Enhanced luminescence efficiency of aqueous dispersible NaYF4:Yb/Er nanoparticles and the effect of surface coating

In a general approach, we designed and synthesized monodisperse, well-defined, highly emissive and aqueous dispersible NaYF4:Yb/Er upconversion nanoparticles, and thereafter their surfaces were coated with inert NaYF4 and silica layers. The crystalline phase, morphology, composition and optical properties of the as-synthesized samples were well characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-Vis absorption (UV-Vis), optical band gap energy, Fourier-transform infrared spectroscopy (FT-IR) and upconversion luminescence spectra, respectively. It is found that the synthesized hexagonal phase nanoparticles have a highly crystalline spherical shape, and are monodisperse with narrow size distribution. They can easily disperse in nonpolar cyclohexane solvent to form transparent colloid solutions. The optical band gap energy clearly shows the effect of surface coating of inert inorganic and porous silica layers surrounding the surface of seed core-nanoparticles due to the increase the crystalline size. The upconversion luminescence intensity was remarkably improved after the formation of a passive NaYF4 layer due to the decrease of non-radiative rate arising from the surface/defects of the particles in the form of surface dangling bonds and capping agents. The growth of the silica shell after inert shell formation and the emission intensity of Er3+ transitions were little affected with respect to inert shell coated core/shell nanoparticles, indicating that silica has been effectively grafted surrounding the core/shell nanoparticles. Our results indicate that surface coating of inactive and silica shells is a key step in producing upconversion nanocrystals with increased brightness for a variety of upconversion luminescence bioimaging and biosensing applications.