A novel photoluminescent Cd(ii)–organic framework exhibiting rapid and efficient multi-responsive fluorescence sensing for trace amounts of Fe3+ions and some NACs, especially for 4-nitroaniline and 2-methyl-4-nitroaniline

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Abstract
The first photoluminescent Cd(II)–organic framework {[Cd(5-asba)(bimb)]}n (1) based on a multi-functional ligand, 2-amino-5-sulfobenzoic acid (H25-asba), has been successfully constructed. The structure of 1 was determined by single crystal X-ray diffraction analysis, revealing that 1 exhibits a six-connected α-polonium network with a Schläfli symbol of 412·63. The phase purity of the bulk sample of 1 was further confirmed by elemental analysis and powder X-ray diffraction pattern (PXRD) which was consistent with the simulated phase purity calculated from single crystal X-ray diffraction data. The optical property of 1 was characterized by analysis of the solid state UV-vis diffuse reflectance spectrum, indicating that 1 may be a potential candidate for wide gap semiconductor materials. The photoluminescence properties of 1 well dispersed in various solvents have been investigated in detail, indicating that 1 shows distinct solvent-dependent photoluminescence. 1 belongs to the rare category of MOFs that exhibit multi-responsive photoluminescence sensing properties. It can be used as a rapid and efficient selective fluorescent sensor for Fe3+ ions in environmental and biological systems at about a one ppm level. For the photoluminescence responses of 1 dispersed in aqueous solution with incrementally increased 4-nitroaniline (4-NA) and 2-methyl-4-nitroaniline (2-M-4-NA) concentrations, photoluminescence quenching was detectable even at a very low concentration of 0.52 ppm for 4-NA and 0.68 ppm for 2-M-4-NA, indicating that 1 exhibits extremely high sensitivities for 4-NA and 2-M-4-NA. A further systematic study reveals that 1 shows more efficient sensing of nitroaniline homologues (4-NA and 2-M-4-NA) over other electron-deficient nitroaromatic compounds (NACs). 1 represents the first case of a MOF-based photoluminescent probe for 4-NA and 2-M-4-NA. Furthermore, 1 can discriminate the nitrophenol isomers (2-, 3- and 4-NP) through fluorescence sensing. To our best knowledge, 1 is the first Cd(II) MOF based on the multi-functional ligand H25-asba that can be used as a multi-responsive efficient sensory material for Fe3+ ions and some NACs, especially for nitroaniline homologues (4-NA and 2-M-4-NA).

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