Electrode passivation by reaction products of the electrochemical and enzymatic oxidation of p-phenylenediamine

The electrochemical and enzymatic oxidation of p-phenylenediamine (PPD) was studied under various conditions to evaluate its reversibility and stability when used as a laccase redox mediator. In accordance with published results, PPD oxidation during cyclic voltammetry showed that a passivation occurred with cycling in static systems without laccase. Such passivation was observed both in McIlvaine and acetate buffers on glassy carbon and platinum electrodes. Our results suggest that the oxidised form of PPD reacts with other PPD molecules in their reduced state to form a polymer on the surface of the electrode. When laccase is present in solution, PPD is only found in its oxidised state and another behaviour is observed at the electrode, with the appearance of a redox couple with an E’ centered at about −25 mV vs. Ag/AgCl in 0.1 M acetate buffer. These redox waves are attributed to the formation of soluble PPD oligomers. RRDE experiments showed no passivation, meaning that in order to form a film, the reaction products from the first oxidation of PPD must be further oxidised in a slow, homogeneous reaction. Results from these experiments have a significant importance in the design of efficient enzyme-modified electrodes since PPD diffusion in the thick polymer layers used to immobilise enzymes is a much slower process than in free solution, which allows enough time for the oxidation products to further react at the electrode and to polymerise on its surface. Passivation can therefore be observed with modified electrodes, even under hydrodynamic conditions, while similar conditions will provide a reversible system on bare electrodes. An example of this consequence of the reactivity of PPD oxidation products on the mediator's reversibility is given through experiments with a modified glassy carbon RDE functionalised with a thick layer of poly(ethyleneimine) microcapsules. In such case, the cyclic voltammograms showed that the oxidation and reduction processes are broader than at a bare electrode and that higher scan rates or rotation rates must be used to avoid passivation.