Electrochemical characterisation of mixed monolayer assemblies of thiol analogues of cholesterol and fatty acids on gold

• A self-assembled monolayer (SAM) on gold prepared from a binary mixture of a thiol analogue of cholesterol (thiocholesterol, TC) and a functionalised alkanethiol (11-mercaptoundecanoic acid, MUA) has been investigated by voltammetry. The voltammetric results are in agreement with previously reported spectroscopic data and show that the geometric arrangement and composition of the molecules in the mixed monolayer controls the heterogeneous electron transfer process of Fe(CN)(6)(3-) across the assembly. The quantitative description of the influence of TC on the electron transfer rate constant is given through Tafel plots. At the pure MUA SAM electrode, the electron transfer is governed by penetration through the monolayer. The introduction of TC into the SAMs creates defects giving rise to diffusion controlled electron transfer in addition to penetration. By raising the TC content the electron transfer rate constant increases due to diffusion, This behaviour can be explained by a model in which the assembly goes from a penetrative but defect-free film barrier (pure MUA SAM) via a structure with defects in the mixed composition regime to a defect-rich structure consisting of an array of ultramicroelectrodes (pure TC SAM). (C) 1997 Elsevier Science S.A.

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