Effect of Nb⁵⁺ Doping on Structural, Electronic, and Antibacterial Properties of Hydroxyapatite

Herein, we address the limited understanding of how Nb⁵⁺ doping governs the defect chemistry and structural characteristics of hydroxyapatite (Hap) and demonstrate its use to tailor their functionalities. Niobium-doped Haps (HapNbx) were synthesized using the coprecipitation method, with Nb doping concentrations of x = 1, 3, 5, 7, and 10 mol %. The effect of dopant concentration on the structural and electronic properties of the samples was systematically investigated and correlated with their functionalities. Structural characterization and Rietveld refinements revealed that single-phase apatite-structured materials were obtained up to 7 mol % Nb, whereas phase segregation occurred at 10 mol %. Incorporation of Nb⁵⁺ into the Hap lattice was revealed by Raman, UV-vis, and XPS spectroscopic analyses and further confirmed by XANES spectroscopy at the P K edge and Nb L-edge. XPS analysis indicated the presence of this cationic species on the surface, and XANES probed the preferential coordination sites for Nb⁵⁺ cations. The antibacterial activity against Staphylococcus aureus and Escherichia coli was evaluated, and the sample containing 5 mol % Nb demonstrated a promising inhibitory effect. Our findings reveal that Nb⁵⁺ preferentially occupies the 7-coordinate Ca(II) sites in the Hap lattice, thereby modifying the structural and electronic properties of HapNbx compounds, affecting their biocidal function.