Synthesis and Characterization of Gold− Silica Nanoparticles

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Synthesis and Characterization of Gold-Silica Nanoparticles Incorporating a Mercaptosilane Core-Shell Interface Michael Ming Yu Chen and Alexander Katz* Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720-1462 Received June 11, 2002. In Final Form: August 6, 2002 The first synthesis and characterization of gold-silica core-shell nanoparticles with a mercaptosilane interface between the core and shell is reported. These nanoparticles exploit the strong interaction between thiols and gold to create a well-defined interface of functional group organization between core and shell. This requires the synthesis of a mercaptosilane monolayer on the colloidal gold surface, which is accomplished indirectly using 1 as a protected precursor to 3-mercaptopropyltriethoxysilane. 1 binds to gold, undergoes sol-gel hydrolysis and condensation, and subsequently deprotects to a thiol via gold-catalyzed thioester hydrolysis. This process results in a monolayer of condensed mercaptosilane on the nanoparticle surface without inducing colloidal instability, which was observed upon direct mercaptosilane addition at the same surface coverage. Similar results were obtained for the previously reported thioester 2, which also binds to gold and deprotects in the bound state, as does 1, but lacks sol-gel active functional groups. Binding experiments with 1 show that its surface-bound thioester has a significantly higher affinity to gold compared with that of 2, which is consistent with polysiloxane formation upon binding 1. Titration of 1 with colloidal gold shows a similar packing density at saturation coverage for 1 as that observed for 2, comprising a footprint of approximately 25 Å2 per molecule, and strongly suggests formation of a two-dimensional polysiloxane network on the nanoparticle surface at saturation coverage of 1. Thioester hydrolysis of surface-bound 1 is catalyzed by gold with an apparent activation energy of 7.8 kcal/mol, which is approximately half of the value observed for 2, and a preexponential factor of 967 s-1, which is more than four decades smaller than the value for 2. These differences in the kinetic parameters are consistent with a significant mass transport limitation for thioester hydrolysis in surface-bound 1, resulting from the cross-linked polysiloxane network, which is not present in the case of 2. Growth of a silica shell after mercaptosilane monolayer synthesis can be observed via transmission electron microscopy. The resulting gold-silica core-shell nanoparticles are robust in that they can withstand organic solvent environments, as well as short-chain thiols, and maintain colloidal stability.

Introduction There has been much interest in the synthesis of goldsilica core-shell nanoparticles for diverse applications,1 which include the templating of material syntheses for molecular confinement and optoelectronics.2 The general synthetic approach relies on the binding of an organosilane to citrate-stabilized colloidal gold.1 The organosilane binds to the nanoparticle surface, and silanols rapidly form upon alkoxide hydrolysis and condense during silica shell growth.3,4 The organosilane thus forms the interface between core and shell. Although aminosilanes such as 3-aminopropyltrimethoxysilane have been successfully used for this purpose,1d,2-4 the relatively weak coordination of amines to gold can ultimately lead to some degree of disorder at the core-shell interface.5 Our objective is to (1) (a) Mulvaney, P.; Liz-Marza´n, L. M.; Giersig, M.; Ung, T. J. Mater. Chem. 2000, 10, 1259. (b) Yau, S.-T.; Mulvaney, P.; Xu, W.; Spinks, G. M. Phys. Rev. B 1998, 57, 124. (c) Dick, K.; Dhanasekaran, T.; Zhang, Z.; Meisel, D. J. Am. Chem. Soc. 2002, 124, 2312. (d) Liz-Marza´n, L. M.; Mulvaney, P. New J. Chem. 1998, 22, 1285. (e) Liz-Marza´n, L. M.; Philipse, A. P. J. Colloid Interface Sci. 1995, 176, 459. (f) Giersig, M.; Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. Adv. Mater. 1997, 9, 570. (g) Philipse, A.; van Bruggen, M. P. B.; Pathmamanoharan, C. Langmuir 1994, 10, 92. (2) Makarova, O. V.; Ostafin, A. E.; Miyoshi, H.; Norris, J. R.; Meisel, D. J. Phys. Chem. B 1999, 103, 9080. (3) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (4) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Chem. Commun. 1996, 731. (5) (a) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (b) Bharathi, S.; Fishelson, N.; Lev, O. Langmuir 1999, 15, 1929.

synthesize gold-silica core-shell nanoparticles with the highest possible degree of functional group organization at the core-shell interface, for subsequent templating of material syntheses, and involves exploiting the strong interaction between thiols and gold to accomplish this. Several thiols have been previously reported to form monolayers on colloidal gold that has been stabilized with citrate and/or tannic acid.6 Among them, mercaptosuccinic acid, mercaptopropionic acid, sodium 4-mercaptobenzoate, sodium mercaptoethanoate, sodium 10-mercaptodecanesulfonate, 11-mercaptoundecanoic acid, and sodium 3-mercaptopropionate have all been used for this purpose.6 There is another category of thiols, however, which, in contrast to the first, has been reported to induce colloidal instability upon addition to gold sol, which can be ascertained by a shift in the surface plasmon resonance absorption band to the infrared (indicative of aggregation).7,8 These include sodium 3-mercaptopropanesulfonate,6a 1-butanethiol (vide infra), as well as other sulfur-containing molecules such as thionicotinamide.8 Although the synthetic method developed by Brust et al. has been applied to anchor a variety of thiols onto gold nanoparticles,9 it is limited to rather small nanoparticle core diameters (typically below 5.2 nm), and using larger excesses of thiols such as (6) (a) Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846. (b) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408. (7) Thomas, K. G.; Kamat, P. V. J. Am. Chem. Soc. 2000, 122, 2655. (8) Fujiwara, H. F.; Yanagida, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 2589.

