Hafnium Oxide as a Nanoradiosensitizer under X‑ray Irradiation of Aqueous Organic Systems: A Model Study Using the Spin-Trapping Technique and Monte Carlo Simulations

The development of the physicochemical basis for applications of nanoradiosensitizers for targeted treatment of tumors is one of the crucial issues of modern radiotherapy. Ceramic nanoparticles (NPs) composed of heavy metal oxides are considered as prospective sensitizers, particularly for X-ray treatment. This study reports a novel approach for experimental simulations of the radiosensitizing effect of NPs in biomimetic systems based on the quantification of radicals produced from organic components in concentrated aqueous organic solutions using the spin-trapping technique with electron paramagnetic resonance detection. This approach was first applied to X-ray irradiation (45 kVp) of aqueous methanol solutions systems containing different concentrations of hafnium oxide nanoparticles with an average diameter of ca. 84 nm (up to 1.8 wp). It was found that the amount of radicals produced from methanol at the same exposition time increased linearly with the increasing content of HfO2 NPs. The effect can be reasonably explained by the physical enhancement mechanism associated with efficient transfer of absorbed energy from the NPs to aqueous organic medium. The Monte Carlo simulations were applied to calculate the absorbed dose in the studied systems as a function of NP concentration. The experimental enhancement factor in the formation of radicals (0.71 wp–1) was found to be slightly lower than the calculated coefficient of the absorbed dose enhancement (0.80 wp–1), which can be explained by partial self-absorption of generated secondary electrons inside rather bulky HfO2 nanoparticles. The proposed model approach may provide a rational ground for comparative studies of different nanoradiosensitizers and the optimization of the NP size, photon energy, and other factors.