Waste‐to‐MOF: Upcycling of Waste Polyimide Into Europium‐Based Metal‐Organic Framework as Efficient Fluorescence Sensor of Biogenic Amines

The fluorescence sensing method based on lanthanide metal-organic frameworks (Ln-MOFs) offers high sensitivity, selectivity, and rapid, real-time detection of biogenic amines (BAs). However, their high cost, complex synthesis, and unclear sensing mechanism limit practical application. Herein, waste polyimide is utilized as a precursor to synthesize Eu-1,2,4,5-benzenetetracarboxylic acid (Eu-BTEC) via a two-step hydrothermal-solution method. Under optimum conditions, the obtained Eu-BTEC exhibits excellent fluorescence selectivity and sensitivity toward cadaverine, with an apparent constant of 1.58 ×10⁶ L·mol^-1 and a detection limit of 0.17 µmol·L^-1. Density functional theory calculations reveal that the amino groups of cadaverine donate electrons to Eu³⁺, blocking the intramolecular electron transfer from the benzene ligand to Eu³⁺. Owing to the absence of chromophores in cadaverine, this electron transfer leads to luminescence quenching of Eu-BTEC. To achieve visual and portable detection, Eu-BTEC-based fluorescence test strips were fabricated by spraying its suspension onto filter paper, enabling visual quantification of cadaverine (0.50 µmol·L^-1) under UV light. This study not only provides a sustainable route for converting waste polyimide into Ln-MOFs but also elucidates the BA sensing mechanism through HOMO-LUMO band-gap and electron-density analyses, offering new insights into the fluorescence quenching process of Ln-MOFs.