Synthesis of Stable, Low-Dispersity Copper Nanoparticles and

Aug 25, 2010 - 2145 Sheridan Road, EVanston, Illinois 60208. ReceiVed: June 17, 2010; ReVised Manuscript ReceiVed: July 31, 2010. Low-polydispersity ...
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Synthesis of Stable, Low-Dispersity Copper Nanoparticles and Nanorods and Their Antifungal and Catalytic Properties Yanhu Wei, Steven Chen, Bartlomiej Kowalczyk, Sabil Huda, Timothy P. Gray, and Bartosz A. Grzybowski* Department of Chemical and Biological Engineering, Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208 ReceiVed: June 17, 2010; ReVised Manuscript ReceiVed: July 31, 2010

Low-polydispersity copper nanoparticles (NPs) and nanorods (NRs) were synthesized by thermal decomposition of copper(II) acetylacetonate precursors in the presence of surfactants. Exchange of weakly bound alkylamine ligands for alkanethiols increased the stability of the NPs and, depending on the thiols’ terminal functionality, rendered them soluble in organic solvents or in water. The water-soluble nanoparticles stabilized with positively charged thiols exhibited long-term (months) stability and antifungal properties. The NPs and NRs stabilized with weakly bound alkylamine ligands are catalytically active in alkyne coupling reactions. Introduction The ability to synthesize nanostructures of controlled sizes and shapes is at the heart of modern nanotechnology and is important for their applications in optics, biodetection, and catalysis.1-11 While for most noble metals (Au,12 Ag,12a,13 Pt,14 Pd15) various methods are known that yield monodisperse nanoparticles and other types of nanostructures (rods,16 plates,17 cubes,18 wires,19 multibranches20), this nanosynthetic repertoire is not as broad for copper particles. Interest in copper nanostructures is spurred by their useful electrical, catalytic, optical, and antifungal/antibacterial properties.21-26 However, synthesis of stable and low-polydispersity Cu nanoparticles (CuNPs) has proven challenging mainly because of the rapid air oxidation of metallic copper to Cu2+ ions or CuxO oxides (x ) 1 or 2).27 Consequently, most of the existing methods of CuNP synthesis produce particles of large polydispersity and/or limited stability (see refs 27a and 28 and also Supporting Information, section 1, for comparison with several recent methods). For copper nanorods (NRs), various methods based on template-based reduction/electrochemical deposition,27b,29 vacuum vapor deposition,30 structure-directing surfactants,31 and hydrothermal reduction31b,32 were reported, but many of these procedures suffer from the same limitations as those for CuNP synthesis (poor size and shape control, low stability). Here, we report a straightforward procedure based on the thermal decomposition of precursor Cu salts27b in the presence of oleylamine (OAM) that yields low-polydispersity CuNPs or CuNRs of various sizes. The Cu nanostructures prepared by this route can be further functionalized by exchanging weakly bound surfactants to tighter binding alkanethiols. This functionalization allows for the control of surface chemistry and solubility of CuNPs/CuNRs in various solvents and yields nanostructures that are stable in solution for prolonged periods of time. Notably, copper nanoparticles functionalized with positively charged thiols are stable in water for several months, during which they exhibit excellent antifungal properties. On the other hand, Cu NPs and NRs stabilized with weaker binding oleylamine ligandssthough less * Corresponding author: e-mail: [email protected], Tel: (+1)847491-3024.

