Facile Synthesis of High-Concentration, Stable Aqueous Dispersions

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Facile Synthesis of High-Concentration, Stable Aqueous Dispersions of Uniform Silver Nanoparticles Using Aniline as a Reductant Jiping Yang,*,† Huajie Yin,† Jingjing Jia,† and Yen Wei‡,§ †

Key Laboratory of Aerospace Materials and Performance, Ministry of Education, School of Materials Science and Engineering, Beihang University, Beijing 100191, PR China ‡ Department of Chemistry, Tsinghua University, Beijing 100084, PR China § Department of Chemistry, Drexel University, Philadelphia, Pennsylvania 19104, United States

bS Supporting Information ABSTRACT: A facile method was developed for preparing uniform silver nanoparticles with small particle sizes of less than 10 nm at high concentrations, in which aniline was used to reduce silver nitrate (AgNO3) to silver nanoparticles in the presence of dodecylbenzenesulfonic acid (DBSA) as a stabilizer. Upon the addition of excess NaOH to the DBSA anilineAgNO3 (DAA) system, the formation of silver nanoparticles was almost complete in just 2 min at 90 °C (in 94% yield). The average size of those resultant silver nanoparticles was 8.9 ( 1.1 nm, and the colloids were stable for more than 1 year at ambient temperature. A possible mechanism for the formation of silver nanoparticles was proposed to be related to two factors: one was the mesoscopic structures of the DAA system in which silver ions were restricted in the dispersed phases composed of DBSA and aniline; the other was Ag2O nanocrystallites generated in situ that could be readily reduced by aniline to small silver nanoparticles at high concentrations.

’ INTRODUCTION Metal nanoparticles have been intensvely investigated in recent years.16 Among them, metallic silver nanoparticles exhibit many unique optical, electronic, and chemical properties and have been exploited for applications in photonics,7 catalysis,8 sensing,911 antibacterial materials,1215 medicine,16 and so forth. Because these properties are very sensitive to the size, size distribution, and shape of silver nanoparticles,1721 it is crucial to prepare silver nanoparticles of controllable monodisperse sizes. Besides the physical separation methods, a rich variety of recipes are available for preparing silver nanoparticles as stable colloidal dispersions in water and organic solvents, including the polyol process,1723 seed-mediated synthesis,24,25 surfactantassistant reduction in polar or nonpolar solvents,26,27 and the microemulsion method.28 Currently, it is possible to produce silver nanoparticles with good control of their sizes and structures. However, there are very few methods capable of producing high-concentration aqueous dispersions of uniform silver nanoparticles with sizes below 10 nm. Because most water-based wet chemical methods are homogeneous nucleation processes, it is difficult to prepare silver nanoparticles at high concentration with monodisperse small sizes in one reaction system. It is indeed a “Catch-22” situation. To keep the size of a nanoparticle small, the initial concentration of silver salt in a reaction system must be low and the reducing agent should be strong enough, generally by using sodium borohydride (NaBH4) and hydrazine hydrate (N2H4 3 H2O) to effect a fast nucleation. Obviously, a low initial r 2011 American Chemical Society

concentration of silver salt may lead to a low content of silver nanoparticles in the colloidal product, which is undesirable for economic reasons. However, if the initial concentration is high, then silver particles may grow too large. Therefore, it remains a formidable challenge to find a simple way to generate monodisperse silver nanoparticles with small sizes of less than 10 nm at high concentration. In this article, we presented a facile method to achieve this goal. In this method, aniline, a common, low-cost compound, was used as a reductant and dodecylbenzenesulfonic acid (DBSA) served as a surfactant/stabilizer. The initial concentration of the silver precursor (i.e., silver nitrate, AgNO3) was 20 mmol/L, which was much higher than those around 102100 mmol/L as currently used.15,29,30 By adding NaOH to the reaction system at 90 °C, silver nanoparticles of 8.9 ( 1.1 nm diameter were generated in just 2 min in high yield. It was found that the mesoscopic structures, formed by DBSA, aniline, and AgNO3, and the in situ-generated Ag2O nanocrystallites might be key factors in the quick formation of silver nanoparticles with small uniform sizes. It is worth noting that, to the best of our knowledge, there is only one report in the literature using aniline as a reductant to prepare silver nanoplates in the presence of poly(vinyl pyrrolidone),31 which is different from our reaction system. The research by Tan et al. employed aniline only as a postsynthesis Received: January 3, 2011 Revised: March 12, 2011 Published: March 24, 2011 5047

