Ag Composite ... - ACS Publications

11692

J. Phys. Chem. C 2007, 111, 11692-11698

Novel Method to Fabricate SiO2/Ag Composite Spheres and Their Catalytic, Surface-Enhanced Raman Scattering Properties Ziwei Deng, Min Chen, and Limin Wu* Department of Materials Science and the AdVanced Coatings Research Center of China Educational Ministry, AdVanced Materials Laboratory, Fudan UniVersity, Shanghai 200433, P. R. China ReceiVed: May 11, 2007; In Final Form: May 27, 2007

This paper presents a novel method for the fabrication of SiO2/Ag composite spheres with the aid of the reducing and stabilizing function of polyvinylpyrrolidone (PVP). In this approach, [Ag(NH3)2]+ ions were first adsorbed on the surfaces of silica spheres via electrostatic attraction between the silanol groups and ions; these [Ag(NH3)2]+ ions adsorbed on silica spheres were then reduced and protected by PVP to obtain SiO2/Ag composite spheres. Neither additional reducing agent nor core surface modification was needed; the particle size and the coverage degree of silver nanoparticles on the silica spheres could be easily tuned by altering the concentration of the precursor-[Ag(NH3)2]+ ions. UV-visible spectrometer analysis showed these composite spheres had very good catalytic property; Raman spectrometer measurement showed that these composite spheres exhibited excellent surface-enhanced Raman scattering (SERS) performance.

Introduction Recently, the core-shell composite spheres with a dielectric solid sphere (e.g., silica, polystyrene) covered by a metallic nanoshell have attracted increasing interest.1-9 Especially, more and more research studies have been focused on the core-shell composite spheres with Ag or Au nanoshells since they have great potential applications in catalysis, chemical and biological sensing, optoelectronics, photonic crystals, the plasmonics and surface-enhanced Raman scattering-based analytical devices, and so forth.10-14 Several processes have been explored to synthesize coreshell composite spheres with Ag nanoshell successfully,8,9,15-20 and two of them are particularly interesting due to the formation of a controllable Ag nanoshell. In the first process, namely, an electroless plating/seeding method, the compound or metal such as SnCl2,17 gold,8,9 or palladium21 is first used to activate the dielectric core surface, and then the Ag nanoshell is coated on the core surface. For example, Liz-Marzan et al.17 employed a SnCl2-mediated electroless plating method for deposition of silver nanoparticles on colloidal silica spheres. To obtain a denser coating on the surface, two-step processes had to be repeated, for example, the adsorption of Sn2+ ions occurred on the surfaces of the silica particles, and the Ag+ ions were reduced subsequently into metallic Ag nanoparticles, which attached on the silica surface. The second process, termed the layer-by-layer (LbL) self-assembly technique, has become a very attractive topic of investigation since the pioneering work done by Caruso.22 The basis of this process is the electrostatic association between alternately deposited, oppositely charged species. For example, Caruso et al.18 fabricated core-shell composite spheres with a dielectric solid sphere core covered with a continuous metallic Ag shell by the LbL self-assembly of octa(3-aminopropyl)silsesquioxane-stabilized Ag nanoparticles and PSS onto colloid spheres. However, it is quite difficult to form a complete Ag nanoshell with the first approach. In order to form a complete metal Ag * Corresponding author. E-mail: [email protected], [email protected].

