Type I Collagen-Mediated Synthesis and Assembly of UV

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J. Phys. Chem. C 2007, 111, 1976-1982

Type I Collagen-Mediated Synthesis and Assembly of UV-Photoreduced Gold Nanoparticles and Their Application in Surface-Enhanced Raman Scattering Gang Wei, Li Wang, Lanlan Sun, Yonghai Song, Yujing Sun, Cunlan Guo, Tao Yang, and Zhuang Li* State Key Lab of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun, 130022, China ReceiVed: September 8, 2006; In Final Form: NoVember 29, 2006

We reported a simple method to synthesize gold nanoparticles (NPs) by photoreducing HAuCl4 in acetic acid solution in the presence of type I collagen. It was found that the collagen takes an important role in the formation of gold NPs. The introduction of collagen made the shape of the synthesized gold nanocrystals change from triangular and hexangular gold nanoplates to size-uniform NPs. On the other hand, thanks to the special characters of collagen molecules, such as its linear nanostructure, are positively charged when the pH < 7, and the excellent self-assembly ability, photoreduced gold NPs were assembled onto the collagen chains and formed gold NPs films and networks. A typical probe molecule, 4-aminothiophenol, was used to test the surface-enhanced Raman scattering activity of these gold NPs films and networks and the results indicated good Raman activity on these substrates.

1. Introduction Knowing about the physical and chemical properties of inorganic nanoparticles (NPs) is essential and important for developing facile methods for their synthesis and organization. Studies of noble metal NPs, such as gold, silver, and platinum NPs are particularly important for their potential applications in catalysis,1-3 spectroscopy,4-7 plasmonic waveguides,8-11 and sensors.12-15 Photoreduction method is a facile and effective method to synthesize different silver, gold, and platinum nanocrystals. Silver NPs,16-20 silver nanoplates,21-23 silver nanoprisms,24-26 gold NPs,27-30 gold nanoplates,31 and platinum NPs,32-34 have been prepared by the photoreduction strategy. Recently, Berti et al. reported a very simple, rapid, and effective way to form strings of silver NPs on a DNA template by photoreducing DNA-AgNO3 complexes with 253.7-nm UV light. They thought that DNA plays an active role in the reduction process, which functions as a photosensitizer.35 It is possible to prepare some complex inorganic nanostructures by the biomoleculemedicated self-assembly of inorganic NPs. Collagen, one of the most important and abundant structural proteins in the extracellular matrix (ECM), has been widely used in biomedical and biomaterial applications.36-38 It shows special self-assembly character in solution and solid matrix by adjusting the concentration of collagen, as well as the pH and temperature of the system, and the collagen molecules can form fibers, networks and films with fine strength and stability through its self-aggregation and cross-linking.39-41 Type I collagen is one of the several types of collagen that were used usually. It was reported that type I collagen was a versatile liquid crystal biological template for Si,42 (Ti, Sn)O2,43 and other metal structures44 from nano- to microscopic scales. We reasoned that it would be possible to synthesize gold NPs by photoreducing collogen-HAuCl4 complexes in solution. In * To whom correspondence should be addressed. Fax: +86 431 5262057. Tel.: +86 431 5262057. E-mail: [email protected].

