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Monolayer-Protected Gold Nanoparticle Coalescence Induced by Photogenerated Radicals Arnold J. Kell,† Abdolhamid Alizadeh,‡ Li Yang,§ and Mark S. Workentin*,† Department of Chemistry, The University of Western Ontario, London, Ontario, Canada, N6A 5B7 Received June 20, 2005. In Final Form: July 28, 2005 Acyl and alkyl radicals generated photochemically in a solution containing monolayer-protected gold nanoparticles are shown to efficiently liberate the alkylthiolate ligands into the solution as the thioacetyl or alkyl sulfide, respectively. This radical-induced reaction initiates a coalescence of the gold cores to facilitate an increase in core size. The photoinitiated radical reaction also liberates monolayers from twodimensional gold surfaces.

Monolayer-protected nanoparticles (MPNs) possess unique features allowing them to be utilized in applications ranging from catalysis and chemical sensing to possible media for the delivery and controlled release of multiple substrates from their surfaces. Some applications require particular control of the size range of the metal cores. For example, sensing processes often rely on detecting changes in the plasmon absorption band, a phenomenon notable in metal cores in excess of 2-3 nm in diameter, upon a substrates interaction with the MPN1 or monolayer anchored to the nanoparticle.2-4 Although there are a number of useful methods for the preparation of organicsoluble MPNs,5-10 there remains a need to develop alternative procedures capable of targeting larger monodisperse MPNs that can be more difficult to access through the present synthetic methodologies. Zhong and coworkers11-13 and Miyake and co-workers14,15 have recently * To whom correspondence should be addressed. E-mail: [email protected]. † Current address: Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Canada. ‡ Visiting graduate student from Department of Chemistry, BuAli Sina University, Hamadan, Iran, 65174. § Laboratory Director of the Nanoimaging Laboratory, Simon Fraser University, Burnaby BC. (1) Thomas, K. G.; Zajicek, J.; Kamat, P. V. Langmuir 2002, 18, 37223727. (2) Storhoff, J. J.; Mirkin, C. A. Chem. Rev. 1999, 99, 1849-1862. (3) Chakrabarti, R.; Klibanov, A. M. J. Am. Chem. Soc. 2003, 125, 12531-12540. (4) Dragnea, B.; Chen, C.; Kwak, E.-S.; Stein, B.; Kao, C. C. J. Am. Chem. Soc. 2003, 125, 6374-6375. (5) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 801-802. (6) 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, 17-30. (7) Garcia-Martinez, J. C.; Crooks, R. M. J. Am. Chem. Soc. 2004, 126, 16170-16178. (8) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. J. Phys. Chem. B. 2005, 109, 692-704. (9) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515-7520. (10) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 2003, 15, 935-942. (11) Zhong, C.-J.; Zhang, W. X.; Leibowitz, F. L.; Eichelberger, H. H. Chem. Commun. 1999, 1211-1212. (12) Maye, M. M.; Zhong, C. J. J. Mater. Chem. 2000, 10, 18951901. (13) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K.; Zhong, C.-J. Langmuir 2000, 16, 490-497. (14) Teranishi, T.; Hasegawa, S.; Shimizu, T.; Miyake, M. Adv. Mater. 2001, 13, 1699-1701.

