Mercaptocalixarene-Capped Gold Nanoparticles via Postsynthetic

Evidence for interactions between π-electron-rich calixarene cavities and the surface of gold nanoparticles is demonstrated using a comparative synth...
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2009, 113, 1137–1142 Published on Web 12/31/2008

Mercaptocalixarene-Capped Gold Nanoparticles via Postsynthetic Modification and Direct Synthesis: Effect of Calixarene Cavity-Metal Interactions Jeong-Myeong Ha,† Alexander Katz,*,† Andriy B. Drapailo,‡ and Vitaly I. Kalchenko‡ Department of Chemical Engineering, UniVersity of California at Berkeley, Berkeley, California 94720-1462, and Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Murmanska str., 5, 02660, KyiV-94, Ukraine ReceiVed: September 13, 2008; ReVised Manuscript ReceiVed: NoVember 5, 2008

Evidence for interactions between π-electron-rich calixarene cavities and the surface of gold nanoparticles is demonstrated using a comparative synthetic approach involving the capping of the nanoparticles with thiol ligands. In one approach, thiol ligands are systematically titrated onto existing 5.3 nm gold nanoparticles in solution (postsynthetic synthesis approach), whereas in the other a gold solution is reduced in the presence of thiol ligands (direct synthesis approach). The calixarene cavity is placed in close proximity and facing toward the gold nanoparticle surface upon adsorption of ligand 5,11,17,23-tetrakis-mercaptomethyl-25,26,27,28tetrapropoxylcalix[4]arene (MMC), which comprises four upper-rim thiol groups. The other two ligands used in this investigation contain either four thiol groups on the calixarene lower-rim, 25,26,27,28-tetrakis(4mercaptobutyloxy)calix[4]arene (MBC), or a model monodentate thiol, 4-methoxybenzenemethanethiol (MBM), as controls. The orientation of the cavity in MMC is toward the gold surface upon adsorption, which is in complete contrast to its isomer MBC in which the cavity points away from the surface and is located further away. Control ligand MBM lacks a calixarene cavity though still possessing an electron-rich arene ring in close proximity to the gold nanoparticle surface upon adsorption. The surface plasmon resonance absorption (SPR) band red shifts ∼5 nm in wavelength and increases ∼10% in absorption intensity during titration of the gold surface with MMC, as monitored via UV-vis spectroscopy, until the onset of surface site saturation, which corresponds to a molecular footprint of ∼116 Å2 per MMC molecule. Using MBC under identical conditions produces no change in either the SPR band wavelength or absorbance. Adsorption of ligand MBM to the gold nanoparticle surface results in a similar increase as MMC on the SPR absorption intensity and no change in SPR band wavelength. These results can be understood on the basis of interactions between the π-electron-rich calixarene cavity and metal surface. The influence of the calixarene cavity on SPR band wavelength was also observed in nanoparticles prepared by the direct synthesis approach, in which gold reduction occurred in the presence of thiol ligand. The MMC-capped nanoparticles had a higher SPR band wavelength relative to those capped with MBC and MBM. Altogether, these results demonstrate that even for colloids that are relatively large compared with molecular clusters, subtle characteristics of the capping ligand such as the presence of a π-electron rich cavity influence surface microenvironment upon adsorption. Introduction Catalysis, being a surface phenomenon, is often controlled in a critical fashion by the state of atoms on a metal surface, which for a nanoparticle comprises a significant fraction of atoms relative to bulk materials. These surface atoms are thought to be significantly influenced by the nature of ligand. We are interested in tuning gold as a catalytic metal because of the broad range of selective transformations, spanning both oxidation1 and reduction,2 that gold is known to catalyze. Our overarching hypothesis is that heterogeneous catalysis with gold can be controlled by tuning the state of surface atoms using an organic ligand, and that this holds true for both clusters as well as larger nanoparticles. * To whom correspondence should be addressed. E-mail: katz@ cchem.berkeley.edu. Fax: (510) 642-4778. † University of California at Berkeley. ‡ National Academy of Sciences of Ukraine.