nanoshell on the core surface, the subsequent seeding growth process is usually used, and the compound or metal such as SnCl2, gold, or palladium will exist in the final composite spheres as an impurity. Recently, Zhang et al.4 and Jiang et al.11 used metal silver nanoparticles tethered on the dielectric sphere as seed particles for the growth of a silver nanoshell overlayer, which avoided the effects caused by additional metal nanoparticles as seed particles. The second approach seems to be too time-consuming for practical application. Therefore, how to develop facile and feasible methods to prepare spheres coated with uniform and complete metallic Ag nanoshells still remains a great challenge to materials scientists. In this work, we present a novel method for the fabrication of SiO2/Ag composite spheres with the aid of polyvinylpyrrolidone (PVP). PVP was usually employed as a stabilizer for the preparation of composite spheres based on the work of our group23,24 and other researchers.4,25,26 Very recently, Xia et al. and Hoppe et al. successfully used PVP to reduce the aqueous solution of some metal salts (Au, Ag, Pd, and Pt) and prepared these noble metal nanoparticles or nanoplates, respectively.27-29 In this paper, we further combined the reducing function of PVP with its stabilizing function to prepare SiO2/Ag composite spheres successfully. First, [Ag(NH3)2]+ ions were adsorbed on the surfaces of silica spheres via electrostatic attraction between the negatively charged silanol groups and the positively charged ions, and then these [Ag(NH3)2]+ ions adsorbed on silica spheres were reduced and protected by PVP at the same time to obtain SiO2/Ag composite spheres. Neither additional reducing agent nor core surface modification was needed; the particle size and the coverage degree of silver nanoparticles on the silica spheres could be easily tuned by altering the concentration of the precursor-[Ag(NH3)2]+ ions. This method should be much simpler than the current typical fabrication processes for coreshell composite spheres with a Ag nanoshell, and to the best of our knowledge it has not yet been reported. TEM, SEM, XPS, XRD, and UV-vis spectroscopy were used to investigate the morphology, surface composition, crystallinity, and surface plasmon resonance absorbance of the composite spheres,

10.1021/jp073632h CCC: $37.00 © 2007 American Chemical Society Published on Web 07/13/2007

Fabrication of SiO2/Ag Composite Spheres

J. Phys. Chem. C, Vol. 111, No. 31, 2007 11693

Figure 2. TEM images of SiO2/Ag composite spheres prepared by PVP (a) in ethanol media, (b) in water media. PVP: 2.5 × 10-4 mol/ L; [Ag(NH3)2]+ ions: 2.4 × 10-1 mol/L. Figure 1. TEM image of silica spheres (480 nm).

respectively. UV-visible spectrometer and Raman spectrometer analyses showed that these nanocomposite spheres had very good catalytic and surface-enhanced Raman scattering (SERS) performances. Experimental Section Materials. Polyvinylpyrrolidone (PVP, MW ) 40 000) was purchased from Fluka and was used as received. Silver nitrate (AgNO3, g99.8%), tetraethoxysilane, ethanol, aqueous ammonia solution (28 wt %), potassium borohydride (KBH4, g95%), Rhodamine B (RB), Reactive Brilliant Red K-2BP (BR K-2BP), and 4-aminobenzenethiol (4-ABT, g98%) were used in the experiment. Ultrapure water (>17 MΩ cm-1) from a Milli-Q water system was used throughout the experiment. Synthesis of Silica Particles. Monodisperse silica particles were prepared using the Sto¨ber method from the sol-gel process of TEOS under base catalysis, centrifuged at 10 000 rpm for 15 min with a Beckman instrument (Allegra 64R centrifuge), as shown in Figure 1, and then redispersed in absolute ethanol at 2-3% silica concentration for subsequent uses.

Preparation of SiO2/Ag Composite Spheres. The typical strategy used to fabricate SiO2/Ag composite spheres is shown in Scheme 1. A 10 mL quantity of freshly prepared [Ag(NH3)2]+ ion solution was quickly added to 10 g of silica dispersion under magnetic stirring at room temperature. After the [Ag(NH3)2]+ ions were absorbed onto the surfaces of silica spheres for around 1 h via electrostatic attraction between [Ag(NH3)2]+ ions and the negatively charged Si-OH groups,23,24,30 the dispersion was added into 50 mL of ethanol containing PVP (5 × 10-4 mol/L) in a 250-mL three-neck flask equipped with a magnetic stirrer and stirred at 70 °C for around 7 h. The product was collected by centrifugation and then redispersed in ethanol for further examination. Catalytic Property of SiO2/Ag Composite Spheres. In the catalytic study, 10 mL of SiO2/Ag alcoholic solution was centrifuged and then redispersed in 500 mL of water for catalytic examination. After that, a given amount of the SiO2/Ag aqueous solution was mixed with 10 mL of 2 × 10-5 mol/L dye solution, and then injected rapidly with 1 mL of KBH4 solution under stirring. The color of the mixture vanished gradually, indicating the reduction of the dye; the catalytic performance of SiO2/Ag composite spheres was studied by monitoring the variation in

