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Deposition of silver nanoparticles on silica spheres via ultrasound irradiation
Applied Surface Science 253 (2007) 6264–6267 www.elsevier.com/locate/apsusc
Deposition of silver nanoparticles on silica spheres via ultrasound irradiation Xiaoyun Ye, Yuming Zhou *, Jing Chen, Yanqing Sun School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China Received 3 December 2006; received in revised form 22 January 2007; accepted 22 January 2007 Available online 3 February 2007
Abstract Silver-decorated silica spheres of submicrometer-sized silica spheres with a core-shell structure were obtained based on a seed-mediated growth process, where silver nanoparticles were firstly formed from reducing Ag+ to Ag0 in N,N-dimethylformamide (DMF) in the presence of poly(vinylpyrrolidone) (PVP) as protective agent under ultrasound irradiation, followed by the growth of silver shell served silver nanoparticles as nucleation sites and formaldehyde as reducer. The results revealed that the terms of PVP addition and ultrasonic surroundings had great influence on the fabrication of silver seeds. # 2007 Elsevier B.V. All rights reserved. Keywords: Silica; Silver; Nanoparticle; Deposition; Ultrasound
1. Introduction
2. Experimental
Metal-coated colloidal core-shell composite particles are a class of materials widely used in many fields of colloid and materials science, due to their unique and tailored properties for various applications as catalysts, sensors, colloidal entities with unique optical properties, and substrates for surface-enhanced Raman scattering [1–4]. Up to now, a variety of techniques has been employed on the deposition of silver or gold nanoparticles on silica spheres, including the inverse micelle method [5], pretreatment of electroless plating deposition [6], sono-chemical synthesis [7,8], and surface functionalization deposition [3,9]. The desired metal shell can be taken from a successive deposition process. Unlike the cases mentioned, this paper reports a seedmediated particles growth process for fabricating silica coresilver shell particles under a mild reaction condition by ultrasound irradiation, comprising the reduction of silver ions to silver metal particles acting as nucleation sites and the further growth of a silver shell. To the best of our knowledge, no previous studies have employed the combination of N,Ndimethylformamide (DMF) reduction system and sonochemical methods to explore the growth of a silver shell on silica using silver nanoparticles as seeds.
2.1. Materials
* Corresponding author. Tel.: +86 25 52090617; fax: +86 25 52090617. E-mail address: [email protected] (Y. Zhou). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.01.111
Tetraethoxysilane (TEOS, 99%) was obtained from Shanghai Chemical reagent Company and distilled under reduced pressure prior to use. Silver nitrate (AgNO3, 99.5%), poly(vinylpyrrolidone) (PVP, K30, polymerization degree 360), N,N-dimethylformamide (DMF), aqueous ammonia (28%) and formaldehyde (37%), also from Shanghai Chemical reagent Company, were used as received without further purification. In all preparations ethanol (99.7%) and deionized water were used. 2.2. Methods Silica core-silver shell particles were synthesized in a simple two-step reaction process. Scheme 1 shows the typical strategy used to prepare a silver nanoshell. First, in the presence of PVP, silver ion was reduced to zerovalent metal atom by DMF. Here, PVP acts as a protective agent for the formation of colloid Ag nanoparticles. At the same time, the colloid particles were deposited on the surface of silica sphere without pre-treatment by ultrasound irradiation, which then served as seeds for the growth of a silver nanoshell layer. The process can be repeated to increase the Ag nanoparticle density on the surface. Subsequently Ag nanoparticles on decorated silica sphere core grew, using formaldehyde as a reducing agent.
X. Ye et al. / Applied Surface Science 253 (2007) 6264–6267
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Scheme 1. Deposition procedure of silver nanopartilces on the silica sphere.
