Highly Stable Zn(II) Metal–Organic Framework as an “On–Off–On” Fluorescence Sensor toward H2PO4– Anion in the pH-Dynamic System

In a dynamic system, fluorescent probes with dual signals not only efficiently identify target analytes but also resist interference from the system, aiding in further system differentiation. A new three-dimensional (4,6)-connected Zn(II)-based metal-organic framework with the formula {[Zn₂(BTDI)(3,5-BBIP)]·2.5H₂O·2DMF}_n (JXUST-50) was solvothermally used with 5,5'-(benzothiadiazole-4,7-diyl)diisophthalic acid (H₄BTDI), which incorporates benzothiadiazole luminescent groups. JXUST-50 displays a fluorescence "turn-on" effect and a blue-shift behavior toward the H₂PO₄^- anion in formamide solution, with a limit of detection of 0.50 μM. Remarkably, in a pH-dynamic system regulated by NaOH/HCl, JXUST-50 demonstrates a fluorescence "on-off-on" effect and a blue-red-blue-shift behavior toward the H₂PO₄^- anion. Furthermore, the fluorescence "turn-on" and blue-shift effect observed between JXUST-50 and H₂PO₄^- may be attributed to host-guest interaction and an absorbance-caused enhancement mechanism.