Nearly Complete Oxidation of Au° in Hydrophobized Nanoparticles to

Sep 8, 2007 - Au° atoms in hydrophobized gold nanoparticles were oxidized to Au3+ ions, nearly quantitatively, simply by treatment with N-bromosuccin...
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14348

J. Phys. Chem. C 2007, 111, 14348-14352

Nearly Complete Oxidation of Au° in Hydrophobized Nanoparticles to Au3+ Ions by N-Bromosuccinimide Sanjay Singh and B. L. V. Prasad* Materials Chemistry DiVision, National Chemical Laboratory, Pune - 411 008, India ReceiVed: May 15, 2007; In Final Form: July 3, 2007

Au° atoms in hydrophobized gold nanoparticles were oxidized to Au3+ ions, nearly quantitatively, simply by treatment with N-bromosuccinimide and sonication. 1H NMR results indicate that the octadecylamine molecules are detached from the Au° surface by NBS. The bromine molecules released by NBS are suggested to be the species responsible for the oxidation of Au° to Au3+, which is supported by the observation that addition of molecular bromine also leads to similar results. A gratifying feature is that the Au3+ ions could be subsequently reduced back to Au nanoparticles.

Introduction Synthesis of metal nanoparticles constitutes one of the most active areas of research these days.1 While the aqueous mediumbased methods have been reported and are being practiced for many decades now,1c preparation of gold nanoparticles in an organic medium is gaining attention which is pioneered by Brust et al.2 Apart from the Brust protocol methods such as reverse micelle synthesis,3 digestive ripening procedure4 and phase transfer of Au nanoparticles from aqueous medium to organic medium5 are also in vogue for obtaining these so-called monolayer protected gold nanoparticles (Au NPs). These are envisaged to be important for a vast variety of applications inclusive of catalysis,6 optoelectronic devices,7 sensors,8 diagnostic devices, etc.9 One very important attribute of these Au NPs is that they can be obtained in powder form and redispersed in solvents as desired.2-4 While the number of reports of the synthesis of Au NPs is increasing on one hand, the oxidative stability of these nanoparticles is also receiving keen interest from researchers these days.10 This is important, not only to understand the factors governing the stability of nanoparticles under various conditions but also assumes significance, as oxidation can provide a means of recycling the unused batches of hydrophobized nanoparticles. Recycling is also important because of the threat that powdered forms pose when these nanoparticles are airborne.11 It is well-known that metallic gold even in the bulk form can be dissolved either in very strong acids such as aquaregia or cyanides12 and obtain the corresponding gold salts are obtained. The same holds for Au nanoparticles also. However, both aquaregia and cyanides need special precautions to handle. With this premise it would be interesting if we could use routinely available laboratory oxidizing agents to oxidize Au° atoms in nanoparticles to Au3+ ions. These then could be reduced again to get a fresh batch of nanoparticles if desired. Accordingly in this paper the nearly quantitative oxidization of octadecylamine (ODA)-capped gold nanoparticles using a common oxidizing agent N-bromosuccinimide (NBS) to Au3+ ions is presented, and it is also shown that these can subse* Author to whom correspondence should be addressed. E-mail: [email protected], phone: +91 20 25902013, fax: +91 20 25902636.

