Bilayer pH-responsive intelligent active packaging film based on gelatin/Na-doped rice bran carbon dots and chitosan/shikonin for real-time salmon freshness monitoring and preservation

Aggregated carbon dots exhibit weak antibacterial and antioxidant activities, limiting their application in packaging systems. Metal doping is an effective strategy to enhance their performance. Herein, Na-doped rice bran carbon dots (Na-RBCDs) were synthesized hydrothermally from rice bran and NaOH. A pH-sensitive bilayer film was fabricated via layer-by-layer casting, comprising an outer gelatin layer containing Na-RBCDs and an inner chitosan layer loaded with shikonin for indication/sustained release. Optimal antioxidant and antibacterial activities of Na-RBCDs were achieved at a 10:1 rice bran/NaOH mass ratio, and they exhibited no cytotoxicity toward HaCaT cells below 2 mg/mL. Scanning electron microscopy confirmed their uniform dispersion within the film matrix, ensuring a smooth surface and strong interlayer adhesion. Fourier transform infrared spectroscopy and thermogravimetric analysis indicated that Na-RBCDs formed hydrogen bonds with the polymer matrix, thereby improving the film’s thermal stability. Incorporating 5% Na-RBCDs enhanced UV-blocking ability, hydrophilicity (water contact angle >75°), and tensile strength (1.7-fold increase). The film demonstrated effective sustained release of both active components, alongside improved antioxidant, antibacterial, and pH-responsive colorimetric properties. When applied to refrigerated salmon, the film containing 5% Na-RBCDs extended the shelf life from 3 to 9 days by enhancing chemical and microbial stability. A pronounced color change was observed between days 9 and 12 (ΔE from 15.7 to 30.6), demonstrating its ability to monitor salmon freshness. This gelatin/chitosan-based intelligent active packaging system shows promising applicability for muscle food preservation and freshness monitoring. Further research is needed to establish a precise correlation between colorimetric response and specific spoilage indicators.