Investigation of the Core−Shell Interface in Gold@Silica Nanoparticles

Nov 24, 2005 - Solid-state 13C CPMAS NMR spectroscopy on these materials demonstrates that the amount of amine immobilization must be less than 10% of...
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Langmuir 2005, 21, 12348-12356

Investigation of the Core-Shell Interface in Gold@Silica Nanoparticles: A Silica Imprinting Approach Saran Poovarodom,† John D. Bass,† Son-Jong Hwang,‡ and Alexander Katz*,† Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720-1462, and Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received July 23, 2005. In Final Form: September 17, 2005 The nature of the self-assembled core-shell interface in gold@silica nanoparticles synthesized via a 3-aminopropyltrimethoxysilane (APTMS) route is investigated using materials synthesis as a sensitive tool for elucidating interfacial composition and organization. Our approach involves condensation of the gold@silica nanoparticles within a silica framework for synthesis of a composite gold-silica material containing ∼30 wt % gold. This material contains one of the highest gold loadings reported, but maintains gold core isolation as ascertained via a single surface plasmon resonance absorption band frequency corresponding to that of gold nanoparticles in dilute aqueous solution. The immobilized gold cores are subsequently etched using cyanide anion for the synthesis of templated porosity, which corresponds to the space that was occupied by the gold. Characterization of immobilized amines is performed using probe molecule binding experiments, which demonstrate a lack of accessible amines after gold removal. Solidstate 13C CPMAS NMR spectroscopy on these materials demonstrates that the amount of amine immobilization must be less than 10% of the expected yield, assuming that all of the APTMS becomes bound to the gold nanoparticle template. These results require a core-shell interface in the gold@silica nanoparticles that is predominantly occupied by inorganic silicate species, such as Si-O-Si and Si-OH, rather than primary amines. Such a result is likely a consequence of the weak interaction between primary amines and gold in aqueous solution. Our method for investigating the core-shell interface of gold@silica nanoparticles is generalizable for other interfacial structures and enables the synthesis of bulk imprinted silica using colloidal templates.

Introduction Gold@silica nanoparticles are versatile colloidal building blocks for the synthesis of advanced materials.1,2 The isolated nature of colloidal gold in these nanoparticles has been exploited in a variety of technological applications including nonlinear optical materials,3 optical filters,4,5 and single electron capacitors.6 Gold isolation within these nanoparticles is directly monitored via the surface plasmon resonance absorption band4,5 and has permitted the study of the surface plasmon resonance temperature dependence7 and of the size dependence of colloidal gold melting temperature.8 Our goal is to investigate organosilane organization at the core-shell interface of these nanoparticles and, by doing so, evaluate colloidal gold as a functional template for the imprinting of bulk silica with primary amines.9-11 † ‡

University of California at Berkeley. California Institute of Technology.

(1) Mulvaney, P.; Liz-Marzan, L. M. Top. Curr. Chem. 2003, 226, 225-246. Hall, S. R.; Davis, S. A.; Mann, S. Langmuir 2000, 16, 14541456. (2) Mulvaney, P.; Liz-Marzan, L. M.; Giersig, M.; Ung, T. J. Mater. Chem. 2000, 10, 1259-1270. (3) Selvan, S. T.; Hayakawa, T.; Nogami, M.; Kobayashi, Y.; LizMarzan, L. M.; Hamanaka, Y.; Nakamura, A. J. Phys. Chem. B 2002, 106, 10157-10162. Hamanaka, Y.; Fukuta, K.; Nakamura, A.; LizMarzan, L. M.; Mulvaney, P. J. Lumin. 2004, 108, 365-369. Anija, M.; Thomas, J.; Singh, N.; Nair, A. S.; Tom, R. T.; Pradeep, T.; Philip, R. Chem. Phys. Lett. 2003, 380, 223-229. (4) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, 3441-3452. (5) Salgueirino-Maceira, V.; Caruso, F.; Liz-Marzan, L. M. J. Phys. Chem. B 2003, 107, 10990-10994. (6) Yau, S. T.; Mulvaney, P.; Xu, W.; Spinks, G. M. Phys. Rev. B 1998, 57, R15124-R15127. (7) Liz-Marzan, L. M.; Mulvaney, P. New J. Chem. 1998, 22, 12851288.

The objective of bulk silica imprinting is the specific organization of chemical functional groups within pores of a size and shape that are controlled by the template. These pores have heretofore been microporous (less than 2 nm) and have contained up to three chemical functional groups per imprinted site.9-11 While colloids have been used routinely as templates for creating porosity in materials via lost-wax types of approaches,12 imprinting aims to selectively organize chemical functional groups within the templated pore space. An imprinted material containing mesoporous templated porosity and chemical functional group organization contained therein represents a new advanced material, which can be used for the specific nucleation of matter in porous solids, with applications ranging from photoluminescent materials13 to tailored catalysts14 and media for large-molecule separations.15 (8) Dick, K.; Dhanasekaran, T.; Zhang, Z. Y.; Meisel, D. J. Am. Chem. Soc. 2002, 124, 2312-2317. (9) Bass, J. D.; Anderson, S. L.; Katz, A. Angew. Chem., Int. Ed. 2003, 42, 5219-5222. (10) Bass, J. D.; Katz, A. Chem. Mater. 2003, 15, 2757-2763. (11) Katz, A.; Davis, M. E. Nature 2000, 403, 286-289. (12) Imhof, A.; Pine, D. J. Nature 1997, 389, 948-951. Johnson, S. A.; Ollivier, P. J.; Mallouk, T. E. Science 1999, 283, 963-965. Goltner, C. G. Angew. Chem., Int. Ed. 1999, 38, 3155-3156. Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 9554-9555. Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453-457. Tessier, P.; Velev, O. D.; Kalambur, A. T.; Lenhoff, A. M.; Rabolt, J. F.; Kaler, E. W. Adv. Mater. 2001, 13, 396-400. Cornelissen, J. J. L. M.; Connor, E. F.; Kim, H. C.; Lee, V. Y.; Magibitang, T.; Rice, P. M.; Volksen, W.; Sundberg, L. K.; Miller, R. D. Chem. Commun. 2003, 1010-1011. Li, Z. J.; Jaroniec, M. J. Am. Chem. Soc. 2001, 123, 9208-9209. Li, Z. J.; Jaroniec, M. Chem. Mater. 2003, 15, 1327-1333. (13) Yun, F.; Hinds, B. J.; Hatatani, S.; Oda, S.; Zhao, Q. X.; Willander, M. Thin Solid Films 2000, 375, 137-141.

