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UV irradiation induced formation of single-crystal gold nanonetworks with controllable pore distribution
Colloids and Surfaces A: Physicochem. Eng. Aspects 340 (2009) 131–134
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
UV irradiation induced formation of single-crystal gold nanonetworks with controllable pore distribution Wenjun Tong, Shengchun Yang ∗ , Bingjun Ding Non-equilibrium Condensed Matter and Quantum Engineering Laboratory, the Key Laboratory of Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, Shann Xi, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 4 January 2009 Received in revised form 4 March 2009 Accepted 5 March 2009 Available online 19 March 2009 Keywords: Gold nanonetworks UV irradiation Porous nanoplate
a b s t r a c t Single-crystal gold nanonetworks and nanoplates with novel porous structures were synthesized through a continuous UV irradiation method. The structures of the porous nanonetworks and the nanoplates were found to be citric acid concentration dependent. Transmission electron microscopy (TEM) showed that the two-dimensional (2-D) nanonetworks prepared at the lower citric acid concentration (0.5 mM) had irregular pores and bigger area. Increasing the citric acid concentration resulted in formation of gold nanoplates with hexagonal, triangular or truncated triangular pores. When the acid concentration came to 2 mM, the nanoplates with single and double pores were observable. The selected area electron diffraction (SAED) patterns showed that both the nanonetworks and porous nanoplates were singlecrystal. The presence of 1/3{4 2 2} reflections indicated that the surface of the gold nanonetwork and nanoplates is atomically flat. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recent researches in the fields of nanoscience and nanotechnology have shown that the optical, electronic and magnetic properties of metal nanoparticles are strongly dependent on their size and shape [1]. Although a great progress has been made in the morphological control of nanomaterials, investigating new morphologies and structures of nanoparticles is still a challenge for researchers in this field, which mainly attributes to that a novel morphology and structure might imply new properties, and the potential applications of nanomaterials could be found. For example, the extinction peak of gold nanobox and nanocages could be tuned from 400 nm to the near-infrared (1200 nm) region of the electromagnetic spectrum and characterized by controllable cross-sections for both absorption and scattering through controlling their dimensions. This hollow nanostructure was expected to detect the tumor in early stage and as therapeutic agent for cancer treatment [2]. The porous gold nanospheres could improve the fluorescence in cell staining by offering an enhanced surface area for binding of the fluorescent dye for application in cell imaging [3]. The branched gold nanocrystals exhibit a shape-dependent plasmon resonance that is red-shifted by 130–180 nm from the spherical particle wavelength, and particles with such shape could be useful for achieving large surface Raman enhancements [4]. As a result, various metal nanoparticles with novel nanostructures were
∗ Corresponding author. E-mail address: [email protected] (S. Yang). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.03.012
synthesized, such as single-crystal cubes and tetrahedrons of silver with truncated corners/edges [5], nanodisks/nanoplates [6–9], tadpole [10] and bipyramid-shaped nanoparticles [11], branched nanocrystals [12]. However, there are few examples for the preparation of porous metal nanoparticles [13]. Besides the porous nanocages mentioned above, the synthesis of other porous nanostructures, especially two-dimension nanostructures, mainly based on the organization of nanoparticles into nanowires with a networked structure and the formation process was suggested to follow a hit-to-stick-tofusion model [14,15]. In addition, the porous nanonetworks were also synthesized by irradiation of intense pulsed laser onto gold nanoparticles [16]. Lasia and co-workers [17] described several methods to prepare porous gold electrodes, and these methods involved thermal decomposition, leaching or dissolution of surface amalgam. Recently, Hakamada and co-workers [18] reported a simple and spontaneous synthesis of nanoporous gold prism microassembly with highly dense skins, which is achieved just by immersing nanoporous gold into concentrated hydrochloric acid. The long-range periodic porous gold nanostructures [19], nanorings [20] and nanoframework [21] structures could be synthesized by template method or a galvanic replacement reaction. In our previous works, an UV induced morphology transformation of gold nanostructures from small particles to nanonetworks was reported. [22] But how to finely control the pore distribution of nanonetworks was still a remained question. In this paper, we report a facile method to control the pore distribution in the singlecrystal gold nanonetworks, in which the concentration of citric acid
