Relative Contributions of Experimental Parameters to NIR-Absorption Spectra of Gold Nanoshells

Transcription

1 J. Ind. Eng. Chem., Vol. 13, No. 1, (2007) Relative Contributions of Experimental Parameters to NIR-Absorption Spectra of Gold Nanoshells Sangeun Park, Minyim Park, Pokeun Han, and Sangwha Lee Department of Chemical & Bio Engineering, Kyungwon University, Gyeonggi-do , Korea Received August 1, 2006; Accepted October 2, 2006 Abstract: Gold nanoshells are new constructional units consisting of dielectric core encapsulated by gold nanolayer that exhibits a plasmon-derived optical resonance typically shifted to near-infrared (NIR) wavelengths ( nm). The fabrication of gold nanoshell was optimized based on the relative contributions of the aging of gold colloids, concentration of reducible gold salts, and dosages of formaldehyde reducing agents. One-week-aged gold colloids produced a relatively monodisperse deposition of gold seeds onto silica nanoparticles, whereas one-day-aged gold colloids produced a heterogeneous deposition of gold seeds on the silica nanoparticle surfaces. At the optimal dosages of reducing agents, the thickness of the gold layer increased with the increment of the concentrations of the reducible gold salts, consequently leading to the smoother morphologies and stronger plasmon resonances of the gold nanoshells. The excessive addition of gold salts and/or reducing agents, however, resulted in a significant reduction of the plasmon resonance because of the increased aggregation between the gold nanoparticles. Keywords: gold nanoshell, aging, near-infrared (NIR), plasmon resonance Introduction 1) Core-shell nanocomposites represent new types of constructional units that are currently the subject of active research areae [1]. Surface plasmon resonance (SPR) is the term given to the collective oscillations of free electrons at the metallic interface surface. These oscillations can give rise to the intense colors of nanoparticle solutions because of their strong plasmon-derived absorption bands. A gold nanoshell is a layered nanoparticle consisting of a dielectric core surrounded by a gold metallic shell; these particles exhibit a plasmon-derived optical resonance typically shifted to much longer wavelength than the corresponding plasmon resonance of gold nanoparticles [2]. As shown in Figure 1, the fabrication of gold nanoshells consists of the following consecutive steps: i) synthesis of gold colloids and APTMS-functionalized silica nanoparticles, ii) attachment of gold colloids onto the functionalized silica surface, and iii) gold shell growth over the silica cores. The silica surface must be modified with pos- To whom all correspondence should be addressed. ( lswha@kyungwon.ac. kr) itively charged amino groups to avoid electrostatic repulsions between gold and silica nanoparticles having the same surface charges. After the gold colloids are attached onto the aminated silica surface, the gold-deposited silica (Au-APTMS/SiO 2 ) is further reduced by reaction with gold salts in the presence of reducing agents. Finally, the continuous gold layer is formed over the silica cores via ucleate-induced cluster coalescence [2,3]. Gold nanoshells with strong near-infrared (NIR) absorption can be applied to hyperthermia cancer therapy and photo-thermal-induced drug delivery because plasmon resonance at NIR wavelength exhibits optimal tissue transmissivity. Human breast carcinoma cells incubated with gold nanoshells have undergone photo-thermally induced morbidity on exposure to NIR light [3]. Yiwen Wang s group explored the feasibility of using metal nanoshells as intravascular optical contrasts in a rat brain. In contrast to NIR absorbing dyes, the optical properties of nanoshells, which are dependent upon a rigid metallic structure, are not susceptible to the photo-bleaching that is an inevitable problem when using organic dyes [4]. The practical applications of gold nanoshells, however, are limited by the penetration depth of the NIR laser (800