10.1021/la052006d CCC: $30.25 © 2005 American Chemical Society Published on Web 11/24/2005

Core-Shell Interface in Gold@Silica Nanoparticles

We use gold@silica nanoparticles that are synthesized via a 3-aminopropyltrimethoxysilane (APTMS) route as the imprint for our approach because this type of coreshell nanoparticle has been widely used as a building block for advanced materials. The synthesis of these gold@silica nanoparticles was originally described in 1996 by LizMarza´n, Giersig, and Mulvaney and relies on treatment of aqueous colloidal gold sol with APTMS, which has been demonstrated to facilitate silica shell nucleation and growth. Upon addition of silica, usually as sodium silicate, condensation around the metal core ensues and uniform core-shell nanoparticles are synthesized. These can be further grown to larger shell thicknesses under Sto¨berlike conditions.16 A bulk silica imprinting approach using gold@silica nanoparticles synthesized via an APTMS route is schematically represented in Figure 1a,b, relying on condensation of the nanoparticles for synthesis of composite goldsilica solid 1. Material 1 consists of a high density (∼30 wt %) of colloidal gold immobilized within a silicate framework with an average pore size considerably smaller than the nanoparticle diameter. This framework serves to keep the colloidal gold isolated by preventing direct contact between adjacent gold cores. The gold cores in this material are subsequently etched using cyanide anion for the synthesis of templated porosity, with the possibility of organized amino groups therein, which is schematically represented as 2 in Figure 1b. The silica imprinting approach depicted in Figure 1a,b requires the specific organization of primary amines on the colloidal gold surface in the core-shell interface of the gold@silica nanoparticle imprint. Aminopropyl functionality has been thought to occupy this interface;16 however, this has remained as a hypothesis. There has yet to be a rigorous demonstration of primary amines at the core-shell interface within these nanoparticles. The first step in the gold@silica nanoparticle synthesis has been suggested to be adsorption of APTMS on the gold surface followed by alkoxide hydrolysis and condensation.16 This is similar to the process when thioesters and thiocarbonates are used for gold@silica nanoparticle synthesis,17 which were shown to have an association constant for the colloidal gold surface in excess of 5 × 107 M-1 in aqueous solution.18 However, unlike thiols and protected thiols, the interaction between primary amines and both planar and colloidal gold surfaces is relatively weak. Xu et al. reported that C10 alkylamine monolayers on planar gold surfaces are stable in the gas phase, but decompose in polar condensed phases such as alcohols and water,19 which are present for gold@silica synthesis. The latter observation is supported by a study of a C18 alkylamine adsorption on a planar gold substrate by Bain et al., who were unable to synthesize stable amine monolayers on gold using ethanol as solvent.20 Kurth and Bein investigated 3-aminopropyltriethoxysilane (APTES) adsorption on a planar gold substrate. They concluded that in the strict absence of water there was no coordination between the APTES (14) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642-7643. (15) Nykypanchuk, D.; Strey, H. H.; Hoagland, D. A. Science 2002, 297, 987-990. (16) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Chem. Commun. 1996, 731-732. Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329-4335. (17) Chen, M. M. Y.; Katz, A. Langmuir 2002, 18, 8566-8572. (18) Chen, M. M. Y.; Katz, A. Langmuir 2002, 18, 2413-2420. (19) Xu, C. J.; Sun, L.; Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1993, 65, 2102-2107. (20) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164.

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Figure 1. Two opposing scenarios schematically representing material 1, consisting of isolated gold nanoparticles that are immobilized in silica, and its use in bulk silica imprinting. Materials in (a) and (b) contain ∼600 aminopropyl functional groups organized around the interfacial region occupied by the colloidal gold, whereas in (c) and (d) there is no aminosilane in this interface. Etching of the gold cores in 1 synthesizes material 2. Legend: silica (white), colloidal gold (light gray), and interface under investigation (dark gray). Double arrows represent a distance of 12.5 nm.

and gold surfaces.21 An adsorbed layer of APTES on gold could only be formed by first preadsorbing water on the gold surface, which presumably causes hydrolysis and condensation of the adsorbing APTES, in a preferred orientation having the primary amino groups point away from the gold surface, toward the top surface of the film, and the hydrolyzed Si-OH and condensed Si-O-Si moieties interacting with the metal surface.21 The structure of monolayers reported in this study is reminiscent of C18 alkyl trichlorosilane monolayers on a planar gold (21) Kurth, D. G.; Bein, T. Langmuir 1995, 11, 3061-3067.

