Colour tuneable upconversion photonic materials for anti-counterfeiting security inks

Ultimate generation of luminescent security inks have been used to detect counterfeiting including applications in banknotes, quick response codes (QR codes), barcodes, security documents, drug packaging and food security. Anti-counterfeiting strategies are competitively developed against fast- growing counterfeit markets. So far, most security inks are based on the down-conversion effect, that is transforming UV incident radiation into visible light. But the substrates are sensitive to this UV "reading light", which leads to a reduction of the contrast between the substrate and the printed sample. Therefore, rare-earth doped upconversion luminescent materials present significant advantages compared to standard fluorescent dyes, such as invisibility in ambient light, excitation by low cost commercial NIR irradiation and a lack in background noise, due to negligible auto-fluorescence from the surface. However, more attention needs to be paid on the codification of overall emitted luminescence, so it will be more difficult to mimic by ever increasing sophisticated counterfeiters. Thus, we present upconversion emissions in rare-earth doped nano-glass-ceramics (nGCs) under NIR excitation at 980 nm. Intensity ratios among UV and VIS upconversion emission bands can be tailored by modifying doping concentration level. Specific doping level, excitation wavelength and focusing conditions give rise to additional security features based on light-responsive encryption security patterns. A multi-digit code based on these intensity ratios can be set from resulting spectra, providing a proof-of-concept test for light-responsive encryption security patterns.