Ratiometric Fluorescent Recognition of Sarin Simulant Enabled by a [Eu4(OH)4] Cluster-Based Metal–Organic Framework

The rapid and sensitive detection of nerve agent simulants, such as diethyl chlorophosphate (DCP), is critical for public security and environmental monitoring. In this work, we report a novel europium metal-organic framework (SNNU-372) constructed from a cubane-type [Eu4(OH)4] cluster and a 2,5-diaminoterephthalate (2,5-(NH2)2-BDC). SNNU-372 exhibits a three-dimensional ecu topology with regular cages and uncoordinated amino groups lining the pore surfaces. The material serves as a ratiometric fluorescence sensor for DCP, showing a 65-fold emission enhancement at 492 nm upon addition of 20 ppm DCP, with a remarkable increase in the I492/I420 ratio from 0.5 to 7.0. The limit of detection (LOD) is as low as 0.01 ppm (49.9 nM), with fast response (100 s), excellent recyclability (≥5 cycles), and high selectivity against common interferents including HCl. Mechanistic studies reveal that DCP protonates the pendant -NH2 groups, suppressing ligand-to-metal charge transfer (LMCT) and intramolecular charge transfer (ICT), thereby turning on the ligand-centered fluorescence with a visible color change from nearly colorless to green under UV light.