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substrate, as investigated by Finklea et al., who also report that a thin film of water on gold is necessary for alkylsilane monolayer formation.22 This monolayer was shown to consist of a silyl moiety adjacent to the gold surface and all-trans conformation of the hydrocarbon chain extending vertically from the surface with a small average tilt. This is exactly the opposite of the orientation for 3-mercaptopropyltrimethoxysilane adsorption on gold, which forms monolayers with the sulfur atom adjacent to the gold surface and methoxy headgroups oriented parallel and away from the surface.23 More recently, using colloidal rather than planar gold as a substrate, Thomas et al. investigated the interaction between primary amines and colloidal gold in toluene, and calculated an association constant of ∼5 × 104 M-1 for primary amines to gold at room temperature.24 The aqueous stability of primary amine monolayers on colloidal gold has been investigated by Heath et al., who showed that, although again stable in toluene, these monolayers were unstable in water and resulted in an aggregated gold film when in contact with aqueous solution.25 The weak binding between primary amines and gold in water suggests that siloxy functionality, which is present during gold@silica nanoparticle synthesis in large excess, may competitively adsorb on gold surface sites during the core-shell nanoparticle synthesis. This could lead to the type of siloxy interfacial composition depicted as 1 in Figure 1c for the material after gold@silica nanoparticle condensation, resulting in the absence of imprinted amines as in 2 represented in Figure 1d after gold core etching. The primary amine-gold interface in Figure 1a and the siloxy-gold interface in Figure 1c are both examples of extreme possible cases. Despite the weak interaction between isolated primary amines and gold surfaces in water, it is possible that a condensed polyamine species adsorbs to the gold surface during the gold@silica nanoparticle synthesis. There are several examples involving this type of interaction and resulting in the adsorption of gold nanoparticles onto aminopropyl-functionalized glass surfaces from aqueous solution.26 There is also the possibility for the mixed case, having both some organized amino groups at the gold surface as in Figure 1a, while permitting some randomly oriented amino groups in the bulk silica network away from the gold, as proposed by Bharathi and Lev.27 Rigorous proof remains unavailable on which one is most accurate of the three possible scenarios: either the structure shown in Figure 1a, the structure shown in Figure 1c, or the mixed case between these two extremes, as proposed by Bharathi and Lev. (22) Finklea, H. O.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986, 2, 239-244. (23) Thompson, W. R.; Cai, M.; Ho, M. K.; Pemberton, J. E. Langmuir 1997, 13, 2291-2302. (24) Thomas, K. G.; Zajicek, J.; Kamat, P. V. Langmuir 2002, 18, 3722-3727. (25) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 47234730. (26) Caruso, F.; Spasova, M.; Saigueirino-Maceira, V.; Liz-Marzan, L. M. Adv. Mater. 2001, 13, 1090. Fan, H. Y.; Zhou, Y. Q.; Lopez, G. P. Adv. Mater. 1997, 9, 728-731. Rubin, S.; Bar, G.; Taylor, T. N.; Cutts, R. W.; Zawodzinski, T. A. J. Vac. Sci. Technol., A 1996, 14, 1870-1877. Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629-1632. Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353-2361. Fleming, M. S.; Walt, D. R. Langmuir 2001, 17, 48364843. Pham, T.; Jackson, J. B.; Halas, N. J.; Lee, T. R. Langmuir 2002, 18, 4915-4920. Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396-5401. Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243-247. (27) Bharathi, S.; Lev, O. Chem. Commun. 1997, 2303-2304. Bharathi, S.; Fishelson, N.; Lev, O. Langmuir 1999, 15, 1929-1937.

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Our objective is to differentiate between the types of functional group organization at the gold-silica interface in 1 and 2. Our approach is to remove the gold cores from 1 and characterize the number density and organization of the immobilized primary amines. We use spectroscopic signatures of covalently bound probe molecules, which we have previously used for studying amines in imprinted silica,9,10 as well as solid-state nuclear magnetic resonance (NMR) spectroscopy for characterizing local environment and quantifying amine functional group number density in materials after gold core template removal.10,11 To the best of our knowledge, aside from elucidating organosilane organization in the gold@silica nanoparticles under investigation, the experiments described herein are the first to demonstrate a materials synthesis methodology that can be used to enable the imprinting of silica using colloidal templates. This has been challenging to accomplish in part by the difficulty of removing the colloidal template from bulk silica under conditions that preserve the integrity and attachment of anchored organic functional groups on silica.28 In addition, the gold-silica composite materials synthesized here represent one of the highest loadings of isolated colloidal gold in bulk silica synthesized to-date.27,29-31 This high loading of gold in silica is necessary in order to provide enough sensitivity for characterizing the number density of amine functionality after gold core template removal. Experimental Section General. UV-vis spectroscopy was performed on a Varian Cary 400 Bio UV-vis spectrophotometer equipped with a Harric Praying Mantis accessory for diffuse-reflectance measurements on solids at room temperature. Optical microscopy was performed at the Biological Imaging Facility at University of California at Berkeley (UCB) using a Zeiss Axiophot optical microscope equipped with a grayscale Photometrics Quantix camera (12 bit KAF1401E CCD Kodak chip). Transmission electron microscopy (TEM) samples were prepared by drying a drop of solution on carbon-coated copper grids. Solid-state NMR spectroscopy was performed at the Caltech Solid-State NMR Facility. 13C crosspolarization magic-angle-spinning (CPMAS) NMR spectra were collected using a Bruker DSX-500 spectrometer operating at 125.4 MHz for the 13C nucleus, and using a Bruker 4 mm CPMAS probe. A contact time of 1.0 ms was used for all CP experiments and samples were spun at 6.0 kHz. The chemical shifts were referenced externally to tetramethylsilane. Gas chromatography was performed on an Agilent 6890 system equipped with a flame ionization detector. Thermogravimetric analysis was performed on a TA Instruments 2950 system. Nitrogen physisorption was performed on a Quantachrome Autosorb-1 using samples that had been degassed for at least 24 h at a temperature of 120 °C. Measurement of gold concentration in materials was performed using inductively coupled plasma (ICP) by QTI Inc. (Whitehouse, NJ). All vacuum environments refer to a pressure of 50 mTorr or less. (28) Makarova, O. V.; Ostafin, A. E.; Miyoshi, H.; Norris, J. R.; Meisel, D. J. Phys. Chem. B 1999, 103, 9080-9084. (29) Zhu, H. G.; Lee, B.; Dai, S.; Overbury, S. H. Langmuir 2003, 19, 3974-3980. Khushalani, D.; Hasenzahl, S.; Mann, S. J. Nanosci. Nanotechnol. 2001, 1, 129-132. (30) Madler, L.; Stark, W. J.; Pratsinis, S. E. J. Mater. Res. 2003, 18, 115-120. Shi, H. Z.; Zhang, L. D.; Cai, W. P. Mater. Res. Bull. 2000, 35, 1689-1695. Nooney, R. I.; Dhanasekaran, T.; Chen, Y. M.; Josephs, R.; Ostafin, A. E. Adv. Mater. 2002, 14, 529. Kozuka, H. In Sol-Gel Processing of Advanced Materials; Pope, E. J. A., Ed.; American Ceramic Society: Westerville, OH, 1998; Vol. 81, pp 263-270. Konya, Z.; Puntes, V. F.; Kiricsi, I.; Zhu, J.; Ager, J. W.; Ko, M. K.; Frei, H.; Alivisatos, P.; Somorjai, G. A. Chem. Mater. 2003, 15, 1242-1248. Martino, A.; Yamanaka, S. A.; Kawola, J. S.; Loy, D. A. Chem. Mater. 1997, 9, 423429. Cheng, S.; Wei, Y.; Feng, Q. W.; Qiu, K. Y.; Pang, J. B.; Jansen, S. A.; Yin, R.; Ong, K. Chem. Mater. 2003, 15, 1560-1566. Nooney, R. I.; Thirunavukkarasu, D.; Chen, Y. M.; Josephs, R.; Ostafin, A. E. Langmuir 2003, 19, 7628-7637. (31) Kobayashi, Y.; Correa-Duarte, M. A.; Liz-Marzan, L. M. Langmuir 2001, 17, 6375-6379.