addition, based on the collagen’s characters of forming fibers, networks and films, it is possible to prepare collagen-templated gold nanostructures by the electrostatic interaction between collagen and photoreduced gold NPs. In this work, gold NPs with uniform size were prepared by reducing HAuCl4 with 253.7-nm UV light in acetic acid solution in the presence of collagen. By adjusting the molar ratio of collagen to HAuCl4, the size and shape of prepared gold NPs can be controlled simply. At the same time, collagen can provide an effectual template for the self-assembly of gold NPs to form NPs films and networks. It is found that these gold NPs films and networks can be used as good surface-enhanced Raman scattering (SERS)active substrates with 4-aminothiophenol (4-ATP) as a test probe. 2. Experimental Section 2.1. Materials. Chloroauric acid (HAuCl4‚4H2O, AR) and acetic acid (CH3COOH, AR) were purchased from Beijing Chemical Co. (Beijing, China). 4-ATP was purchased from Aldrich. Type I collagen form calf shin (acid soluble) was supplied by Sigma-Aldrich Inc. (U.S.A.) and used as received. Acetic acid (0.2 M) was used to dissolve the collagen to different concentration from 2 to 50 ng/µL for next experiments. 2.2. Photoreduction of Collagen-HAuCl4 Complexes to Form Gold Nanocrystals Film and Networks. A 2 mL 1 mM aqueous HAuCl4 amount was first added into a 10-mL beaker, and then collagen solutions with different volumes and concentrations were added into the beaker, and the final volume for the mixed solution was kept about 4 mL. After mixing them equally, UV irradiation was carried out in the air at room temperature. The samples were placed in a photochemical reactor (home made) and irradiated by 253.7-nm UV light with a 15-W lamp for varying amounts of time. The irradiation intensity is larger than 30 µW/cm-2 when the distance between lamp and sample is in the scale of 100 cm, as provided by the specification. The distance between the samples and the lamp is about 3-4 cm.

10.1021/jp065868b CCC: $37.00 © 2007 American Chemical Society Published on Web 01/11/2007

UV-Photoreduced Gold Nanoparticles

Figure 1. UV-vis spectra of gold nanocrystals synthesized by photoreducing the mixed solution of 2 mL of 1 mM HAuCl4 with (a) 2 mL of H2O, (b) 2 mL of 0.2 M acetic acid, (c) 2 mL of 2 ng/µL collagen, and (d) 2 mL of 10 ng/µL collagen. The optical length is 1 cm. The final volume is 4 mL by adding 0.2 M acetic acid for b-e. The irradiation time is 1 h. The digital image shows the change of the color for the gold nanocrystals in solution.

For comparison, a gold NP film formed by 35-nm Au NPs was prepared. The 35-nm Au NPs were prepared as previously method reported by Freeman et al.45 and Mulvaney et al.46 In brief, the synthesis includes two main steps: (1) preparation of 12-nm gold seeds: 20 mL of 1 mM HAuCl4 was brought to a rolling boil with vigorous stirring. Rapid addition of 2 mL of 38.8 mM sodium citrate to the vortex of the solution resulted in a color change from pale yellow to burgundy. (2) Synthesis of 35-nm gold NPs: 3 mL of 12 mM HAuCl4 was added to every 97 mL of H2O, and the mixed solution was brought to a boil under vigorous stirring, then 1 mL of a 12 nm gold colloid and 0.5 mL of 1% sodium citrate were added and boiled for 10 min. The gold NPs films or networks were prepared by depositing 80 µL gold colloids onto the indium-tin oxide (ITO) glass. 2.3. Instruments. UV-vis absorbance spectra were acquired using a Cary 500 UV-vis NTR spectrophotometer (Varian). Scanning electron microscopy (SEM) images were obtained by an XL30 ESEM FEG field emission scanning electron microscope (SEM, FEI Co. with 20 kV operating voltage); ITO glasses were used as the substrates for all FESEM experiments in this work. The Raman experiments were conducted on a FTRaman 960 spectrometer (Thermo Nicolet), which was equipped with an InGaAs detector and an Nd/VO4 laser (1064 nm) as an excitation line. The laser power used is 400 mW. The number of sample scans is 1024, and the resolution is 8.00 for all the measurements. In this work, two parallel experiments were applied to prepare the SERS substrates (gold NPs networks and films), and three random points were selected and detected on every substrate. 3. Results and Discussion 3.1. The Importance of Introduction of Acetic Acid and Collagen to the Formation of Gold NPs. UV-vis absorption spectra have been proved to be very sensitive to the formation of gold and silver colloids. Figure 1a shows the UV-vis spectrum of gold nanocrystals synthesized by irradiating HAuCl4 aqueous solution at room temperature for 1 h. Two absorption