10.1021/jp808165f CCC: $40.75

Relevant supporting data demonstrating ligand control of surface atoms in gold colloids has been previously demonstrated for small gold clusters consisting of up to 11 atoms. The UV-vis spectra of these clusters depend sensitively on the type of surface ligand.3 Larger (up to 4 nm) gold nanoparticles also exhibit significant changes in their physical and spectral characteristics upon ligand adsorption, which has been carefully studied via XANES and XPS to be driven in part by the depletion of d orbital charge on the metal surface upon thiol ligand adsorption.4,5 This situation in principle creates a driving force for interactions between charge-depleted gold atoms on the thiol ligand-bound surface and an electron rich auxiliary functional group on the ligand. Here, our goal is to use the functional group diversity available in calixarenes as macrocyclic ligands to investigate the importance of the calixarene cavity as such a π-electronrich group. Macrocycles are particularly attractive as surface  2009 American Chemical Society

1138 J. Phys. Chem. C, Vol. 113, No. 4, 2009

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SCHEME 1: Capping Gold Nanoparticles with MBM, MMC, and MBC

ligands for nanoparticles due to their ability to chelate to the surface with a high degree of affinity,6-11 as well as the unique potential to create void spaces between bulky adsorbed macrocycles, which can be useful for coadsorbing smaller molecular species, something that may be particularly useful for applications in catalysis.12-15 Calixarenes in the cone conformation are sterically bulky and rigid macrocylic oligomers of phenol, which are known to possess π-electron rich cavities that have a perturbed refractive index, resulting in a higher optical dielectric constant relative to the corresponding monomeric unit.16 These π-electron rich cavities complex a variety of guests such as C6017 and can be synthetically tailored to specifically control extent of guest binding.18,19 Calixarenes adsorbed to nanoparticle surfaces have been used for applications in sensing and molecular recognition,8,9,20,21 enhancing colloidal stability,7,22,23 and tuning semiconductor nanoparticle optical absorption10 and photoluminescence properties.24 The goal of this study is to understand how the calixarene cavity affects gold nanoparticles upon calixarene adsorption to the surface by addressing the following question: is there a significant difference in the surface microenvironment of capped gold nanoparticles depending on calixarene cavity proximity and orientation relative to the metal surface? Such a phenomenon may be driven by interactions between the π-electron rich calixarene cavity and electron-depleted metal surface atoms that arise upon thiol adsorption,4,5 and such interactions have been previously observed between π-rich calixarene cavities and C60.17 Further evidence for relevant cation-π interactions has been observed between Ag(I) cations and lower-rim capped calixarenes in the cone and partial cone conformations, when using single-crystal X-ray diffraction. In the case of the partialcone system, the cation is observed to interact with three aromatic rings, whereas in the cone system, the cation is positioned to be in the middle of the cone calixarene cavity.25,26 Our approach is to measure the SPR wavelength and absorbance

intensity as relevant and sensitive indicators of interactions at the ligand-nanoparticle interface. This approach is not trivial given the many factors that are known to influence the SPR band absorbance and wavelength according to Mie theory. 27-33 Interactions involving partial transfer of charge are also known to affect the SPR band.28 Previous studies have noticed significant changes in the SPR band wavelength and absorbance intensity upon binding calixarene macrocycles to the surface of gold nanoparticles and in some cases observing SPR band wavelength shifts of up to 25 nm upon calixarene adsorption;34 however, the role of calixarene cavity-metal interactions in these shifts has not been systematically addressed. In this manuscript, our approach is to use synthesis as a tool to peel away the complexity of the many known factors influencing the SPR band and thereby elucidate the effect of calixarene cavity proximity and orientation on gold nanoparticle properties upon calixarene adsorption. We have carefully chosen three model thiol ligands that, when taken together, enable this to be performed. These compounds are represented in Scheme 1: (i) MMC (5,11,17,23-tetrakis-mercaptomethyl-25,26,27,28tetrapropoxylcalix[4]arene) represents a calixarene consisting of four thiol groups on the upper rim; (ii) MBC (25,26,27,28tetrakis(4-mercaptobutyloxy)calix[4]arene) represents a similar array of four thiol groups except tethered off of the lower rim; and (iii) MBM (4-methoxybenzenemethanethiol) represents a monomeric version of MMC. Isomers MMC and MBC represent a similar tetrathiol functional group array for binding to gold and have similar ligand thicknesses of ∼10 Å, which are defined by the same calixarene height for a bidentate or higher degree of calixarene chelation to the gold surface (such a degree of coordination is predicted based on observations in other calixarene12 and chelating thiolate ligands on gold surfaces). MMC and MBC are expected to have similar refractive indices;12,35 however, they differ in their upper rim orientation to the gold surface upon adsorption as shown in Scheme 1.

