Properties of HO₂^•radicals induced by γ-ray irradiation in silica nanoparticles

We report an experimental investigation on the effects of γ-ray irradiation in several types of silica nanoparticles previously loaded with O₂molecules. They differ in specific surface and average diameter. By electron paramagnetic resonance (EPR) measurements we observe the generation of about 10¹⁸HO₂^•/cm³interstitial radicals. These radicals are induced by reaction of interstitial O₂molecules with radiolytic H atoms, as previously suggested for O₂-loaded bulk a-SiO₂samples. However, at variance with respect to bulk materials, our experimental evidences suggest a different generation process of HO₂^•radical. In fact, by a detailed study of samples exposed to D₂O, our results prove that radiolytic hydrogen atoms reacting with O₂to produce HO₂^•mainly arise from a radiation induced breaking of H₂O molecules in the layers surrounding the nanoparticles or in the interstices. Also, by the correlation of HO₂^•paramagnetic centers concentration, determined by EPR measurements, and O₂Raman/PL signal we further considered the issue of the direct estimation of the O₂concentration in silica nanoparticles from Raman/PL spectra giving an independent conversion factor (the ratio between these latter two quantities), which is in good agreement with those previously proposed by other authors basing on optical measurements.