One-step synthesis and characterization of water-soluble NaYF4:Yb,Er/Polymer nanoparticles with efficient up-conversion fluorescence

Up-conversion nanoparticles (UCNPs), which can convert a longer wavelength radiation to a shorter wavelength radiation, have great potential use as bio-labels in biological detection. However, these NPs usually cannot be used directly unless their surface is further modified. Herein, we developed a novel and straightforward method for the synthesis of monodisperse NaYF4:Yb,Er/Polymer UCNPs. The resultant UCNPs exhibited a pure hexagonal phase, strong UC fluorescent intensity, and a uniform size of about 40 nm, which were characterized by XRD, luminescence spectroscopy and TEM, respectively. FT-IR spectra revealed that these NPs were coated with a layer of hydrophilic polymers (including PVP, PEG, PAA and PEI), thus the NPs could be dispersed in water easily. More importantly, the PAA and PEI coated NPs were functionalized by –COOH and –NH2 groups, respectively, which were expected to be used for further bio-conjugation.