Letters

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1139

SCHEME 2: Synthesis of MBC

Adsorption studies using these compounds are performed in two regimes defined by postsynthetic modification and direct synthesis. The former refers to addition of calixarene ligand to surfactant-stabilized gold nanoparticles, whereas the latter represents the synthesis of gold nanoparticles in the presence of calixarene ligand via Au(III) reduction using NaBH4. By studying the effect of these surface ligands on SPR band properties of coated gold nanoparticles, it becomes possible to elucidate the active role of the calixarene cavity in controlling the surface microenvironment upon calixarene adsorption. The molecular footprint of the calixarene when adsorbed to the gold surface is also determined. The methods used are generalizable insofar as their ability to be used in conjunction with other nanoparticles and macrocyclic ligand combinations. Results and Discussion Synthesis of MBC. Calixarene MBC was synthesized with satisfactory yields by the interaction of the tetrakis(4bromobutyloxy)calix[4]arene with thiourea, followed by decomposition of formed thiouronium salts by sodium hydroxide (Scheme 2). The 1H NMR spectrum (CDCl3, 25 °C) of MBC demonstrates that the calix[4]arene exists only in the cone conformation, which is confirmed by two doublets of the AB spin system for the axial and equatorial protons of the methylene bridges of the macrocyclic skeleton. This means that the intramolecular rotation of the benzene rings through the macrocyclic annulus is blocked by bulky substituents at the narrow rim. Gold Nanoparticle Postsynthetic Modification. Titration of gold nanoparticle surfaces with organic ligands has been previously used to measure saturation coverages, allowing calculation of adsorbed ligand areas, and ligand interactions with the surface.36,37 Here, gold nanoparticles stabilized with tetraoctylammonium bromide (TOAB) were synthesized in a biphasic mixture of toluene and water,38-40 and the nanoparticles dissolved in the toluene phase were diluted with tetrahydrofuran (THF) to obtain a 0.4 mM solution of gold atoms in a mixture of 80% (v/v) THF and 20% (v/v) toluene (Supporting Information). The results shown in Figure 1 correspond to titration experiments at a fixed gold nanoparticle concentration of 0.4 mM, which are conducted by adding successive quantities of thiol ligand at room temperature. During this process, the gold particle size remains the same for all ligands, which is confirmed by an almost constant (within 2 nm) full width at half-maximum of the SPR band41,42 as well as TEM images of titrated nanoparticles (Supporting Information). The wavelength of the SPR band (λmax) red shifts by 5 nm during postsynthetic titration of MMC, as demonstrated in Figure 1a, whereas the same experiment with MBC and MBM results in almost no changes in λmax, as shown in Figure 1b,c. Comparison of MMC and MBC provides compelling evidence for the effect of interactions between the calixarene cavity and