SCHEME 1: Schematic Diagram of SiO2/Ag Composite Spheres Prepared by Reduction and Stabilization of PVP

11694 J. Phys. Chem. C, Vol. 111, No. 31, 2007

Deng et al.

Figure 4. SEM images of SiO2/Ag composite spheres prepared with various concentrations of [Ag(NH3)2]+ ion: (a) 1.2 × 10-1 mol/L; (b) 1.8 × 10-1 mol/L; (c) 2.4 × 10-1 mol/L; (d) 5.9 × 10-1 mol/L.

Figure 3. TEM images of Ag nanoparticles supported on silica spheres (480 nm) prepared with various concentrations of [Ag(NH3)2]+ ion: (a, b) 5.9 × 10-2 mol/L; (c,d) 1.8 × 10-1 mol/L; (e,f) 2.9 × 10-1 mol/L; (g, h) 5.9 × 10-1 mol/L.

optical density at the wavelength of the absorbance maximum (λmax) of the dye with a UV-visible spectrometer. SERS Property of SiO2/Ag Composite Spheres. The SiO2/ Ag composite particles were washed with the acetone/water solution (v/v ) 3:1) and then redispersed in ethanol, and followed by drying the SiO2/Ag alcoholic solutions on silica wafers to form SiO2/Ag films as SERS-active substrates. Different concentrations of 4-ABT were dropped onto SiO2/ Ag substrates as target molecules and detected for the SERS property with a Raman spectrometer. Characterization. TEM ObserVation. A transmission electron microscope (TEM Hitachi H-600, Hitachi Corp.) was used to

observe the morphologies of the obtained silica colloid spheres and composite spheres. The dispersions were diluted with ethanol and ultrasonicated at 25 °C for 15 min and then dried onto carbon-coated copper grids before examination. SEM ObserVation. The morphologies of the obtained composite spheres were further characterized with a field-emission scanning electron microscope (SEM Philips XL30 apparatus) operated at an accelerating voltage of 20 kV. Analysis was performed on samples deposited onto silicon wafers under ambient conditions. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) measurement was carried out on a PHI-5000C ESCA system with Mg KR radiation (hν ) 1253.6 eV), the X-ray anode was run at 250 W, and the high voltage was kept at 14.0 kV with a detection angle at 54°. All the binding energies were calibrated by using the containment carbon (C 1s ) 284.6 eV). X-ray Diffraction. Crystal structure identification was carried out using a Rigaku D/max-rB X-ray diffractometer with Cu KR radiation (λ ) 1.54056 Å) at a scanning rate of 0.02 degrees per second in the 2θ ranging from 30° to 90°. The samples for XRD were supported on glass substrates. UV-Visible Spectrum. UV-visible absorption spectra were recorded using a UV-visible spectrophotometer (Hitachi UV3000, Japan). The samples were placed in a 1 cm × 1 cm × 3 cm quartz cuvette, and spectra were recorded at room temperature. SERS Mmeasurement. SERS spectra of 4-ABT adsorbed on SiO2/Ag substrates were obtained on a LabRam-1B Raman spectrometer (Dilor Instrument, France) equipped with 632.8 nm He-Ne laser. Approximately 1 mW of laser irradiation was used to excite the samples.The signal collection time was 10 s. Results and Discussion Reduction and Stabilization of PVP. Several methods have been reported to reduce silver salts to zero-valent silver, for example, strong or weak reducing agents (KBH4, hydrazine, sodium citrate, ascorbic acid), irradiation (γ-ray, ultraviolet, microwave, ultrasound),31-33 or reducing solvent media (dimethylformamide,34 alcohols, or polyols35,36). Recently, Xia et al. and other authors found that PVP had a reduction function for some metal salts (Au, Ag, Pd, and Pt) and prepared these