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Materials. Tetrachloroauric acid was purchased from Acros. All other chemicals were purchased at the highest possible level of purity from Aldrich and were used as received unless stated otherwise. The purity of 3-aminopropyltrimethoxysilane (APTMS) was verified via 1H NMR and Fourier transform infrared spectroscopies. Silica used for the synthesis of all control materials was Selecto silica gel (average pore diameter 60 Å and surface area 500 m2/g). Water used in these experiments was distilled at least once and treated with a Barnstead Nanopure Infinity system to possess at least 18 MΩ purity. Synthesis of Colloidal Gold. Citrate-stabilized colloidal gold was synthesized according to previously published procedures.32 The gold sol (concentration of 7.8 × 10-4 M Au; 13 nM gold colloid) diluted with four parts of water per part of colloid had a measured λmax of 518 nm (516-520 nm literature specification) and a full peak width at half-maximum of 84 nm (80-90 nm literature specification). TEM was performed on a random sampling of particles and verified the presence of spherical 12.5 nm colloidal gold particles. Synthesis of Gold-Silica Core-Shell Nanoparticles. Synthesis of gold@silica nanoparticles via the APTMS route was performed according to previously published procedures.16 Two liters of colloidal gold sol was diluted with an equal volume of water to synthesize 4 L of stock solution. Twenty milliliters of a freshly prepared 1 mM aqueous solution of APTMS was added to the 4 L of stock solution. The mixture was vigorously stirred for 30 min. A silica solution was separately prepared by diluting sodium silicate solution (27 wt % SiO2) with water to synthesize a 0.54 wt % SiO2 solution. The silica solution (160 mL) was added to the stock solution containing APTMS. Two different pHs (high and low) of the 0.54 wt % SiO2 solution were used for the silica coating process: a high pH silica coating solution, which led to a pH of 9.4 upon addition to the stock solution, and a lower pH silica coating solution, which led to a pH of 7.2 upon addition to the stock solution. The higher pH solution was synthesized by adding 0.54 wt % SiO2 solution at a pH of 11.0-11.2 to the stock solution. The lower pH solution was synthesized by addition of Dowex 50WX8 ion-exchange resin (H form) to the silica solution immediately prior to its addition to stock solution. The resulting mixture was vigorously stirred for 1 week and then left to stand at room temperature for at least 4 weeks for silica coating, after which time the synthesis of gold@silica composite material via gelation procedure was begun. During silica coating, the pH of the two stock solutions dropped from 9.4 to 8.5 and from 7.2 to 6.3 after 4 weeks for the high and low pH silica-coated colloids, respectively. Gold@silica nanoparticles were characterized via TEM (see Supporting Information). Synthesis of Gold-Silica Composite Material. Immediately before the gelation procedure below, the high pH silicacoated colloid was adjusted from a pH of 8.5 to a pH of 6.0-6.5 by adding ∼0.15 mL per 500 mL of gold@silica colloid solution of a 1 M aqueous citric acid solution. The low pH coated colloid was not pH adjusted prior to gelation. In batches of 500 mL, the gold@silica colloid was concentrated to between 25 and 50 mL using a rotary evaporator operating at a temperature between 40 and 50 °C. The concentrated colloid was allowed to stand at room temperature until gelation occurred during the course of 1 week. After gelation, a deep purple gold-silica composite gel settled under a head of clear liquid. The resulting solution containing gel and liquid was centrifuged at 1000-2000 rpm for 15 min. The liquid was decanted and replaced with water. Centrifugation and water rinsing was repeated at least five times before the solid gel was collected via filtration. Removal of all soluble salts (citrate) from the gel was followed via thermogravimetric analysis. The resulting gel was dried in an oven at 105 °C and under vacuum at room temperature for 24 h before storage in a desiccator. Typical silica yields were approximately 75% of the theoretical amount (∼0.16 g of composite gold-silica material per 500 mL of coated colloid solution was recovered). Gold Core Removal via Etching. Gels were crushed into a powder (40-200 µm particle size as determined via optical microscopy) using a granite mortar and pestle. The gold-silica material was etched with freshly made 0.1 M aqueous KCN

One of the key features of an imprinting strategy is ensuring site isolation.33 For colloidal gold, this can be probed directly via the surface plasmon resonance absorption band, which is known to red shift as the degree of gold core aggregation increases.34 We investigated the degree of imprint isolation in bulk gold-silica composite material 1 using diffuse-reflectance UV-vis spectroscopy. Results are shown in Figure 2 for materials before (1) and after gold etching (2). For 1 derived from both the low pH (silica coating final pH 6.3) and high pH (silica coating final pH 8.5) silica coating procedures, a maximum in the absorbance spectrum at 518 nm is observed that is coincident with the maximum in the surface plasmon resonance band of isolated 12.5 nm gold nanoparticles in aqueous solution. Upon gold core etching with cyanide from 1 for the synthesis of 2, the surface plasmon band disappears, and the featureless spectra shown in Figure 2d (material derived from low pH silica coating) and Figure 2e (material derived from high pH silica coating) result.

(32) Keating, C. D.; Musick, M. D.; Keefe, M. H.; Natan, M. J. J. Chem. Educ. 1999, 76, 949-955.

