Preparation and Characterization of Gold Nanoparticles Dispersed in

Langmuir 2002, 18, 8255-8259

Preparation and Characterization of Gold Nanoparticles Dispersed in Poly(2-hydroxyethyl methacrylate) Nancy N. Kariuki, Li Han, Nam K. Ly, Melissa J. Patterson, Mathew M. Maye, Guojun Liu,† and Chuan-Jian Zhong* Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902 Received May 24, 2002. In Final Form: July 30, 2002

Introduction The encapsulation of very small metal or oxide particles, especially gold and silver, with an organic monolayer (e.g., alkanethiolate) possesses a number of unique attributes, including size monodispersity, core-shell processability, air stability, supramolecular-like solubility in common organic solvents, and intriguing optical, electronic, magnetic, catalytic and chemical/biological phenomena.1-2 A challenging task is to assemble the core-shell nanoparticles into macroscopic materials with desired nanoscale properties. A number of approaches have been reported in the past several years, including placeexchange derivatization,3 stepwise molecular or complex assembling,4-6 DNA-directed linking,7 incorporation into polymer layers,8 one-step molecular exchange-cross* To whom correspondence should be addressed. Phone: 607777-4605. E-mail: [email protected]. † Ultraphotonics, 48611 Warm Springs Blvd., Fremont, CA 94539. (1) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Gittins, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000, 408, 67. (2) (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27 and references therein. (b) Whetten, R. L.; Shafigulin, M. N.; Khoury, J. T.; Schaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (c) Storhoff, J. J.; Mirkin, C. Chem. Rev. 1999, 99, 1849. (d) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18. (e) Zhong, C. J.; Maye, M. M. Adv. Mater. 2001, 13, 1507. (f) Han, L.; Daniel, D. R.; Maye, M. M.; Zhong, C. J. Anal. Chem., 2001, 73, 4441. (3) (a) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212. (b) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (c) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (d) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906. (e) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S. W.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845. (4) (a) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14 (4), 5425. (b) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C, J. Adv. Mater. 1995, 7, 795. (c) Bethell, D.; Brust, M.; Schiffrin D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137. (d) Brust, M.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. J. Am. Chem. Soc. 1998, 120, 12367. (e) Baum, T.; Bethell, D.; Brust, M.; Schiffrin, D. J. Langmuir 1985, 1 (5), 866. (f) Horswell, S. L.; O’Neil, I. A.; Schiffrin, D. J. J. Phys. Chem. B 2001, 105, 941. (5) (a) Musick, M. D.; Pena, D. J.; Botsko, S. L.; McEvoy, T. M.; Richardson, J. N.; Natan, M. J. Langmuir 1999, 15, 844. (b) Keating, C. D.; Musick, M. D.; Lyon, L. A.; Brown, K. R.; Baker, B. E.; Pena, D. J.; Feldheim, D. L.; Mallouk, T. E.; Natan, M. J. ACS Symp. Ser. 1997, 679, 7. (c) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (6) (a) Zamborini, F. P.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 4514. (b) Templeton, A. C.; Zamborini, F. P.; Wuelfing, W. P.; Murray, R. W. Langmuir 2000, 16, 6682. (c) Lahav, M.; Shipway, A. N.; Willner, I. J. Chem. Soc., Perkin Trans. 1999, 2, 1925. (7) (a) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (b) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (c) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (d) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609.

