Reaction Time Induced a Two-Step Dissolution and Recrystallization Structural Transformation with Three Eu Metal–Organic Frameworks: Crystal Structures and Multiresponsive Fluorescence Detection

Under solvothermal conditions, three 3D lanthanide metal-organic frameworks (Ln-MOFs): [Eu(H₂DHTA)_1.5(DMF)₂]·DMF (<b>1</b>), [Eu(H₂DHTA)_0.5(DHTA)_0.5(DMF)(H₂O)]·2H₂O (<b>2</b>), and Eu(HCOO)₃ (<b>3</b>) (H₄DHTA = 2,5-dihydroxyterephthalic acid) have been synthesized by different reaction times. Interestingly, induced by reaction time, compounds <b>1-3</b> underwent a two-step dissolution and recrystallization structural transformation (DRST) reaction. Investigations on the DRST processes were carried out, and the transformation pathway was deduced, which was verified by XRD analyses. Notably, compound <b>2</b> demonstrates pronounced luminescence as well as high stability in water and other organic solvents. The fluorescent detection of furan antibiotics can serve as turn-off effects, and glutamic acid (Glu), aspartic acid (Asp), and riboflavin (VB₂) can serve as the turn-on effect. To explain the enhancing and quenching mechanisms, XRD, UV-visible absorption spectroscopy, electrochemistry, IR spectra, theoretical calculation, fluorescence lifetimes, and XPS were discussed. Additionally, MOF-coated test strips were utilized to detect these analytes, exhibiting excellent agreement with fluorescence spectroscopy. This work provides an example for more effective designs to employ Ln-MOFs as multiresponsive fluorescent sensors for detection of environmental pollutants in aqueous solution.