A luminescent cationic MOF for bimodal recognition of chromium and arsenic based oxo-anions in water

Abstract

Water pollution from heavy metals and their toxic oxo-anionic derivatives such as CrO₄²⁻, Cr₂O₇²⁻, HAsO₄²⁻, and HAsO₃²⁻ has become one of the most critical environmental issues. To address this, herein, we report a new hydrolytically stable luminescent Zn(II) based cationic metal organic framework (MOF), iMOF-4C, which further successfully exhibited a rare dual “turn off/on” fluorescence response toward Cr(VI), As(V) and As(III) based oxo-anions respectively in water medium. In addition, iMOF-4C was found to maintain its superior selectivity in the presence of other concurrent anions (e.g. SO₄²⁻, Cl⁻, Br⁻, ClO₄⁻, NO₃⁻, SCN⁻ and CO₃²⁻). More importantly, iMOF-4C exhibited an excellent selective and sensitive luminescence “turn-off” response towards CrO₄²⁻ and Cr₂O₇²⁻ anions in water medium with the quenching constant (K_sv) values as high as 1.31 × 10⁵ M⁻¹ (CrO₄²⁻) and 4.85 × 10⁵ M⁻¹ (Cr₂O₇²⁻), which are found to be the highest among the values reported in the regime of MOFs. Interestingly, iMOF-4C showed fluorescence “turn-on” response toward HAsO₄²⁻ and HAsO₃²⁻ with an enhancement coefficient (K_ec) of 1.98 × 10⁴ M⁻¹ and 3.56 × 10³ M⁻¹ respectively. The high sensitivity and low detection limits make iMOF-4C more feasible for real-time sensing of such toxic oxo-anions in an aqueous medium. Furthermore, the probable sensing mechanism has been investigated by DFT calculation studies and discussed in detail.