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linking,9 and hydrogen-bonding intershell linking.10 The latter two approaches, termed as the exchange-crosslinking-precipitation method,9-10 create a simple and versatile route for self-assembling core-shell nanoparticles into thin films. The viability of the exchange reaction was first demonstrated by Murray’s group3 for introducing functionalized components into the shell structure. The exchange-cross-linking-precipitation method further exploits the functionalized components toward cross-linking to produce a thin film assembly.10 We note that complexation chemistry between carboxylates and divalent metal ions is recently demonstrated for assembling gold nanoparticles via a layer-by-layer pathway.6,8 In some practical applications, environmentally or biologically compatible polymer matrixes are desired for dispersing the functionalized nanoparticles. The compatibility of a variety of polymers with biological systems and their ability to withstand harsh environmental conditions have been known.11 Most of the existing approaches employ in-situ formation of nanoparticles during polymerization or in the presence of polymers and metal monomer precursors.12 The fine-tuning of the nanoparticles in terms of size and surface properties prior to polymeric thin film formation is rather difficult for controlling the nanoscale functional properties of the resulting materials. This difficulty can be overcome by pre-engineering the shell with alkanethiols with carboxylic acid groups,13 as recently demonstrated for assembling nanoparticle thin films by utilizing polyelectrolytes in a layer-by-layer pathway.8 In this paper, we report the results from an investigation of the preparation of alcoholsoluble gold nanoparticles toward the dispersion into poly(2-hydroxyehtyl methacrylate) (PHM). The choice of PHM is largely based on the fact that PHM and related copolymers have found a wide range of applications, including ophthalmology (e.g., contact lenses), plastic surgery, controlled drug delivery, microencapsulation of insulin-producing cells, diagnostic carriers, biosensors, replacements of bone tissues, and other biomedical applications.14 Experimental Section Poly(2-hydroxyethyl methacrylate) (PHM) was obtained from Scientific Polymer Products (Optical grade; average MW, 300 000; (8) Hicks, J. F.; Seok-Shon, Y.; Murray, R. W. Langmuir 2002, 18, 2288. (9) (a) Zhong, C. J.; Zheng, W. X.; Leibowitz, F. L. Electrochem. Comm. 1999, 1, 72. (b) Leibowitz, F. L.; Zheng, W. X.; Maye, M. M.; Zhong, C. J. Anal. Chem. 1999, 71, 5076. (10) (a) Zheng, W. X.; Maye, M. M.; Leibowitz, F. L.; Zhong, C. J. Anal. Chem. 2000, 72, 2190. (b) Han, L.; Maye, M. M.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J. J. Mater. Chem. 2001, 11, 1259. (c) Israel, L. B.; Kariuki, N. N.; Han, L.; Maye, M. M.; Luo, J.; Zhong, C. J. J. Electroanal. Chem. 2001, 517, 69. (11) (a) Hsiue, G.-H.; Guu, J.-A.; Cheng, C.-C. Biomaterials 2001, 22, 1763. (b) Tiyaboonchai, W.; Woiszwillo, J.; Middangh, C. R. J. Pharm. Sci. 2001, 90, 902. (12) (a) Mayer, A. B. R. Polym. Adv. Technol. 2001, 12, 96. (b) Mayer, A. B. R. Mater. Sci. Eng. C 1998, 6, 155. (c) van der Zande, B. M. I.; Pages, L.; Hikmet, R. A. M.; van Blaaderen, A. J. Phys. Chem. B 1999, 103, 5761. (d) Teranishi, T.; Kiyokawa, I.; Miyake, M. Adv. Mater. 1998, 10, 596. (e) Underhill, R. S.; Liu, G. J. Chem. Mater. 2000, 12, 3633. (f) Mayer, A. B. R.; Mark, J. E. Polymer 2000, 41, 1627. (13) (a) Boal, A. K.; Rotello, V. M. Langmuir 2000, 16, 9527. (b) Kim, Y.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 4. (c) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (d) Johnson, S. R.; Evans, S. D.; Brydson, R. Langmuir 1998, 14, 6639. (e) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 3944. (f) Hao, E.; Lian, T. Chem. Mater. 2000, 12, 3392. (14) (a) Kost, J.; Langer, R. Adv. Drug Delivery Rev. 2001, 46, 125. (b) Horak, D. Chem. Listy, 1992, 86, 681.

