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

[Display omitted] • A dual-sensitive 1D PC was developed for use in encryptable anticounterfeiting films. • The PC showed humidity-sensitive color responses that can be tuned by UV irradiation. • The humidity sensitivity of the PC was decreased when exposed to aldehydes. • The aldehyde exposed PC could be recovered to its pristine by immersing it in HCl. • The prepared PC-based anticounterfeiting film was suitable for high-level security. Although considerable progress has been made in anticounterfeiting film technologies, developing difficult-to-imitate novel anticounterfeiting films is still challenging. Herein, we report novel photopatternable one-dimensional 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 TiO 2 –copolymer hybrid (Ti70) and primary-amine-containing copolymer (P(AEMA 97 - co -BPA 3)–NH 2) were prepared and utilized to fabricate 11-layered 1D PC (TiNH 2 PC). In addition, using UV post-treatment with customized photomasks, TiNH 2 PCs with hidden photopatterns that showed tunable humidity-responsive bright color contrast could be fabricated. However, after exposure to aldehydes, the TiNH 2 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)–NH 2 layers decreased when they converted to P(AEMA 97 - co -BPA 3)-imine layers under aldehyde exposure. However, the aldehyde-exposed TiNH 2 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. [ABSTRACT FROM AUTHOR]