The electrochemistry of the ferri/ferrocyanide couple at a calix[4]resorcinarenetetrathiol-modified gold electrode as a study of novel electrode modifying coatings for use within electro-analytical sensors

The electrochemistry of the ferri/ferrocyanide redox couple has been studied at Au electrodes modified with calix[4]resorcinarenetetrathiol. Cyclic voltammetry in Fe(CN)63− solutions yields three separate pairs of faradaic peaks. Evidence is given for these redox couples corresponding to the reduction of Fe(CN)63− and the subsequent re-oxidation of Fe(CN)64− in three differing steric arrangements. One pair of peaks suggest that when the Fe(CN)63− ion resides within the calix[4]resorcinarene bowl, electron transport is facilitated by the calix[4]resorcinarene acting as a charge transfer mediator; in this arrangement the activation energy is found to be lowered by ∼24 kJ mol−1. Another pair of peaks is thought to correspond to the reduction of Fe(CN)63− as it approaches the Au electrode by packing itself in-between adjacent calix[4]resorcinarene molecules. The third pair of redox peaks is attributed to the reduction and subsequent re-oxidation of Fe(CN)63−/Fe(CN)64− when the ion resides above a saturated calix[4]resorcinarene coating; in this case the activation energy was raised by ∼45 kJ mol−1. FTIR spectroscopy of calix[4]resorcinarene-coated Au electrodes and calix[4]resorcinarene-coated Au electrodes exposed to Fe(CN)63− lends further support to this argument, by demonstrating that the Fe(CN)63− ion resides within at least two and possibly three differing environments. Calix[4]resorcinarene modified electrodes previously exposed to ferricyanide lose the calix[4]resorcinarene coating together with a surface layer of gold when subsequently scanned in a phosphate buffer. It therefore appears that the calix[4]resorcinarene/Fe(CN)63− association is stronger than the Au binding to the underlying glass material.