Metal-organic frameworks are a class of attractive materials for fluorescent sensing. Improvement of hydrolytic stability, sensitivity, and selectivity of function is the key to advance application of fluorescent MOFs in aqueous media. In this work, two stable MOFs, [Zr6O4(OH)8(H2O)4(L1)2] (BUT-14) and [Zr6O4(OH)8(H2O)4(L2)2] (BUT-15), were designed and synthesized for the detection of metal ions in water. Two new ligands utilized for construction of the MOFs, namely, 5',5‴-bis(4-carboxyphenyl)-[1,1':3',1″:4″,1‴:3‴,1''''-quinquephenyl]-4,4''''-dicarboxylate (L1) and 4,4',4″,4‴-(4,4'-(1,4-phenylene)bis(pyridine-6,4,2-triyl))tetrabenzoate (L2), are structurally similar with the only difference being that the latter is functionalized by pyridine N atoms. The two MOFs are isostructural with a sqc-a topological framework structure, and highly porous with the Brunauer-Emmett-Teller (BET) surface areas of 3595 and 3590 m2 g-1, respectively. Interestingly, they show intense fluorescence in water, which can be solely quenched by trace amounts of Fe3+ ions. The detection limits toward the Fe3+ ions were calculated to be 212 and 16 ppb, respectively. The efficient fluorescent quenching effect is attributed to the photoinduced electron transfer between Fe3+ ions and the ligands in these MOFs. Moreover, the introduced pyridine N donors in the ligand of BUT-15 additionally donate their lone-pair electrons to the Fe3+ ions, leading to significantly enhanced detection ability. It is also demonstrated that BUT-15 exhibits an uncompromised performance for the detection of Fe3+ ions in a simulated biological system.
Keywords: Fe3+ ion detection; fluorescent quenching; ligand design; metal−organic framework; water system.