Rare earth doped lanthanum fluoride nanocrystals (LaF₃:5% Yb, 2% Er) with ultra-small particle size and uniform shape were successfully synthesized via a solvothermal method in octadecylene. In this synthetic system, oleic acid and octadecylene was used as surfactant and solvent, respectively. After the surfactant oleic acid, the precursor of rare earth ions and HF-oleylamine solution were added in octadecylene orderly, the mixture solution was then heated to 310 ℃ and maintained at this temperature for 1 h under a protective nitrogen flow throughout. The morphology, crystal structures and average size of the LaF₃: 5% Yb, 2% Er nanocrystals were characterized by transmission electron microscope (TEM), powder X-ray diffraction (XRD), and dynamic light scattering (DLS). The XRD patterns show a representative refiection of hexagonal phase LaF₃ nanocrystals, which was also confirmed by high-resolution TEM (HRTEM) analysis. Based on over 100 nanoparticles from the TEM image, the average size of the monodispersible nanoparticles was 6 nm with a standard deviation of 0.4 nm. Under the irradiation of 980-nm diode laser, these small nanocrystals emit strong green upconversion luminescence. The green emissions at 524 and 544 nm can be assigned to the energy level transition of Er³⁺ ions from ²H_11/2 to ⁴I_15/2 and ⁴S_3/2 to ⁴I_15/2, respectively. Another obvious emission bands at 655 nm is the result of transition from ⁴F_9/2 to ⁴I_15/2 (red) of Er³⁺ ions. Before the biomedical applications as biomarkers, these hydrophobic nanocrystals were successfully functionalized with poly(amino acid) to render them water-dispersible, bioconjugatable and biocompatible by means of a novel and facile encapsulation strategy. The TEM imaging shows the hydrophobic nanocrystals were successfully transferred into water with one particle per micelle without aggregation and the DLS characterization show the average hydrate particle size was 14.1 nm. The above results indicate that these water soluble materials were well monodispersible. Due to their ultrasmall particle size and novel upconversion luminescence, these nanocrystals were bioconjugated with folic acid and successfully utilized as biomarkers for targeted cancer cell imaging.

Key words: lanthanum fluoride, upconversion luminescence, nanocrystals, cell imaging