Cerium oxide nanoparticles: A study of Ce3+ ion influence on radioprotection

In 2011 Celardo et al. [1] demonstrated that the antioxidant capacity of cerium oxide nanoparticles (CeOx NPs) was dependent on the population of Ce3+ ions within the nanoparticle. In the cellular environment the nanoparticles were able to quench free radicals through cyclic redox reactions of the Ce3+ ions. The consequence being the larger the Ce3+ ion population of included NPs the larger the resultant cell growth of the assayed cell line. The goal of this thesis was to reproduce these findings and determine whether this dependency also translated to radiation protection efficacy under megavoltage (MV) x-ray radiation. Two-thirds of cell death inflicted by MV x-rays is achieved through the generation of free radical species. It therefore stands that controlling the population of Ce3+ ions within CeOx NPs should control their ability to protect against radiation damage. Cerium oxide nanocrystalline particles were synthesised by spray pyrolysis. To modulate the population of Ce3+ ions the NPs were doped with iron (Fe3+). 5 %, 10 % and 20 % atomic concentrations variants were synthesised. The nanoparticles were characterised using x-ray diffraction (XRD), optical emission spectroscopy (OES), transmission electron microscope (TEM) and x-ray photoelectron spectroscopy (XPS). XRD showed all crystallites to be of a single phase and built of a cubic fluorite structure consistent with the formation of cerium oxide. Crystallites were all within the range of 5-10 nm. A reducing lattice parameter with increased iron doping confirmed the inclusion of the Fe3+ ions within the cerium oxide lattice. Rietveld refinement determine the minimum lattice parameter to be 5.416 Å suggesting inclusion of Ce3+ ions for all particles. Actual Fe3+ doping was found to be within 0.5 % of target concentration for all particles except the 20 % iron doped variant which experienced 1% iron loss as a result of surpassing the crystal solubility limit. Global assessment of nanocrystalline particles using XPS showed a decreasing population of Ce3+ ions with increasing Fe3+ ion inclusion. 18 % atomic concentration of Ce3+ ions in pure cerium oxide particles reduced to ~8 % for the 20 % iron doped variant. Clonogenic assays using 9L gliosarcoma cells assessed the ability of CeOx NPs to boost cell growth and protect against megavoltage x-ray damage. With no radiation pure CeOx NPs provided a 10 % boost to cell growth. When 8 Gy of 10 MV x-rays was delivered to the cell monolayer the CeOx NP inclusion improved cell survival by 75 %. Increasing the radiation survival fraction from 0.12 to 0.22. The inclusion of the 5 % Fe doped CeOx NPs boosted cell growth by 6 % and increased the radiation survival fraction by 20 %. 10 % Fe doped CeOx NPs did not significantly boost cell growth or improve radiation survival fraction. 20 % Fe doped CeOx NPs reduced cell growth by 12 % and were not used in radiation studies. Overall these studies suggest that radioprotection efficacy of cerium oxide nanoparticles is dependent on the population of Ce3+ ions. The correlation between nanoparticle Ce3+ population, cell growth and cell survival fraction provide clear and comprehensive evidence of this dependency