Dual-Responsive Nanoparticles and their Self-Assembly

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1 Dual-Responsive Nanoparticles and their Self-Assembly Sanjib Das, Priyadarshi Ranjan, Pradipta Sankar Maiti, Gurvinder Singh, Gregory Leitus, and Rafal Klajn * Self-assembly has emerged as the method of choice for constructing micrometer-sized objects from nanoscopic components. [ 1 ] Among the various stimuli employed to guide the self-assembly of nanoparticles (NPs), magnetic fields and light are arguably the most attractive as they can be delivered remotely and momentarily, and lead to dynamic superstructures that is, structures capable of existing only as long as the stimulus is applied. [ 2 ] Although self-assembly using both of these stimuli applied independently has been achieved, the resulting structures were previously limited to spherical assemblies and onedimensional chains in the case of light [ 3 ] and magnetic fields, [ 4 ] respectively. The ability to engineer double-responsiveness in one type of building block opens the door to generate a whole spectrum of self-assembled structures, depending on the relative contributions of the two types of stimuli. Photoswitchable magnetic NPs have previously been reported, [ 5 9 ] however, their response to external stimuli in the context of self-assembly has not been investigated. Herein we report the design and preparation of doublyresponsive materials, whereby the magnetic interactions between NPs originate from their superparamagnetic (SPM) Fe 3 O 4 cores, and the light-induced interactions are derived from monolayers of photoresponsive ligands on the surfaces of these cores. The ability to control interparticle interactions using these two orthogonal external stimuli enables us to assemble NPs into one-dimensional superstructures with controllable aspect ratios, which has previously been unachievable by solution self-assembly. Due to their dynamic nature, the structures of these assemblies can further be manipulated remotely. To render SPM NPs photoswitchable, we synthesized a ligand comprising an azobenzene group and a catechol anchor for NPs (azobenzene-catechol; AC, Figure 1a; see the Supporting Information (SI) for experimental details). The 3,4-dihydroxyphenyl (catechol) moiety has recently emerged as a ligand of choice for the stabilization and surface functionalization of iron oxide NPs. [ ] Monodisperse, single-domain, [ 15 ] magnetite NPs, 11 nm in diameter (Figure 1 b), were synthesized as described previously, [ 16 ] and functionalized with AC by incubating the Dr. S. Das, P. Ranjan, P. S. Maiti, Dr. G. Singh, Dr. R. Klajn Department of Organic Chemistry Weizmann Institute of Science Rehovot 76100, Israel rafal.klajn@weizmann.ac.il Dr. G. Leitus Department of Chemical Research Support Weizmann Institute of Science Rehovot 76100, Israel DOI: /adma oleic acid-capped Fe 3 O 4 NPs with an excess of AC (see SI for details). We have verified that the functionalization procedure affected neither the size nor shape of the NPs (Figure S4,6). In order to determine the surface area occupied by a single AC molecule on the surface of iron oxide NPs, one can take advantage of the high absorbance of AC in the near-uv region (Figure S2). In a series of experiments, we incubated iron oxide NPs with an excess of AC, and, following selective removal of functionalized NPs, spectrophotometrically evaluated the amount of AC removed from the solution (on the surfaces of NPs). Knowing the average diameter of the NPs, their total mass and density (5.17 g/cm 3 for magnetite [ 17 ] ), we could calculate that a single AC occupies 0.49 ± 0.03 nm 2 on the surface of Fe 3 O 4 (see SI for details). This number is in agreement with previously estimated values of 0.42 nm 2 (by TGA) and 0.56 nm 2 per molecule (by XPS) for a structurally similar, dopamine-based ligand on the surface of magnetite NPs. [ 12 ] Importantly, the average surface