Fluorescent Metal–Organic Framework Nanoparticles for Explosive Detection

Fluorescent metal-organic framework (MOF) nanoparticles were prepared by doping zeolitic imidazolate framework-8 (ZIF-8) with fluorescein through ″one-pot″ synthesis. The resulting fluorescein@ZIF-8 (F@ZIF-8) material exhibits a luminescent "turn-off" response toward nitroaromatic explosives. We performed a combined experimental and computational study to understand the structural properties and formation mechanisms of the prepared F@ZIF-8 composite and to explain the solid-state fluorescence and quenching in the presence of a nitroaromatic analyte. We confirmed that fluorescein can be encapsulated within the ZIF-8 cages through electrostatic interactions between dianionic fluorescein and Zn open metal sites. The scaffold of ZIF-8 can reduce the aggregation-caused quenching (ACQ) effect of fluorescein and increase the quantum yield to 69.7 ± 0.1%. The photophysical mechanism explains the fluorescence emission of F@ZIF-8 in the solid state and the fluorescence quenching in the presence of nitroaromatic analytes, including 2,4,6-trinitrophenol (TNP), trinitrotoluene (TNT), 4-nitrotoluene, nitrobenzene, and 2-nitropropane. Our material exhibits a high sensitivity toward TNP with a limit of detection (LOD) of 2 μM. This work presents a new opportunity for designing luminescent porous materials for "signal-off" sensing and demonstrates the potential of using luminescent MOFs as an analytical tool for explosive detections.