(33) Katz, A.; Davis, M. E. Macromolecules 1999, 32, 4113-4121. (34) Quinten, M. Appl. Phys. B: Lasers Opt. 2001, 73, 317-326.

solution. The pH was maintained at 9.0 to reduce the solubility of silica in water, by addition of Dowex 50WX8 ion-exchange resin (H form) to KCN solution prior to addition of gold-silica composite material. CAUTION: This procedure should only be conducted in a wellventilated fume hood because toxic HCN may be produced. Approximately 50 mL of the etchant was used per gram of the gold-silica composite material (3-fold excess of cyanide anion per gold atom). The etching process was conducted for up to 6 days under magnetic stirring. The resulting material was filtered and washed with excess water for removing residual KCN (wash with 3 L of water was per gram of material). It was subsequently dried in an oven at 105 °C and placed under vacuum at room temperature for 24 h before storage in a desiccator. The same etching procedure was performed with a positive control material (synthesized as described below). Synthesis of Surface-Functionalized Positive Control Material. A control material containing approximately 0.023 mmol/g of aminopropyl functionality on the surface of silica was prepared as follows. A 7.0 g sample of Selecto silica gel was suspended in 300 mL of benzene, and 0.5 mL of 0.35 M aminopropyltriethoxysilane in benzene was added to this suspension. The mixture was stirred for 24 h. The material was collected via filtration and Soxhlet extracted in dry acetonitrile for 24 h. It was subsequently dried under vacuum at room temperature and stored in a desiccator. A portion of this material was subjected to the etching procedure detailed above. Salicylaldehyde Binding. A 1.0 mL volume of a 2.22 mM solution of salicylaldehyde in acetonitrile was added to 50 mg of silica. This represents a minimum molar ratio of salicylaldehyde to anchored amine of 2.0. After an equilibration time of 4 days, the materials were collected via filtration and washed with a combination of 100 mL of acetonitrile, 100 mL of chloroform, and 50 mL of pentane. The materials were subsequently Soxhlet extracted in chloroform for at least 16 h and dried under vacuum at room temperature. The amount of salicylaldehyde covalently bound was quantified using gas chromatography on syringefiltered samples and 1,3,5-trimethoxybenzene as an internal standard. TNBS Binding. Materials were treated with a 0.04 wt % solution of 2,4,6-trinitrobenzenesulfonic acid (TNBS) in dimethylformamide at room temperature for 1 week. In a typical procedure, 40 mg of sample was treated with 1.50 mL of TNBS solution, representing a minimum of 2 equivalents of TNBS per anchored amine. The materials were filtered, washed with dimethylformamide (20 mL) and chloroform (30 mL), and Soxhlet extracted in chloroform for 24 h prior to drying under vacuum at room temperature.

Results

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Figure 2. Solid-state UV-vis spectra of (a) 1 derived from low pH silica coating procedure, (b) 1 derived from high pH silica coating procedure, (c) isolated gold nanoparticles in aqueous solution that were used as precursors for 1, (d) 2 derived from low pH silica coating procedure, (e) 2 derived from high pH silica coating procedure.

Figure 3. Physical adsorption/desorption isotherms of nitrogen at 77 K on materials derived from low pH silica coating procedure (s) and high pH silica coating procedure (‚‚‚) for (a) 1 and (b) 2 (materials in (a) following gold core etching). Inset shows the corresponding BJH pore-size distribution based on the desorption branch of the isotherm derived from low pH silica coating procedure (s) and high pH silica coating procedure (‚‚‚).

Nitrogen porosimetry was used to investigate the porosity in 1 and 2, as it is this porosity that ultimately permits transport that is necessary for etching of gold and access to imprinted sites in 2. Nitrogen adsorption/ desorption isotherms at 77 K are shown in Figure 3 for 1 and 2 derived from low and high pH silica coatings. Included within the inset is a pore-size distribution based on the Barrett-Joyner-Halenda (BJH) model of the desorption branch of the isotherm,35 which shows a mean mesoporosity for 1 that is significantly smaller than the gold core diameter (12.5 nm), along with some microporosity. Material 1 derived from the low pH silica (35) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373-380.

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coating procedure has a slightly smaller average mesopore diameter of 7.4 nm compared with the 8.8 nm diameter for materials derived from the high pH silica coating procedure. It is not possible to reliably subtract isotherms for 1 and 2 for calculating the volume of templated porosity synthesized upon gold etching, as previously performed for templated porosity in hydrophobic bulk imprinted silica,11 due to its small fraction relative to the total mesopore volume in the material (this fraction is less than 0.025). The relatively small changes to the bulk network porosity as a result of aqueous conditions employed here for gold core etching outweigh the expected volume change due to synthesis of templated porosity. BrunauerEmmett-Teller (BET)37 surface areas for the respective low and high pH silica coating procedures were measured to be 330 and 353 m2/g in 1 and 369 and 339 m2/g in 2. Figure 4 shows snapshots of the mass-transport limited gold etching process, as viewed on a single gold-silica composite material particle (∼250 µm in diameter) using an optical microscope. Figure 4a shows the particle before etching, which is opaque due to the distributed colloidal gold within the composite gold-silica material. Figure 4b shows the same particle approximately 6.5 h later, consisting of an unetched central region and an optically clear etched silica shell surrounding it. Finally, Figure 4c shows the same particle after 22 h of etching without visible colloid gold. The etching process was also followed by powder X-ray diffraction, which shows the disappearance of peaks associated with metallic gold upon etching (Supporting Information). Au ICP analysis was used to quantify the gold mass fraction in these materials. The mass fraction for 1 was measured to be 0.31 and 0.33 for materials derived from silica coating procedures at low pH and high pH, respectively. The gold mass fraction remaining in 2 after etching was below the 0.05% detection limit of Au ICP, indicating a removal of greater than 99.99% of gold in 1 during synthesis of 2. The number density of amine functional groups in 2 was investigated both quantitatively and qualitatively using probe molecule binding experiments. Both positive and negative control materials were used for comparison purposes. The negative control material consists of a mesoporous silica scaffold without immobilized amine functional groups that had undergone the same washing and treatments, including etching, as 2. The positive control material contains the maximum number density of amine functional groups assuming that all of the amine added during synthesis of 1 becomes covalently incorporated into the material (∼0.023 mmol/g). A portion of this control material was subjected to the same etching treatment as 2 in order to study the effect of etching on the attachment of anchored aminopropyl functionality on silica. The covalent binding of salicylaldehyde as a probe molecule for primary amines produces a chromophore that can be characterized using solid-state UV-vis spectroscopy.9,10 The binding of salicylaldehyde is also used to determine the number density of immobilized amines by monitoring the change in concentration before and after binding via gas chromatography.10 These data are provided for all materials studied in Table 1. Salicylaldehydetreated materials visibly turned yellow upon salicylaldehyde binding to primary amines. The UV-vis bands (36) Lecloux, A. J.; Bronckart, J.; Noville, F.; Dodet, C.; Marchot, P.; Pirard, J. P. Colloids Surf. 1986, 19, 359-374. (37) Brunauer, S. E.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309-319.