10.1021/la026055r CCC: $22.00 © 2002 American Chemical Society Published on Web 10/03/2002

Mercaptosilane Core-Shell Interface

dodecanethiol and butanethiol in the synthesis has been reported to produce irreversible colloidal aggregation.10 Our current approach therefore follows literature precedent and uses the larger citrate-stabilized colloidal gold as a scaffold for core-shell nanoparticles.1 The first synthetic step requires that mercaptosilane chemisorb on the surface of colloidal gold and self-assemble into a monolayer. This has been previously demonstrated on two-dimensional surfaces for the purpose of forming an interfacial adhesive layer between gold and silica.11-13 However, upon adding various mercaptosilanes directly to citrate-stabilized gold sol, we visually observed colloidal aggregation after several minutes following addition. We postulated that the cause of colloidal instability in this case may be due to the time scale associated with thiol monolayer self-assembly on gold,14,15 which has been reported to have kinetics of several hours6b,16 and is significantly longer than the time scale associated with nanoparticle aggregation. To prepare stable gold nanoparticles with a thiolate monolayer in aqueous solution, a protected thiol in the form of a thioester is used, which has been previously used as a precursor to thiolate monolayers on two-dimensional gold surfaces.12a,b,17 Building on our previous experience with the interaction between thioesters and gold,18 1 was synthesized as a precursor to 3-mercaptopropyltriethoxysilane. The pyrene fluorescence tag in 1 provides a sensitive probe of local environment, which we have previously used for demonstrating organosulfur functional group binding to colloidal gold surfaces using steady-state fluorescence.18 The butyl tethers that are attached to the sulfur maintain a certain degree of hydrophobicity within the layer of bound molecules, so as to promote organization into hydrophobic domains within the monolayer, while simultaneously facilitating a limited degree of water solubility. Thioester 1 is expected to self-assemble on colloidal gold in a fashion similar to that of the previously reported 1-pyrenebutanethioic acid S-butyl ester (2), which lacks sol-gel active functional groups.18 Surface-bound 1 is expected to rapidly (9) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655. (c) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359. (d) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (e) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (f) 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. (g) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262. (h) Sarathy, K.; Raina, G.; Yadav, R.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (10) Templeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564. (11) (a) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Chem. Phys. Lett. 1999, 300, 651. (b) Fleming, M. S.; Walt, D. R. Langmuir 2001 17, 4836. (12) (a) Wasserman, S. R.; Biebuyck, H.; Whitesides, G. M. J. Mater. Res. 1989, 4, 886. (b) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621. (c) Goss, C. A.; Charych D. H.; Majda, M. Anal. Chem. 1991, 63, 85. (13) (a) Thompson, W. R.; Pemberton, J. E. Chem. Mater. 1995, 7, 130. (b) Thompson, W. R.; Cai, M.; Ho, M.; Pemberton, J. E. Langmuir 1997, 13, 2291. (14) Heller, W.; Pugh, T. L. J. Polym. Sci. 1960, 47, 203. (15) Heller, W.; Pugh, T. L. J. Polym. Sci. 1960, 47, 219. (16) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (17) (a) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 37. (b) Skulason, H.; Frisbie, C. D. J. Am. Chem. Soc. 2000, 122, 9750-9760. (18) Chen, M. M.-Y.; Katz, A. Langmuir 2002, 18, 2413.

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undergo silicon alkoxide hydrolysis and condensation,4 in addition to gold-catalyzed thioester hydrolysis.18 In the current study, we demonstrate that the sol-gel condensation and resulting formation of a polysiloxane of 1 on the nanoparticle surface has profound effects on its binding affinity to gold, as well as its rate of gold-catalyzed thioester hydrolysis relative to 2. Using this process, the synthesis of a mercaptosilane monolayer on the surface of a stable colloidal gold nanoparticle in aqueous solution is accomplished. The resulting assembly is subsequently used as a nucleation site for silica shell growth, to synthesize the first gold-silica core-shell nanoparticles incorporating a mercaptosilane interface between the core and shell. The overall process reported here for core-shell nanoparticle synthesis is schematically represented in Figure 1.

Figure 1. Schematic illustration of gold-silica core-shell nanoparticle synthesis. (a) 12.5 nm citrate-stabilized colloidal gold nanoparticles (circle filled with dots) are treated with 1, which binds to the nanoparticle surface and undergoes rapid sol-gel hydrolysis and condensation to a two-dimensional polysiloxane network comprising a monolayer of thioester on gold (annulus with diagonal lines). (b) Gold-catalyzed thioester hydrolysis converts the thioester monolayer in part (a) to a thiolate monolayer (annulus with heavy concentration of dots) and concomitantly releases 1-pyrenebutyric acid. (c) Sto¨ber conditions are used to preferentially nucleate silica shell (annulus with sparse concentration of dots) growth on the nanoparticle assembly following part (b) and complete the synthesis of a gold-silica core-shell nanoparticle incorporating a mercaptosilane interface between the core and shell.

