Facile synthesis of 5 nm NaYF4:Yb/Er nanoparticles for targeted upconversion imaging of cancer cells

Abstract

5nm intense green emission NaYF4:Yb/Er upconversion nanoparticles (UCNPs) with pure β phase was synthesized with a simple “green” strategy for the first time. Traditional organic solvothermal method is often applied to prepare the high-quality and uniform UCNPs, but the preparation of lanthanide-oleate complexes is laborious as heating and multistep post-treatment for purification are often required. The water-alcohols solvothermal method is environmentally friendly, but the fabricated UCNPs have big size, poor biocompatibility and high cytotoxicity, which limited their application for cell imaging. Herein, NaYF4:Yb/Er UCNPs were prepared with rare-earth nitrates RE(NO3)3 (RE=Y0.80 Yb0.18 Er0.02) as precursors and diethylene glycol (DEG)/ethylene glycol (EG)/water as the solvent. A facile green solvothermal method with the temperature being controlled at 300°C was developed. The as-prepared NaYF4:Yb/Er UCNPs were characterized and were found to have enhanced UC emission and controllable particle size. The as-prepared UCNPs were further functionalized via folic acid coating for the targeted imaging and improved bio- compatibility. It was made the UCNPs potential for upconversion bioimaging of living cells by the strong upconversion luminescence, the excellent biocompatibility, and the super-small size. The good colloidal stability and low cell cytotoxicity of the as-prepared UCNPs and the developed synthesis protocol might advance both the fields of UCNPs and biomolecule-based nanotechnology for future studies.