Branched Gold Nanochains Facilitated by Polyvinylpyrrolidone and

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Branched Gold Nanochains Facilitated by Polyvinylpyrrolidone and their SERS Effects on p-Aminothiophenol Dong-Feng Zhang,† Li-Ya Niu,† Li Jiang,† Peng-Gang Yin,† Ling-Dong Sun,‡ Hua Zhang,† Rui Zhang,† Lin Guo,*,† and Chun-Hua Yan*,‡ School of Chemistry and EnVironment, Beijing UniVersity of Aeronautics and Astronautics, Beijing 100083, People’s Republic of China and State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: April 10, 2008; ReVised Manuscript ReceiVed: July 18, 2008

Stable gold nanochains with branched features were fabricated at room temperature with the assistance of polyvinylpyrrolidone (PVP). An attachment and fusion process was believed to responsible for the formation of the chain-like structure. During the process, PVP acted both as a structure-directing agent and as a stabilizer to inhibit continuous size increase. UV-vis-NIR study revealed that the unique structure shifted the longitudinal surface plasma resonant band (LSPR) to as far as 1457 nm, which may endow the product with new applications. The surface-enhanced Raman scattering activity was investigated with p-aminothiophenol as probe molecule. The enhancement factor (EF) of the products deposited as film on the Si substrate was estimated to be (1.03 ( 0.74) × l06 for the a1-type band, while it was (1.32 ( 0.15) × l07 for b2-type band. The enhancement of a1 modes was attributed to the electromagnetic mechanism, whereas the additional EF for the b2 mode is presumed to contribute from the chemical effect. Introduction Au nanocrystals have been an important member of the materials family because of their unique optical properties and excellent biocompatibilities. Surface plasmon resonance (SPR), an optical phenomenon arising from the oscillation between electromagnetic wave and the conduction electrons, is one of the most important basis properties for the application of Au nanocrystals. It has been proven theoretically and experimentally that the surface plasmon absorption bands can be tuned throughout the visible and near-infrared region, depending on their size, shape, aggregation pattern, and environmental dielectric constant.1-4 For example, one-dimensional Au nanocrystals typically possess two surface plasmon absorption bands owing to the transverse and longitudinal oscillations of electrons. SPR peak shift was also observed in the nanoparticle assembly owing to the interparticle coupling. If there were molecules adsorbed on the nanocrystals’ surface, the localized surface plasmon resonance and the coupling between the nanocrystals’ plasmons and the molecules’ electronic states would enhance the Raman scattering cross section of the adsorbed molecules and resulted in the surface-enhanced Raman scattering (SERS) effects.5-9 The distinct features promoted Au nanocrystals as promising candidates for application in a wide range covering from biodiagnostic10 to potential optoelectronic devices such as photonic wave-guiding and optical switching.11 Although excellent studies have been carried out with respect to the precise size and shape control of Au nanocrystals (i.e., nanoparticles, nanorods, and nanoplates), the ability to assemble these primary building blocks into architectures with desired properties is still limited.12 Kang et al. organized poly(styreneblock-acryclic acid) (PS-b-PAA) embedded Au nanoparticles * To whom correspondence should be addressed. Phone & Fax: +8610-82338162. E-mail: [email protected]; [email protected]. † Beijing University of Aeronautics and Astronautics. ‡ Peking University.