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W. Tong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 340 (2009) 131–134
was found to be the most significant effect on the formation of the pores and its distribution in the gold nanonetworks. 2. Experimental details UV irradiation was performed in a tubular coil made of quartz tube with an internal diameter of 2 mm as described in our previous work [23]. A twin-channel peristaltic pump is used for simultaneously introducing the chloroauric acid and citric acid solution into the tube. A low-pressure mercury lamp (14 W) for 254 nm wavelength irradiation is used as light source. The irradiation time could be well controlled through the solution flow rate. In our experiment, citric acid was employed to initiate the photochemical reactions even at room temperature effectively, because citric acid could transform as the reducing radicals for the AuCl4 − reduction and might also act as the stabilizer during particles formation [24]. The synthesis of single-crystal porous gold nanoplates was carried out as following: the chloroauric acid solution (0.4 mM) and the dilute citric acid solution were simultaneously introduced into the reaction tube for 48 min irradiation. High-resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) patterns were recorded on a JEOL JEM-2000 transmission electron microscope with an accelerating voltage of 300 kV. 3. Results and discussion The concentration of citric acid was found to play a significant role in the formation of porous gold nanonetworks. Fig. 1a presents a typical TEM image of the nanonetworks obtained by irradiating the solution containing 0.5 mM citric acid and 0.4 mM HAuCl4 for
48 min. The enlarged TEM image (Fig. 1b) shows the part I in Fig. 1a, which indicates that the networks with bigger area are formed by the single-crystal belts to weld each other through the projecting tips (as shown by the arrows in Fig. 1b). Fig. 1c and d shows the enlarged images of part II and part III in Fig. 1a. The corresponding the SAED patterns in Fig. 1c reveals that the crystal lattice orientations of the fragments in network are various, showing that the connected fragments are randomly oriented. Fig. 1d shows the ED pattern of part III in Fig. 1a, which is a typical ED pattern of the single-crystal gold nanoplates, indicating that the recrystallization of randomly oriented primary fragments is expected to ultimately improve the growth of nanonetworks to porous nanoplates. The formation of the single-crystal belts could be attributed to citric acid molecule was preferentially adsorbed on the {1 1 1} facets and suppressed the growth in the 1 1 1 direction. However, what quite differs from the previous reports is that the extending direction of {1 1 1} facets is random during the anisotropic growth of nanoplates. Ultimately, the branched nanobelts are formed (Fig. 1b), which may be due to the lower additive concentration in the experiment. The spots could be indexed based on the face-centredcubic (f.c.c.) structure of gold, and this hexagonal symmetry of the diffracted spots suggests that the top crystal face of the networks must be {1 1 1} facets. The presence of the 1/3{4 2 2} reflections (circled spot) can be observed only from a gold (or silver) sample which is atomically flat [8] and the growth of the networks must be induced by the twin plan [25]. Fig. 2 shows typical TEM images of porous gold nanoplates synthesized at the citric acid concentration 1.0 mM with 48 min UV irradiation. The results indicate that most of the pores in nanoplates are hexagonal, triangular or truncated triangular. Fig. 2a represents
Fig. 1. (a) Typical TEM images of the product obtained by irradiating the solution containing 0.5 mM citric acid and 0.4 mM HAuCl4 for 48 min; (b) the enlarged image of part I in (a); (c and d), the corresponding electron diffraction of part II and III in a.
