Journal of Colloid and Interface Science 322 (2008) 152–157 www.elsevier.com/locate/jcis

Seedless synthesis of octahedral gold nanoparticles in condensed surfactant phase Cuong Cao a , Sungho Park b , Sang Jun Sim a,∗ a Department of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, South Korea b Department of Chemistry, BK21 School of Chemical Materials Science & SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University,

Suwon 440-746, South Korea Received 4 November 2007; accepted 9 March 2008 Available online 20 March 2008

Abstract We report a seedless synthetic method of gold octahedral nanoparticles in an aqueous phase. Eight facets with {111} crystalline structures of octahedral nanoparticles could be formed in an aqueous medium when the gold salt was reduced by ascorbic acid at room temperature in the presence of cetyltrimethylammonium bromide as a shape-inducing agent, and hydrogen peroxide as a reaction promoter. The growth kinetics and surface crystalline structures were characterized by UV–vis spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy. © 2008 Elsevier Inc. All rights reserved. Keywords: Template-directed synthesis; Non-seeding process; H2 O2 -dependent growth; Octahedra; Gold nanoparticles

1. Introduction Synthesis of metal nanoparticles is an attractive goal of nanomaterial technology in achieving their many potential applications in photonics [1,2], microelectronics [3,4], catalysis [5,6], biology [7,8], or development of sensor devices [9–12]. These applications strongly depend on size, shape, composition, and dielectric properties of the metal nanoparticles [13]. The presence of sharp edges or convex tips has been shown to increase the electromagnetic field, or light intensity relative to the incident field, and that is very important to applications of the nanomaterials as sensing constituents [13,14]. Therefore, diverse methods including seed-mediated [15,16], electrochemical [17], and biological [18] reductions have been used to synthesize anisotropic nanoparticles with controlled size and shape to exploit their unique physio-optical properties. In a seed-mediated synthesis, particle growth has been considered to arise from surfactant-templated processes where its shape is physically constrained by the surfactant [15,16]. Obvi* Corresponding author. Fax: +82 31 290 7272.

E-mail address: [email protected] (S.J. Sim). 0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2008.03.031

ously, faceting tendency of the stabilizing surfactant impacting on crystallographic planes of the nanoseeds, along with kinetics of the crystal growth, will stipulate the shape formation. For example, single crystal surfaces {111}, {100} and {110} represent the most frequently studied surface planes, and are also the most commonly occurring surfaces of fcc metals such as gold. Stability of the individual surface planes decreases in the order {111} > {100} > {110} based on their surface atom densities and coordination number of the surface atoms [19,20]. Therefore, by using different concentrations of surfactants to control crystallographic surface exposure of gold seeds to deposition of Au0 , a variety of gold nanoparticles with the highly faceted shapes can be fabricated using the seed-mediated growth [16,21–23]. It is also well known that at the initial stages of seedless crystal growth in the presence of high surfactant concentration, small tetrahedra or octahedra having the most stable {111} surfaces are formed [24]. In case of low surfactant concentration, the nanoclusters will continuously grow and facilitate the deposition of Au0 onto the {111} faces, leading to formation of less stable structures such as truncated octahedra having both {111} and {100} faces, or cubic nanoparticles that have all of the six planes of {100} type [16,24]. Therefore, to maintain the exis-