Experimental Section General. 1H and 13C NMR were performed on Bruker AMX 300 and 400 MHz machines at UCB. FAB mass spectra were recorded at the UCB Mass Spectrometry Facility. UV/vis spectroscopy was performed on a Varian Cary 400 Bio UV/vis spectrophotometer using a water baseline correction. Transmission electron microscopy (TEM) was performed at the Electron Microscopy Laboratory at UCB on a FEI Tecnai 12 instrument using an accelerating voltage of 120 kV. TEM samples were prepared by drying a drop of solution on a carbon-coated copper grid.

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Materials. All chemicals were purchased at the highest possible level of purity and were used as received unless stated otherwise. Tetrachloroauric acid was purchased from Acros. Absolute ethanol was distilled under nitrogen in a glass apparatus prior to usage. 1-Pyrenebutyric acid was purchased from Aldrich at the highest available purity and subsequently recrystallized three times from benzene prior to use. Thioester 2 was synthesized as described previously.18 Unless stated otherwise, water that was used in these experiments was distilled, purified by a Barnstead Nanopure Infinity system to possess at least 18 MΩ purity, and filtered through a 0.2 µm membrane. For gold colloid syntheses, this water was further distilled three times in a glass distillation apparatus. 1-Pyrenebutanethioic Acid S-[3-(Triethoxysilyl)propyl] Ester (1). Carbonyldiimidazole (0.44 g, 2.72 mmol) was dissolved in 6 mL of anhydrous THF in a dry airless flask, to which was added 2 mL of a 1.3 M solution of 1-pyrenebutyric acid in anhydrous THF. The resulting solution was stirred overnight at room temperature. In a 250 mL two-neck flask, sodium ethoxide (0.4 g, 5.88 mmol) was suspended in 50 mL of THF and subsequently 3-mercaptopropyltriethoxysilane (0.743 g, 3.12 mmol) was added to this mixture. The contents of the airless flask were mixed into the two-neck flask, and the reaction mixture was carefully heated with an oil bath maintained at 40 °C for approximately 30 min and was monitored using thin-layer chromatography. Subsequently, the reaction mixture was cooled to room temperature and 450 mL of ether and 6 g of silica gel were added. The mixture was filtered, concentrated to a dark yellow oil, and purified by silica chromatography (Silica Gel 60 and 10.0/1.0 v/v hexane/ethyl acetate) to yield an oil (0.37 g, 0.7013 mmol, yield 27%). 1H NMR (CDCl3): 0.725 (2H, t, J ) 8.4 Hz, CH2); 1.210 (9H, t, J ) 7.1 Hz, OCH2CH3); 1.712 (2H, m, CH2); 2.228 (2H, m, CH2); 2.704 (2H, t, J ) 7.2 Hz, CH2); 2.942 (2H, t, J ) 7.4 Hz, CH2); 3.386 (2H, t, J ) 7.8 Hz, CH2); 3.807 (6H, q, J ) 7.0 Hz, OCH2CH3); 7.850-8.262 (9H, m, Ar-H). 13C{1H} NMR (CDCl ): 10.05 (CH ); 18.31 (OCH CH ); 23.39 3 2 2 3 (CH2); 27.47 (CH2); 31.84 (CH2); 32.60 (CH2); 43.68 (CH2); 58.44 (OCH2CH3); 123.28 (Ar-C); 124.80 (Ar-C); 124.94 (Ar-C); 125.00 (Ar-C); 125.11 (Ar-C); 125.86 (Ar-C); 126.75 (Ar-C); 127.36 (ArC); 127.45 (Ar-C); 127.5 (Ar-C); 128.73 (Ar-C); 130.02 (Ar-C); 130.91 (Ar-C); 131.42 (Ar-C); 135.57 (Ar-C); 199.28 (CdO). Mass spectrum (FAB) m/z 508.2105 (C29H36O4SSi 508.2104). Preparation of Gold Sol. The gold sol was synthesized according to previously published procedures19 and characterized via transmission electron microscopy and UV/vis spectroscopy. The gold sol diluted with four parts of water per part of sol had a measured λmax of 519 nm (516-520 literature specification), an absorbance of 0.7 (0.7-0.9 literature specification), and a full peak width at half maximum of 86 nm (80-90 nm literature specification). Synthesis of Gold-Silica Core-Shell Particles. A 0.246 mM fluorophore stock solution of 1 in absolute ethanol was freshly prepared. 48 mL of water, 1.75 mL of gold sol, and 0.25 mL of stock solution were added to a scintillation vial. The solution was maintained at 40 °C during thioester deprotection. The total extent of deprotection was monitored via steady-state fluorescence, which indicated that complete deprotection was achieved after approximately 72 h. To the fully deprotected solution, either 0.5 mL (low [SiO2]/[Au] ratio) or 2 mL (high [SiO2]/[Au] ratio) of 0.54 wt % sodium silicate at pH 10-11 was added. The resulting dispersion was allowed to stand for a period up to 28 days for silica nucleation and growth at room temperature. 4 mL aliquots were periodically removed after 48 h and tested for coagulation upon exposure to 1-butanethiol using UV/vis spectroscopy to monitor silica growth. Dialysis was subsequently performed with a 10 000 MWCO dialysis membrane (Pierce) that was submersed in 2 L of deionized water for 24 h (2 cycles × 12 h per cycle), and the colloidal gold was concentrated via centrifugation at 4500 rpm for 15 min. Gold Sol Stability Analysis. Stock solutions at a fixed concentration of 0.246 mM using 1-butanethiol, 3-mercaptopropyltrimethoxysilane, and 1 in absolute ethanol were freshly prepared and stored in the dark at 5 °C. 3.86 mL of water, 0.14 mL of gold sol, and 20 µL of the corresponding stock solution were mixed. The UV-vis spectrum of the mixture was collected after 24 h except in the case of 1, which was collected following

