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

Abstract A set of 10 nanocrystalline carbonate-containing calcium hydroxyapatites (HAs) with various mean crystal sizes below 100 nm was prepared by dry milling in air of commercial HA (platelets ca. 100 nm 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 60 Co γ-source, a 7 kGy dose, an irradiation rate of 0.7 kGy/h) and studied using electron paramagnetic resonance (EPR). Four different radiogenic radicals were observed: CO 3 3 - in the crystal lattice (in B sites formerly occupied by PO 4 3 - ), CO 3 - and O − radicals on the crystal surface, while dosimetric CO 2 - 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.