se muestran los patrones de SAXS y SANS de los nanovarillas de oro estabilizados con el surfactant 16-EO1-16 ([Au]=6.0 mm and [16-EO1-16] = 0.5 mm): ( )

197 196 Resumen SAXS; (Δ) SANS en D2O; Las lineas se corresponden con las curvas teóricas obtenidos con los parámetros dados en la Tabla 1. Recuadro derecha: Imágenes de TEM de los nanovarillas de oro. Recuadro izquierda: Imagen que ejemplifica el carácter de la bicapa de surfactante sobre la nanopartícula de oro. Se diseñaron dos experimentos diferentes, con dos concentraciones de oro diferentes (2.5mM y 6mM) manteniendo la concentración de surfactante constante para cada tipo de surfactante CTAB y 16-EO1-16. Se realizaron las medidas de SAXS (Ilustración R- 1) y se han ajustado con los datos que previamente habíamos obtenido de las imágenes de microscopía electrónica (ver Tabla R-1). Surfactant (concentration) [Au], mm (UV) R 0 (Å) L 0(Å) u (for TEM) v v (SANS) T, (Å) CTAB (6 mm) (10) 16-EO1-16 (6 mm) (10) CTAB (2.5 mm) EO1-16 (2.5 mm) Tabla R-1: Parametros obtenidos a partir de las imágenes de TEM para los nanovarillas de oro. Estos parámetros son los usados para calcular los patrones SAXS y SANS correspondientes. Debido a la variación de contraste que nos produce el SANS con respecto al SAXS, se puede determinar la bicapa de surfactante que está rodeando al nanovarilla de oro ajustando los datos obtenidos en el SANS con los datos ajustados del SAXS (Ilustración R- 2). Se observó para ambos casos un espesor de la bicapa de 32 ± 2 Å con una densidad de la cadena alquílica del %. Estos resultados confirman la presencia de una bicapa de surfactante sobre la superficie de los nanovarillas de oro, un resultado era ampliamente aceptado, pero que nunca se había confirmado de manera directa.

198 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 197 Ilustración R- 2: Efecto del espesor t en la intensidad de dispersion SANS para los mismos valores de dimensiones de los nanovarillas de oro. Nanopartículas estabilizadas en CTAB ([Au] = 6.0 mm, [CTAB] = CMC): (t = 40 Å (línea negra punteada), t = 34 Å (línea roja), t = 30 Å (línea verde), t = 20 Å (línea marrón)). El recuadro muestra la representación I q 2.9 frente a q. Sintesis de nanovarillas tipo nucleocorteza de oro-plata: estabilización de las caras 100 En la bibliografía hay una gran cantidad de artículos centrados en el crecimiento de nanopartículas metálicas mediante semillas, para así poder controlar la forma y tamaño de las mismas. [3],[11],[12] La forma de la partícula final será consecuencia de la estructura cristalina de la semilla usada, así como

199 198 Resumen por la presencia de ligandos que ayuden a estabilizar ciertas caras cristalográficas. El método de crecimiento de semillas puede ser aplicado en medios acuosos, gracias a la presencia de surfactantes, [13] o en medios no acuosos, tales como la N, N-dimetilformamida (DMF) o los procesos conocidos con el nombre de poliol. [14] Ilustración R- 3: Imágenes de TEM de las diferentes nanopartículas de oro usadas como semillas (columna de la izquierda, escala 100nm), las correspondientes partículas después de la deposición de plata (escala 100nm) e imágenes representativas de HAADF-STEM de las partículas núcleo-corteza (columna de la derecha). Para una mejor comprensión del mecanismo de crecimiento hemos decidido estudiar por separado la deposición de la plata en función de la estructura cristalina de la semilla. Deposición de plata en semillas de oro monocristalinas: Como semillas de oro monocristalinas han sido elegidos dos tipos de nanopartículas; octaedros de oro

