Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label

• Hydrophilic NaYF4:Yb,Er nanoparticles were prepared by a facile hydrothermal route.
• They are hydrophilic due to the presence of polyethylene glycol ligands.
• They exhibit bright upconverting luminescence and can be used as biological label.

A prerequisite for biomedical applications of rare-earth upconverting nanophosphors (RUNPs) is to prepare hydrophilic RUNPs with strong upconverting luminescence. Herein, we developed a simple one-pot synthesis strategy for hydrophilic NaYF4:Yb,Er nanophosphors by hydrothermally treating water/ethylene glycol mixture containing different rare-earth ions, BF4−, Na+ and polyethylene glycol (MW=6000, abbreviated as PEG-6000). NaYF4:Yb,Er sample consisted of irregular spherical nanoparticles with average size of ~50 nm, and it could be easily dispersed in water due to the presence of PEG-6000. Importantly, the aqueous dispersion of NaYF4:Yb,Er nanoparticles exhibited bright upconversion luminescence and could be used for imaging HeLa cells. Therefore, these hydrophilic NaYF4:Yb,Er nanoparticles have great potential as luminescent labeling materials for biological applications.