Electrochemical Oxidation of Adenine: A Mixed Adsorption and Diffusion Response on an Edge-Plane Pyrolytic Graphite Electrode

Abstract

Understanding the oxidation of the purines adenine and guanine is primary to improving electrochemical methods of DNA detection and analysis. Adenine in the solution phase is reported to undergo a complex electrochemical oxidation mechanism that is overall a -6H+, -6e- process, involving irreversible chemical steps. The observed voltammetry associated with the oxidation of adenine is strongly dependent upon the electrode used; this is a reflection of both the kinetics of oxidation and strength of bonding to the electrode surface, both of which are surface specific. Two main cases are presented within the article, one in which the adsorption of adenine to the electrode surface is strong, as is the case with gold and one in which the bonding is weaker, as found with graphitic surfaces. In the case of gold, adsorption is strong enough to prevent the adenine oxidation to be observed within the electrochemical window and further to this the formed monolayer prevents oxidation of the gold surface. With graphitic surfaces adsorption is weaker and as such oxidation of adenine is observed, this oxidative signal is demonstrated to be due to the oxidation of both surface bound and solution phase species. A generic method for analyzing the peak currents for reversible electron transfers coupled with an irreversible chemical process is presented. The analysis is dependent upon knowledge of the number of electrons transferred prior to the first chemically irreversible process and the total number of electrons transferred during the redox process. In addition to this, the analysis provides a description for the peak height of voltammetric waves that have contributions from both adsorbed and solution phase species, with these two processes being resolved through their differing dependencies with scan rate. The methodology is then used for the analysis of the oxidation of adenine on an edge-plane pyrolytic graphite electrode, where the influences on peak current from adsorption and diffusion are demonstrated. The diffusion coefficient for adenine is found to be (1.25 ± 0.2) × 105 cm s -1, which is in close agreement to that found by independent measurements reported in the literature. The adsorption of adenine at low concentrations to the electrode surface, is shown to exhibit a linear dependence of coverage with the solution phase concentration, where the surface coverage is given by Γ = KC* with a measured K value of (1.7 ± 0.1) × 10-3 cm and C* is the solution phase concentration.