Deep eutectic solvent-regulated starch films integrated with CuZn-MOF for ammonia-responsive and visual food...

To circumvent the complex and unstable processes of natural-pigment indicator films, a green starch-based carrier was designed to load CuZn-MOF-74, yielding responsive and antibacterial films for food-freshness monitoring. When choline chloride-urea-glycerol (ChCl-U-G) was used as a plasticizer, the toughness of the hydroxypropyl starch film was increased by 3.08 times compared with that of the unplasticized film (St), while the tensile strength was slightly reduced. The film almost completely disintegrated within 10 days. Combined with XRD, FT-IR, and DSC characterization, ChCl-U-G was found to weaken the intermolecular hydrogen-bond network of hydroxypropyl starch, causing an 84 cm^-1 redshift of the -OH stretching vibration, as well as reductions in crystallinity (13.96%) and ΔH (29.3%), indirectly evidencing the plasticizer-hydroxypropyl starch matrix interaction. The resulting CuZn/ChCl-U-G-St film exhibited high antibacterial activity (inhibition rate up to 95%), sensitive ammonia response, and color stability. In practical applications, the film could indicate pork freshness in real time via a characteristic color change (ΔE > 15), and ΔE exhibited a quadratic polynomial relationship with total volatile basic nitrogen (TVB-N) (R² > 0.91). Based on electrostatic potential predicted structural stability, quantitative binding energy calculations showed that ChCl-U-G-St had the strongest binding energy (-1615.56 kcal/mol).