Cerium-based azide- and nitro-functionalized UiO-66 frameworks as turn-on fluorescent probes for the sensing of hydrogen sulphide

Abstract

A new and an existing Ce-based metal–organic framework (MOF) having a UiO-66 framework topology and incorporating azide and nitro functional groups in their frameworks have been successfully used as turn-on fluorescent probes for the sensing of H₂S under physiological conditions. The azide (1-N₃) and nitro (2-NO₂) functionalized Ce MOFs have been synthesized under similar solvothermal conditions (100 °C, 15 min) using ammonium cerium(IV) nitrate and H₂BDC-X (BDC = 1,4-benzenedicarboxylate; X = –N₃ for 1-N₃ and –NO₂ for 2-NO₂) linkers in DMF/H₂O (DMF = N,N-dimethylformamide) mixtures. The phase purity of both compounds has been confirmed by X-ray powder diffraction (XRPD) analyses, infrared spectroscopy and thermogravimetric (TG) analyses. The thermally activated forms of both compounds (1′-N₃ and 2′-NO₂) show fast response time, excellent selectivity and sensitivity for the detection of H₂S under physiological conditions (HEPES buffer, pH 7.4) through the fluorescence ‘turn-on’ mechanism. The detection limits (12.2 μM for 1′-N₃ and 34.8 μM for 2′-NO₂) of both materials lie within the range of the H₂S concentration observed in biological systems. The materials can selectively detect H₂S even in the presence of other competing biomolecules. Apart from the sensing of H₂S, both compounds exhibit high uptake of CO₂ (2.6 mmol g⁻¹ for 1′-N₃ and 3.7 mmol g⁻¹ for 2′-NO₂) at 0 °C and 1 bar. Thus, the materials are promising candidates in the fields of H₂S sensing and CO₂ capture.