Synthesis of Redox-Active Terbium(III)-Based Metal–Organic Framework Crystals and Visual Fluorescence Sensing for Vanillylmandelic Acid

Fluorescent sensing metal–organic framework crystalline materials have potential applications in the detection of biomarkers for disease. Two highly thermally and chemically stable redox-active Tb-MOF and Tb-MOF-Ox crystalline materials based on the tetracarboxylic acid ligand were successfully synthesized and characterized, and the Tb-MOF nanocrystals also were prepared, which can quantitatively identify vanillylmandelic acid (VMA), an early pathological feature of pheochromocytoma (PCh). Tb-MOF shows high sensitivity and high selectivity with a low detection limit of 0.044 μM for VMA. Tb-MOF-Ox was obtained through the oxidation of Tb-MOF, which has better fluorescence performance and lower detection limit of 0.025 μM. Further, synthesized Tb-MOF nanocrystals also have better fluorescence properties and a lower detection limit of 0.026 μM. Tb-MOF and Tb-MOF-Ox can be fabricated into portable test strips and LED light beads or doped into sodium alginate to prepare hybrid sensing membranes (Tb@SA), which exhibit excellent VMA sensing ability. Moreover, Tb-MOF exhibits an exceptionally high proton transport capacity with a value of 1.20 × 10^–3 S·cm^–1 at 60 °C and 98% RH. Therefore, Tb-MOF is a bifunctional material for fluorescence sensing and proton conduction.