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Figure 1. (A) UVvis spectra of the reaction system after reacting at 90 °C for different reaction times. (The samples were diluted 200-fold at room temperature for spectral measurements.) The inset is an amplification of the top of the UVvis spectra. (B) Relationship between the SPR intensity of silver nanoparticles and the reaction time.

stabilizer of silver nanoparticles.32 Therefore, the formation mechanism using aniline as a reductant to prepare silver particles has not been fully understood until now. Our results suggested that the pH value (e.g., addition of NaOH) in the reaction system had a great influence on the generation of silver nanoparticles.

’ EXPERIMENTAL SECTION Materials. Aniline (Shanghai Wulian Chemical Engineering Factory, China) was distilled under reduced pressure and stored at low temperature prior to use. Silver nitrate (AgNO3, purity g98%), dodecylbenzenesulfonic acid (DBSA, purity g98%), sodium hydroxide (Shanghai Chemical Reagent Company, China), and other chemicals were used as received without further purification. All aqueous solutions were prepared with distilled water. Preparation of Silver Nanoparticles. In a typical synthesis, DBSA (3.26 g, 10.0 mmol) was dissolved in 90 mL of distilled water, followed by the addition of aniline (0.184 g, 2.0 mmol) under vigorous stirring until a transparent solution formed. Next, a AgNO3 aqueous solution (10 mL, 0.20 mol/L) was added to the solution, and the resultant mixture was stirred for 15 min to form a DBSAanilineAgNO3 aqueous system (DAA system). For a room-temperature reaction, NaOH aqueous solution (4.5 mL, 3.0 mol/L) was added to the above DAA system rapidly at room temperature, and then this mixture was left to stand for a long time with occasional shaking. The completion of the reaction was monitored by the addition of a 1.0 mol/L NaCl solution to the above reaction system until a white AgCl precipitate was not observed, implying nearly 100% yield of silver from the AgNO3 precursor. For a 90 °C reaction, the above DAA mixture was heated to 90 °C, followed by the addition of NaOH aqueous solution (4.5 mL, 3.0 mol/L). The mixture was kept at 90 °C under stirring for 1 h until the white AgCl precipitate was not observed when a 1.0 mol/L NaCl solution was added to the above reaction system, which implied a nearly 100% yield of silver from the AgNO3 precursor. Silver colloids with varying particle morphologies and sizes could be prepared by changing the contents of the NaOH and AgNO3 solutions in the reaction system. Characterization. UVvis adsorption spectra were measured at room temperature on a TU1901 UVvis spectrometer from 250 to 900 nm using 1 cm path length quartz cuvettes. UVvis samples were prepared by quickly removing a small amount of the reaction mixture and immediately transferring it to room-temperature water for dilution. Morphology studies were carried out using an FE-SEM JSM 6700 scanning electron microscope (SEM) operating at 10 kV. The energydispersive X-ray spectrometer (EDS) measurements were carried out

using an Oxford Instruments Link ISIS coupled with a Hitachi S-530 SEM. Transmission electron microscope (TEM) images and selected-area electron diffraction (SAED) patterns were measured with a JEM-2100F electron microscope at a working voltage of 200 kV with a point resolution of 0.23 nm and a lattice resolution of 0.10 nm. Samples for TEM analysis were prepared by dripping particle colloids onto the carbon-coated copper grids and drying in air at room temperature. Wide-angle X-ray diffraction (XRD) patterns of particles were recorded using a Rigaku DMAX2200 with Ni-filtered Cu KR radiation over a scanning range of 5 to 60° at an X-ray power of 40 kV and 40 mA.

