Enhancing visual luminescence detection of tetracycline antibiotics through regulation of nitrogen-containing ligand and antenna effect of Ln3+†

Two fluorescent Cd-MOFs were obtained by a solvothermal process using a flexible carboxylic acid ligand of 4-(3,5-dicarboxylatobenzyloxy) benzoic acid (H₃L) and flexible/rigid tripodal N-containing ligands of 1,3,5-tris(1,2,4-triazol-1-ylmethyl)-2,4,6-trimethylbenzene (ttytb) and 2,4,6-tris(4-pyridyl)-1,3,5-triazine (tpytz), namely, {[Cd(HL)(ttytb)]·DMF·3H₂O}_n (LCU-127) and {[Cd(HL)(tpytz)(H₂O)]}_n (LCU-128). LCU-127 and LCU-128 both display two-dimensional layered structures with an interesting intercalation mode. Due to the rigid conjugated rings in tpytz, LCU-128 showed stronger blue emission than LCU-127. Accordingly, although both LCU-127 and LCU-128 could detect tetracycline antibiotic tetracycline (TC) through luminescence quenching, the visualization effect under UV light irradiation was much improved in the suspensions of LCU-128, which demonstrated a limit of detection (LOD) of 0.18 μM. Simultaneously, LCU-127 exhibited higher selectivity and sensitivity toward Tb³⁺ due to stronger antenna effect. When Tb³⁺ was added to the suspension of LCU-127, the visual detection of oxytetracycline (OTC) was significantly improved with LOD of 45.51 μM. The rigidity or conjugation of the nitrogen-containing ligands may enhance the conjugation of the whole framework and further the electron transfer ability. Meanwhile, the antenna effect caused by the interaction between active sites in LCU-127 and Tb³⁺ could improve the luminescence performance. Furthermore, LCU-128 and Tb³⁺ + LCU-127 were effective in mixed matrix membranes (MMMs), which endows them with potential for application in rapid visual detection of antibiotics. The measurements conducted using real water samples revealed the recovery rates toward OTC and TC in the range of 94.7 to 103.0%. Our work indicates that visual detection could be achieved by the employment of more rigid conjugated moieties in the framework and introduction of Ln³⁺ emitters by the antenna effect.