Colorimetric ammonia (NH3) sensor based on Cu2+ doped sodium alginate hydrogel and its application in visual monitoring of shrimp freshness

A portable, low-cost, and efficient sensing strategy is developed for real-time, non-destructive monitoring ammonia (NH 3 ) released to evaluate the shrimp spoilage. In this study, a colorimetric sensing hydrogel with ammonia-responsive properties is prepared by incorporating Cu 2+ into sodium alginate hydrogel. The sensor exhibits a detection limit of 1.19 ppm for NH 3 and a response time of approximately 10 min, demonstrating good sensitivity. The color changes of the colorimetric ammonia sensor are captured with a smartphone and its response values are analyzed via software. When monitoring NH 3 accumulation in shrimp stored at 4 °C, the colorimetric hydrogel shows a color change from light green to dark blue, with results indicating a positive correlation between color variation and TVB-N value changes in shrimp meat. This study demonstrates the great potential of colorimetric hydrogel gas sensors in the field of food safety supervision.