Facile synthesis of β-NaGdF4:Yb/Er@CaF2 nanoparticles with enhanced upconversion fluorescence and stability via a sequential growth process

Upconversion fluorescent nanostructures with controlled sizes and morphologies have attracted much attention due to their potential and important applications in bioimaging, photodynamic therapy, solar cells and so on. In this work, we describe a facile successive process to fabricate β-NaGdF4:Yb,Er nanocrystals coated with an ultrathin layer of CaF2 with enhanced upconversion fluorescence and stability. The as-prepared β-NaGdF4:Yb,Er@CaF2 nanoparticles were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM, JEM 21000), fluorescence spectroscopy (Hitachi High-Technology Corporation, F-2700) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The as-prepared core–shell nanoparticles show enhanced upconversion fluorescence and superior magnetic resonance (MR) relaxivity. In addition, the as-prepared β-NaGdF4:Yb,Er@CaF2 nanoparticles show enhanced chemical stability after surface functionalization and transfer into aqueous solutions.