W. Tong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 340 (2009) 131–134
133
Fig. 2. (a, c and e) TEM images of the product obtained by irradiating the solution containing 1.0 mM citric acid and 0.4 mM HAuCl4 for 48 min; (b), the corresponding electron diffraction pattern of (a); (d) the HRTEM images of the pore edges shown by the circle in c.
the most typical pores distribution in the nanoplates: a pore with bigger size locates in the center of the plates and surrounded by the smaller ones which might be called ‘satellite-pores’. Fig. 2c and e shows other two different pores distributions models: linear and scattered. The latter is found with bigger porosity than the former. However, some pores with irregular shape still could be observed in the nanoplates as shown by the arrows in the figures. A high-resolution TEM image of an as-prepared porous gold nanoplate shows a crystal structure with clearly resolved lattice fringes (Fig. 2d); the distance of about 0.24 nm between these fringes is in reasonable correlation to the d111 of 0.235 nm for the f.c.c. crystal structure for the nanoplates; it reveals that the nanoplates are covered by {1 1 1} facets; the inner angle between
the adjacent edges of the pores is measured 120◦ which should correspond to the {1 1 0} and {1 0 1} planes. The SAED pattern (Fig. 2b) is found to be quite accordant with that of Fig. 1d showing the 1/3{4 2 2} reflections. It indicates that both nanonetworks and porous nanoplates should be atomically flat [8], and the growth of these plates reasonably induced by the twin plane between two (1 1 1) planes [25]. Fig. 3 shows the TEM image of the product obtained by irradiating the solution containing 2 mM citric acid and 0.4 mM HAuCl4 for 48 min. The gold nanoplates with single and double pores are observed. It indicates that higher concentration of citric acid results in closing the pores in the nanoplates. As one can see from the figures, the pore would rather locate in the center of the plates than
134
W. Tong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 340 (2009) 131–134
Fig. 3. TEM images of the product obtained by irradiating the solution containing 2 mM citric acid and 0.4 mM HAuCl4 for 48 min.
the edges, which might be attributed to the preferable closure of the pores in the edges. 4. Conclusion In summary, the structures of the porous nanonetworks and the nanoplates can be controlled by changing citric acid concentration as well as the irradiation time. Lower citric acid concentration lead to the formation of nanonetworks with irregular shapes. The single-crystal nanoplates with triangle and hexagon pores can be synthesized at higher citric acid concentration and longer irradiation time. These Au nanoparticles with unusual nanostructures may have important applications in other fields. Acknowledgment This work was supported by the National Natural Science Foundation of China (no. 50471033) References [1] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176. [2] J.Y. Chen, B. Wiley, Z.Y. Li, D. Campbell, F. Saeki, H. Cang, L. Au, J. Lee, X.D. Li, Y.N. Xia, Adv. Mater. 17 (2005) 2255. [3] S. Shukla, A. Priscilla, M. Banerjee, R.R. Bhonde, J. Ghatak, P.V. Satyam, M. Sastry, Chem. Mater. 17 (2005) 5000.
