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 film integrating volatile-amine-responsive colorimetric sensing and sustained antibacterial release. • Bromocresol purple/fluorescein enabled dual colorimetric and fluorescent responses over a broad pH range. • Carvacrol/tea tree oil-loaded L/HKUST-1 inhibited both Gram-positive and Gram-negative bacteria. • The film showed reversible responses to NH 3 , DMA, and TMA, while ΔE correlated well with beef TVB-N ( R 2  = 0.9417). • The bilayer film delayed refrigerated beef deterioration by about 4 days and enabled freshness monitoring. 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 μ M, 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. Download: Download high-res image (212KB) Download: Download full-size image