Emission colour changes in the CaF2 sub-microspheres doped with Yb3+, Er3+ and Mn2+ ions
A series of CaF2-doped phosphors was synthesized in situ in hydrothermal conditions (in a DAB- 2 reactor), in the presence of sodium citrate as a complexing agent preventing premature precipitation. The materials were doped with Yb3+/Er3+ pair of ions, which resulted in up-conversion emission. Additionally, Mn2+ ions were incorporated into the structure of the CaF2 to analyse their influence on the chromaticity of the luminescence colour, in view of their application in bioimaging. Detailed structural analysis showed that the structure of the nanoparticles obtained resembled the core@shell like structure. Finally, the emission colour was tuned from green to orange – red. It was confirmed that the quantity of Mn2+ ions incorporated in the CaF2 matrix definitely differs from the expected one. The real concentration of Mn2+ ions in the structure is frequently ignored, which leads to incorrect conclusions. For this reason, a detailed structural and spectroscopic analysis was conducted and the effect of Mn2+ ions on the structure and the average size of the CaF2-doped phosphors was studied. Moreover, the in vitro cytotoxicity of the nanoparticles was evaluated. Obtained results showed that CaF2:15%Yb3+, 2%Er3+, 2.6%Mn2+ reduces cell viability of fibroblast at lower concentrations (50, 100 and 200 μg/ml) than CaF2:15%Yb3+, 2%Er3+ (200 μg/ml). Moreover, CaF2:15%Yb3+, 2%Er3+ caused increase of the fibroblast proliferation rate at the lowest tested concentration (10 μg/ml).