Benzothiadiazole Tetracarboxylate-Based HoIII Metal–Organic Frameworks with Reversible Single-Crystal-to-Single-Crystal Transformations, Good Stabilities, and Fluorescence Sensing for Amino Acids in Aqueous Solutions

The structures, stabilities, and fluorescence sensing properties of rare earth metal–organic frameworks (MOFs) can be regulated by reversible single-crystal-to-single-crystal (SCSC) transformations, which attract much attention from chemical researchers. In this work, a novel dinuclear-based three-dimensional Ho^III MOF with the formula [Ho₄(BTDI)₃(DMF)₄]_n (JXUST-51) (H₄BTDI = 5,5′-(benzothiadiazole-4,7-diyl)diisophthalic acid, DMF = N,N-dimethylformamide) has been successfully prepared with a benzothiadiazole tetracarboxylate ligand and fully characterized. Very interestingly, the reversible SCSC transformations between JXUST-51 and {[Ho₄(BTDI)₃(H₂O)₄]_n·4H₂O·solvents}_n (JXUST-51a) can be induced by the temperature and solvent. Both JXUST-51 and JXUST-51a exhibit good thermal, solvent, and acid-basic stabilities in the pH range of 1–12. More importantly, the transformed crystalline sample of JXUST-51a remains stable after soaking in boiling water for 24 h. Remarkably, the presence of Ho^III ions and fluorescent BTDI^4– ligands leads to potential candidates as fluorescent sensors for JXUST-51 and JXUST-51a. l-Lysine (Lys) and l-glutamic acid (Glu) in aqueous solutions can be detected by JXUST-51 and JXUST-51a via fluorescence turn-on and turn-off effects with high selectivity and sensitivity, respectively. Moreover, JXUST-51 and JXUST-51a’s amino acid fluorescence sensing mechanism may be explained by the interaction between Lys/Glu and the framework. Significantly, fluorescent composite films and test papers with JXUST-51 have been further developed for the simple and rapid detection of amino acids in practical applications.