developed procedures where the core size of MPNs can be evolved through a heat treatment procedure where preformed dodecanethiolate-modified MPNs (C12MPNs), prepared via the Brust method,5 and tetraoctylammonium bromide (TOAB) are heated in concentrated toluene solutions or the molten TOAB itself. Generally, these thermally induced evolutions in core size require the MPNs to be heated to a temperature in excess of 150 °C, where it is proposed that surface melting of the metal MPN cores lead to a coalescence and evolution of core size.11-15 For other applications, the ability to release the monolayer ligands from the metal surface is important. Currently, this is accomplished chemically via thiolate oxidation by reactive oxygen species16 or I2,17 reductive desorption,18-22 or through thermally breaking the Au-thiolate bond.23,24 However, the liberation of the ligands is generally difficult to control in the cases described above. During the course of our investigations into using photochemical probes as a means of elucidating mobility and reactivity constraints imposed on substrates that are incorporated onto MPNs,25-28 we discovered that radicals generated in solution were capable of efficiently abstracting the alkylthiolate substrates from gold MPN surfaces without precipitation of elemental gold. Here we report results of a photochemically induced radical abstraction process to liberate efficiently substrates from MPN (and (15) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Miyake, M. J. Phys. Chem. B 2003, 107, 2719-2724. (16) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 33423343. (17) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906-1911. (18) Zhang, Y.; Salaita, K.; Lim, J.-H.; Mirkin, C. A. Nano Lett. 2002, 2, 1389-1392. (19) Shepherd, J. L.; Kell, A. J.; Chung, E.; Sinclar, C.; Workentin, M. S.; Bizzotto, D. J. Am. Chem. Soc. 2004, 126, 8329-8335. (20) Quinn, B. M.; Kontturi, K. J. Am. Chem. Soc. 2004, 126, 71687169. (21) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687-2693. (22) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359. (23) Kim, T.; Chan, K. C.; Crooks, R. M. J. Am. Chem. Soc. 1997, 119, 189-193. (24) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782-3789. (25) Kell, A. J.; Donkers, R. L.; Workentin, M. S. Langmuir 2005, 21, 735-742. (26) Kell, A. J.; Stringle, D. L. B.; Workentin, M. S. Org. Lett. 2000, 2, 3381-3383. (27) Kell, A. J.; Workentin, M. S. Langmuir 2001, 17, 7355-7363. (28) Kell, A. J.; Montcalm, C. C.; Workentin, M. S. Can. J. Chem. 2003, 81, 484-494.

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Figure 3. Thermogravimetric analysis results for the C12MPN prior to irradiation of 0.1 M pinacolone and after 90 and 210 min of irradiation, respectively.

Figure 1. The 1H NMR spectra of: (A) a C12MPN prior to irradiation; (B) after irradiation in the presence of 0.09 M pinacolone. Note the resonances associated with S-(dodecyl)ethanethioate (a), dodecyl-tert-butyl sulfide (b), isobutylene (c), and unreacted pinacolone (d), labeled in spectrum B. (C) and (D) show 1H NMR spectra after the liberated substrates have been washed away from the solution containing the C12MPNs after irradiation of pinacolone in their presence for 90 and 210 min, respectively.

Figure 2. The UV/vis absorption spectra of the C12MPN prior to irradiation of 0.1 M pinacolone and after 90 and 210 min of irradiation, respectively.

self-assembled monolayer, SAM) surfaces in a controlled and mild reaction and further how it can be employed to controllably increase the size of MPN cores. Results As a proof of principle experiment, we first generated radicals in the presence of MPNs with simple alkylthiolate ligands. The starting MPNs with hexanethiolate, dodecanethiolate, and octadecanethiolate ligands, C6MPN, C12MPN, and C18MPN respectively, were synthesized following the Brust procedure5 with modifications reported by Murray and co-workers.6 Briefly, the CxMPN was prepared from the sodium borohydride-promoted reduction of a 1:3 ratio of hydrogen tetrachloroaurate/alkanethiol in the presence of tetraoctylammonium bromide at 0 °C, which generates an MPN with an average core diameter of 1.7 ( 0.5 nm as determined by TEM analysis. Additional characterization by 1H NMR spectroscopy (Figure 1), UV/vis spectroscopy (Figure 2) and thermogravimetric analysis (Figure 3) also suggest the MPN core to be smaller than 2 nm and the solution free of any nonbonded thiol or disulfide ligand. Following preparation and purification, 12 mg of the CxMPN was dissolved in 0.7

