Solvothermal synthesis and modification of NaYF₄:Yb/Er@NaLuF₄:Yb for enhanced up-conversion luminescence for...

Water-soluble NaYF₄:Yb/Er@NaLuF₄:Yb up-converting nanoparticles (UCNPs) with a strong green emission were successfully prepared by a solvothermal method in a short period of time and at a low temperature. First, the hydrophobic UCNPs were prepared by a simple solvothermal method, then modified using a polyetherimide (PEI) surfactant or oxidation of the oleic acid ligands with the Lemieux-von Rudloff reagent. The modified UCNPs, having an average particle diameter of 60 ± 5 nm, showed a high dispersity. The oleic acid ligand on the sample surface was oxidized azelaic acid (HOOC(CH₂)₇COOH), identified from Fourier transform infrared (FTIR) spectroscopy, which results in the generation of free carboxylic acid, hence conferring a high solubility in water. The 3-4,5-dimethylthiazol-2-yl-2,5-diphenyl tetrazolium bromide (MTT) method and cell-targeted labeling proved that oleic acid-capped UCNPs after oxidation (UCNPs-OAO) have a higher biocompatibility than polyetherimide-capped UCNPs (UCNPs-PEI). Therefore, the UCNPs-OAO have a great potential in biomedical applications, such as multimodal imaging, targeted therapy, and gene therapy.