Design and characterization of soy protein isolate/alginate/N-doped carbon dot composite film for dual colorimetric and fluorescence sensing in intelligent packaging

This study reports an eco-friendly sensing film composed of soy protein isolate, zinc alginate, eggplant peel extract, and N-doped carbon dots designed for pork freshness monitoring. The film integrates fluorescence and colorimetric responses to enable dual-mode, pH-sensitive detection and exhibits stable optical behavior across a wide pH range (2.0–12.0). The film also responds linearly to NH 3 over a concentration range of 0–1000 ppm (R 2  = 0.97528, LOD = 8.68 ppm), with distinct and visible color changes observed at higher concentrations (500–5000 ppm). The film effectively preserved pork quality, as indicated by a 50% reduction in fluorescence intensity at 25 °C after 48 h, and maintained stable fluorescence for up to 36 h at 5 °C prior to spoilage onset. Under 5 °C, it contributed to a 3.9% reduction in pH. This film provides an effective and non-invasive solution for real-time, on-site meat freshness assessment and food waste reduction.