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glass transition temperature, 55 °C; density, 1.19; refractive index, 1.5119). Decanethiolate (DT)-capped gold nanoparticles of 2-nm core diameter (DT/Au2-nm) were synthesized using the standard two-phase protocol by Schiffrin and co-workers1a and the modified procedure,15 which produced nanoparticles with a core size of 1.9 ( 0.7 nm (Au2-nm), as determined from TEM. Gold nanoparticles of 5.2 ( 0.3 nm core size (Au5-nm) were derived from the above nanoparticles by a thermally activated processing route that was recently developed in our laboratory.16 The MUAderivatized gold nanoparticles were prepared by dissolving films produced via exchange-cross-linking-precipitation 9,10 in two methods. (A) Using hexane with minimum ethanol to dissolve MUA (typically 92% hexane/8% ethanol), the product from the exchange-cross-linking reaction between MUA and DT-capped gold nanoparticles was mainly precipitated as a thin film on the wall of the reaction vessel or on glass slides.10 (B) Using a solvent consisting of 92% hexane/8% ethanol, the reaction proceeded under stirring and the product was partly dissolved in ethanol and partly precipitated as powders or films on the wall of reaction vessel. Both reactions were conducted at room temperature. The typical concentrations were 8 µM for DT/Au2-nm and 1.6 or 4 mM MUA. For DT-capped Au5-nm particles, the concentrations were 1.0 µM for DT/Au 5-nm and 0.5 or 10 mM for MUA. The reaction time ranged from a few hours to 2 days, depending on concentrations and particle sizes. The precipitated products were rinsed thoroughly with hexane. For preparing polymer films, the MUAderivatized gold nanoparticles were dissolved in an ethanol solution containing PHM. The concentration of PHM ranged from 0.04% to 2% (wt %). Thin films were then prepared by casting or spin-coating of the nanoparticle/PHM mixed solution on different substrates. The films were dried at room temperature. The 1H and 13C NMR spectra were collected at 300 and 360 MHz, respectively, on a Bruker (AC) spectrometer. Deuterated DMSO or ethanol was used as solvent. UV-vis spectra were acquired with a HP 8453 spectrophotometer. Thin films were prepared using cover glasses as substrates. Infrared spectra (IR) were acquired with a Nicolet 760 ESP FT-IR spectrometer. Transmission electron microscopy (TEM) was performed on a Hitachi H-7000 electron microscope (100 kV). Thin films were deposited on a carbon-coated copper grid by immersing it in a very dilute ethanolic solution of nanoparticles and PHM. The grid was emersed from the solution and dried with air.

Results and Discussion The initial DT-capped gold nanoparticles are soluble in nonpolar solvent (e.g., hexane) but not in polar solvent. In contrast, the MUA-derivatized particles became insoluble in the nonpolar solvent but soluble in polar solvent (e.g., ethanol). The solubility of the MUA-derivatized particles in ethanol also depends on the nanocrystal size. While the Au2-nm is soluble, the Au5-nm is not or only slightly soluble (product from method B). We note that while spectral characterizations of MUA-assembled thin films of gold nanoparticles have been reported previously,10a the characterization of the nanoparticles after dissolution of the films in ethanol, which is reported herein for the first time, is needed to establish the structural properties for subsequent mixing with polymer in ethanol solution toward preparation of the nanoparticle/PHM films. We also note that recent work 2,8 has demonstrated that the exchange reaction between MUA and hexade(15) (a) Wuelfing, W. P.; Templeton, A. C.; Hicks, J. F.; Murray, R. W. Anal. Chem. 1999, 71, 4069-4074. (b) Chen, S. W.; Murray, R. W. J. Phys. Chem. B 1999, 103, 9996. (c) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (d) 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. (e) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (16) (a) Maye, M. M.; Zheng, W. X.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J. Langmuir 2000, 16, 490. (b) Maye, M. M.; Zhong, C. J. J. Mater. Chem. 2000, 10, 1895.

Notes Table 1. Assignments (cm-1) of the FTIR Spectra for the DT-Capped and the MUA-Derivatized DT/Au Nanoparticles MUA-derivatized DT/Au

DT/Au

Au2-nm νCsH: (CH2): 2923 (νa), 2852 (νs) νCsH: (CH2): 2920 (νa), 2849 (νs) (CH3): 2960 (νa), 2876 (νs) (CH3): 2957 (νa), 2869 (νs) νCdO: 1732 δCsH: ∼1400 νCsO: 1280 Au5-nm νCsH: (CH2): 2925 (νa), 2855 (νs) νCsH: (CH2): 2918 (νa), 2847 (νs) (CH3): 2961 (νa), 2871 (νs) (CH3): 2957 (νa), 2899 (νs) νCdO: 1730 δCsH: ∼1400 νCsO: 1280