stable than their thiol-protected counterpartssare catalytically active in alkyne coupling reactions. Experimental Section Materials. Copper acetylacetonate (Cu(acac)2), iron(0) pentacarbonyl (Fe(CO)5), oleylamine (OAM), oleic acid (OA), 1,2tetradecanediol, 1-hexadecanethiol, 3,4-dihydroxyhydrocinnamic acid, ascorbic acid, phenyl ether, and 1-octadecene were purchased from Sigma-Aldrich. Hydrogen tetrachloroaurate trihydrate (HAuCl4 · 3H2O) was purchased from DF Goldsmith. N,N,N-Trimethyl(11-mercaptoundecyl)ammonium chloride (TMA, SH(CH2)11N(CH3)3Cl) was a generous gift of ProChimia Surfaces (Gdansk, Poland). All solvents and reagents were used without further purification. Synthesis of CuNPs and NRs. In a typical experiment, 0.125 mmol of copper acetylactonate, Cu(acac)2, was dissolved in 5 mL of phenyl ether and 6 mL of OAM. To this so-called royal blue solution 2 mmol of 1,2-tetradecanediol was added. The solution was stirred and degassed by three vacuum pump/backfill cycles under Ar. The temperature of the solution was slowly increased (at ∼2 °C/min) over 1 h to 155 °C and held there for another hour. Afterward, the solution was allowed to cool to room temperature, and the particles were precipitated from phenyl ether with ethanol and redispersed in toluene or hexane for further use. Cu nanorods were obtained by the same route as CuNPs, except that 0.1 equiv of dodecylammonium bromide, DDAB, was added as structure-directing surfactant. Culturing of Stachybotrys Chartarum Fungus. A strain of S. chartarum (isolated from a house not associated with idiopathic pulmonary hemorrhage, Cleveland, OH) was obtained from ATCC and cultured on a solid media composed of cornmeal extract, 5% V8 juice, 3 g/L CaCO3, and 7.5 g/L bacteriological agar as solidifying agent with pH adjusted to 5.6-6.0 at 25 °C (ATCC medium 309). Minimum Inhibitory Concentration Assay. After the growth medium (20 mL per plate) was autoclaved for sterilization, it was mixed with CuNPs of concentrations 0.01-2 mM (in terms of metal atoms) and allowed to solidify. Colonies of S. chartarum were picked using a cotton swab and were added to sterile water and then brought to a concentration of roughly 107 CFU. Next, 100 µL of this suspension was spread uniformly

10.1021/jp1055683  2010 American Chemical Society Published on Web 08/25/2010

Low-Dispersity Cu Nanoparticles and Nanorods

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Figure 1. TEM images of Cu nanoparticles prepared with (a) χ ) 9 (d ) 34 nm, σ ) 32%), (b) χ ) 25 (d ) 27 nm, σ ) 27%), (c) χ ) 50 (d ) 21 nm, σ ) 18%), and (d) χ ) 100 (d ) 9 nm, σ ) 15%). (e) UV-vis spectra of CuNPs in toluene: 1, freshly prepared CuNPs coated with OAM; 2, Cu/OAM NPs stored under argon for 10 days; 3, freshly prepared CuNPs protected with 1-undecanethiol; 4 and 5, CuNPs coated with 1-undecanethiol and stored under argon for 10 and 20 days, respectively. (f) A representative TEM image of CuNPs coated with 1-undecanethiol after storing in toluene under argon for several weeks. (g) UV-vis spectra of freshly prepared CuNPs in H2O (solid curve) and the same NPs stored in water for 2 months (dotted curve). (h) A representative image of Cu/TMA NPs stored in water for two months. (i) The HRTEM image of alkylthiol-coated CuNPs (synthesized at χ ) 100) stored in toluene under argon for 2 weeks. The distance between two adjacent lattice planes is 2.2 Å. Scale bars in (a) and (b) are 50 nm, those in (c), (d), (f), and (h) are 20 nm, and that in (i) is 2 nm.

onto each of the medium/CuNPs plates and left in an incubator at 27 °C for 72 h to grow. The lowest concentration of copper nanoparticles which exhibited no mold growth was determined as the minimum inhibitory concentration (MIC). Upon determining the MIC of copper nanoparticles, different ratios of latex paint to the MIC (1:1, 1:2, 1:5, 1:10, 1:20) were mixed with the MIC of copper nanoparticles and the growth medium, and the MIC assay was repeated. This was done to test whether the addition of paint would affect the MIC. Disk Diffusion Assay. Suspension of S. chartarum (100 µL of ∼107 CFU/mL) in sterile water was applied uniformly on the surface of the growth medium plate. Wells (1 cm in diameter) were hollowed out in each of the growth-medium agar plates tested and served as reservoirs for loading copper nanoparticle suspensions (concentrations in terms of Cu atoms 0.06, 0.08, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, and 1 mM) mixed with a commercial latex paint. The plates were then left in an incubator at 27 °C for 48 h to grow. The zone of inhibition (ZoI) surrounding the hole was determined on the basis of five measurements for each CuNP concentration studied. Cu NP-Catalyzed Alkyne Coupling Reaction. For experimental details see section 3 of the Supporting Information. Results and Discussion Copper NPs were synthesized in phenyl ether using Cu(acac)2 as a salt precursor, oleylamine (OAM) as a surfactant, and 1,2tetradecanediol as a reducing agent. The solution was degassed and stirred while its temperature was raised at a rate of ∼2 °C/

