Comparative study of the voltammetric behaviour of guanine at carbon paste and glassy carbon electrodes and its determination in purine mixtures by differential-pulse voltammetry

Abstract

Cyclic voltammetry and differential-pulse voltammetry (DPV) were used in a comparative investigation into the electrochemical oxidation of guanine at a carbon paste electrode (CPE) and a planar glassy carbon electrode (GCE). The DPV peak potential and peak current were found to be dependent on the pH of the 0.5 mol dm^–3 phosphate buffer over the range 3.0–9.0 for both electrodes. The effect of ionic strength was investigated over the concentration range 0.05–0.5 mol dm^–3(pH 5.0). Slightly larger peak currents were obtained with 0.5 mol dm^–3 supporting electrolyte at both electrodes; however, an approximate 2.5-fold increase in the anodic response was observed with the CPE. The electro-oxidation of guanine was found to occur with the production of one voltammetric peak; the product probably undergoes further oxidation to yield two peaks on the second anodic scan. Under these conditions the electrode reaction was identified as being irreversible. Systematic studies into the possible adsorption of guanine were performed on a 1 × 10^–5 mol dm^–3 guanine solution in 0.5 mol dm^–3 phosphate buffer by DPV, utilizing CPEs and GCEs over the pH range 5.0–9.0. The peak currents did not increase with various accumulation potentials and times, indicating that the parent compound does not adsorb at either of the electrode surfaces studied. The optimum medium for quantitative determination at a CPE or a GCE by DPV was found to be 0.5 mol dm^–3 phosphate buffer (pH 5.0). Anodic peak currents were found to be linearly related to concentration over the range 1 × 10^–5–1 × 10^–7 mol dm^–3 and 1 × 10^–5–7.5 × 10^–7 mol dm^–3 for the CPE and GCE, respectively. Guanine was successfully determined in a deoxyribonucleic acid sample, following acid hydrolysis, with use of DPV. The developed method is more sensitive than those of previous voltammetric studies.