The monolayer thickness dependence of quantized double-layer capacitances of monolayer-protected gold clusters

This report-describes how the electrochemical double-layer capacitances of nanometer-sized alkanethiolate monolayer-protected An dusters (MPCs) dissol ved in electrolyte solution depend on the alkanethiolate chain length (C4 t o C16), The double-layer capacitances of individual MPCs (C-CLU) are suffic iently small (sub-attoFarad, aF) that : their metal core potentials change by > 0.1 V increments for single electron transfers at the electrode/soluti on interface. Thus, the current peaks observed are termed "quantized double layer charging peaks", and their spacing on the potential axis caries with C-CLU. Differential pulse voltammetric measurements of C-CLU in solutions of core-size-fractionated (i.e., monodisperse) MPCs are compared to a simpl e theoretical model, which considers the capacitance as governed by the thi ckness of a dielectric material (the monolayer, whose chain length is varie d) between concentric spheres of conductors (the Au core and the electrolyt e solution). The experimental results fit the simple model remarkably well. The prominent differential pulse voltammetric charging peaks additionally establish this method, along with high-resolution transmission electron mic roscopy and laser ionization-desorption mass spectrometry, as a tool for ev aluating the degree of monodispersity of MPC preparations. We additionally report on a new tactic for the preparation of monodisperse MPCs with hexane thiolate monolayers.