Multi-channel information encryption based on the structural color and photoluminescence capability of cellulose nanocrystal flexible films†

Traditional single-channel message encryption can only store a limited amount of encrypted information, and the lower level of security makes the decryption process very vulnerable. In this regard creating a multi-channel message encryption combining structural color, photoluminescence, and chiral optical properties is necessary. In this work, a cellulose nanocrystal (CNC)/glycerol (Gly)/polyvinylpyrrolidone (PVP) ternary cellulose nanocrystal composite film is designed with excellent optical properties, unique structural features and good mechanical properties, which can effectively respond to left-hand circularly polarized light, viewing angle and relative humidity changes. Microanalysis showed that the presence of Gly and PVP would not hinder the self-assembly of CNCs in aqueous solution and would enhance the mechanical properties and improve the structural color of the composite films. Afterwards, the fluorescent dye spiropyran (SP) was introduced into the system, and the composite film SP-CNC produces richer color effects. Based on this, we constructed a multi-channel message encryption model by combining the excellent optical properties of CNCs, the photoluminescence properties of SP dyes, and the classical Vigenère cipher. This greatly improves the security level of message encryption and provides a powerful strategy for large-capacity and high-security message encryption.