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

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. CO₃ ²⁻ 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.

Abstract

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 (CO₃ ²⁻). 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 μM (0.04 ppm) via quenching. On the contrary, CO₃ ²⁻ in an aqueous medium significantly enhances the luminescence of UiO-66-NH₂-IM, which is detected with extremely high sensitivity (LOD=1.16 μM, i. e., 0.07 ppm). Large Stern-Volmer constant, K_sv, 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 CO₃ ²⁻. Moreover, the optical behavior of the post-synthetic material toward analytes is recyclable.

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