Forensic-level security using non-imitable anticounterfeiting films: Humidity-sensitive 1D photonic crystals with UV-tunable color response and their encryption using small aldehydes

Although considerable progress has been made in anticounterfeiting film technologies, developing difficult-toimitate novel anticounterfeiting films is still challenging. Herein, we report novel photopatternable onedimensional photonic crystal (1D PC)-based anticounterfeiting films, whose colors can be changed by human breath and which can be encrypted and decrypted using small aldehydes and hydrochloric acid, respectively. A photo-crosslinkable TiO2-copolymer hybrid (Ti70) and primary-amine-containing copolymer (P(AEMA(97-co)BPA(3))-NH2) were prepared and utilized to fabricate 11-layered 1D PC (TiNH2 PC). In addition, using UV posttreatment with customized photomasks, TiNH2 PCs with hidden photopatterns that showed tunable humidityresponsive bright color contrast could be fabricated. However, after exposure to aldehydes, the TiNH2 PC showed reduced or no color shifts in response to further changes in the relative humidity. This is because the hydrophilicity of the P(AEMA(97-co-)BPA(3))-NH2 layers decreased when they converted to P(AEMA(97-co-)BPA3)imine layers under aldehyde exposure. However, the aldehyde-exposed TiNH2 PC could be reversibly recycled to its pristine state by simply immersing it in a hydrochloric acid solution (1 M) for 30 min. These findings provide insights into fabricating various colorimetric sensors as non-imitable anticounterfeiting films with selective encryption/decryption for achieving forensic-level security.