4-hydroxythiophenol-protected gold nanoclusters in aqueous media

Gold nanoclusters protected by a monolayer of 4-hydroxythiophenol (HOPhSAu MPCs) were synthesized and characterized by various analytical techniques. The particles were found to be soluble in alkaline aqueous solutions, whereas at low pH, flocculation of particles started to occur. Acid/base titration of particle solutions indicated that the pK(a) of particle-bound phenol moieties was close to 10, about 1 unit higher than that of free monomers (ca. 8.9). Transmission electron microscopy (TEM) was employed to measure the average particle size (ca. 5 nm) and size dispersity (ca. 30%), where the resulting size histograms exhibited two major populations centered around 6 and 4 nm. Upon decreasing solution pH, partial fractionation was effected, with smaller particles showing a higher solubility at lower pH. W-vis spectroscopic measurements showed a surface-plasmon band at about 530 nm in various organic media. The red-shift of this band, relative to that of alkanethiolate or arenethiolate MPCs, might be due to particle flocculation as well as the electronic interactions between the phenyl moiety and the gold core that were facilitated by the hydroxy functional groups. In aqueous solutions, the surface-plasmon band position shows a slight red-shift with decreasing solution pH, and the absorption intensity monitored at this position exhibited a titration-like pH-dependent feature. A cyanide decomposition study of these particles showed a rather rapid rate constant, which might be ascribed to the enhanced partition of cyanide into the aromatic monolayers. Electrochemical studies of an aqueous solution of HOPhSAu particles and self-assembled monolayers of HOPhSH on a gold electrode surface both exhibited a pair of(quasi)reversible voltammetric waves, where the peak position shifted cathodically with increasing solution pH. A reaction mechanism was proposed which involved the formation of a carbonium intermediate in the electro-oxidation of the phenol moieties.