Typically, monodisperse silica cores were prepared by the well-known Sto¨ber–Fink–Bohn method [10]. This method yielded the colloidal solution of silica particles with a narrow size distribution over a wider range, and the particle size depended on initial reagent concentrations, temperature and solvent. The initial deposition of Ag nanoparticles on the silica surface was performed following an ultrasonic procedure processed as follows. At the beginning, 5 mL of AgNO3 (0.8 mM) aqueous solution was added to a 45 mL PVP solution in DMF ([AgNO3]/[PVP] = 0.1), following the addition of silica sphere (0.02 wt%). The mixture was exposed to highintensity ultrasound irradiation for 30 min at 28 8C. Ultrasound irradiation was accomplished with a high-intensity ultrasonic probe (Xinzhi Co., China, Ti-horn, 20 kHz, 800 W/cm2) immersed directly in the reaction solution. When the reaction was finished, a dark brown precipitation was observed. The mixture was centrifugally separated from the suspension and ultrasonically washed with water. The above procedures were repeated several times in order to increase the density of silver nanoparticles on the surface. To further growth of a silver shell, 10 mg of the seeded colloid particles aqueous solution (0.1 wt%) was mixed with 10 mL of a solution of AgNO3
(0.15 mM), and then 25 mL of formaldehyde was added, immediately followed by 25 mL of concentrated ammonia. 2.3. Characterization Transmission electron microscopy (TEM) was performed with a Hitachi H-600 microscope operating at 120 kV. Samples were prepared by placing drops of the colloids dispersion on a Cu grid and removing onto a filter paper for solvent evaporation at room temperature. X-ray diffractometry (XRD) was performed with an X-ray diffractometer (XD-3A) operating at 40 kV and 30 mA with Cu Ka radiation. UV–vis absorption spectra were measured with a Shimadzu UV-2201 spectrophotometer. Atomic force microscopy (AFM) was measured with a veeco DI IIIa system operating with a 1-mm scanner. 3. Results and discussion 3.1. Ag seeds deposition The silica cores prepared by the Sto¨ber–Fink–Bohn method are shown in Fig. 1a. Fig. 1b and c shows the TEM images of
Fig. 1. TEM micrographs of silica spheres during the successive deposition steps: (a) bare silica; (b and c) first and second deposition of silver particles on the silica surface; (d) the growth of silver shell on the silica surface.
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Fig. 2. UV–vis absorption spectra of silica spheres by the successive deposition steps. The symbols correspond to Fig. 1.
Fig. 3. XRD pattern of silver shell coating on the silica surface.
silica spheres by successive silver deposition steps. In the presence of PVP, silver ions were reduced to zerovalent metal (as seeds) by DMF under ultrasound irradiation condition. After the first deposition, an innumerability of Ag nanoparticles with a diameter of approximately 20 nm uniformly distributed on the surface of silica spheres gains, compared to Konno et al.’s work [11]. To deposit a larger amount of Ag nanoparticles, a second coating step was carried out the same as the first step. TEM (Fig. 1c) shows that the surface coverage by Ag nanoparticles was increased, resulting from not only the freshly formation of Ag nanoparticles but also the growth of previously existing Ag nanoparticles. Obviously, the size of current Ag particles ranged from about 15 to 60 nm. Fig. 2 shows the UV–vis spectra of the silica colloids before and after silver deposition. No distinct peaks were observed for bare silica (Fig. 2a) owing to the strong scattering from the silica colloid. After the deposition of Ag nanoparticles, a broad absorption peak (Fig. 2b and c) around 425 nm appeared, due to surface plasmon excitation of metal silver nanoparticles. With the increase of the silver density on silica spheres, the position of the peak slightly red shifted, partly attributed to the formation of larger particles. Again, it is clear that no shoulder peak appeared. Since the ratio of silica core to silver shell influences the optical properties of the composite particles [9]. Submicrometer-sized silica core may screen the plasmon band by the strong scattering from the silica, which is in agreement with Ref. [6].
Such a broaden peak is attributed to more metal Ag deposition on the surface of the silica sphere [12]. Fig. 3 shows a typical X-ray diffraction (XRD) pattern of the composite particles of silver shell and silica core. Sharp diffraction peaks were observed corresponding to the facecentered cubic (fcc) structure of metallic Ag with the diffraction peaks corresponding to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, indicating the formation of pure silver of high crystallinity (JCPDS file, No. 4-783). The roughness of the particle surface was performed by AFM. Seen from Fig. 4a and b, the surface of silica spheres is rough but relatively homogeneous after the formation of silver shell compared to bare silica surface. And the Ag nanoparticles size from the AFM image is basically in accord with that obtained from TEM (Fig. 1d).
3.2. Ag shell growth Once the silica sphere surface has been covered with a high density of Ag nanoparticles, Ag shell could cover with the silica by reduction of additional AgNO3 with formaldehyde in the presence of ammonia. Fig. 1d shows the TEM image of the silica sphere covered with a silver shell, though it is of some incontinuous. In the UV–vis absorption spectrum (Fig. 2d), the magnitude of the extinction spectrum remarkably increases.