Chen and Katz full thioester deprotection, as monitored via steady-state fluorescence. A similar procedure was used for the core-shell nanoparticles. Steady-State Fluorescence. Measurements were performed on a Hitachi F-4500 fluorimeter equipped with a 150 W Xe-lamp source. The signal-to-noise ratio was greater than 100/1 using the Raman band of water as a standard. All spectra were corrected using a Rhodamine B standard unless noted otherwise. Wavelength accuracy was checked using the Xenon line at a wavelength of 450.1 nm with a diffuser cell and was within (2.0 nm. A thermostated extended temperature compartment with a temperature range of 5-60 °C was used to control the cell temperature. The F-4500 was interfaced to a computer through a National Instrument PCI-GPIB interface card, and data acquisition and analysis software was provided by the Hitachi F-4500 Fluorescence Spectroscopy FL solutions software (2001 release). Emission experiments were conducted with an excitation wavelength of 278.0 nm, which is at a maximum ultraviolet absorbance for all of the fluorophores investigated. The entry beam was passed through a 320 nm filter (Hoya UV-32) to eliminate secondorder Rayleigh scatter from the excitation beam. A PMT voltage of 950 V and an excitation slit width of 5.0 nm were employed in all fluorescence experiments unless specified otherwise. All emission scans were collected from 360 to 600 nm. The fluorescence cuvettes used were purchased from Hitachi Instruments (Hitachi AN0-1804) and comprised 10 mm2 by 45 mm high ultrahigh-quality quartz cells with four sides polished and no background fluorescence. There was no significant background fluorescence measured from the colloidal gold under the conditions used in this investigation. Approximately 30 s of argon purging was performed on all samples prior to measurement. Concentration-Dependence Studies. Samples for titrating colloidal gold at a fixed fluorophore concentration were prepared as follows. A 0.246 mM fluorophore stock solution of 1 in absolute ethanol was freshly prepared. 4 mL of water was added to clean scintillation vials, and a set of solutions containing 0, 3, 7, 10, 15, 20, and 29 µL of gold sol was added to these vials. 2 µL of fluorophore stock solution was added to these vials immediately prior to measurement. Separate sets of solutions for cases using 5 and 8 µL of the fluorophore stock solution were also prepared. Upon preparing a solution for fluorescence measurement, the solution was transferred from the scintillation vial to a fluorescence cuvette via a glass pipet. The cuvette was then placed in a thermostated cell holder, which was maintained at a fixed temperature of 7 °C. The fluorescence emission spectra were measured for each solution, and the emission slit width was adjusted to maximize the signal-to-noise ratio (set to 2.5 nm for solutions containing 8 µL of 1 and 5.0 nm for solutions containing 2 µL and 5 µL of 1 stock solution). For studies of equilibrium limited binding involving dilution, solutions of varying concentrations were made that had a fixed ratio of 7.25/1 v/v gold sol/ fluorophore stock solution. The emission slit width was maintained fixed at 2.5 nm, and the temperature was also maintained at 7 °C. Kinetics Studies. A 0.246 mM fluorophore stock solution of 1 in absolute ethanol was freshly prepared and stored in the dark at 5 °C. 3.12 mL of water, 0.88 mL of gold sol, and 2 µL of fluorophore stock solution (corresponding to approximately 1.65% of saturation coverage) were mixed in a clean scintillation vial. The pH of the final solution was measured to be 6.3. The solution was transferred from the vial to a fluorescence cuvette via a glass pipet. Emission scans were recorded at 3-min intervals using the time-delay feature of the fluorescence software with an emission slit width of 5 nm. The experiment was performed at temperatures of 7, 15, 25, and 35 °C. Note that, in all cases investigated, thioester binding to gold occurred on a time scale that was instantaneous compared with the fluorescence measurement (∼30 s).