canethiolate-capped gold nanoparticles carried out in THF solvent produced soluble nanoparticles. In comparison, the nanoparticle products produced from our nonpolar solvent system were easily isolated from the reaction solution and readily cleaned using nonpolar solvent. FTIR characterization revealed (Table 1) vibrational bands diagnostic of the carbonyl stretching of -CO2H groups and the C-H stretching of methyl and methylene groups, which is consistent with early results for gold nanoparticles of ∼2-nm core sizes capped by alkanethiolates of other chain lengths and their exchange products17 and thin films.10 The detection of the band at 1730 cm-1 is assigned to the carbonyl stretching band of the -CO2H group, which has an envelop of two components due to free acid and hydrogen-bonded dimer.10 The band at 1632 cm-1 could be due to a combination of polymeric hydrogenbonding of -CO2H groups and crystalline water in the powder samples. The band at 1280 cm-1 can be assigned to C-O stretching of the -CO2H group. The asymmetric and symmetric C-H stretching bands for the MUAderivatized nanoparticles were similar to those for DTcapped nanoparticles. While the relative ratio of these bands varies, the shift in methylene stretching bands (3-7 cm-1) after derivatization suggests that the chain-chain packing for the MUA-encapsulation is relatively more disordered than the DT-encapsulation. Qualitatively, the detection of the methyl band is indicative of the presence of unexchanged DT molecules on the particles. For MUAderivatized Au5-nm particles, spectral features are qualitatively comparable to those of the MUA-derivatized Au2-nm. The MUA-DT exchange reaction on both nanoparticles leads to a mixed monolayer encapsulation. A comparison of the UV-vis spectra for MUA-derivatized Au2-nm or Au5-nm particles dissolved in ethanol solution with the spectrum for DT-capped Au2-nm or Au5-nm particles in hexane (Figure 1) revealed similarity; both display an identifiable but weak SP band envelope around ∼520 nm. The shape and position of this band are in agreement with those previously reported for particles of similar sizes.15 The results, in combination with TEM observation of the unchanged core size, indicate that the exchange process does not induce an evolution of the core. Both the leaving of DT molecules and the binding of MUA molecules take place on the nanocrystal surface. The structural difference of these two encapsulating molecules does not significantly alter the SP band property. NMR characterization of the soluble nanoparticles after the derivatization reaction provided further insight about the extent of the exchange. We note that extensive NMR characterizations have been reported15d,19 for place(17) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (18) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706.

Notes

Figure 1. UV-visible spectra: (A) DT-capped Au2-nm in hexane (1) and the MUA-exchanged Au2-nm in ethanol (2). The absorbance of spectrum 2 is scaled up by ×4. (B) DT-capped Au5-nm in hexane (1) and the MUA-exchanged Au5-nm in ethanol (2). The absorbance of spectrum-2 is scaled up by ×2.

Figure 2. 1H NMR spectra of the MUA-exchanged Au2-nm in d6-DMSO solvent (a) and DT-capped Au2-nm in C6D6 solvent (b). Peak assignments (ppm): (a) 2.7 (-S-CH2-), 1.5 (-SCH2-CH2-), 1.2 (-S-CH2-CH2-(CH2)8-), 2.2 (-CH2-CO2H), 2.5 (DMSO solvent); (b) 1.5 (-S-(CH2)n-), 1.0 (-CH3).

exchange derivatization of nanoparticles carried out under different synthetic conditions. Figure 2 shows 1H spectra for MUA-derivatized Au2-nm dissolved in DMSO solvent (a) and for DT-capped Au2-nm in benzene solvent (b). According to recent work,15d the peak broadening and the absence of peaks corresponding to the -CH2 group next to the S-Au bond are due to a combination of factors. For DT-capped nanoparticles, the dense packing and solidlike property of -CH2 next to the S-Au bond experience fast spin relaxation from dipolar interactions, whereas -CH2 groups furthermost from the Au core experience freedom of motion and spin relaxations more similar to those of dissolved species. In addition, spin-spin relaxation broadening depends on the rate of tumbling of particles in solution, and the tumbling rate is significantly reduced for alkanethiolate molecules on the particle surface. The spectral feature is consistent with those reported earlier (19) (a) Zamborini, F. P.; Gross, S. M.; Murray, R. W. Langmuir 2001, 17, 481. (b) 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., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (c) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262. (d) Hasan, M.; Bethell, D.; Brust, M. J. Am. Chem. Soc. 2002, 124, 1132.

