Cu-MOFs Nanozymes with Ascorbate Oxidase and Peroxidase-like Activity for Sensitive Fluorometric Detection of Total Antioxidant Capacity in Fruits

In this work, two-dimensional copper-based metal–organic frameworks (Cu-MOFs) nanozymes, including cuprous oxide-tetrakis (4-carboxyphenyl) porphyrin (Cu2O-TCPP) and copper-cuprous oxide-tetrakis (4-carboxyphenyl) porphyrin (Cu-Cu2O-TCPP), were synthesized, which exhibit dual ascorbate oxidase (AO) and peroxidase (POD)-like activities. The reductants, such as ascorbic acid (AA), can be oxidized by the cascade AO and POD catalysis on Cu-MOFs to oxidize p-phthalic acid (PTA) and generate fluorescence. Consequently, a fluorescence sensing platform for AA and other reducing substances was established. This platform offers potential for efficient and selective monitoring of reductive species and related antioxidant levels in food systems. The results showed that the two Cu-MOFs displayed favorable linear relationships (R2 ≥ 0.99) for the detection of AA, glutathione (GSH) and L-cysteine (L-Cys). Their limits of detection (LOD) were 5.3 μM for Cu2O-TCPP and 92.5 μM for Cu-Cu2O-TCPP. Finally, by detecting real samples of vitamin C tablets and fruits, the accuracy of the two Cu-MOFs nanos enzymes was validated, with Cu2O-TCPP showing higher accuracy.