Punctual investigation of surface sites of HA and magnesium-HA

IR spectroscopy of adsorbed CO is used for the first time to characterize surface cationic centres of pure and magnesium doped hydroxyapatite (HA and MHA, respectively). In air, water molecules saturate the coordination sphere of exposed surface cations by a strong coordinative interaction. At first, the materials were outgassed at 300 °C in order to remove water molecules adsorbed on the surface. Following this treatment, surface sites able to adsorb CO are produced. The vibrational frequency of adsorbed CO is sensitive to the features of the adsorbing sites. The IR spectra of CO adsorbed on HA revealed the presence of surface Ca²⁺ ions differing for the degree of coordinative unsaturation and of P OH groups. On MHA, CO was able to monitor the presence of surface Mg²⁺ ions, the amount of which increased by aging the material in air.