3.3. Mechanism for the deposition of Ag nanoparticles on silica spheres by ultrasound irradiation in the presence of PVP Silver seeds for the growth of a silver shell were formed through the reduction of Ag+ ions with DMF, using PVP as a protective agent. Three steps of PVP protective mechanism of the silver reduction are as follows [13,14]: (a) the formation of coordinative bonding between PVP and silver ions; (b) PVPpromoted silver nucleation; (c) PVP prohibiting grain growth and particle aggregation. PVP could promote the nucleation of the Ag nanoparticles in the reaction. To test this hypothesis, reactions with PVP and without PVP were carried out respectively at the same reagent concentration and reaction condition. In this case, the solution without PVP remained transparent, while the other one became dark brown, indicating the formation of Ag particles. Additionally, in despite of the reduction of Ag+ ions by DMF can carry through at room temperature, the rate is very slow for over weeks [15]. At the same time, the rate is much higher when the temperature is increased. In particular, ‘‘interfacial region’’ exists during the ultrasonic reaction process, where required temperature for the
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Fig. 4. AFM images of (a) bare silica spheres; (b) silver-decorated silica spheres (scan size: 1.5 mm).
reaction is supplied [7,16]. Moreover, the Ag nanoparticles bonded to the silica surface is related to the microjets and shock waves created at the inner environment of the ultrasonic solution [17], which results in pushing the nanoparticles toward the surface of silica spheres at a high speed and adhesion of the nanoparticles to the silica. 4. Summary Nanoparticles of silver were uniformly deposited by ultrasonic irradiation method on the surface of submicrometer-sized silica spheres from a basic aqueous solution containing Ag+ ions by the addition of PVP. The density of Ag nanoparticles could be tailored by repeating the cycles of silver deposition process. Silver shell was grown on the basis of silver seeds dispersed on the surface of silica spheres by reduction of Ag+ ion with formaldehyde in the presence of ammonia. The advantages of the process described above are that it is simple and efficient to achieve the uniformly deposition of Ag nanoparticles in the condition of low temperature of 28 8C and short reaction time of 30 min. PVP plays a crucial role to promote the nucleation and formation of Ag nanoparticles. The two factors of additive PVP and ultrasound are indispensable in this report for the formation of silver seeds. Acknowledgements The authors are grateful to ‘the New Century Talents Program’ of ministry of education of China (NCET-04-0482),
the National Nature Science Foundation of China (50377005) and ‘the Six Top Talents’ of Jiangsu Province of China (06-A033) for their support of this investigation. References [1] F. Caruso, M. Spasova, V. Salgueirino-Maceira, L.M. Liz-Marzan, Adv. Mater. 13 (2001) 1090. [2] D.I. Gittins, A.S. Susha, B. Schoeler, F. Caruso, Adv. Mater. 14 (2002) 508. [3] T. Pham, J.B. Jackson, N.J. Halas, T.R. Lee, Langmuir 18 (2002) 4915. [4] S.J. Oldenburg, J.B. Jackson, S.L. Westcott, N.J. Halas, Appl. Phys. Lett. 75 (1999) 2897. [5] P. Lianos, J.K. Thomas, J. Colloid Interface Sci. 117 (1987) 505. [6] Y. Kobayashi, V. Salgueirino-Maceira, L.M. Liz-Marzan, Chem. Mater. 13 (2001) 1630. [7] V.G. Pol, D.N. Srivastava, O. Palchik, V. Palchik, M.A. Slifkin, A.M. Weiss, A. Gedanken, Langmuir 18 (2002) 3352. [8] V.G. Pol, A. Gedanken, J. Calderon-Moreno, Chem. Mater. 15 (2003) 1111. [9] Z.J. Jiang, C.Y. Liu, J. Phys. Chem. B 107 (2003) 12411. [10] W. Sto¨ber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [11] Y. Kobayashi, Y. Tadaki, D. Nagao, M. Konno, J. Colloid Interface Sci. 283 (2005) 601. [12] J.B. Jackson, N.J. Halas, J. Phys. Chem. B 105 (2001) 2743. [13] Z.T. Zhang, B. Zhao, L.M. Hu, J. Solid State Chem. 121 (1996) 105. [14] I. Pastoriza-Santos, L.M. Liz-Marzan, Langmuir 18 (2002) 2888. [15] I. Pastoriza-Santos, L.M. Liz-Marzan, Langmuir 15 (1999) 948. [16] A. Sanchez-Iglesias, I. Pastoriza-Santos, J. Perez-Juste, B. RodriguezGonzalez, F.J. Garcia de Abajo, L.M. Liz-Marzan, Adv. Mater. 18 (2006) 2529. [17] K.S. Suslick, G.J. Price, Annu. Rev. Mater. Sci. 29 (1999) 295.
