Hybridization of soy protein isolate with Cu-BTC for the development of colorimetric spoilage-sensing composite films for intelligent packaging

A novel colorimetric indicator (Cu-BTC@SPI@AF) was developed by incorporating a copper-based metal–organic framework (Cu-BTC) and soy protein isolate into an agar matrix to produce a biocompatible sensing platform. The indicator showed long-term stability and irreversible color responses to hydrogen sulfide (H₂S) and ammonia (NH₃), with low detection limits of 0.69 and 1.17 μM. Langmuir and Freundlich models confirmed a high adsorption capacity (qₘ = 5.08 cm³/g) and a Langmuir constant (K L = 0.92), attributed to its mesoporous structure (10.38 nm) and surface area (4.44 m²/g). FTIR verified chemisorption through Cu–S and Cu–N bonds, while FE-SEM revealed morphological changes after gas exposure. Furthermore, the incorporation of the Cu-BTC@SPI hybrid significantly improved the film's functional performance, increasing tensile strength from 30.45 to 79.13 MPa, enhancing elongation at break from 16.64% to 43.96%, and reducing water vapor permeability from 3.18×10 -7 to 1.15 10 -7 g/(m h Pa). Copper ion migration into food simulants was diffusion-controlled and reached its lowest level in vegetable oil, remaining strictly below European regulatory limits (5 mg/kg). During actual fish storage, the indicator displayed distinct color changes from deep blue to golden-orange that correlated linearly with conventional freshness indices (pH, TVB-N, and TVC). Kinetic analysis followed a pseudo-second-order model (R² > 0.99), with higher affinity toward H₂S (k₂ = 2.61 min⁻¹). Consequently, the developed indicator enables non-destructive, high-sensitivity, and real-time monitoring of seafood freshness for intelligent packaging applications.