A versatile pH-sensitive hydrogel based on a high-performance dye: Monitoring the freshness of milk and chicken meat

There is a growing interest in soft-matter sensors at the research and application level. Colorimetric hydrogel-based sensors can work as smart tools for the fast, simple, and low-cost monitoring of food freshness and spoilage. Herein we developed a hydrogel pH indicator working from pH 2–12 with a specific colorimetric pattern. A fast-responsive, easily formed and mouldable hydrogel was obtained by adding a minimal amount of a single pH-sensitive dye to an agar matrix. Optical, mechanical, and morphologic characteristic of the hydrogel were examined in detail. An easy-to-use smartphone app ensures color tone capture and correlation of color changes to pH levels. Practical applications of the versatile gel sensor were tested in solution and in the gaseous phase. The sensor was employed to assess the freshness of milk samples and to monitor the volatile basic nitrogen compounds resulting from packaged chicken spoilage.