Mechanistic insights into the electrochemical oxidation of dopamine by cyclic voltammetry

Due to its presence in many biological systems, the oxidation of dopamine to polydopamine is of high interest with regard to biomedical and technical applications. Many variations in the reaction mechanisms have been discussed in the scientific literature, indicating a strong influence of the particular reaction conditions chosen. In this paper a cyclic voltammetric study on a gold electrode in phosphate buffered saline at pH 5.8 and 7.0 has been performed to elucidate the anodic oxidation at physiologic pH conditions. Calculation of the pH dependent species distribution of dopamine was used to identify the relevant species involved in the first step of the anodic oxidation. In the pH range 5.8-7.0 the phenolic groups of dopamine are not present in dissociated form and also the amino group is present in protonated form. The oxidation at the gold electrode is an electrochemically reversible process and leads to formation of a semiquinone as an intermediate. The follow-up reactions of the anodically formed intermediates depend on the solution pH. At pH 7.0 the formation of brown polymeric products on the electrode is faster and compared to experiments at pH 5.8 a lower cathodic peak current (I-p)(c). is observed during the reverse scan. The semiquinone intermediate explains formation of ring-bridged dimers as oligomeric precursors in polydopamine formation. Even at pH 7 the amino group still is present as ammonium ion, thus Michael addition, Schiff type reactions and cyclisation to indole ring containing products are expected to proceed as follow-up reactions with lower reaction rate.