Figure 5. XPS scans of SiO2/Ag composite spheres. (a) Survey spectrum; (b) C 1s spectrum; (c) O 1s spectrum; (d) N 1s spectrum; (e) Ag 3d spectrum; (f) Si 2p spectrum.

noble metal nanoparticles or nanoplates successfully. Moreover, the reduction mechanisms have been elucidated well.27-29 Here, we used PVP as both reductant and stabilizer to prepare SiO2/ Ag composite spheres. Figure 2a displays the TEM image of SiO2/Ag composite spheres prepared by PVP in the ethanol media. Compared with the pure silica particles in Figure 1, relatively rougher surfaces of spheres were observed, indicating that Ag nanoparticles had deposited on silica spheres through PVP reduction of [Ag(NH3)2]+ ions. To demonstrate the reduction and stabilization of PVP in this work, two control experiments were carried out. One was performed with all the ingredients but PVP, and a lot of aggregations were observed since the negatively charged SiOH groups of silica particles were neutralized and not protected. Meanwhile, TEM illustrated that SiO2/Ag composite spheres did not form in the ethanol media without the aid of PVP, and the dispersion did not show a plasmon resonance peak at around 420 nm by UV-visible spectroscopy. Another control experiment was carried out to further confirm the reduction of PVP with all the ingredients but using water not ethanol as the media,

A TEM image illustrated that Ag nanoparticles could also form on the surfaces of silica spheres very well (see Figure 2b). These facts strongly indicated that PVP had a dual function as both reductant and stabilizer in this research. Effect of the Concentrations of [Ag(NH3)2]+ Ions. Figure 3 shows the TEM images of SiO2/Ag composite spheres prepared at different concentrations of [Ag(NH3)2]+ ions. When the concentration of [Ag(NH3)2]+ ions was low, for example, 5.9 × 10-2 mol/L, the Ag nanoparticles formed on the silica spheres with lower coverage (see Figure 3a,b). As the concentration of [Ag(NH3)2]+ ions increased, the Ag nanoparticles with higher coverage were coated on the surfaces of silica cores (see Figure 3c-f). When the concentration of [Ag(NH3)2]+ ion was increased to 5.9 × 10-1 mol/L, a much denser Ag nanoshell formed on the surfaces of the spheres (see Figure 3g,h). Similar results could be further observed using 280 nm silica particles as cores (see Supporting Information, Figure S1). This meant that the coverage degree of Ag nanoparticles on the silica spheres could be easily tuned by altering the concentration of the precursor-[Ag(NH3)2]+ ions.


Deng et al.

TABLE 1: Binding Energy Values of Pure PVP, Silica, and SiO2/Ag Composite Spheres binding energy (eV) samples

C 1s

O 1s

N 1s Ag 3d5/2 Ag 3d3/2 Si 2p3

PVP

284.6 (1) 530.7 399.6 285.8 (2) 287.8 (3)

