Flexible multiple-stimulus-responsive photonic films based on layer-by-layer assembly of cellulose nanocrystals and deep eutectic solvents

Photonic crystal materials responding to environmental stimuli are important in sensors, intelligent detection, and anti-counterfeiting. However, traditional photonic materials usually lack tunable color and sufficient color suppleness, hindering their practical applications. Here, we report a facile strategy to achieve mechanical and color-tunable photonic films by doping deep eutectic solvents (DES) into cellulose nanocrystals (CNC) using layer-by-layer assembly. The amount of DES and several assembly layers were vital in determining the structure color and mechanical and response properties in the CNC photonic films. The experimental results revealed that DES could endow CNC suspension systems with high zeta potential, high viscoelasticity, and low transmittance. The tiny DES compensated for the insufficient flexibility of the CNC film, resulting in a wide color range, high solvent discrimination ability, and multi-color at several view angles. Moreover, photonic crystal film also played an important role in intelligent display and could be applied to traffic reflective signs to remind pedestrians and drivers of road safety and as smart labels for indoor humidity detection. These characteristics of the CNC photonic films could provide a promising strategy for developing advanced intelligent sensors, detection, and anti-counterfeiting materials.