2 66 Sangeun Park, Minyim Park, Pokeun Han, and Sangwha Lee Figure 2. Size variation of silica nanoparticles with respect to the ammonium ion dosages (28 wt%) at room temperature and 48 o C. The standard deviation of silica size is denoted by the error bars. 37 wt%), hydrogen tetrachloroaurate(iii) hydrate (HAuCl 4, %), potassium carbonate (K 2 CO 3, 99.7 %), absolute ethanol (C 2 H 5 OH, 99.5 %), and sodium hydroxide (NaOH, semiconductor grade). Figure 1. Schematic representation of gold layer formation over silica cores nm) that is used to activate plasmon-derived local heating on the gold surface. In a recent study, Halas found that the temperature rise induced by the embedded gold nanoshells in deep tissues was relatively lower than those in shallow tissues [5]. The intensity and position of the NIR-absorption spectra determines the effectiveness of gold nanoshells in biomedical applications. We have systematically investigated the relative contributions of various experimental parameters (such as aging of gold colloids and the amounts of gold reducible salts and formaldehyde reducing agents) to the surface morphology and plasmon resonance of gold nanoshells during the nucleate-mediated fabrication process. The structural morphology and optical properties of gold nanoshells were characterized using TEM, SEM, and UV-Vis spectroscopy. Experimental Materials All of the following reagents were purchased from Aldrich and used as received: 3-aminopropyl trimethoxysilane (APTMS, 97 %), tetraethyl orthosilicate (TEOS, %), tetrakis (hydroxymethyl) phosphonium chloride (THPC, 80 % solution in water), ammonium hydroxide (NH 4 OH, 28 % NH 3 in water), formaldehyde (HCHO, Systhesis Silica nanoparticles were prepared via hydrolysis and condensation of TEOS using the sol-gel process. An ammonium hydroxide solution was used as the catalyst to cause the formation of spherical silica particles [6]. To prepare gold colloids of 1 3 nm diameter, THPC/NaOH reducing agents were mixed with 1.0 wt% aqueous tetrachlroloaurate(iii) trihydrate. Under alkaline conditions, THPC is a reducing agent powerful enough to reduce gold salts by the derivation of formaldehyde [7]. An excess of APTMS was then added to the solution of silica nanoparticles. Next, functionalized silica nanoparticles (0.5 ml) were added to the gold colloids for the deposition of gold colloids onto the silica nanoparticles. Gold-deposited silica (Au-APTMS/SiO 2 ) was further reduced by reacting with reducible gold salts to form the gold layer over the silica cores. The reducible gold salts were prepared through the addition of potassium carbonate to 1.0 wt% HAuCl 4 solution; the color of the solution slowly changed from yellow to colourless. Results and Discussion Preparation of Colloidal Silica Spheres Silica nanoparticles ( nm) prepared by Stöber method at 48 o C exhibited zeta-potentials of ca mv; they were very stable during two-week storage at 4 o C. The size of the silica nanoparticles was controlled by the addition of 28 wt% ammonium hydroxide solution at a fixed reaction temperature. As shown in Figure 2, the part-

