Portable plasmonic nanoparticle sensor for rapid and visual monitoring of food freshness.

Biogenic amines (BAs) are important indicators of food spoilage, yet rapid and visual detection remains challenging. Herein, a portable optical freshness sensor (NanoOFS) was developed by assembling Ag nanoparticles (NPs), Au NPs and hollow gold nanospheres (HGNs) as monolayers on a flexible white polyethylene terephthalate substrate. During food spoilage, released BAs adsorb onto NP surfaces, altering the local refractive index and inducing localized surface plasmon resonance shifts, enabling both naked-eye colorimetric readouts and quantitative UV-vis-NIR reflectance analysis. The Ag NP-based films exhibited rapid color transitions, discernible by the naked eye within 15 s, whereas the Au-HGN hybrid films responded with pronounced spectral red-shifts detectable by reflectance measurements. Additionally, the sensor produced clear optical responses to multiple BAs, with negligible response to common atmospheric gases. In putrescine-spiked salmon and naturally spoiled salmon and beef, NanoOFS enabled rapid visual monitoring of freshness, indicating potential for on-site food quality assessment.