mL benzene-d6 and a 1H NMR spectrum was run to represent the sample prior to irradiation. For example, Figure 1A shows that the 1H NMR spectrum of the starting C12MPN has only broad resonances at 0.95 ppm attributed to the protons on the terminal methyl group and 1.151.85 ppm attributed to the methylene protons along the dodecanethiolate chain. As a convenient source of radicals, 3,3-dimethyl-2-butanone (pinacolone) was added to this solution (0.09 M solution), deoxygenated with nitrogen, and then irradiated at wavelengths above 300 nm with a medium-pressure mercury arc lamp contained in a water jacket and a circulating water bath maintaining a temperature of 24 ( 3 °C for 2 h. The irradiation of pinacolone results in the generation of acyl and tert-butyl radicals by the Norrish type I photoreaction.29-31 These radicals subsequently react with the sulfur atom of the alkanethiolate substrate anchored to the MPN surface to liberate the ligand as the S-alkyl ethanethioate and alkyl-tertbutylthioether, respectively (Scheme 1). As shown in Figure 1B for the irradiation of C12MPN, there is 1H NMR spectroscopic evidence for the generation of S-dodecyl ethanethioate (a) and t-butyldodecyl sulfide (b), where the ratio of a/b is 8:1. The only other new resonances appearing in the 1H NMR spectrum are consistent with the generation of isobutylene (c), presumably from the disproportionation of either two tert-butyl radicals or a tert-butyl radical and acyl radical and acetaldehyde from H-atom abstraction by the generated acyl radical. Though a significant concentration of isobutylene is generated during the course of the irradiation, there is very little acetaldehyde generated (there is a 17:1 ratio of isobutylene/ acetaldehyde), suggesting that the acyl radical reacts with the alkylthiolate-anchored ligand more efficiently than by hydrogen-atom abstraction reactions, whereas the tertbutyl radical has other, more competitive reaction channels. The higher reactivity of the acyl radicals with the passivating ligand may also be a result of its more-favored mobility through the alkyl monolayer. Both a and b were isolated from the reaction mixture and characterized by NMR and IR spectroscopy and mass spectrometry. Similar results were obtained using the C6MPN and C18MPN, with the liberation of the analogous sulfides. Pivalophenone (2,2-dimethyl-1-phenylpropanone) can also be employed as the radical precursor, though significantly more aldehyde (4:1 ratio of thiobenzoate/benzaldehyde) was observed during the course of this reaction; this is likely due to the benzoyl radical having a decreased rate of reaction with the MPN compared to the acyl radical, possibly due to sterics hindering its ability to reach the surface efficiently to react with the thiolate. It is important to note that irradiating the CxMPN in the presence of (29) Lewis, F. D.; Magyar, J. G. J. Org. Chem. 1972, 37, 2102-2107. (30) Lewis, F. D.; Hoyle, C. E.; Magyar, J. G.; Heine, H.-G.; Hartmann, W. J. Org. Chem. 1975, 40, 488-492. (31) Lewis, F. D.; Lauterbach, R. T.; Heine, H.-G.; Hartmann, W.; Rudolph, H. J. Am. Chem. Soc. 1975, 97, 1519-1525.

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Scheme 1. Cartoon Representation of the Photochemically Induced Radical Size Evolution of the MPN Cores Facilitated by the Norrish Type I Photochemical Reaction of t-Butyl Ketones and Their Subsequent Abstraction of Alkylthiolate Ligands from the MPN Surface

pinacolone (or pivalophenone) at wavelengths corresponding to the plasmon absorption band at 450-550 nm (and wavelengths where pinacolone has no absorption) using an appropriate filter results in no reaction. Though the photochemically generated radicals abstract the alkylthiolate passivating ligands from the MPN surface, the solution remains dark brown in color and there is no evidence of a precipitate or elemental gold, the latter would result from a complete loss of the passivating alkylthiolate ligands. Because the alkylthiolate shell is responsible for the solubility of the MPN in benzene, it appears as though the large concentration of alkylthiolate abstracted from the surface does not render the MPN insoluble. To elucidate how the CxMPN Au core itself is affected by this reaction, the photochemistry was followed as a function of irradiation time in the presence of 0.1 M radical precursor using a number of identically prepared samples that were removed after various times during the irradiation. Following 1H NMR analysis, the resulting CxMPN solutions were concentrated and the liberated thioacetate, dialkylsulfide and isobutylene products, along with any unreacted pinacolone, were washed away from the remaining CxMPN using acetonitrile. The NMR spectra show that the resonances associated with the MPN-bound alkylthiolates get progressively broader, consistent with CxMPNs of progressively larger sizes as the irradiation time proceeds. Representative 1H NMR spectra of the isolated C12MPN are shown in Figures 1C and D. Also notably, the UV/vis spectra, thermogravimetric analyses (TGA), and transmission electron micrographs (TEM) of the isolated CxMPN also suggest that the MPN cores are increasing in size as the irradiation proceeds. These data are shown for C12MPN in Figures 2, 3, and 4, respectively. The TEM images, as well as the histograms derived from them (Figure 4), provide valuable information concerning how the MPN cores are increasing in size over the course of the irradiation. In the case of the irradiation of the pinacolone in the presence of the C12MPNs, there is a size increase from 1.7 ( 0.5 nm prior to irradiation to 2.7 ( 0.8 and 5.1 ( 0.9 nm after 90 and 210 min, respectively. Closer examination indicates that the larger nanoparticles are formed at the expense of smaller MPNs. For example, prolonged irradiation of C12MPN results in the disappearance of the smallest nanoparticles (1.5, 1.8, and 2.0 nm) as larger nanoparticles are generated. As the C12MPN is irradiated for 210 min, many of the nanoparticles up to diameters as large as 3.2 nm have coalesced into much larger nanoparticles. The core size evolution was also followed by UV/vis spectroscopy, where the plasmon absorption band at 518 nm increases in intensity as the MPN core size increases (Figure 2).6 Prolonged irradiation times (exceeding about 300 min) of the MPN solutions containing ∼0.1 M pinacolone result in a broadening and dampening of the