[4] E. Hao, R.C. Bailey, G.C. Schatz, J.T. Hupp, S.Y. Li, Nano Lett. 4 (2004) 327. [5] B. Wiley, T. Herricks, Y.G. Sun, Y.N. Xia, Nano Lett. 4 (2004) 1733. [6] E. Hao, K.L. Kelly, J.T. Hupp, G.C. Schatz, J. Am. Chem. Soc. 124 (2002) 15182. [7] R. Jin, Y.C. Cao, E. Hao, G.S. Métraux, G.C. Schatz, C.A. Mirkin, Nature 425 (2003) 487. [8] R.C. Jin, Y.W. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Science 294 (2001) 1901. [9] A. Callegari, D. Tonti, M. Chergui, Nano Lett. 3 (2003) 1565. [10] J.Q. Hu, Y. Zhang, B. Liu, J.X. Liu, H.H. Zhou, Y.F. Xu, Y.X. Jiang, Z.L. Yang, Z.Q. Tian, J. Am. Chem. Soc. 126 (2004) 9470. [11] M.Z. Liu, P. Guyot-Sionnest, J. Phys. Chem. B 109 (2005) 22192. [12] T.K. Sau, C.J. Murphy, J. Am. Chem. Soc. 126 (2004) 8648. [13] X.W. Teng, X.Y. Liang, S. Maksimuk, H. Yang, Small 2 (2006) 249. [14] L.H. Pei, K. Mori, M. Adachi, Langmuir 20 (2004) 7837. [15] G. Ramanath, J.D. Arcy-Gall, T. Maddanimath, A.V. Ellis, P.G. Ganesan, R. Goswami, A. Kumar, K. Vijayamohanan, Langmuir 20 (2004) 5583. [16] F. Mafune, J. Kohno, Y. Takeda, T. Kondow, J. Phys. Chem. B 107 (2003) 12589. [17] R. Jurczakowski, C. Hitz, A. Lasia, J. Electroanal. Chem. 572 (2004) 355. [18] M. Hakamada, M. Mabuchi, Nano Lett. 6 (2006) 882. [19] O.D. Velev, P.M. Tessier, A.M. Lenhoff, E.W. Kaler, Nature 401 (1999) 548. [20] J. Aizpurua, P. Hanarp, D.S. Sutherland, M. Käll, G.W. Bryant, F.J. García de Abajo, Phys. Rev. Lett. 90 (2003) 057401. [21] G.S. Métraux, Y.W.C. Cao, R.C. Jin, C.A. Mirkin, Nano Lett. 3 (2003) 519. [22] S.C. Yang, T.W. Zhang, L. Zhang, Q.F. Wang, R.L. Zhang, B.J. Ding, Nanotechnology 17 (2006) 5639. [23] S.C. Yang, T.W. Zhang, L. Zhang, S.X. Wang, Z.M. Yang, B.J. Ding, Colloid Surf. A 296 (2006) 37. [24] C.H. Wu, P.L. Su, C.S. Yeh, J. Phys. Chem. B 107 (2003) 14240. [25] C. Lofton, W. Sigmund, Adv. Funct. Mater. 15 (2005) 1197.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
UV irradiation induced formation of single-crystal gold nanonetworks with controllable pore distribution Wenjun Tong, Shengchun Yang ∗ , Bingjun Ding Non-equilibrium Condensed Matter and Quantum Engineering Laboratory, the Key Laboratory of Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, Shann Xi, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 4 January 2009 Received in revised form 4 March 2009 Accepted 5 March 2009 Available online 19 March 2009 Keywords: Gold nanonetworks UV irradiation Porous nanoplate
a b s t r a c t Single-crystal gold nanonetworks and nanoplates with novel porous structures were synthesized through a continuous UV irradiation method. The structures of the porous nanonetworks and the nanoplates were found to be citric acid concentration dependent. Transmission electron microscopy (TEM) showed that the two-dimensional (2-D) nanonetworks prepared at the lower citric acid concentration (0.5 mM) had irregular pores and bigger area. Increasing the citric acid concentration resulted in formation of gold nanoplates with hexagonal, triangular or truncated triangular pores. When the acid concentration came to 2 mM, the nanoplates with single and double pores were observable. The selected area electron diffraction (SAED) patterns showed that both the nanonetworks and porous nanoplates were singlecrystal. The presence of 1/3{4 2 2} reflections indicated that the surface of the gold nanonetwork and nanoplates is atomically flat. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recent researches in the fields of nanoscience and nanotechnology have shown that the optical, electronic and magnetic properties of metal nanoparticles are strongly dependent on their size and shape [1]. Although a great progress has been made in the morphological control of nanomaterials, investigating new morphologies and structures of nanoparticles is still a challenge for researchers in this field, which mainly attributes to that a novel morphology and structure might imply new properties, and the potential applications of nanomaterials could be found. For example, the extinction peak of gold nanobox and nanocages could be tuned from 400 nm to the near-infrared (1200 nm) region of the electromagnetic spectrum and characterized by controllable cross-sections for both absorption and scattering through controlling their dimensions. This hollow nanostructure was expected to detect the tumor in early stage and as therapeutic agent for cancer treatment [2]. The porous gold nanospheres could improve the fluorescence in cell staining by offering an enhanced surface area for binding of the fluorescent dye for application in cell imaging [3]. The branched gold nanocrystals exhibit a shape-dependent plasmon resonance that is red-shifted by 130–180 nm from the spherical particle wavelength, and particles with such shape could be useful for achieving large surface Raman enhancements [4]. As a result, various metal nanoparticles with novel nanostructures were
