Surface and structural characterization of Cu-exchanged hydroxyapatites and their application in H₂O₂ electrocatalytic reduction

We report on the preparation of hydroxyapatite (HA) nanoparticles with high specific surface area, terminated with calcium-rich {0 1 0} facets, and their subsequent surface functionalization with copper by cationic exchange with copper nitrate solutions at different concentrations. Elemental analysis highlighted a progressive increase of the amount of copper incorporated into the HA, reaching a maximum of ∼ 7 wt%. A combined X-ray diffraction and solid-state NMR investigation showed no significant structural differences after the Cu functionalization, confirming that the copper exchange occurs mainly at the surface until saturation and, for the higher Cu concentrations, also in the sub-surface/bulk layers of the material, without altering the HA crystal structure. The gradual substitution of surface Ca²⁺ by Cu²⁺ was studied also by IR spectroscopy using carbon monoxide as probe molecule. Finally, we assessed the catalytic activity of the materials testing the electrochemical reduction of H₂O₂ by cyclic voltammetry. We observed a progressive increase in catalytic activity correlated with the amount of Cu, suggesting the possible application of copper-exchanged HA as electrochemical H₂O₂ sensors.