A Novel Differential Pulse Voltammetric (DPV) Method for Measuring the Antioxidant Capacity of Polyphenols-Reducing Cupric Neocuproine Complex

Abstract

A novel differential pulse voltammetric (DPV) method is presented, using a chromogenic oxidizing reagent, cupric neocuproine complex (Cu(Nc)(2)(2+)), for the assessment of antioxidant capacity of polyphenolic compounds (i.e., flavonoids, simple phenolic acids, and hydroxycinnamic acids), ascorbic acid, and real samples for the first time. The electrochemical behavior of the Cu(Nc)(2)(2+) complex was studied by cyclic voltammetry at a glassy carbon (GC) electrode. The electroanalytical method was based on the reduction of Cu(Nc)(2)(2+) to Cu(Nc)(2)(+) by antioxidants and electrochemical detection of the remaining Cu(II)-Nc (unreacted complex), the difference being correlated to antioxidant capacity of the analytes. The calibration curves of individual compounds comprising polyphenolics and vitamin C were constructed, and their response sensitivities and linear concentration ranges were determined. The reagent on the GC electrode retained its reactivity toward antioxidants, and the measured trolox equivalent antioxidant capacity (TEAC) values of various antioxidants suggested that the reactivity of the Cu(II)-Nc reagent is comparable to that of the solution-based spectrophotometric cupric ion reducing antioxidant capacity (CUPRAC) assay. This electroanalytical method better tolerated sample turbidity and provided higher sensitivity (i.e., lower detection limits) in antioxidant determination than the spectrophotometric assay. The proposed method was successfully applied to the measurement of total antioxidant capacity (TAC) in some herbal tea samples such as green tea, sage, marjoram, and alchemilla. Results demonstrated that the proposed voltammetric method has precision and accuracy comparable to those of the spectrophotometric CUPRAC assay.