A pH responsive fluorescent/visible light-emitting film for real time visual monitoring of food freshness

A dual-layer pH-responsive fluorescent film was developed using hydroxypropyl methylcellulose (HPMC), zinc alginate (ZA), fluorescein isothiocyanate (FITC), and dragon fruit peel extract (DE) for real-time food freshness monitoring. The outer protective layer served as a hydrophobic barrier to reduce moisture interference, while the inner indicator layer acted as a pH-sensitive interface, integrating FITC's fluorescence and DE's visible color change for dual-mode sensing. The film exhibited good thermal stability, antimicrobial activity, and improved hydrophobicity. Optical responses were evaluated across a wide pH range (2.0–12.0), showing stable visible color under ambient light and enhanced fluorescence under 365 nm UV. Upon exposure to ammonia vapor, fluorescence intensity increased linearly from 0 to 1000 ppm (R 2  = 0.9895; LOD = 6.73 ppm), while visible color changes became evident at 500–5000 ppm, with R/B and R/G ratios exhibiting strong linearity (R 2  = 0.96176 and 0.96747; LODs = 1126 ppm and 958 ppm, respectively). Application trials with pork revealed distinct color changes after 4 days at 25 °C or 2 days at 5 °C. These results confirm the film's effectiveness for dual-mode, non-destructive, and real-time monitoring of pork freshness.