C. Cao et al. / Journal of Colloid and Interface Science 322 (2008) 152–157

tence of the structures of {111} type, it is rational to assume that the surfactant molecules should be excessively dominated in the non-seeding synthesis. However, under such a high concentration of surfactant, the initial nonmetallic clusters are strongly protected by surfactant ligands that make the precursors inaccessible to the catalytic growth of reductants [25]. In turn, the inaccessibility will inhibit the particle enlargement, especially if weak reducers are used (e.g. sodium citrate, ascorbic acid, H2 O2 ). Hereafter, we wish to report on a one pot, non-seeding, room temperature, high-yield, and highly reproducible, templatedirected synthesis of convex gold octahedra in aqueous condensed surfactant phase. The octahedral nanoparticles were systematically prepared by reduction of HAuCl4 by ascorbic acid (AA) in high concentration of cetyltrimethylammonium bromide (CTAB) as the stabilizing and templating surfactant. CTAB and AA have been widely utilized to synthesize many shapes of gold nanoparticles by a seed-mediated method due to the fact that AA could reduce CTAB-Au3+ complex to Au+ which could be subsequently reduced to Au0 at the surface of small Au seeds [15,16]. In this case, the Au seeds act as catalysts for the growth of particles, and they decrease the threshold energy of the reaction. We observed that without the Au seeds, gold nanoparticles are hardly to be produced, especially in the densely surfactant-containing solution. In fact, colloidal particles were obtained with very low concentration and extremely slow growth kinetics because AA is a mild reductant while the process was not catalyzed by any pre-synthesized Au seeds in our experiment. Surprisingly, beautiful Au octahedra with very high yield were produced if a small volume of H2 O2 is added to the reaction. In addition to its strong oxidizing property, H2 O2 has been also known as a reducer of gold ions to synthesize gold nanoparticles [26–29]. However, we found that H2 O2 acts as an activator to prompt the reduction–oxidation reactions of AA rather than a direct reducer of HAuCl4 . Mechanism of growth of Au octahedra and the functional relations of AA and H2 O2 in the catalytic process are also discussed in this paper. 2. Materials and methods 2.1. Materials Hydrogen tetrachloroaurate (HAuCl4 ), ascorbic acid (AA), hydrogen peroxide (H2 O2 ) and cetyltrimethylammonium bromide (CTAB) were purchased from Sigma–Aldrich. Ultra pure water (18.2 M cm−1 ) was used throughout the experiments. 2.2. Preparation of gold octahedra by non-seeding method Gold octahedra were systematically prepared by reduction of HAuCl4 using AA with an orientation that concentration of CTAB (templating surfactant) should be maintained excessively. A small amount of H2 O2 was found to be very necessary for the growth of nanocrystals. 11.058 ml solution consisting of final effective concentrations of 1×10−1 M CTAB, 1×10−2 M AA, 1.4 × 10−4 M HAuCl4 , and 0.025% H2 O2 was prepared in a 20 ml vial. The solution was gently mixed by inversion of

153

the vial after the addition of every component. The nanocrystals were fully grown within 3 h. During this period, color of the synthesizing samples was gradually changed from yellow to colorless, and finally violet. To control the uniformity of the octahedra, the solution containing octahedra was sonicated at 40 ◦ C for 90 min using an ultrasonic bath with temperature controlled. CTAB is easily removed from the synthesized nanoparticles by centrifuging at 15,000 rpm at 20 ◦ C for 30 min. The particles were collected and re-dispersed in deionized water. 2.3. Instrumentation The UV–vis absorption spectra were recorded using an Optizen 2120 UV plus spectrophotometer with a 1 nm resolution. High resolution transmission electron microscopy (HR-TEM) and fast Fourier transformation (FFT) measurements were performed on a JEOL model JEM-2100F instrument operated at an accelerating voltage of 200 kV. Samples for TEM were prepared by placing a drop of the gold colloidal solution on a carbon-coated TEM copper grid, and then the mixtures were allowed to dry at room temperature for 10–12 h. Field-emission scanning electron microscopy (FE-SEM) measurements were performed on a JEOL model JSM6700F instrument operated at an accelerating voltage of 15 kV. Samples for SEM were prepared by placing a drop of the gold colloidal solution on a silicon wafer, and then the mixtures were allowed to dry at room temperature for 10–12 h. X-ray diffraction (XRD) pattern was obtained on a Brucker model D8 Discover instrument with CuKα1 radiation (λ = 1.54056). The sample was prepared as same as the SEM sample. 3. Results and discussion 3.1. Synthesis and characterizations of gold octahedra As mentioned earlier, surfactants play a very important role in the kinetic growth of highly faceted shapes. To maintain the existence of the structures of {111} type, it is rational to orient that the templating CTAB molecules should be excessively dominated in the non-seeding synthesis. Therefore, saturated concentration of CTAB was kept throughout the synthesizing process while the concentrations of other reactants were varied to find the best conditions for the reaction. We found that H2 O2 is a-must-be-added-in reagent to synthesize of gold octahedra in this reaction condition. In fact, a large proportion of the particles that have an uniform square dipyramid D4h symmetry with eight equilateral triangles, defined as convex octahedra of Platonic solid (Fig. 1A) [21,30–33], was produced at 1 × 10−1 M CTAB, 1 × 10−2 M AA, 1.4 × 10−4 M HAuCl4 , and 0.025% H2 O2 . As seen, yield of the synthesis is very high (about 86–90%), and the rests are minor by-products comprising of triangular prisms, pentagonal or hexagonal shapes. However, length between two symmetric tips of the octahedra was observed with very high variation ranging from 19 to 60 nm. Therefore, cavitation effect of ultra-