area occupied by one AC in our system is larger than 0.40 nm 2, which is the minimum surface area an azobenzene group needs in order to isomerize efficiently. [ 18 ] Indeed, we found that the switching kinetics for AC isomerization in solution and on NPs were almost identical (SI, Section 7) i.e., it took the same time of UV irradiation to reach the photostationary state. Furthermore, the compositions of the photostationary states were similar: the population of cis- - AC reversibly comprised up to 93% for free and up to 88% for immobilized AC under the same irradiation conditions. When the NPs were dissolved in solvents of low dielectric constant (e.g., toluene), azobenzene trans cis isomerization induced attractive electric dipole interactions between the NPs and induced their self-assembly [ 3, 18, 19 ] into spherical aggregates. [ 20, 21 ] We followed this self-assembly process by dynamic light scattering (DLS), UV-Vis spectroscopy, and scanning electron microscopy (SEM) (Figure 1 c-f). The latter technique showed that, upon exposure to UV light only, the initially formed spherical aggregates (obtained at t = 150 s; Figure 1f, top ) stuck together to give larger, ill-defined aggregates at t > 300 s (Figure 1 f, bottom ). The formation of the aggregates was fully reversible we saw no indication of fatigue even after performing 100 assembly-disassembly cycles; the high stability of the azobenzene photoswitch was likely due to the low intensity of UV light we used ( 1.0 mw/cm 2 at λ = 339 nm). At the same time, our NPs are expected to respond to external magnetic fields due to their superparamagnetic cores. [ 23 ] Magnetic dipole interaction energy between two particles can be expressed as [ 24 ] E d μ 0 4π μ 2 σ 3, where μ 0 is the permeability of free space, μ magnetic dipole moment ( μ = M V, where M is the magnetization, and V volume of the particle), and σ effective NP diameter, consisting of the diameter of the NP core and the ligand shell. For our 11.3 nm Fe 3 O 4 NPs functionalized with 2 nm long AC ligands, E d J. This interaction 422

2 Figure 1. Light-induced self-assembly of magnetite NPs. a) Structural formula of an azobenzene-terminated catechol ( AC ). b) TEM images of AC -functionalized 11 nm magnetite NPs. c,d) Changes in the particle size distribution (c) and optical properties (d) accompanying the exposure of AC -coated NPs in toluene to low-intensity UV light. e) Reversible switching of absorbance ( A ) at λ = 444 nm ( top ) and average hydrodynamic diameter ( bottom ) upon exposure to UV and visible light. A was followed at 444 nm since the change between spectra before and after irradiation is the largest at this specifi c wavelength. Each cycle corresponds to 3 min of UV light followed by 45 s of visible light. For SEM images of samples after multiple cycles of assembly-disassembly, refer to SI, Section 8. f) SEM images of an individual pseudospherical NP aggregate formed upon exposing AC-coated NPs to UV light for t = 150 s ( top ), and of larger aggregates formed at t = 300 s ( bottom ). energy is not sufficient to overcome the thermal energy, k B T at room temperature J, which explains why these NPs do not aggregate even upon exposure to an external magnetic field ( Figure 2a, third from the left ). At the same time, the magnetic dipole interaction is proportional to r 6 /σ 3 (where r is the radius of the NP core), and it is expected to be very sensitive to the particle size. Consequently, unlike free NPs, the spherical clusters induced by UV irradiation readily assembled into extended aggregates. In other words, the magnetic interactions can be turned on using UV light. To better understand the nature of magnetic interactions within the spherical aggregates, we investigated their magnetic properties, along with the magnetic properties of free NPs, using a superconducting quantum interference device (SQUID) magnetometer. The dependence of sample