Electrochemical Study of DPPH Incorporated in Carbon Paste Electrode as Potential Tool for Antioxidant Properties Determination

The electrochemical behaviour of 2,2-diphenyl-1-picrylhydrazyl (DPPH) incorporated in a carbon paste electrode (CPE/DPPH1%) was studied. Two electrochemical systems assigned to the free-radical were perfectly defined, including System I where DPPH acts as reducing agent and System II where it acts as an oxidant. Although System II was reversible and remains stable after several consecutive potential scans, System I shows high peak current ratio (Ipa/Ipc=1.4) and a significant decrease in signal intensity when performing consecutive potentials scans. This phenomenon was due to the chemical reaction of the oxidized DPPH and the hydroxide ions present at the vicinity of the electrode surface. This new compound formed was responsible of a new system (System III). Systems II and III were highly pH-dependent, in agreement with the proton exchange involved in these electrochemical transformations. When experiments were performed in the presence of ascorbic acid (AAH(2)), only System I was found to be ideal for monitoring the antioxidant activity of AAH(2). System I clearly shows a linear dependency between the peak currents of DPPH as a function of AAH(2) concentration. This linear response was used to determine the IC50 of ascorbic acid (0.56M) in the described experimental condition. This effect was confirmed when controlled amount of tea extracts were used instead of AAH(2), confirming the efficiency of CPE/DPPH1% for the electrochemical evaluation of antioxidant properties.