ELECTROCHEMICAL CHARACTERIZATION OF MIXED MONOLAYER ASSEMBLIES OF THIOL ANALOGS OF CHOLESTEROL AND FATTY-ACIDS ON GOLD

A self-assembled monolayer (SAM) on gold prepared from a binary mixtur e of a thiol analogue of cholesterol (thiocholesterol, TC) and a funct ionalised alkanethiol (11-mercaptoundecanoic acid, MUA) has been inves tigated by voltammetry. The voltammetric results are in agreement with previously reported spectroscopic data and show that the geometric ar rangement and composition of the molecules in the mixed monolayer cont rols the heterogeneous electron transfer process of Fe(CN)(6)(3-) acro ss the assembly. The quantitative description of the influence of TC o n the electron transfer rate constant is given through Tafel plots. At the pure MUA SAM electrode, the electron transfer is governed by pene tration through the monolayer. The introduction of TC into the SAMs cr eates defects giving rise to diffusion controlled electron transfer in addition to penetration. By raising the TC content the electron trans fer rate constant increases due to diffusion, This behaviour can be ex plained by a model in which the assembly goes from a penetrative but d efect-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 Sci ence S.A.