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Water-Soluble, Sulfonic Acid-Functionalized, Monolayer-Protected Nanoparticles and an Ionically Conductive Molten Salt Containing Them Young-Seok Shon, W. Peter Wuelfing, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received August 22, 2000. In Final Form: November 21, 2000 Sulfonic acid-functionalized monolayer-protected gold clusters (SO3-MPCs) have been synthesized by the reaction of the Bunte (thiosulfate) salt of 2-acrylamido-2-methyl-1-propanesulfonic acid (SO3) with tetrachloroaurate and borohydride in aqueous acetic acid. The nanoparticles produced have an average core diameter, obtained by transmission electron microscopy (TEM), of 2.2 ( 1.1 nm, and a 30 wt % ligand content, obtained by thermogravimetric analysis. These parameters, assuming an ideal closed-shell truncated octahedral nanoparticle, correspond to an average nanoparticle formula of Au314SO3111. Proton NMR, Fourier transform infrared (FTIR), and UV/vis spectroscopies are consistent with nanoparticle formation, and acid/base titrations are consistent with the strong acid character of the SO3-MPC monolayer. These nanoparticles add to a growing family of water-soluble MPCs. Synthesis of a salt of SO3-MPC with a polyether-tailed triethylammonium countercation yields an ionically conductive molten salt, which is studied by alternating current impedance spectroscopy in the solid state (neat melt) and solution form.
Monolayer-protected gold clusters (MPCs) are becoming recognized for their diverse chemical and physical properties1-5 and properties potentially relevant to nanoscale electronic devices, optic devices, magnetic materials, multifunctional catalysts, chemical recognition, and biosensors,1,6-10 among others. Since Brust et al.2a described the synthesis of isolable, stable, alkanethiolatepassivated gold nanoparticles, such nanoparticles have been functionalized by simple chemical transformations,2-5,11-14 with a wide variety of structural units, (1) (a) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 101-117. (b) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202-218. (2) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655-1656. (c) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795-797. (d) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. J. Electroanal. Chem. 1996, 409, 137-143. (3) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36 and references therein. (4) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397-406. (5) (a) Badia, A.; Demers, L.; Dickinson, L.; Morin, F. G.; Lennox, R. B.; Reven, L. J. Am. Chem. Soc. 1997, 119, 11104-11105. (b) Badia, A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682-2692. (6) (a) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904-13910. (b) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924-1926. (c) Ahmadi, T. S.; Wang, Z. L.; Henglein, A.; El-Sayed, M. A. Chem. Mater. 1996, 8, 1161-1163. (7) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335-1338. (8) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41-53. (9) (a) Schmid, G.; Maihack, V.; Lantermann, F.; Peschel, S. J. Chem. Soc., Dalton Trans. 1996, 589-595. (b) Schmid, G. Chem. Rev. 1992, 92, 1709-1727. (10) Matijevic, E. Curr. Opin. Colloid Interface Sci. 1996, 1, 176183. (11) (a) Johnson, S. R.; Evans, S. D.; Mahon, S. W.; Ulman, A. Langmuir 1997, 13, 51-57. (b) Johnson, S. R.; Evans, S. D.; Brydson, R. Langmuir 1998, 14, 6639-6647. (12) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1997, 119, 12384-12385. (13) (a) Chen, S.; Murray, R. W. Langmuir 1999, 15, 682-689. (b) Chen, S. Langmuir 1999, 15, 7551-7557. (c) Chen, S.; Huang, K. Langmuir 2000, 16, 2014-2018.