3 Relative Contributions of Experimental Parameters to NIR-Absorption Spectra of Gold Nanoshells 67 Figure 3. UV-Vis spectra of gold colloids prepared using THPC/ NaOH reducing agents plotted with respect to the aging time at 4 o C. (a) (b) Figure 4. TEM images of Au-APTMS/SiO 2 samples prepared using (a) one-day-aged and (b) one-week-aged gold colloids. icle size did not increase linearly because of various influencing factors, such as the molar ratio of [H 2 O]/[TEOS], the feeding rates, and the ph. The increased ammoniumion-dosages led to the formation of larger silica nanoparticles; high contents of residual water caused a rapid nucleation rate via hydrogen-bonding-induced agglomeration between silica nanoparticles [8]. In comparison to the results at room temperature, an elevated reaction temperature (48 o C) resulted in smaller particles with narrower size distributions due to enhanced nucleation rates (when other reaction conditions were kept constant) [9]. Aging Effects of Gold Colloids THPC-induced gold colloids (1 3 nm) exhibited zetapotentials in the range of mv. Figure 3 exhibits the variations of the UV-Vis spectra of gold colloids with various aging times. The absorption intensity increased slightly at ca. 520 nm with increasing aging times, indicating a slight variation of colloidal stability and particle size [10]. The ph of one-week-aged gold colloids gradually decreased to ph 7 8 from an initiall ph 10, possibly because the residual OH - ions were gradually consumed by THPC reducing agents during the aging period at 4 o C. Figure 5. UV-Vis spectra of Au-APTMS/SiO 2 samples prepared using gold colloids of different aging times. To attach the gold colloids onto the silica cores, the silica surface was self-assembled using APTMS with amino groups having positive zeta potentials. The attachment of gold colloids was achieved through electrostatic attraction between the aminated silica nanoparticles and the gold colloids having negative charges. Two types of gold colloids (i.e., one-day-aged and one-week-aged gold colloids) were used in the preparation of the gold-deposited silica nanoparticles (Au-APTMS/SiO 2 ). Figure 4 compares TEM images of the Au-APTMS/SiO 2 samples prepared using the one-day-aged and one-week-aged gold colloids. The sizes of gold seeds attached to the silica nanoparticles was roughly estimated from the TEM images. The oneweek-aged gold colloids particle size: ca. 3.5 nm exhibited a monodisperse deposition of gold seeds on the silica nanoparticle surfaces. In contrast, the Au-APTMS/ SiO 2 sample prepared using the one-day-aged gold colloids exhibited a relatively heterogeneous deposition of gold seeds, with a particle size of ca nm that was much larger than the mean particle size of the THPC-induced gold colloids. Gold-deposited silica samples exhibit UV-Vis spectra that reflect the characteristics of gold seeds attached to the silica nanoparticle surface. As shown in Figure 5, peaks in the plasmon-derived absorption spectra were positioned around 534 and 509 nm, respectively, for the one-dayaged and one-week-aged gold colloids. The differences in peak positions and absorption intensities were caused by the cluster sizes of the deposited gold seeds on the silica nanoparticles, i.e., the stronger plasmon resonance was caused by larger-sized gold clusters. However, it is important to prepare monodisperse gold-deposited silica for the fabrication of strong NIR-absorbing gold nanoshells [11]. Hence, one-week-aged gold colloids were used in the following experiments to determine the growth process of the gold layers over the silica cores.

