Synthesis of NaLuF4-based nanocrystals and large enhancement of upconversion luminescence of NaLuF4:Gd, Yb, Er by coating an active shell for bioimaging

Abstract

A series of NaLuF₄-based hexagonal phase upconversion nanocrystals (UCNs) were synthesized by a facile solvothermal method and the properties of the UCNs were investigated. The results show that the as-prepared nanocrystals exhibit pure hexagonal lattice structures, uniform morphologies, high monodispersities and excellent upconversion luminescence. The upconversion luminescence (UCL) intensities of the UCNs can be enhanced by coating with a shell of NaLuF₄. More interestingly, the UCL intensities of active-shell coated nanocrystals (NaLuF₄:Gd, Yb, Er@NaLuF₄:Yb, Ho and NaLuF₄:Gd, Yb, Er@NaLuF₄:Yb) are remarkably higher than that of inert-shell coated nanocrystals (NaLuF₄:Gd, Yb, Er@NaLuF₄), and NaLuF₄:Gd, Yb, Er@NaLuF₄:Yb, Ho is higher than NaLuF₄:Gd, Yb, Er@NaLuF₄:Yb. The mechanisms of upconversion luminescence enhancement are discussed in detail. The bioimaging application of the nanocrystals showed that bright upconversion luminescence was observed when UCNs-labeled HeLa cells were excited with 980 nm light. This study presents a facile method for the synthesis of NaLuF₄-based upconversion nanocrystals with intense luminescence that can be used as potential fluorescent probes for sensitive bioimaging, and the suggested mechanism could provide new insights into fabrication of upconversion materials with high upconversion fluorescence.