∗ Corresponding author. E-mail address: [email protected] (S. Yang). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.03.012
synthesized, such as single-crystal cubes and tetrahedrons of silver with truncated corners/edges [5], nanodisks/nanoplates [6–9], tadpole [10] and bipyramid-shaped nanoparticles [11], branched nanocrystals [12]. However, there are few examples for the preparation of porous metal nanoparticles [13]. Besides the porous nanocages mentioned above, the synthesis of other porous nanostructures, especially two-dimension nanostructures, mainly based on the organization of nanoparticles into nanowires with a networked structure and the formation process was suggested to follow a hit-to-stick-tofusion model [14,15]. In addition, the porous nanonetworks were also synthesized by irradiation of intense pulsed laser onto gold nanoparticles [16]. Lasia and co-workers [17] described several methods to prepare porous gold electrodes, and these methods involved thermal decomposition, leaching or dissolution of surface amalgam. Recently, Hakamada and co-workers [18] reported a simple and spontaneous synthesis of nanoporous gold prism microassembly with highly dense skins, which is achieved just by immersing nanoporous gold into concentrated hydrochloric acid. The long-range periodic porous gold nanostructures [19], nanorings [20] and nanoframework [21] structures could be synthesized by template method or a galvanic replacement reaction. In our previous works, an UV induced morphology transformation of gold nanostructures from small particles to nanonetworks was reported. [22] But how to finely control the pore distribution of nanonetworks was still a remained question. In this paper, we report a facile method to control the pore distribution in the singlecrystal gold nanonetworks, in which the concentration of citric acid
132
W. Tong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 340 (2009) 131–134
was found to be the most significant effect on the formation of the pores and its distribution in the gold nanonetworks. 2. Experimental details UV irradiation was performed in a tubular coil made of quartz tube with an internal diameter of 2 mm as described in our previous work [23]. A twin-channel peristaltic pump is used for simultaneously introducing the chloroauric acid and citric acid solution into the tube. A low-pressure mercury lamp (14 W) for 254 nm wavelength irradiation is used as light source. The irradiation time could be well controlled through the solution flow rate. In our experiment, citric acid was employed to initiate the photochemical reactions even at room temperature effectively, because citric acid could transform as the reducing radicals for the AuCl4 − reduction and might also act as the stabilizer during particles formation [24]. The synthesis of single-crystal porous gold nanoplates was carried out as following: the chloroauric acid solution (0.4 mM) and the dilute citric acid solution were simultaneously introduced into the reaction tube for 48 min irradiation. High-resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) patterns were recorded on a JEOL JEM-2000 transmission electron microscope with an accelerating voltage of 300 kV. 3. Results and discussion The concentration of citric acid was found to play a significant role in the formation of porous gold nanonetworks. Fig. 1a presents a typical TEM image of the nanonetworks obtained by irradiating the solution containing 0.5 mM citric acid and 0.4 mM HAuCl4 for