metal nanoparticle surface, because the ligand thicknesses and refractive indices for MMC and MBC are expected to be almost identical (vide supra) and therefore cannot be responsible for the stark contrast in the observed effects on the SPR band upon ligand adsorption. These interactions are expected to be facilitated by the proximity and orientation of the MMC calixarene cavity relative to the nanoparticle surface upon its adsorption. It has been previously observed that thiol ligand adsorption to gold results in subvalent surface gold atoms (vide supra). We hypothesize that the calixarene cavity in MMC interacts with the gold surface via π-electron charge transfer to these subvalent gold surface atoms upon adsorption. This type of reasoning is consistent with previous explanations of aromatic organosulfur ligand substituent effects in which strongly electron-donating groups on the ligand have been observed to increase the Au-ligand bond strength in systems consisting of thiolate monolayers on gold nanoparticles as well as twodimensional gold surfaces.43,44 A general inference from this hypothesis is that π-electron donation from the arene portion of the ligand to the gold surface needs to be explicitly taken into consideration when describing interactions between an aromatic organosulfur ligand and gold surface. Systematic titration of gold nanoparticles with MMC further demonstrates a plateau in λmax with increasing MMC concentration, which corresponds to a surface saturation regime. The concentration of MMC at the onset of this plateau corresponds to an adsorbed MMC molecular area of 116 Å2, which is consistent with the approximate calculated MMC area of ∼100 Å2 (see Supporting Information for a detailed calculation). The experimentally determined surface area per thiol group (116/4 ) 29 Å2) is slightly larger than the literature value of 20 Å2 for MBM on Au(111) and 20-38 Å2 for other aromatic thiols on Au(111)45 and, as expected, is significantly larger than 17.2 and 15.2 Å2 for adsorbed monodentate 1-tetradecanethiol and octadecanethiol, respectively, on gold nanoparticles.16 This MMC area is about 30% higher than molecular footprints for similar tetrathiolated resorcinarenes on the surface of gold nanoparticles, which were calculated on the basis of XPS measurements that employ a large excess of resorcinarene.12 The measured calixarene density upon saturation of MMC corresponds to approximately 87.7 calixarenes per 5.3 nm diameter gold nanoparticle (Supporting Information). The absorbance intensity of the SPR band of MMC- and MBM-capped nanoparticles also exhibits a plateau regime during titration after increasing in magnitude by approximately 11% from values prior to titration (Figure 1e). The onset of the plateau regime during titration of MMC coincides with the calculated molecular footprint of MMC when monitoring λmax above, suggesting that the same phenomena that govern λmax trends also control the intensity of the SPR band upon MMC adsorption. In contrast to behavior observed for MMC and

1140 J. Phys. Chem. C, Vol. 113, No. 4, 2009

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Figure 1. Representative UV-vis results of postsynthetic gold nanoparticles in a mixture of toluene and THF, treated with (a) MMC (s, no thiol added; ----, 0.005 mM of MMC added; · · · · , 0.01 mM of MMC added), (b) MBM (s, no thiol added; ----, 0.02 mM of MBM added; · · · · , 0.04 mM of MBM added), and (c) MBC (s, no thiol added; ----, 0.005 mM of MBC added; · · · · , 0.01 mM of MBC added). (d) Wavelength of SPR bands, λmax, and (e) normalized UV-vis absorbance at λmax depending on the concentration of thiols. The UV-vis absorbance was normalized at λmax of the TOAB-stabilized gold nanoparticles (2; MMC-capped particles, 9; MBM-capped particles, b; MBC-capped particles). Each solution contains 0.4 mM of gold atoms and 40 mM of TOAB.

MBM, the absorbance and λmax of MBC-capped nanoparticles remains unchanged during titration; however, this is not due to lack of MBC binding. MBC adsorption to the gold nanoparticle surface is confirmed by the lack of a measurable shift in λmax upon addition of a monolayer equivalent of MMC to gold nanoparticles that had been previously titrated with a monolayer of MBC (Supporting Information). The lack of changes to the absorbance intensity upon binding MBC, in contrast to the behavior observed with MMC and MBM, further highlights the importance of interactions between arene rings in adsorbed MMC and MBM, and the metal surface. In ligand MBM, like MMC, close proximity of the aromatic ring to the gold nanoparticle surface enables interactions between the electronrich arene ring of adsorbed MBM and gold surface; this is something that is not possible with multidentate binding in MBC. Besides the effect of a π-electron rich arene ring system, comparison of the different effects of ligands MMC, MBC, and MBM on SPR band properties upon adsorption is also governed by a combination of optical dielectric function and thickness of the adsorbed layer. The calixarene cavities in MMC and MBC are expected to create a higher refractive index environment relative to MBM. This effect is supported by examining the refractive index of p-tert-butylcalix[4]arene (which is 1.5616) in comparison with the monomer, p-tert-butylphenol (which is 1.4846). The refractive index is one effect known to influence the SPR band;28,29 in addition the tilt of MBM when adsorbed to the gold surface results in a shorter thickness of 0.52 nm,45 compared with the multidentate calixarene ligands (vide supra).12,35