Results UV/Vis Spectroscopy of Colloidal Stability. To assess the relative stability of colloidal gold that has been treated with various organosulfur compounds, UV/vis spectroscopy was performed in the wavelength region of the surface plasmon resonance band. Citrate-stabilized

Mercaptosilane Core-Shell Interface

gold sol was treated with an equal coverage corresponding to approximately one monolayer of 1, 3-mercaptopropyltrimethoxysilane, and 1-butanethiol. 1 was allowed to undergo complete thioester hydrolysis, as monitored via steady-state fluorescence of released 1-pyrenebutyric acid. Mixtures of short-chain thiols and gold sol were investigated after a period of 24 h and were used to differentiate between effects of a silanol versus hydrophobic methyl monolayer terminus. Large red shifts and broadening of the surface plasmon band were observed upon addition of these short-chain thiols to gold in Figure 2, whereas relatively little change was observed for the case of the fully hydrolyzed 1. Figure 3 shows changes in the UV/vis spectrum accompanying thioester hydrolysis of 1 in the wavelength region that is associated with pyrene electronic transitions. Significant shifts are observed during deprotection, in going from surface-bound thioester to the fully deprotected state, as monitored via steady-state fluorescence. The spectrum corresponding to deprotected 1 overlaps identically with bands for an authentic sample of 1-pyrenebutyric acid in gold sol (vertical dashed line marks one of the bands for visual clarity). The ordinate in Figure 3 represents a single linear absorbance scale for all four spectra, with the baseline for each spectrum translated for ease of visual comparison between spectra. Steady-State Fluorescence Spectrscopy of Fluorophore Binding. The fluorescence emission spectrum of 1 in aqueous solution is amplified significantly upon addition of colloidal gold, as observed previously with 2.18 Figure 4 shows a titration of 1 at a fixed concentration of 123.5 nM with varying amounts of colloidal gold. The total area of the emission spectrum (comprising mostly excimer band for the bound thioester) was calculated at each colloidal gold concentration to generate the titration curve. Control experiments corresponding to titrations with varying concentrations of 1 were performed and showed that the amount of colloidal gold necessary to achieve surface saturation with 1 increased commensurately with the fluorophore concentration as shown in the inset of Figure 4. The binding constant for 1 to gold was investigated by successively diluting samples at a fixed surface coverage of 1 on colloidal gold corresponding to saturation (full monolayer coverage). The steady-state fluorescence emission spectrum was measured at several increments of decreasing total concentration for this system, in an attempt to observe departure from dilution behavior (representing an infinite binding constant) due to equilibrium binding limitations. Figure 5 represents the total fluorescence emission band area (comprising mostly excimer emission) versus the fluorophore concentration from these measurements. The concentration range for this study was chosen on the basis of previously observed equilibrium limitations for the binding of 2 to colloidal gold.18 Steady-State Fluorescence of Thioester Hydrolysis of 1 on Gold. The kinetics of thioester hydrolysis for surface-bound 1 were investigated by measuring fluorescence emission spectra under pseudo-first-order conditions, as described previously.18 The inset of Figure 6 shows the dynamic evolution of these spectra during the course of the hydrolysis reaction for a system that was initially at saturation coverage of 1. Rate coefficients based on the measured initial rate of surface-bound 1 consumption are represented in Figure 6. Characterization of Gold-Silica Core-Shell Nanoparticles. The multistep synthesis of the gold-silica core-shell nanoparticles was followed by UV/vis spec-

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troscopy of the surface plasmon resonance band. Figure 7 shows the UV/vis spectra of colloidal gold nanoparticles incorporating a thiolate monolayer, which results from the hydrolysis of 1 on the gold surface, gold-silica coreshell nanoparticles incorporating a mercaptosilane interface between the core and shell, and a 1-butanethioltreated sample of the core-shell nanoparticles. Figure 8 shows TEM images of core-shell nanoparticles that were synthesized using lower (Figure 8a) and higher (Figure 8b-d) [SiO2]/[Au] ratios during silica shell synthesis. Discussion The first step in core-shell nanoparticle synthesis comprises monolayer formation on the surface of colloidal gold and typically involves the direct addition of organosilane to gold sol. This step can be characterized using UV/vis spectroscopy, since the surface plasmon resonance band of citrate-stabilized gold nanoparticles is known to shift to the near-infrared upon loss of colloidal stability and thus provides a sensitive probe for the integrity of nanoparticle isolation within a sol.7,8,14,15 Figure 2 shows the effect of adding 1-butanethiol and 3-mercaptopropyltrimethoxysilane to a citrate-stabilized gold sol. A loss of colloidal stability that is marked by a pronounced broadening of the surface plasmon band to the near-infrared region is observed. These results can be corroborated visually by the loss of the characteristic red sol color to either a colorless solution with fine precipitate shortly after 1-butanethiol addition or a light blue solution after 3-mercaptopropyltrimethoxysilane addition. Both of these observations are indicative of colloidal aggregation.20 The homologous 2-mercaptoethyltriethoxysilane leads to a similar loss of colloidal stability, as do mercaptosilanes with other alkoxides. Although the reasons for colloidal instability upon thiol addition in these instances are not fully understood, it is known that the interaction of thiol compounds with gold sol often leads to aggregation.8 This may be a result of the relatively large time scale required for monolayer formation with a thiol on gold,6b,16 although a complete mechanism involving short-range interactions at the gold/water interface during thiol monolayer formation in a colloidal system is currently unavailable.21 The results above necessitate a different approach for synthesizing a mercaptosilane monolayer on the surface of citrate-stabilized colloidal gold. Our hypothesis involved circumventing the limitations on colloidal stability above by using a thioester instead of a thiol during monolayer self-assembly. We have previously reported the synthesis of thiolate monolayers on the surface of citrate-stabilized colloidal gold in aqueous solution by using 2 and reasoned that 1 can be used as the analogous protected precursor for 3-mercaptopropyltriethoxysilane. 1 is expected to bind as a thioester to the gold surface, as previously reported for 2,18 and the silicon alkoxide is expected to hydrolyze quickly upon thioester adsorption to form silanol triols that can subsequently condense.4 Thioester hydrolysis of 1 can be observed in the pyrene UV/vis absorption bands before and after complete reaction (monitored via steady-state fluorescence). A blue shift of approximately 3-5 nm in the position of several bands is observed for surface-bound 1 during the course of thioester hydrolysis, with the spectrum after reaction overlapping well with that of 1-pyrenebutyric acid in gold sol (hydrolysis product standard) in (19) Keating, C. D.; Musick, M. D.; Keefe, M. H.; Natan, M. J. J. Chem. Educ. 1999, 76, 949. (20) Enu¨stu¨n, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317. (21) Biggs, S.; Mulvaney, P.; Zukoski, C. F.; Grieser, F. J. Am. Chem. Soc. 1994, 116, 9150.