Deposition of silver nanoparticles on silica spheres via ultrasound irradiation Xiaoyun Ye, Yuming Zhou *, Jing Chen, Yanqing Sun School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China Received 3 December 2006; received in revised form 22 January 2007; accepted 22 January 2007 Available online 3 February 2007
Abstract Silver-decorated silica spheres of submicrometer-sized silica spheres with a core-shell structure were obtained based on a seed-mediated growth process, where silver nanoparticles were firstly formed from reducing Ag+ to Ag0 in N,N-dimethylformamide (DMF) in the presence of poly(vinylpyrrolidone) (PVP) as protective agent under ultrasound irradiation, followed by the growth of silver shell served silver nanoparticles as nucleation sites and formaldehyde as reducer. The results revealed that the terms of PVP addition and ultrasonic surroundings had great influence on the fabrication of silver seeds. # 2007 Elsevier B.V. All rights reserved. Keywords: Silica; Silver; Nanoparticle; Deposition; Ultrasound
1. Introduction
2. Experimental
Metal-coated colloidal core-shell composite particles are a class of materials widely used in many fields of colloid and materials science, due to their unique and tailored properties for various applications as catalysts, sensors, colloidal entities with unique optical properties, and substrates for surface-enhanced Raman scattering [1–4]. Up to now, a variety of techniques has been employed on the deposition of silver or gold nanoparticles on silica spheres, including the inverse micelle method [5], pretreatment of electroless plating deposition [6], sono-chemical synthesis [7,8], and surface functionalization deposition [3,9]. The desired metal shell can be taken from a successive deposition process. Unlike the cases mentioned, this paper reports a seedmediated particles growth process for fabricating silica coresilver shell particles under a mild reaction condition by ultrasound irradiation, comprising the reduction of silver ions to silver metal particles acting as nucleation sites and the further growth of a silver shell. To the best of our knowledge, no previous studies have employed the combination of N,Ndimethylformamide (DMF) reduction system and sonochemical methods to explore the growth of a silver shell on silica using silver nanoparticles as seeds.
2.1. Materials
* Corresponding author. Tel.: +86 25 52090617; fax: +86 25 52090617. E-mail address: [email protected] (Y. Zhou). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.01.111
Tetraethoxysilane (TEOS, 99%) was obtained from Shanghai Chemical reagent Company and distilled under reduced pressure prior to use. Silver nitrate (AgNO3, 99.5%), poly(vinylpyrrolidone) (PVP, K30, polymerization degree 360), N,N-dimethylformamide (DMF), aqueous ammonia (28%) and formaldehyde (37%), also from Shanghai Chemical reagent Company, were used as received without further purification. In all preparations ethanol (99.7%) and deionized water were used. 2.2. Methods Silica core-silver shell particles were synthesized in a simple two-step reaction process. Scheme 1 shows the typical strategy used to prepare a silver nanoshell. First, in the presence of PVP, silver ion was reduced to zerovalent metal atom by DMF. Here, PVP acts as a protective agent for the formation of colloid Ag nanoparticles. At the same time, the colloid particles were deposited on the surface of silica sphere without pre-treatment by ultrasound irradiation, which then served as seeds for the growth of a silver nanoshell layer. The process can be repeated to increase the Ag nanoparticle density on the surface. Subsequently Ag nanoparticles on decorated silica sphere core grew, using formaldehyde as a reducing agent.
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Scheme 1. Deposition procedure of silver nanopartilces on the silica sphere.