min to 155 °C and was held there for an additional 1 h. During heating, the color of the reaction mixture changed from blue to dark red, signifying the formation of CuNPs. The diameters, d, and the polydispersities, σ, of the CuNPs depended on the ratio, χ, of OAM to Cu(acac)2. TEM images of particles prepared at four different values of χ are shown in Figure 1a-d. The surface plasmon resonance (SPR) band of freshly prepared CuNPs was at ∼590 nm (Figure 1e) and did not shift much upon the size change from 34 to 9 nm (Figure 1a-d). Both d and σ decreased with increasing χ, and the polydispersity was as low as 15% (for d ) 9 nm NPs at χ ) 100). The OAM surfactant can play two roles in the synthesis: (i) as complexing agent with copper ion and (ii) as capping agent for stabilizing formed particles. Here, the capping action likely dominates and causes the decrease of the particle size with an increase of the OAM concentration. However, these surfactant-coated NPs were stable for only short time. For instance, the color of toluene/hexane CuNP solution (degassed and under Ar) changed gradually from dark brown-red to blue within 2 weeks. This change was accompanied by a decay of the intensity of the SPR band at around 590 nm (curves 1 and 2 in Figure 1e) and by increasing polydispersity of the sample, as verified by TEM. Decomposition of the NPs was even more rapid (1-2 days) if the solution was exposed to atmospheric oxygen that caused Cu oxidation. The particles became more stable after exchange of weak OAM ligands with tighter binding thiols. The improved NP stability was evidenced by TEM images of CuNPs stored for

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Figure 2. (a) SEM image of Cu nanorods, χ ) 100, average aspect ratio, R ≈ 13 (average length/width ) 841.6 nm/63.5 nm). (b) TEM image of Cu nanorods, χ ) 60, R ≈ 64 (average length/width ) 1338 nm/21 nm). (c) HRTEM image of Cu nanorods, χ ) 100; distance between two adjacent lattice planes is 2.2 Å. (d) TEM image of Cu nanorods coated with 1-undecanethiol after storing under atmosphere for several months. Scale bars in (a) 400 nm, (b, d) 200 nm, and (c) 2 nm for the left and 200 nm for the right images.

several weeks (Figure 1f) and by the corresponding UV-vis spectra (curves 3, 4, and 5 in Figure 1e). The HRTEM image of alkylthiol-capped CuNPs (after storing in toluene under argon for 2 weeks) indicated that the distance between two adjacent lattice planes of CuNPs is 2.2 Å (Figure 1i), which is consistent with the reported value of copper fcc {111}.30,31,33 The SPR band of thiolated CuNPs shifted with increased storage time from ∼590 to ∼690 nm (at 20 days) because of partial aggregation/merging of CuNPs. The NPs stabilized with 1-undecanethiol were readily soluble in nonpolar solvents such as toluene or hexane; on the other hand, stabilization with positively charged thiols, TMA, gave particles soluble in water. In the latter case, the particles remained stable for several weeks and even months (especially in the presence of 0.1 M ascorbic acid, preventing Cu oxidation), which was confirmed by TEM imaging (Figure 1h) and was evidenced by only minute changes in the UV-vis spectra (Figure 1g). Interestingly, attempts to prepare negatively charged NPs covered with deprotonated carboxylic acid thiols (e.g., HS-(CH2)10-COO-) proved unsuccessful, likely because the coordination of copper to COO- head groups shifts the equilibrium from Cu(0) to Cu(II). The procedure for the synthesis of CuNRs was identical to that of CuNPs with the exception that a small amount (∼0.1 equiv) of didecyldimethylammonium bromide (DDAB) was used as a structure-directing surfactant. The aspect ratio, R ) length/width, of the Cu NRs depended on the ratio, χ, of OAM to Cu(acac)2. Figures 2a,b show SEM and TEM images of Cu NRs prepared at two different values of χ. As χ was decreased from 100 to 60, the value of R increased from 13 to 64, with the rods becoming both longer and thinner. In addition, the value of R increased slightly with increasing the concentration of DDAB. In this case, however, the yield and monodispersity of NRs became markedly worse. High-resolution TEM images of Cu NRs show that the lattice spacing on the sides of CuNRs is 2.2 Å (Figure 2c, left), indicating that these planes are fcc {111}.30,31,33 While we do not know the details of the mechanism of CuNR formation, this crystallographic finding suggests that DDAB controls the growth of nanorods by stabilizing (111) facets of copper17asin the presence of such a stabilization, crystal growth is favored from higher surface energy “tips” of the rods. The color of freshly prepared Cu nanorods was usually brown-pink, and the rods were more stable than the corresponding CuNPs stabilized with OAM; still, they gradually oxidized after several weeks. After ligand exchange, however, the rods

