A role of the membrane|solution interface in electron transfer at self-assembled monolayer modified electrodes

• The redox of ubiquinone-10 and vitamin K1 incorporated in SAMs was studied.
• The redox was significantly slowed by the presence of the SAM|solution interface.
• The adsorption of hydrophobic cations on the SAM caused the structural disorder.
• The adsorption of cations could be confirmed by capacity measurements.

A quinone derivative XQ (= ubiquinone-10 or vitamin K1) was incorporated into alkanethiol self-assembled monolayers (SAMs) formed on a gold electrode and the effect of the SAM|solution interface on the electron-transfer kinetics of XQ was investigated by cyclic voltammetry. It was confirmed that when a longer alkanethiol (e.g., 1-octadecylmercaptan) was used to form the SAM, the redox reactions of XQ were slowed significantly not only by the separation of XQ from the Au|SAM interface but also by the inhibition of the electron-transfer-coupled proton transfer across the SAM|solution interface. When a strongly hydrophobic supporting-electrolyte cation such as tetrabutylammonium or tetrapentylammonium was added to the aqueous solution, however, the redox reaction of XQ at the SAM modified electrode became faster. In order to explain this reason, we have proposed a model in which the adsorption of strongly hydrophobic cations on the SAM|solution interface induces a structural disorder of the monolayer and then facilitates the supply of protons from the solution to XQ in the monolayer. An evidence of the adsorption of the strongly hydrophobic cations on the SAM|solution interface was obtained from the measurements of the double-layer capacitance of the SAM modified electrode by alternating-current voltammetry.

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