Highly efficient and bifunctional Cd(II)-Organic Framework platform towards Pb(II), Cr(VI) detection and Cr(VI) photoreduction

Lead and chromate/dichromate are widely used in manufacturing industry, but Pb(II) and Cr(VI) ions pose fatal threats to human health and the water ecological environment. It is thus highly desirable to develop novel and valid strategies for rapid detection and effective decontamination of these heavy metal ions. Design and assembly of luminescent Metal-Organic Framework (LMOFs) for effective recognition and removal of Pb(II) and/or Cr(VI) from water provide a feasible avenue to address this issue. Herein, a novel Cd(II)-MOF, [(Cd3L2)]·(solvent)x (labeled as compound 1) with bright blue fluorescence was assembled under the solvothermal reaction of triangular ligand 5'-(5-carboxy-1H-benzo [d]imidazole-2-yl)-[1,1':3′,1″-terphenyl]-4,4″-dicarboxylic acid (H3L) and Cd(NO3)2. Given that the unique structure features of 3D stacking framework, i. e. strong π···π conjugative effects of organic linkers, plentiful free imidazole N atoms and carboxyl O atoms, Cd-MOF of 1 was deliberately deployed as bifunctional platform to sense Pb(II)/Cr(VI) and photochemically reduce Cr(VI) to Cr(III). Notably, 1 emits durable bright blue fluorescence and performs rapid quenching sense for Pb2+, Cr2O72− and CrO42−, with ultra low detection limits of 1.89, 4.83 and 2.84 ​ppb, respectively. Moreover, highly toxic Cr(VI) could be rapidly and thoroughly reduced to Cr(III) ions by 1 under the irradiation of mercury lamp, with reliable recycling ability. The possible photoluminescence, quenching and photocatalytic reduction mechanisms also were tentatively proposed.