Redox-Active Monolayers on ITO Prepared by Post-Amidation and Direct Esterification and Their Spectroelectrochemical Characterization

A redox-active monolayer on an optically transparent electrode constitutes a typical platform for spectroelectrochemical sensing. The necessity for its sophistication arises from the availability of multi-dimensional sensing signals. Simultaneous monitoring of the redox current and color change synchronized with the oxidation state change significantly enhances sensitivity and selectivity. This study aimed to elucidate the modification of an indium tin oxide (ITO) electrode with a viologen monolayer with an ordered orientation. Novel methods were developed to immobilize a viologen molecule bearing a carboxyl group to form assembled monolayers through a condensation reaction using 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide with N-hydroxy-succinimide (EDC/NHS). In the two methods of immobilization, one utilizes a two-step process to firstly form an aromatic siloxane base layer and subsequently attach the viologen derivative through an amide linkage by post-amidation. The other employs a direct ester linkage between the hydroxyl groups of the ITO surface and the carboxyl group of the viologen derivative. The latter method was also applied to immobilize a ferrocenyl group at a very short distance from the ITO surface. Potential-modulated UV-visible transmission absorption spectral measurement techniques with oblique incidence of plane-polarized light were employed to determine the orientation of the longitudinal axis of the reduced form of the viologen. The frequency dependence data of the potential-modulated transmission absorption signals were utilized to analyze the electron transfer kinetics. The performance of the two viologen-modified electrodes was compared to that of an ITO modified by post-amidation to the most commonly used base layer prepared with 3-aminopropyl triethoxysilane.