’ RESULTS AND DISCUSSION Reaction at 90 °C. It was known that aniline, a weak reductant, did not easily reduce silver ions when the ions were isolated in acidic solution.33 When the DBSAanilineAgNO3 aqueous system (DAA system) was heated to 90 °C with stirring, no reaction occurred even after heating for dozens of hours. However, the appearance of silver nanoparticles could be triggered by the addition of NaOH aqueous solution. When 4.5 mL of a 3.0 mol/L NaOH aqueous solution was added in one portion to 100 mL of the above DDA system at 90 °C, the system immediately became dark red. The reaction system variation was monitored using UVvis spectra and is shown in Figure 1A, in which the samples for UVvis spectra were diluted 200-fold at room temperature before spectral measurements were made. The absorbance of the surface plasmon resonance (SPR) of silver nanoparticles at around 410 nm became very strong after just 2 min, suggesting that most silver nanoparticles had already been formed in a very short period of time (in 94% yield). During the next hour of reaction, the growth of the SPR absorbance reached a plateau as shown in Figure 1B. It is noteworthy that the commonly observed induction period for forming silver nuclei34,35 is absent in our DAA reaction system. Figure 2A,B shows representative TEM images of the resultant silver nanoparticles in the reaction system after adding NaOH at 90 °C for 1 h. Apparently, the formed nanoparticles had selfassembled into 2D arrays on the TEM grid, and nearly monodisperse, spherical silver nanoparticles were observed. These silver nanoparticles had an average size of 8.9 ( 1.1 nm with a narrow distribution (a total 600 particles were counted) as shown in Figure 2C. Their XRD pattern (Figure 2D) showed characteristic diffraction peaks for metallic silver [111], [200], [220], and [311] facets.24,25,36 5048

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Figure 2. (A, B) TEM images at two magnifications of silver nanoparticles collected from the reaction system after adding NaOH at 90 °C for 1 h. (C) Corresponding histogram of the silver nanoparticle size distribution. (D) XRD pattern of silver nanoparticles.

Figure 3. (A) Photographs of as-synthesized silver nanoparticle colloids without dilution and after dilution by 50-, 100-, and 200-fold from left to right. (B) UVvis spectra of the silver nanoparticle colloids with dilution ratios from 100- to 500-fold with an interval of 50-fold. The inset is the plot of SPR absorbance against the concentrations of silver in colloids.

Figure 3A shows that the resultant silver colloid without dilution was opaque because of a high concentration of silver nanoparticles. The dark-red appearance is different from the grayish or yellow color of larger silver nanoparticles at high concentrations.37 With an increase in the dilution ratio, the formed colloid became transparent and appeared bright yellow in color when it was diluted more than 100-fold. Figure 3B shows the UVvis spectra of the silver nanoparticle colloids with dilution ratios of 100- to 500-fold at an interval of 50-fold. According to the dilution ratios, the concentrations of silver in the colloids could be calculated. The inset of Figure 3B plots the SPR absorbance against the concentrations of silver in colloids, which exhibits a small standard deviation (SD) and a high linear correlation coefficient. Such a good linear relationship suggests a potential application of this preparation method in sensing/detecting trace silver ions on the order of micrograms in solution. Furthermore, the colloids were very stable and could be stored at room temperature for 1 year without any precipitation. The color and UVvis spectra did not change during storage,

indicating the good protecting effect of DBSA such that the aggregation between neighboring nanoparticles or Ostwald ripening might have been prevented by the monolayer surfactant coatings on the nanoparticle surfaces.38 It should be emphasized again that there were two main distinctive features in this system that were different from those in most existing systems with respect to the preparation of silver nanoparticles.2932,3441 First, though a weak reductant was used in this system, the formation of silver nanoparticles was very fast without an induction period. The SPR ratio of colloids after reaction for 2 min to that for 1 h reached 0.944, indicating a very high formation rate. In contrast, when some weak reductants such as saccharides,15 amino acids,39 aliphatic amines,40 and citrate41 were used, the reaction often took several hours to achieve a reasonable yield of silver nanoparticles. Second, a high silver precursor concentration of 20 mM was used, but a stable colloid of monodisperse silver nanoparticles with a small size of 8.9 ( 1.1 nm was generated. 5049

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Langmuir Reaction at Room Temperature. To understand the abovedescribed features further, the same reaction was run at room temperature but was relatively slow, taking several days to complete the production of silver nanoparticles instead of minutes for the reaction at 90 °C. Such a slow process made it easier to investigate the initial and intermediate states of the reaction. The color change of the reaction system at different stages is shown in Figure 4. DBSA (0.10 mol/L in water) was a transparent, homogeneous, pale-yellow solution (Figure 4A). After aniline was added, the solution was still transparent and homogeneous with a slightly deeper color (Figure 4B). It was believed that aniline molecules were located in the micellewater interface of DBSA micelles in the form of anilinium ions, as reported by Kim et al.42 Adding AgNO3 solution to the above-mentioned DBSAaniline aqueous solution followed by stirring for 15 min resulted in a translucent, milky-white DAA system (Figure 4C). The appearance change from transparence to translucence indicated that the

Figure 4. Photographs of a reaction solution system at different stages taken at room temperature.

