Edible cellulose-based photonic crystals with low-temperature response for food sensing.

Responsive photonic crystals offer the advantages of zero energy consumption and easy recognition, making them ideal for food sensing applications. Temperature monitoring during food storage is crucial for ensuring food safety. However, non-toxic, safe, food-grade temperature sensors are still limited, and tuning the responsive range to accommodate extremely low temperatures remains a significant challenge. Here, we demonstrate edible photonic crystal materials with low-temperatures response for food sensing, fabricated through the co-assembly of hydroxypropyl cellulose (HPC) with edible ethanol. The ethanol effectively reduces freezing point of the HPC photonic crystals, allowing them to maintain the responsiveness at temperatures as low as -35 °C. Moreover, adjusting the ethanol concentration allows for modulation of the structural color and operational temperature range. The moldability of the photonic crystals facilitates the design of intricate patterns or informative labels, enabling the development of a colorimetric sensor capable of visually monitoring storage conditions at -20 °C. These green, safe, and responsive characteristics make cellulose-based photonic crystals promising candidates for applications such as displays, health monitoring, and smart sensing.