Photonic Inks with Dual‐Layer Security Features by Encapsulation of Color Tunable Fluorescent Dyes in PMMA Colloidal Microspheres

Abstract To counter economic terrorism by preventing counterfeit currency, documents and high‐value commercial products, new‐generation security inks with multiple safety features are required. Herein, color‐tunable pyrylium and pyridinium dye‐encapsulated polymethyl methacrylate (PMMA) colloidal microspheres are reported to exhibiting brilliant emission and photonic properties. A combination of the PMMA colloidal photonic ink having structural color variation and the dye‐encapsulated colloidal photonic ink with fluorescence modulation is used for security labeling. The angle‐dependent structural color variations, a remarkable 250‐fold fluorescence enhancement, non‐toxicity, and the rare earth‐free formulation have made the ink novel and suitable for dual‐layer high‐security printing. Covert security patterns and labels are made overt under 365 nm UV light, while also exhibiting angle‐dependent structural color. The increased level of security with developed photonic colloidal inks is demonstrated with dual‐layer screen‐printed images and patterns on flexible substrates.