J. Phys. Chem. C, Vol. 111, No. 5, 2007 1977 peaks can be found in this spectrum. One narrow peak at 308 nm indicates the existence of unreduced AuCl4- ions in the solution after photoreduction process,27 and another broad peak at about 561 nm and the long tail to the long wavelength scale show the formation of large NPs and/or aggregation or assemblies of NPs.47 The SEM image (Figure 2a) of this product agrees well with the result of UV-vis spectrum shown in Figure 1a. There are some triangular gold nanoplates (including deficient triangular nanoplates) except gold NPs with large size (about 400 nm). The addition of acetic acid increases the kinetics of reduction. As shown in Figure 1b, the disappearance of the band at 308 nm in the UV-vis spectrum indicates the AuCl4- ions in the solution have been reduced completely after 1 h. Also, there is an obvious increase for the intensity of characteristic peak of gold NPs at 552 nm, which also can be testified by the SEM image showed in Figure 2b. From this image, we found that the diameter of the synthesized gold NPs (about 150 nm) is smaller than that in Figure 2a. A special phenomenon which should be mentioned is that nearly all the gold nanoplates are intact triangular forms but not like the deficient triangular form revealed in Figure 2a. It seems that this SEM image is not in agreement with the UV-vis spectrum showed in Figure 1a, which reveals relative small scattering. In the products, the gold NPs are predominant compared to the gold triangular plates. Because the acetic acid molecules cannot protect the gold NPs and nanoplates effectively, the synthesized nanocrystals will aggregate and form sediment within 1 h. It is possible that the aggregates and sediment were generated on the substrates during the water evaporation. Indeed, what Figure 2b showed is the image of aggregated gold NPs and triangular plates. Furthermore, the introduction of acetic acid can greatly accelerate the reaction rate of AuCl4- changes to Au(0), an obvious hint being the change in the color from colorless to orange after 10 min of irradiation. A control experiment was done to check the reduction activation of acetic acid to AuCl4- ions under the irradiation of UV light. First, collagen (2 ng/µL) dissolved with 0.2 M HCl was mixed with 1 mM HAuCl4 with the same volume for 10 min. Then the mixed solution was irradiated under the 253.7nm UV light for 1 h. There was no change for the color of the solution, even though the irradiation time was increased to 2 h. Moreover, there were no other characteristic peaks appeared at the range from 500 nm to the longer wavelength direction (data not shown). Negishi et al. demonstrated that 2,3-dimercaptosuccinic acid (DMSA) can reduce HAuCl4 to Au(0) at ambient temperature with forming small gold NPs.48 They thought that the SH moiety but not -COOH groups took important role in the reduction of AuCl4- ions to gold nanocrystals under their experimental conditions. In the present work, the acetic acid is not directly reducing AuCl4- ions. A possible mechanism is CH3COOH (or CH3COO-) can scavenge radicals to form ‚CH2COOH (or ‚CH2COO-), which is a reducing radical. It is also possible that the -COOH groups complex the gold ions, and this can help their reduction. We suggested that acetic acid serves as a suitable reducing agent for the AuCl4- ions to generate gold nanocrystals under the irradiating of UV light. Parts c and d of Figure 1 show the UV-vis absorption spectra of photoreduced gold NPs when the concentration of collagen in the final mixed solution is 1 and 5 ng/µL, respectively. After 1 h of UV light irradiation, the colors for the products reveal obvious red and carmine as shown in the digital image in Figure 1. When the collagen concentration in the final mixed solution is 1 ng/µL, above 95% of the products are monodisperse gold

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Figure 2. FESEM images of the photoreduced gold nanocrystals. The SEM images are corresponding to those UV-vis spectra in Figure 1.

