A CuO–Nitrogen‐Doped Carbon Dot Hybrid Nanozyme with Peroxidase‐Mimicking Activity for Colorimetric Analysis of Ascorbic Acid in Strawberries

ABSTRACT Ascorbic acid (AA) is an essential exogenous vitamin possessing tremendous antioxidant properties, rendering its detection in food samples increasingly important. In this study, copper oxide/nitrogen‐doped chlordiazepoxide hydrochloride carbon dot (CuO/N‐CD) nanozymes were used for AA detection via a colorimetric method. The synthesized CuO/N‐CDs were characterised using various analytical and spectroscopic techniques. The synthesised nanozyme demonstrated a peroxidase (PO)‐like activity that could catalyse the oxidation of m‐amino benzoic acid (MABA) to the oxidized form (oxMABA) in the presence of hydrogen peroxide (H 2 O 2 ), resulting in a colour change of the solution. The PO‐like activities of CuO/N‐CDs could be strongly inhibited by AA owing to the scavenging of free radicals generated catalytically from H 2 O 2 by CuO/N‐CDs. The concentration of AA was monitored by measuring the colour change of the oxMABA solution (absorbance change) in the linear range of 0.1–5.0 mg L −1 with a detection limit of 0.02 mg L −1 . Moreover, a low concentration (0.5 mg L −1 ) of AA could be detected by the naked eye. As a result of its high selectivity, rapidness, and simplicity, the proposed method was successfully used to quantify AA in fresh and stored strawberries. The results of the analysis of real samples indicated recoveries varying from 96.7% to 99.9% with a relative standard deviation of 0.05%–1.05%, which showed the validity of the proposed method for AA quantitation in real samples.