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A synthesis strategy to obtain monodisperse hexanethiolate-protected Au38 clusters based on their resistance to etching upon exposure to a hyperexcess of thiol is reported. The reduction time in the standard Brust-Schiffrin two-phase synthesis was optimized such that Au38 were the only clusters that were fully passivated by the thiol monolayer which leaves larger particles vulnerable to etching by excess thiol. The isolated Au38 was characterized by mass spectrometry, thermogravimetric analysis, optical spectroscopy, and electrochemical techniques giving Au38(SC6)22 as the molecular formula for the cluster. These ultrasmall Au clusters behave analogously to molecules with a wide energy gap between occupied (HOMO) and unoccupied levels (LUMO) and undergo single-electron charging at room temperature in electrochemical experiments. Electrochemistry provides an elegant means to study the electronic structure and the chemical stability of the clusters at different charge states. We used cyclic voltammetry and scanning electrochemical microscopy to unequivocally demonstrate that Au38 can be reversibly oxidized to charge states z = +1 or +2; however, reduction to z = -1 leads to desorption of the protecting thiolate monolayer. Although this reductive desorption of thiol from the cluster surface is superficially analogous to electrochemical desorption of planar self-assembled monolayers (SAMs) from macroscopic electrodes, the molecular details of the process are likely to be complicated based on the current view that the thiolate monolayer in clusters is in fact composed of polymeric Au-S complexes.

Solutions of monodisperse monolayer-protected clusters (MPCs) of gold can beused as multivalent redox mediators in electrochemical experiments due to their quantized double-layer charging properties. We demonstrate their use in scanning electrochemical microscopy (SECM) experiments wherein the species of interest (up to 2-electron reduction or 4-electron oxidation from the native charge-state of the MPCs) is generated at the tip electrode, providing a simple means to adjust the driving force of the electron transfer (ET). Approach curves to perfectly insulating (Teflon) and conducting (Pt) substrates are obtained. Subsequently, heterogeneous ET between MPCs in 1,2-dichloroethane and an aqueous redox couple (Ce(IV), Fe(CN)[SUB6][SUP3-/4-], Ru(NH[SUB3])[SUB6][SUP3+], and Ru(CN)[SUB6][SUP4-]) is probed with both feedback and potentiometric mode of SECM operation. Depending on the charge-state of the MPCs, they can accept/donate charge heterogeneously at the liquid-liquid interface. However, this reaction is very slow in contrast to ET involving MPCs at the metal-electrolyte interface. [ABSTRACT FROM AUTHOR]

Thiol-protected gold nanoparticles, so-called monolayer protected clusters (MPCs), are unique in that they can be repeatedly isolated and dispersed without irreversible aggregation. This extraordinary stability is due to the strength of the gold-thiol bond. From extensive research on 2D self-assembled thiol monolayers on Au (SAMs), the gold-sulfur bond is broken only under extreme oxidative conditions, at high temperatures or by electrochemical reduction. MPCs are essentially 3D SAMs and have been termed freely diffusing nanoelectrodes.

Describes the preparation of hexanethiol-capped Au (C6S-Au) particles by the Brust method using the modification proposed by Murray and colleagues to obtain particles of small core radius and improved monodispersity. Methods used to investigate the monolayer protected clusters (MPC) redox properties; Current response; Differential pulse voltammetry response; Oxidation states for monolayer protected gold nanoparticles.

This article reports the use of the scanning electrochemical microscope (SECM) to investigate the electronic properties of Langmuir monolayers of alkane thiol protected gold nanocrystals (NCs). A substantial increase in monolayer conductivity upon mechanical compression of the Au NC monolayer is reported for the first time. This may be the room temperature signature of the insulator to metal transition previously reported for comparable silver NC monolayers. Factors influencing the conductivity of the monolayer NC array are discussed. [ABSTRACT FROM AUTHOR]