into 1D chains with the assistance of cross-linker agents such as NaCl, CaCl2, and 1-(3-dimethylamino)propyl)-3-ethyl-carbodiimide methiodide (EDC).12a Mann’s group reported the fabrication of chain-like Au nanocrystals based on the ligand exchange of citrate ions stabilized Au nanoparticles by 2-mercaptoethyl alcohol (MEA).12b The end-to-end assembly of Au nanorods was demonstrated by taking advantage of bifunctional molecules such as cysteine, glutathione, and mercaptocarboxylic acid.12c-e With dodecanethiol (C12H25SH) as a capping agent, Lin et al. assembled the Au nanoparticles into 1D, 2D, and 3D superlattices.12f In most cases, Au nanoparticles are separated by layers of organic species. And it is difficult to maintain the chain-like framework after the removal of the organic linkers. In this work, we report the fabrication of branched Au nanochains by a simple and mild process with the assistance of polyvinylpyrrolidone (PVP), which serves as both a structure directing and a stabilizing agent. To be noted, the branched chain-like structure shifted the longitudinal plasma band of Au nanostructure to the region as far as 1400∼1600 nm. Stimulated by the junction sites presented in the branched chainlike structure, the SERS activity was investigated with p-aminothiophenol (p-ATP) as probe molecule. Experimental Section Synthesis. The preparation of the seed solution: 0.15 mL of freshly prepared 0.1 M NaHB4 solution was added into the mixture of 0.1 mL of 0.025 mol · L-1 sodium citrate in water and 4.95 mL of 1 × 10-3M HAuCl4 in glycol under vigorous stirring. The solution color changed from colorless to dark blue. The growth of chain-like structure: 0.0044 g Polyvinylpyrrolidone (PVP, MW 58 000) was dissolved into 9.9 mL of 1 × 10-3 mol · L-1 AuCl4- in glycol to form transparent light yellow solution. After the above solution was stirred for 1 h, 1.0 mL of seed solution was added and subsequently 0.3 mL of freshly prepared 0.1 mol · L-1 NaBH4 was introduced. Stirring lasted

10.1021/jp803102h CCC: $40.75  2008 American Chemical Society Published on Web 09/23/2008

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Figure 1. Morphology and structure of the as-prepared branched Au nanochains. (a) TEM image, (b) EDAX spectrum, (c) high-magnification TEM image, and (d) HRTEM image recorded at the black-framed region in part c.

for 15 min to complete the reaction. The color of the solution changed into reddish brown. The separation of the chain-like structure: the resulting solution was centrifuged at 13 000 rpm for 20 min. After the supernatant was poured off and washed several times with distilled water, a dark blue precipitate was produced, which was dispersed into 1.0 mL of ethanol for further TEM, UV-vis absorption, and SERS activity characterization. SERS Sample Preparation. The SERS substrate was prepared by dropping 10 µL of the above-prepared sample onto a carefully cleaned silicon plate, which was allowed to dry naturally in air. After repeating the dropping 3×, the substrate was immersed into 1 × 10-5 mol · L-1 p-aminothiophenol (pATP) ethanol solution for 30 min. After drying in a dark at room temperature, it was then rinsed with deionized water and absolute ethanol several times to remove the free p-ATP molecules. The SERS measurement on aqueous solution was carried out using the following procedure. First, a 0.0015-g portion Au of nanochains was dispersed into 1.0 mL of distilled water. Second, an equal volume of the solution, consisting of p-ATP and distilled water in a molar ratio of 1:1000, was added to the above Au suspension. After being sonicated for 15 min, the mixture was transferred into a quartz cell for the further Raman characterization. Instrumentation. Scanning electron microscopy (SEM) observations were carried out with an Hitachi S-4800 operated at an acceleration voltage of 5 kV. High-resolution transmission electron microscopy (HRTEM) characterizations were per-

formed with a JEOL JEM-2100F microscope used at 200 kV. UV-vis-NIR characterization was performed on a PerkinElmer Lamdba 950 spectrometer. Raman spectra were recorded on a Jobin Yvon (Laboratory RAM HR900) spectrometer employing a 632.8-nm laser line as the excitation source. The Raman band of a silicon wafer at 520 cm-1 was used to calibrate the spectrometer. The spectra were obtained by using a 50× objective lens to focus the laser beam onto a spot with ∼1 µm diameter. All of the spectra reported were the result of a single 50 s accumulation. Results and Discussion Among the existing literature on the size- and shapecontrolled synthesis of Au nanocrystals, seed-mediated growth13 is a mature one, which generally involves two-steps: First, the preparation of gold seed by borohydride sodium (NaBH4) reduction of gold salt with citrate as capping agent; and second, the subsequent growth process in a gold ion-containing aqueous solution in the presence of a weaker reducing agent, such as ascorbic acid, and a structure directing agent, usually as cetyltrimethylammonium bromide (CTAB). Our previous studies revealed that the presence of PVP facilitated the formation of chain-like structure in an appropriate kinetic regime.14 It occurred to us that if we use PVP instead of CTAB in the growth stage, Au nanoparticles might be directed into chain-like structures by adjusting the reaction parameters, which was definitely confirmed by the experiments.

