An investigation in the production of singlet oxygen by titanium dioxidenanoparticles under ionizing photon radiation

Abstract

Most modern cancer treatments are a combination of therapies. In approximately 50% of these combinations radiation therapy plays a prominent role. Although external radiation therapy is very effective,
it still damages healthy tissue. Therefore, ways to increase radiation dose deposition at the tumor are sought. The interaction of radiation with the photocatalytic nanoparticle titanium dioxide, TiOኼ NPs, is investigated in effort to improve the effectiveness of radiation therapy, minimizing collateral damage to healthy tissue.
TiOኼ NPs, upon the absorption of light, are capable of producing reactive oxygen species, which are the main contributor to DNA strand breakage and cell damage through oxidative reactions in radiation therapy. Singlet oxygen is the most potent of these reactive oxygen species. This research aims to
determine whether TiOኼ NPs are capable of producing singlet oxygen when interacting with ionizing radiation. Efforts are made in uncovering the mechanisms and dependencies behind the production of singlet oxygen using the fluorescent probe Singlet Oxygen Sensor Green, SOSG. The oxidative capabilities of the TiOኼ NPs in combination with ionizing radiation are looked into with the probe Mythelene blue, MB, which degrades under oxidative reactions.
The results for aerobic conditions show a significant rise in fluorescent intensity by the probe SOSG when irradiated in the presence TiOኼ NPs. The rise in fluorescence appears to be linked to the presence of singlet oxygen but this could not be confirmed. SOSG has also shown a rise in fluorescence in
anaerobic circumstances not suitable for singlet oxygen production. This signal response is attributed to the increased presence of hydrated electrons, in line with hypothesized sensitivity of SOSG towards hydrated electrons. The mechanisms behind the production of singlet oxygen could not be uncovered
however the production does favour low photon energies and does not depend on dose rate.
The results from the MB degradation experiments indicate no significant amount of degradation is caused by the photocatalytic activity of TiOኼ NPs concentrations used in this thesis. The degradation that does occur is due to radiolysis, which favours low photon energies due to the increased absorbance.
The production of singlet oxygen through the oxidation of superoxide radicals, facilitated by the interaction of ionizing photon irradiation with TiOኼ NPs, is expected but could not be verified. Future research may verify the origin of the rise in fluorescencent SOSG to be hydrated electrons or singlet oxygen.