silica SiO2/Ag composite spheres

532.0 284.6 (1) 530.7 399.6 285.7 (2) 531.7 287.8 (3) 532.0

367.8

373.8

103.2 103.2

Figure 4 further presents the SEM images of SiO2/Ag composite spheres prepared at different concentrations of [Ag(NH3)2]+ ions. Obviously, when the concentration of [Ag(NH3)2]+ ions was increased, the coverage degree of Ag nanoparticles on the silica spheres increased gradually. The SEM images with higher magnification were further observed and showed that the size of Ag nanoparticles increased with an increase in the concentration of [Ag(NH3)2]+ ions (see Supporting Information, Figure S2). XPS Measurements. Figure 5 shows the XPS scans of SiO2/ Ag composite spheres, and the quantitative results are summarized in Table 1. The occurrence of nitrogen (N 1s) and carbon (C 1s) signals further confirmed the presence of PVP on the surfaces of SiO2/ Ag composite spheres since PVP molecules have a strong interaction with silver nanoparticles and silica spheres.26,33,37 Comparing with the XPS spectra of pure PVP (see Supporting Information, Figure S3), no significant difference was observed in the peaks of C 1s and N 1s (Figure 5b,d). The XPS spectrum of O 1s was fitted by multiple Gaussians, as shown in Figure 5c; the peak of O 1s could be deconvoluted into three peaks with binding energies of 532.0, 531.7, and 530.7 eV. The O 1s peaks at 530.7 and 532.0 eV are similar to those of pure PVP and silica powders (see Supporting Information, Figure S4), respectively. The peak at 531.7 eV, higher binding energy compared with pure PVP, should be attributed to both the interaction between the oxygen in the carboxyl groups of PVP with Ag nanoparticles and the hydrogen bonding between PVP and SiO2.38 The XPS spectrum of Ag 3d of the spheres, as shown in Figure 5e, demonstrates two peaks at 367.8 and 373.8 eV with a spin-orbit separation of 6.0 eV, corresponding to the binding energies of Ag 3d5/2 and Ag 3d3/2, respectively, which is consistent with the reported results of PVP-capped silver nanowires.37,39 The peaks at Ag 3d5/2 and Ag 3d3/2 of the composite spheres were shifting to lower binding energy in comparison with pure Ag0 (368.2 eV for Ag 3d5/2, 374.2 eV for Ag 3d3/2), further confirming that there was an interaction between the carboxyl groups of PVP and Ag nanoparticles. X-ray Diffraction Patterns. The typical XRD pattern of the composite spheres, as illustrated in Figure 6, exhibited peaks at 2θ angles of 37.9°, 44.1°, 64.3°, 77.2°, and 81.4° corresponding to the reflections of (111), (200), (220), (311), and (222) crystalline planes of the fcc structure of Ag (JCPDS No.04-0783), indicating that the Ag nanoparticles with crystallinity could be obtained by PVP reduction of [Ag(NH3)2]+ ions. UV-Visible Spectroscopy. The UV-visible absorption spectra of SiO2/Ag composite spheres prepared at various concentrations of [Ag(NH3)2]+ ions are shown in Figure 7. All the samples were centrifuged and then redispersed in ethanol

Figure 6. XRD pattern of SiO2/Ag composite spheres prepared by stabilization and reduction of PVP.

Figure 7. UV-visible absorption spectra of bare silica colloids (a) and SiO2/Ag composite spheres prepared at various concentrations of [Ag(NH3)2]+ ions: (b) 5.9 × 10-2 mol/L; (c) 1.2 × 10-1 mol/L; (d) 1.8 × 10-1 mol/L; (e) 2.4 × 10-1 mol/L; (f) 2.9 × 10-1 mol/L; (g) 5.9 × 10-1 mol/L.

for the UV-visible scan measurement. The bare silica colloids did not show any UV-visible absorption (curve a), but the composite spheres displayed an obvious absorption peak at around 420 nm due to the Mie plasmon resonance excitation from the silver nanoparticles.40-42 As the concentration of [Ag(NH3)2]+ ions increased, the plasmon resonance peak was becoming red-shifted gradually, and the peak was becoming broader due to much larger silver nanoparticles and higher coverage forming on silica spheres as indicated in Figure 4 and Supporting Information, Figure S2. The absorption spectra of the supernatants of SiO2/Ag composite spheres after centrifugation were also measured by a UV-visible spectrometer; no plasmon resonance peak at around 420 nm was observed (not presented here), confirming that few free Ag nanoparticles appeared in dispersions; most of the Ag nanoparticles were formed on the silica spheres. Catalytic Property of SiO2/Ag Composite Spheres. It has been experimentally demonstrated that metal nanoparticles have high catalytic activities for hydrogenation, hydroformylation, carbonylation, and so forth.43-45 However, in most cases, they would coalesce during catalytic processes, because nanosized metal particles in the solution are active and tend to coalesce due to Van der Walls forces and high surface energy unless they are protected.