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volume of the dispersed phases had grown large enough to scatter visible light, and the diameters were estimated to vary from 50 to 200 nm.43 This suggested that after Agþ was added to the DBSAaniline aqueous system the original micellar structures would disrupt and agglomerate to a microemulsion system, which was consistent with previous observations in the literature.44 Neighboring micelles in the DBSAaniline aqueous system would connect to one another because of the interaction between Agþ and SO3 ions, and the silver ions were restricted in the resultant enlarged dispersed phases. A schematic illustration of the mesoscopic structure changes of the thus-prepared DAA system is given in the Supporting Information (Figure S1). The silver particle generation at room temperature was also initiated by adding a NaOH aqueous solution in one portion to the DAA system, and the translucent system turned dark yellow immediately as shown in Figure 4D. (The diluted colloid was transparent yellow as shown in Figure S2.) In contrast to the reaction at 90 °C, the color changed to dark red very slowly during the reaction at room temperature (Figure 4E). Typical UVvis spectra during the reaction at room temperature are shown in Figure 5A, in which the samples were diluted 100-fold at room temperature before spectral measurements were made. No SPR absorbance appeared at the beginning after adding NaOH (time = 0 h) though the appearance was colloidlike. Then the absorbance of silver nanoparticles at around 420 nm increased gradually, and the final SPR bandwidth was much broader than that obtained at 90 °C. Figure 5B plots the SPR absorbance against reaction time, which is similar to the kinetic curve obtained at 90 °C but the reaction rate was much slower. The process of silver nanoparticle formation was also monitored by means of TEM as shown in Figures 68. In the

Figure 5. (A) UVvis spectra of a reaction system prepared at room temperature and different reaction times. (The samples were diluted 100-fold at room temperature before spectral measurements were made.) (B) Relationship between the SPR absorbance of silver nanoparticles and reaction time.

Figure 6. (A) TEM image of dispersed phases of the DAA system after the addition of NaOH at room temperature and time = 0 h. (B) The middle of Figure 6A at a higher magnification. (C) SAED pattern of the dispersed phases (Ag2O). 5050

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Langmuir beginning after the addition of NaOH (time = 0 h), many spherelike aggregates with different sizes appeared (Figure 6A). The distribution was quite broad, ranging from 40 to 220 nm with an average size of 123.2 ( 33.8 nm as given in Figure S3. More details of the aggregates are shown in Figure 6B. It was shown that the aggregates were composed of small particles and that most of these small particles were irregular spheres under 10 nm. The absence of SPR absorbance at 420 nm (Figure 5A) demonstrated that these initially emerging small particles were different from the final silver nanoparticles. The addition of NaOH to the reaction system should first result in the formation of Ag2O. In the 100 mL DAA system, the total molar mass of DBSA and AgNO3 was 1.2 mmol, which was slightly less than the 1.35 mmol of NaOH added, so the pH of the reaction system increased to around 10. In such an alkaline environment, Ag2O should be formed,4547 as evidenced by the selected area electron diffraction (SAED) pattern (Figure 6C) and the energy-dispersive X-ray (EDX) profile (Figure S4). The corresponding d value of each diffraction ring was calculated using the equation d = lλ/r, where l was the distance over which the diffraction pattern was directed (l = 80 cm in this pattern), λ was the electron wavelength (λ = 0.0025 nm at 200 kV), and r was the radius of the diffraction ring. The calculated d values were 0.2740, 0.2381, 0.1667, and 0.1493 nm, respectively, which corresponded well to the (111), (200), (220), and (311) diffraction facets of Ag2O nanocrystallites, respectively.46 In a word, the SAED pattern indicated that Ag2O nanocrystallites with an fcc structure were formed.46,47 The EDX profile (Figure S4) showed the existence of O, S, and Na elements besides the silver element, where the oxygen element might come from Ag2O compared

