The electrochemistry of the ferri/ferrocyanide couple at a calix4resorcinarenetetrathiol-modified gold electrode as a study of novel electrodemodifying coatings for use within electro-analytical sensors

摘要

The electrochemistry of the ferri/ferrocyanide redox couple has been studied atAu electrodes modified with calix[4]resorcinarenetetrathiol. Cyclic voltammetryin Fe(CN)63- solutions yields three separate pairs of faradaic peaks. Evidenceis given for these redox couples corresponding to the reduction of Fe(CN)63- andthe 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 theactivation energy is found to be lowered by ~24kJmol-1. Another pair of peaks isthought to correspond to the reduction of Fe(CN)63- as it approaches the Auelectrode 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~45kJmol-1. FTIR spectroscopy of calix[4]resorcinarene-coated Au electrodes andcalix[4]resorcinarene-coated Au electrodes exposed to Fe(CN)63- lends furthersupport to this argument, by demonstrating that the Fe(CN)63- ion resides withinat least two and possibly three differing environments. Calix[4]resorcinarenemodified electrodes previously exposed to ferricyanide lose the calix[4]resorcinarene coating together with a surface layer of gold when subsequentlyscanned in a phosphate buffer. It therefore appears that the calix[4]resorcinarene/Fe(CN)63- association is stronger than the Au binding to theunderlying glass material.