S.S. Ishchenko

13C hyperfine interactions of CO2- in irradiated tooth enamel as studied by EPR.

The EPR spectrum of tooth enamel caused by 13C hyperfine interactions of the CO2- radical were studied on gamma-irradiated powdered samples annealed for 40 min at different temperatures up to 250 degrees C. The lineshape and hyperfine splitting of the spectra were found to depend on the annealing temperature. Experimental spectra were compared with calculated ones assuming that EPR spectra are formed by two CO2- species--axial (rotating) and orthorhombic (braked) radicals. We assumed that the axial CO2- radicals are centers located in perfect areas of the hydroxyapatite crystals of tooth enamel whereas the orthorhombic CO2- radicals are rotating centers which are braked by defects. The thermal treatment of enamel samples leads to defective annealing and transformation of the orthorhombic centers into axial ones. This results in an increasing axial CO2- radical contribution to the EPR spectrum with increase of annealing temperature.

NO 3 2− and CO 2 − centers in synthetic hydroxyapatite: Features of the formation under γ- and UV-irradiations

The EPR studies of synthetic hydroxyapatite containing carbonate and nitrate ions exposed to γ-ray and UV irradiations have been performed. It has been found that γ irradiation leads to the formation of both NO32− and CO2− paramagnetic centers, while the UV irradiation induces only NO32− centers. To explain this fact, the hypothesis has been proposed, according to which in the hydroxyapatites studied, there coexist complexes consisting of nitrate ions and shallow electron traps that serve as sources of secondary electrons during UV irradiation. The EPR spectroscopy parameters (g and A) of the detected centers have been determined and compared with similar centers in hydroxyapatite with a different impurity composition. The study of the thermal stability of the centers has demonstrated that, in the temperature range 20–300°C, the NO32− centers formed by UV irradiation are more stable than the same centers created by γ-ray irradiation.

NO32- centers in synthetic hydroxyapatite

The structure and properties of NO32− paramagnetic complexes formed by γ-rays and UV irradiation in synthetic hydroxyapatite have been investigated using EPR spectroscopy. The formation of NO32− centers under UV irradiation has been revealed for the first time. It has been found that NO32− complexes in synthetic hydroxyapatite form two types of paramagnetic centers. Models of the centers have been proposed, their EPR spectroscopy parameters have been determined, and the influence of thermal annealing has been investigated. As the microwave power increases, the inversion of the EPR spectra has been observed.

Structure of Biocompatible Coatings Produced from Hydroxyapatite Nanoparticles by Detonation Spraying.

Detonation-produced hydroxyapatite coatings were studied by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), Raman spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy. The source material for detonation spraying was a B-type carbonated hydroxyapatite powder. The coatings consisted of tetracalcium phosphate and apatite. The ratio depended slightly on the degree of crystallinity of the initial powder and processing parameters of the coating preparation. The tetracalcium phosphate phase was homogeneous; the apatite phase contained defects localized on the sixfold axis and consisted of hydroxyapatite and oxyapatite. Technological factors contributing to the transformation of hydroxyapatite powder structure during coating formation by detonation spraying are discussed.

Radiation-induced defects in annealed carbonate-containing hydroxyapatite

The effect of preliminary (before irradiation) annealing of synthetic carbonate-containing hydroxyapatite powders on the formation of paramagnetic centers under γ-ray and ultraviolet irradiation has been investigated. Annealing of the samples has been performed in the temperature range from 100 to 700°C. It has been found that electron paramagnetic resonance spectra of radiation-induced defects depend substantially on the annealing temperature. The paramagnetic centers CO2− dominate in the samples annealed to 250°C (γ-ray irradiation) and 500°C (ultraviolet irradiation). In the samples annealed above 400°C, other defects, in particular, the O− and CO33− centers, play a significant role. Annealing at some temperatures leads to an increase in the radiation sensitivity of the material. The observed effects can be associated with the escape of molecular water from the annealed hydroxyapatite samples and with the corresponding transformation of the defect subsystem of the material.

Spatial distribution of radiation defects in tooth enamel

The spatial distribution of radiation defects in tooth enamel has been investigated using EPR imaging. Plates of enamel irradiated with γ rays and electrons with energy 1.2 and 3.8 MeV have been studied. A falloff of the radiation-defect concentration in the direction in which the radiation acts is detected in the electron-irradiated plates, with the slope of the falloff decreasing with increasing electron energy. The defect distribution was uniform in the γ-irradiated plates. It is shown that the study of tooth enamel by means of EPR imaging can be used to determine the type and energy characteristics of the ionizing radiation that acts on a living organism.

Synthesis, Characterization and EPR Investigation of γ-Induced Defects of Nanoparticles of (MI, CO3)-Containing Apatites (MI – Na, K)

The nanoparticles of (Na, CO3)- and (K, CO3)-containing Calcium phosphates have been prepared by a wet precipitation method in aqueous media. The influence of nature of alkaline metal and of samples composition on particles size and peculiars of their thermal transformation have been studied using such physical-chemical methods as FTIR, XRD, SEM and TPM MS analysis. Chemical composition of powders have been determined by ICP analysis and the degree of substitution of Calcium atoms by alkaline metals depends on CO32-/PO43- molar ratio in initial solutions. The differences in the formation of γ-induced defects in (Na, CO3)- and (K, CO3)-containing apatites have been found using EPR spectroscopy.