154

C. Cao et al. / Journal of Colloid and Interface Science 322 (2008) 152–157

Fig. 1. HR-TEM images of Au nanoparticles and their corresponding colors under different conditions: (A) gold octahedra synthesized without sonication, and (B) with sonication; (C) FE-SEM image of Au octahedra synthesized with the non-seeding method under sonication; (D) UV–vis absorption spectra for the gold octahedral nanoparticles treated with sonication (red solid line), and without sonication (black dashed line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

sound, which has been previously exploited to fabricate metal nanoparticles [21], was rationally utilized to control the uniformity, or to decrease the size deviation, of the reproducible synthesis. Figs. 1B and 1C show morphology of the sonicated octahedra taken by TEM and SEM, respectively. The yield is obviously improved up to ∼92% of the octahedra with dominant size of about 52.3 nm. Several Au nanoprisms are insignificantly produced during the sonication, they have also been reported as a by-product by others [16,21,24,33]. Fig. 1D shows that the maximum absorption peaks (λmax ) are obtained due to the localized surface plasmon resonance at 559 nm for octahedra synthesized without sonication. On applying sonication to control the size deviation of octahedra, the absorption spectrum is shifted to 555 nm with a sharper peak indicating that their homogeneity is improved in size and shape. As seen in Figs. 2A and 2B, the size distribution of gold octahedral nanoparticles was narrower after being treated with sonication, and it is also well agreed with the UV–vis spectra. HR-TEM images of a single octahedron at different positions are given in Figs. 3A and 3B. As shown, the synthesized octahedron is a quasi-square-dipyramid of eight equilateral triangles because its vertices and edges are slightly rounded. Edge length (a) and height (h) of the illustrated octahedron as measured by TEM are about 34 and 47 nm, respectively, with a relationship that is fully satisfied with geometric calculation for a square dipyramid √ h = 2a. (1) These two parameters of other octahedra also show the same relationship as described by Eq. (1).

Fig. 2. Size distribution histograms of gold octahedral nanoparticles synthesized without sonication (A), and with sonication (B).

Fig. 3. (A) and (B) are HR-TEM images of an individual octahedron at different positions; (C) FFT pattern of the octahedron. (D) was taken at the square area of the particle shown in (A); (E) XRD pattern of the same batch of sample confirming the formation of pure fcc gold octahedra with all 8 facets belonging to {111} family.

