Redox-Active Green Electrode: Plant-Based Betanin Immobilized on Carbon Black for Drift-Free Voltammetric and Potentiometric pH Sensor Applications

Abstract

The development of eco-friendly chemicals and material-based electrode systems with a reduced carbon footprint is a novel initiative for future technological applications. While electrochemical systems based on plant phytochemicals meet the requirements, comprehending the fundamental electron-transfer reactions and preparing stable surface-confined redox systems pose significant research challenges. In this study, we have demonstrated an in situ electrochemical reaction-assisted entrapment of redox-active betanin molecular species from native beetroot plants on a carbon black-modified glassy carbon surface (GCE/CB@Betn-Redox, where Betn-Redox stands for redox-active betanin molecular species) in a pH 2.2 KCl-HCl solution. In general, direct access to native plant phytochemicals is a formidable task due to the matrix effect. Isolating the desired phytochemicals necessitates a series of time-consuming chemical separation steps. Unlike previous literature reports on the unstable nature of Betn, GCE/CB@Betn-Redox exhibited a stable and well-defined proton-coupled electron-transfer peak at an apparent electrode potential, Eo’ = 0.4 V vs Ag/AgCl, with a surface-excess value of 17.02 × 10-9 mol cm-2. Using several physicochemical techniques (transmission electron microscopy (TEM), Fourier-transform infrared (FTIR), and Raman), molecular techniques (UPLC), and electrochemical methods (in situ electrochemical quartz crystal microbalance (EQCM) and scanning electrochemical microscopy (SECM)), we have demonstrated the biomimicking electron-transfer functionality of CB@Betn-Redox. The unique feature of CB@Betn-Redox is its nonmediated effect on common biochemicals. This advantage makes it an interesting option for use as a selective pH sensor system without the complications of voltage drift that occur with the mediated oxidation/reduction functionality. We have successfully demonstrated highly selective and stable voltammetric and potentiometric pH sensor applications with practical real samples.