Up-conversion emission and in vitro cytotoxicity characterization of blue emitting, biocompatible SrTiO3 nanoparticles activated with Tm3+ and Yb3+ ions

Abstract

Functional SrTiO₃ nanoparticles activated with a broad concentration range of Tm³⁺ and Yb³⁺ ions were obtained utilizing the citric route. The effect of the sintering temperature and optically active co-dopant concentration on the structural and up-conversion emission properties was studied by using XRD (X-ray powder diffraction) and spectroscopic techniques (emission, power dependence, emission kinetics). The particle size was verified by the TEM technique being in the range of 20–90 nm depending on the annealing temperature. It was shown that cross-relaxations contribute to depopulation of both ¹G₄ and ³H₄ levels, whereas some of them populate the ³H₄ level. These processes are strongly dependent on the concentration of Tm³⁺ and are responsible for the specific interplay between blue and NIR emissions. Extraordinary short decays were recorded due to the relatively high concentrations of rare earths, structural features of the host matrix, extended surface area and contribution of the non-radiative processes. The cytotoxic activity of the nanoparticles to J774.E murine macrophages and U2OS human osteosarcoma cells was assessed showing that SrTiO₃:Tm³⁺/Yb³⁺ nanoparticles are biocompatible and can be potentially used in further bio-related applications such as active implant layers or injectable medical cement ingredients.