magnetization ( M ) on the external magnetic field ( H ) at room temperature is typical of superparamagnetic materials (no hysteresis in the M-H curves can be seen), and it does not change as a result of aggregation (Figure 2 b). Likewise, temperature dependence of sample magnetization, as manifested in the zero field-cooled-field-cooled (ZFC-FC) curves, indicates that the aggregation process does not affect the sample s blocking temperature ( T B ), with T B 120 K for both free and aggregated NPs (Figure 2 c). These observations indicate that the exchange interactions between NPs comprising the aggregates are negligible. Exchange coupling in our system is prohibited because of the self-assembled monolayers of AC, each 2 nm-thick, decorating the NPs indeed, the range of exchange interactions is typically 2 nm. [ ] Instead, attractive magnetic interactions between the NPs are due to the long-range dipole-dipole forces. To monitor the formation of the extended NP assemblies (Figure 2 d f), we quenched the process at different time intervals in a set of experiments conducted both in the presence and in the absence of an external magnetic field. In both cases, the shortest time needed to observe aggregates was 150 s. While in the absence of a magnetic field these structures were spherical, 100 nm in diameter (cf. Figure 1 f), most of those formed in the presence of the magnetic field were elongated, already at t = 150 s, with an average aspect ratio of 3 and the same thickness of 100 nm. This observation suggests that as soon as the spherical aggregates form, they interact by magnetic dipole interactions, and, since NPs within them are held together by reversible non-covalent interactions, readily coalesce with one another. The resulting extended aggregates further join with one another in an end-to-end fashion (cf. Figure 2 g), thereby generating threads of even longer lengths, whose thickness, however, is preserved throughout the process (at 100 nm). Remarkably, one can visualize this hierarchical [ ] nature of self-assembly with the help of an SEM: occasionally, we observed structures in which two extended segments were joined in an end-to-end manner by thin bridges composed of a small number of NPs (Figure 2 h). These structures represent an intermediate step on the way towards the final product that is, smooth threads. As the length of the threads increased, the magnetic force [ 21 ] acting on them overcame the Brownian force, [ 32 ] and the threads were attracted towards the regions of high magnetic field gradients, eventually concentrating near the magnets. At the same time, they slowly sedimented from the solution and were deposited onto arbitrary substrates for example, Figure 2 i shows oriented threads, many of them several tens of micrometers long, deposited on the surface of the silicon wafer. We also investigated the magnetic characteristics of these aligned 1D superstructures with the external magnetic field applied in the direction parallel to the main axes of the threads. Figure 2 j and k further indicate superparamagnetic behavior, with no hysteresis in the M-H curves, and T B 120 K. 423

3 Figure 2. Combining the effects of light and magnetic fi eld. a) Visual changes accompanying exposure to UV light ( left ), magnetic fi eld ( second from right ), and both stimuli ( right ). Only in the latter case, rapid aggregation of NPs is observed. b,c) Comparison of magnetic properties of free (red) and aggregated (blue) NPs: room temperature M-H curves (b) and temperature dependence (c). d) 1D assemblies formed as a result of exposure of the AC-NPs to both light and magnetic fi eld. Despite their various lengths, the assemblies all have roughly the same thickness. e) SEM image of an individual 1D assembly. f) SEM side view of several assemblies. Additional images taken at various tilt angles can be found in the SI, Figure S11. g,h) Visualizing the hierarchical nature of self-assembly