Radiation induced free radicals as molecular probes in synthetic apatites

Free radicals generated in synthetic apatitic calcium phosphates by X-ray radiation were investigated by electron spin resonance (ESR) spectroscopy. Among the species stable enough at -188 degrees C to be identified were hydrogen atoms, phosphate radicals, and oxygen anion radicals. The ESR spectra were markedly dependent on the specific surface of the mineral. Oxygen radicals dominated the spectra of low specific surface samples while phosphate radicals were the predominant species at higher specific surfaces. Our studies suggest that the oxygen radicals are more stable in the bulk of the crystal while the hydrogen atoms and the phosphate radicals are stabilized at or near the crystal surface. It was concluded that the surface species are potentially capable of serving as probes of biologically relevant mineral-organic interfaces.