Effect of SnO2 nanoparticles on the formation of radicals under the X-ray irradiation of aqueous organic system: an evidence for new mechanism of chemical enhancement

Abstract

The increasing rate of radical production under X-ray irradiation of aqueous-organic systems in the presence of metal and oxide nanoparticles (NPs) attracts considerable attention in view of various potential applications. This phenomenon is usually rationalized in terms of physical and chemical enhancement (PE and CE, respectively). In the present study using spin trapping technique, it was shown that the rate of radical production in a model oxygen-free aqueous organic system (water + methanol) irradiated with X-rays (45 kVp) increases by the factor of (1.45 ± 0.11) in the presence of 0.13 wp SnO₂ nanoparticles (NPs) with an average diameter of 5–6 nm (as compared to the reference samples without NPs). The effect becomes negligible for the larger NPs. In this case the role of PE is definitely minor, because the absorbed dose in the presence of NPs increases only by the factor of 1.02 as shown by the Monte-Carlo simulations. Based on kinetic consideration and observed size effect, the result was explained by a specific kind of CE mechanism, which implies reduction of SnO₂ at the NP surface yielding (SnO₂)_n^•‒ followed by generation of additional ^•H atoms eventually reacting with methanol. The proposed mechanism is important for better understanding of the role of oxide NPs in the radical reactions occurring under X-ray irradiation of aqueous organic media and biological systems.