Synthesis, Optical Properties, and Sensing Applications of LaF3:Yb3+/Er3+/Ho3+/Tm3+ Upconversion Nanoparticles

Abstract

Herein, we successfully synthesized a series of LaF₃:Yb³⁺/Er³⁺/Ho³⁺/Tm³⁺ upconversion nanoparticles (UCNPs) and LaF₃:Yb³⁺_0.20, Er³⁺_0.02@LaF₃:Yb³⁺_0.20 core/shell UCNPs by modifying the amount of NaOH and the reaction time. Hexagonal LaF₃ nanocrystals with uniform particle sizes and bright UC emissions were obtained. The crystal structures of the lanthanide-doped LaF₃ 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 LaF₃ nanocrystals could be tuned by altering the doping ratio of Er³⁺, Ho³⁺, and Tm³⁺. 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 LaF₃ nanocrystals were obtained. After surface modification by ligand exchange, the hydrophobic LaF₃:Yb³⁺_0.20, Er³⁺_0.02@LaF₃:Yb³⁺_0.20 core/shell UCNPs became water-dispersible. These colloid UCNPs could be utilized as a fluorescent probe for the detection of Hg²⁺ ions under 980 nm near-infrared irradiation.

Keywords: LaF3; UCNP; core/shell; hexagonal; ligand exchange.