by SEM. Additional examples can be found in the SI, Figure S12. i) Optical microscopy image of long threads deposited on the surface of a silicon wafer and aligned in an external magnetic fi eld. j) Room temperature M-H curves, and k) ZFC-FC curves for 1D assemblies. Similarly to the spherical aggregates, the 1D superstructures could be assembled and disassembled over multiple cycles. We verified that exposure to 15 seconds of visible light ( I 1.0 W/cm 2 ) was sufficient for both types of aggregates to quantitatively disassemble. However, despite the fact the NPs within the threads are held together by a combination of magnetic dipole and electric dipole interactions, only the latter type of interactions is necessary to preserve the structure of the assemblies. Consequently, the threads remained stable when the magnetic field was removed, but they disintegrated rapidly when UV light was turned off. In other words, while both UV and magnetic stimuli are necessary to induce the formation of the threads, only UV light is required to keep them alive. The fact that our NPs are responsive to both UV light and magnetic field suggests that the structure of the resulting assemblies can be controlled using both of these stimuli. To verify this hypothesis, we conducted two sets of experiments in which we varied i ) the strength of the magnetic field in the range of 200 G down to 0.5 G (the geomagnetic field) while keeping the intensity of UV light constant (at 1.03 mw/cm 2 ), and, conversely, ii ) the intensity of light from 1.03 mw/cm 2 down to 0.10 mw/cm 2 while keeping the magnetic field at 200 G ( Figure 3a). In i ), as the contribution of the magnetic dipole interactions (favoring linear NP aggregates) decreased, the NPs maximized the number of cis-azobenzene-cis-azobenzene interactions, and the aggregates assumed increasingly spherical shapes (Figure 3 b). This strategy enabled us to precisely tune the aspect ratio of the threads (Figure 3 c). On the other hand, in ii ) the dimensions of the threads were not sensitive to the intensity of UV light provided it was high enough to induce attractive electric dipole interactions between the NPs in the first place (Figure 3 d,e). The latter result again confirms that UV light, and not magnetic field, is necessary to initiate the growth of the assemblies. In summary, we have demonstrated that self-assembly of NPs can be controlled independently and/or cooperatively by light and magnetic field. This methodology could be used to fabricate devices whose electronic properties could independently be modulated using these two orthogonal types of external stimuli. In the context of self-assembly, the results reported herein can be used to construct dynamically self-assembling magnetic objects which can be guided, using external magnetic fields, to desired locations, where their disassembly can be initiated at will, using visible light. We expect that in the long run, this strategy will enable controlled capture, delivery, and release of molecular/nanosized cargo. Experimental Section Synthesis of AC, preparation, and functionalization of iron oxide NPs are described in detail in the Supporting Information. Self-assembly of NPs was carried out in a setup shown in Figure 3 a, where the intensity of UV light and the magnetic fi eld were modulated by the distances d 1 and d 2 in the range of 1.03 to 0.10 mw/cm 2 and 200 to 0.5 G, respectively. For every experiment, of AC-functionalized Fe3O 4 NPs dissolved in 300 μ L of toluene were used. As the light source, we used a 4 W hand-held UV lamp (UVP, LLC; Upland, CA; model number UVGL-25). As the magnetic fi eld source, we used a small NdFeB magnet (Stanford Magnets, Irvine, CA). For inspection, a drop of the solution was applied onto a silicon wafer, the solvent was evaporated at once, and the samples were examined by an SEM (we have used an ULTRA 55 fi eldemission SEM (Carl Zeiss Microscopy, LLC), as well as a SUPRA 55VP fi eld-emission SEM (Carl Zeiss Microscopy, LLC), both operating at 5 kv). 424

