Interfacial electron shuttling processes across Kolliphor®EL monolayer grafted electrodes

Covalently grafted Kolliphor®EL (a poly-ethylene-glycol based "transporter molecule" for hydrophobic water-insoluble drugs; MW ca. 2486; diameter ca. 3-5 nm) at the surface of a glassy carbon electrode strongly affects the rate of electron transfer for aqueous redox systems such as Fe(CN)63-/4-. XPS data confirm mono-layer grafting after electrochemical anodisation in pure Kolliphor®EL. Based on voltammetry and impedance measurements, the charge transfer process for the Fe(CN)63-/4- probe molecule is completely blocked after Kolliphor®EL grafting and in the absence of a "guest". However, in the presence of low concentrations of suitable ferrocene derivatives as "guests", mediated electron transfer across the mono-layer via a "shuttle mechanism" is observed. The resulting amplification of the ferrocene electroanalytical signal is investigated systematically and compared for 5 ferrocene derivatives. The low concentration electron shuttle efficiency decreases in the sequence dimethylaminomethyl-ferrocene > n-butyl-ferrocene > ferrocene-dimethanol > ferrocene-acetonitrile > ferrocene-acetic acid.