200 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 199 (nanopartículas isotrópicas formadas por ocho caras {111}), y nanovarillas de oro los cuales son anisotrópicos y tienen ocho caras laterales {520}. La Ilustración R- 3 muestra una imagen TEM de los octaedros de oro usados como semilla (aproximadamente 60 nm de diámetro). Las partículas presentan principalmente una sección transversal hexagonal, pero podría ser también cuadrada, como consecuencia de las diferentes proyecciones del octaedro. [15] La deposición de plata se realiza a una concentración de tensioactivo baja, lo cual determinará que el precursor de la plata sea AgBr como recientemente propuso Kyoungweon por Park y colaboradores, [16] siendo el ácido ascórbico, el agente reductor. El proceso se lleva a cabo a 60 º C para facilitar una reducción controlada de plata. Después de la deposición de plata todas las partículas presentan una sección transversal cuadrada con una longitud de lado media de 102 nm (véase la Ilustración R- 3). Para entender cómo es la deposición de la plata sobre los octaedros de oro, se realizaron imágenes de alta resolución de STEM (véase la Ilustración R- 3). La reconstrucción de la partícula se muestra en la Ilustración R- 3, en la que se puede apreciar que el octaedro de oro está rodeado por un cubo de plata y las seis puntas del octaedro se enfrentan a las seis caras del cubo (ver Ilustración R- 3). La FFT correspondiente de la partícula de muestra que las puntas de los octaedros de oro, así como las seis caras del cubo de plata, son [100], [010] y [001], respectivamente. Un análisis similar se realizó en el caso de los nanovarillas de oro recubiertos con plata (véase la Ilustración R- 3). Los nanovarillas de oro presentan sección transversal octogonal con la presencia altos índices de Miller en las caras laterales. [17] La Ilustración R- 3 muestra que después de que el recubrimiento de plata, la proyección 2D de las partículas muestra principalmente una forma rectangular, pero un vistazo más de cerca revela la presencia de partículas de pie perpendiculares a la rejilla de TEM revelando una sección transversal cuadrada. El patrón de difracción indica que el

201 200 Resumen nanovarilla de oro está orientado a lo largo de un eje de zona [100] y la ausencia de escisión indica el crecimiento epitaxial de la plata sobre los nanovarillas de oro. Por lo tanto, la morfología externa de la partícula estará formada por seis caras {100}. Una vez que el recubrimiento de plata induce la transformación de los nanovarillas de oro en un prisma rectangular con seis caras {100}, la deposición de la plata será más o menos uniforme en todas las caras lo que conducirá a una disminución gradual en la relación de aspecto del prisma que idealmente se transformaría en un cubo. Deposición de plata sobre semillas de oro policristalinas: Para analizar la influencia de la estructura cristalina de las semillas en la deposición de plata se han elegido nanovarillas de oro policristalinos ya que tienen una estructura cristalina bien definida, y al mismo tiempo, permiten el estudio de la influencia de la anisotropía de las semillas en la deposición de plata (véase la Ilustración R- 3). Hoy en día, se acepta que los nanovarillas de oro policristalinos están formados por cinco caras laterales {100} y diez caras en las puntas {111}. [18] La deposición de la plata se realizó bajo condiciones experimentales similares a las semillas de oro monocristalinas (Ilustración R- 3). Se puede ver claramente en las imágenes de microscopia, que la deposición de plata es preferentemente en las puntas de los nanovarillas de oro aumentando así su relación de aspecto, estando el nanovarilla inicial en el centro de la nanopartícula final. La relación de aspecto de los naoncilindros tipo núcleocorteza se puede controlar variando la relación entre el nitrato de plata y las semillas de oro. La Ilustración R- 3 muestra una imagen de STEM de un nanovarilla núcleo-corteza Au-Ag, donde se puede distinguir un pentágono interior el cual se corresponde con el nanovarilla de oro y que refleja la sección transversal pentagonal de este. Pero también se puede distinguir con un brillo menor el pentágono correspondiente al recubrimiento de plata. El proceso implica el crecimiento de las caras {111} y la estabilización de las caras laterales {100}.