4 Figure 3. Orthogonal control of the self-assembly process by magnetic fi eld and light. a) Schematic representation of the experimental setup. b,c) Aspect ratio of the NP assemblies can be controlled by the strength of the magnetic fi eld. d,e) Variation of intensity of UV light does not affect the dimensions of the assemblies provided the intensity is high enough to initiate aggregation of free NPs. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work was supported by the European Union Marie Curie Reintegration Grant, the G. M. J. Schmidt-Minerva Center for Supramolecular Architectures, the Helen and Martin Kimmel Center for Molecular Design, and the Minerva Foundation with funding from the Federal German Ministry for Education and Research. We thank Mr. Shachar Lerer (Prof. Gil Markovich group; Tel Aviv University) for the assistance with SQUID measurements. We thank Dr. Guohua Jia (Prof. Uri Banin group; Hebrew University of Jerusalem) for the assistance with XRD measurements. The EM studies were conducted at the Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging at the Weizmann Institute. R. K. is the incumbent of the Robert Edward and Roselyn Rich Manson Career Development Chair. Received: April 29, 2012 Revised: August 8, 2012 Published online: August 30, 2012 [1 ] M. Grzelczak, J. Vermant, E. M. Furst, L. M. Liz-Marzan, ACS Nano 2010, 4, [2 ] M. Fialkowski, K. J. M. Bishop, R. Klajn, S. K. Smoukov, C. J. Campbell, B. A. Grzybowski, J. Phys. Chem. B 2006, 110, [3 ] R. Klajn, P. J. Wesson, K. J. M. Bishop, B. A. Grzybowski, Angew. Chem. Int. Ed. 2009, 48, [4 ] M. Vilfan, A. Potocnik, B. Kavcic, N. Osterman, I. Poberaj, A. Vilfan, D. Babic, Proc. Natl. Acad. Sci. USA 2010, 107, [5 ] R. Mikami, M. Taguchi, K. Yamada, K. Suzuki, O. Sato, Y. Einaga, Angew. Chem. Int. Ed. 2004, 43, [6 ] M. Suda, M. Nakagawa, T. Iyoda, Y. Einaga, J. Am. Chem. Soc. 2007, 129, [7 ] M. Suda, N. Kameyama, M. Suzuki, N. Kawamura, Y. Einaga, Angew. Chem. Int. Ed. 2007, 47, 160. [8 ] M. Taguchi, K. Yamada, K. Suzuki, O. Sato, Y. Einaga, Chem. Mater. 2005, 17, [9 ] M. Suda, Y. Miyazaki, Y. Hagiwara, O. Sato, S. Shiratori, Z. Einaga, Chem. Lett. 2005, 34, [10 ] A. K. L. Yuen, G. A. Hutton, A. F. Masters, T. Maschmeyer, Dalton Trans. 2012, 41, [11 ] M. D. Shultz, J. U. Reveles, S. N. Khanna, E. E. Carpenter, J. Am. Chem. Soc. 2007, 129, [12 ] E. Amstad, T. Gillich, I. Bilecka, M. Textor, E. Reimhult, Nano Lett. 2009, 9, [13 ] H. Wei, N. Insin, J. Lee, H. S. Han, J. M. Cordero, W. H. Liu, M. G. Bawendi, Nano Lett. 2012, 12, 22. [14 ] D. Ling, W. Park, Y. I. Park, N. Lee, F. Li, C. Song, S. G. Yang, S. H. Choi, K. Na, T. Hyeon, Angew. Chem. Int. Ed. 2011, 50, [15 ] R. F. Butler, S. K. Banerjee, J. Geophys. Res. 1975, 80, [16 ] Y. Ridelman, G. Singh, R. Popovitz-Biro, S. G. Wolf, S. Das, R. Klajn, Small 2012, 8, 654. [17 ] W. M. Haynes, Handbook of Chemistry and Physics, 92nd Edition ; CRC Press, [18 ] R. Klajn, Pure Appl. Chem. 2010, 82, [19 ] R. Klajn, K. J. M. Bishop, B. A. Grzybowski, Proc. Natl. Acad. Sci. USA 2007, 104, [20 ] R. Klajn, J. F. Stoddart, B. A. Grzybowski, Chem. Soc. Rev. 2010, 39, [21 ] R. Klajn, K. J. M. Bishop, M. Fialkowski, M. Paszewski, C. J. Campbell, T. P. Gray, B. A. Grzybowski, Science 2007, 316, 261. [22 ] R. Klajn, A. O. Pinchuk, G. C. Schatz, B. A. Grzybowski, Angew. Chem. Int. Ed. 2007, 46, [23 ] C. T. Yavuz, J. T. Mayo, W. W. Yu, A. Prakash, J. C. Falkner, S. Yean, L. L. Cong, H. J. Shipley, A. Kan, M. Tomson, D. Natelson, V. L. Colvin, Science 2006, 314, 964. [24 ] S. Morup, M. F. Hansen, C. Frandsen, Beilstein J. Nanotechnol. 2010, 1, 182. [25 ] A. Mamedov, J. Ostrander, F. Aliev, N. A. Kotov, Langmuir 2000, 16, [26 ] G. A. Held, G. Grinstein, H. Doyle, S. H. Sun, C. B. Murray, Phys. Rev. B 2001, 64, Art. No

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