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

Lanthanide-doped NaYF 4 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 NaYF 4 :Ln 3+ @NaYF 4 :Yb 3+ (e.g. NaYF 4 :Er 3+ /Tm 3+ @NaYF 4 :Yb 3+ , NaYF 4 :Yb 3+ /Er 3+ @NaYF 4 :Yb 3+ , and NaYF 4 : Yb 3+ /Tm 3+ @NaYF 4 :Yb 3+ ) nanophosphors with enhanced near-infrared (NIR)-to-visible upconversion luminescence by coating Yb 3+ -doped NaYF 4 shell (active-shell) on NaYF 4 :Ln 3+ upconversion core nanoparticles . The structure, morphology, and elemental composition of NaYF 4 :Ln 3+ @NaYF 4 :Yb 3+ core/active-shell nanoparticles (CASNs), NaYF 4 :Ln 3+ @NaYF 4 core/inert-shell nanoparticles, and NaYF 4 :Ln 3+ core nanoparticles were characterized and compared systematically to reveal their structure-property relationship. Collective data showed that coating of a NaYF 4 :Yb 3+ active shell on NaYF 4 :Ln 3+ cores significantly enhanced upconversion luminescent intensity by ~21 times, while an inert (pure NaYF 4 ) shell coated on the surface of NaYF 4 :Ln 3+ core only resulted in 5.2 times luminescence enhancement. The enhanced upconversion luminescence intensity was mainly attributed to (i) the NaYF 4 shell-reduced quenching effect and (ii) the promoted transformation of the absorbed NIR light from the pump source to the core nanoparticles by Yb 3+ 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 NaYF 4 materials for anti-counterfeiting applications.