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Whilst nanotechnology and the use of nanomaterials in research and industry are growing rapidly, the understanding of interactions between nanomaterials and biological systems is still in its infancy. Nanomaterials of high interest to scientists are nanoparticles due to their high surface-to-volume ratio and hence increased surface reactivity. When in contact with biological matter, nanoparticles are ingested by cells and tissue, accumulate and travel within the living system. This is why nanoparticles not only attract great interest in medicine but also create great concern amongst toxicologists. The in vitro and in vivo mechanisms and pathways of nanoparticles are subject to intense research and exposure to nanoparticles may pose a serious public health risk. Although radiophysical and radiobiological effects of ionising radiation are well understood, research into interactions between electromagnetic radiation and nanoparticles has been sparse. Whilst medicine articulates strong interests in nanoparticles as radiosensitisers, understanding biophysical interactions of nanoparticles and radiation still demands further research. This thesis aims to address fundamental interaction mechanisms between nanoparticles and radiation and to establish the biological impact of photoelectron production as a consequence of nanoparticle irradiation. Our data confirms that X-ray irradiation of gold nanoparticles produces secondary electrons. However, secondary electron production had no perceptible biological effects in terms of cell survival or DNA damage. The absence of DNA damage can be explained by the fact that nanoparticles accumulate in lysosomal compartments and electrons generated there are unable to reach the nuclear target. Free electron production entails the generation of reactive oxygen species, a key component when assessing nanoparticle toxicity. This study confirms that an increase in reactive oxygen species due to nanoparticle irradiation is particle size dependent and detectable at the chemical level. However, in vitro experiments show that such an increase in reactive oxygen species due to particle size is irrelevant. This is likely to be caused by nanoparticle agglomeration in cytoplasmic vesicles. The agglomerated particles act as a cluster rather than individual particles leading to increased reabsorption of secondary electrons by nearby particles. In conclusion, this work confirms the size and energy dependent production of secondary electrons by nanoparticles at the physical and chemical level. In a biological context, however, and with the parameters used in this study, secondary electron production seems to play a minor role with little impact for therapeutic applications or potential carcinogenic effects of background radiation.

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