202 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 201 Contrariamente a lo que sucede con las semillas monocristalinas el crecimiento de las semillas policristalinas implica un aumento de la relación de aspecto y por lo tanto, un aumento en la anisotropía final de la nanopartícula. Autoensamblaje de nanovarillas de oro Au@Ag gracias a surfacantes Gemini para un aumento SERS-mediante supercristales La investigación sobre nuevos sustratos más eficientes para el uso en espectroscopia de dispersión Raman aumentada por superficie (SERS) se ha centrado recientemente en la obtención de supercristales tridimensionales (3D) plasmónicos, [19] con el objetivo de obtener superredes cristalinas de nanopartículas metálicas con efectos de antena intensos y controlado, que resultan en una gran concentración del campo eléctrico. [20] El autoensamblaje de nanopartículas metálicas se basa en las interacciones entre las moléculas situadas en las interfaces de los nanocristales. [21] Recientemente hemos investigado el papel de los surfactantes tipo gemini sobre la síntesis de nanovarillas de oro en la se auto-ensamblan de forma vertical para formar superredes de 2D y 3D mediante simple deposición de la gota. [6] Teniendo en cuenta la mayor eficiencia plasmónica de nanopartículas de plata, [22] y alentados por el necesidad de la optimización del proceso de auto-ensamblaje que puede permitir el diseño de supercristales a gran escala y de gran actividad óptica para la detección de SERS, informamos en este capítulo sobre un método directo para obtener supercristales plasmónicos con núcleo de oro y recubiertos de plata (Au@AgNRs). Au@AgNRs se han sintetizado a partir de semillas monodispersas de nanovarillas de oro siguiendo un protocolo descrito por R. Vaia. [16] La mezcla

203 202 Resumen de agentes tensioactivos originalmente empleados durante el crecimiento de la plata fue sustituido por el agente tensioactivo gemini bis(hexadecil dimetilamonio) diethyl ether bromide (16-SO1-16). [23] En la Ilustración R- 4 se muestra el éxito de la síntesis, donde a la vez se pueden observar los espectros de absorbancia de UV/Vis de las semillas nanovarillas de oro y el resultado final correspondiente Au@AgNR. Deposición de plata dio lugar a un aumento de la intensidad y un desplazamiento hacia el azul de los LSPR correspondeintes a las bandas longitudinal y transversal, apareciendo a la vez dos modos más de mayor energía descritos por Cortie. [24] Ilustración R- 4: Panel superior: Espectro de absorbancia UV/Vis de los nanovarillas de oro usados como semillas (negra) y las partículas finales Au@AgNRs (rojo)

