Boric Acid Functional Fluorescent Covalent–Organic Framework for Sensitive and Selective Visualization of CH3Hg+

Methylmercury (CH₃Hg⁺) recognition remains a challenging and imperative task due to its high toxicity and wide existence in the ecosystem. Herein, a novel fluorescent covalent–organic framework containing a boric acid functional group (COF-BA) was prepared by a postmodification strategy for CH₃Hg⁺ detection. COF-BA served as a sensing platform for CH₃Hg⁺ with fluorescence static quenching accompanied by fluorescence color changing from intense blue to colorless, and the detection limit was determined as 1.68 μM in a relatively narrow concentration range. COF-BA also exhibited superior selectivity toward CH₃Hg⁺ detection. Furthermore, the spiked and recovery test in real water samples showed its efficient detection practicality. The detection mechanism of COF-BA toward CH₃Hg⁺ was investigated. The recognitive boric acid group in COF-BA was first replaced by CH₃Hg⁺. Then, the quinoline structure that served to limit the rotation of the imine bond was disrupted, leading to dramatic fluorescence quenching. The boric acid functional COF fluorescent probe can be a promising sensing platform for the detection of methylmercury and also provides new ideas for the construction of new fluorescent COF materials.