Bioinspired Bilayer Hydrogel Colorimetric Sensor Array for Low-Temperature Food Freshness Analysis

Sensitive monitoring of food freshness is essential for ensuring food safety, yet remains technically challenging under low-temperature and high-humidity cold-chain conditions. Herein, we report a bioinspired bilayer hydrogel colorimetric sensing platform that mimics the capture-transport-response mechanism of the olfactory mucosa. The upper adsorption layer, consisting of hollow ZIF-8 nanomaterials (H-ZIF-8 NMs) embedded in a hydrophilic agar hydrogel, serves as an artificial mucus interface to preconcentrate trace volatile analytes and amplify weak gaseous signals. The underlying sensing layer, based on a PVA/PAM hydrogel integrated with a chemical dye array, enables selective colorimetric transduction toward multiple volatile organic compounds (VOCs). Owing to the synergistic enrichment-reaction design, the platform achieves an ultralow detection limit of 0.02 ppm for trimethylamine and allows rapid and discriminative identification of representative spoilage markers, including 2-nonanone, glutaraldehyde, and trimethylamine. In addition, the sensor array enables sensitive recognition of common foodborne pathogens (Staphylococcus aureus, Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella) down to 2 log CFU mL^-1. When integrated with smartphone-based imaging, the system provides a portable and on-site analytical tool for early stage freshness assessment of refrigerated foods. This work establishes a general analytical strategy for gas preconcentration-reaction under humid and low-temperature environments and offers a bioinspired, functionally stratified sensing paradigm for cold-chain food freshness monitoring.