Typically, monodisperse silica cores were prepared by the well-known Sto¨ber–Fink–Bohn method [10]. This method yielded the colloidal solution of silica particles with a narrow size distribution over a wider range, and the particle size depended on initial reagent concentrations, temperature and solvent. The initial deposition of Ag nanoparticles on the silica surface was performed following an ultrasonic procedure processed as follows. At the beginning, 5 mL of AgNO3 (0.8 mM) aqueous solution was added to a 45 mL PVP solution in DMF ([AgNO3]/[PVP] = 0.1), following the addition of silica sphere (0.02 wt%). The mixture was exposed to highintensity ultrasound irradiation for 30 min at 28 8C. Ultrasound irradiation was accomplished with a high-intensity ultrasonic probe (Xinzhi Co., China, Ti-horn, 20 kHz, 800 W/cm2) immersed directly in the reaction solution. When the reaction was finished, a dark brown precipitation was observed. The mixture was centrifugally separated from the suspension and ultrasonically washed with water. The above procedures were repeated several times in order to increase the density of silver nanoparticles on the surface. To further growth of a silver shell, 10 mg of the seeded colloid particles aqueous solution (0.1 wt%) was mixed with 10 mL of a solution of AgNO3
(0.15 mM), and then 25 mL of formaldehyde was added, immediately followed by 25 mL of concentrated ammonia. 2.3. Characterization Transmission electron microscopy (TEM) was performed with a Hitachi H-600 microscope operating at 120 kV. Samples were prepared by placing drops of the colloids dispersion on a Cu grid and removing onto a filter paper for solvent evaporation at room temperature. X-ray diffractometry (XRD) was performed with an X-ray diffractometer (XD-3A) operating at 40 kV and 30 mA with Cu Ka radiation. UV–vis absorption spectra were measured with a Shimadzu UV-2201 spectrophotometer. Atomic force microscopy (AFM) was measured with a veeco DI IIIa system operating with a 1-mm scanner. 3. Results and discussion 3.1. Ag seeds deposition The silica cores prepared by the Sto¨ber–Fink–Bohn method are shown in Fig. 1a. Fig. 1b and c shows the TEM images of
Fig. 1. TEM micrographs of silica spheres during the successive deposition steps: (a) bare silica; (b and c) first and second deposition of silver particles on the silica surface; (d) the growth of silver shell on the silica surface.
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Fig. 2. UV–vis absorption spectra of silica spheres by the successive deposition steps. The symbols correspond to Fig. 1.
Fig. 3. XRD pattern of silver shell coating on the silica surface.
silica spheres by successive silver deposition steps. In the presence of PVP, silver ions were reduced to zerovalent metal (as seeds) by DMF under ultrasound irradiation condition. After the first deposition, an innumerability of Ag nanoparticles with a diameter of approximately 20 nm uniformly distributed on the surface of silica spheres gains, compared to Konno et al.’s work [11]. To deposit a larger amount of Ag nanoparticles, a second coating step was carried out the same as the first step. TEM (Fig. 1c) shows that the surface coverage by Ag nanoparticles was increased, resulting from not only the freshly formation of Ag nanoparticles but also the growth of previously existing Ag nanoparticles. Obviously, the size of current Ag particles ranged from about 15 to 60 nm. Fig. 2 shows the UV–vis spectra of the silica colloids before and after silver deposition. No distinct peaks were observed for bare silica (Fig. 2a) owing to the strong scattering from the silica colloid. After the deposition of Ag nanoparticles, a broad absorption peak (Fig. 2b and c) around 425 nm appeared, due to surface plasmon excitation of metal silver nanoparticles. With the increase of the silver density on silica spheres, the position of the peak slightly red shifted, partly attributed to the formation of larger particles. Again, it is clear that no shoulder peak appeared. Since the ratio of silica core to silver shell influences the optical properties of the composite particles [9]. Submicrometer-sized silica core may screen the plasmon band by the strong scattering from the silica, which is in agreement with Ref. [6].
Such a broaden peak is attributed to more metal Ag deposition on the surface of the silica sphere [12]. Fig. 3 shows a typical X-ray diffraction (XRD) pattern of the composite particles of silver shell and silica core. Sharp diffraction peaks were observed corresponding to the facecentered cubic (fcc) structure of metallic Ag with the diffraction peaks corresponding to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, indicating the formation of pure silver of high crystallinity (JCPDS file, No. 4-783). The roughness of the particle surface was performed by AFM. Seen from Fig. 4a and b, the surface of silica spheres is rough but relatively homogeneous after the formation of silver shell compared to bare silica surface. And the Ag nanoparticles size from the AFM image is basically in accord with that obtained from TEM (Fig. 1d).
3.2. Ag shell growth Once the silica sphere surface has been covered with a high density of Ag nanoparticles, Ag shell could cover with the silica by reduction of additional AgNO3 with formaldehyde in the presence of ammonia. Fig. 1d shows the TEM image of the silica sphere covered with a silver shell, though it is of some incontinuous. In the UV–vis absorption spectrum (Fig. 2d), the magnitude of the extinction spectrum remarkably increases.
