Stabilization of Radicals by Doping from Aqueous Solutions into Crystals of Hydroxyapatite

Unstable superoxide, · O 2 - , and semiquinone can be stabilized when hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 is crystallized from aqueous solutions containing these radicals. The inorganic microcrystals surround the radicals and stabilize them for nearly a year at room temperature. Radicals adsorbed on the surface of apatite showed spectra different from those obtained by doping. Semiquinone radicals in apatite were almost stable up to about 600°C. Effects of additional Fe 3+ and F - impurities on the electron spin resonance (ESR) spectrum of superoxide-doped apatite were studied to determine the site of the radicals in the lattice. Superoxide may be substituted at the OH - site.