Triple‐State Invisible Photonic Crystal Pattern Encrypted in Hollow‐Silica/Polyurethane Film for Anticounterfeiting Applications

An invisible photonic crystal (PC) pattern encrypted at the dry state and decrypted by solvent is a new anticounterfeiting material system, but its diversity and complexity in response are still required to be increased to enhance the anticounterfeiting effect. Herein, a triple‐state invisible pattern encrypted in hollow‐silica/polyurethane (h‐SiO 2 /PU) film with two decryption mechanisms is developed based on the asymmetric solvent response of h‐SiO 2 /PU with different SiO 2 shell thicknesses. The pattern is printed via etching SiO 2 shell in the designed region, which creates an etched “pattern” containing thin‐shell h‐SiO 2 particles and an unetched “background” containing thick‐shell particles. Different from the traditional invisible PC patterns, the current system possesses triple states, including a “dry” invisible state composed of two close inverse opal structures, a “wet” invisible state composed of inverse opals filled with water, and an ethanol‐soaking visible state due to the prevention of solvent diffusion by the thicker shell. The invisible patterns are reversibly and randomly switched among these three states, which suggest that they are decrypted from the dry invisible state through selective solvent infiltration or from the wet invisible state through selective extraction. Such increased diversity and complexity make this invisible PC pattern ideal material for anticounterfeiting applications.