Incorporating amidine groups into photonic cholesteric polymer networks for food quality sensing

Abstract

Concerns about food safety and food quality have been the driving forces behind the development of smart packaging and food quality sensors. Cholesteric liquid crystal polymers are excellent candidates for this purpose due to their structural colours and ease of processing in the fluid phase. Designing new types of responsiveness in these materials is essential for realizing their potential in the development of responsive photonic materials. Here, we report on the preparation of cholesteric polymer networks based on a new reactive mesogen containing an amidine group, which responds to organic acid often found in deteriorating food products. The new mesogen was designed, synthesized, characterized, and then incorporated into a cholesteric network with structural colour that responds to the presence of aqueous acid and acid vapor. It is demonstrated that the colours and extent of the colour shift can be precisely and independently controlled by changing the quantity of the chiral component and free volume in the system. As a potential application, the detection of the spoiling of fresh milk is demonstrated. The new material is expected to play a large role in the future development of cholesteric sensors.