Synthesis of NaYF₄:Yb³⁺, Er³⁺ upconversion nanoparticles using hydrogel networks as a template

Hydrogel network structures were used as nano-sized reactors for synthesizing NaYF₄:Yb³⁺, Er³⁺ upconversion nanoparticles with controlled size. By adjusting the crosslinking density, hydrogel networks with different sizes were prepared and used as reactor to control the size of synthesized nanoparticles. The particle size and upconversion luminescence property of the nanoparticles were characterized using XRD, TEM and photoluminescence (PL) spectra. The results show that the size of synthesized NaYF₄:Yb³⁺, Er³⁺ nanoparticles is around 10 nm. The diameter and the luminescence intensity decrease with increase of hydrogel crosslinking density. The increased crosslinking density reduces Ostwald ripening and increases pore pressure, which limit the growth of nanoparticles. The decreased particle size results in broken bonds on the particle surface and particle scattering. Besides, the hydrogel may also quench the fluorescence of the nanoparticles. This research provides a simple and environmentally friendly method for synthesizing uniform upconversion nanoparticles.