Electrocatalytic kinetics and mechanistic insights into indole oxidation reaction on highly oriented pyrolytic graphite electrodes

Abstract

The kinetics and mechanism of the indole oxidation reaction (IOR) on a highly oriented pyrolytic graphite (HOPG) electrode were investigated using various electrochemical techniques. The HOPG electrode exhibited excellent electrocatalytic activity, reaching current densities of up to 35 µA/cm2 over a concentration range of 0.1 and 200.0 µM and a detection limit of 0.02 µM. Chronoamperometric transients were analyzed through a kinetic model that described the relationship between the observed current density and the distribution of free and occupied active sites on the electrode surface. The number of active sites was quantified, revealing a moderate turnover frequency (TOF) under non-saturating conditions, showing the efficiency of the electrode during IOR. Electrochemical impedance spectroscopy (EIS) was employed to characterize the electrode surface and electrode/electrolyte interface. The EIS spectra, fitted using an equivalent circuit model, allowed the evaluation of charge transfer resistance and effective capacitance as a function of indole concentration. This study highligThe kinetics and mechanism of the indole oxidation reaction (IOR) on a highly oriented pyrolytic graphite (HOPG) electrode were investigated using various electrochemical techniques. The HOPG electrode exhibited excellent electrocatalytic activity, reaching current densities of up to 35 µA/cm2 over a concentration range of 0.1 and 200.0 µM and a detection limit of 0.02 µM. Chronoamperometric transients were analyzed through a kinetic model that described the relationship between the observed current density and the distribution of free and occupied active sites on the electrode surface. The number of active sites was quantified, revealing a moderate turnover frequency (TOF) under non-saturating conditions, showing the efficiency of the electrode during IOR. Electrochemical impedance spectroscopy (EIS) was employed to characterize the electrode surface and electrode/electrolyte interface. The EIS spectra, fitted using an equivalent circuit model, allowed the evaluation of charge transfer resistance and effective capacitance as a function of indole concentration. This study highlights the technological importance of understanding IOR, especially in the development of sensors for clinical and environmental applications requiring precise detection and quantification of indole.hts the technological importance of understanding IOR, especially in the development of sensors for clinical and environmental applications requiring precise detection and quantification of indole.