Facile Epoxidation Strategy for Producing Amphiphilic Up-Converting Rare-Earth Nanophosphors as Biological Labels

The biological application of up-converting rare-earth nanophosphors (UCNPs) is still restricted because of unfavorable surface properties. Herein, the development of a simple, effective, and versatile method for converting hydrophobic UCNPs into amphiphilic ones, based on epoxidation of the surface oleic acid (OA) ligand and further coupling with polyethylene glycol monomethyl ether (mPEG-OH), is described. The successful epoxidation and effective coating of the surface of the UCNPs by mPEG-OH have been confirmed by H-1 NMR spectrometry, Fourier-transform infrared (FTIR) spectroscopy, and dynamic light scattering (DLS) studies. Investigations by transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy showed no obvious variations in the morphologies and luminescent properties of the UCNPs during the epoxidation and functionalization processes. In particular, the low cytotoxicity and good cell membrane permeability of the resulting amphiphilic mPEG-UCNPs make them promising candidates for use as bioimaging probes.