NPs, which can be revealed in the UV-vis spectrum (Figure 1c) and the SEM image (Figure 2c). But because of the selfassembly of collagen and its special character as a interaction template, the monodisperse gold NPs were adsorbed onto the collagen chains and formed gold NPs network. When the collagen concentration in the mixed solution was increased to 5 ng/µL, the intensity of the characteristic peak of gold NPs decreased and a new broad peak at about 800 nm appeared (see Figure 1d). Figure 2d gives the SEM image of this product. In this image, gold NPs showed larger size than that in Figure 2c, and they were aggregated to form accumulated gold NPs film on the ITO glass surface. When more collagen was added (12.5 ng/µL), the color of the product turned into blue-green and some precipitates were generated in the solution. A SEM image reveals the deep aggregation of gold NPs caused by the superfluous collagen molecules (data not shown). 3.2. The Influence of Different Irradiation Time to the Formation of Gold NPs. In our experimental conditions, we think the photoreduction action rate for the formation of gold NPs is very fast. In order to reveal it, the photochemical formation of gold NPs was monitored by UV-vis absorption spectra. Figure 3a gives the UV-vis spectrum of the solution before irradiation, and there is only a high-intensity characteristic peak for the AuCl4- at 301 nm. When the irradiation time is 10 min, there are two obvious changes for the UV-vis absorption spectrum (Figure 3b). First, there is a red shift for the peak of AuCl4- from 301 to 308 nm, and the intensity of this peak decreases about 0.5-fold compared with the peak in Fiugre 3a. Second there is a new peak appears at 543 nm, which indicates the generation of gold NPs in the solution. By increasing the irradiation time, the absorption peak assigned for the NPs shifts toward short wavelengths and its intensity is steeply enhanced, as revealed in parts c-e of Figure 3. When the irradiation time reached 1 h, the peak at 301 nm disappears totally, which indicates the AuCl4- in the solution was used out. On the other hand, there are two new peaks at about 267 and 400 nm in parts d and e of Figure 3. We suggest that it could be due to the oxidation products of CH3COOH (or ‚CH2COOH) caused by the UV radiation.

Figure 3. UV-vis spectra of gold NPs synthesized by photoreducing mixed solution of 2 mL of 1 mM HAuCl4 with 2 ng/µL collagen for (a) 0 min, (b) 10 min, (c) 30 min, (d) 1 h, and (e) 1.5 h. The final volume is 4 mL by adding 0.2 M acetic acid. The spectra exhibit an important diffusion due to the presence of large particles. The inset is a plot of the spectrum intensity (at about 540 nm) of gold NPs vs the irradiation time.

The UV-vis absorption intensity at about 540 nm for the gold NPs vs irradiation time is plotted in Figure 3 (see inset). After a certain irradiation time, such as 90 min in this experiment, the UV absorption spectrum reaches a state of stagnation, and there will no further changes either in the maximum absorption wavelength or the intensity. 3.3. The Effect of Different Collagen Concentrations to the Size and Shape of Gold NPs and the Formation of Gold NPs Networks. As revealed in the above control experiments, we can see some factors, such as the introduction of acetic acid and collagen, and the irradiation time will affect the formation of gold NPs. In this experiment, in order to investigate the effect of different collagen concentrations to the formation of gold NPs networks, the final volume of mixed solution was controlled at 4 mL, and the concentration of collagen in the mixed solution was adjusted by adding different amount of collagen. Figure

UV-Photoreduced Gold Nanoparticles

Figure 4. UV-vis spectra of gold NPs synthesized by photoreducing mixed solution of 2 mL of 1 mM HAuCl4 with (a) 0.1 mL of 2 ng/µL, (b) 0.5 mL of 2 ng/µL, (c) 2 mL of 2 ng/µL, and (d) 0.5 mL of 10 ng/µL collagen solution. The final volume is 4 mL by adding 0.2 M acetic acid and keeping the final concentration of collagen 0.05, 0.25, 1, and 1.25 ng/µL for a-d, respectively. The irradiation time is 1 h for all the samples. The digital image shows the change of color for the different gold NPs, respectively.

