Heterojunctions for food freshness monitoring: interfacial engineering, sensing mechanisms, and intelligent packaging

The growing demand for real-time, non-destructive, and intelligent food quality assessment has spurred rapid advances in chemical sensors that monitor freshness deterioration during storage, transportation, and retail. Among emerging sensing platforms, heterojunction-based sensors are particularly attractive because their well-defined interfacial architectures regulate charge transport, surface adsorption, catalytic activation, and signal amplification. These features are highly desirable for detecting volatile spoilage biomarkers, including ammonia (NH 3 ), hydrogen sulfide (H 2 S), trimethylamine (TMA), and other bioamines, which are continuously released during the microbial and enzymatic degradation of protein-rich foods such as meat, fish, seafood, and dairy products. This review first elucidates the formation mechanisms of key spoilage biomarkers, including NH 3 , H 2 S, TMA, and other bioamines, in high-protein foods (meat, aquatic products, and dairy products), and clarifies their basis as freshness fingerprints. We then systematically summarize major heterojunction-based sensing platforms, including metal oxide and sulfide heterostructures, noble-metal-modified heterostructures, carbon- and MXene-based hybrids, porous framework-based composites, and conductive polymer heterostructures. Particular attention is given to p–n, n–n, p–p, and Schottky junctions, with a clear distinction between interfacial signal amplification driven by band bending, depletion-layer modulation, and built-in electric fields, and molecular selectivity governed primarily by analyte-dependent surface chemistry, defect sites, catalytic activity, and pore structure. Heterojunction formation enhances signal transduction and response kinetics and may improve selectivity when it preferentially alters the adsorption or reaction pathways of the target analyte relative to interfering species. Furthermore, representative applications in food analysis, intelligent packaging, flexible devices, wireless monitoring, and cold-chain early warning are highlighted to demonstrate the translation from materials design to practical freshness monitoring. Finally, we analyze critical challenges related to humidity and matrix interference, cross-sensitivity, signal drift, reproducibility, food-contact safety, scalable fabrication, standardized evaluation, data reliability, and sustainable integration. This review establishes a mechanistic framework linking spoilage chemistry, heterointerface engineering, sensing performance, and practical deployment, and provides design principles for next-generation intelligent food-freshness monitoring platforms.