Fig. 3D illustrates the magnified image of the square area in Fig. 3A, and it shows the parallel lattice planes of the gold nanocrystal. FFT pattern in Fig. 3C obtained by directing an electron beam perpendicular to the convex tip of individual

C. Cao et al. / Journal of Colloid and Interface Science 322 (2008) 152–157

155

Fig. 5. Kinetic plots of gold plasmon absorbance at 555 nm versus time. Production of gold colloids was performed by AA + [H2 O2 ]0.025% (green dashed line); AA (red line); and [H2 O2 ]0.025% (black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. (A) UV–vis absorption spectra for the gold nanoparticles produced by AA after different time periods; (B) UV–vis absorption spectra of: HAuCl4 (blue solid line); HAuCl4 + [H2 O2 ]0.025% (black dotted line); HAuCl4 + [H2 O2 ]0.025% + Au seeds (red dashed line) in CTAB solutions after 3 h. The maximum absorption bands at 395 nm indicated the presence of ligand-substituted anions, and no typical peaks of gold nanoparticles were observed in these conditions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

octahedron reveals a 4-fold symmetry, and shows that the octahedron is a single crystal. Moreover, the XRD pattern can be utilized to assess the overall quality, purity and crystal structure of the synthesized particles (Fig. 3E). A strong peak from atomic lattices of the octahedra is observed at 2θ = 38.264◦ that can be indexed to (111) Bragg diffraction planes of the fcc gold crystals with a d-spacing of 2.35 Å for adjacent lattice planes, and a lattice constant of 4.071 Å. The values are highly consistent with the Joint Committee on Power Diffraction Standards (JCPDS 04-0784). The other (200) diffraction peak is also observed, however its intensity is much lower than that of (111) peak indicating that all eight facets of the single crystalline octahedra belong to {111} family, and preferential growth of the particles along 111 direction as particle size increases. 3.2. Kinetics of the particle growth Fig. 4A illustrates absorption spectra for the gold nanoparticles after different time periods. As seen, the growth rate of

particles is extremely slow if AA is used as a sole reductant of HAuCl4 . After 2 h 30 min of the colloidal growth, the λmax at 555 nm is about 0.02 and no typical peaks of gold colloids were found. After 4 h 30 min, the gold colloids/or gold seeds were seemingly formed as shown by a raising peak at λ555 nm . The typical peak of gold nanoparticles was only observed after 8 h 30 min of growth at which the λ555 nm was about 0.08 showing that gold colloids/or gold seeds were produced insignificantly. It is because the nonmetallic Au clusters are strongly protected by the surfactant ligands that lead to inaccessibility of AA to the Au ionic clusters in this circumstance [25]. From literatures, it is evident that H2 O2 has been utilized as oxidizing as well as reducing agent to synthesize gold nanoparticles [26–29]. Therefore, possibility of using H2 O2 as the sole reductant of HAuCl4 should be examined to elucidate its function. Fig. 4B shows that H2 O2 could not catalyze for the reduction of HAuCl4 in the high concentration of CTAB. The yellowish color of sample containing [H2 O2 ]0.025% (black dotted line) has a maximum absorption band at 395 nm suggesting the presence of ligand-substituted anions, such as [AuCl3 Br]− , or CTAB-Au3+ complexes, or both [15]. Similar plasmon bands were also obtained for the H2 O2 -free sample (blue solid line) and the seeding sample (red dashed line) in which no typical peaks of gold nanoparticles were seen after 3 h of the colloidal growth indicating that the gold nanoparticles were not produced, regardless of the presence of small gold seeds (5– 6 nm) acting as catalysts (red dashed line). The kinetics of particle growth is shown in Fig. 5 where absorbance of the gold plasmon band at 555 nm was used to monitor the crystallization as a function of time. As clearly seen, the gold colloids are insignificantly produced by AA after 3 h, whereas it is not detectable in case of using [H2 O2 ]0.025% . However, very beautiful gold octahedra were synthesized when a mixture of AA and the amount of H2 O2 were simultaneously introduced into the reaction. The sigmoidal shape of the plot