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Figure 2. UV/vis spectra in the wavelength region of the surface plasmon resonance band for citrate-stabilized colloidal gold control (- ‚ -) treated with a monolayer coverage of 1 (after complete thioester deprotection) (s), 1-butanethiol (‚ ‚ ‚), and 3-mercaptopropyltrimethoxysilane (- - -).

Figure 3. UV/vis spectra in the wavelength region of pyrene electronic transitions showing (a) 1-pyrenebutyric acid in colloidal gold sol, (b) monolayer coverage of surface-bound 1 in colloidal gold sol after complete thioester deprotection, as monitored via steady-state fluorescence, (c) monolayer coverage of surface-bound 1 in colloidal gold sol immediately after addition (before thioester deprotection), and (d) colloidal gold sol control. The ordinate represents a linear absorbance scale, with the baseline for each spectrum (a-d) translated accordingly for visual clarity. The dashed vertical line is provided for reference to mark the band position.

Figure 3. The synthesis of 1-pyrenebutyric acid has been corroborated using two-dimensional topographic fluorescence emission data that comprise a fluorescence emission intensity landscape versus excitation and emission wavelength, as reported previously for 2.18 These topographic data provide a fingerprint that qualitatively resembles the information in Figure 3, along with additional information that is incorporated from the variation of the fluorescence emission wavelength (wavelength in transmission experiment is similar to fluorescence excitation wavelength). The solid curve in Figure 2 shows no changes indicative of colloidal instability upon complete hydrolysis of a monolayer coverage of 1 to a thiolate on the surface of colloidal gold. The wavelength of the surface plasmon band is slightly red shifted (3 nm), and only a small increase in the breadth of the band is observed, which is similar to results with aminosilanes.3 Visually, the gold sol retains the same red color after complete thioester hydrolysis of 1 to a thiolate as before addition. Similar results indicative of a stable colloidal suspension after thioester hydrolysis are also observed with a monolayer coverage of 2 with the

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Figure 4. Normalized fluorescence emission band area (total area from 360 to 600 nm, corresponding to both monomer and excimer bands) versus amount of gold nanoparticles per molecule of 1. A smooth curve is fit to the data for visualization purposes. A fixed fluorophore concentration of 123.4 nm was used in the titration experiment. Inset: Relationship between the highest colloidal gold concentration corresponding to saturation (maximum in emission area versus colloids per fluorophore in the titration experiment of Figure 4) and the concentration of 1. Each data point represents a separate titration experiment, and the result of a linear regression of the inset data is drawn as a solid line, which is used to estimate the packing density of 1 at saturation coverage of the nanoparticle.

Figure 5. Fluorescence emission band area (total area from 360 to 600 nm, corresponding to both monomer and excimer bands) versus concentration of 1. The ratio of fluorophore molecules to colloidal gold nanoparticles was held fixed at 2000 for all data points investigated, which represents the saturation coverage packing density (monolayer coverage). The line labeled K ) ∞ represents the behavior expected due to dilution alone, without a finite equilibrium constant for binding, and it represents the best linear fit to the data under the constraint of passing through the origin.

same gold sol. Altogether, these results suggest that kinetics (as opposed to thermodynamics) of monolayer formation are responsible for colloidal instability that is observed after the direct addition of 1-butanethiol and 3-mercaptopropyltriethoxysilane to citrate-stabilized gold sol, since the corresponding end-products of a thiolate monolayer on colloidal gold obtained with 1 and 2 are the same. The results of titrating 1 with colloidal gold are similar to those reported previously for 2.18 A discrete saturation coverage is observed in Figure 4 at approximately 2000 molecules of 1 per gold nanoparticle, which corresponds to the same cross-sectional area of 25 Å2 per 1 as previously reported for 2.18 Another similarity between Figure 4 and the titration curve reported previously for 2 involves the fluorescence intensity ratio between saturation coverage

Mercaptosilane Core-Shell Interface

Figure 6. Plot of the natural logarithm of the first-order rate constant versus the inverse of temperature for the deprotection of 1 in the presence of excess colloidal gold. Lines represent linear regression of the experimentally measured data to the Arrhenius equation. Inset: Dynamical evolution of fluorescence emission spectra due to the hydrolysis of surface-bound 1 on colloidal gold. The initial coverage is at a packing density corresponding to saturation of the nanoparticle surface with 1 (monolayer coverage). Data were collected initially and after 24, 48, and 72 h, after which time thioester deprotection was complete. The temperature was maintained at 40 °C during the course of thioester deprotection.