4a shows the UV-vis absorption spectrum of gold nanocrystals formed by photoreducing the mixed solution in which contains 0.5 mM HAuCl4 and 0.05 ng/µL collagen. An obvious character is that there is a broadband appears at 563 nm, and there is also a long tail extends to the long wavelength scale. It is suggested that large gold particles maybe exist in the product. The SEM image (Figure 5a) makes it clear that triangular gold nanoplates (the length for the side is about 500 nm) coexist with the gold NPs in the final product. Figure 6a shows the histogram of the gold NPs in Figure 5a, and a statistical analysis shows the size is about 101.1 ( 24.6 nm. When the collagen concentration in the final mixed solution was increased, the trends revealed in the UV-vis spectra lie in

J. Phys. Chem. C, Vol. 111, No. 5, 2007 1979 two aspects. One aspect is the intensity of the characteristic peak for the gold NPs increased steeply; another aspect is this peak has obvious blue shift from 563 to 557, 539, and 533 nm for parts b-d of Figure 4, respectively. The change of color for the gold NPs caused by introducing collagen solution with different concentrations can be seen in the inset digital image in Figure 4. Figure 5b reveals the SEM image of photoreduced gold NPs when the collagen concentration in the final mixed solution is 0.25 ng/µL. Compared with the NPs in Figure 5a, the size of the gold NPs in this image decreased, and the mean diameter and standard deviation of the gold NPs were estimated to be 87.0 and 24.8 nm, respectively. In this image, triangular gold nanoplates still can be found, but their size is smaller than that in Figure 5a. When the collagen concentration in the mixed solution was increased to 1 ng/µL, gold NPs with much smaller size, about 26.4 ( 10.1 nm were generated, moreover, there were no triangular gold nanoplates formed, as indicated in Figure 5c. When the collagen concentration in the mixed solution was increased to 1.25 ng/µL, the diameter of the synthesized gold NPs increased but not decreased, the average size for the gold NPs is about 37.2 ( 6.9 nm. Referenced as the images in Figure 2, it can be concluded that the diameter of the photosynthezied gold NPs reveals a decrease when the collagen concentration increased from 0.05 to 1 ng/µL, and it also shows a increase when the collagen concentration increased from 1 to 5 ng/µL. We suggest that collagen played an important role in the formation of gold NPs network. Coffer et al.49 and Berti et al.35 also reported the same phenomena when they deposited semiconducting or metal NPs on the DNA templates by chemical reduction. They thought the DNA base sequence, and more specifically the content of the base adenine, had a significant effect on the size of the NPs formed. Under the irradiation of high-intensity UV light, the gold salt was reduced to gold atoms. In the absence of collagen, part of the gold atoms serve as nucleation sites for generation of NPs with isotropic growth, and part of the gold atoms serve as the nucleation sites for formation of nanoplates with anisotropic growth. Kim et al. have reported the successful preparation of gold platonic nanocrystals (cube, tetrahedron, and icosahedron), and proved

Figure 5. FESEM images of gold NPs. The SEM images are corresponding to those UV-vis spectra in Figure 4.

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Figure 6. Size histograms of the gold NPs synthesized in parts a-d of Figure 5.

Figure 7. Typical FT SERS spectra of 4-ATP (1 × 10-5 M) on different substrates: (a) NPs network in Figure 2c, (b) film in Figure 2d, and (c) 35-nm gold NP film.

Figure 8. Typical FT SERS spectra of 4-ATP (1 × 10-5 M) on gold NP networks in parts a-d of Figure 5, respectively.

that the exquisite shape control can be achieved through careful growth rate regulation along different crystallographic directions.47 Mirkin et al. also reported the photoinduced synthesis of silver nanoprisms by controlling anisotropic NPs growth trough plasmon excitation.24 We believe that the introduction of collagen in the current process can significantly inhibit this kind of growth rate regulation. When collagen was introduced into the system, the positively charged collagen chains show strong electrostatic interaction with negatively charged AuCl4ions. Gold atoms were generated along the collagen templates when the collagen-AuCl4- complexes were irradiated with UV light, the anisotropic NPs growth was inhibited and all these gold atoms serve as nucleation sites for further isotropic growth of NPs on the collagen templates. On the other hand, collagen

molecules reveal exciting characters in that they can form selfassembly structures, such as fibers, networks, and films on solid substrates by adjusting the pH and temperature of the systems and the concentration of collagen solution.39-41 All these factors provide the possibility for the formation of gold NPs networks and films by simply irradiating the collagen-HAuCl4 complexes and depositing the products on the ITO substrate. 3.4. SERS Measurements of 4-ATP Molecules on the Collagen-Templated Gold NPs Film and Networks. Similar to the silver nanostructures, gold nanostructures are also important for their applications in SERS. In the recent years, some gold nanostructures, such as single gold NPs,50 hollow NPs,51 NPs aggregates,52 NPs film,53 shells,54 nanorods/nanowires films,55 and other new gold nanostructures,56 have been used

