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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 freestanding 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.

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Recent advances in structural-color cellulose nanocrystal (CNC) materials have been made toward chemical sensing applications; however, such materials lack sufficient color chroma for naked-eye observation, and their selective recognition to given chemicals as well as the corresponding mechanism has rarely been reported. Here, a dopamine-infiltration and post-polymerization approach is proposed to construct vivid structural-color composite films. The chiral nematic structure of CNC enables the structural coloration, while the strong light absorption of the polymeric co-phase, polydopamine (PDA) enhances the color chroma and visibility. By controlling the PDA amount, the composite films can detect organic solvents quantitatively and selectively via visible color changes. From the viewpoint of the compatibility and similitude principle, notably, a critical solubility parameter distance (R 0) between PDA and “active” solvents is defined with a three-dimensional Hansen solubility sphere; this well constructs a rule for the sensing selectivity of the chemochromic composite films. The findings pave the foundation for the design of colorimetric sensors with specifically testing objects.