A Water-Stable Eu III -Based Coordination Polymer and Its Composite Film for Luminescent Sensing of Diquat

A water-stable Eu III -based coordination polymer, namely [EuL(H 2 O) 4 ]·H 2 O ( EuL ), was successfully synthesized under solvothermal conditions using a phenylpyrazole tricarboxylate ligand (H 3 L). The structure features a rail-like chain due to the T-shaped three-connectivity of fully deprotoned L 3– and Eu 3+ . EuL exhibits characteristic Eu III -centered red emission alongside residual ligand-based fluorescence. To enhance its luminescence performance and portability, EuL was doped into a poly(methyl methacrylate) (PMMA) matrix to form a flexible EuL @PMMA composite film. Both EuL in aqueous suspension and the EuL @PMMA film serve as highly selective and sensitive luminescent sensors for diquat (DQ) via a quenching effect. The detection limits are 0.58 μM and 0.13 μM, respectively. A synergistic mechanism is proposed in which photoinduced electron transfer (PET) dominates the quenching process, with competitive absorption (CAM) and inner filter effect (IFE) playing secondary yet significant roles, jointly suppressing the efficient L 3– → Eu 3+ sensitization. This work represents the first example of a Eu III -based coordination polymer and its composite film for discriminative detection of DQ with excellent recyclability and real-sample applicability.