quently be reduced to Au nanoparticles again. The choice of NBS was based on literature reports that halides can induce shape transformations in Au nanoparticles, especially dispersed in aqueous media, which is attributed to the lattice strain caused on the Au (111) surfaces by the adsorbed layers of halides.13 As mentioned in a previous report when we tested different N-halosuccinimides, NBS was not only able to transform the shapes of organically dispersed nanoparticles14 but at appropriate concentrations lead to the results reported here. Similar results could be obtained with molecular bromine as well but keeping the difficulty in handling molecular bromine compared to NBS, only the results with NBS treatment are presented here. Experimental Section Synthesis. To 500 mL of 1 × 10-4 M of choloauric acid was added 0.05 g of sodium borohydride under vigorous stirring conditions. The solution immediately turned burgundy red in color. This solution was aged for 24 h in order to ensure complete reaction. This solution of gold nanoparticles (500 mL) was then shaken with 50 mL of 10-3 M ODA in chloroform. This results in the transfer of Au NPs from aqueous to chloroform phase and is seen as a transfer of burgundy red color from water to chloroform. After the completion of phase transfer, the organic phase was separated from the aqueous phase. UVvis-NIR absorbance spectra were recorded from this solution, which shows the appearance of a surface plasmon resonance band at about 520 nm, which is characteristic feature of spherical Au NPs. The aqueous phase turns colorless, and no feature around 520 nm is observed with UV-vis spectral analysis, indicating that the above-mentioned procedure leads to complete phase transfer of Au NPs from aqueous phase to CHCl3 phase. Analysis. UV-vis spectra was monitored on Jasco V-570 UV\Vis\NIR spectrophotometer operated at a resolution of 2 nm. TEM measurements were done on a JEOL model 1200EX instrument operated at an accelerating voltage of 80 kV. X-ray photoemission spectroscopy (XPS) measurements of films HAuCl4 and NBS-treated hydrophobized AuNPs cast on to silicon wafers were carried out on a VG Microtech ESCA 3000 instrument at a pressure better than 1 × 10-9 Torr. The general scan and C1s, Au4f, Cl2p, and Br3d core level spectra were recorded with un-monochromatized Mg KR radiation (photon energy ) 1253.6 eV) and electron takeoff angle (angle between

10.1021/jp073717i CCC: $37.00 © 2007 American Chemical Society Published on Web 09/08/2007

Oxidation of Au° in Hydrophobized Nanoparticles

J. Phys. Chem. C, Vol. 111, No. 39, 2007 14349

Figure 1. (A) Optical spectra of ODA-capped Au NPs in CHCl3 (curve 1), NBS-treated Au NPs (curve 2), and pure NBS in CHCl3 (curve 3). The corresponding images of the sample vials are given in the inset. (B) Optical spectra of solution obtained by shaking NBS-treated Au NPs in CHCl3 with water (solid line), HAuBr4 (dotted line), and that of the product formed by adding NBS to ODA (dashed line). All the spectra are taken after transferring the respective products into water.

electron emission direction and surface plane) of 60°. The overall resolution was ∼1 eV for the XPS measurements. The core level spectra were background corrected using the Shirely algorithm,15 and the chemically distinct species were resolved using a nonlinear least-square fitting procedure. The core level binding energies (BE) were aligned with the adventitious carbon binding energy of 285 eV. Atomic absorption spectroscopy (AAS) studies were performed on an AAS 201 Chemito instrument the following way. A 8 mL batch of 1 × 10-3 M ODA-capped Au NPs in chloroform was made as explained earlier. Of this amount, 4 mL was kept aside and to another 4 mL was added freshly recrystallized NBS solid so that the final NBS concentration in solution was 10-2 M. The yellow solution obtained was then shaken with 4 mL of water. The water layer immediately turned yellow, this water layer was separated and subjected to AAS analysis along with the chloroform layer containing ODA-capped Au NPs, and the Au content was estimated. 1H NMR spectra were obtained in the following way. CHCl3 from a 10 mL batch of freshly phase-transferred Au NPs was removed by rotary evaporation. A known weight of this powder was taken and dispersed in 1 mL of CDCl3 to give a clear burgundy red colored solution of 1 × 10-3 M Au NP solution. Two such batches were prepared. To this was added NBS so that the final concentration of NBS was 1 × 10-2 M. One batch was subjected to UV-vis analysis, and just when the complete oxidation of Au° was observed, the second batch of solution was subjected to 1H NMR analysis. 1H NMR spectra of octadecylamine, NBS, and octadecylamine-capped Au NPs were also recorded for comparison. All these were solubilized/ dispersed in CDCl3. The 1H NMR spectra were recorded on a 400 MHz Bruker NMR spectrometer. Results and Discussion NBS was freshly recrystallized and was added to the solution of 1 × 10-3 M ODA-capped Au NPs in chloroform such that its final concentration is 1 × 10-2 M (Au:NBS ) 1:10). Figure 1A presents the optical absorbance spectra of Au NPs (curve 1) overlaid with those treated with NBS (curve 2) and pure and freshly recrystallized NBS in chloroform (curve 3). The image of the corresponding sample vials is given in the inset. Au NPs display (vial 1, Figure 1A inset) a beautiful burgundy red color and show a prominent peak around 520 nm in the electronic

