Fe-doped carbon dots as a fluorescent peroxidase-like nanozyme for a dual-mode assay of ascorbic acid

Reliable quantification of ascorbic acid (AA) is essential for pharmaceutical quality control. Herein, we report Fe-doped carbon dots (Fe-CDs) as a fluorescent nanozyme for the colorimetric/fluorometric dual-mode quantification of AA. Fe doping endows CDs with intrinsic peroxidase-like activity to catalyze H₂O₂-mediated oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxidized TMB (oxTMB), quenching the fluorescence of Fe-CDs via the inner filter effect. The introduction of AA reduces the oxTMB and leads to a decrease in absorbance and recovery of fluorescence. Such Fe-CD based dual-mode method achieves high sensitivity for AA quantification with a limit of detection of 89.1 ng mL^-1 for the colorimetric mode and 90.9 ng mL^-1 for the fluorometric mode, and exhibits satisfactory quantification accuracy in commercial AA tablets, demonstrating its applicability as a reliable, sensitive, and point-of-need tool for quality control of pharmaceutical tablets.