UV-Photoreduced Gold Nanoparticles as SERS substrates and applied in the detection of chemical and biological molecules. In our previous studies, we investigated the self-assembly of positively charged silver NPs on the artifical λ-DNA networks, and explored the applications of these nanoporous silver NPs networks in the SERS-active substrate primarily.57 We also found that these substrates showed great Raman enhancement ability for some probe molecules and proved that it is a feasible and simple strategy to prepare NP films for the application in SERS by assembling the gold and silver NPs on the biomacromolecules templates, such as DNA, proteins, and polymers. For one basic application, we also hope these collagen-templated photoreduced gold NPs film and networks formed in this work can be used as SERS-active substrates. In order to detect the possibility of these photoreduced gold NPs films and networks for the applications in SERS-active substrates, 20 µL 1 × 10-5 M 4-ATP (dissolved by ethanol) was dropped onto these substrates and detected with a Fourier transform (FT) Raman spectrometer. Parts a and b of Figure 7give typical comparative FT SERS spectra of 1 × 10-5 M 4-ATP on the gold NPs network and film showed in parts c and d of Figure 2, respectively. It is obvious that the gold NPs film in Figure 2d shows greater enhancement ability than the gold NPs network in Figure 2c for 4-ATP probe molecules. As these two SERS spectra, four important bands, υ(CC) for 1584 cm-1, υ(CC) for 1077 cm-1, and δ (CH) for 1178 and 392 cm-1, which is assigned to one of the vibrational modes of C-S bond, are all dominated with the in-plane (a1) vibrational mode.58 Wang et al. also reported this phenomenon, and they thought the predominance of the a1 mode in the FT SERS spectra may imply the enhancement via the electromagnetic mechanism is significant.56 In order to identify the Raman enhancement ability of collagen-medicated gold NPs networks and films, we prepared a gold NPs film formed by 35-nm gold colloids. Figure 7c shows the typical FT SERS spectrum of 1 × 10-5 M 4-ATP on the 35-nm gold NPs film. It is obvious that the substrates shown in the present work reveal a 2-4-fold increase compared with the 35-nm gold NPs film at the band of 1077 cm-1. Parts a-d of Figure 8 show the typical FT SERS spectra of 1 × 10-5 M 4-ATP on gold NP networks prepared by adjusting the final collagen concentration is 0.05, 0.25, 1, and 1.25 ng/ µL, respectively. In this figure, an obvious trend in the enhancement ability increases with the increasing of the collagen concentration, the Raman intensity of three characteristic bands at 1077, 1178, and 1581 cm-1 have the same increase trend, and the intensity at 1077 cm-1 in parts b-d of Figure 8 is 1.3-, 2.0-, and 4.6-fold the intensity shown in Figure 8a. It should be noted that, in the two parallel experiments, the Raman spectra collected from the three random points reveal similar characters and intensity. At the same time, these substrates showed the same SERS spectra even if they were placed on the air for two weeks. We suggested that our collagen-mediated method provided in this work may be a feasible strategy for the preparation of SERS-active substrates for the practical applications in chemical and biological analysis. First, the good signal-to-noise for the SERS spectra of 4-ATP on the gold NPs networks and film indicates that these substrates are very suitable as SERS substrates. In this work, we thought the Raman enhancement mainly comes from three factors: the rough surface on nanometer scale, the aggregation of gold NPs caused by collagen molecules, and the relatively large surface area provided by the network structures.57 Second, the collagen template has the advantage over other biological molecules in that it presents

