Electrochemistry of redox probes at thin films of carbon nano-onions produced by thermal annealing of nanodiamonds

Abstract: Electrochemical data for pristine carbon nano-onions (CNO), radio frequency Ar/O2 plasma oxidized CNO (CNO-RF) and chemically oxidized CNO (CNO-OXI) cast on glassy carbon electrodes (GCE) are presented. The electrochemical response was evaluated by cyclic voltammetry using [Fe(CN)6]3-, [Ru(NH3)6]3+, [Fe(H2O)6]2+ and ferrocenemethanol (FcCH2OH) as redox probes. The electron transfer kinetics for these probes was influenced by the nature of functional groups on the electrode material, adsorption processes and the electrostatic interactions between the redox probes and the surface of the different carbon nano-onions. The electrocatalytic properties for the reduction of O2 to H2O2 at CNO, CNO-RF and CNO-OXI modified electrodes were also evaluated using linear sweep voltammetry. The electroreduction of O2 shifted to more positive potential (up to -0.16 V vs Ag/AgCl) as the oxidation degree of CNO increased. In the same way, the sensitivity for the electrochemical detection of H2O2 was enhanced up to 50-fold respect to unmodified GC electrode. Furthermore, the electrocatalytic oxidation of ascorbic acid (AA), dopamine (DA) and uric acid (UA) was studied by cyclic voltammetry and differential pulse voltammetry at CNO, CNO-RF and CNO-OXI modified electrodes. The electrodes displayed excellent electrocatalytic activity for the oxidation of AA, DA and UA. These results demonstrate that CNO enhance the electroactive properties of redox probes and are thus promising materials for electrochemical detection and metal-free electrocatalysts of different reactions. Additionally, the electrochemical data obtained here can serve to understand the performance of carbon nano-onion thin films as electrode materials.