4 68 Sangeun Park, Minyim Park, Pokeun Han, and Sangwha Lee Table 1. Optical Characteristics of Gold Nanoshells Prepared at Different Dosages of Gold Salts and Reduction Agents Core Diameter Formaldehyde Dosage Gold Salt Volume (Concentration) Gold Shell Thickness Core/Shell Ratio Spectral Position (Maximal Peak) Absorption Spectra Difference (Δλ a ) 185 nm 3.4 µl 4 ml (0.38 mm) nm nm 13.4 µl 4 ml (0.38 mm) 20 nm nm nm 3.4 µl 4 ml (0.38 mm) 22 nm nm 26.8 µl 4 ml (0.38 mm) 27 nm nm 120 µl 3 ml (1.52 mm) 28 nm nm nm 120 µl 4 ml (1.52 mm) 23 nm nm nm 120 µl 5 ml (1.52 mm) 26 nm nm nm 50 µl 4 ml (1.52 mm) 14 nm nm nm 80 µl 4 ml (1.52 mm) 30 nm nm nm 120 µl 4 ml (1.52 mm) 30 nm nm nm 80 µl 4 ml (1.52 mm) nm nm 26.8 µl 4 ml (1.14 mm) nm nm 40 µl 4 ml (1.14 mm) nm nm 53.6 µl 4 ml (1.14 mm) nm nm 53.6 µl 4 ml (1.52 mm) nm nm 67 µl 4 ml (1.52 mm) nm nm 80 µl 4 ml (1.52 mm) 28 nm nm 1.09 a Absorbance difference between the heights of plasmon resonance bands. * - denotes unavailable (or immeasurable) experimental data resulting from the limited resolution of the SEM instrument and/or some agglomeration of gold nanoshells. Figure 6. UV-Vis spectra of gold nanoshell recorded during a growth process of 5 min. The solid line is the initial UV-Vis spectrum of Au-APTMS/SiO 2. Other lines indicate the evolution of the optical resonance as the gold layers grew over the silica cores. Concentration Effects of Gold Reducible Salts The gold seeds attached to the silica nanoparticles were further reduced by reacting with gold-reducible salts, consequently leading to continuous gold layers on the silica nanoparticle surface via coalescing of neighboring gold clusters. Figure 6 displays the evolution of the UV-Vis spectra during the growth process of the gold layers over the silica cores within five minutes. The optical plasmon absorption of the gold nanoshells red-shifted strongly with the progressive coverage of the gold layer on the silica nanoparticle surface; the color of the solution changed from colorless to blue and then to dark-red during the five minute period. The formation of gold layers on the silica cores was conducted at several concentrations of gold-reducible salts at optimal dosages of formaldehyde reducing agents. Figure 7 shows SEM images of the gold nanoshells prepared by the addition of gold salts (from 0.38 to 1.52 mm) at a fixed dosage of formaldehyde (1.07 mmol). When the available gold ion concentration was too low (in the case of 0.38 mm gold salts), the gold clusters formed on the silica surface were not fully grown to cover the whole silica nanoparticle surface, as shown in Figure 7(a). When a two-fold concentration (0.76 mm) of gold salts was added, the coverage of the gold layer increased, as shown in Figure 7(b). When the concentration of gold salts was increased to 3 4 times of the initial 0.38 mm, the gold layer almost covered the silica nanoparticle surface, as shown in Figure 7(c), and the thickness of the gold layer increased correspondingly. The excessive addition of gold salts (more than 2.0 mm) and/or formaldehyde reducing agent (more than 1.6 mmol), however, led to the decrease of the plasmon-derived absorption intensity because of the

5 Relative Contributions of Experimental Parameters to NIR-Absorption Spectra of Gold Nanoshells 69 (a) (b) (c) Figure 7. SEM images of gold nanoshells (110 nm silica core) prepared at different concentrations of gold salts: (a) 0.38 mm gold salts, 1.07 mmol HCHO; (b) 0.76 mm gold salts, 1.07 mmol HCHO; (c) 1.52 mm gold salts, 1.07 mmol HCHO; (d) 1.52 mm gold salts, 0.67 mmol HCHO. (d) Figure 8. UV-Vis spectra of gold nanoshells formed at (a) different concentrations of gold salts and (b) different dosages of reducing agents. agglomeration between gold nanoparticles (data not shown). Figure 7(d) shows the SEM image of gold nanoshells formed at 1.52 mm gold salts and 0.67 mmol formaldehyde. In comparison to the gold nanoshells (shell thickness = 30 nm) shown in Figure 7(c), the decrease of the reducing agents led to a smaller gold cluster and a thinner gold layer (shell thickness = 14 nm) on the silica nanoparticle surface.