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for gold nanoparticles of similar core sizes with alkanethiolates of a different chain length.15d In comparison with the two peaks observed for DT-Au2-nm nanoparticles, the 1.2 (-CH2-) and 0.9 (-CH3) ppm peaks observed for MUA-exchanged particles are much more narrow. The slight shift of these two peaks may reflect a more disordered shell structure after the exchange and a solvent effect. The result indicates the presence of DT in the MUAexchanged shell structure. The 2.2 ppm peak is characteristic of MUA (i.e., -CH2- adjacent to -CO2H) whereas the 0.9 ppm peak is characteristic of DT (i.e., -CH3). On the basis of their ratio, we estimated the degree of the exchange. A value of 46% is found for the displacement of DT by MUA in the MUA/Au2-nm particles, suggesting a mixed monolayer encapsulation. This value is remarkably comparable with values reported for gold nanoparticles of an average core diameter 1.6 nm with monolayers composed of hexanethiolate ligands prepared by “placeexchange” reactions by stirring the particles in a solution of MUA in THF for ∼4 days, during which ligand exchange occurs between the C6 thiolates and the MUA in solution.8 The NMR determination of the exchange extent involved quantitatively desorbing the ligands from the particle core as disulfides by the addition of a crystal of iodine, which yielded a displacement of 38%.8 The difference in chain length is likely one of the factors contributing to the difference in exchange degree. We also note that preferential binding of one thiol component over another from solution mixtures of alkanethiols has been well-known in the spontaneous assembly of 2D monolayers on gold surfaces. It is interesting that the use of a solvent in which the product is insoluble can lead to an extent of exchange comparable with that from using other solvents in which the product is soluble.3,8 We have also compared 1H spectra for MUA-derivatized Au5-nm and DT-capped Au5-nm. The peak broadening for DT-capped Au5-nm in comparison with the data for the DT-capped Au2-nm is consistent with previous observation,15d due to a combination of the solid-state effect of the nanoparticles and spin-spin relaxation broadening from the slow rate of tumbling of the particles in solution.15d The poor solubility of the MUA-derivatized Au5-nm particles however prevented us from further quantitative characterization of the degree of exchange at this time. The MUA/Au2-nm/PHM solution appears to be the same brown color as in the absence of the polymer. The stability of the nanoparticle/polymer solution was evidenced by the fact that there was no apparent precipitation or color change in an at least one-year period. This indicates that the polymer does not displace the encapsulating shell molecules on the nanoparticle. The solution can be easily cast as a thin film of (MUA/Au2-nm)/PHM onto a flat substrate by dip-coating or spin-coating. The thin film can also be redissolved in ethanol at an elevated temperature, demonstrating the reversibility of the film formation. The UV-vis spectrum of a (MUA/Au2-nm)/PHM film shows that the absorption in the visible region characteristic of the surface plasmon resonance band for the 2 nm gold nanoparticles remains unchanged in the thin film. This is also true for thin films cast from different PHM concentrations. The spectral characteristic is very similar to that obtained for a MUA/Au2-nm thin film,10a indicating that the integrity of the individual nanoparticles remains intact in these two different microenvironments. Figure 3A shows a representative set of TEM micrographs for a thin film of (MUA/Au2-nm)/PHM prepared by dip-coating from two ethanolic solutions of PHM, 0.04% with ∼0.6 µM MUA/Au2-nm (a) and 0.18% with ∼0.2 µM MUA/Au2-nm (b). The MUA/Au2-nm (∼1 µM) particles were

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Notes

Figure 4. TEM micrographs of a thin film cast (dip coated) on a carbon-coated TEM grid from an ethanolic solution of 2.0% PHM with 2.6 µM MUA/Au2-nm. Inset: a magnified view of the indicated area.

Figure 3. (A) TEM micrographs of thin films cast (dip coated) on carbon-coated TEM grids from an ethanolic solution of PHM: 0.04% with ∼0.6 µM MUA/Au2-nm (a); 0.18% with ∼0.2 µM MUA/Au2-nm (b). (B) TEM micrograph of nanoparticles cast (dip coated) on a carbon-coated TEM grid from MUA-Au2-nm with ∼1 µM in ethanol solution.

