Improvement of dispersion stability and characterization of upconversion nanophosphors covalently modified with PEG as a fluorescence bioimaging probe

Upconverting (UC) phosphors (UCPs) are ceramic materials doped with rare earth ions. These materials can absorb and upconvert infrared (IR) radiation to emit visible light by the stepwise excitation among discrete energy levels of the rare earth ions. UCPs are potentially useful reagents for use in bioimaging since the use of low energy photons avoids photo-toxicity. The use of UCP nanoparticles as bioimaging probes requires surface modifications in an effort to improve dispersion stability in aqueous milieu. In this study, we covalently attached poly(ethylene glycol) (PEG) to the surface of Er-doped Y2O3 nanoparticles and firstly demonstrated that PEG covalently bound to the Y2O3 surface markedly improved dispersion stability in water. UC emission of PEG-modified Er-Y2O3 nanoparticles excited with IR light was successfully observed. We also showed that PEG-modified Er-Y2O3 nanoparticles exhibit no cell-toxicity. These observations lend strong support to the potential use of PEG-modified UCP nanoparticles as bioimaging tools.