spectra attributed to the surface plasmon resonance.16 Immediately after treatment with NBS this color is lost and a brownish yellow color develops. Upon intermittent sonication over 30 min this finally turns to a clear yellow color solution (vial 2, Figure 1A inset)). No precipitate could be seen in the reaction vessel, indicating that the Au NPs have reacted with NBS, and this is not a salting out effect where the Au NPs would have simply precipitated out. The optical absorbance spectra clearly support this with complete absence of 520 nm peak upon NBS treatment of Au NPs. Instead a distinct peak around 330 nm with an appreciable shoulder centered around 460 nm developed (solid line in Figure 1B). The important point to note here is that NBS does not have any absorbance in this region (and a freshly recrystallized sample looks white and a solution in chloroform is colorless vial 3, Figure 1A inset). However, under ambient conditions, NBS does develop brownish color in about 12 h arising from release of free bromine. Nevertheless, this solution has its absorbance at ∼440 nm (Figure S1, Supporting information), clearly suggesting that the peaks in curve 2 are probably due to the presence of Au3+ ions and other reaction products due to the reaction between ODA-capped Au NPs and NBS. TEM images of ODA-capped Au NPs clearly reveal the presence of nicely ordered particles (Figure 2, Supporting information). On the other hand, the drop-casted film of NBS-treated Au NPs hardly displayed any regions with dark contrast, asserting that almost all the nanoparticles have disappeared. To further appreciate the oxidation of Au° to Au3+ by NBS, the yellow colored solution obtained after NBS treatment of Au NPs was shaken with water, and it was observed that the water layer immediately became yellow, suggesting the phase transfer of bromoaurate ions that could have formed upon NBS treatment. The UV-vis spectra of this yellow aqueous layer (solid line) is overlaid with the spectra obtained from HAuBr4 solution obtained by the addition of HBr to HAuCl4 (dotted line) and that of the products formed by adding NBS to pure ODA (dashed line) in Figure 1B. Two major peaks can be seen in the spectra denoted by a solid line in Figure 1B, one at 315 nm and another centered at 380 nm. The peak centered at 380 nm is in complete agreement with earlier reports where it has been allied to the formation of AuBr4- ions,17 and our results from HAuBr4 (formed when excess HBr is added to HAuCl4)

14350 J. Phys. Chem. C, Vol. 111, No. 39, 2007

Singh and Prasad

Figure 2. (A) XPS spectra from Au 4f core levels obtained from HAuCl4 and (B) the same recorded from NBS-treated Au NPs. (C) XPS spectra from Cl 2p (fitted to two spin-orbit pairs of 2p3/2) and (D) Br 3d core levels (fitted to two spin-orbit pairs of 3d5/2) obtained from HAuCl4 and NBS-treated Au NPs, respectively. The recorded spectra of Br 3d core level (inset in C) from HAuCl4 sample and the Cl 2p spectra recorded from NBS-treated Au NPs after phase transfer to water (inset in D) are also displayed (see text for details).