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Figure 2. Micrograph of the products obtained in the absence of PVP at different duration stage. (a) 1 min and (b) 30 min.

Figure 1a is a typical transmission electron micrograph (TEM) image of the as-prepared product, which clearly shows the chainlike structure with distinct branched features. Although the complex geometry confused the identification of its length, its diameter can be determined as ∼12 nm in average. The X-ray energy dispersive analysis (EDAX) gave signals of Au, Cu, and C (Figure 1b). The C and Cu came from the carbon-coated copper grid used to support the samples. Thus, EDAX confirmed that the chain-like structure is composed of Au with high purity. Closer observation (Figure 1c) revealed that besides the chainlike structure, nanoparticles smaller than 3 nm in diameter were also observed, which could be separated after several iterations of centrifugation. High-resolution TEM (HRTEM) images (Figure 1d) recorded at the joint regions of the Au nanochains provided solid evidence that the branches had fused into, rather than physically adsorbed onto, each other. Although the lattice fringe in different regions possessed different orientations, the measured spacing is 0.236nm for all sets, corresponding to (111) plane of cubic Au. Few locations lacking of lattice fringe can also be observed in the nanochains, illustrating its polycrystalline nature. The results were similar to the nanowires reported by Peng15 and Lee16 et al. very recently. In their works, Au nanowires were observed as intermediates and soon evolved into sphere-like particles with increasing size. However, the chain-like structure in our case kept stable for at least three months. We tried to trace the morphologic evolution of the chainlike structure by TEM observation at different duration intervals. It revealed that the formation of the chain-like structure was nearly instantaneous with the introduction of the NaBH4. Hardly any change was detectable. To learn the detailed growth kinetics, control experiments were conducted. First, we carried out the experiment under similar conditions but without the addition of PVP. TEM observations (Figure 2a) revealed that isolated nanoparticles with diameter in the range of 4-6 nm were the predominant morphologies in the very early reaction stage. After several minutes, the initially transparent solution began to agglomerate into dark blue particle suspension. As shown in Figure 2b, irregularly-shaped aggregates with large sizes were produced. Obviously, the initially formed nanoparticles experienced a fast aggregation process. Coming back to the chainlike structure formed in the presence of PVP, it can be concluded that the chain-like structure should be formed by the attachment of initially produced nanoparticles. In fact, there have been many reports regarding the formation of one-dimensional (1D) nanomaterial by the attachment of nanoparticles in an oriented manner.17 However, according to the HRTEM characterization shown in Figure 1d, the lattice fringes of two adjacent particles are randomly oriented in our case.

Figure 3. TEM image of the product obtained by using ascorbic acid as a reducing reagent.

Further investigation indicated that the concentration of the initially formed nanoparticle might be crucial for the formation of the chain-like structure. Under similar conditions, with the exception of the substitution of ascorbic acid for NaBH4 as the reducing reagent in the growth stage, irregular shaped nanoparticles with sizes in the range of 30∼50 nm were obtained, as shown in Figure 3. It is well-known that the reduction potential of the ascorbic acid is not high enough to directly reduce Au3+ ions into metallic particles. Thus, the concentration of the Au nuclei at the early stage of the growth is rather low, which might not be sufficiently high for the particle interconnection. The pre-introduced seeds triggered the growth of metallic Au on their surface, which led to size increase rather than interparticle fusion. The above discussion suggested to us that attachment and fusion are intrinsic behaviors for nanoscaled Au particles at appropriate kinetic regimes. However, since the products exhibited as irregular aggregations with broad size distributions in the absence of PVP, we believed PVP had played an important role during the formation of the chain-like structure. Lee’s group also observed short chain-like structures in their study16 with NaBH4 as reducing reagent, although it was difficult to be isolated due to the fast reaction kinetic. In conjunction with their results, we supposed that the adsorption of the PVP on the Au surface prevented the inter- or intraparticle ripening, and thus stabilized the chain-like nanostructures. However, it is well-known that PVP itself is a chain-like polymer,18 which has been successfully employed to direct the growth of chainlike structures in previous studies.14 Compared with the short chain-like structure obtained by Lee et al.,16 we believed PVP had also served as a structure-directing agent during the formation of the Au nanochains.