3.3. Mechanism for the deposition of Ag nanoparticles on silica spheres by ultrasound irradiation in the presence of PVP Silver seeds for the growth of a silver shell were formed through the reduction of Ag+ ions with DMF, using PVP as a protective agent. Three steps of PVP protective mechanism of the silver reduction are as follows [13,14]: (a) the formation of coordinative bonding between PVP and silver ions; (b) PVPpromoted silver nucleation; (c) PVP prohibiting grain growth and particle aggregation. PVP could promote the nucleation of the Ag nanoparticles in the reaction. To test this hypothesis, reactions with PVP and without PVP were carried out respectively at the same reagent concentration and reaction condition. In this case, the solution without PVP remained transparent, while the other one became dark brown, indicating the formation of Ag particles. Additionally, in despite of the reduction of Ag+ ions by DMF can carry through at room temperature, the rate is very slow for over weeks [15]. At the same time, the rate is much higher when the temperature is increased. In particular, ‘‘interfacial region’’ exists during the ultrasonic reaction process, where required temperature for the
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Fig. 4. AFM images of (a) bare silica spheres; (b) silver-decorated silica spheres (scan size: 1.5 mm).
reaction is supplied [7,16]. Moreover, the Ag nanoparticles bonded to the silica surface is related to the microjets and shock waves created at the inner environment of the ultrasonic solution [17], which results in pushing the nanoparticles toward the surface of silica spheres at a high speed and adhesion of the nanoparticles to the silica. 4. Summary Nanoparticles of silver were uniformly deposited by ultrasonic irradiation method on the surface of submicrometer-sized silica spheres from a basic aqueous solution containing Ag+ ions by the addition of PVP. The density of Ag nanoparticles could be tailored by repeating the cycles of silver deposition process. Silver shell was grown on the basis of silver seeds dispersed on the surface of silica spheres by reduction of Ag+ ion with formaldehyde in the presence of ammonia. The advantages of the process described above are that it is simple and efficient to achieve the uniformly deposition of Ag nanoparticles in the condition of low temperature of 28 8C and short reaction time of 30 min. PVP plays a crucial role to promote the nucleation and formation of Ag nanoparticles. The two factors of additive PVP and ultrasound are indispensable in this report for the formation of silver seeds. Acknowledgements The authors are grateful to ‘the New Century Talents Program’ of ministry of education of China (NCET-04-0482),
the National Nature Science Foundation of China (50377005) and ‘the Six Top Talents’ of Jiangsu Province of China (06-A033) for their support of this investigation. References [1] F. Caruso, M. Spasova, V. Salgueirino-Maceira, L.M. Liz-Marzan, Adv. Mater. 13 (2001) 1090. [2] D.I. Gittins, A.S. Susha, B. Schoeler, F. Caruso, Adv. Mater. 14 (2002) 508. [3] T. Pham, J.B. Jackson, N.J. Halas, T.R. Lee, Langmuir 18 (2002) 4915. [4] S.J. Oldenburg, J.B. Jackson, S.L. Westcott, N.J. Halas, Appl. Phys. Lett. 75 (1999) 2897. [5] P. Lianos, J.K. Thomas, J. Colloid Interface Sci. 117 (1987) 505. [6] Y. Kobayashi, V. Salgueirino-Maceira, L.M. Liz-Marzan, Chem. Mater. 13 (2001) 1630. [7] V.G. Pol, D.N. Srivastava, O. Palchik, V. Palchik, M.A. Slifkin, A.M. Weiss, A. Gedanken, Langmuir 18 (2002) 3352. [8] V.G. Pol, A. Gedanken, J. Calderon-Moreno, Chem. Mater. 15 (2003) 1111. [9] Z.J. Jiang, C.Y. Liu, J. Phys. Chem. B 107 (2003) 12411. [10] W. Sto¨ber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [11] Y. Kobayashi, Y. Tadaki, D. Nagao, M. Konno, J. Colloid Interface Sci. 283 (2005) 601. [12] J.B. Jackson, N.J. Halas, J. Phys. Chem. B 105 (2001) 2743. [13] Z.T. Zhang, B. Zhao, L.M. Hu, J. Solid State Chem. 121 (1996) 105. [14] I. Pastoriza-Santos, L.M. Liz-Marzan, Langmuir 18 (2002) 2888. [15] I. Pastoriza-Santos, L.M. Liz-Marzan, Langmuir 15 (1999) 948. [16] A. Sanchez-Iglesias, I. Pastoriza-Santos, J. Perez-Juste, B. RodriguezGonzalez, F.J. Garcia de Abajo, L.M. Liz-Marzan, Adv. Mater. 18 (2006) 2529. [17] K.S. Suslick, G.J. Price, Annu. Rev. Mater. Sci. 29 (1999) 295.