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with the EDX profile of the final silver nanoparticles without the O element (Figure S5). Figure 7 depicts TEM images of dispersed phases after reaction for 8 h in which the aggregate size distribution was quite broad from 10 to 400 nm. Most importantly, large Ag2O aggregates appeared to disaggregate (as shown by the arrows in Figure 7A). Several phenomena, such as collision and amalgamation, also took place among these Ag2O aggregates, where the boundary became blurry in the boxes in Figure 7B. These changes agreed with the emergence and increased SPR absorbance of silver nanoparticles at 420 nm in Figure 5A. Because Ag2O was very unstable, which could be reduced by some weak reductants to form silver nanoparticles,45 Ag2O nanocrystallites were reduced to metallic silver gradually by aniline. The change from Ag2O to metallic silver might alter the inner “environment” of the original Ag2O aggregates, and more DBSA molecules tend to coat newly formed silver nanoparticles because of bonding interactions between DBSA molecules and the nanoparticle surface.38 This transformation might be one of the reasons for the disaggregation of Ag2O aggregates. When more and more Ag2O was reduced to silver, those Ag2O aggregates finally no longer existed in the system. Alternatively, individual silver nanoparticles formed and dispersed in the reaction system as shown in Figure 8A. The corresponding size distribution (Figure 8B) shows the polydispersity of the silver nanoparticles with an average diameter of 11.0 ( 3.7 nm, which agreed with its expanded SPR band. The inset of Figure 8A was the SAED pattern of silver nanoparticles. The calculated d values were 0.2451, 0.2083, 0.1497, and 0.1282 nm, respectively, which could be assigned to the (111), (200), (220), and (311) facets of metallic silver, respectively, and reflected the fcc structure of metallic silver in the polycrystalline state.48 The high-resolution TEM image (Figure S6) confirmed the polycrystalline structure of the resultant silver nanoparticles. However, the oxidation product of aniline under alkaline conditions was irregularly substituted soluble oligoanilines with Scheme 1. Proposed Reaction Equations in the Preparation of Silver Nanoparticles in the DAA System

Figure 7. (A, B) TEM images of the dispersed phases after reaction for 8 h at room temperature.

Figure 8. (A) TEM image of the silver nanoparticles after reaction at room temperature for 122 h and the inset showing the corresponding SAED pattern. (B) Corresponding histogram of the silver nanoparticle size distribution. 5051

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Scheme 2. Proposed Formation Mechanism of Silver Nanoparticles as Prepared from the DAA System upon the Addition of NaOH

Figure 9. (A, B) TEM images of the resultant silver nanoparticle prepared at pH 7 and 90 °C for 5 h at AgNO3 concentrations of 20 and 10 mmol/L, respectively. (C) Corresponding histograms of resultant silver nanoparticle size distributions prepared at pH 7 and 90 °C for 5 h with different AgNO3 concentrations.

possible phenazine cyclic structure.49 Thus, the reactions related to the formation of silver nanoparticles in the DAA system could be depicted in Scheme 1. Proposed Formation Mechanism of Silver Nanoparticles. On the basis of the above results, we postulated the formation mechanism of silver nanoparticles in a three-stage model as shown in Scheme 2. First, when excess NaOH was added to the DAA system, OH ions would neutralize DBSA and react with Agþ to generate Ag2O aggregates in situ. Second, because of its high reactivity, Ag2O was reduced to silver nanoparticles by aniline combining with the amalgamation/disaggregation of Ag2O aggregates. Finally, all Ag2O aggregates were disaggregated and reduced to silver nanoparticles, which were dispersed individually and stably in the system at a high concentration of 2.0 mg/mL for more than 1 year. According to this mechanism, the polydispersity of the silver nanoparticles prepared at room temperature should be attributed to the presence of inhomogeneous Ag2O aggregates initially generated in the system (Figure 6A,B). However, at 90 °C, the initially generated Ag2O nanocrystallites would adopt highly dispersed structures because of the fast reaction rate. As a result, as-synthesized silver nanoparticles presented a uniform morphology with a narrow size distribution when prepared at 90 °C. Furthermore, this mechanism suggested that the formation of silver nanoparticles from the DAA system might take a heterogeneous nucleation pathway because of the in situ formation of Ag2O nanocrystallites. Compared to homogeneous nucleation, the barrier to form initial silver nuclei could be easily overcome in a heterogeneous process.50 Therefore, the “induction period” for forming silver nuclei was absent in the DAA reaction system. In addition, the heterogeneous nucleation of this DAA system

