A MOF-based luminometric sensor for ultra-sensitive and highly selective detection of chromium oxyanions

Sensitization of Cr(VI) oxyanions in environmentally or industrially relevant aquatic media is highly desired owning to their biological toxicity and essential role in nuclear fuel cycle. However, many chemosensors of CrO₄^2- and Cr₂O₇^2- suffer from critical drawbacks, including insufficient sensitivity, selectivity, and/or hydrolytic stability. In this work, we prepared a hydrolytically stable metal-organic framework, namely Hf-BITD, which can retain its crystallinity and structural integrity in solutions over a wide pH range (0-12) and in 3 M HCl. The strong emission via rigidifying fluorescent linkers allows for sensing of CrO₄^2- and Cr₂O₇^2- in a luminescence quenching manner, with excellent linear correlations (I₀/I = 1+ K_sv [Q]) in the ranges of 0-80 μM and 0-50 μM for CrO₄^2- and Cr₂O₇^2-, respectively. The adsorption of Cr(VI) oxyanions and the concomitant resonance energy transfer between framework and analysts efficiently turn the emission of Hf-BITD off, which allows for selective recognition of CrO₄^2- and Cr₂O₇^2- with detection limits of 0.38 nM and 0.33 nM, respectively. Furthermore, fabrication of Hf-BITD incorporating PVDF membrane makes Hf-BITD@PVDF a promising candidate for facile and effective sensitization of Cr(VI) oxyanions.