Thermal sensitivity of the Nd3+/Yb3+ co-doped β-BaB2O4 nanoparticles for luminescence nanothermometry

Abstract

In this work, the synthesis and characterization of the β-BaB2O4 (β-BBO) phase single-doped with Nd3+ and co-doped with Nd3+/Yb3+ were performed. This work focuses on luminescence nanothermometry application in the near-infrared (NIR) window and the temperature range between 25 and 55 °C to evaluate potential technological applications in nanomedicine. The polymeric precursor method was used for the synthesis of Nd3+ doped and Nd3+/Yb3+ co-doped β-BBO nanoparticles (0.2 mol% of each doping ion) crystallized after a heat-treatment. The structural and thermal properties of these nanoparticles were characterized using thermal analysis (Thermogravimetry and Differential Scanning Calorimetry) to specify the temperature of heat treatment (20 h at 750 °C) for the formation of pure crystalline phases characterized by X-ray diffraction and Raman spectroscopy. Transmission Electron Microscopy was used to assess the shape (parallelepiped needles) of the particles, their length (∼214 nm) and average width (∼18 nm), as well as the orientation of the atomic planes of the crystalline planes by electron diffraction. Diffuse reflectance spectroscopy allowed to specify the transparency range of the pure crystalline matrix and to investigate the absorption bands of the doping ions, as well as to determine the bandgap using the Kubelka-Munk equation. In addition, the temperature dependence of the luminescence intensity ratios of Stark components from both Nd3+ and Yb3+ ions were analyzed. This led to a relative thermal sensitivity value of 0.06%·oC−1 for single-doped Nd3+ sample and 0.23%·oC−1 for Nd3+/Yb3+ co-doped sample from emissions in the range 1050.5 nm–1067.5 nm. Finally, a value of 0.28%·oC−1 was found for Nd3+/Yb3+ co-doped sample from the intensity ratio between emissions at 979.0 nm and 1058.5 nm. Thus, the relative thermal sensitivity of the β-BBO phase co-doped with the ions Nd3+ and Yb3+ at the concentration of 0.2 mol.% was superior to that of some matrices found in the literature. This allows us to conclude that Nd3+/Yb3+ co-doped β–BBO nanoparticles can be used as an efficient luminescent nanothermometer in the near-infrared.