Europium(III) Functionalized Covalent Organic Framework as Sensitive and Selective Fluorescent Switch for Detection of Uranium

Uranium poses severe health risks due to its radioactivity and chemical toxicity if released into the environment. Therefore, there is an urgent demand to develop sensing materials in situ monitoring of uranium with high sensitivity and stability. In this work, a fluorescent Eu³⁺-TFPB-Bpy is synthesized by grafting Eu³⁺ cation onto TFPB-Bpy covalent organic framework (COF) synthesized through Schiff base condensation of monomers 1,3,5-tris(4-formylphenyl)benzene (TFPB) and 5,5'-diamino-2,2'-bipyridine (Bpy). The fluorescence of Eu³⁺-TFPB-Bpy is enhanced compared with that of TFPB-Bpy, which is originated from the intramolecular rotations of building blocks limited by the bipyridine units of TFPB-Bpy coordinated with Eu³⁺. More significantly, Eu³⁺-TFPB-Bpy is a highly efficient probe for sensing UO₂²⁺ in aqueous solution with the luminescence intensity efficiently amplified by complexation of UO₂²⁺ with Eu³⁺. The turn-on sensing capability was derived from the resonance energy transfer occurring from UO₂²⁺ to the Eu³⁺-TFPB-Bpy. The developed probe displayed desirable linear range from 5 nM to 5 μM with good selectivity and rapid response time (2 s) for UO₂²⁺ in mining wastewater. This strategy provides a vivid illustration for designing luminescence lanthanide COF hybrid materials with applications in environmental monitoring.