Multi-shelled upconversion nanostructures with enhanced photoluminescence intensity via successive epitaxial layer-by-layer formation (SELF) strategy for high-level anticounterfeiting†
Owing to the ability to convert near-infrared (NIR) incident light into high-energy ultraviolet or visible light photons, lanthanide-doped upconversion nanoparticles (UCNPs) have attracted great attention in anticounterfeiting applications as appealing and unparalleled agents. To facilitate the efficient energy transfer and overcome the bottleneck of low upconverted photoluminescence, we report a one-pot successive epitaxial layer-by-layer formation (SELF) strategy based on ion layer adsorption and Ostwald ripening to synthesize a series of high-quality monodispersed multi-shelled UCNPs with narrow size distribution (coefficient of variation less than 5%). Up to 30 layers of uniform shell are successfully deposited by successive introduction of the shell precursor solutions, which results in fold change of 300 and 200 in upconverted emission intensities for Er3+ and Tm3+-doped multi-shelled UCNPs, respectively. Using as-prepared multi-shelled UCNPs via SELF approach in conjunction with fabricated downconversion nanoparticles (DCNPs), we develop a facile and cost-effective strategy based on dual-modal manipulation of luminescence in anticounterfeiting to provide extra high-level security protection. The genuine pattern with true information is easier to visualize with the naked eye under excitation of 980 nm near-infrared (NIR) laser, while the false information could be readily read out when exposed to ultraviolet (UV) light. Meanwhile, latent fingerprint recognition with low background interference and distinguishable details in ridge patterns is achieved taking advantage of the significantly improved brightness in multi-shelled UCNPs.