Nitrogen-Doped Carbon Dots as a Fluorescent "Off-On" Probe for Selective Ascorbic Acid Detection via H₂O₂-Mediated...

Nitrogen-doped carbon dots (NCDs) exhibiting superior fluorescence characteristics were synthesized employing o-phenylenediamine and 2-methylimidazole as precursors. The synthesized NCDs exhibited yellow photoluminescence with an excitation/emission maxima of 410/554 nm with a quantum yield of 28.41%. The presence of pyridinic N, pyrrolic N, graphitic N, and amino N functionalities on the NCDs' surface provided strong evidence for the successful nitrogen doping of the carbon dots. Upon exposure to hydrogen peroxide (H₂O₂), the NCDs exhibited a significant reduction in fluorescence intensity, which could be restored by the addition of ascorbic acid (AA), demonstrating a quantitative relationship between ascorbic acid and fluorescence efficiency. A novel fluorescence "off-on" system utilizing these NCDs was developed for the quantification of AA. The sensing mechanism relies on H₂O₂-induced fluorescence quenching via the selective oxidation of the NCDs' surface, followed by fluorescence restoration upon AA addition due to the reduction in surface defects. Meanwhile, further experiments confirmed that the quenching mechanism was static quenching. The NCDs demonstrated a limit of detection (LOD) of 0.605 μM for AA detection. The use of NCDs for AA sensing was validated through the analysis of commercially available beverages. This study aimed to establish a simplified method for ascorbic acid detection. The experimental findings indicated that the developed technique exhibited high accuracy in quantifying ascorbic acid. These findings suggest that the developed NCDs possess considerable potential as a multifunctional sensing tool for various analytical applications.