In this report, we evaluate the present state of the rapidly emerging field of monolayer-protected cluster (MPC) molecules with regard to their synthesis and monolayer functionalization, their core and monolayer structure, their composition, and their properties. Finally, we canvass some of the important remaining research opportunities involving MPCs.
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 properties 1-5 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, 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 Au 55 clusters with strong acid ligands (Au 55 [PPh 2 C 6 H 4 SO 3 H]Cl 6 ) 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 understood 20 that solid-state cation conductivity (1) (a) Schon, G.; Simon, U. C...
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