Iron-doped hydroxyapatite by hydrothermal synthesis: Factors modulating the Fe²⁺, Fe³⁺ content

The present study focuses on tailoring the relative content of Fe³⁺ and Fe²⁺ ions incorporation into hydroxyapatite (HA) lattice, employing a hydrothermal approach in a closed vessel to minimize Fe²⁺ oxidation and secondary phase formation. Citrate molecules are used to regulate nanoparticle formation/stability, creating a mild reducing environment, while the impact of a stronger reducing agent, hydroxylamine, is explored. Fe³⁺ insertion was found to be less favoured than Fe²⁺, possibly due to charge imbalance. Iron doping significantly alters stoichiometry and crystallinity of HA, with Fe³⁺ enhancing OH⁻ depletion. Morphological analysis reveals differences among samples, as induced by the different Fe ions incorporation: particularly Fe²⁺ ion incorporation is found to maintain rod-like structures, which changes upon Fe³⁺ presence. Overall, this study provides insights into controlled doping of HA with iron ions, vital for developing stable, redox-responsive nanomaterials applicable in cancer therapy and other applications where surface activity plays a relevant role.