Re‐Printable Chiral Photonic Paper with Invisible Patterns and Tunable Wettability

Abstract The construction of invisible patterns via high‐resolution printing and the independent encoding/decoding of complex information can lead to promising applications in steganography and watermarking for optical encryption. Herein, a rewritable chiral photonic paper formed by cholesteric cellulose nanocrystals and polycation is reported. The chemically crosslinked polycation network interpenetrates in the cholesteric structure while retaining the optical properties of the photonic crystals. The film exhibits controllable wettability via anion exchange, leading to extremely low contrast in the dry state but high contrast by a rapid wetting response. Triple invisible information is independently encoded on the films, including invisible patterns caused by reversible counterion‐controlled wettability, permanent fluorescent labels based on fluorescent counterions, and polarization‐dependent structural colors based on cholesteric structures. Full color patterns can be reversibly constructed via inkjet printing, with a high resolution of 100 µm. In addition, the circular polarization characteristics of the cellulose nanocrystals, liquid crystals, endow the system with complex and independent responses, realizing a wetting/polarization double‐key decryption. This work provides a simple and effective optical technique for coding complex information on a single material platform and expands the techniques available to achieve invisible patterns for sensing and anti‐counterfeiting.