6 70 Sangeun Park, Minyim Park, Pokeun Han, and Sangwha Lee The plasmon-based absorption bands were distinctly observed with the increment of gold salts up to 1.52 mm. Gold nanoshells formed at concentrations of higher gold salts than 1.14 mm exhibited a slight blue-shift of their plasmon resonance (from 830 to 810 nm), probably due to the increased gold layer thickness, as shown in Figure 8(a). Gold nanoshells formed at lower dosages of reducing agents gave rise to a more-red-shifted plasmon resonance (from 825 to 964 nm) due to the decreased gold layer thickness, as shown in Figure 8(b). Because the plasmon resonance is a collective oscillation of excited electrons from gold nanolayers, the intensity of the optical resonance over increased with the increment of the gold layer thickness interfaced with the dielectric silica core [12]. In summary, the optical resonance of gold nanoshells was optimized by considering the relative contributions of the aging of gold colloids, the concentration of gold salts, and the dosage of reducing agents. To obtain strong NIR-absorbing nanoshells, it is important to prepare complete and thick gold layers onto the silica cores. Table 1 exhibits the optical characteristics of the gold nanoshells prepared under various conditions. The position of the absorption peak of the gold nanoshells increased asymptotically with the increasing core-shell ratio, which is in accord with the theoretical predictions of Mie scattering reported by Halas and coworkers [13]. Conclusions The morphologies of gold-deposited silica nanoparticles were determined by adjusting the aging times of THPCinduced gold colloids. One-week-aged gold colloids produced a relatively monodisperse deposition of gold seeds onto the silica nanoparticles; in contrast, one-day-aged gold colloids produced a heterogeneous deposition of gold seeds on the silica nanoparticle surface. Gold nanoshells were fabricated through the monodisperse deposition of gold seeds onto silica nanoparticles by varying the concentrations of reducible gold salts and reducing agents. The gold nanoshells (core diameter = 150 nm; core/shell = 5) exhibited strong plasmon-derived absorption bands at ca nm. Increasing the amount of reducible gold salts led to an increase of the gold layer thickness over the silica cores, accompanied by a distinct increase of the plasmon-derived optical resonance. Excessive amounts of reducible gold salts and/or reducing agents, however, induced the agglomeration of gold nanoshells, resulting in a decreased extinction absorption intensity of the plasmon-derived optical reson- ance. Acknowledgments The study was supported by GRRC Project of Gyeonggi Provincial Government and partially also supported by Regional Innovation Center for Nanoparticles at Kyungwon University. References 1. a) M. Zhu, G. Qian, Z. Hong, Z. Wang, X. Fan, and M. Wang, J. Phys. Chem. Solids, 66, 748 (2005). b) Y. H. Kim, Y. S. Kang, and B. G. Jo, JIEC, 10, 739 (2004). 2. J. B. Jackson and N. J. Halas, J. Phys. Chem., 105, 2743 (2001). 3. L. R. Hirsch, R. J. Stafford, J. A. Bankson, S. R. Sershen, B. Rivera, R. E. Price, J. D. Hazle, and N. J. Halas, Proc. Natl. Acad. Sci. U.S.A. 100, (2003). 4. Y. Wang, X. Xie, X. Wang, G. Ku, K. L. Gill, D. P. O Neal, G. Stoica, and L. V. Wang, Nano Lett., 4, 1689 (2004). 5. C. Loo, A. Lin, L. Hirsch, M. H. Lee, J. Barton, N. Halas, J. West, and R. Drezek, Technol. Cancer Res. Treatment, 3, 33 (2004). 6. W. Stber, and A. Fink, J. Colloid Interface Sci., 26, 62 (1968). 7. D. G. Duff and A. Baiker, Langmuir, 9, 2310 (1993). 8. S. K. Park, K. D. Kim, and H. T. Kim, J. Ind. Eng. Chem., 6, 365 (2000). 9. S. K. Park, K. D. Kim, and H. T. Kim, Colloids Surf., 197, 7 (2002). 10. J. D. Grunwaldt, C. Kiener, C. Wögerbauer, and A. Baker, J. Catal., 181, 223 (1998). 11. S. E. Park, M. Y. Park, P. K. Han, and S. W. Lee, Bull. Korean Chem. Soc., 27, 1341 (2006). 12. N. J. Halas, in Proc. NASA Conference Publication, , pp.301 (1999). 13. S. J. Oldenburg, R. D. Averitt, S. L. Westcott, and N. J. Halas, Chem. Phys. Lett., 288, 243 (1998).