Highly Stable Acid-Induced Emission-Enhancing Cd-MOFs: Synthesis, Characterization, and Detection of Glutamic Acid in Water and Fe Ions in Acid.

Two novel 3D fluorescent metal-organic frameworks (MOFs), [Cd(L)(bbibp)] _n ( 1 ) and [Cd(L)(bbibp) _0.5 ] _n ( 2 ), where H ₂ L = 4,4'-(4,4'-bipyridine-2,6-diyl)dibenzoic acid and bbibp = 4,4'-bis(benzoimidaz-1-yl)biphenyl, were acquired through a conventional method and characterized via IR spectra, single-crystal X-ray diffraction, elemental analysis, thermogravimetric analysis, powder X-ray diffraction (PXRD), scanning electron microscopy, N ₂ adsorption-desorption isotherms, and X-ray photoelectron spectroscopy (XPS). The crystal framework of Cd-MOF 1 remained stable in the range of pH = 1.0-12.0. Interestingly, the emission peak of 1 showed a red shift and exhibited a fluorescence turn-on effect in an acidic environment. X-ray diffraction measurement revealed that the crystal structure of 1 remained unchanged after immersion in a pH = 1.0 solution. In addition, Cd-MOFs 1 and 2 displayed fluorescent quenching to l-glutamic acid with high sensitivity and selectivity. Meanwhile, 1 showed high selectivity in recognizing Fe ³⁺ under acidic conditions, which made 1 capable of detecting Fe ³⁺ in acidic industrial wastewater. Finally, the fluorescent sensing mechanism was carefully studied by PXRD, transient fluorescence lifetime, XPS, and UV spectroscopy.