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Figure 4. UV-vis-NIR spectra of the branched Au nanochains.

As shown in Figure 4, the as-prepared chain-like nanostructure dispersed in ethanol solution possessed two adsorption bands, one located at around 536 nm, and the other at around 1457 nm. The former can be assigned to the transverse SPR (TSPR) adsorption band, whereas the latter is believed to originate from the longitudinal SPR (LSPR) coupling. Since for Au nanoparticles with an average diameter of 9 nm, the SPR band was expected at around 517 nm,19 the large red shift in our observations can be attributed to morphology effect. As depicted in Figure 1, the as-prepared products were of chainlike structure with distinct branch features. The complex morphology resulted in a distribution in aspect ratio. The branches with relatively small aspect ratios resulted in the broadening and shift of the SPR from 517 to 536 nm, while the plasmonic coupling between particles in chains of large aspect ratios produced the LSPR mode around 1457 nm. To be noted, the reported LSPR is usually located in the 800∼1200 nm regime. The band as far as ∼1457 nm in our case might endow the product with new applications such as communication. The SPR behavior of the products in the powder state was further investigated by UV-vis-NIR diffuse reflection spectrum (UV-vis-IR DRS) with BaSO4 as a reflectance standard (as shown in Figure S2 of the Supporting Information). It indicated that in comparison with Figure 4, the LSPR band ranging from 1400 to 1600 nm remained and predominated the spectrum, while the 536 nm band was hardly identified and a weak and broad absorption appeared at around 635 nm. The difference can be elucidated from the viewpoint of the aggregation pattern of the samples. For UV-vis-NIR DRS measurement, it is necessary to press the mixture of the powdered sample and BaSO4 into a plate. Therefore, the Au nanocrystals exhibited as a condensed aggregation of chain-like structures. The surface plasmon coupling along the chain produced the longitudinal band at around 1430 nm, which is similar to that in the SPR spectrum of the ethanol dispersion. However, as has been proven in the previous reports,20 the densely packed aggregates would greatly decrease the TSPR band and shift it to the lower energy side, which is responsible for the weak band at about 635 nm. Promoted by the superior sensitivity and molecular specificity, surface enhanced Raman scattering has been a powerful tool to label the bond vibrations of molecules. Chemical effect (CE) and electromagnetic (EM) effect were generally accepted as two major mechanisms responsible for the SERS activity with Au as substrate. Calculations based on the electromagnetic enhancement predicted that SERS intensity would increase significantly at sharp discontinuities on a metal surface. Obviously, the junction presented in the branched chain-like Au nanostructure would serve as “hot” sites to facilitate such an enhancement. With a He-Ne 632.8 nm laser line as the excitation source, we investigated the SERS behaviors of the as-prepared Au nano-

Figure 5. Normal Raman and SERS spectra of (a) solid p-ATP and (b) p-ATP on branched Au nanochains. The inset is the SEM image of the Au nanochains deposited on silicon.

