EPR studies of radicals generated by γ-radiation in nanocrystalline hydroxyapatites prepared by dry milling

► Nanocrystalline CO32--containing Ca-hydroxyapatites were prepared by dry milling. ► The materials exhibited radicals before and after γ-irradiation. ► The CO2--radicals were distributed between crystal surface and lattice B-sites. ► The strongest EPR signal was found for the crystal sizes as in the bone mineral. A set of 10 nanocrystalline carbonate-containing calcium hydroxyapatites (HAs) with various mean crystal sizes below 100nm was prepared by dry milling in air of commercial HA (platelets ca. 100nm in size). The obtained crystallites had prolate plate-like shapes as in the bone mineral. The apatites were then irradiated at room temperature by γ-rays (a 60Co γ-source, a 7kGy dose, an irradiation rate of 0.7kGy/h) and studied using electron paramagnetic resonance (EPR). Four different radiogenic radicals were observed: CO33- in the crystal lattice (in B sites formerly occupied by PO43-), CO3- and O− radicals on the crystal surface, while dosimetric CO2- radicals were distributed between both those compartments. Concerning the milling time, the radicals were accumulated and disappeared in the specific sequence, related to the crystal surface area, crystal size (crystallinity) and carbonate content. The strongest EPR signal was observed for the crystal dimensions comparable to those in the bone mineral.