Wei et al.

Figure 3. Antifungal and catalytic properties of CuNPs. (a) The radii of the zones of inhibition surrounding CuNP reservoirs increase with nanoparticle concentration. Scale bar ) 3 cm. (b) Radii of the inhibition zones plotted as a function of CuNP concentration. The solid curve is a theoretical, logarithmic fit. Red markers correspond to experimental data. Error bars are from five independent experiments for each condition. The inset gives a semilog plot of the data. Critical inhibitory concentration 55.46 µM is estimated from the y-intercept. (c) Various w/w ratios of the fungus medium applied onto commercial paints without CuNPs (top row) and containing 40 µM CuNP (bottom row). In all cases, CuNPs inhibit the growth of the mold. Scale bar ) 3 cm. (d) The CuNP catalyzed coupling of phenylacetylene. (e) An optical image of PDMS cubes loaded with catalytic CuNPs. Scale bar ) 3 mm.

covered with thiolate SAMs remained stable in solution (even open to atmosphere and without antioxidants) for several months. This was confirmed by TEM images in Figure 2d, by only slight decrease of the SPR band intensity (see Supporting Information, section 2), and by the unchanged color of Cu NR toluene solution stored under atmosphere for several months. One of the most promising applications of CuNPs and NRs is as antifungal agents. The water-soluble nanoparticles synthesized by our method are especially useful for this purpose owing to their long-term stability. Interestingly, while the NPs themselves are not antifungal, they slowly oxidize and release cupric ions (Cu2+), which have the ability to generate toxic hydroxyl free radicals when near the lipid membrane. These free radicals cause oxidative degeneration of lipids comprising cell membrane,34,35 leakage of intracellular substances such as K+ ions,36 alteration of key biochemical processes inside of the cell,37-39 and, ultimately, cell death. We have verified by inductively coupled plasma mass spectrometry (ICP-AES) analysis of the aqueous NP solutions that Cu NPs and NRs stabilized by TMA thiols release ca. 2.3% of Cu atoms per month at an approximately constant rate. We then tested the antifungal efficiency of the CuNPs/NRs against Stachybotrys chartarum fungi (a common indoor house mold that has been linked to various health effects and the “sick building syndrome”40) using the so-called disk diffusion assay (DDA) and the minimum inhibitory concentration (MIC) assay described in the Experimental Section. In the first assay, the radii, r, of the inhibition zones surrounding CuNP reservoirs depended on the concentration of the nanoparticles, c (Figure 3a). Following the standard procedure,41 these radii were fitted (Figure 3b) to the solution of the one-dimensional, radial diffusion equation, (r - r0)2 ) 4Dt ln(c/c0), where r0 ) 5 mm is the radius of the NP reservoir, D is diffusion constant of CuNPs/Cu2+, t stands for time, and c0 is the critical NP concentration required to inhibit any growth of the fungus. Rearranging this equation and plotting ln c ) (r - r0)2/4Dt + ln c0 gives the critical

