Application of Electrochemiluminescence for the Evaluation of the Antioxidant Capacity of Some Phenolic Compounds Against Superoxide Anion Radicals

Abstract

This paper for the first time reports on novel and non-enzymatic method for studying the free radical-scavenging properties of phenolic compounds against superoxide anion radicals (O₂∙^-) by using the cathodic electrochemiluminescence (ECL) of lucigenin (Luc²⁺). The ECL of Luc²⁺ at a glassy carbon (GC) electrode is observed in an aeration electrolytic solution (pH 7), which is believed to be due to the reaction of a one-electron reduced form of Luc²⁺(i.e. a radical cation, Luc∙⁺) with in situ electrogenerated O₂. The ECL intensity is dependent on the concentration of dissolved oxygen, and is suppressed dramatically by superoxide dismutase (SOD), a typical O₂∙^- scavenger. Since the coexisting hydrogen peroxide (H2O2) has no influence on the cathodic ECL of Luc²⁺, it is thus suggested that the ECL signal specifically reflected the O₂∙^- concentration level generated at the electrode surface. When phenolic compounds were added into the solution, this resulted in the inhibition of ECL signals due to the elimination of O₂∙^-. The ECL inhibition rate measured at each concentration was compared against the SOD equivalent (U mL⁻¹), and the relative antioxidant efficiency, K_ao(U mmol⁻¹ equivalent SOD), was used to evaluate the antioxidant activity of some phenolic compounds, including flavonoids, in this study. Structurally different water-soluble phenols were compared, and those compounds containing to catechol skeletal structure are found to present the higher antioxidant capacity.