Design of core/active-shell NaYF4:Ln3+@NaYF4:Yb3+ nanophosphors with enhanced red-green-blue upconversion luminescence for anti-counterfeiting printing

Lanthanide-doped NaYF4 upconversion luminescent nanophosphors have attracted great interest for anti-counterfeiting applications. However, low luminescent intensity caused by surface defects and low extinction coefficients of lanthanide dopants greatly limits their practical applications. Herein, we developed core/active-shell NaYF4:Ln3+@NaYF4:Yb3+ (e.g. NaYF4:Er3+/Tm3+@NaYF4:Yb3+, NaYF4:Yb3+/Er3+@NaYF4:Yb3+, and NaYF4: Yb3+/Tm3+@NaYF4:Yb3+) nanophosphors with enhanced near-infrared (NIR)-to-visible upconversion luminescence by coating Yb3+-doped NaYF4 shell (active-shell) on NaYF4:Ln3+ upconversion core nanoparticles. The structure, morphology, and elemental composition of NaYF4:Ln3+@NaYF4:Yb3+ core/active-shell nanoparticles (CASNs), NaYF4:Ln3+@NaYF4 core/inert-shell nanoparticles, and NaYF4:Ln3+ core nanoparticles were characterized and compared systematically to reveal their structure-property relationship. Collective data showed that coating of a NaYF4:Yb3+ active shell on NaYF4:Ln3+ cores significantly enhanced upconversion luminescent intensity by ~21 times, while an inert (pure NaYF4) shell coated on the surface of NaYF4:Ln3+ core only resulted in 5.2 times luminescence enhancement. The enhanced upconversion luminescence intensity was mainly attributed to (i) the NaYF4 shell-reduced quenching effect and (ii) the promoted transformation of the absorbed NIR light from the pump source to the core nanoparticles by Yb3+ sensitizer in the active shell. Moreover, by tailoring dopant pairs and molar ratios of lanthanide ions in the core nanoparticles, three CASNs with enhanced red, green, and blue upconversion luminescence were obtained and fabricated into environmental benign luminescent inks. The inkjet printing of RGB CASNs inks on papers produced multiple-color, high-resolution, complex luminescent patterns under 980 nm laser. This work expands the lanthanide-doped NaYF4 materials for anti-counterfeiting applications.