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Figure 5. Solid-state UV-vis spectra of (a) positive control silica after etching procedure and treatment with salicylaldehyde, (b) 2 derived from low pH silica coating procedure after treatment with salicylaldehyde, (c) 2 derived from high pH silica coating procedure after treatment with salicylaldehyde, (d) negative control silica after treatment with salicylaldehyde, (e) positive control silica after etching procedure and treatment with TNBS, (f) 2 derived from low pH silica coating procedure after treatment with TNBS, (g) 2 derived from high pH silica coating procedure after treatment with TNBS, and (h) negative control silica after treatment with TNBS.

Figure 4. Optical micrographs of gold etching dynamics as viewed on a single macroscopic gold-silica composite material particle. The scale bar represents a distance of 50 µm. The particle before etching (a) consists of a single opaque region representative of the colloidal gold. The concentration profile of colloidal gold during etching follows a classical shrinking core model, as shown in (b) after 6.5 h of etching, even though the gold cores remaining in (b) consist of a continuum of isolated colloidal gold particles that are immobilized within a porous silica matrix. The particle is completely etched as shown in (c) after a period of 22 h.

did not diminish in intensity following Soxhlet extraction in chloroform, confirming the covalent nature of salicylaldehyde attachment.9,10 Corresponding solid-state UVvis spectra of salicylaldehyde-treated materials after etching are shown in Figure 5a-d.

To further corroborate the salicylaldehyde binding results, another probe molecule that is known to specifically bind to amines, TNBS, was also used. This compound arylates amines in proteins (lysine residues) and imprinted silica containing a silanol-rich silica framework environment forming a chromophore that has an absorbance between 340 and 410 nm.10 Figure 5e-h shows the solidstate UV-vis spectra of materials treated with TNBS. Whereas the probe molecule experiments described above are sensitive to the presence of accessible amino functional groups immobilized on silica, they do not provide information on the absolute presence or absence of immobilized aminopropyl functionality, because some amines may be inaccessible. To address the question of the absolute presence or absence of immobilized aminopropyl functionality in 2, we performed 13C CPMAS solidstate NMR spectroscopy on a positive control material containing the theoretical number of primary amines after the etching procedure as well as material 2 derived from low and high pH silica coating procedures. These spectra are represented in Figure 6.

Table 1. Salicyaldehyde Binding Results on Several Materials as Measured via Gas Chromatography material 2 derived from low pH silica coating 2 derived from high pH silica coating control materials negative control silica positive control silica before etching procedure positive control silica after etching procedure

expected amine density (mmol/g)

salicyaldehyde bound (mmol/g)

0.023 0.023

0.000 ( 0.002 0.000 ( 0.002

0.000 0.025 0.025

0.000 ( 0.002 0.027 ( 0.002 0.026 ( 0.002

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Figure 6. Solid-state 13C CPMAS NMR spectra of (a) positive control silica after etching procedure, (b) 2 derived from low pH silica coating procedure, and (c) 2 derived from high pH silica coating procedure. Data were recorded over 100 000 transients with a 2 s recycle delay for each sample and normalized to the sample weights. The contact time was 1.0 ms and the MAS spinning speed was 6 kHz.

Discussion Achieving colloidal gold isolation during synthesis of gold-silica composite materials has proven to be difficult to accomplish even at smaller gold mass fractions.29 Our procedure for the synthesis of 1 employs gold@silica nanoparticles as building blocks for maintaining isolation of colloidal gold. Similar methods have been previously used for the synthesis of bulk gold-silica composite materialssalbeit at smaller gold mass fractions of up to ∼0.006.31 For these materials,synthesis conditions were chosen in order to optimize the formation of isolated gold colloid while avoiding conditions in which multiple metal cores can agglomerate. Previous detailed studies of the effect of pH on the adsorption of SiO3- in metal@silica nanoparticles show that a pH range of 5-10 during silica shell growth is desirable for maintaining isolated metal cores during the silica coating process.38 In synthesizing 1, we chose a high (8.5) and a low (6.3) pH silica coating procedure during silica shell formation, that were within the narrow window above. APTMS is known to be important as a primer during silica coating of colloidal gold to maintain sol stability.16,31 When synthesis of 1 was conducted in the absence of APTMS, we observed the onset of colloidal instability marked by visible gold nanoparticle aggregation during the concentrating step using a rotary evaporator. This suggests that APTMS facilitates the growth of a silica shell thicker than 2-4 nm, which is known to increase the robustness and ability of gold@silica nanoparticles to withstand the high ionic strengths present in solution during silica gelation.16 The single surface plasmon resonance band frequency observed for 1 (Figure 2) corresponds to that of isolated gold nanoparticles in solution. Aggregation of gold cores would result in a red shift due to different plasmon resonances in the longitudinal and transverse directions.39 Both low and high pH variants of 1 contain a slight broadening in their corresponding spectrum in Figure 2 relative to isolated gold nanoparticles in solution. It is highly unlikely that this broadening is due to gold nanoparticle aggregation, because TEM micrographs (Supporting Information) fail to show evidence for this during synthesis of 1. Moreover, the observed broadening in the spectrum for 1 in Figure 2 is very similar to that (38) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740-3748. (39) Norman, T. J.; Grant, C. D.; Magana, D.; Zhang, J. Z.; Liu, J.; Cao, D. L.; Bridges, F.; Van Buuren, A. J. Phys. Chem. B 2002, 106, 7005-7012.

