Design and electrochemical study of merocyanine dyes: Influence of substituents on the redox behaviors and fouling propensity at ubiquitous electrode surfaces

Abstract

Investigations into the electrochemical behaviours of merocyanine dyes containing the N-methylbenzothiazolium (Btz) acceptor vinyl linked to phenolic (PhO) donors with electron-donating substituents (chlorine atoms), electron-withdrawing substituents (methoxy groups), or without any substituents at their ortho positions on common glassy carbon electrodes (GCEs), as well as gold and platinum surfaces were performed. It was found that the sensing signals and fouling propensities were heavily dependent on the electrode being used. The inclusion of substituent functional groups also minimized the electrochemical fouling of the dyes and further affected the electron transfer kinetics. The reaction mechanisms of the dyes were examined using electrochemical analysis and DFT computations. For both the Cl2PhOBtz and OMe2PhOBtz (Scheme 1), the radical species underwent a partial rearrangement to form radical species in the ortho position following the loss of the first electron. Following the first electron loss for the PhOBtz (Scheme 1), the formed radical species was unstable and underwent an instantaneous self-coupling reaction with a similar radical species to produce diquinone methide. This resulted in an irreversible oxidation reaction and increased the fouling propensity of the formed species on the electrodes. By understanding the influences of various substituents and taking into consideration of the fouling propensities, reversibility and sensing signals with respect to specific surfaces, novel aptamer-binding merocyanine dyes may be designed and synthesized for the development of highly robust and sensitive electrochemical sensors for medical and environmental applications.