Ratiometric fluorescent detection system based on dual-driving catalysis of CuO nanozyme with a classical univariate calibration for the determination of ascorbic acid in serum and fruits

•OPD as the sole substrate constructs a ratiometric fluorescence system.•CuO NPs have both oxidase and ascorbate oxidase mimetic activity in the system.•Successfully realized the detection of AA in human serum and diverse fruits.•Scanning measurement analysis is realized through the App on smartphones.