Formation of polymer-modified electrodes from 2-mercaptobenzoxazole in aqueous solution

Abstract

A polymer-modified electrode can be formed from 2-mercaptobenzoxazole (MBO) on glassy carbon (GC) electrodes in aqueous solution. Film formation occurs during prolonged cycling with positive scan limits above +600 mV vs. SCE. The main redox process is the oxidation of MBO to bis(benzoxazolyl) disulfide and the re-reduction to MBO. However, several side reactions including polymerization are observed. A density functional calculation of the MBO radical shows that there is a considerable spin density not only at the sulfur atom but also at the nitrogen atom and at some of the carbon atoms. Therefore ring-ring coupling products other than the disulfide can be formed which may further react to the polymer film. Notably, FTIR spectra indicate substitution at the nitrogen atom. The coupling would explain the occurrence of both thione and bridging sulfur as well as amine and imine nitrogen in the formed polymer. These binding states in the film have been identified by X-ray photoelectron spectroscopy (XPS). Elemental sulfur could not be detected by cyclic voltammetry or XPS of cooled samples. The polymer film is not redox active and non-conducting, as illustrated by the self-limiting growth and the diminishing currents during the potentiodynamic film growth. The film is impermeable for anions. At pH 7 the film is permeable for cations, while it is impermeable for anions and cations at pH 4.