plasmon absorption in the UV/vis absorption spectrum and the precipitation of larger MPNs. However, when the irradiation is monitored and stopped prior to any significant broadening of the plasmon absorption band, the MPNs of size 5.1 ( 0.9 nm remain soluble in nonpolar organic solvents even three months after storage. A valuable technique allowing the determination of the MPN composition is TGA.6 In studying a series of MPNs synthetically tailored to have increasingly larger gold core diameters, Murray established that as the core diameter increases the percentage organic decreases.6 For example, a 140-gold-atom nanoparticle surrounded by dodecanethiolate is 27% organic by weight, whereas a 2951-goldatom nanoparticle surrounded by dodecanethiolate is 11% organic by weight.6 The decreased percentage organic by weight is related to both differences in the packing density of alkylthiolates on the increasingly larger MPN surfaces and the fact that, as the core increases in size, a significant number of gold atoms are present in the core that are not involved in binding alkylthiolates to the surface. According to the models proposed by Murray which take into account the diameter of the MPN core by TEM analysis and the percentage organic determined from the TGA experiments (Figure 3), the average stoichiometry of the starting C12MPN employed in this study is ca. Au140(SC12)53 and that of the size-evolved MPN after 90 min of irradiation is ca. Au870(SC12)163.6 This corresponds to ∼6 MPN cores coming together to make one, but interestingly, for the resulting MPN to be covered by a full shell of alkanethiolate, ∼50% of the substrate can be is abstracted by the photochemically generated radicals. When the irradiation is carried out for 210 min, the average stoichiometry of the MPN is ∼Au4794(SC12)506, which corresponds to the coalescence of ca. 34 Au140(SC12)53 nanoparticles and would require the liberation of approximately 75% of the surface ligands, if the resulting nanoparticle retained good surface coverage. The solid-state IR spectra of the MPNs recorded postirradiation suggest that there is good dodecanethiolate surface coverage, as evidenced by the stretching frequencies for the methylene units on the dodecanethiolate substrates anchored to the MPN all being centered close to 2920 and 2850 cm-1, respectively, whereas those for liquid (disordered methylenes) are at higher frequency (Figure 5).32,33 There is very little difference in the course of the reaction with the starting MPN, C6MPN, C12MPN, and C18MPN, with the exception that the reaction proceeds more rapidly with the shorter alkyl chain. Zhong and co-workers11-13 and Miyake and co-workers14,15 propose that the evolution in size for the MPN cores subjected to heat-treatment in the presence of tetraoctylammonium bromide involves a sequential de(32) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (33) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.

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Figure 4. The transmission electron micrograph (TEM) images acquired for the C12MPN (A) prior to irradiation, (B) following irradiation of 0.1 M pinacolone in the presence of C12MPN for 90 and (C) 210 min. The average diameter of the C12MPN at each time is provided on the histogram. Please note that the scale in Figure A is 20 nm and that in B and C is 10 nm.

