Synthesis and Inkjet Printing of NaYF₄:Ln³⁺@NaYF₄ Core–Shell Nanoparticles with Enhanced Upconversion Fluorescence for Anti-Counterfeiting Applications

Recently, lanthanide-doped upconversion luminescent materials have showed great potential in optical data storage, information encryption and anti-counterfeiting. However, the low upconversion luminescence still limited their applications. In this work, we fabricated RGB NaYF₄:Ln³⁺@NaYF₄ core–shell nanoparticles (CSNPs) with enhanced upconversion luminescence by coating an inert NaYF₄ shell onto NaYF₄:Ln³⁺ core nanoparticles via thermal decomposition method. The effect of increased shell temperature and addition of shell precursors on crystal phase, morphology and luminescent property of the synthesized CSNPs were systematically investigated. It was demonstrated that high shell growing temperature facilitated the formation of pure β-NaYF₄ CSNPs. Upon increasing amount of shell precursors, the morphologies of hexagonal phase NaYF₄ CSNPs changed from nanorod to nanocube and showed different luminescent properties. Pure hexagonal phase NaYF₄ CSNPs with highest upconversion luminescence of about 15 times higher than NaYF₄:Ln³⁺ core nanoparticles can be prepared at 310 °C with the addition of shell precursors at 3 mmol. Moreover, three-primary-color (RGB) CSNPs with enhanced upconversion luminescence were successfully prepared by changing the doping pair of lanthanide ions in core. The synthesized RGB CSNPs were fabricated into environment friendly luminescent ink by sequential surface modification by PAA ligand and dispersing in mixture solvent of ethanol, water and glycerol. Comparative results showed that the fluorescence enhanced RGB CSNPs inks were more suitable for inkjet printing of multicolored, complex and high resolution luminescent anti-counterfeiting patterns on paper substrates.