Dual-Mode Hydrogels with Tunable Structural Color and Fluorescence for Temperature-Responsive Multilevel Encryption

Dual-mode anticounterfeiting technology enhances information security by utilizing both fluorescence and structural color. However, both modes are typically regulated by the same mechanism, leading to identical information in both reflection and emission modes. To address this, we propose dual-mode hydrogels with tunable structural color and fluorescence intensity based on temperature-responsive stimuli, enabling the generation of different information for multilevel encryption. In this study, we polymerized N-isopropylacrylamide (NIPAM), N-acryloyl-l-phenylalanine (Aphe) monomer, and fluorescent 4-boronate-1,8-naphthalimide derivatives (FNI) via precipitation emulsion polymerization to form a series of P(NIPAM-Aphe-FNI) nanogels, abbreviated as P(NAF) nanogels. Additionally, P(NF) nanogels without the Aphe monomer were also prepared. The nanogels can self-assemble into colloidal photonic crystals and were embedded into a polyacrylamide (PAM) matrix to form P(NAF)/PAM and P(NF)/PAM hydrogels. The P(NAF)/PAM hydrogel maintains its structural color with a minimal fluorescence intensity change upon heating, while the P(NF)/PAM hydrogel lost its structural color and exhibited low fluorescence intensities. Based on these differences, we developed dual-mode hydrogels with dynamically tunable fluorescence and structural color. The dual-mode information can be displayed in separate channels or superimposed, with the information being reversibly revealed or concealed. Thus, through a programmed decryption process, the dual-mode hydrogels offer strong information storage and encoding capabilities, enabling multiple information encryption and decryption methods via temperature-responsive stimuli. This dual-mode system with reversible and programmable responses offers a promising strategy for high-security optical encryption and advanced anticounterfeiting applications.