also support this (dotted line in Figure 1B). The peak around 315 nm could be coming from NBS+ODA products, and indeed when we recorded the spectra of a 1:1 mixture of (1 × 10-3 M) of ODA and NBS (dashed curve in Figure 1B), a peak centered around 310 nm is seen grossly matching the peak at 315 nm observed in the product of NBS added to Au NPs. We wish to point out here that NBS addition to ODA-capped Au NPs results in products different than that formed when ODA is simply added to NBS (Vide infra), and that could explain the small differences in the position of these two peaks. The most important point, however, is that in these spectra no features around 380 nm are observed, giving credence to the conclusion that the 380 peak comes from AuBr4- ions. A conclusive proof for the complete oxidation of Au° to Au3+ comes from XPS studies. The Au 4f XPS spectra obtained (Figure S3, Supporting Information) from the ODA-capped Au NPs is broadly in agreement with the earlier reports.19e The XPS spectra could be fit to two spin-orbit pairs with binding energies 83.5 and 85.2 eV corresponding to the Au0 and Au+ states. While in the earlier reports19e it has been suggested that the Au nanoparticles are covered by AuCl4- or AuCl2- ions no signal from the respective Au3+ or Au+ ions could be detected. Figure 2A displays the XPS spectra of Au 4f core levels from HAuCl4 (commercially obtained sample), and Figure 2B displays the same core level spectra obtained after treating the Au NPs with NBS and phase transferring to water. In both cases the 4f7/2 levels could be deconvoluted into two spin-orbit pairs with splitting energies of 3.65 eV. In HAuCl4 the binding energies for the two pairs are deduced to be 85.3 and 87.7 eV, while in Au NPs treated with NBS the two peaks appear at 84.7 and 86.8 eV. The presence of two pairs in each case is attributed to the presence of both Au(I) and Au(III) species respectively. Gold is one of the most easily reducible metals, and thus the presence of Au(I) could be attributed to the partially reduced Au(III) either due to the exposure to surrounding atmospheric conditions

during the sample preparation or the X-rays during XPS measurement. We suggest this possibility against a partial oxidation of Au° to Au(I) by NBS keeping the presence of Au(I) in the spectra of commercially obtained HAuCl4 in view. Similar observations were made in earlier detailed studies on different halogen-gold complexes.18 However, the absence of any peak corresponding to Au° at 83.5 eV clearly points to the complete oxidation of Au°. NBS treatment of Au NPs is expected to form AuBr3, which probably gets converted to HAuBr4 while phase transferring to the aqueous media. The small difference in the binding energies in the two cases studied here (HAuCl4 and NBS-treated Au NPs) could be related to the differences of electronegativites of chlorine and bromine. The lower panel of Figure 2 displays the spectra obtained from HAuCl4 that could be fit to the two spin-orbit pairs of Cl 2p3/2 (Figure 2C). The curves in Figure 2D are from HAuBr4 (resulting from the NBS treatment of Au NPs) and could be fitted to two spin-orbit pairs of Br 3d5/2. As expected, there were no peaks observed from Br 3d levels in HAuCl4 (inset Figure 2C). The interesting point to note here is the absence of peaks from Cl 2p levels in NBS-treated Au NPs (inset Figure 2D). This clearly exemplifies that after NBS treatment and subsequent phase transfer to water the compound formed is HAuBr4 and hence there is no trace of chlorine. The presence of two spin-orbit pairs in bromine and chlorine peaks could be also ascribed to the binding of these species to both Au(I) and Au(III) or the presence of free Cl2 and Br2. Then, in order to get some quantitative estimates of how much Au° is getting converted to Au3+ and getting phase transferred to water, we did atomic absorption spectroscopy (AAS) studies. Here, a 1 × 10-3 M 4 mL CHCl3 solution of ODA-capped Au NPs yielded ∼108 ppm of Au corresponding to 8 × 10-4 M concentration (considering that density of CHCl3 is 1.498), very close to what we have taken. Determination of Au content after NBS reaction with ODA-capped Au NPs and the subsequent

Oxidation of Au° in Hydrophobized Nanoparticles

Figure 3. 1H NMR spectrum of octadecylamine capped on a gold nanoparticle surface (Spectrum 1) and 1H NMR spectrum obtained after treating octadecylamine-capped Au NPs with N-bromosuccinimide (Spectrum 2). Molecular structures of two reaction products (apart from octadecylamine) that could have formed in this reaction are shown in the inset.

