Multifunctional chitosan/gelatin/zinc-doped carbon dot composite films crosslinked by genipin: Structural...

Genipin-crosslinked chitosan/gelatin composite films incorporating zinc-doped carbon dots (Zn-CDs) were fabricated by solvent casting at genipin concentrations of 0-1.0% (w/w), with parallel Zn-CD-free films serving as controls to decouple the individual and synergistic contributions of chemical crosslinking and Zn-CD incorporation to film properties. Successful Zn doping of the carbon dots was confirmed by comparative FTIR analysis, which revealed characteristic shifts in the carboxyl/hydroxyl stretching region and an emerging ZnO-related feature relative to the undoped precursor. Scanning electron microscopy revealed a progressive transition from rough, phase-separated surfaces to compact, homogeneous matrices with increasing genipin content, reflecting formation of a denser covalent network. Zn-CD incorporation fundamentally altered the mechanical response from monotonic to biphasic, peaking at 0.5% genipin (tensile strength 39.73 ± 0.67 MPa; elongation at break 19.34 ± 0.60%), attributable to a dual physical-chemical crosslink network. Water vapor transmission rate reached a minimum of 3.03 g·m^-2·24 h^-1 at 0.5% genipin, lower than Zn-CD-free films even at their best value (4.20 g·m^-2·24 h^-1 at 1.0% genipin), demonstrating that Zn-CDs enhance barrier performance independent of film thickness. FTIR and XRD confirmed molecular-level integration of genipin into the amorphous biopolymer network via covalent amine linkages. Thermal stability increased monotonically with crosslink density, as shown by TGA and DSC. Antioxidant assays on solid-state film extracts showed DPPH scavenging of approximately 50% at the effective genipin/Zn-CD loading, substantially exceeding values obtained from film-forming solutions (4.1-8.6%), confirming meaningful food-contact-relevant antioxidant performance. The films exhibited a multi-stage pH-responsive color transition (blue-green at pH 3-6, near-black at pH 7-8, red-yellow at pH 9-11), corroborated by CIELAB analysis, supporting their potential as naked-eye freshness indicators. MTT/CCK-8 assays on fibroblast cells showed viability above the ISO 10993-5 non-cytotoxicity threshold across all formulations, supporting the preliminary safety of the system for food-contact use. Biodegradability decreased inversely with crosslink density, with all films undergoing meaningful mass loss over 18 days. Collectively, these results establish genipin-crosslinked chitosan/gelatin/Zn-CD films as multifunctional materials combining mechanical integrity, superior barrier performance, functional antioxidant activity, colorimetric freshness sensing, preliminary biocompatibility, and biodegradability for sustainable food packaging.