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."
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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.
http://engine.scichina.com/publisher/scp/journal/SCMs/doi/10.1007/s40843-018-9341-8?slug=full%20text
https://link.springer.com/article/10.1007/s40843-018-9341-8
Science China Materials