Figure 8. UV-visible spectra of dyes reduced by KBH4 as functions of reaction time and catalyst. (a,b) 2 × 10-5 mol/L RB, 1 × 10-2 mol/L KBH4; (d,e) 2 × 10-5mol/L BR K-2BP, 1 × 10-3 mol/L KBH4; (a,d) 0.25 mL SiO2/Ag solution; (b, e) 0.5 mL SiO2/Ag solution.

Here, we further investigated the catalytic property of the SiO2/Ag composite spheres whose coalescing tendency could be decreased (even eliminated), by monitoring the variation in optical density at the wavelength of the absorbance maximum (λmax) of the dye. Two different kinds of dyes such as Reactive Rhodamine B (Basic dye) and Brilliant Red K-2BP (Reactive dye) were selected. Their evolution of the UV-visible spectra during the reduction catalyzed by SiO2/Ag composite spheres are illustrated in Figure 8; their evolution of the UV-visible spectra during the reduction without SiO2/Ag composite spheres are presented in Supporting Information, Figure S5, for the sake of comparison. Very obviously, the system contained SiO2/Ag composite spheres, the absorbance at λmax of the dyes quickly decreased with reaction time, and the reduction of dyes by KBH4 finished within a few minutes. As shown in Figure 8b,e, with increasing the concentration of SiO2/Ag composite spheres, the reduction rate of the dyes increased. However, when the dyes were reduced with KBH4 solutions in the absence of SiO2/Ag composite spheres, the color of dyes remained unchanged even for 24 h (see Supporting Information, Figure S5). This evidence confirmed that the SiO2/Ag composite spheres had a very good catalytic performance. The catalytic mechanism could be explained as follows. The silver nanoparticles supported on silica spheres serve as an electron relay in the system for an oxidant and a reductant, and electron transfer occurs via the supported metal nanoparticles. Usually, dyes are electrophilic and BH4ions are nucleophilic with respect to the silver nanoparticles. In the reaction, the nucleophile KBH4 can donate electrons to

silver nanoparticles, and the electophile dyes would capture electrons from silver nanoparticles. So the silver nanoparticles served as an electron relay for catalytic reduction of dyes in KBH4 solution.45-47 SERS Property of SiO2/Ag Composite Spheres. Since SERS was very sensitive to the roughness of metal surfaces,41 the SiO2/Ag composite spheres covered with larger silver nanoparticles and higher coverage (the sample as in Figure 4d) were used as the efficient SERS-active substrates to examine the SERS property. Figure 9 shows the SERS spectra of 4-ABT deposited onto SiO2/Ag substrates; the spectrum of 4-ABT solution deposited on a silica wafer was also scanned for the sake of comparison. It was clearly found that no Raman peak was identified at all at 5 × 10-3 mol/L of 4-ABT in the absence of SiO2/Ag composite spheres, as shown in Figure 9a, but the Raman signals of 4-ABT at 5 × 10-3 mol/L could be magnified significantly in the presence of SiO2/Ag composite spheres, as seen in Figure 9e. The observed strong and medium-strong bands at 1578, 1440, 1392, and 1144 cm-1 were assigned to the fundamental benzene ring vibrations (ν8b, ν19b, ν3, and ν9b, respectively) of 4-ABT and agreed well with literature values.48,49 Moreover, the Raman peaks of 4-ABT also appeared in the SERS spectra even if the concentration of 4-ABT solution decreased to 5 × 10-4, 5 × 10-5, and 5 × 10-6 mol/L corresponding to Figure 9d, 9c, and 9b, respectively. This evidence indicated that the SiO2/Ag composite spheres as SERS substrates had excellent SERS performance.