Figure 5. The IR spectrum (a) of C12H25SH and the IR spectra (d), (c), and (b) are the C12MPN prior to irradiation of 0.1 M pinacolone and after the radical abstraction reaction of dodecanethiolate from its surface by the photochemically generated acyl and tert-butyl radicals after 90 and 210 s, respectively. Note that the stretching frequencies for dodecanethiolate anchored to the MPN surface are at lower wavenumber, indicative of a more crystalline environment for the substrate when anchored to the MPN surface.

sorption of dodecanethiolate (either as thiolate or disulfide) that serves to expose a portion of the gold core, the eventual coalescence of gold cores as a result of surface exposure, and the reattachment of alkylthiolate or dialkyl disulfide to the nanoparticle surface. It is believed that the tetraoctylammonium bromide partially passivates the MPN surface during this growth process. In concert with this surface exposure, it is proposed that the metal

nanoparticles have size-dependent surface-melting properties,34,35 and this melting phenomenon is believed to drive the coalescence of the cores. However, our approach to the core size evolution is different because there is no temperature elevation, so the only way that the cores can coalesce is through the liberation of enough substrate from the surface to allow a number of partially exposed cores to coalesce into one. When the irradiation is performed in the presence of tetraoctylammonium bromide to provide weak, transient protection to the MPN surface, no differences in results were detected, unlike the heattreatment methodologies where it is vital to the growth procedure.11-15 In explaining how the place-exchange reaction occurs, Murray and co-workers suggest that an incoming ligand first displaces an MPN-anchored substrate from edge and vertex sites.24,36,37 We postulate that the abstraction reaction is also occurring most efficiently at the edge and vertex sites of the MPN surface. In an attempt to elucidate where the abstraction does occur most readily, a selective place-exchange reaction was utilized to populate predominantly the edge and vertex sites of the C12MPN with 11-methoxyundecanethiol through a short place-exchange (34) Wang, Z. L.; Petroski, J. M.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 6145-6151. (35) Dick, K.; Dhanasekaran, T.; Zhang, Z.; Meisel, D. J. Am. Chem. Soc. 2002, 124, 2312-2317. (36) Song, Y.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 70967102. (37) Donkers, R. L.; Song, Y.; Murray, R. W. Langmuir 2004, 20, 4703-4707.

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reaction on a C12MPN.25 It was hoped that we would see, at shorter irradiation times, a larger concentration of the newly introduced 11-methoxyundecanethiolate liberated from the surface; however, the ratio of the different ethanethioates in solution matched that originally anchored to the MPN surface. This could mean that the reaction is indiscriminate on the MPN surface or, more likely, that the liberation of an initial substrate likely from an edge or vertex allows better access to interior terrace-bound substrates because there are open spaces once the abstraction occurs. In addition, we considered the possibility that the mechanism involves the acyl radical binding to the gold surface and subsequently reacting with a thiolate ligand to liberate an ethanethiolate ligand. Nakamoto and co-workers recently demonstrated that gold nanoparticles can be passivated with alkyl chains without thiols or amines as the binding group through gold-carbon bonds.38 This suggests the possibility of the radicals generated here being able to form bonds directly with the gold surface; however, we see no evidence for the presence of carbonyl groups in the IR spectra of the size evolved CxMPNs. Currently, we feel the best description for the size evolution simply involves the stripping of a significant portion of one face of the MPN surface and the subsequent coalescence of at least two MPN cores. In agreement with a number of recent reports where core sizes are evolved through a heat-treatment procedure at temperatures below 110 °C,9-13 we estimate the maximum core size we can achieve where the MPN remains readily soluble in nonpolar solvents is ∼5-6 nm, at least with the alkanethiolate we utilized as the passivating ligand. The ability for the acyl and benzoyl radicals to promote the liberation of organic substrates from the MPN surface through a mild photochemical reaction makes this process attractive to liberate SAM ligands from two-dimensional surfaces as well. One can imagine this process being a useful means of patterning SAMs, which has received attention over the past decade.39-46 To test the ability for the acyl and tert-butyl radicals to abstract thiolates from two-dimensional surfaces, we did a simple experiment where the pinacolone was irradiated in the presence of a dodecanethiolate (C12)-modified polycrystalline gold electrode. Cyclic voltammetry was used to monitor the reaction between the photochemically generated radicals and the C12-modified SAM electrode. This was accomplished by taking advantage of the ability for alkylthiolate monolayers on gold electrodes to block the reversible redox coupling of ferricyanide (Fe2+/Fe3+).23,47,48 As shown in Figure 6, prior to the formation of a monolayer on the gold electrode, there is a reversible redox couple for the 1 mM ferricyanide in aqueous solution with 1 M sodium perchlorate as electrolyte. When the gold electrode is soaked in a 1 mM solution of dodecanethiolate in absolute ethanol (38) Nakamoto, M.; Yamamoto, M.; Fukusumi, M. Chem. Commun. 2002, 1622-1623. (39) Brewer, N. J.; Rawsterne, R. E.; Kothari, S.; Leggett, G. J. J. Am. Chem. Soc. 2001, 123, 4089-4090. (40) Zhang, Y.; Terrill, R. H.; Bohn, P. W. Chem. Mater. 1999, 11, 2191-2198. (41) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305-5306. (42) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626-628. (43) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024-1032. (44) Hu, J.; Liu, Y.; Khemtong, C.; El Khoury, J. M.; McAfoos, T. J.; Taschner, I. S. Langmuir 2004, 20, 4933-4938. (45) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395-4404. (46) Frisbie, C. D.; Wollman, E. W.; Wrighton, M. S. Langmuir 1995, 11, 2563-2571. (47) Fox, M. A.; Wooten, M. D. Langmuir 1997, 13, 7099-7105. (48) Wolf, M. O.; Fox, M. A. Langmuir 1996, 12, 955-962.