was different from a seed-mediated process or silver halide as the nucleation centers. In this DAA system, Ag2O nanocrystallites were generated in situ to serve as nucleation centers, resulting in very small sizes of the prepared silver nanoparticles owing to a large number of nucleation centers. In the meantime, Ag2O would gradually disappear and be reduced to silver by aniline. Thus, using the in situ-generated Ag2O nanocrystallites as heterogeneous nucleation centers could represent a versatile strategy for preparing small silver nanoparticles in high concentrations. Moreover, the nucleation pattern of silver nanoparticles prepared from the DAA system could be easily tuned by the molar mass of NaOH. If the molar mass of added NaOH was just equal to that of DBSA, then the pH value of the reaction system was about 7. Under this neutral condition, Ag2O did not emerge in the reaction system and silver nanoparticles formed through a homogeneous nucleation or self-nucleation process. Figure 9A shows the resultant silver nanoparticles prepared from the DAA system with less NaOH (pH of about 7 in the reaction system) after reacting for 5 h at 90 °C. Most silver particles were quasispheres with an average size of 84.6 ( 14.0 nm (Figure 9C), which was much greater than those prepared from a heterogeneous nucleation process. Furthermore, if the concentration of AgNO3 was reduced from 20 to 10 mmol/L, then the size of the resultant silver nanoparticles decreased to 36.3 ( 8.2 nm as shown in Figure 9B. These results were consistent with the homogeneous nucleation mechanism.3436 These approaches demonstrated the difference between the homogeneous and heterogeneous nucleation processes and reflected the potential ability of using the DAA system to synthesize silver nanoparticles 5052

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Langmuir in high concentrations over wide ranges of controllable size and shape.

’ CONCLUSIONS We presented a facile and efficient method of preparing monodisperse silver nanoparticles in small sizes from a DBSA anilineAgNO3 (DAA) aqueous system with the addition of NaOH. After adding excess NaOH to the DAA system, silver nanoparticles in about 94% yield formed in 2 min at 90 °C. The average size of the resultant silver nanoparticles was 8.9 ( 1.1 nm, and their colloids at a high concentration of more than 2 mg/mL were stable for more than 1 year under ambient conditions. A possible formation mechanism of silver nanoparticles was proposed. The DAA system formed mesoscopic structures in which silver ions were restricted in the dispersed phases composed of DBSA and aniline. After excess NaOH was added to the DAA system, Ag2O nanocrystallites were generated in situ, which might be responsible for the formation of small silver nanoparticles in high concentrations via a heterogeneous nucleation pathway. ’ ASSOCIATED CONTENT

bS

Supporting Information. Characterization of silver nanoparticles and the DAA system. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þþ86 10 82338475. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (NSFC grant no. 50573004 to J.Y.). Y.W. is grateful to the Ministry of Science and Technology of China for funding through the 973 Project Program (grant no. 2011CB935700). ’ REFERENCES (1) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (2) Eustis, S.; El-Sayed, M. A. Chem. Soc. Rev. 2006, 35, 209. (3) Willets, K. A.; Van Duyne, R. P. Annu. Rev. Phys. Chem. 2007, 58, 267. (4) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. Acc. Chem. Res. 2008, 41, 1578. (5) Biesso, A.; Qian, W.; Huang, X.; El-Sayed, M. J. Am. Chem. Soc. 2009, 131, 2442. (6) Lu, X. F.; Chen, J. Y.; Chao, D. M.; Zhang, W. J. Solid State Phenom. 2007, 121123, 183. (7) Liu, B.; Zhao, X.; Zhu, W.; Luo, W.; Cheng, X. Adv. Funct. Mater. 2008, 18, 3523. (8) Rashid, Md. H.; Mandal, T. K. J. Phys. Chem. C 2007, 111, 16750. (9) Murray, B. J.; Walter, E. C.; Penner, R. M. Nano Lett. 2004, 4, 665. (10) Jiang, X. C.; Yu, A. B. Langmuir 2008, 24, 4300. (11) Dubas, S. T.; Pimpan, V. Talanta 2008, 76, 29. (12) Ruparelia, J. P.; Chatterjee, A. K.; Duttagupta, S. P.; Mukherji, S. Acta Biomater. 2008, 4, 707. (13) Smetana, A. B.; Klabunde, K. J.; Marchin, G. R.; Sorensen, C. M. Langmuir 2008, 24, 7457.

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