Cd-MOF and Its Ln3+-Post Modification Products: Regulation of Luminescence Properties and Improved Detection of Uric Acid, Quinine, and Quinidine

One 3D Cd-MOF, namely, {[(HDMA)₂][Cd₃(L)₂]·5H₂O·2DMF}_n (LCU-124, LCU indicates Liaocheng University), was synthesized from an ether-containing ligand 1,3-bis(3,5-dicarboxylphenoxy)benzene (H₄L). Its Ln³⁺-postmodified samples, Eu³⁺@LCU-124 and Tb³⁺@LCU-124, were obtained through cation exchange of dimethylamine cation (HDMA) with Eu³⁺ and Tb³⁺. The successful entry of rare earth into LCU-124 by cation exchange modification was verified by IR, XRD, XPS, EDS mapping, and luminescence spectra. The proportion of Eu³⁺/Tb³⁺ was adjusted during the modification process, leading to fluorescent materials with different emissions. Luminescence measurements indicated that these complexes exhibited interesting multiresponsive sensing activities toward biomarkers urine acid (UA), quinine (QN), and quinidine (QND). First, LCU-124 has a pronounced quenching effect toward UA with the detection limit of 31.01 μM. After modification, the visualization of the detection was improved significantly and the detection limit of Eu³⁺@LCU-124 was reduced to 0.868 μM. Second, when QN and QND were present in the suspensions of Eu³⁺@LCU-124 and Tb³⁺@LCU-124, strong blue light emission peaks occurred, while the characteristic emission of Eu³⁺/Tb³⁺ decreased, forming ratiometric fluorescent sensors with the detection limit in the range of 0.199-9.49 μM. The fluorescent probes have high selectivity, excellent sensitivity recycling, and fast response time (less than 1 min). Besides, a simple logic gate circuit and a range of luminescent mixed matrix membranes were designed to provide simple and fast detection of above biomarkers. Our work indicated that modification of Eu³⁺/Tb³⁺ could improve the detection ability significantly.