Controlled synthesis and upconversion luminescence properties of highly uniform and monodisperse β-NaYF₄:Yb³⁺,Tm³⁺ nanocrystals

Ytterbium, thulium (Yb³⁺,Tm³⁺) co-doped β-NaYF₄ upconversion nanocrystals (U-NCs) were synthesized by a convenient hydrothermal method using sodium citrate as a capping agent. X-ray diffraction (XRD) analysis revealed that NCs phase structures were dependent on reaction time. Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) results showed that the morphology of U-NCs could be controlled by adjusting the concentration of sodium citrate added in the hydrothermal reaction. Moreover, the emission spectra of the U-NCs were investigated to evaluate upconversion efficiency. Strong luminescence intensity of the U-NCs was observed after tuned with optimized concentration of sodium citrate. Furthermore, the U-NCs were incubated with MCF-7 cells at 37°C for 24 h. Under irradiation of a 980 nm laser, the upconversion blue emission of the NCs in MCF-7 cells can be clearly observed through a confocal microscope with an upconversion imaging system and high quality upconversion luminescence images can be acquired. Thus, prepared β-NaYF₄:Yb³⁺,Tm³⁺ NCs provide us with highly efficient luminescent probes which can be applied for diverse bio-applications.