EPR Properties of Carbonated Hydroxyapatite Synthesized through Calcium Phosphate Cements
Objectives: To investigate the reproducibility and stability of radiation-induced Electron Paramagnetic Resonance (EPR) properties of carbonated hydroxyapatite (CHA) cements and assess the feasibility of using these materials as EPR calibration references and, potentially, as dosimeters.
Methods: CHA discs (3.5 mm D × 3 mm H) were prepared by mixing a cement powder (tetracalcium phosphate, dicalcium phosphate anhydrous, and sodium bicarbonate) with a cement liquid (0.5 mol/L disodium hydrogen phosphate) at a powder-to-liquid ratio of 5:2. The hardened discs were analyzed for phase composition (X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR)) and for carbonate and sodium contents. The CHA discs were gamma-ray irradiated at doses in the range of 10-3-102 kGy. The EPR spectra of the irradiated discs were collected on a Bruker ELEXSYS E500 EPR spectrometer.
Results: XRD results showed that all samples consisted only of a low-crystalline apatite. The discs contained 0.92% sodium and 1.86% carbonate. FTIR analyses revealed two peaks at 1415 cm-1 and 1454 cm-1, indicating that B-type CHA was formed in all cases. EPR spectra showed characteristic axial peaks of CO-2 free radicals (g⊥=1.9976, g‖=2.0030). Peak-to-peak intensities of CHA discs that received 10 kGy irradiation were highly reproducible (37.80±0.89; n=3) and indicated the identical and homogeneous composition of all specimens. Peak-to-peak intensities immediately after irradiation and at various time points post-irradiation were not statistically different (p>0.05), indicating no detectable deterioration of the signals following irradiation. The CHA exhibited EPR signals proportional to the irradiation dose.
Conclusions: Irradiated CHA cement samples exhibited distinct, reproducible and stable EPR signals. These materials have the potential for use as EPR calibration references and as bio-dosimeters.