prepared using method A, as described in the Experimental Section. For the nanoparticles evaporated on the TEM grid in the absence of the polymer, that is, the MUA/ Au2-nm (Figure 3B), there is a predominant clustering phenomenon for the particles, presumably due to the strong tendency of intershell hydrogen-bonding. The network branching of the particles can also be observed in domains with a submonolayer coverage. In contrast, two types of unique features are evident for the cast (MUAAu2-nm)/PHM samples (Figure 3A, a and b). First, the particles are relatively well dispersed, though small domains of interparticle packing can be identified. Second, we have observed spherical packing of the nanoparticles. Such an assembly was detected for several samples, which showed relatively loose packing with an average diameter of ∼180 nm. Upon increasing the concentration of PHM from 0.04 (a) to 0.18% (b), the small domains appear to become more isolated with each other. In the meantime, the spherical assembly seems to become more compact, which is reflected by the decrease of the average diameter of the spheres (∼130 nm). In comparison with in-situ formation of nanoparticles reported for many polymeric systems,12,14,21 our findings demonstrate that it is viable to manipulate the dispersion of pre-engineered nanoparticles in the polymeric matrix in terms of interparticle spacing and arrangement. It is intriguing that upon further increasing the concentrations of the polymer and the particles (e.g., 2.0% PHM and 2.6 µM MUA/Au2-nm) the interparticle morphology displays a remarkable evolution (Figure 4). The small domains with individually dispersed nanoparticles (20) Lee, W. F.; Lin, Y. H. J. Appl. Polym. Sci. 2001, 81 1360. (21) Scherble, J.; Thomann, R.; Ivan, B.; Mulhaupt, R. J. Polym. Sci. B 2001, 39, 1429.

as observed in Figure 3 largely disappeared; the number of spherical assemblies showed a significant increase. The packing density of the particles in these spherical assemblies (Figure 4) appears much higher (or more compact). A closer examination of these spherical assemblies also reveals that these assemblies have a welldefined outline and an average diameter of 11.2 nm with a standard deviation of 5.2 nm. Interestingly, the basic feature of these spheres shows some resemblance to those of gold nanoparticle assemblies linked by tetrathioethers, as discovered recently in our laboratory,22 and those reported for other nanoparticle assemblies using thiol or polymeric molecular recognition linkers.23 One of the novel aspects in our (MUA/Au2-nm)/PHM system is the fact that some of the spherical assemblies seem to exhibit a hollow interior (see insert of Figure 4), which is to our knowledge not reported for any other systems. The propagation effect due to polymeric conformational flexibility, as demonstrated recently for a gold nanoparticle/polymer spherical assembly system,23a and the balancing effects such as mass transport inside and outside the sphere and osmotic pressures, as revealed in a recent study of gold nanoparticles capped with mercaptoacetate in water,23b could be some of the possible driving forces that keep the assembly spherical for our MUA-capped nanoparticles in the polymeric PHM environment. We are carrying out an indepth study of these effects. Our further study of the morphology of the dispersed nanoparticles in the polymer is directed to address how the interactions between the MUA shells and between the MUA shell and the polymeric hydroxyl or carbonyl groups are balanced. Nevertheless, the interparticle hydrogen-bonding is believed to play a significant role. This understanding will have important implications for designing potential applications of the nanostructured thin films in biological fluids. By further delineating the concentration or ratio of the polymer and the nanoparticles, the approach of pre-engineering nanoparticles followed by simple dispersion in polymers may eventually (22) Maye, M. M.; Chun, S. C.; Han, L.; Rabinovich, D.; Zhong, C. J. J. Am. Chem. Soc., 2002, 124, 4958. (23) (a) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (b) Adachi, E. Langmuir 2000, 16, 6460. (c) Jin, J.; Iyoda, T.; Cao, C. S.; Song, Y. L.; Jiang, L.; Li, T. J.; Ben Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 2135. (d) Maya, L.; Muralidharan, G.; Thundat, T. G.; Kenik, E. A. Langmuir 2000, 16, 9151.

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lead to a versatile strategy for the creation of novel multifunctional nanomaterials such as membrane-type responsive materials. Conclusions In conclusion, we have shown that decanethiolatecapped gold nanoparticles of two different core sizes can be derivatized via an exchange-precipitation method using alkanethiols functionalized with carboxylic acid groups, leading to the formation of shells with a mixed monolayer of methyl and carboxylic acid functional groups. The dispersion of the functionalized nanoparticles into the hydrophilic poly(2-hydroxyethyl methacrylate) thin film can be achieved via simple solution mixing and subsequent casting. The solution was very stable, and the cast film appeared to reveal well-dispersed and uniform size. By controlling the polymer or nanoparticle concentration, we

have found the formation of an intriguing spherical assembly of the nanoparticles in the polymeric environment. We are currently investigating how the interaction forces are balanced between the nanoparticles and the polymer structure that determines the morphological properties of the dispersed nanoparticles in the polymer. The general methods reported herein are potentially applicable to systems involving nanoparticles of different core-shell composition and polymers of different structural properties. Acknowledgment. Financial support of this work from the Petroleum Research Fund administered by the American Chemical Society and the 3M Corporation is gratefully acknowledged. LA0259860