Low-Dispersity Cu Nanoparticles and Nanorods concentration (from the y-intercept) of 55.46 µM in terms of Cu atoms,42 which is close to the minimum inhibitory concentration of 40 µM determined by the MIC assay (see Experimental Section). This MIC value is lower than the Stachybotrys chartarum MIC values reported for carneic acid (75 µM, 25 µg/mL), penicillin G (299 µM, 100 µg/mL), or itraconazole (142 µM-142 mM; 0.1-100 mg/mL)43,44 but higher than 2.5 µM (3.12 µg/mL)43 for actinomycin D (which, however, is highly toxic). The efficiency of the CuNPs as antifungal agents is also evidenced by the results of experiments where these particles were mixed with commercial acrylic paints (Figure 3c). Another potential area of application of copper nanostructures is in catalysis. In this case, however, stabilization of the particles with tightly binding thiols is undesirable as it poisons the surfacesindeed, we verified that thiol-stabilized CuNPs or NRs are not catalytically active in a model alkyne coupling reaction. This reaction, however, proceeds well in the presence of Cu NPs/NRs stabilized with weak OAM ligands. Unfortunately, these NPs are unstable, and a significant fraction of them decomposes during the reaction. To solve this problem, we occluded the particles in a poly(dimethylsiloxane), PDMS, matrix which efficiently protects Cu NPs from oxidation in air.45 Figure 3d summarizes the results for the system in which 33 g, 3 × 3 × 3 mm3 cubes of PDMS (Figure 3e) containing 0.1 equiv (in terms of Cu atoms) of Cu NPs (or Cu NRs) were added to a mixture of 1.25 mmol of phenylacetylene in 5 mL of pyridine and 10 mL of freshly distilled tetrahydrofuran (THF, or dichloromethane, DCM) and were stirred at room temperature and under argon for 36 h. Since THF swells PDMS matrix,46 the substrates were able to diffuse into the cubes47 where they reacted on the NPs. The yield of the coupling product, 1,4diphenyl-1,3-dibutyne, was as high as ∼90% (after column purification). We make the following comments about these results: (i) first, the relative bulkiness of the NPs prevented their outflow/leaking from the cubes upon solvent swelling. This was in contrast to previously published systems,47 where Cu(OAc)2 salt was occluded in PDMS but leaked during the reaction reducing the reusability of the cubessin our case, where leaking was minimal, the same cubes could be used to catalyze reactions in several consecutive batches of reactants. (ii) Second, Cu NPs exhibited higher catalytic activities compared with Cu NRs (∼50% conversion after 36 h in THF). (iii) Third, we note that the obvious practical advantage of NP-loaded PDMS cubes over free NPs is that the former can easily retrieved from the mixture upon reaction completion. Conclusions In summary, we described a straightforward synthetic method leading to stable and relatively low-dispersity copper nanoparticles and nanorods which, via ligand exchange, can be made soluble in either hydrophobic solvents (e.g., toluene or hexane) or in water and can be used as antifungal agents or catalysts. The yields and monodispersities of the Cu NRs could be improved further by adjusting the concentrations of DDAB and/ or OAM or by using other structure-directing reagents (e.g., CTAB or AgNO3).48 In the future, it would be interesting to quantify catalytic activities of copper nanostructures in other reactions including “click,”49 Ullmann,50 or Sonogashira.51 Last but not least, the procedures described here can be, after minor modifications, extended to the synthesis of other types of nanostructures (see Supporting Information, section 4). Acknowledgment. This work was supported by the Pew Scholarship in the Biomedical Sciences, the Sloan Foundation