This change in thickness is also expected to influence the SPR band, as evident in previous studies of a series of thiolate ligands of varying chain length and similar refractive index adsorbed on silver nanoparticles, which observed changes in λmax of up to 40 nm that varied systematically as a function of ligand thickness.47 Directly Synthesized Ligand-Capped Gold Nanoparticles. The UV-vis spectra of gold nanoparticles that are directly synthesized in the presence of either MMC, MBC, or MBM as thiol ligand are represented in Figure 2 in the region of the SPR band. The decreasing intensity of the SPR band upon increasing thiol concentration used during synthesis has been observed previously in other systems using alkylthiol ligands39,47,48 and can be attributed to the active role of the thiol ligand in gold nanoparticle nucleation and growth, where smaller nanoparticles are formed at higher thiol synthesis concentrations. The flexible thiols tethered on MBC afford the smallest nanoparticles compared to the more rigid benzyl thiols comprising MMC and MBM as ligands, based on their lower intensity of the SPR band at a given ligand concentration. The TEM images of the nanoparticles also confirm that nanoparticles capped with MBC are the smallest of the three ligands (Supporting Information).49 Figure 2 represents absorption spectra in the region of the SPR band for directly synthesized gold nanoparticles that have been synthesized with a variety of thiol concentrations. Accurate values of λmax were obtained from these spectra by performing spectral deconvolutions, which rely on deconvoluting the spectral window shown in Figure 2 as a sum of two Gaussian

Letters

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1141 in which the calixarene cone axis of symmetry is perpendicular to the gold surface. This corresponds to approximately 88 calixarenes per 5.2 nm diameter gold nanoparticle. Comparison of nanoparticles synthesized via the postsynthetic versus direct route demonstrates a smaller nanoparticle size for the former that presumably results from interactions between thiol ligand and gold during nanoparticle nucleation and growth. Acknowledgment. The authors are grateful to the National Science Foundation (NSF DMR 0444761) and Chevron Corporation for their generous financial support. The authors acknowledge support of the National Center for Electron Microscopy, Lawrence Berkeley Laboratory, which is supported by the U.S. Department of Energy under Contract No. DEAC02-05CH11231. The authors thank graduate student S. Miroshnichenko (IOCh NASU) for synthesis of tetrakis-(4bromobutyloxy)-calix[4]arene.

Figure 2. UV-vis results of directly synthesized gold nanoparticles capped with (a) MMC, (b) MBM, and (c) MBC in toluene. Each solution, all containing 2 mM of gold atoms, has 0.1 (s), 0.24 (----), 0.4 ( · · · · ) and 0.8 (- · - · -) mM of thiols. A vertical line emphasizes λmax of nanoparticles capped with 0.24 mM of MMC and 0.1 mM of MBC. The absorbances are normalized at λ ) 800 nm.

Supporting Information Available: Supporting Information includes experimental details, more evidence of formation of MBC-capped gold nanoparticles, calculated molecular conformations of MMC and MBC, calculation of refractive index of solvent mixture, transmission electron micrographs of gold nanoparticles, and deconvoluted UV-vis spectra of directly synthesized calixarene-capped gold nanoparticles. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

curves (Supporting Information). It is evident from this data and deconvolutions of these spectra that λmax of MBC-capped nanoparticles is red-shifted relative to MMC- and MBM-capped nanoparticles. Indeed, the observed trend in λmax versus ligand type for the directly synthesized gold nanoparticles is the same as for the postsynthetically modified gold nanoparticles above. MMC-capped nanoparticles exhibit the largest λmax, followed next by MBM-capped nanoparticles. Because of the small size of the nanoparticles, λmax is not expected to be influenced by small changes in the gold nanoparticle size,27,39,50 as it is for nanoparticles with diameters greater than 30 nm,51 and can therefore be expected to reliably reflect properties of the ligandnanoparticle interface. Therefore, the lowest λmax for MBCcapped nanoparticles is consistent with their relative absence of interactions between arene and subvalent gold surface atoms (vide supra). Conclusions Gold nanoparticles capped with mercaptocalixarene ligands (MMC and MBC) and a monomeric control (MBM) are synthesized using postsynthetic surface modification and direct synthesis approaches. Nanoparticles capped with MMC exhibit the greatest λmax, independent of synthetic approach, and when data is taken altogether using the three ligands there is evidence for the role of the calixarene cavity as a π-electron rich system that interacts with the gold surface upon thiol ligand adsorption. While other potential effects on λmax, such as digestive ripening of the nanoparticles upon ligand adsorption, cannot be strictly ruled-out, it is highly unlikely that this is affecting results due to control experiments (e.g., lack of changes in fwhm during postsynthetic titration, batch-to-batch reproducibility, lack of affect of particle size effect on λmax for gold nanoparticles smaller than 30 nm50,51). The onset of surface saturation occurs at an adsorbed area of 116 Å2 per MMC, which is consistent with a bidentate or higher degree of chelation of the macrocycle

