The mean size of the gold (Au) core in the synthesis of dodecanethiolate-stabilized Au cluster compounds can be finely adjusted by choice of the Au:dodecanethiolate ratio and the temperature and rate at which the reduction is conducted. The Au clusters have been examined with a large number of independent analytical tools, producing a remarkably consistent picture of these materials. Average cluster and core dimensions, as ascertained by 1H NMR line broadening, high-resolution transmission electron microscopy, small-angle X-ray scattering, and thermogravimetric analysis, vary between diameters of 1.5 and 5.2 nm (∼110−4800 Au atoms/core). The electronic properties of the Au core were examined by UV/vis and X-ray photoelectron spectroscopy; the core appears to remain largely metallic in nature even at the smallest core sizes examined. The alkanethiolate monolayer stabilizing the Au core ranges with core size from ∼53 to nearly 520 ligands/core, and was probed by Fourier transform infrared spectroscopy, differential scanning calorimetry, contact-angle measurements, and thermal desorption mass spectrometry. The dodecanethiolate monolayer on small and large core clusters exhibits discernable differences; the line dividing “3-dimensional” monolayers and those resembling self-assembled monolayers on flat Au (2-dimensional monolayers) occurs at clusters with ∼4.4 nm core diameters.

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Rotated disk electrode voltammetry is described for CH2Cl2 solutions of cluster molecules with nanometer-sized gold cores and stabilizing ligand shells consisting of mixed monolayers of octanethiolate and ω-ferrocenyloctanethiolate ligands in molar ratios ranging from 2:1 to 24:1. Voltammograms for the cluster molecules exhibit a ferrocene oxidation wave with a limiting current that is under hydrodynamic mass transport control. The current−potential curves preceding (“prewave”) and following (“postwave”) the ferrocene wave, which are ideally flat, are decidedly sloped. The Δi/ΔE slopes are proportional to the square root of electrode rotation rate, i.e., are also under hydrodynamic control. The Δi/ΔE slopes are due to the charging of the electrical double layers of the cluster molecules, showing them to act as diffusing, molecule-sized “nanoelectrodes”. A theoretical analysis is presented of the transport control of the double layer charging. Possible reasons that the values of the cluster molecule capacitance (per unit surface area of cluster molecule, which entails use of models for the shape of the Au core of the cluster) are somewhat larger than the literature expectation for octanethiolate monolayers on flat gold surfaces are discussed. The tiny capacitances of the cluster molecules means that changing their charges by small potential increments can require an average of less than a single electron per cluster molecule.

Voltammetry of nonspecifically adsorbed and of freely diffusing monolayer protected clusters (MPCs) is presented. The MPC monolayers are mixtures of n-octanethiolates and ω-functionalized ferrocenyloctanethiolates. MPC adsorption coverages range from a few percent to roughly a full monolayer of cluster molecules. Rotated disk electrode voltammetry of the ferrocenated MPCs has two principal features:  the ferrocene oxidation wave and sloping current baselines at prewave and postwave potentials. Each MPC molecule can have multiple ferrocene units; characteristics of the ferrocene wave indicate that the polyelectron-transfer oxidations occur as a rapid sequence of single electron transfers. Comparisons of different modes of polyelectron transfer suggest that the present one involves rotational diffusion. Deviation from ideal Nernstian one-electron-transfer behavior is modeled as a Gaussian distribution of E 0‘ values. The sloping current baselines are attributed to double layer charging of the cluster cores that is controlled by hydrodynamic mass transport. Further aspects of previously described relations governing the core-charging property are compared to and found to be consistent with experimental behavior. Finally, preliminary experiments for determining MPC diffusion coefficients by the Taylor dispersion method are described.