structures. p-Aminothiophenol (p-ATP) was chosen as the probe molecule to form a self-assembled monolayer (SAM) on the nanostructures upon the binding between thiol moiety and Au. Figure 5, parts a and b, displays the normal Raman spectrum of solid p-ATP and the SERS spectrum of p-ATP on the Au nanochains deposited on silicon, respectively. Compared to the normal Raman spectrum, marked changes in frequency shifts and relative intensity occurred for most of the bands in SERS spectrum. On the basis of the well-documented solid data,8,21 the bands at 1592, 1182, 1090, and 467 cm-1 can be assigned to the vibration of νCC, 8a (a1), δCH, 9a (a1), νCS, 7a (a1) and νCCC, 7a (a1), respectively. The νCS vibration shifts from 1090 cm-1 in Figure 5a to 1079 cm-1 in part b, suggesting that the thiol group in p-ATP is directly bonded to Au surface. The band at 467 cm-1 is hardly identified, while a middle strong band at 392 cm-1 appears in the SERS spectrum, which corresponds to the small band at 393 cm-1 in the normal spectrum. Similar phenomena were also observed by Wang22 and Zheng23 et al. They believed it was originated from the bending mode of the C-S bond. It is noteworthy that apart from the modes with a1 symmetry, the bands related to the b2 symmetry scattering were also greatly enhanced. For example, peaks at 1575, 1433, 1389, and 1143 cm-1, attributed respectively to 8b2, 19b2, 3b2, and 9b2 modes of p-ATP, were invisibly weak in the normal Raman spectrum, as shown in Figure 5a, whereas they were compatible in intensity with the dominant band at 1076 cm-1 in Figure 5b. The enhancement mechanism for a1 modes is believed to be different from that of the b2 ones. To evaluate the mechanism more quantitatively, δCH (9a1) at 1094 cm-1 and δCH (9b2) at 1138 cm-1 were selected to determine the enhancement factor (EF), which is usually estimated according to the following equation:22,24

EF ) (ISERS ⁄Nads) ⁄ (Ibulk ⁄Nbulk)

(1)

where ISERS and Ibulk are the measured vibration intensity in the SERS or norm Raman spectra, respectively, Nbulk and Nads are the molecule number of solid or adsorbed PATP in the laser illumination volume, respectively. In our experimental condition, the laser spot area can be determined as ∼2 µm2 and the penetration depth about 15 µm.25 Thus, Nbulk was calculated as 1.69 × l011 considering the density of p-ATP as 1.17 g.cm-3. Owing to the complex geometry, it is of some difficulty to determine the surface area of the Au nanochains. Let us regard the Au nanochain as the assembly of nanoshperes with average diameters of 12 nm, and then the Nads can be acquired via eq 2:

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Nads ) Nd Alaser AN/σ

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(2)

where Nd is the number density of the Au nanoshperes, Alaser is the area of the focal laser spot, AN is the area of one individual nanosphere (calculated as 1.1 × 10-4 µm2) and σ is the surface area occupied by one adsorbed p-ATP molecule, which was about 0.20 nm2/molecule on the basis of the literature.26 Assuming that a layer of Au nanochain deposited on the substrate homogeneously, the number density of the Au nanospheres was counted as ∼3000 particles/µm2 according to the high magnified TEM image (Figure S1 of the Supporting Information). As revealed by SEM observation (inset in Figure 5), the surface coverage of Au nanochains on the silicon substrate is approximately 50%. Thus, the number of probed molecules in the probed volume (Nads) is about 1.15 × 106. According to the statistical results of the repeated SERS measurements, the intensity ratio (Isurf to Ibulk) of δCH (9a1) and δCH (9b2) were measured as 7.0 ( 0.5 and 90 ( 10, respectively. Therefore, the EF can be calculated to be (1.03 ( 0.74) × l06 and (1.32 ( 0.15) × l07 by substituting the values of the variables into eq 1. It is generally accepted that the EM mechanism mainly accounts for the enhancement of a1 modes.20b,22 In comparison with the EF value derived from the a1 modes, it is at least 10× higher for that of the b2 modes. We believed that the excessive enhancement of the b2 type bands can be attributed to the chemical mechanism associated with the strong Au-S interaction. In our case, the junction in the branched chains may serve as the hot sites. Alternatively, due to the highly branched feature, interlinking among the chains is inevitable. As evidenced by the emergence of the 635 band in the UV-vis-NIR DRS spectrum of Au nanochain film (Figure S2 of the Supporting Information), the coupling of the localized surface plasmon of the aggregated nanoparticles existed. The coincident of the 635 band with the 632.8 nm excitation line would also account for the EM enhancement. To eliminate the contribution from the new junctions formed by the aggregation of the branched Au nanostructures deposited onto Si substrate, we also carried out the SERS measurement by dispersing the product into the ethanol solution of p-ATP. However, the Raman scatterings of ethanol are so strong that hardly any signal from p-ATP can be identified. Then, we tried the investigation by dispersing a minimum of p-ATP into the aqueous solution of Au nanostructures under sonicating. It is known that p-ATP is insoluble in water. However, unexpectedly, the SERS measurement demonstrated that the typical vibration modes of p-ATP could be clearly resolved as shown in Figure S3 of the Supporting Information. Since the melting point of p-ATP is rather low (38-42 °C), the heat generated by the sonication of the solution would melt the p-ATP into dispersed liquid droplets. When the droplets met the Au nanostructures, the Au-S interaction would result in the preferentially adsorption of p-ATP molecules onto the surfaces of the Au nanostructures. In the absence of the Au nanostructures, no signals were detected. Thus, the results confirmed the high SERS activity of the as-prepared chain-like nanostructures. Conclusions In this paper, we presented a simple method to fabricate stable Au nanochains with a highly branched feature. HRTEM characterization demonstrated that the nanochains were formed from the attachment and fusion of nanoparticles, which was believed to be an intrinsic behavior for Au nanoparticles under relatively high concentration. During the process, PVP func-

