Lanthanide-doped
upconversion nanoparticles with a suitable surface coating are appealing
for biomedical applications. Because high-quality upconversion nanoparticles
are typically prepared in an organic solvent and passivated by hydrophobic
oleate ligands, a convenient and reliable method for the surface modification
of upconversion nanoparticles is thus highly desired to satisfy downstream
biological investigations. In this work, we describe a facile and
versatile strategy for displacing native oleate ligands on upconversion
nanoparticles with a diversity of hydrophilic molecules. The ligand-exchange
procedure involves the removal of original oleate ligands followed
by the attachment of new ligands in a separate step. The successful
coating of relevant ligands was confirmed by Fourier transform infrared
spectroscopy, thermogravimetry analysis, and ζ-potential measurement.
The surface-modified nanoparticles display high stability and good
biocompatibility, as revealed by electron microscopy, photoluminescence
spectroscopy, and cytotoxicity assessment. Our study demonstrates
that functional biomolecules such as biotin can be directly immobilized
on the nanoparticle surface using this approach for the quick and
effective detection of streptavidin.