J. Phys. Chem. C, Vol. 114, No. 37, 2010 15615 Research Fellowship, and the Camille & Henry Dreyfus Teacher-Scholar Award (to B.A.G.). Supporting Information Available: (1) Comparison with the previously reported procedures of CuNP/NR syntheses, (2) UV-vis spectra of fresh and aged nanorods, (3) experimental details of Cu NP catalyzed alkyne coupling, and (4) extension of NP synthesis to other materials. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Kalsin, A. M.; Pinchuk, A. O.; Smoukov, S. K.; Paszewski, M.; Schatz, G. C.; Grzybowski, B. A. Nano Lett. 2006, 6, 1896–1903. (b) Wei, Y.; Bishop, K. J. M.; Kim, J.; Soh, S.; Grzybowski, B. A. Angew. Chem., Int. Ed. 2009, 48, 9477–9480. (2) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59–61. (3) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025–1102. (4) Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487–490. (5) Huang, X.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. J. Am. Chem. Soc. 2006, 128, 2115–2120. (6) Haes, A. J.; Van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596– 10604. (7) Joo, S. H.; Park, J. Y.; Tsung, C. K.; Yamada, Y.; Yang, P.; Somorjai, G. A. Nature Mater. 2009, 8, 126–131. (8) Lee, I.; Delbecq, F.; Morales, R.; Albiter, M. A.; Zaera, F. Nature Mater. 2009, 8, 132–138. (9) Xie, J.; Lee, J. Y.; Wang, D. I. C.; Ting, Y. P. Small 2007, 3, 672–682. (10) (a) Wei, Y.; Klajn, R.; Pinchuk, A. O.; Grzybowski, B. A. Small 2008, 4, 1635–1639. (b) Wei, Y.; Soh, S.; Apodaca, M. M.; Kim, J.; Grzybowski, B. A. Small 2010, 6, 857–863. (11) Shen, C.; Hui, C.; Yang, T.; Xiao, C.; Tian, J.; Bao, L.; Chen, S.; Ding, H.; Gao, H. Chem. Mater. 2008, 20, 6939–6944. (12) (a) Klajn, R.; Bishop, K. J. M.; Grzybowski, B. A. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 10305–10309. (b) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 20, 3315–3322. (c) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293–346. (13) (a) Dawn, A.; Mukherjee, P.; Nandi, A. K. Langmuir 2007, 23, 5231–5237. (b) Pietrobon, B.; Kitaev, V. Chem. Mater. 2008, 20, 5186– 5190. (c) Kalsin, A. M.; Kowalczyk, B.; Smoukov, S. K.; Klajn, R.; Grzybowski, B. A. J. Am. Chem. Soc. 2006, 128, 15046–15047. (14) (a) Rioux, R. M.; Song, H.; Hoefelmeyer, J. D.; Yang, P.; Somorjai, G. A. J. Phys. Chem. B 2005, 109, 2192–2202. (b) Zhao, M.; Crooks, R. M. AdV. Mater. 1999, 11, 217–220. (c) Niesz, K.; Grass, M.; Gabor, A. Nano Lett. 2005, 5, 2238–2240. (15) (a) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. 1999, 38, 364– 366. (b) Kim, S. W.; Park, J.; Jang, Y.; Chung, Y.; Hwang, S.; Hyeon, T. Nano Lett. 2003, 3, 1289–1291. (c) Son, S. U.; Jang, S. Y.; Yoon, K. Y.; Kang, E.; Hyeon, T. Nano Lett. 2004, 4, 1147–1151. (d) Zelakiewicz, B. S.; Lica, G. C.; Deacon, M. L.; Tong, Y. J. Am. Chem. Soc. 2004, 126, 10053– 10058. (e) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340–8347. (16) (a) Ahrenkiel, S. P.; Micic, O. I.; Miedaner, A.; Curtis, J. C.; Nedeljkovic, J. M.; Nozik, A. J. Nano Lett. 2003, 3, 833–837. (b) Dumestre, F.; Chaudret, B.; Amiens, C.; Respaud, M.; Fejes, P.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2003, 42, 5213–5216. (c) Busbee, B. D.; Obare, S. O.; Murphy, C. J. AdV. Mater. 2003, 15, 414–416. (17) (a) Klajn, R.; Pinchuk, A. O.; Schartz, G. C.; Grzybowski, B. A. Angew. Chem., Int. Ed. 2007, 46, 8363–8367. (b) Xue, C.; Meı´traux, G. S.; Millstone, J. E.; Mirkin, C. A. J. Am. Chem. Soc. 2008, 130, 8337–8344. (c) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874–12880. (18) (a) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176–2179. (b) Gou, L. F.; Murphy, C. J. Nano Lett. 2003, 3, 231–234. (c) Lifshitz, E.; Bashouti, M.; Kloper, V.; Kigel, A.; Eisen, M. S.; Berger, S. Nano Lett. 2003, 3, 857–862. (19) (a) Yu, H.; Buhro, W. E. AdV. Mater. 2003, 15, 416–419. (b) Yu, H.; Li, J. B.; Loomis, R. A.; Gibbons, P. C.; Wang, W. L.; Buhro, W. E. J. Am. Chem. Soc. 2003, 125, 16168–16169. (20) (a) Chen, S. H.; Wang, Z. L.; Ballato, J.; Foulge, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186–16187. (b) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nature Mater. 2003, 2, 382–385. (c) Zitoun, D.; Pinna, N.; Frolet, N.; Belin, C. J. Am. Chem. Soc. 2005, 127, 15034–15035. (21) Chan, G. H.; Zhao, J.; Hicks, E. M.; Schatz, G. C.; Van Duyne, R. P. Nano Lett. 2007, 7, 1947–1952.

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