Theranostic Applications of Nanostructured Silicate-Substituted Hydroxyapatite Codoped with Eu³⁺ and Bi³⁺ Ions—A Novel Strategy for Bone Regeneration

In this paper, nanocrystalline silicate-substituted hydroxyapatites (nSi-HAps) codoped with Eu³⁺ were functionalized with Bi³⁺ ions. Biomaterials were synthesized using a microwave-assisted hydrothermal method and heat-treated at 700 °C. The concentration of Eu³⁺ ions was established at 1 mol %, and the concentration of Bi³⁺ was in the range of 0.5-2 mol %. The physicochemical properties of the obtained biomaterials were determined using previously established methods, including X-ray powder diffraction, scanning electron microscopy techniques, and IR spectroscopy. Particle sizes obtained in this study were in the range of 22-65 nm, which was established by the Rietveld method. The luminescence properties of the Eu³⁺ ion-doped silicate-substituted apatite were recorded depending on the bismuth(III) concentration. The cytocompatibility of obtained biomaterials was tested using the model of mouse pre-osteoblasts cell line, that is, MC3T3-E1. We showed that the obtained biomaterials exerted anti-apoptotic effect, reducing the number of early and late apoptotic cells and decreasing caspase activity and reactive oxygen species accumulation. The transcripts levels of genes associated with apoptosis confirmed the anti-apoptotic effect of the biomaterials. Increased metabolic activity of MC3T3-E1 in cultures with biomaterials functionalized with Bi³⁺ ions has been observed. Moreover, the determined profile of osteogenic markers indicates that the obtained matrices, that is, Eu³⁺:nSi-HAp functionalized with Bi³⁺ ions, exert pro-osteogenic properties. The biological features of Eu³⁺:nSi-HAp modified with Bi³⁺ ions are highly desired in terms of functional tissue restoration and further efficient osteointegration.