Water-soluble NaYF4:Yb/Er upconversion nanophosphors: Synthesis, characteristics and application in bioimaging

Water-soluble and citrate (cit) coated rare-earth upconversion nanophosphors (UCNPs) were prepared by an efficient surface ligand exchange of the oleic acid capped hydrophobic UCNPs in diethylene glycol (DEG) at high temperature. The successful surface ligand exchange of the UCNPs with citrate was confirmed by Fourier-transform infrared (FTIR) spectroscopy. Further characterizations such as TEM and XRD showed that the ligand exchange reaction had no obvious influence on chemical properties of UCNPs, such as morphology and crystallization. Furthermore, bright UC luminescence was observed when UCNPs-labeled cells were excited with 980nm near-infrared (NIR) light proving that Cit-UCNPs can be used as non-specific labels for imaging HeLa cells. These results indicate that the Cit-UCNPs are a promising candidate for use as bioimaging probes.