Bilayer TPU films integrating pH-responsive bromocresol purple-fluorescein indicators and carvacrol/tea tree oil-loaded HKUST-1@alkaline lignin: Visualization, controlled release, and meat preservation.

A bilayer TPU-based intelligent packaging film integrating colorimetric sensing and antibacterial preservation was developed. The film consisted of a dense cast TPU indicator layer containing a bromocresol purple/fluorescein indicator and an electrospun TPU antibacterial layer loaded with lignin-modified HKUST-1 loaded with carvacrol and tea tree oil. The indicator showed reversible responses to ammonia, dimethylamine, and trimethylamine, with detection limits of 0.37, 0.15, and 0.18 μM, respectively. Lignin modification improved the aqueous structural retention of HKUST-1, and the electrospun layer exhibited sustained, pH-dependent release of the antibacterial components. Water vapor permeability was further evaluated under both 25 and 4 °C conditions. The film showed antibacterial activity against the four tested bacterial strains (Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Bacillus subtilis), while its extracts caused no detectable cytotoxicity under the tested conditions. In refrigerated beef, the selected TBF3-T-LH@CT formulation delayed quality deterioration by approximately 4 days and provided visible freshness indication from yellow to orange-red. The color variation correlated well with beef spoilage indicators, supporting visual freshness monitoring. These results demonstrate the potential of the bilayer film for intelligent active food packaging.