Upconversion and ligand-sensitized downshifting from active inert shell in Ln-doped core–shell nanocrystals for anticounterfeiting applications

Luminescent nanocrystals (NCs), with their ability to emit multiple colors in response to various excitation sources, have gained significant importance in various fields, especially in information storage and anticounterfeiting applications. Here, we engineered lanthanide-doped core-shell and core-shell-shell NCs using the thermal decomposition method. These core-shell NCs exhibit blue and green emissions from Er/Tm ions when sensitized by Yb ions under 980 nm laser irradiation. Similar emissions are observed from core-shell-shell NCs, where Yb and Nd ions sensitize emissions under 980 and 800 nm excitations, respectively. However, due to surface defects on the core and core-shell NCs, their upconversion efficiency is very low, limiting their applications. To address this, we improved the upconversion luminescence efficiency by introducing an active inert layer as a shell on the core and core-shell NCs. This active inert layer, doped with Eu/Tb ions, acts as a protective layer, enhancing radiant efficiency by mitigating defects on the NC surface. Moreover, we functionalized the NC surface with 2,6-PDA ligands using the hydrothermal method, enabling excitation of the NCs under UV light irradiation. Upon 282 nm excitation, strong red/green emissions are observed from Eu/Tb ions via 2,6-PDA ligand sensitization. Using these NCs, we developed stable fluorescent inks mediated by carboxymethyl cellulose. NC-based fluorescent inks were employed to print various images on nonfluorescent paper using a simple and cost-effective screen-printing method. These flexible printed patterns remain invisible under ambient light but exhibit different colors upon exposure to 980 or 800 nm NIR and 254 nm UV light. This demonstrates the potential of dual-mode excitable multi-shell NCs for anticounterfeiting applications.