phase transfer to 4 mL water yielded ∼121 ppm that corresponds to 6 × 10-4 M. Thus it is clear that, within experimental error, NBS is able to oxidize the Au° atoms in Au NPs to Au3+ ions nearly quantitatively. While the above experimental results clearly indicate the oxidation of Au° to Au3+, the fate of NBS and the other reaction products still remains unclear. To understand this, 1H NMR spectra of all the reagents and the reaction products were analyzed. Spectrum 1 in Figure 3 corresponds to the ODAcapped Au NPs, while spectrum 2 is obtained just after all the Au° has been oxidized to Au3+, as confirmed by UV-vis spectroscopy, and the details are provided in the Experimental Section. The main features that stand out when the spectrum of ODA-capped Au NPs is compared with pure ODA spectrum (Figure S4, Supporting Information) are the vanishing peak at 1.20 ppm and broadening of peaks corresponding to the R (∼2.7 ppm) and β (∼1.45 ppm) CH2 groups. Similar features have been observed earlier and ascribed to the proximity of this group to the metal surface.19 This along with the fact that the initially hydrophilic Au NPs are rendered hydrophobic by stirring with ODA confirms the capping of Au NPs by ODA molecules through the NH2 group. Dramatic changes in the 1H NMR spectra can be seen once the oxidation of Au° to Au3+ is complete. The spectrum obtained reveals several peaks from the different reaction products overlapping with each other. However, the main features that can be discerned are that all the peaks corresponding to free ODA seem to have been regenerated, indicating that the ODA molecules are no longer attached to the gold surface. The major reaction product coming from the NBS side is succinimide (structure A, inset Figure 3) as indicated clearly by a broad peak at ∼8.5 ppm corresponding to the imide proton, suggesting the release of bromine from this reagent. Note that this peak is not observed in the 1H NMR spectrum of freshly crystallized NBS (Figure S5, Supporting Information), signifying that the succinimide is formed after the reaction with Au° and not present initially. Several other peaks of low intensity can be seen between 1.5 and 2.5 ppm, 3.0-3.5 ppm, and 4.0-5.5 ppm. These can be attributed to a molecule formed (molecular structure shown in structure B of Figure 3 inset) by covalent linkage of an opened structure of succinimide with octadecylamine and is well supported by distinct peaks attributable to the C(dO)-NH2 protons and the C(dO)-NH protons at 5.16 and 7.75 ppm, respectively. An