(maximum in Figure 4) and the absence of colloidal gold (zero on abscissa of Figure 4). This ratio is calculated to be 23 from the data in Figure 4, whereas the same ratio for 2 (at the same fluorophore concentration) is 20.18 The similarity of these ratios suggests a nearly-identical environment for the pyrenes within the monolayer for both surface-bound 1 and 2, and it is consistent with the formation of a monolayer of 1 on the nanoparticle surface, thus disfavoring the possibility of multiple layers comprising three-dimensional polysiloxane networks on the gold surface. If such multiple layers were formed, there would not be good correspondence in the packing density at saturation coverage between 1 and 2 (multiple layers impossible at saturation coverage). The formation of a two-dimensional monolayer here is similar to that reported previously for octadecyltrichlorosilane monolayers on gold, where three-dimensional polymer networks were ruled out on the basis of ellipsometric data.22 The binding affinity of 1 to gold was studied by diluting samples of colloidal gold that are saturated with 1 and measuring the excimer fluorescence emission intensity as a function of concentration. The emission intensity is expected to be a linear function of concentration at intermediate concentrations due to dilution; however, at small concentrations, there may be deviation from dilution behavior (line labeled K ) ∞), which is reflective of equilibrium binding limitations of fluorophore to gold. In a previous study of the binding of 2 to gold, a departure from dilution behavior at concentrations less than 0.3 µM was observed, which was indicative of a binding constant of somewhat less than 108 M-1.18 In contrast to this result and a similar one for the isostructural thiocarbonate fluorophore,18 Figure 5 does not show deviation from dilution behavior for the binding of 1 to colloidal gold, indicating that the binding constant is considerably larger than that previously reported for the other fluorophores. This is consistent with the condensation of 1 on the nanoparticle surface, to synthesize a cross-linked polysiloxane network, which interacts in a multidentate fashion with an overall binding constant that is greater than the (22) Finklea, H. O.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986, 2, 239.

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Figure 7. UV/vis spectra in the wavelength region of the surface plasmon resonance band for a monolayer of 1 adsorbed to citrate-stabilized colloidal gold after complete thioester deprotection (‚ ‚ ‚), a gold-silica core-shell nanoparticle incorporating a mercaptosilane interfacial layer at the core-shell interface (- - -), and the core-shell nanoparticle above treated with an excess of 1-butanethiol (s). The core-shell nanoparticles used for this figure were synthesized with the higher [SiO2]/[Au] ratio (see Experimental Section).

Figure 8. (a) Gold-silica core-shell nanoparticles incorporating a mercaptosilane interface between the core and shell synthesized with a low [SiO2]/[Au] ratio (see Experimental Section). The scale bar shown represents a distance of 135 nm. (b-d) TEM images of gold-silica core-shell nanoparticles incorporating a mercaptosilane interface between the core and shell and synthesized with a high [SiO2]/[Au] ratio (see Experimental Section). The scale bars shown represent a distance of 100 nm.

sensitivity of the fluorescence measurement used here. The time scale required to hydrolyze and condense the silicon alkoxides of 1 for polysiloxane network formation is expected to be much shorter than that required for thioester hydrolysis.3,4 The kinetics of thioester hydrolysis for surface-bound 1 are shown in Figure 6. The inset shows the dynamical evolution of the pyrene emission spectrum in proceeding from a saturated surface of bound 1 to 1-pyrenebutyric acid in aqueous solution during the course of a 3-day period at 40 °C. These thioester deprotection kinetics are significantly slower than those previously observed for 2 in

