BCNO Quantum Dots and MnO2 Nanosheets-Based Fluorescence and Digital Image Colorimetry Sensing for the Detection of 2, 4-Dichlorophenoxyacetic Acid

2, 4-dichlorophenoxyacetic acid is a widely used pesticide, necessitating the development of a low-cost, reliable, and rapid detection method. In this work, BCNO quantum dots were synthesized by a one-step microwave method. A fluorescence “turn-off-on” probe for 2, 4-dichlorophenoxyacetic acid detection was developed using MnO₂ nanosheets due to their strong oxidation and broad absorption spectrum. MnO₂ nanosheets can quench the fluorescence of BCNO quantum dots through internal filter effect. Alkaline phosphatase hydrolyzes trisodium L-ascorbate-2-phosphate to produce ascorbic acid, which decomposes MnO₂ nanosheets, restoring the fluorescence of BCNO quantum dots. With the increase of alkaline phosphatase concentration, the ascorbic acid produced gradually increased; thus, the fluorescence intensity of BCNO quantum dots increased. 2, 4-dichlorophenoxyacetic acid inhibits alkaline phosphatase activity, preventing the decomposition of MnO₂ nanosheets and hindering fluorescence recovery. The probe exhibits a linear detection range of 1.0–40.0 mg/L and a detection limit of 0.067 mg/L. Color changes corresponding to 2, 4-dichlorophenoxyacetic acid concentration were analyzed using a smartphone application, establishing a digital image colorimetry method. The results demonstrated a good linear relationship within the range of 0–50 mg/L. The method was validated with real samples, demonstrating its utility in environmental and food safety monitoring.