Flexible Photonic Cellulose Nanocrystal Films as a Platform with Multisensing Functions

Biomimetic optical cellulose nanocrystal (CNC) materials have shown great potential for application in colorimetric sensing, anticounterfeiting, and decorative coatings because of simple recognition by the naked eye; however, how to simultaneously solve the inherent brittleness of CNCs as well as achieve multisensing functions is still a big challenge. Here, we propose a new coassembly strategy of CNCs and citric acid (CA) to fabricate free-standing photonic CNC films. The chiral nematic structure and visible structural colors can be adjusted in a wide color range by varying the CA content. Owing to the plasticizing effect, the resulting CNC-CA films display high flexibility and can be folded freely. Notably, such films can sense different external signals, including compression, ethanol and alkali, by changing apparent structural colors. Additionally, for volatile chemicals, the color changes are reversible, ensuring repeating applications. Given the high mechanical performance and multisensing performance, this method represents a simple but effective way to construct highly flexible and multifunctional photonic CNC materials.