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the same reaction,18 and this may be due to diffusional limitations. The apparent activation energy of 7.8 kcal/ mol calculated from Figure 6 is slightly greater than half of the activation energy of 14 kcal/mol previously determined for 2 in the same reaction.18 The pre-exponential factor of 967 s-1 calculated from Figure 6 is more than four decades lower than the value for 2 in the same reaction.18 These differences in the activation energy and entropy can be explained by either one of two scenarios below. The first assumes the same chemical reaction mechanism for thioester hydrolysis between surface-bound 1 and 2 and involves a significant mass transport limitation for diffusion of water within surface-bound 1, which is likely due to the cross-linked polysiloxane network. The apparent activation energy in the case of internal diffusional limitations represents a combination of activation energies for chemical kinetics and activated diffusion.23 The rather high apparent activation energy represented here (slightly more than 7 kcal/mol), relative to typical activation energies for activated diffusion, suggests a transition regime in which both diffusion within the cross-linked network and intrinsic chemical kinetics of thioester hydrolysis limit the observed rate (no single rate-limiting process). A second scenario involves changes in the chemical reaction mechanism for thioester hydrolysis between surface-bound 1 and 2. Although a rigorous justification for one of these two scenarios cannot be provided at this time, the first is consistent with the concomitant decreases in activation energy and preexponential factor that are observed here. It is important to note that the kinetic data in Figure 6 (not including inset) were collected under pseudo-first-order conditions at a dilute initial loading of 1 that comprises about 33 molecules of thioester per gold nanoparticle. Thus, the possibility of forming an extended two-dimensional polymeric network encompassing the entire nanoparticle periphery was not investigated under these conditions. Upon thioester hydrolysis of surface-bound 1, monitored by steady-state fluorescence, a mercaptosilane monolayer on colloidal gold results. The molecular conformation and packing within such a monolayer has been investigated previously on a two-dimensional gold surface. It was found that the propyl chains are largely in a trans conformation with a small fraction of silanols remaining (fewer than 5% relative to the amount of alkoxide present initially), indicating a highly cross-linked polysiloxane network comprising the monolayer.13b This network should form a good adhesive layer for silica shell growth around the nanoparticle, as observed previously on flat substrates.11-13 Figure 7 shows the effect of silica coating on the surface plasmon resonance band, for the synthesis of the coreshell nanoparticles shown in Figure 8b-d, which use a higher [SiO2]/[Au] ratio during shell growth. The wavelength of this band is red-shifted by 7.2 nm and has a significantly greater intensity compared to that of the band for uncoated colloidal gold control. These observations are consistent with trends predicted from Mie theory as the shell volume fraction increases.10 The resulting core-shell nanoparticles are robust in that they no longer coalesce upon exposure to even a large excess of short-chain thiols. This is expected given the high density of condensed mercaptosilane on the gold surface and is corroborated by the data shown in Figure 7, which demonstrate that the intensity and breadth of the surface plasmon band for the core-shell nanoparticles do not appreciably change upon treatment with 1-butanethiol. This is in stark contrast to results shown in Figure 2 for the uncoated colloidal gold control.

Figure 8 shows several TEM images of the core-shell nanoparticles. The initial experimental conditions chosen for silica shell growth were based on those reported previously in the literature and are based on the method of Sto¨ber.1d These core-shell nanoparticles failed to show the presence of a clear silica shell via TEM, as shown in Figure 8a, which used a [SiO2]/[Au] ratio that was approximately 4-fold higher than the literature ratio used with aminosilanes.1d Therefore, to favor thicker shell formation around the thiolate-coated gold cores while simultaneously preventing homogeneous silica nucleation,24 the [SiO2]/[Au] ratio was increased another factor of 4-fold, up to the levels used here (about 16-fold higher than the literature ratio1d). A silica shell that is significantly thicker surrounding the nanoparticle cores is seen in Figure 8b-d and varies in thickness between 5 and 50 nm. Almost all of the core-shell nanoparticles imaged maintain a certain degree of isolation, as shown in Figure 8, and this observation is independent of the [SiO2]/[Au] ratio used during silica shell growth. On the contrary, control colloidal gold that does not contain an interfacial monolayer of mercaptosilane and was directly treated with sodium silicate with the higher [SiO2]/[Au] ratio shows a broad range of behavior on the TEM. Most of this coated control silica is in an agglomerated state and shows a thick silica shell encompassing a multitude of nanoparticles within a continuous filmlike structure. In conclusion, the synthesis of gold-silica core-shell nanoparticles incorporating a mercaptosilane interface between the core and shell is reported. Thioester 1 is synthesized as a protected precursor to 3-mercaptopropyltriethoxysilane and affords the required mercaptosilane monolayer on the surface of colloidal gold. The binding constant of surface-bound 1 to colloidal gold is significantly stronger than that reported previously for 2, and this is attributed to the condensation of 1 to form polysiloxane on the nanoparticle surface immediately after binding. Titration of colloidal gold with standard solutions at a fixed concentration of 1 shows a similar saturation coverage for 1 on the nanoparticle surface as reported previously for 2, strongly suggesting a lack of multilayered condensation and formation of a cross-linked twodimensional polysiloxane network on the nanoparticle surface at saturation coverage. Gold-catalyzed thioester hydrolysis kinetics of surface-bound 1 show a smaller apparent activation energy than previously reported for 2 and a pre-exponential factor that is more than four decades smaller than that previously reported for 2. These changes are consistent with significant mass transport limitations in the case of thioester hydrolysis with surfacebound 1, resulting from the cross-linked polysiloxane network. Growth of a silica shell onto the thiolate monolayer resulting from hydrolyzed 1 is observed via TEM. The resulting gold-silica core-shell nanoparticles are robust in that they can withstand organic solvent environments, as well as short-chain thiols, and maintain colloidal stability.

(23) (a) Wheeler, A. Adv. Catal. 1951, 3, 249. (b) Weisz, P. B.; Prater, C. D. Adv. Catal. 1954, 6, 143.

(24) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740.

Acknowledgment. The authors are grateful to Ms. Rina Zalpuri at the EML facility at UCB for providing helpful assistance with TEM experiments. The authors acknowledge the Petroleum Research Fund Type G Grant (PRF # 37698-G5) and UCB Department of Chemical Engineering Start-up for funding. LA026055R