including aromatic thiolates,2b,11-13 ω-substituted alkanethiolates,14a and polyhetero-ω-functionalized alkanethiolates.14b These materials exhibit solubility in organic solvents of polarity correlated with that of the monolayers but lack water solubility, which is desirable for a variety of purposes including use in conjunction with biological systems.15 Progress in designing water-soluble MPCs has been steady over the past several years and now includes MPCs with monolayers of tiopronin,16a,b poly(ethylene glycol),16c glutathione,17 4-hydroxythiophenol,2b,13b mercaptobenzoic acid,11b and mercaptosuccinic acid.18 Solubilities of 4-hydroxythiophenol, mercaptobenzoic acid, and mercaptosuccinic acid functionalized clusters are both limited and pH-dependent. Tiopronin [N-(2-mercaptopropionyl)glycine] and glutathione MPCs are freely water-soluble and the former additionally has been further functionalized by ligand place exchange and by amide coupling reactions of its carboxylic acid groups.16b Small Au55 clusters with strong acid ligands (Au55[PPh2C6H4SO3H]Cl6) have been reported.19 There has, however, been no well-characterized MPC reported with strong acid-functionalized thiolate ligand monolayers. In the field of molten salts known as polymer electrolytes, it is understood20 that solid-state cation conductivity (14) (a) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (b) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175-9178. (15) Colloidal Gold: Principles, Methods and Applications; Hayat, M. A., Ed.; Academic Press: San Diego, CA, 1989; Vols. 1 and 2. (16) (a) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66-76. (b) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081-7089. (c) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 12696-12696. (17) Schaaff, T. G.; Knight, G.; Shafigullin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 10643-10646. (18) Chen, S.; Kimura, K. Langmuir 1999, 15, 1075-1082. (19) Simon, U.; Schmid, G.; Schon, G. Mater. Res. Soc. Symp. Proc. 1992, 272, 167-175. CdS clusters protected with sodium 3-mercaptopropanesulfonate have also appeared: Miyake, M.; Torimoto, T.; Nishizawa, M.; Sakata, T.; Mori, H.; Yoneyama, H. Langmuir 1999, 15, 2714-2718.
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is enhanced by polymeric anions that are salts of strong acids. Metal and inorganic nanoclusters stabilized by polymeric materials have been of interest due to their electrical, magnetic susceptibility, dielectric, charge storage, electrochromic, and catalytic properties.21-23 We have described the solid-state ionic conductivity of MPCs passivated with a monolayer of thiolated polymer: R-methoxy-ω-mercaptopoly(ethylene glycol) (PEG-SH, MW 5000).16c The ionic conductivity of a polymer electrolyte made by dissolving LiClO4 in this material was larger than that of a polyether phase not containing the metal nanoparticles. This paper presents the first example and detailed characterization of a strong acid-functionalized, thiolate monolayer-protected nanoparticle that is freely watersoluble and can be dried of solvent and redissolved without aggregation of the Au cores. This nanoparticle, abbreviated SO3-MPC, bears ligands of 3-thiolatopropanamido-2methyl-1-propanesulfonic acid that are incorporated onto its surface by reactions of the Bunte (thiosulfate) salt of 2-acrylamido-2-methyl-1-propanesulfonic acid. (The polymeric form of this precursor is well-known in biological and polymeric chemistry,24-27 for electrophoresis,24 chromatography,25 biological responses,26,27 etc.) The anionic SO3-MPC nanoparticles are combined with a polyethertailed ammonium cation to produce a nanoparticlecontaining, ionically conductive molten salt. This new material is a semisolid having an ionically conductive nanophase surrounding a metallic core. Experimental Section Chemicals. Hydrogen tetrachloroaurate (HAuCl4‚xH2O) was synthesized by a literature method.28 Sodium thiosulfate (Na2S2O3‚5H2O, Mallinckrodt), 2-acrylamido-2-methyl-1-propanesulfonic acid (Aldrich), sodium borohydride (Aldrich), acetic acid (Fisher Scientific), ethyl alcohol (Aaper Alcohol and Chemical Co.), and tetrabutylammonium hydroxide (Aldrich) were used as received. Water was purified with a Barnstead Nanopure water system, model 4754. Polyethylene glycol monomethyl ether (MN ) 350 g/mol, MePEG, Aldrich) was dried in a vacuum oven at 60 °C prior to use, while triethylamine (Aldrich) was dried by passing through an activated alumina column. p-Toluenesulfonyl chloride (Aldrich) was stored in a desiccator under N2. Polyethertailed triethylammonium hydroxide (MePEG350-Et3N+OH-) was synthesized by a previously published method.29 Dowex 1 · 8-400 ion-exchange resin (Aldrich) in the hydroxide form was prepared in a conventional manner. (20) Dickinson, E. V.; Masui, H.; Williams, M. E.; Murray, R. W. J. Phys. Chem. B 1999, 103, 11028. (21) Mayer, A. B.; Mark, J. E.; Morris, R. E. Polym. J. 1998, 3, 197205. (22) Chan, Y. N. C.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 24-27. (23) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608-622. (24) Yu, C.; Svec, F.; Frechet, J. M. J. Electrophoresis 2000, 21, 120127. (25) (a) Peters, E. C.; Lewandowski, K.; Petro, M.; Svec, F.; Frechet, J. M. J. Anal. Commun. 1998, 35, 83-86. (b) Peters, E. C.; Petro, M.; Svec, F.; Frechet, J. M. J. Anal. Chem. 1998, 70, 2288-2295. (c) Peters, E. C.; Petro, M.; Svec, F.; Frechet, J. M. J. Anal. Chem. 1998, 70, 22962302. (26) (a) Bellini, M. P.; Manchester, K. L. Anal. Biochem. 1999, 268, 21-29. (b) Hu, N. F.; Rusling, J. F. Langmuir 1997, 13, 4119-4125. (c) Bellini, M. P.; Manchester, K. L. Anal. Biochem. 1999, 268, 21-29. (27) (a) Liekens, S.; Leali, D.; Neyts, J.; Esnouf, R.; Rusnati, M.; Dell’Era, P.; Maudgal, P. C.; DeClercq, E.; Presta, M. Mol. Pharmacol. 1999, 56, 204-213. (b) Liekens, S.; Neyts, J.; Degreve, B.; DeClercq, E. Oncol. Res. 1997, 9, 173-181. (c) Mohan, P.; Schols, D.; Baba, M.; DeClercq, E. Antivir. Res. 1992, 18, 139-150. (28) (a) Handbook of Preparative Inorganic Chemistry; Brauer, G., Ed.; Academic: New York, 1965; pp 1054-1059. (b) Block, B. P. Inorg. Synth. 1953, 4, 14-17. (29) (a) Dickinson, E., V.; Williams, M. E.; Hendrickson, S. M.; Masui, H.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 613-616. (b) Long, J. W.; Kim, I. K.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 11510.
Shon et al. Synthesis of Bunte Salts of 2-Acrylamido-2-methyl-1propanesulfonic Acid. 2-Acrylamido-2-methyl-1-propanesulfonic acid (20.7 g, 100 mmol) and 24.8 g (100 mmol) of Na2S2O3‚ 5H2O in 200 mL of water were refluxed together in a 500 mL round-bottom flask for 24 h and the water was then removed under vacuum. The crude product was dissolved in hot water/ ethanol (3:1 v/v). The insoluble materials were filtered off and the filtrate was recrystallized from cold water/ethanol. 1H NMR (200 MHz, D2O): δ 3.28 (s, 2 H, CH2SO3-), 3.15 (t, 2 H, CH2S2O3-), 2.58 (t, 2 H, CH2CONH), 1.35 [s, 6 H, (CH3)2C]. Synthesis of Sulfonic Acid-Functionalized Water-Soluble Gold Clusters (SO3-MPCs). The synthesis of nanoparticles from Bunte salts of 2-acrylamido-2-methyl-1-propanesulfonic acid is analogous to that of tiopronin-MPCs.16a In a typical reaction, 0.31 g of HAuCl4‚xH2O (0.80 mmol) and 0.88 g of the Bunte salt of 2-acrylamido-2-methyl-1-propanesulfonic acid (2.40 mmol) were codissolved in 36 mL of 6:1 water/acetic acid and stirred for ca. 10 min at room temperature. NaBH4 (0.61 g, 16.0 mmol) in 15 mL of Nanopure water was added over a period of ca. 30 s (vigorous reaction!). The solution immediately darkens and the temperature increases to ca. 45 °C during the borohydride addition. After it was stirred for 90 min, the pH of the reaction mixture was adjusted to 1 by dropwise addition of concentrated HCl; then the mixture was purified by dialysis (tubing ) Spectra/ Por CE, MWCO ) 10,000) for >5 days and water was removed under vacuum at