tioned both as a structure-directing agent and a stabilizer to inhibit continuous size increase. The UV-vis-NIR spectrum of the branched Au nanochains consisted of a TSPR band centered about 536 nm and a LSPR at as far as 1457 nm. Promoted by the junction sites presented in the branched chainlike structure, the SERS activity was investigated with p-ATP as the probe molecule. The enhancement factor of the products deposited as film on Si substrate was estimated to be (1.03 ( 0.74) × l06 for a1-type bands, while it was (1.32 ( 0.15) × l07 for b2-type band. Assuming that the contribution from the electromagnetic mechanism is equal for a1 and b2 vibrations, the additional EF for the b2 mode is presumed to be attributed to the chemical enhancement associated with the strong Au-S interaction. Acknowledgment. The authors are thankful for financial support from 863 projects of China (2006AA03Z326), Research Fund for the Doctoral Program of Higher Education of China (No. 20070006016), National Natural Science Foundation of China (50725208), and the Opening Foundation of State Key Laboratory of Rare Earth Materials Chemistry and Applications. Supporting Information Available: High-magnified TEM image of the Au nanochains, absorption spectrum of the powdered sample, and the SERS spectrum of p-ATP aqueous solution in the presence of Au nanochains. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hu, M.; Chen, J. Y.; Li, Z. Y.; Au, L.; Hartland, G. V.; Li, X. D.; Marqueze, M.; Xia, Y. N. Chem. Soc. ReV 2006, 35, 1084, and references therein. (2) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J. X.; Gou, L. F.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (3) Sosa, I. O.; Noguez, C.; Barrera, R. G. J. Phys. Chem. B 2003, 107, 6269. (4) Jain, P. K.; Eustis, S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 18243. (5) Schatz, G. C. Acc. Chem. Res. 1984, 17, 370. (6) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (7) Campion, A.; Ivanecky, J. E., III; Child, C. M.; Foster, M. J. Am. Chem. Soc. 1995, 117, 11807. (8) Kim, K.; Lee, H. S. J. Phys. Chem. B 2005, 109, 18929. (9) Ghosh, S. K.; Pal, T. Chem. ReV. 2007, 107, 4797. (10) (a) Reinhard, B. M.; Siu, M.; Agarwal, H.; Alivisatos, A. P.; Liphardt, J. Nano Lett. 2005, 5, 2246. (b) El-Sayed, I. H.; Huang, X.; ElSayed, M. A. Nano Lett 2005, 5, 829. (c) Skrabalak, S. E.; Chen, J. Y.; Au, L; Lu, X. M.; Li, X. D.; Xia, Y. N. AdV. Mater. 2007, 19, 3177. (11) (a) Ozbay, E. Science 2006, 311, 189. (b) Imura, K.; Okamoto, H.; Hossain, M. K.; Kitajima, M. Nano Lett. 2006, 6, 2173. (c) Sukharev, M.; Seideman, T. J. Chem. Phys. 2007, 126, 204702. (12) (a) Kang, Y. J.; Erickson, K. J.; Taton, T. A. J. Am. Chem. Soc. 2005, 127, 13800. (b) Lin, S.; Li, M.; Dujardin, E.; Girard, C.; Mann, S. AdV. Mater. 2005, 17, 2553. (c) Zhang, S. Z.; Kou, X. S.; Yang, Z.; Shi, Q. H.; Stucky, G. D.; Sun, L. D.; Wang, J. F.; Yan, C. H. Chem. Commun. 2007, 1816. (d) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 13066. (e) Sudeep, P.K.; Joseph, S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2005, 127, 6516. (f) Lin, J.; Zhou, W.; O’Connor, C. J. Mater. Lett. 2001, 49, 282. (13) (a) Jana, N.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (b) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389. (c) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (d) Gou, L. F.; Murphy, C. J. Chem. Mater. 2005, 17, 3668. (e) Iqbal, M.; Chung, Y. I.; Tae, G. J. Mater. Chem. 2007, 17, 335. (14) Guo, L.; Liang, F.; Wen, X. G.; Yang, S. H.; He, L.; Zheng, W. Z.; Chen, C. P.; Zhong, Q. P. AdV. Funct. Mater. 2007, 17, 425. (15) Ji, X. H.; Song, X. N.; Li, J.; Bai, Y. B.; Yang, W. S.; Peng, X. G. J. Am. Chem. Soc. 2007, 129, 13939. (16) Pong, B. K.; Elim, H. I.; Chong, J. X.; Li, W.; Trout, B. L.; Lee, J. Y. J. Phys. Chem. C 2007, 111, 6281. (17) (a) Penn, R. L.; Banfield, J. F. Geochim. Cosmochim. Acta 1999, 63, 1549. (b) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188. (18) Xu, Z.; Xie, Y.; Xu, F.; Xu, D.; Liu, X. H. Inorg. Chem. Commun. 2004, 7, 417.

