Ln3+ ions doped upconversion materials possess intrinsic upconversion fluorescence properties than conventional fluorescence materials, including anti-Stocks fluorescence (converted long wavelength to short wavelength), tunable multicolor emissions under near-infrared (NIR) light excitation, and long fluorescence lifetime, which are excellent candidates for security application.
Wei Wu and co-workers, who were devoted to the preparation of double-model NIR irradiation NaYF4 upconversion inks and their applications in information encoding and security application, from Wuhan University, demonstrated a modified high-temperature coprecipitation to synthesize core-triple-shell Ln3+ ions-doped NaYF4 upconversion nanocrystals for screen-printing designable, complex and multicolor patterns, recently published in SCIENCE CHINA Materials (2018, doi: 10.1007/s40843-018-9341-8).
Recently, in order to achieve high-level security, incorporating multiple model emissions into a system for replacing single model emissions has been implemented. Wu, the leader of the research group, tells us "Such core-triple-shell NaYF4 upconversion nanocrystals (UCNCs) demonstrate excellent upconversion fluorescence properties of independent emitting colors under 808 or 980 nm laser excitation, which could be used in the anti-counterfeiting fields to update the encoding level."
He emphasized, "Additive mixing three kinds of 808 nm emitted red-green-blue (RGB) UCNCs colloid solution can precisely regulate the emissions of the suspension for achieving full-color display. As-obtained RGB three-primary colors induced by 808 nm lasers accomplish broader color gamut than traditional standard RGB (sRGB) model and printing cyan-magenta-yellow (CMY) model."
The unique upconversion fluorescence features of core-triple-shell UCNCs endow its application in full-color security designs, verifying by a series of screen-printed multi-color patterns of Chinese zodiac and large-area complex images. And transparent PVC self-adhesive anti-counterfeiting label was fabricated for the wine package, underlining the full-color anti-counterfeiting level.
Prof. Wei Wu tells us "The full-color and multiple encoding patterns based on core-triple-shell UCNCs can open the door to substitute current single model encoding for security application."
This research was funded by the NSFC (51471121), Basic Research Plan Program of Shenzhen City and Wuhan University.
See the article: Weijing Yao, Qingyong Tian, Bin Tian, Mengxiao Li, Huanjun Wang, Pan Zeng, Li Liu, He Zheng, Wei Wu. "Dual upconversion nanophotoswitch for security encoding."
Sci. China Mater. 2018, doi: 10.1007/s40843-018-9341-8.