156

C. Cao et al. / Journal of Colloid and Interface Science 322 (2008) 152–157

shows that the growth of octahedra is initiated by the nucleation phase that happened during 1 h 20 min of the synthesizing reaction. A fast autocatalytic growth took place within next 30 min once the gold nuclei were formed, and followed by the saturation phase. As a result, fully grown gold octahedra were obtained after 2 h 30 min. Therefore, it is rational to say that H2 O2 plays a very important role to induce or to facilitate the formation of gold octahedra in the catalytic reaction. Comprehensively, AA is the main reductant to produce Au octahedra while H2 O2 plays a crucial role of activator to enhance reduction–oxidation reactions of AA, or to decrease the activation energy of the reduction of AuCl− 4 under the saturated concentration of CTAB. 3.3. Proposed mechanism of the particle growth In this synthesis, Au octahedra are crystallized by nucleation and enlargement processes. The nucleation phase is initiated by producing free hydrated electrons from the reducer (AA), and then the electrons are subsequently utilized to reduce the substrate (HAuCl4 ). On the basis of UV–vis absorption spectra and electron microscopic studies, we observed that AA is able to reduce CTAB-Au3+ complex to produce the Au colloids. When AA was added to the CTAB-Au3+ solution, the color of solution changed from yellow to colorless suggesting that the Au3+ complex was reduced to Au+ which could be subsequently reduced to Au0 [15]. However, the kinetics of reaction is extremely slow. This may be because the weak reducer AA releases electrons gradually leading to insufficient electron potential to overcome the activation energy of the reaction. Another possibility, which has also been observed by the previous studies [15,16,25], is that the Au ionic clusters are strongly protected by micro-emulsion sheath of the saturated CTAB ligands that turn the Au clusters inaccessible to the catalytic growth by the hydrated electrons. Therefore, the catalytic activity of AA should be prompted to overcome the energy barrier of the reaction. To accomplish the task, H2 O2 is brought to consideration because of its strong oxidizing property. In a system containing AA and H2 O2 , reduction–oxidation reactions of AA will be rapidly facilitated by H2 O2 until H2 O2 becomes exhausted H2 O2

AA  Dehydro-AA + 2e− .

(2)

Indeed, after the addition of H2 O2 , a color change from colorless (Au+ ) to violet indicated that Au nanoparticles were produced by the reduction of AA. It is because the hydrated electrons from Eq. (2) are continuously produced and accumulated in order to reduce Au ions to produce the gold nanoseeds. Finally, the metallic Au0 seeds act as the nuclei for the deposition of Au ions. In this enlargement process, the Au nuclei are self-catalysts by receiving and transferring the electrons produced from AA to Au ions for their crystalline growth. This process is summarized by the following equations, AA/H2 O2 Au ions + e− −−−−−−→Au0 (seed),

(3) − AA

Au ions + Au0n=1 (seed, atom = 1) + e −→ Au0n=2 .

(4)

Fig. 6. (A) HR-TEM image of rounded-shape decahedra, all the ten facets of the gold decahedra belonging to the {111} family as confirmed by XRD (inset); (B) Kinetics of the decahedal growth in different conditions. In this case, the Au nuclei act as the catalysts to speed up the crystalline growth rapidly, and H2 O2 does not show strong effect on the seeding process.

Above, we have addressed the mechanism in which AA reduces Au ions in condensed CTAB phase, and the introduction of H2 O2 is very crucial to the initial phase of particle growth. The final piece of evidence to support for the proposed mechanism is supplied by a complementary experiment where presynthesized small gold seeds are utilized as catalysts to replace the role of H2 O2 in stimulating catalytic growth of the {111} nanoparticles. 100 µl of gold nanoseeds (5–6 nm) was added to the synthesizing reaction as described in materials and methods. Due to differences in crystallization dynamics between seeding and non-seeding syntheses, Au nanodecahedra which consist of five equal tetrahedral subunits bound together by five twinned (111) planes and five twinned axes have been obtained (Fig. 6A). However, the growth of nanoparticles was much faster than that of Au octahedra, it occurred almost instantaneously and fully saturated after 4 min, regardless of addition