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observed in the solid-state UV-visible spectra of other gold-silica composite materials that have been synthesized at much lower gold mass fractions, for which gold nanoparticle aggregation should be even less likely than in 1.1,30,31 Ung et al. demonstrate that the gold volume fraction in materials synthesized from gold@silica building blocks made from 13.2 nm diameter colloidal gold must be below about 0.15 in order to prevent interactions between gold cores.4 For 1, the gold volume fraction is less than 0.025, well below the threshold predicted by Ung et al. for shifts in the surface plasmon resonance band frequency due to interactions between gold cores. This suggests that the gold mass fraction in 1 can still be increased significantly before this threshold is met. The silica shell porosity shown in the nitrogen physisorption data in Figure 3 is similar to the expected porosity of gold@silica nanoparticles, which has been analyzed previously via TEM and has been suggested to be bimodal, consisting mostly of mesopores along with some microporosity.2,16,40 The isotherms in Figure 3 exhibit a sharp uptake of nitrogen at relative pressures of less than 0.01, which is indicative of microporosity, and hysteresis between adsorption and desorption branches that is indicative of cylinder-shaped mesoporosity.36 Most mesoporosity in 1 likely arises as the result of voids between silica shells on adjacent gold@silica nanoparticles comprising the bulk material network, in much the same manner that voids between nanometer-sized colloidal silica particles are responsible for mesoporosity in base-catalyzed silicates.36 The insets of Figure 3 clearly represent the bimodal pore size distributions in 1 and 2. Etching of gold cores in 1 for the synthesis of 2 was performed using cyanide anion treatment at room temperature. We have taken care to avoid a high pH during cyanide etching in order to minimize silica dissolution, by balancing the pH of the etching solution with acid ionexchange resin at values of about 9.0-9.5, which corresponds to significantly reduced silica solubility relative to pH values above 10.0.41 Cyanide anion treatment has been previously used for etching ∼5 nm diameter surfacesupported gold nanoparticles on ceria,42 and for the preparation of hollow shells from individual core-shell nanoparticles,2,28,40,43,44 but it has not been used previously for removing gold cores from gold@silica nanoparticles immobilized in a bulk material. The dynamic concentration profile of colloidal gold within the macroscopic gold-silica particle of Figure 4 follows a classical shrinking core type of profile. This is consistent with a mass-transfer-limited process of cyanide and oxygen delivery to the gold cores in 1 followed by a fast gold etching reaction. The diffusion time scale of the etching process can be estimated from Figure 4b by the ∼40 µm penetration depth that has been achieved via etching during the course of the first 6.5 h of etching. This corresponds to a diffusivity during etching of ∼10-10 cm2/s, which is approximately 5 orders of (40) Giersig, M.; Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Adv. Mater. 1997, 9, 570-&. (41) Brinker, C. J. S.; G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990. (42) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935-938. (43) Ostafin, A. E.; Siegel, M.; Wang, Q.; Mizukami, H. Microporous Mesoporous Mater. 2003, 57, 47-55. Rosemary, M. J.; Suryanarayanan, V.; Reddy, P. G.; Maclaren, I.; Baskaran, S.; Pradeep, T. Proc. Indian Acad. Sci., Chem. Sci. 2003, 115, 703-709. Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846-6852. (44) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C.; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518-8522.

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magnitude smaller than the expected liquid-phase diffusivity for cyanide and oxygen in bulk aqueous solution of ∼10-5 cm2/s.45 This significantly depressed value of the diffusion coefficient during etching of 1 is consistent with previous studies that are based on TEM observations on single gold@silica nanoparticles and UV-vis studies of etching of gold@poly(pyrrole) nanoparticles.2,44 We suggest that the slow rate of gold etching in 1, as well as the observed shrinking core behavior in Figure 4, is governed by the slow diffusion of gold-cyanide complex out of the material and into bulk solution. Turning attention now to the characterization of amines after etching in materials, Figure 5 shows solid-state UVvis spectra of materials that have been treated with salicylaldehyde. The 392 nm band of the positive control material is consistent with amines surrounded by a silica framework that contains free acidic silanols, which shifts the equilibrium of the bound salicylaldehyde to the zwitterionic tautomer, as observed previously on studies of imprinted amines on silica.9,10 In contrast, 2 that was treated with salicylaldehyde did not show bands corresponding to covalently bound probe, exhibiting neither the neutral phenolic form (320 nm band expected)10 nor the zwitterionic form of the bound salen. The quantitative salicylaldehyde binding data in Table 1 correspond to the expected number of amines in the control materials and the absence of salicylaldehyde accessible amines in 2. The etching treatment changed the amine number density negligibly in the positive control (less than 10% difference between before and after etching and within error of measurement), and this result is consistent with the general stability of covalently immobilized amines on silica surfaces in aqueous solution within the pH range used here for postsynthesis materials processing.46 The spectrum of the TNBS-treated positive control after etching procedure in Figure 5e exhibits the expected bands in the range of 340 and 410 nm for amines surrounded by a hydrophilic silica framework.10 However, the data in Figure 5e-h do not show evidence of covalent binding of TNBS to amines in 2, pointing to the same qualitative conclusions as in the salicylaldehyde study. There are multiple potential reasons for a lack of probe molecule binding in 2. Imprinted primary amine inaccessibility is one possibility, despite our careful pH control during etching and lack of evidence for significant pore volume collapse40 in materials after etching as judged by the surface area and nitrogen physisorption isotherm of 2 in Figure 3b. There could also be unfavorable probe molecule partitioning in the amine-containing active-site regions of 2, as observed previously with TNBS.10 Both of these possibilities do not rule out the presence of imprinted amines as schematically represented by 2 in Figure 1b. An alternative possibility, however, is that there is an entire lack of amines, with 2 having an interfacial structure as schematically represented in Figure 1d. Such an outcome would preclude the possibility of organized aminosilane at the colloidal gold surface during gold@silica nanoparticle synthesis. To further discriminate between the two opposing outcomes represented by Figure 1b,d, we acquired the 13C CPMAS NMR spectra of 2 for assessing an absolute measure of primary amine density. The cross-polarization technique allows us to amplify resonances of atoms with attached protons, thus allowing imprinted aminopropyl (45) Sun, X. W.; Guan, Y. C.; Han, K. N. Metall. Mater. Trans. B 1996, 27, 355-361. (46) Strother, T.; Hamers, R. J.; Smith, L. M. Nucleic Acids Res. 2000, 28, 3535-3541.

