A polydiacetylene-based colorimetric sensor as an active use-by date indicator for milk.

Current food date marking tools fail to take into account variables that impact shelf life, leading to food waste and posing human health risks. Herein, we develop a polydiacetylene/zinc oxide (PDA/ZnO) colorimetric sensor that can indicate milk freshness in real time. The PDA/ZnO system is engineered to indicate the freshness of full cream milk by monitoring its pH based on lactic acid concentration, an indicator of bacterial spoilage. The sensitivity is tuned by selection of the diacetylene monomers from which it is composed. Designs constructed from shorter chain diacetylenes are found to undergo chromatic transitions at a lower lactic acid concentration. Two new methods to tune PDA/ZnO sensitivity to lactic acid are explored, including relocation of the diacetylene functional group and lipid doping. A system constructed from 5,7-hexadecadiynoic acid (HDDA) and ZnO can discriminate between fresh (pH 6.8–6.0), spoiling (pH 6.0–4.5), and spoiled milk (pH 4.5–4.0) by a respective blue to purple to red color change. The chromatic stability of the PDA/ZnO sensor in food is studied as previous reports have shown the system to be unstable. We present a new method that stabilizes the nanocomposite by pre-exposure to the food matrix prior to photopolymerization.