Novel reagentless chronopotentiometric method for assessing antioxidant activity using chemically modified working electrode

Abstract

In my studies working with carbon electrodes chemically modified with a reversible redox mediator layer, it was learned that the confined mediator layer could easily be electrochemically brought into its oxidized or reduced redox state. This guided us into investigating the applicability of the mediator layer as an analytical redox reagent. Furthermore, potentiometry could be used to detect the progress of heterogeneous redox reactions. These encouraged us to experiment with working out practically applicable “reagentless” analytic methods. Based on literature information and on earlier or on my recent 2 experimental findings we concentrated our efforts in dealing with determination of the action of reductive species (of antioxidant character) in liquid samples. In living organisms the oxidizing materials, like reactive oxidizing species (ROS) or free radicals have important beneficial roles in different biochemical reactions, in signaling or in self-defenses. On the other hand however, their elevated level can be extremely harmful. It can mediate generation of oxidative stress a cell damaging condition. Presence of certain bioactive species, called antioxidants can counteract these unwanted seriously aging and multiple pathogen condition by adding electron or hydrogen to the oxidizing molecule or to the radical, respectively. Therefore determination of antioxidants in physiological samples or in nutrients has high practical importance. Depending on their source, solubility, action mechanism and chemical structure the physiologically important antioxidants are usually sorted in different groups. Real samples taken for chemical analysis usually contain a mixture of antioxidant species. We can say that the antioxidant character is a collective property. Therefore the results of the analysis are often given in relation to the action of a species used for calibration as antioxidant activity or capacity. The measured values obtained for food, beverage, plant tissue or physiologic samples with different methods usually are not in good correlation. The collective antioxidant activity (AOA) is related to reaction rate. Higher activity shows higher reaction rate in radical quenching or electron exchange. (The antioxidant capacity reflects how many oxidant molecule is reduced by certain amount of the sample). In my work a reagentless method was worked out. It uses for gaining analytical signal the rate of a heterogeneous redox reaction by detecting the initial change of open circuit redox potential in the first few milliseconds. The reagent reacting with the analyte is a reversible redox mediator confined on the surface of a carbon working electrode. A controlled potential step is used for setting the redox state of the reagent. It needs to be mentioned here that most of the advanced electrochemical workstations are equipped with potentiometric measuring program that is capable of executing the measuring protocol. It was hoped that the obtained signal can be used well for estimating the antioxidant activity of certain samples.