Post-Synthetic Modification of Zr-based Metal-Organic Frameworks with Imidazole: Variable Optical Behavior and Sensing

UiO-66-NH2-IM, a fluorescent metal-organic framework (MOF), was synthesized by post-synthetic modification of UiO-66-NH2 with 2-imidazole carboxaldehyde via a Schiff base reaction. It was examined using various characterization techniques (PXRD, FTIR, NMR, SEM, TGA, UV-Vis DRS, and photoluminescence spectroscopy). The emissive feature of UiO-66-NH2-IM was utilized to detect volatile organic compounds (VOCs), metal ions, and anions, such as acetone, Fe3+, and carbonate (CO32-). Acetone turns off the high luminescence of UiO-66-NH2-IM in DMSO, with the limit of detection (LOD) being 3.6 ppm. Similarly, Fe3+ in an aqueous medium is detected at LOD=0.67 & mu;M (0.04 ppm) via quenching. On the contrary, CO32- in an aqueous medium significantly enhances the luminescence of UiO-66-NH2-IM, which is detected with extremely high sensitivity (LOD=1.16 & mu;M, i. e., 0.07 ppm). Large Stern-Volmer constant, Ksv, and low LOD values indicate excellent sensitivity of the post-synthetic MOF. Experimental data supported by density functional theory (DFT) calculations discern photo-induced electron transfer (PET), resonance energy transfer (RET), inner filter effect (IFE), or proton abstraction as putative sensing mechanisms. NMR and computational studies propose a proton abstraction mechanism for luminescence enhancement with CO32-. Moreover, the optical behavior of the post-synthetic material toward analytes is recyclable. Post-synthetic modification (PSM) with imidazole makes UiO-66-NH2 metal-organic framework (MOF) luminescent. This enables it to detect health-hazardous pollutants such as acetone, aq. Fe3+, and aq. CO32- by luminescence ON/OFF. This PSM MOF exhibits the highest sensitivity for pollutants among other no rare-earth element MOFs reported thus far in the literature.+image