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 Eu-TFPB-Bpy is synthesized by grafting Eu 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 Eu-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 Eu. More significantly, Eu-TFPB-Bpy is a highly efficient probe for sensing UO in aqueous solution with the luminescence intensity efficiently amplified by complexation of UO with Eu. The turn-on sensing capability was derived from the resonance energy transfer occurring from UO to the Eu-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 UO in mining wastewater. This strategy provides a vivid illustration for designing luminescence lanthanide COF hybrid materials with applications in environmental monitoring.