(1) (a) Cha, G. Y.; Parravano, G. J. Catal. 1970, 18, 200. (b) Cant, N. W.; Fredrickson, P. W. J. Catal. 1975, 37, 531. (c) Haruta, M.; Date´, M. Appl. Catal., A 2001, 222, 427. (2) (a) Sermon, P. A.; Bond, G. C.; Wells, P. B. J. Chem. Soc., Faraday Trans. 1 1979, 75, 385. (b) Okumura, M.; Akita, T.; Haruta, M. Catal. Today 2002, 74, 265. (3) Grant, C. D.; Schwartzberg, A. M.; Yang, Y.; Chen, S.; Zhang, J. Z. Chem. Phys. Lett. 2004, 383, 31. (4) Zhang, P.; Sham, T. K. Phys. ReV. Lett. 2003, 90, 245502. (5) Zhang, P.; Sham, T. K. Appl. Phys. Lett. 2002, 81, 736. (6) Stavens, K. B.; Pusztay, S. V.; Zou, S.; Andres, R. P.; Wei, A. Langmuir 1999, 15, 8337. (7) Wei, A. Chem. Commun. 2006, 1581. (8) Arduini, A.; Demuru, D.; Pochini, A.; Secchi, A. Chem. Commun. 2005, 645. (9) Tshikhudo, T. R.; Demuru, D.; Wang, Z.; Brust, M.; Secchi, A.; Arduini, A.; Pochini, A. Angew. Chem., Int. Ed. 2005, 44, 2913. (10) Misra, T. K.; Liu, C.-Y. J. Colloid Interface Sci. 2007, 310, 178. (11) Liu, J.; Ong, W.; Kaifer, A. E. Langmuir 2002, 18, 5981. (12) Balasubramanian, R.; Kim, B.; Tripp, S. L.; Wang, X.; Lieberman, M.; Wei, A. Langmuir 2002, 18, 3676. (13) Pusztay, S. V.; Wei, A.; Stavens, K. B.; Andres, R. P. Supramol. Chem. 2002, 14, 291. (14) Coffer, J. L.; Chandler, R. R.; Cutsche, C. D.; Alam, I.; Pinizzotto, R. F.; Yang, H. J. Phys. Chem. 1993, 97, 696. (15) Crooks, R. M.; Lemon III, B. I.; Sun, L.; Yeung, L. K.; Zhao, M. Top. Curr. Chem. 2001, 212, 81. (16) Shirshov, Yu. M.; Zynio, S. A.; Matsas, E. P.; Beketov, G. V.; Prokhorovich, A. V.; Venger, E. F.; Markovskiy, L. N.; Kalchenko, V. I.; Soloviov, A. V.; Merker, R. Supramol. Sci. 1997, 4, 491. (17) Ikeda, A.; Suzuki, Y.; Yoshimura, M.; Shinkai, S. Tetrahedron 1998, 54, 2497. (18) Nechifor, A. M.; Philipse, A. P.; de Jong, F.; van Duynhoven, J. P. M.; Egberink, R. J. M.; Reinhoudt, D. N. Langmuir 1996, 12, 3844. (19) Yanagi, A.; Otsuka, H.; Takahara, A. Polymer J. 2005, 37, 939. (20) Leyton, P.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Domingo, C.; Campos-Vallette, M.; Saitz, C.; Clavijo, R. E. J. Phys. Chem. B 2004, 108, 17484. (21) Li, H.; Zhang, Y.; Wang, X.; Xiong, D.; Bai, Y. Mater. Lett. 2007, 61, 1474. (22) Kim, B.; Balasubramanian, R.; Perez-Segarra, W.; Wei, A. Supramol. Chem. 2005, 17, 173. (23) Balasubramanian, R.; Kwon, Y.-G.; Wei, A. J. Mater. Chem. 2007, 17, 105. (24) Notestein, J. M.; Iglesia, E.; Katz, A. Chem. Mater. 2007, 19, 4998.

