Color Halftone Quick Response Code Based on Gravure Printing of Upconversion Hexagonal-Phase NaYF4Crystals for Multilevel Anti-Counterfeiting

The smartphone recognition of color halftone quick response (QR) codes with upconversion luminescence was demonstrated, where the codes, with excellent reliability and attractive appearance, were achieved via high-quality gravure printing of hexagonal phase (β-) NaYF4 crystals. Firstly, the three-primary-color (RGB) waterborne upconversion inks were formulated by incorporating the synthesized β-NaYF4 crystals with excellent dispersion and corresponding emission signals. The secondary colors (yellow, magenta, and cyan) and tertiary colors (white and black), with accurate chromaticity, were garnered in accordance with the additive color synthesis. Then the color halftone QR codes could be generated by embedding regular QR codes into pre-defined halftone images as ciphertexts. Produced by independent deposition of the RGB inks via high-quality gravure printing, the color halftone QR codes were invisible under ambient condition, while presented bright multi-color halftone patterns upon near-infrared (NIR) irradiation. The ciphertexts could be decoded with a common smartphone under the 980 nm excitation for the authenticity verification. The developed technology offered significant advantages in multiple anti-counterfeiting, tracking, and visual effects, particularly possessing merits in the respect of low cost, high throughput, excellent concealment, and pro-environment, paving the way for the upconversion materials in the anti-counterfeiting application.