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Figure 6. The liberation of dodecanethiolate (C12) from the surface of a SAM prepared on a polycrystalline gold electrode by radicals generated after the irradiation of pinacolone. (a) The cyclic voltammagram (CV) of 1 mM ferricyanide on the unmodified gold electrode (b) the CV of ferricyanide on the electrode modified with a C12 thiolate monolayer and (c) the CV obtained after irradiation of pinacolone in the presence of the C12-modified electrode for 180 s.

for 24 h, there is a large current decrease and the electrochemical activity of the ferricyanide is no longer detected because the electrode has been passivated by the dodecanethiolate monolayer. This current decrease and the observed cyclic voltammagrams of the blocked gold electrodes are consistent with those published for other organic thiol SAM systems using the same redox probe.23,47-51 The C12-modified electrode was then placed in a nitrogen-saturated solution of benzene with 3 × 10-2 M pinacolone, and the solution of pinacolone was irradiated with a medium-pressure Hanovia arc lamp for intermittent times over 180 s. As shown in Figure 6, the irradiation of pinacolone in the presence of the C12SAM results in the recovery of the current consistent with loss of the dodecanethiolate ligand from the surface. The redox wave does not recover to the original values most likely because released substrates can physisorb to the electrode surface or the substrates of a partially covered surface may lay down on the surface, which would serve to partially block the electrode surface. Direct irradiation of the C12-modified gold electrode in a benzene solution in the absence of the radical precursor showed no evidence for a decomposition and loss of blocking. The photochemical generation of acyl and tert-butyl radicals in the presence of alkylthiolate-modified gold MPNs has been shown to be a simple and versatile method of both controllably liberating substrates from MPN surfaces and evolving the size of MPN cores. The reaction is very efficient, resulting in the generation of a thioacetate and tert-butyl thioether that can be easily separated from the MPN, while the MPN core can be increased to diameters in excess of 5 nm. We are currently unable to determine if the ligand liberation is a radical abstraction process or one that involves reduction of the incoming radical by the Au, followed by a nucleophilic displacement of ligand. Generally speaking though, this reaction provides a mild route for the evolution of core sizes in MPNs and provides an efficient process whereby MPNs can be employed as scaffolds for the controlled release of (49) Li, W.; Lynch, V.; Thompson, H.; Fox, M. A. J. Am. Chem. Soc. 1997, 119, 7211-7217. (50) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (51) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatini, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660-3667.

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substrates into solution and opens a new avenue for general surface modifications. We are currently exploring these possibilities. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada (NSERC), the Province of Ontario for a Premier’s Research Excellence Award, the Canadian Foundation of Innovation (CFI), the Ontario Research and Development Challenge Fund (ORDCF), and The University of Western Ontario (ADF) for supporting this research. A.J.K. thanks NSERC Canada and the Province of Ontario for graduate scholar-

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ships. A.A. thanks the Iranian Ministry of Science and Technology for a Visiting Graduate Fellowship Grant, and his supervisor, Professor D. Habibi, for the opportunity to study abroad. TEM was performed at the Nanoimaging Facility at Simon Fraser University. TGA was carried out by Elizabeth Turner. Supporting Information Available: Experimental details and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. LA051655M