Synthesis and characterization of upconversion nanoparticles with shell structure and ligand-free hydrophilic modification

Abstract

Ytterbium and erbium co-doped sodium yttrium fluoride (NaYF₄:Er³⁺, Yb³⁺) rare-earth upconversion nanoparticles (UCNPs) with different phases were prepared and characterized. The luminescence intensity of UCNPs can be enhanced by coating a uniform layer of the shell. The crystal structure, morphology and upconversion spectra of the sample were investigated using X-ray powder diffractometry, transmission electron microscopy, and laser upconversion spectrometry with a 980 nm diode laser. Finally, UCNPs with core–shell structure were modified to be hydrophilic by ligand-free hydrophilic modification. Moreover, the water dispersible UCNPs have much stronger luminescence compared with hydrophobic UCNPs.