Synthesis and optical properties of water soluble upconversion NaYF₄: Yb, Tm nanoparticles

Water soluble NaYF₄:20%Yb, 0.5%Tm upconversion nanoparticles(UCNPs) were prepared by a facile solvothermal approach using polyvinylpyrrolidone (PVP) as the surfactant. X-ray diffraction (XRD) analyses showed that the UCNPs were pure cubic phase NaYF₄ nanocrystals. The size and morphology of the UCNPs were characterized by a scanning electron microscopy (SEM). The SEM images proved that almost all the UCNPs were nanospheres with the size of ~40 nm. The UCNPs could be well dispersed in water to form a clearly transparent solution which could be kept for at least two days and there was no obvious precipitation. Pumped with 980-nm diode laser, the solution presented bright naked eye-visible violet/blue upconversion luminescence and the upconversion mechanism of the UCNPs was also given in this paper. These results showed that the UCNPs had potential applications in biological field as luminescence labeling probes.