Figure 9. Raman spectra of 4-ABT. (a) 5 × 10-3 mol/L, no SiO2/Ag spheres; (b) 5 × 10-6 mol/L, with SiO2/Ag spheres; (c) 5 × 10-5 mol/ L, SiO2/Ag spheres; (d) 5 × 10-4 mol/L, SiO2/Ag spheres; (e) 5 × 10-3 mol/L, SiO2/Ag spheres.

Conclusion On the basis of this study, the SiO2/Ag composite spheres could be prepared with the aid of the dual function of PVP as both reductant and stabilizer; neither additional reducing agent nor the core surface modification was needed. TEM, SEM, and UV-visible spectroscopy confirmed the formation of SiO2/Ag composite spheres; the particle size and the coverage of silver on the silica spheres could be easily controlled by altering the concentration of the precursor-[Ag(NH3)2]+ ions. XPS showed that there was interaction between the carboxyl groups of PVP and Ag nanoparticles. XRD indicated that the Ag nanoparticles were crystalline. UV-visible spectrometer and Raman spectrometer analyses showed these nanocomposite spheres had very good catalytic and surface-enhanced Raman scattering (SERS) performances. This method presented a new paradigm for preparation of core-shell dielectric core spheres (silica, polystyrene, etc.) coated with different metallic (Au, Pd, Pt, etc.) nanoshells. On the basis of this technique, many kinds of composite spheres coated with various metal nanoparticles could be possibly obtained, and these composite spheres could be possibly used in catalytic reactions and biosensor devices. Acknowledgment. The financial support from the Foundation of Science and Technology of Shanghai, Trans-century Outstanding Talented Person Foundation of China Educational Ministry, and Shuguang Scholar-Tracking Foundation of Shanghai for this research is appreciated. Supporting Information Available: TEM and SEM images of the composite spheres, XPS spectra of pure PVP and silica powder. UV-visible spectra of RB and BR K-2BP solutions mixed with KBH4 solution in the absence of SiO2/Ag composite spheres. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gittins, D. I.; Susha, A. S.; Schoeler, B.; Caruso, F. AdV. Mater. 2002, 14, 508. (2) Ji, T. H.; Lirtsman, V. G.; Avny, Y.; Davidov, D. AdV. Mater. 2001, 13, 1253. (3) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (4) Zhang, J. H.; Liu, J. B.; Wang, S. Z.; Zhan, P.; Wang, Z. L.; Ming, N. B. AdV. Funct. Mater. 2004, 14, 1089.

