A simple and efficient synthetic route for preparation of NaYF4 upconversion nanoparticles by thermo-decomposition of rare-earth oleates

Hexagonal-phase NaYF4 nanocrystals with good uniformity and monodispersity have been successfully obtained through a thermal decomposition of rare-earth oleate complexes. By co-doping upconverters (Yb/Er, Yb/Tm or Yb/Ho) or downconverters (Eu or Ce/Tb), multicolor upconversion (UC) luminescence under 980 nm laser excitation or downconversion luminescence under UV irradiation could be obtained. For the first time, we systematically investigate the effects of various parameters including reaction temperature, time, NaF to rare-earth ions ratio and oleic acid concentration on the size, morphology, phase purity and UC emission properties using Yb/Er co-doped NaYF4 UC nanoparticles as a typical example. The results demonstrated that this strategy is a simple yet efficient route for fabrication of UCNPs with good uniformity and monodispersity, and enriches the synthetic routes for production of high quality hexagonal-phase NaYF4 nanocrystals. In addition, a mesoporous silica layer was coated onto the hydrophobic NaYF4:Yb/Er nanoparticles, converting them into hydrophilic ones, which then could be used as a potential luminescent probe for cell imaging and a promising nanocarrier for therapeutic drug delivery, making them a multifunctional platform for simultaneous imaging and therapy.