J. Phys. Chem. C, Vol. 111, No. 39, 2007 14351 expanded spectrum revealing these details is provided as Figure S6, Supporting Information. However, it is not clear at this moment whether this molecule is formed after the oxidation of Au° to Au3+ or during the oxidation. Time-dependent 1H NMR studies can probably reveal more details about this product, and efforts are currently underway in our laboratory to such effect. Nevertheless, mere addition of NBS to ODA does not give either succinimde or the products that were formed as described above (Figure S7, Supporting Information), signifying that these reactions happen only in the presence of Au NPs. Finally to see whether it would be possible to obtain Au NPs again from the Au3+ obtained via NBS oxidation, the chloroform solution obtained after NBS treatment has been exposed to vapors coming from hydrazine hydrate, and almost immediately the characteristic burgundy color of the Au NPs appeared exemplifying that through our procedure we could obtain Au3+ ions which could be reduced back to Au NPs if desired. The TEM grid prepared from this resynthesized nanoparticles again presents many regions loaded with Au NPs (Figure S8, Supporting information). We must admit here that the size control in the rereduction is not so good at this time but probably can be improved with the removal of side products present and by including appropriate capping agents during the rereduction. We also wish to mention here that almost similar color changes and rereduction trends were observed when thiol-capped gold nanoparticles prepared by the digestive ripening method4 were used instead of amine-capped nanoparticles, indicating the generality of this procedure. As mentioned earlier, Au NPs when treated with 10-4 M NBS remained suspended in solution albeit with a change in shape.14 On the other hand, Au NPs treated with N-iodosuccinimide got irreversibly aggregated.14 It has been suggested that upon addition of NBS, NIS, Br2, and I2 to Au NPs, atomic halogen species are generated from the above reagents. Many electrochemical studies on NBS in aprotic solvents also support this.20 These strip off the amine-capping agent and get adsorbed on the nanoparticle surfaces, probably forming ionic species by oxidizing the Au atoms on the surface. It is then possible that during sonication, the surface layer of gold(III) bromide is removed into the solution and generates a fresh surface of Au° on the nanoparticle. This surface is again attacked by the bromine molecules, and this process continues until all the Au atoms of the nanoparticle are consumed, leading to their complete oxidation. The redox potentials of Br2 + 2e- f 2Br(E° ) +1.09) and AuBr4- + 3e- f Au°+4Br- (E° ) +0.854)21 are favorable for a spontaneous reaction between Br2 and Au° leading to AuBr4-. Support for this hypothesis also comes from the fact that molecular bromine is also capable of bringing about similar oxidation of Au° to Au3+. On the other hand, iodide is known to form the compound AuI, and this AuI possibly makes a protective shield on the Au surface, leading to aggregation and precipitation hence hampering the complete oxidation. Conclusions A simple route to the nearly quantitative oxidization of octadecylamine (ODA)-capped gold nanoparticles using a common oxidizing agent N-bromosuccinimide (NBS) to Au3+ ions is presented. It has been shown that the molecular bromine species released probably strip off the ODA molecules from the surface of Au NPs and react with the surface Au° atoms to form gold(III) bromide and removed to the solution, further exposing the new Au° surfaces to fresh bromine molecules finally leading to complete oxidation of Au° to Au3+. It is also shown that these Au3+ ions can subsequently be reduced to Au

14352 J. Phys. Chem. C, Vol. 111, No. 39, 2007 nanoparticles again, in the organic solvent itself, thus making this process useful for the recycling of nanoparticles. Acknowledgment. S.S. thanks the CSIR for a fellowship. This work is partially funded by the DST through DSTUNANST scheme for NCL and a fast track scheme for young scientists to B.L.V.P. and is gratefully acknowledged. We also thank Dr. C. V. Ramana and Dr. T. Ajithkumar, NCL, Pune, for many fruitful discussions and help with NMR spectra. Supporting Information Available: UV-vis spectra of NBS kept at ambient conditions for a long time, TEM images of as-prepared and rereduced Au NPs, 1H NMR spectra of octadecylamine and freshly recrystallized NBS, the expanded spectrum of the reaction products obtained by the treatment of NBS with ODA-capped Au NPs, and the 1H NMR spectra of products obtained by adding NBS to pure ODS. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. ReV. 2004, 104, 3893. (b) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Soc. ReV. 2000, 29, 27. (c) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (3) (a) Arcoleo, V.; Liveri, V. T. Chem. Phys. Lett. 1996, 258, 223. (b) Chen, F.; Xu, G.-Q.; Hor, T. S. A. Mater. Lett. 2003, 57, 3282. (4) (a) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515. (b) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 2003, 15, 935. (5) (a) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001. (b) Corma, A. Chem. ReV. 1997, 97, 2373. (c) Kumar, A.; Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2003, 19, 6277. (6) (a) Chen, M.; Goodman, D. W. Acc. Chem. Res. 2006, 39, 739. (b) Bond, G. C.; Sermon, P. A.; Webb, G.; Buchanan, D. A.; Wells, P. B. J. Chem. Soc., Chem. Commun. 1973, 11, 444. (c) Sermon, P. A.; Bond, G. C.; Wells, P. J. Chem. Soc., Faraday Trans. 1 1979, 75, 385. (d) Cha,

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