Deng et al. (5) Lu, Y.; Mei, Y.; Drechsler, M.; Ballauff, M. Angew. Chem., Int. Ed. 2006, 45, 813. (6) Shi, W.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N. Langmuir 2005, 21, 1610. (7) Averitt, R. D.; Sarkar, D.; Halas, N. J. Phys. ReV. Lett. 1997, 78, 4217. (8) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (9) Jackson, J. B.; Halas, N. J. J. Phys. Chem. B 2001, 105, 2743. (10) Chen, Z. M.; Gang, T.; Yan, X.; Li, X.; Zhang, J. H.; Wang, Y. F.; Chen, X.; Sun, Z. Q.; Zhang, K.; Zhao, B.; Yang, B. AdV. Mater. 2006, 18, 924. (11) Jiang, Z. J.; Liu, C. Y. J. Phys. Chem. B 2003, 107, 12411. (12) Wang, W.; Asher, S. A. J. Am. Chem. Soc. 2001, 123, 12528. (13) Graf, C.; van Blaaderen, A. Langmuir 2002, 18, 524. (14) Siiman, O.; Burshteyn, A. J. Phys. Chem. B 2000, 104, 9795. (15) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396. (16) Dokoutchaev, A.; James, J. T.; Koene, S. C.; Pathak, S.; Prakash, G. K. S.; Thompson, M. E. Chem. Mater. 1999, 11, 2389. (17) Kobayashi, Y.; Salgueirino-Maceira, V.; Liz-Marzan, L. M. Chem. Mater. 2001, 13, 1630. (18) Cassagneau, T.; Caruso, F. AdV. Mater. 2002, 14, 732. (19) Pol, V. G.; Srivastava, D. N.; Palchik, O.; Palchik, V.; Slifkin, M. A.; Weiss, A. M.; Gedanken, A. Langmuir 2002, 18, 3352. (20) Pol, V. G.; Grisaru, H.; Gedanken, A. Langmuir 2005, 21, 3635. (21) Warshawsky, A.; Upson, D. A. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 2963. (22) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (23) Deng, Z. W.; Chen, M.; Zhou, S. X.; You, B.; Wu, L. M. Langmuir 2006, 22, 6403. (24) Chen, M.; Wu, L. M.; Zhou, S. X.; You, B. AdV. Mater. 2006, 18, 801. (25) Imhof, A. Langmuir 2001, 17, 3579. (26) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693. (27) Hoppe, C. E.; Lazzari, M.; Pardinas-Blanco, I.; Lopez-Quintela, M. A. Langmuir 2006, 22, 7027. (28) Xiong, Y. J.; Washio, I.; Chen, J. Y.; Cai, H. G.; Li, Z. Y.; Xia, Y. N. Langmuir 2006, 22, 8563. (29) Washio, I.; Xiong, Y. J.; Yin, Y. N.; Xia, Y. N. AdV. Mater. 2006, 18, 1745. (30) Chen, M.; Zhou, S. X.; You, B.; Wu, L. M. Macromolecules 2005, 38, 6411. (31) Henglein, A.; Giersig, M. J. Phys. Chem. B 1999, 103, 9533. (32) Gao, F.; Lu, Q.; Komarneni, S. Chem. Mater. 2005, 17, 856. (33) Huang, H. H.; Ni, X. P.; Loy, G. L.; Chew, C. H.; Tan, K. L.; Loh, F. C.; Deng, J. F.; Xu, G. Q. Langmuir 1996, 12, 909. (34) Pastoriza-Santos, I.; Liz-Marzan, L. M. Langmuir 2002, 18, 2888. (35) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (36) Wang, Y. Z.; Li, Y. X.; Yang, S. T.; Zhang, G. L.; An, D. M.; Wang, C.; Yang, Q. B.; Chen, X. S.; Jing, X. B.; Wei, Y. Nanotechnology 2006, 17, 3304. (37) Jiang, P.; Li, S. Y.; Xie, S. S.; Gao, Y.; Li, S. Chem. Eur. J. 2004, 10, 4817. (38) Pattanaik, M.; Bhaumik, S. K. Mater. Lett. 2000, 44, 352. (39) Elechiguerra, J. L.; Larios-Lopez, L.; Liu, C.; Garcia-Gutierrez, D.; Camacho-Bragado, A.; Yacaman, M. J. Chem. Mater. 2005, 17, 6042. (40) Mayer, A. B. R.; Grebner, W.; Wannemacher, R. J. Phys. Chem. B 2000, 104, 7278. (41) Kim, K.; Kim, H. S.; Park, H. K. Langmuir 2006, 22, 8083. (42) Mie, G. Ann. Phys. 1908, 25, 377. (43) Ahmadi, T.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-sayed, M. A. Science 1996, 272, 1924. (44) Pal, T.; Sau, T. K.; Jana, N. R. Langmuir 1997, 13, 1481. (45) Jiang, Z. J.; Liu, C. Y.; Sun, L. W. J. Phys. Chem. B 2005, 109, 1730. (46) Jana, N. R.; Sau, T. K.; Pal, T. J. Phys. Chem. B 1999, 103, 115. (47) Jana, N. R.; Pal, T. Langmuir 1999, 15, 3458. (48) Kim, K.; Lee, H. S. J. Phys. Chem. B 2005, 109, 18929. (49) Kim, K.; Yoon, J. K. J. Phys. Chem. B 2005, 109, 20731.

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