Fluorescent Detection of 2,4-Dichlorophenoxyacetic Acid in Food Samples Based on Covalent Organic Frameworks and MnO2 Nanosheets

2,4-Dichlorophenoxyacetic acid (2,4-D) is a typical chlorinated aromatic herbicide which is widely used because of its low cost. Herein, a new fluorescent assay for 2,4-D is developed based on covalent organic frameworks (COFs) and MnO 2 nanosheets. The fluorescence of COFs is quenched effectively by MnO 2 nanosheets. Ascorbic acid 2-phosphate (AAP) is hydrolyzed by alkaline phosphatase (ALP) to produce ascorbic acid (AA), which reduces MnO 2 nanosheets into Mn 2+ , so the quenched fluorescence recovers. Additionally, 2,4-D can inhibit the catalytic activity of ALP. Therefore, the addition of 2,4-D quenches the fluorescence of COF-MnO 2 system again. Under the optimum conditions, this assay displays a wide linear range of 2,4-D from 1 to 150,000 ng/mL with a limit of detection of 0.36 ng/mL. Then, this method is successfully used to measure 2,4-D in rice, millet, and cucumber samples with satisfactory recoveries. Compared with other probes for 2,4-D detection, both of COFs and MnO 2 nanosheets are stable, cost-effective, and easily prepared. The sensitivity and linear range of 2,4-D in this strategy are superior to most reported works. This is the first time that a novel sensing platform using fluorescent COFs and MnO 2 nanosheets is developed for 2,4-D.