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functional groups to be observed with greater sensitivity than what would be otherwise possible. The spectra in Figure 6a clearly show the three expected resonances of the propyl tether in the positive control material after etching procedure. However, the spectra in Figure 6b,c, which were taken under conditions identical to those in Figure 6a and correspond to 2 show very weak signal, if any at all, from aminopropyl resonances. If aminopropyl functionality is present in 2, it must be present in amounts less than 10% of the maximum potential yield and within the noise of spectra in Figure 6b,c in order to be consistent with the NMR results. Our results suggest that there is a preponderance of silanol functionality at the gold-silica interface in 1, and are much more consistent with the scenario schematically represented in Figure 1c,d rather than Figure 1a,b, with the latter scenario accounting for at most 10% of aminopropyl functionality. This result is supported by more recent syntheses of gold@silica nanoparticles, which have successfully replaced APTMS with ammonia, thus demonstrating that an amine polysiloxane layer is not required at the core-shell interface for nanoparticle synthesis.47 The results above do not provide an answer for the fate of APTMS in gold@silica nanoparticle synthesis, which expressed another way requires closing of the amine material balance. To obtain further insight into this question, we have used silica gel as a silica source instead of the sodium silicate used for the synthesis of 1, at the same sodium citrate ion concentration and pH as used for silica coating the gold nanoparticles during the synthesis of 1. The results of this experiment demonstrate that, within experimental error, all of the APTMS covalently attaches to the silica gel surface, as ascertained via salicylaldehyde binding experiments on the silica gel after treatment. This result requires any remaining aminosilane in solution after silica gelation to be attached to the composite gold-silica gel material. Since most (>90%) of the aminosilane was not found in this material, it requires that at some point before silica gelation, presumably during gold@silica nanoparticle synthesis, APTMS must covalently bind to colloidal particles of silica in solution. Because the silica yield for the synthesis of 1 is 75%, it is possible that these colloidal APTMS-functionalized silica particles remain dispersed in water and function as a catalyst for silica condensation during gold@silica nanoparticle synthesis, since polyamine cations are known to serve in this role.48 This would explain the experimentally observed function of APTMS as serving to facilitate the formation of a silica shell around the gold nanoparticle. These APTMS-functionalized silica particles may have been separated from the silica gel 1 after filtration and washing. An additional question that needs to be considered is the following: if the APTMS forms a colloidal polyamine species on a silica particle as suggested above, then why does this polyamine not interact and cause adsorption of the colloidal gold to the polyamine surface? Polysiloxane layers derived from APTMS, as well as a variety of other organosilanes, adsorb gold nanoparticles from aqueous solution.26 While not eliminating this possibility entirely, we suggest that this inevitably reduces to a problem of (47) Mine, E.; Yamada, A.; Kobayashi, Y.; Konno, M.; Liz-Marzan, L. M. J. Colloid Interface Sci. 2003, 264, 385-390. Lu, Y.; Yin, Y. D.; Li, Z. Y.; Xia, Y. A. Nano Lett. 2002, 2, 785-788. (48) Delak, K. M.; Sahai, N. Chem. Mater. 2005, 17, 3221-3227. Menzel, H.; Horstmann, S.; Behrens, P.; Barnreuther, B.; Krueger, I.; Jahns, M. Chem. Commun. 2003, 2994-2995. Rhodes, K. H.; Davis, S. A.; Caruso, F.; Zhang, B. J.; Mann, S. Chem. Mater. 2000, 12, 2832. Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111-1114.

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competitive kinetics and adsorption. The colloidal silicate species present in solution during the silica coating step, by virtue of their larger number density and stronger interaction with the polyamine layer (strong electrostatic bond) relative to colloidal gold, may act to competitively screen the polyamine layer away from the gold nanoparticle surface. The silica thus nucleated would subsequently bind to the gold and produce the silica shell of the gold@silica nanoparticle for synthesis of 1 as per Figure 1c.

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gold@silica nanoparticles synthesized via an APTMS route. We propose that functional groups having stronger interactions with gold, such as thiols49 and protected thiols,18 which have been previously used for synthesizing gold@silica nanoparticles with controlled interfacial composition and structure,17 be implemented for imprinting silica with colloidal gold. The materials synthesis methods described here can be used to investigate organic functional group organization in other metal@silica nanoparticles, and enable the synthesis of a diverse new class of imprinted materials based on colloidal templates.

Conclusion We have investigated the organization of aminosilane in gold@silica nanoparticles synthesized via an APTMS route, while evaluating these nanoparticles as functional templates for the synthesis of bulk imprinted silica. Goldsilica composite material 1 has been synthesized with upwards of 30 wt % gold in silica using gold@silica nanoparticles that have been synthesized via an APTMS route as building blocks. Material 1, while having some of the highest gold loadings reported in the literature to date, still maintains gold core isolation. The gold cores have been successfully removed from 1 via cyanide etching at a pH that significantly reduces silica solubility during gold removal. The resulting materials have been characterized using physisorption and a variety of spectroscopic methods. Probe molecule binding experiments, coupled with results from 13C CPMAS NMR spectroscopy, suggest that covalent amine incorporation into the composite goldsilica materials synthesized has a relatively low yield of less than 10%. This result requires that siloxy (Si-O-Si and Si-OH) functionality and not organized aminosilane functionality is present at the core-shell interface in

Acknowledgment. The authors are grateful to Ms. Rina Zalpuri at the EML facility at UCB for providing helpful assistance with TEM experiments, and to Mr. Steven Ruzin at the Light Microscopy Facility at UCB for technical assistance with optical microscopy. The authors acknowledge the National Science Foundation (DMR 0444761) and a 3M Untenured Faculty Award for funding. Supporting Information Available: Additional photographs of materials synthesis process (demonstrating necessity of APTMS for maintaining colloid stability), X-ray diffraction powder patterns for materials 1 and 2, nitrogen physisorption isotherms for material 2 (derived from low and high pH silica coating) at 77 K, TEM micrographs of gold@silica nanoparticle building blocks, and miscellaneous experimental conditions and calculations. This material is available free of charge via the Internet at http://pubs.acs.org. LA052006D (49) Hasan, M.; Bethell, D.; Brust, M. J. Am. Chem. Soc. 2002, 124, 1132-1133. Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802.