Recyclable luminescence sensor for Cu2+, Cr2O72- and CrO42- in water and acid/base vapor response based on water-stable bipyridyl-based Ln-MOFs

Detecting toxic contaminants in water, such as heavy metal ions and chromate ions, is important for public health and the environment. In the context, luminescence lanthanide-organic frameworks have been the competitive means to recognize such contaminants. Herein, two Ln-MOFs, [Ln(3)(L)(2)(HCOO)(H2O)(5)]center dot 14H(2)O (H4L = 2,6-di(2,4-dicarboxyphenyl)-4-(pyridine-4-yl)pyridine) (Ln = Eu, 1-Eu and Ln = Tb, 1-Tb), have been synthesized solvothermally by using the selection of the bipyridyl-tetracarboxylic ligand (H4L). Single-crystal X-ray diffraction indicates that this two Ln-MOFs are isostructural and contain uncoordinated pyridyl nitrogen atoms and water molecules, which provides the Lewis-base sites for detecting various small molecules. Moreover, both 1-Eu and 1-Tb exhibit excellent chemical stability in aqueous solution with the pH ranging from 2 to 13. Thanks to such characteristics, 1-Eu can be served as a mull-responsive luminescence sensor for Cu2+, Cr2O72- and CrO42- in deionized water, tap water and river water via luminescent quenching. Notably, the sensing processes toward Cu2+, Cr2O72- and CrO42- exhibit high selectivity and sensitivity, and good recyclability. The luminescence sensing mechanism was carefully studied by PXRD, XPS, and UV spectroscopy. In addition, 1-Eu also displays luminescence turn-off/on when alternately exposed to HCl and NH3 gases, which can mainly be attributed to the protonation/deprotonation process by the free bipyridyl N sites. The work illustrates 1-Eu could shed light on the exploration of MOF-based sensors for analytical applications.