Surface-carboxylated cobaltous oxide with oxidase-like activity for conveniently assessing total antioxidant capacity of Cordyceps militaris at neutral pH

Abstract

Reassessment of total antioxidant capacity (TAC) in Cordyceps militaris is imperative for providing dietary guidance and facilitating health monitoring. Herein, surface-carboxylated cobaltous oxide (COOH-Co3O4) nanozyme was achieved via a dual-stage procedure encompassing reduction preparation and surface modification. COOH-Co3O4 exhibited exceptional oxidase-like activity under neutral pH conditions, facilitating conversion of dissolved oxygen into superoxide anion free radicals and subsequently catalyzing oxidation of colorless 3, 3′, 5, 5′-tetramethylbenzidine (TMB) to yield blue oxidized TMB (TMBox) at neutral pH. Additionally, antioxidants can eliminate O2·− produced in the above reaction system. Moreover, the absorbance at 652 nm of the COOH-Co3O4 + TMB reaction system gradually diminishes with an increase in ascorbic acid concentration. Based on it, a colorimetric evaluation system (using ascorbic acid as a reference) for TAC of C. militaris was established. This innovative approach offers an impressively low limitation of detection (LOD) calculated as 0.78 μM. Furthermore, it was employed to evaluate TAC of real C. militaris samples, yielding a satisfactory recovery (96.1 %–102.3 %), thereby demonstrating its exceptional accuracy. The TAC of C. militaris extracted using different solvents was determined, indicating that the highest TAC is observed in C. militaris extracted by deionized water. This study not only offers a rapid, cost-effective, and convenient analytical tool for evaluating TAC of C. militaris but also expands the potential applications of nanozymes.