Dual-mode ZIF-8/carboxylated polyurethane 3D photonic crystals for ultrasensitive visual detection of putrescine and cadaverine in food freshness monitoring

A novel ZIF-8/carboxylated polyurethane 3D photonic crystal sensor is developed for putrescine and cadaverine detection. • Dual sensing mechanism: ZIF-8 enables size-selective adsorption and refractive index modulation, while PU provides carboxyl-amine acid-base swelling. • The optimal sensor (n BDO :n DMBA  = 3:7) achieves a low detection limit of 0.86 ppm for putrescine and high selectivity over common VOCs. • Synergistic signal amplification yields a maximum reflection peak shift of 27.02 nm at 100 ppm. Rapid on-site detection of biogenic amines (e.g., putrescine and cadaverine) is critical for food freshness monitoring. Herein, we report a three-dimensional photonic crystal (3DPC) sensor composed of ZIF-8 nanoparticles and carboxyl-functionalized polyurethane (PU). The PU matrix, with tunable carboxyl content via DMBA/BDO ratio, enables acid-base induced swelling upon exposure to amine vapors, while ZIF-8 provides size-selective enrichment and refractive index modulation. The sensor with an optimal n BDO :n DMBA molar ratio of 3:7 exhibits a low detection limit of 0.86 ppm for putrescine, a sensitivity of 0.2384 nm/ppm, and excellent selectivity over common VOCs. The synergistic dual-mechanism (swelling + refractive index change) yields a maximum reflection peak shift of 27.02 nm at 100 ppm. Moreover, the sensor shows moderate response time (2.5 min) and good long-term stability (35 days). This work provides a new strategy for integrating MOF adsorption and polymer swelling into a visual sensing platform for biogenic amines.