Comparison of Two Strategies for the Synthesis of Upconverting Nanoparticles as Biological labels

Oleic acid-coated upconverting nanoparticles were prepared by hydrothermal and solvothermal synthesis. The properties of as-prepared NaYF4:Yb,Er nanoparticles were characterized, including transmission electron microscope, X-ray diffraction pattern and photoluminescence spectrum. The effects of reaction conditions (reaction time and temperature) and the concentration of rare earth dopants have been investigated in each strategy. Optical properties of upconverting nanoparticles synthesized by two strategies were compared as well. For biological application, the surface ligands were exchanged from oleic acid to hexanedioic acid with carboxyl-functionalized groups. The results indicated upconverting nanospheres (45 +/- 2 nm) and nanoplates (150 +/- 5 nm) prepared by solvothermal synthesis had higher photoluminescence intensity with uniform size distribution, whereas hydrothermal synthesis produced small upconverting nanocubes (8-20 nm). Upconverting nanoparticles in hydrothermal synthesis showed better optical properties while increasing the reaction time and decreasing the molar ratio of water. The ratio of green and red emissions of NaYF4:Yb,Er nanoparticles could be adjusted through changing the concentration of rare earth dopants. Furthermore, hexanedioic acid mediated ligand-exchange process transferred NaYF4 nanoparticles from hydrophobic to hydrophilic with great potential as biological labels.