Dual-atom nanozyme-based self-calibrating test strip for convenient on-site vitamin C quantification in fruits

As a key nutritional indicator, vitamin C (VC) content in fruits is affected by multiple factors, including cultivar and storage conditions, and notably, treatment with toxic artificial ripening agents leads to a pronounced decline in VC levels. Hence, a self-calibrating test strip based on a dual-atom nanozyme (DAzyme) was developed for portable and on-site VC quantification. The sensing mechanism was that VC quenched the TMB-based colorimetric reaction catalyzed by the DAzyme. First, FeCo-N-C DAzyme with high oxidase-like activity (54.6 U mg^-1) was prepared to enable direct juice sample detection without the addition of extra H₂O₂ and pretreatments. Then, a self-calibrating test strip was constructed by integrating a VC-sensing line, a pH-sensing line, and an optical calibration line, which reduced dependence on external buffers and errors from imaging conditions. This design improved the coefficient of determination (R²) by 3.2-fold and lowered the mean absolute error (MAE) by 69%. Further, a machine learning model (R² = 0.93, MAE = 56.4 μM) based on the Gradient Boosting algorithm was developed using 324 test strip images for automatic VC quantification. As a proof-of-concept, the system demonstrated reliable performance in on-site VC detection and successfully identified artificially ripened fruits under controlled experimental conditions.