Improving Upconversion Luminescence by Coating Active Shell: A Case Study on NaYF₄:Gd, Yb, Er@NaYF₄:Yb, Pr.

Upconversion nanocrystals (UCNCs) have a lot of advantages over other fluorescent materials. However, their applications are still limited due to the relatively low upconversion luminescence (UCL). In the present study, a novel core/shell structural upconversion nanocrystal NaYF₄:Gd, Yb, Er@NaYF₄:Yb, Pr was prepared successfully with a facilely solvothermal method and the properties of the nanocrystal were investigated. It was interesting to find that the as-prepared nano-crystal showed excellent UCL. Its UCL intensity was 65, 44, and 20 times higher than those of core nanocrystal NaYF₄:Gd, Yb, Er, core/inert-shell nanocrystal NaYF₄:Gd, Yb, Er@NaYF₄, and core/active-shell nanocrystal NaYF₄:Gd, Yb, Er@NaYF₄:Yb, respectively. Moreover, the measured UCL lifetime of the as-prepared nanocrystal was longer than those of the controls. The mechanism of the UCL enhancement of the product was discussed in detail. The high UCL of the as-prepared product could be ascribed to the function of the shell, energy transfer from the active shell to the core, and Pr³⁺-doping in the shell. This study provided a new insight into the fabrication of core– shell structural upconversion nanocrystals with high UCL. The high UCL potentially increases the overall upconversion nanocrystal detectability for highly sensitive biological, medical, and optical detections.