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}_<i>n</i> (<b>JXUST-50</b>) was solvothermally used with 5,5'-(benzothiadiazole-4,7-diyl)diisophthalic acid (H₄BTDI), which incorporates benzothiadiazole luminescent groups. <b>JXUST-50</b> displays a fluorescence "turn-on" effect and a blue-shift behavior toward the H₂PO₄<b>^-</b> anion in formamide solution, with a limit of detection of 0.50 μM. Remarkably, in a pH-dynamic system regulated by NaOH/HCl, <b>JXUST-50</b> demonstrates a fluorescence "on-off-on" effect and a blue-red-blue-shift behavior toward the H₂PO₄<b>^-</b> anion. Furthermore, the fluorescence "turn-on" and blue-shift effect observed between <b>JXUST-50</b> and H₂PO₄<b>^-</b> may be attributed to host-guest interaction and an absorbance-caused enhancement mechanism.