C. Cao et al. / Journal of Colloid and Interface Science 322 (2008) 152–157

of H2 O2 (Fig. 6B). In this case, the gold seeds act as the catalysts to speed up the crystalline growth rapidly, and H2 O2 does not show strong effect on the formation of the gold decahedra in the presence of pre-synthesized Au nuclei. Moreover, the formation of gold nanoparticles was not found if H2 O2 was used as a sole reducer. The complementary result, yet again, confirms the mechanism and the functions of AA and H2 O2 proposed in the non-seeding, template-directed synthesis of the gold octahedra. Recently, gold octahedra with all 8 faces belonging to {111} type have been reported by several research groups using polyol or thermal decomposition process. Luis M. Liz-Marzán and co-workers have reported that gold octahedra or truncated octahedra could be produced by the reduction of HAuCl4 using N,N -dimethylformamide (DMF) as a reducing reagent and poly(vinylpyrrolidone) (PVP) as a stabilizer in the presence of small (2–3 nm) platinum seeds [21]. Sung Oh Cho and co-workers have developed a modified polyol process to selectively synthesize octahedral Au nanocrystals with yield of about 90% [30]. No pre-synthesized seeds were required for the growth of particles but the synthesizing process should be treated at high temperature for a long time (at 125 ◦ C during 48 h). Another modified polyol process has been reported by Hyunjoon Song and co-workers that gold octahedra could be prepared with different amounts of AgNO3 [31]. Gold octahedra have also been synthesized by a thermal decomposition of HAuCl4 in block copolymers at a temperature of 250 ◦ C in air [32]. These references show that most of up-to-date methods have been related to using pre-synthesized seeds, at a high temperature, or in a long time growth. This study is different from the others because of these reasons as follows. Firstly, our method does not require any pre-synthesized seeds to make the monodispersed gold octahedra. Secondly, the octahedral gold nanoparticles are produced in very mild conditions using one-pot reactions, friendly chemicals, at room temperature, and especially in aqueous solution that is supposed to be more favorable for biological/chemical post-processing. Thirdly, the surfactant-templated method is highly reproducible; one can synthesize the gold octahedra with very high yield and high uniformity in a short time. Lastly, we believe that the method with the proposed mechanism is totally applicable to the synthesis of other metal nanoparticles such as silver. 4. Summary Gold octahedral nanoparticles have been successfully prepared in high yield by a one pot, non-seeding, room temperature, high yield, and highly reproducible method in aqueous solution. The present study is interesting not only due to beautiful shape of the gold octahedra but also because of its simplicity and applicability. The main finding of the study is that the most stable surface plane {111} can be perfectly formed in a template-directed environment containing very high concentration of CTAB surfactant using AA as a main reducer, and this is a H2 O2 -dependent process where H2 O2 acts as an activator to prompt the reduction–oxidation reactions of AA rather than

