An excited-state intramolecular proton-transfer responsive nanoscale MOF for dual sensing of water and chromate ions

Abstract

The sensitive and instantaneous detection of a variety of classes of analytes using a single material exemplifies an environmentally and economically lucrative approach. Herein, the presence of an excited-state intramolecular proton transfer (ESIPT)-sensitive linker (2,5-dihydroxyterephthalate) in a Zn-based MOF [Zn₂(H₂dht)(dht)_0.5(bpee)_0.5(H₂O)]_n(Zn-db) imparts it with intriguing solvent-dependent luminescence properties. Using the coordination modulation method (CMM), three different products of the Zn-db MOF (Zn-db-1, Zn-db-2, and Zn-db-3) with different morphologies (mesosheets, nanorods, and nanoparticles) have been isolated. The nanoscale MOF Zn-db-3 exhibits good solution processability and a BET surface area of 365 m² g⁻¹ which is significantly higher than its bulk analogues, enabling the facile diffusion of analytes into its pores. Taking these features into account, the nanoscale Zn-db-3 was exploited for the rapid and highly efficient detection of moisture content in organic solvents like ethanol, methanol, and tetrahydrofuran. This MOF-based chemosensor can detect a broad range of water contents (0.05–15% v/v in ethanolic medium) almost instantly (<30 s), thereby making it a promising candidate for real-time, industrial applications. Furthermore, the intense emission properties of Zn-db-3 in aqueous media make it a potential chemosensor for the detection of carcinogenic CrO₄²⁻ ions in water via a luminescence ‘turn-off’’ mechanism. Zn-db-3 demonstrated highly selective and instant luminescence quenching upon exposure to CrO₄²⁻ ions in aqueous solution, achieving a detection limit of 12 ± 0.3 ppb, which is way below the contamination standard of 100 ppb in drinking water defined by the Environmental Protection Agency (EPA). Therefore, Zn-db-3 is capable of acting as an active dual-functional sensor for the quick and efficient detection of two different analytes via two different mechanisms.