48 min. The enlarged TEM image (Fig. 1b) shows the part I in Fig. 1a, which indicates that the networks with bigger area are formed by the single-crystal belts to weld each other through the projecting tips (as shown by the arrows in Fig. 1b). Fig. 1c and d shows the enlarged images of part II and part III in Fig. 1a. The corresponding the SAED patterns in Fig. 1c reveals that the crystal lattice orientations of the fragments in network are various, showing that the connected fragments are randomly oriented. Fig. 1d shows the ED pattern of part III in Fig. 1a, which is a typical ED pattern of the single-crystal gold nanoplates, indicating that the recrystallization of randomly oriented primary fragments is expected to ultimately improve the growth of nanonetworks to porous nanoplates. The formation of the single-crystal belts could be attributed to citric acid molecule was preferentially adsorbed on the {1 1 1} facets and suppressed the growth in the 1 1 1 direction. However, what quite differs from the previous reports is that the extending direction of {1 1 1} facets is random during the anisotropic growth of nanoplates. Ultimately, the branched nanobelts are formed (Fig. 1b), which may be due to the lower additive concentration in the experiment. The spots could be indexed based on the face-centredcubic (f.c.c.) structure of gold, and this hexagonal symmetry of the diffracted spots suggests that the top crystal face of the networks must be {1 1 1} facets. The presence of the 1/3{4 2 2} reflections (circled spot) can be observed only from a gold (or silver) sample which is atomically flat [8] and the growth of the networks must be induced by the twin plan [25]. Fig. 2 shows typical TEM images of porous gold nanoplates synthesized at the citric acid concentration 1.0 mM with 48 min UV irradiation. The results indicate that most of the pores in nanoplates are hexagonal, triangular or truncated triangular. Fig. 2a represents
Fig. 1. (a) Typical TEM images of the product obtained by irradiating the solution containing 0.5 mM citric acid and 0.4 mM HAuCl4 for 48 min; (b) the enlarged image of part I in (a); (c and d), the corresponding electron diffraction of part II and III in a.
W. Tong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 340 (2009) 131–134
133
Fig. 2. (a, c and e) TEM images of the product obtained by irradiating the solution containing 1.0 mM citric acid and 0.4 mM HAuCl4 for 48 min; (b), the corresponding electron diffraction pattern of (a); (d) the HRTEM images of the pore edges shown by the circle in c.
the most typical pores distribution in the nanoplates: a pore with bigger size locates in the center of the plates and surrounded by the smaller ones which might be called ‘satellite-pores’. Fig. 2c and e shows other two different pores distributions models: linear and scattered. The latter is found with bigger porosity than the former. However, some pores with irregular shape still could be observed in the nanoplates as shown by the arrows in the figures. A high-resolution TEM image of an as-prepared porous gold nanoplate shows a crystal structure with clearly resolved lattice fringes (Fig. 2d); the distance of about 0.24 nm between these fringes is in reasonable correlation to the d111 of 0.235 nm for the f.c.c. crystal structure for the nanoplates; it reveals that the nanoplates are covered by {1 1 1} facets; the inner angle between
the adjacent edges of the pores is measured 120◦ which should correspond to the {1 1 0} and {1 0 1} planes. The SAED pattern (Fig. 2b) is found to be quite accordant with that of Fig. 1d showing the 1/3{4 2 2} reflections. It indicates that both nanonetworks and porous nanoplates should be atomically flat [8], and the growth of these plates reasonably induced by the twin plane between two (1 1 1) planes [25]. Fig. 3 shows the TEM image of the product obtained by irradiating the solution containing 2 mM citric acid and 0.4 mM HAuCl4 for 48 min. The gold nanoplates with single and double pores are observed. It indicates that higher concentration of citric acid results in closing the pores in the nanoplates. As one can see from the figures, the pore would rather locate in the center of the plates than
134
W. Tong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 340 (2009) 131–134
Fig. 3. TEM images of the product obtained by irradiating the solution containing 2 mM citric acid and 0.4 mM HAuCl4 for 48 min.
