Imidazole-Based Metal-Organic Framework for the "On¹-Off-On²" Fluorescence-Based Detection of Picric Acid and the Adsorption of Congo Red

An imidazole-functionalized metal-organic framework(MOF),{[Ba2L (H3CCOO) (DMF)(2)] (DMF)}( n ), denoted as HBU-168, has beensynthesized and characterized by single-crystal X-ray diffractometry.The synthesized MOF exhibits ligand-based luminescence, which canbe attributed to coordination effects. Due to its excellent fluorescenceperformance, HBU-168 can be used to detect nitroaromaticcompounds (NACs) and metal ions selectively and sensitively. The moststriking properties of HBU-168 are the distinct red shift(similar to 34.8 nm) observed in the presence of picric acid (PA) witha Stern-Volmer constant (K (SV)) of4.17 x 10(4). The mechanism of the change in luminescencein the presence of PA is determined by experiments and density functionaltheory calculations. The results indicate that the electron transferbetween PA and the MOF has a reduction in the frontier orbital energygap. Subsequently, HBU-168 is used to prepare a light-emitting-diode-basedsensor (denoted HBU-168-LED), which can detect PA rapidlyand visually. In addition, we find that HBU-168 can beused to detect metal ions, particularly magnesium ions. The metaldetection mechanism is based on the turn-on fluorescence of HBU-168 caused by weak interactions between the MOF and metalions. Finally, HBU-168 is found to be a good adsorbentfor Congo red, achieving a removal efficiency of 98.89%. Therefore,the multifunctional HBU-168 MOF is a promising luminescentsensor and adsorbent. Amultifunctional Ba-(II)-MOF based on imidazole ligands,namely, HBU-168, has been constructed for "on(1)-off-on(2)" fluorescence-shifteddetection of picric acid (PA). Moreover, HBU-168 is successfullyapplied to the detection of metal ions and achieves efficient removalof Congo red in the water.