Tunable Emissions of Upconversion Fluorescence for Security Applications

The rampant appearance of counterfeiting has a serious adverse effect on every aspect in the global markets. Thus, the development of high-tech security strategies has become an urgent challenge. Recently, lanthanide ions (Ln(3+)) doped materials open new avenues for concealing factual data and shield against counterfeiting because of their unique optical characteristics of color-tunable emissions under near-infrared excitation. The present review surveys the recent advances in Ln(3+)-doped upconversion crystals (UCCs) as fluorescent functional inks toward designable and high-level anti-counterfeiting patterns and graphical encoding. The great achievements of fabrication of versatile security patterns can be ascribed to the combination of the single or multiple colorful UCCs with polymers, additives, and solvents. Moreover, the crucial factors including underlying mechanisms, synthetic methods of upconversion fluorescence materials, and the diverse printing technologies employed for patterning the fluorescence images have been highlighted, and the corresponding challenges and opportunities in this promising research area are presented. This review will help providing a fundamental understanding and guidance to rational design fluorescence optical labels based on Ln(3+)-doped UCCs for broadening their applications in high-level security fields.