204 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 203 sintetizadas en 16.EO Los recuadros muestran imagines de T.E.M. representativas de cada espectro. Panel inferior: Micrografias de SEM de las nanopartículas sobre ITO, obtenidas por medio de la deposición de gota con unas condiciones optimizadas ([16-EO 1-16] = 10-3 M, [Au@AgNR] = 10-6 M, humedad relativa del 90% y temperatura de 20 ºC). a) Vista parcial de la gota. b) Magnificazión del anillo de café en (a), se aprecian tres regiones concéntricas. c) Vista más cercana de la región (1) mostrando ensamblajes desordenados de Au@AgNRs (ancho del anillo < 10 m). d) vista parcial de la región (2) que muestra la existencia de supercristales en 3D de nanovarillas orientados de forma vertical con respecto al sustrato (ancho del anillo ~ 40 m). e) Región (3) en la cual se observan supercristales en 3D de nanovarillas en disposición horizontal con el sustrato (ancho del anillo < 15 m). La optimización de la preparación de supercristales en 3D a partir de Au@AgNRs se llevó a cabo mediante la deposición de una gota de 10 L de la disolución coloidal de nanopartículas ([16-EO1-16] = 10-3 M, [Au@AgNRs] = M), controlando la temperatura (20, 30 y 40 ºC) y humedad (35, 60, y 90%). La formación espontánea de superredes auto-ensambladas de Au@AgNRs se observa fácilmente en condiciones de 20 ºC y 90% de humedad. En concordancia con estudios previos realizados con nanovarillas de oro estabilizados con surfactantes tipo gemini, [6] la deposición de nanopartículas metálicas da como resultado la formación del denominado anillo de café (~ 40 a 70 m de ancho y 5 ~ mm de diámetro exterior, Ilustración R- 4-a y -b muestran este efecto). [25] Nanoparticulas depositadas de forma aleatoria se observan en el borde exterior (1) (Ilustración R- 4-c), con anchos de los anillos inferiores a 10 m, Au@AgNRs auto-ensambladas en supercristales de forma vertical se pueden observar en la siguiente región del anillo (2) (Ilustración R- 4-d). La caracterización SERS de los supercristales Au@AgNR se llevó a cabo utilizando benceno-tiol (BT), una molecula muy utilizada en estos casos ya que posee una alta afinidad por los metales nobles. Para comparar, otro supercristal de nanovarillas de oro fue preparado en las mismas condiciones (Ilustración R- 5-a). Aunque el espectro SERS característica de BT se identificó claramente en ambos supercristales, aquellos compuestos por Au@AgNRs dio 4 veces más intensidad que los compuestos por nanovarillas de oro (Ilustración R- 5-b). Esto es consistente con el hecho de que la plata es ópticamente mucho más eficiente

205 204 Resumen que el oro, y para tamaños de partículas y formas similares va a producir factores de mejora de 2 ó 3 órdenes de magnitud mayores que las de oro. [22] La región (1) se caracteriza por la deposición de nanopartículas de forma aleatoria creando hot spots punta-punta o lado-lado, [26] mientras que las regiones (2) y (3) mostran supercristales de nanopartículas en forma vertical o horizontal al sustrato, respectivamente. Al comparar la intensidad SERS se hace evidente que los nanovarillas metálicos distribuidos al azar no pueden competir con las superestructuras altamente organizadas (Ilustración R- 5-c). Los supercristales con nanovarillas organizados de manera vertical ofrecen factores de mejora de aproximadamente dos órdenes de magnitud mayor que la de los hot spots aleatorios. Ilustración R- 5: a) Perfil de la gota y mapa SERS del BT evaporado en Au@AgNRs y Au@NRs. Deposiciones tipo anillo de café (condiciones optimizadas sobre ITO: 20 ºC y

206 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods % humedad). Linea de excitación laser = 633 nm, potencia en la muestra 1mW, tiempo de adquisición 200 ms, m, intervalo de 1 m 2 y 4050 espectros. b) espectros SERS del BT en la región (2) de los supercristales de Au@AgNRs y Au@NRs. c) Comparación entre las intensidades del SERS del BT a (1072 cm -1 ) en los supercristales Au@AgNRs y Au@NRs a diferentes regiones del anillo (1), (2) y (3). La optimización del auto-ensamblaje de Au@AgNRs de forma vertical creando supercristales ha sido utilizado para fabricar sustratos para detección SERS con grandes zonas de detección ( m 2 ), aumentando la uniformidad del campo eléctrico y la intensidad los hot spots. Supercristales Au@AgNRs se ha demostrado que superan a sus homólogos AuNRs como sustratos para detección SERS. De este modo anticipar el uso de estos nuevos sensores ópticamente activos como un nuevo tipo de sustrato SERS que ofrecen nuevas posibilidades para la detección ultrasensible de los objetivos analíticos de interés para la ciencia médica y ambiental. Ensamblajes quirales de nanovarillas de oro La actividad óptica de las nanopartículas plasmónicas es un campo que ha emergido rápidamente en la frontera entre la nanofotónica y la espectroscopia. [27],[28] Este interés se debe al carácter multidisciplinario que se le pueden dar a las posibles aplicaciones como pueden ser en biología, [29] química, [30] y en conferir nuevas propiedades ópticas a nuevos metamateriales. [31] El desarrollo de la actividad óptica en plasmónica se justifica por la presencia de dos componentes claves. En primer lugar, los estudios de la actividad óptica, dicroísmo circular (CD), en particular, en una muestra quiral pueden revelar una gran cantidad de información estructural. [30] En segundo lugar, las nanopartículas plasmónicas pueden conferir al material anfitrión propiedades ópticas únicas y comprendidas en el rango de UV-VIS-NIR. [12] Los nanovarillas de oro se prepararon mediante el método de crecimiento de semillas, [32] siendo posteriormente recubiertos con el polímero