157

a direct reducer of HAuCl4 in the absence of pre-synthesized nuclei. To our best knowledge, the gold octahedral structure has not been observed by others using CTAB as templating reagent. This work, in accompany with previous reports, are practically realizing the chemical synthesis of metal nanoparticles with well-controlled size and shape. Acknowledgments This work is supported by Ministry of Environment as “The Eco-technopia 21 project” and Korea Institute of Environmental Science and Technology (KIEST). References [1] S.A. Maier, M.L. Brongersma, P.G. Kik, S. Meltzer, A.A.G. Requicha, H.A. Atwater, Adv. Mater. 13 (2001) 1501. [2] Y. Cui, M.T. Bjork, A. Liddle, C. Sonnichsen, B. Boussert, A.P. Alivisatos, Nano Lett. 4 (2004) 1093. [3] G. Schon, U. Simon, Colloid Polym. Sci. 273 (1995) 202. [4] P.V. Kamat, J. Phys. Chem. B 106 (2002) 7729. [5] L.N. Lewis, Chem. Rev. 93 (1993) 2693. [6] R. Narayanan, M.A. El-Sayed, J. Phys. Chem. B 109 (2005) 12663. [7] S.R. Nicewarner-Pena, R.G. Freeman, B.D. Reiss, L. He, D.J. Pena, I.D. Walton, R. Cromer, C.D. Keating, M.J. Natan, Science 294 (2001) 137. [8] Y.W.C. Cao, R.C. Jin, C.A. Mirkin, Science 297 (2002) 1536. [9] J.M. Thomas, Pure Appl. Chem. 60 (1988) 1517. [10] C.R. Yonzon, D.A. Stuart, X. Zhang, A.D. McFarland, C.L. Haynes, R.P. Van Duyne, Talanta 67 (2005) 438. [11] A.J. Haes, R.P. Van Duyne, Anal. Bioanal. Chem. 379 (2004) 920. [12] A.J. Haes, D.A. Stuart, S. Nie, R.P. Van Duyne, J. Fluoresc. 14 (2004) 355. [13] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2003) 668. [14] O.M. Bakr, B.H. Wunsch, F. Stellacci, Chem. Mater. 18 (2006) 3297. [15] C.J. Johnson, E. Dujardin, S.A. Davis, C.J. Murphy, S. Mann, J. Mater. Chem. 12 (2002) 1765. [16] T.K. Sau, C.J. Murphy, J. Am. Chem. Soc. 126 (2004) 8648. [17] M.T. Reetz, W. Helbig, J. Am. Chem. Soc. 116 (1994) 7401. [18] S.S. Shankar, A. Rai, B. Ankamwar, A. Singh, A. Ahmad, M. Sastry, Nat. Mater. 3 (2004) 482. [19] Y. Sun, Y. Xia, Science 298 (2002) 2176. [20] Z.L. Wang, J. Phys. Chem. B 104 (2000) 1153. [21] A.S. Iglesias, I.P. Santos, J.P. Juste, B.R. Gonzalez, F.J.G. de Abajo, L.M.L. Marzán, Adv. Mater. 18 (2006) 2529. [22] C.L. Nehl, H. Liao, J.H. Hafner, Nano Lett. 6 (2006) 683. [23] C.H. Kuo, T.F. Chiang, L.J. Chen, M.H. Huang, Langmuir 20 (2004) 7820. [24] J.M. Petroski, Z.L. Wang, T.C. Green, M.A. El-Sayed, J. Phys. Chem. B 102 (1998) 3316. [25] Y. Xiao, B. Shlyahovsky, I. Popov, V. Pavlov, I. Wilner, Langmuir 21 (2005) 5659. [26] T.K. Sarma, D. Chowdhury, A. Paul, A. Chattopadhyay, Chem. Commun. 10 (2002) 1048. [27] T.K. Sarma, A. Chattopadhyay, Langmuir 20 (2004) 3520. [28] M. Zayats, R. Baron, I. Popov, I. Willner, Nano Lett. 5 (2005) 21. [29] Y. Jin, P. Wang, D. Yin, J. Liu, L. Qin, N. Yu, G. Xie, B. Li, Colloids Surf. A 302 (2007) 366. [30] C. Li, K.L. Shuford, Q.H. Park, W. Cai, Y. Li, E.J. Lee, S.O. Cho, Angew. Chem. Int. Ed. 46 (2007) 3264. [31] D. Seo, J.C. Park, H. Song, J. Am. Chem. Soc. 128 (2006) 14863. [32] J. Zhang, Y. Gao, R.A. Alvarez-Puebla, J.M. Buriak, H. Fenniri, Adv. Mater. 18 (2006) 3233. [33] F. Kim, S. Connor, H. Song, T. Kuykendall, P. Yang, Angew. Chem. Int. Ed. 43 (2004) 3673.