1142 J. Phys. Chem. C, Vol. 113, No. 4, 2009 (25) Ikeda, A.; Tsuzuki, H.; Shinkai, S. J. Chem. Soc., Perkin Trans. 1994, 2, 2073. (26) Iwamoto, K.; Ikeda, A.; Araki, K.; Harada, T.; Shinkai, S. Tetrahedron 1993, 49, 9937. (27) Ghosh, S. K.; Nath, S.; Kundu, S.; Esumi, K.; Pal, T. J. Phys. Chem. B 2004, 108, 13963. (28) Templeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564. (29) Mulvaney, P. Langmuir 1996, 12, 788. (30) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66. (31) Praharaj, S.; Ghosh, S. K.; Nath, S.; Kundu, S.; Panigrahi, S.; Basu, S.; Pal, T. J. Phys. Chem. B 2005, 109, 13166. (32) Kreibig, U.; Genzel, L. Surf. Sci. 1985, 156, 678. (33) Leatherdale, C. A.; Woo, W.-K.; Mikulec, F. V.; Bawendi, M. G. J. Phys. Chem. B 2002, 106, 7619. (34) Xu, S.; Podoprygorina, G.; Bo¨hmer, V.; Ding, Z.; Rooney, P.; Rangan, C.; Mittler, S. Org. Biomol. Chem. 2007, 5, 558. (35) (a) Wei, L.; Tiznado, H.; Liu, G.; Padmaja, K.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Phys. Chem. B 2005, 109, 23963. (b) Park, J.-S.; Vo, A. N.; Barriet, D.; Shon, Y.-S.; Lee, T. R. Langmuir 2005, 21, 2902. (c) Fox, M. A.; Whitesell, J. K.; McKerrow, A. J. Langmuir 1998, 14, 816. (d) Kittredge, K. W.; Minton, M. A.; Fox, M. A.; Whitesell, J. K. HelV. Chim. Acta 2002, 85, 788. (e) Li, Z.; Lieberman, M.; Hill, W. Langmuir 2001, 17, 4887. (f) Beulen, M. W. J.; Bu¨gler, J.; Lammerink, B.; Geurts, F. A. J.; Biemond, E. M. E. F.; van Leerdam, K. G. C.; van Veggel, F. C. J. M.; Engbersen, J. F. J.; Reinhoudt, D. N. Langmuir 1998, 14, 6424. (36) Chen, M. M. Y.; Katz, A. Langmuir 2002, 18, 8566. (37) Chen, M. M. Y.; Katz, A. Langmuir 2002, 18, 2413. (38) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655. (39) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17.

Letters (40) Saunders, A. E.; Sigman, M. B.; Korgel, B. A. J. Phys. Chem. B 2004, 108, 193. (41) Rodrı´guez-Sa´nchez, M. L.; Rodrı´guez, M. J.; Blanco, M. C.; Rivas, J.; Lo´pez-Quintela, M. A. J. Phys. Chem. B 2005, 109, 1183. (42) Kreibig, U.; Fragstein, C. V. Z. Phys. 1969, 224, 307. (43) Donkers, R. L.; Song, Y.; Murray, R. W. Langmuir 2004, 20, 4703. (44) Liao, S.; Schnidman, Y.; Ulman, A. J. Am. Chem. Soc. 2000, 122, 3688. (45) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L. Langmuir 1997, 13, 4018. (46) CRC Handbook of Chemistry and Physics, Internet Version 2007 (87th Edition); Lide, D. R., Eds.; Taylor and Francis: Boca Raton, FL, 2007. (47) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471. (48) (a) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (b) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 882. (c) Shimizu, T.; Teranishi, T.; Hasegawa, So.; Miyake, M. J. Phys. Chem. B 2003, 107, 2719. (49) The trend in the particle diameter as measured by TEM corresponds to the measured fwhm (full width at half-maximum) of SPR bands, which is known to be inversely proportional to the size of the particles (refs 41 and 42). The fwhm’s of 0.025 mM MMC (0.1 mM thiol)-, 0.1 mM MBM-, and 0.025 mM MBC (0.1 mM thiol)- capped gold nanoparticles are 84, 80, and 92 nm, respectively, which indicates the MBC-capped nanoparticles are the smallest, confirming information from TEM. (50) (a) Kreibig, U. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (b) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss Faraday Soc. 1951, 55. (c) Turkevich, J.; Garton, G.; Stevenson, P. C. J. Colloid Sci. 1954, 9, S26. (51) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212.

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