J. Phys. Chem. C, Vol. 111, No. 5, 2007 1981 self-assembly properties, which can provide various templates, such as specific collagen aggregates, fibers, networks, and films, to prepare some inorganic nanostructures. Moreover, the collagen templates can be easily removed by calcinations or other milder methods.42 Third, these SERS substrates (gold NP networks and film) prepared by our method have good stability and reproducibility. Finally, this one-step photoreduction method to prepare gold NP networks and film for SERS applications is very simple and quick compared to some previous methods.53,59 4. Conclusions In summary, a simple method has been used to synthesize gold nanocrystals in the presence of acetic acid and collagen. Under the irradiation of 253.7-nm UV light, acetic acid serves as an appropriate reducing agent for the formation of gold nanoplates, and the introduction of collagen greatly accelerates the generation of gold NPs. Collagen plays an important role not only for controlling the shape and size of formed gold NPs but also for controlling the assembly of NPs as an effective template. By dropping the gold NPs onto the surface of solid matrixes directly, gold NP film and networks can be prepared easily, which all exhibit fine SERS ability by testing with Raman probe molecules. Acknowledgment. This work was supported by the National Natural Science Foundation of China. The authors thank the reviewers for their valuable suggestions and comments. References and Notes (1) Crooks, R. M.; Chechik, V. J. Am. Chem. Soc. 2000, 122, 1243. (2) Mu, Y.; Liang, H.; Hu, J.; Jiang, L.; Wan, L. J. Phys. Chem. B 2005, 109, 22212. (3) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Zaikovski, V.; Klabunde, K. J. J. Am. Chem. Soc. 2003, 125, 10488. (4) Dirix, Y.; Bastiaansen, C.; Caseri, W.; Smith, P. AdV. Mater. 1999, 11, 223. (5) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. ReV. 1999, 99, 2957. (6) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18. (7) Kho, K. W.; Shen, Z. X.; Zeng, H. C.; Soo, K. C.; Olivo, M. Anal. Chem. 2005, 77, 7462. (8) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424, 824. (9) Lal, S.; Taylor, R. N.; Jackson, J. B.; Westcott, S. L.; Nordlander, P.; Halas, N. J. J. Phys. Chem. B 2002, 106, 5609. (10) Maier, S. A.; Kik, P. G.; Atwater, H. A.; Meltzer, S.; Harel, E.; Koel, B. E.; Requicha, A. A. G. Nat. Mater. 2003, 2, 229. (11) Charlton, C.; Katzir, A.; Mizaikoff, B. Anal. Chem. 2005, 77, 4398. (12) Haes, A. J.; Van, Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596. (13) Sonnichsen, C.; Alivisatos, A. P. Nano Lett. 2005, 5, 301. (14) McFarland, A. D.; Van, Duyne, R. P. Nano Lett. 2003, 3, 1057. (15) Cao. Y. W. C.; Jin, R. C.; Mirkin, C. A. Science 2002, 297, 1536. (16) Hada, H.; Yonezawa, Y.; Yoshida, A.; Kurakake, A. J. Phys. Chem. 1976, 80, 2728. (17) Huang, H. H.; Ni, X. P.; Loy, G. L.; Chew, C. H.; Tan, K. L.; Lob, F. C.; Deng, J. F.; Xu, G. Q. Langmuir 1996, 12, 909. (18) Maillard, M.; Huang, P.; Brus, L. Nano Lett. 2003, 3, 1611. (19) Gaddy, G. A.; Korchev, A. S.; McLain, J. L.; Slaten, B. L.; Steigerwalt, E. S.; Mills, G. J. Phys. Chem. B 2004, 108, 14850. (20) Jia, H.; Zeng, J.; Song, W.; An, J.; Zhao, B. Thin Solid Films 2006, 496, 281. (21) Jia, H.; Xu, W.; An, J.; Li, D.; Zhao, B. Spectrochim. Acta, A 2006, 64, 956. (22) Sun, Y.; Xia, Y. AdV. Mater. 2003, 15, 695. (23) Germain, V.; Li, J.; Ingert, D.; Wang, Z. L.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 8717. (24) Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487. (25) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (26) Xue, C.; Li, Z.; Mirkin, C. A. Small 2005, 1, 513. (27) Torigoe, K.; Esumi, K. Langmuir 1992, 8, 59. (28) Eustis, S.; Hsu, H. Y.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 4811.

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