Photochromic Paper Indicators for Acidic Food Spoilage Detection

A photoresponsive microstructured composite is fabricated through the impregnation of cellulosic filter paper (FP) with a spiropyran-modified acrylic polymer. The polymer enwraps uniformly each individual cellulose fiber, increases the thermal stability of cellulose, and ensures the preservation of the composite functionalities even upon removal of the surface layers through mechanical scratching. The photochromic spiropyran moieties of the polymer, even while embedded in the cellulosic sheet, can reversibly interconvert between the colorless spiropyran and the pink merocyanine isomeric states upon irradiation with UV and visible light, respectively. Moreover, the photochromic polymer presents a faster photochromic response and a higher resistance to photodegradation, with an outstanding reusability for more than 100 switching cycles when it is incorporated in the cellulose network. Most importantly, the acidochromism of the modified FP, attributed to the spiropyran molecules after UV activation, allows the real-time optical and visual detection of acidity changes and spoilage in food products, such as wine and milk. Spoilage due to bacterial degradation and oxidation processes generates acidic vapors that induce the protonation of the merocyanine. This results in a visually detectable chromic transition from pink to white of the treated cellulose fibers, corresponding to a blue shift in the absorption spectrum. The developed photoresponsive cellulose composite can serve as cost-effective robust optical component in integrated functional platforms and consumer-friendly indicators for smart food packaging, as well as portable on demand acidoresponsive interfaces for gas monitoring in industrial and environmental applications.