Fe-Functionalized Covalent Organic Frameworks: Charge Transport and Surface Barrier Modulation for Biogenic Amine Sensing and Real-Time Lamb Freshness Monitoring.

Room-temperature (RT) ammonia (NH3) sensing is of great significance for industrial safety and food freshness monitoring. Herein, a Fe3+-functionalized imine-linked covalent organic framework microsphere (TD-Fe) was constructed for high-performance RT NH3 detection and real-time lamb freshness monitoring. Fe3+ coordination induced local framework contraction and electronic structure modulation, thereby improving the pore structure, enhancing charge transport, and increasing the proportion of chemisorbed oxygen species (OC). The optimized TD-Fe sensor exhibited an outstanding response of 4834% toward 500 ppm NH3 at RT, a fast recovery time of 1.3 s, a low limit of detection (LoD) of 0.357 ppm, good stability, and reliable humidity resistance. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Kelvin probe force microscopy (KPFM), in situ X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations revealed that Fe sites enhanced NH3 adsorption, decreased the adsorption energy (Eads.) from -0.13 to -0.96 eV, promoted interfacial charge transfer, and regulated the surface potential barrier. Furthermore, TD-Fe enabled real-time monitoring of lamb spoilage over 7 days, demonstrating its practical potential for food freshness evaluation.