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

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 x 10(5) M-1 (CrO42-) and 4.85 x 10(5) 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 x 10(4) M-1 and 3.56 x 10(3) 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.

Automated summary

A new hydrolytically stable luminescent Zn(ii) based cationic metal organic framework (MOF), iMOF-4C, has been developed for selective and sensitive detection of toxic oxo-anions in water medium. This MOF exhibited a rare dual turn off/on fluorescence response towards Cr(vi), As(v), and As(iii) based oxo-anions, demonstrating high sensitivity and low detection limits. The mechanism of sensing has been investigated through DFT calculation studies.