Synthesis and characterisation of nanophase hydroxyapatite co-substituted with strontium and zinc

Abstract

In order to develop new bioactive calcium phosphate (CaP) materials to repair bone defects, it is important to ensure these materials more closely mimic the non-stoichiometric nature of biological hydroxyapatite (HA). Typically, biological HA combines various CaP phases with different impurity ions, which substitute within the HA lattice, including strontium (Sr2+), zinc (Zn2+), magnesium (Mg2+), carbonate (CO32-) and fluoride (F-), but to name a few. In addition to this biological HA have dimensions in the nanometre (nm) range, usually 60 nm in length by 5–20 nm wide. Both the effects of ion substitution and the nano-size crystals are seen as important factors for enhancing their potential biofunctionality. The driving hypothesis was to successfully synthesise nanoscale hydroxyapatite (nHA), co-substituted with strontium (Sr2+) and zinc (Zn2+) ions in varying concentrations using an aqueous precipitation method and to understand their chemical and physical properties. The materials were characterised using Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM) techniques. The FTIR, XRD and XPS results confirmed that the nHA was successfully co-substituted with Sr2+ and Zn2+, replacing Ca2+ within the nHA lattice at varying concentrations. The FTIR results confirmed that all of the samples were carbonated, with a significant loss of hydroxylation as a consequence of the incorporation of Sr2+ and Zn2+ into the nHA lattice. The TEM results showed that each sample produced was nano-sized, with the Sr/Zn-10%nHA having the smallest sized crystals approximately 17.6 ± 3.3 nm long and 10.2 ± 1.4 nm wide. None of the materials synthesised here in this study contained any other impurity CaP phases. Therefore, this study has shown that co-substituted nHA can be prepared, and that the degree of substitution (and the substituting ion) can have a profound effect on the attendant materials’ properties.