Cobalt-doped carbon dots nanozymes for dual-readout colorimetric detection of ascorbic acid in fruits.

Carbon dots nanozymes, as emerging catalytic materials have drawn increasing attention across diverse research fields. Herein, we report a dual-readout colorimetric strategy based on metal-doped CDs nanozymes for sensitive and visual detection of ascorbic acid (AA), using both UV-Vis spectrophotometry and smartphone-based colorimetric readout. Cobalt-doped CDs (CoCDs) were synthesized via one-pot solvothermal method, showing good water solubility, uniform size and significant cobalt incorporation. CoCDs exhibited potent peroxidase-like activity, efficiently catalyzing 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to blue oxTMB in the presence of H2O2, which can be selectively reduced by AA, resulting in a visible color fade and absorbance decrease. The platform demonstrated high sensitivity with detection limits of 0.75 μM (UV-Vis) and 0.76 μM (smartphone-based), and yielded satisfactory recoveries (87.24-112.00%) in various fruits. This dual-readout platform combines high sensitivity with operational simplicity, offering a feasible approach for rapid AA detection under standardized imaging conditions, with potential applicability for food quality monitoring.