Anti-counterfeiting system based on luminescent varnish enriched by NIR- excited nanoparticles for paper security.

Up-converting nanoparticles can be a demand for requirements in many areas, including bioimaging and conversion of energy, but also in the battle against counterfeiting. The properties of lanthanide ions make falsification difficult or even impossible using appropriately designed systems. The proposition of such an approach is the NaErF ₄ :Tm ³⁺ @NaYF ₄ core@shell up-converting nanoparticles combined with transparent varnishes. Given the spectroscopic properties of Er ³⁺ ions present in the fluoride matrix, the obtained up-converting nanoparticles absorb light by 808 and 975 nm wavelengths. The intentionally co-doped Tm ³⁺ ions enable tuning characteristic green Er ³⁺ emission to red luminescence, particularly desirable in anti-counterfeiting applications. The article includes a thorough analysis of structural and morphological properties. Moreover, this work shows that exclusive luminescent properties of NaErF ₄ :Tm ³⁺ @NaYF ₄ NPs can be given to the transparent varnish, providing an excellent anti-counterfeiting system, revealing red emission under two different excitation wavelengths.
(© 2022. The Author(s).)