207 206 Resumen poli(vinilpirrolidona) (PVP) y dispersadas en etanol. [33] Para la preparación de los nanocompuestos, se añadió una alícuota de la dispersión de nanovarillas de oro concentrados a cualquiera de las fibras P o M, lo que lleva a ensamblaje espontáneo de nanopartículas sobre la superficie de la fibra. Los nanovarillas de oro están preferentemente alineados a lo largo de la dirección longitudinal de las fibras por medio de interacciones no covalentes. Los espectros de absorbancia de UV / Vis de los nanocompuestos en solución revelan las bandas características con las transversales (520 nm) y las longitudinales (720 nm) de los nanovarillas de oro (Ilustración R- 6-f). Los espectros de CD de los nanocompuestos presentan un fuerte efecto de Cotton [34] en la posición de la banda longitudinal de los nanovarillas de oro (Ilustración R- 6-e). Para efectos de comparación, también se han preparado nanocompueston con las fibras P y M pero usando nanoesferas de oro (diámetro promedio 15 nm), sin observarse actividad óptica en la parte del espectro correspondiente a la banda de las nanoesferas de oro ( aprox. 520 nm). Nosotros proponemos que la señal observada SP-CD tiene su origen a partir de un ensamblaje quieral en 3D de los nanovarillas. Modelos teóricos predicen una robusta señal de SP-CD para una estructura quiral que posee partículas alargadas que están orientadas a lo largo de una hélice (Ilustración R- 6-a y -b). Para un aumento en la señal SP-CD se requieren al menos cuatro nanoesferas pero se puede obtener el mismo aumento con sólo dos nanovarillas de oro. Además, la intensidad de la señal SP-CD aumentará rápidamente con el número de nanovarillas que posean las fibras (Ilustración R- 6-g). La actividad óptica de los sistemas quirales se mide a menudo a través del factor de anisotropía (factor g) [ecuación (1)]: [1] donde y son el dicroísmo circular molar y la extinción molar, respectivamente. La Ilustración R- 6-g muestra el aumento en el factor g con el

208 Colloidal synthesis, structural characterization and assembly of plasmonic metal nanorods 207 aumento de la concentración de nanovarillas de oro en el nanocompuesto, alcanzando un valor máximo de 0,022. A una concentración baja de partículas, las imágenes de TEM muestran la cobertura parcial de los nanoclindros de oro adsorbidos en la superficie de las fibras, correspondientes a distancias entre partículas relativamente grandes y, en consecuencia, darán acoplamientos electromagnéticos débiles y una señal moderada en el SP-CD. Al aumentar la concentración de nanovarillas de oro, la cobertura se extiende hasta la saturación completa de la superficie de la fibra, produciendo una señal SP-CD mucho más intensa y alcanzando el máximo en el factor g. Ilustración R- 6: Representación del nanocompuesto y el espectro CD calculado a) esquema general del sistema. b) es lo mismo que en (a) pero solo con dos partículas. c)