Fabrication of Printable Colorimetric Food Sensor Based on Hydrogel at Low Concentration of Ammonia

Due to increased single-person households and social distancing by a covid-19 virus, demand for convenience, instant, and delivery foods is rising. In food products without an expiration date when storing food, frequent disposal occurs due to a lack of information on quality verification and spoilage. Multi-functional smart-packaging sensors have been reported to solve such issues. The smart-packaging sensor previously reported is mainly used by electronic device-based sensors, making them challenging for consumers to use due to high price and low convenience and to understand the state of food through 'naked-eye'. Therefore, recent studies for the colorimetric method have been increased to provide food spoilage visually and instantly [1, 2]. However, the process of fabrication and the components of the sensor are complicated, making it difficult to use yet. For the commercialization and popularization of food sensing technologies, diverse functionalities, low cost, and convenience are required in developing food sensors.<br/>In this work, we fabricated a printable colorimetric food sensor based on the hydrogel using biocompatible polymers. The hydrogel sensor included pH indicators that can visually represent changes, given external stimuli such as pH changes. When the ammonia gas generated by food spoilage was ionized inside an aqueous solution of the hydrogel, it caused a change in internal pH, which changed the sensor's color. Hydrogel consists of a pH indicator with a sulfonate group (SO3-) negative charge and the monomer with trimethylammonium (−N+(CH3)3) positive charge, indicating excellent color reproducibility and stability[3]. Furthermore, we included the polymer to transfer the ammonia faster as a matrix and printed it to improve sensitivity and control the viscosity of the solution. It was possible to detect a very low concentration (~ a few ppb) and fast detection time (~a few sec.). It also showed stable color changes to users through the interaction of cationic monomer and anionic pH indicator. Finally, we utilized the food sensor through the pork spoilage experiment after printing a hydrogel food sensor we wanted to form. We confirmed color variations through the food spoilage experiment depending on the time and its utilization in food. This result shows availability to manufacture in a bar code and QR code, which can easily be mass-produced, making it economical and popular. Therefore, the hydrogel-based food sensor can develop attachable, reusable, and highly available sensors for food packaging containers and play a role in popularizing customized food sensors.<br/>Reference<br/>[1] F. Rastiani<i> et al.</i>, Monitoring the freshness of rainbow trout using intelligent PH-sensitive indicator during storage. Journal of Nutrition and Food Security 4, (2019) 225.<br/>[2] N. J. Halonen<i> et al.</i>, Bio-based smart materials for food packaging and sensors–a review. Frontiers in Materials 7, (2020) 82.<br/>[3] J. Kim, T. N. Cho, G. Valdés-Ramírez, J. Wang, A wearable fingernail chemical sensing platform: pH sensing at your fingertips. Talanta 150, (2016) 622.