Optical nanothermometer of CaF2:Yb3+/Er3+nanocrystals under excitation at the minimum of the NIR-II biological window

Nanoparticles (NPs) for real-time temperature measurements in biological applications with minimum pumpinduced heating effects and minimally invasive are still a hot topic of research in recent years. Therefore, in this work, calcium fluoride (CaF2) NPs doped with Yb3+ and Er3+ have been prepared via the co-precipitation method whose goal is its potential use as luminescence nanothermometers in biological systems. The Yb3+ ions concentration was fixed at 10 mol% and four Er3+ ions concentrations were explored (0.5, 1, 2, and 4 mol %). As a result, the concentration-quenching effect was noticed after 1.0 mol% and the possible reasons were discussed. The potential use of the CaF2:Yb3+/Er3+ NPs as nanothermometers was explored by monitoring the variation of the emission intensity at 660 nm with the temperature. In addition, the effect of the excitation wavelength (980, 1064, and 1470 nm) was comparatively investigated. Particularly, the results indicated that the excitation at 1064 nm is the more indicated to avoid light-heat conversion and consequently tissue damage. Furthermore, a linear positive dependence of the emission at 660 nm with temperature and a thermal sensitivity close to 2.53% degrees C- 1 at 26 degrees C were obtained. Experiments with biological tissues were also performed and the results showed that these NPs make up a promising system for evaluating temperature variations in biological applications.