| Summary: | Herein, we successfully synthesized a series of LaF<sub>3</sub>:Yb<sup>3+</sup>/Er<sup>3+</sup>/Ho<sup>3+</sup>/Tm<sup>3+</sup> upconversion nanoparticles (UCNPs) and LaF<sub>3</sub>:Yb<sup>3+</sup><sub>0.20</sub>, Er<sup>3+</sup><sub>0.02</sub>@LaF<sub>3</sub>:Yb<sup>3+</sup><sub>0.20</sub> core/shell UCNPs by modifying the amount of NaOH and the reaction time. Hexagonal LaF<sub>3</sub> nanocrystals with uniform particle sizes and bright UC emissions were obtained. The crystal structures of the lanthanide-doped LaF<sub>3</sub> UCNPs were investigated using wide-angle X-ray diffraction. The morphologies and particle sizes of the nanocrystals were determined using transmission electron microscopy. The photoluminescence (PL) spectra of the LaF<sub>3</sub> nanocrystals could be tuned by altering the doping ratio of Er<sup>3+</sup>, Ho<sup>3+</sup>, and Tm<sup>3+</sup>. In addition, the PL intensities increased after coating the UCNP cores with an active shell. The fluorescence intensities of the UCNPs synthesized via a one-hour reaction with the addition of 2.5 or 5 mmol NaOH increased by up to 17 times compared with the sample prepared without the addition of NaOH. By modifying the doping ratio of Yb/Tm, UV-emissive LaF<sub>3</sub> nanocrystals were obtained. After surface modification by ligand exchange, the hydrophobic LaF<sub>3</sub>:Yb<sup>3+</sup><sub>0.20</sub>, Er<sup>3+</sup><sub>0.02</sub>@LaF<sub>3</sub>:Yb<sup>3+</sup><sub>0.20</sub> core/shell UCNPs became water-dispersible. These colloid UCNPs could be utilized as a fluorescent probe for the detection of Hg<sup>2+</sup> ions under 980 nm near-infrared irradiation.
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