16016 J. Phys. Chem. C, Vol. 112, No. 41, 2008 (19) (a) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212. (b) Liz-Marza´n, L. M. Langmuir 2006, 22, 32. (20) (a) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959. (b) Lin, S.; Li, M.; Dujardin, E.; Girard, C.; Mann, S. AdV. Mater. 2005, 17, 2553. (21) (a) Osawa, M.; Matsuda, N.; Yoshii, K.; Uchida, I. J. Phys. Chem. 1994, 98, 12702. (b) Kim, K.; Yoon, J. K. J. Phys. Chem. B 2005, 109, 20731. (22) Wang, Y. L.; Zou, X. Q.; Ren, W.; Wang, W. D.; Wang, E. K. J. Phys. Chem. C 2007, 111, 3259. (23) Zheng, J. W.; Li, X. W.; Gu, R.; Lu, T. H. J. Phys. Chem. B 2002, 106, 1019.

Zhang et al. (24) (a) He, P.; Liu, H.; Li, Z.; Liu, Y.; Xu, X.; Li, J. Langmuir 2004, 20, 10260. (b) Taylor, C. E.; Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M. H. Appl. Spectrosc. 1999, 53, 1212. (c) Ren, B.; Lin, X. F.; Yang, Z. L.; Liu, G. K.; Aroca, R. F.; Mao, B. W.; Tian, Z. Q. J. Am. Chem. Soc. 2003, 125, 9598. (25) Gole, A.; Sainkar, S. R.; Sastry, M. Chem. Mater. 2000, 12, 1234. (26) (a) Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J. Anal. Chem. 2005, 77, 3261. (b) Jiang, J.; Bosnick, K.; Maillard, M.; Brus, L. J. Phys. Chem. B 2003, 107, 9964.

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