the edges, which might be attributed to the preferable closure of the pores in the edges. 4. Conclusion In summary, the structures of the porous nanonetworks and the nanoplates can be controlled by changing citric acid concentration as well as the irradiation time. Lower citric acid concentration lead to the formation of nanonetworks with irregular shapes. The single-crystal nanoplates with triangle and hexagon pores can be synthesized at higher citric acid concentration and longer irradiation time. These Au nanoparticles with unusual nanostructures may have important applications in other fields. Acknowledgment This work was supported by the National Natural Science Foundation of China (no. 50471033) References [1] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176. [2] J.Y. Chen, B. Wiley, Z.Y. Li, D. Campbell, F. Saeki, H. Cang, L. Au, J. Lee, X.D. Li, Y.N. Xia, Adv. Mater. 17 (2005) 2255. [3] S. Shukla, A. Priscilla, M. Banerjee, R.R. Bhonde, J. Ghatak, P.V. Satyam, M. Sastry, Chem. Mater. 17 (2005) 5000.
[4] E. Hao, R.C. Bailey, G.C. Schatz, J.T. Hupp, S.Y. Li, Nano Lett. 4 (2004) 327. [5] B. Wiley, T. Herricks, Y.G. Sun, Y.N. Xia, Nano Lett. 4 (2004) 1733. [6] E. Hao, K.L. Kelly, J.T. Hupp, G.C. Schatz, J. Am. Chem. Soc. 124 (2002) 15182. [7] R. Jin, Y.C. Cao, E. Hao, G.S. Métraux, G.C. Schatz, C.A. Mirkin, Nature 425 (2003) 487. [8] R.C. Jin, Y.W. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Science 294 (2001) 1901. [9] A. Callegari, D. Tonti, M. Chergui, Nano Lett. 3 (2003) 1565. [10] J.Q. Hu, Y. Zhang, B. Liu, J.X. Liu, H.H. Zhou, Y.F. Xu, Y.X. Jiang, Z.L. Yang, Z.Q. Tian, J. Am. Chem. Soc. 126 (2004) 9470. [11] M.Z. Liu, P. Guyot-Sionnest, J. Phys. Chem. B 109 (2005) 22192. [12] T.K. Sau, C.J. Murphy, J. Am. Chem. Soc. 126 (2004) 8648. [13] X.W. Teng, X.Y. Liang, S. Maksimuk, H. Yang, Small 2 (2006) 249. [14] L.H. Pei, K. Mori, M. Adachi, Langmuir 20 (2004) 7837. [15] G. Ramanath, J.D. Arcy-Gall, T. Maddanimath, A.V. Ellis, P.G. Ganesan, R. Goswami, A. Kumar, K. Vijayamohanan, Langmuir 20 (2004) 5583. [16] F. Mafune, J. Kohno, Y. Takeda, T. Kondow, J. Phys. Chem. B 107 (2003) 12589. [17] R. Jurczakowski, C. Hitz, A. Lasia, J. Electroanal. Chem. 572 (2004) 355. [18] M. Hakamada, M. Mabuchi, Nano Lett. 6 (2006) 882. [19] O.D. Velev, P.M. Tessier, A.M. Lenhoff, E.W. Kaler, Nature 401 (1999) 548. [20] J. Aizpurua, P. Hanarp, D.S. Sutherland, M. Käll, G.W. Bryant, F.J. García de Abajo, Phys. Rev. Lett. 90 (2003) 057401. [21] G.S. Métraux, Y.W.C. Cao, R.C. Jin, C.A. Mirkin, Nano Lett. 3 (2003) 519. [22] S.C. Yang, T.W. Zhang, L. Zhang, Q.F. Wang, R.L. Zhang, B.J. Ding, Nanotechnology 17 (2006) 5639. [23] S.C. Yang, T.W. Zhang, L. Zhang, S.X. Wang, Z.M. Yang, B.J. Ding, Colloid Surf. A 296 (2006) 37. [24] C.H. Wu, P.L. Su, C.S. Yeh, J. Phys. Chem. B 107 (2003) 14240. [25] C. Lofton, W. Sigmund, Adv. Funct. Mater. 15 (2005) 1197.