Poly(2-isoproprenyl-2-oxazoline)-Based Reactive Hydrophilic Cross-Linked Nanofiber Networks as the Basis for Colorimetric Continuous Meat Freshness Monitoring Sensors

Global food waste is a cross-cutting issue that ranges from agricultural production to storage in households and leads to the loss of one-third of produced food products. Although the currently depicted static expiry, best-by, and due-by dates provide accurate information on when to consume the food product, these time indications are only valid under ideal storage conditions. Therefore, there is a stringent need for easy-to-use sensor applications that allow continuous monitoring of food freshness. This work showcases a straightforward and effortless approach to obtain highly porous cross-linked water-stable and reactive hydrophilic nanofiber membranes based on electrospinning of poly(2-isoproprenyl-2-oxazoline) (PiPOx) with succinic acid as the cross-linker. Subsequent thermal treatment of the produced succinic acid-containing PiPOx nanofiber networks generated sufficient cross-linking to preserve the fibrous morphology in an aqueous environment while retaining the hydrophilic character of the scaffold. The remaining 2-oxazoline side chains of PiPOx are stable at room temperature but provide reactivity for post-cross-linking modification with carboxylic acid-containing compounds, as exploited here for continuous food monitoring by incorporating acid-functionalized dyes to yield a colorimetric sensor for biogenic food spoilage indicators, such as thiol- and amine-containing small molecules.