In the thesis the methods of optical spectroscopy were used to study the mechanisms of formation of defect centers in cerium oxide nanocrystals and mixed oxides. The influence of defect structure on the mechanisms of antioxidant activity of nanocrystals was revealed, and possible ways to control both the defect structure and antioxidant properties of nanoparticles were shown. Several types of defect centers (Ce3+ - Vo -Ce3+ complexes, F0- and F+-centers) were observed, the ratio of which in cerium oxide nanocrystals depends on the atmosphere of heat treatment, the size of the nanocrystals, and concentration of impurity ions.
The content of Ce3+ ions in nanoceria increases significantly at incorporation of both non-isovalent (Y3+, Eu3+, Tb3+) or smaller (Zr4+) ions into ceria lattice. For CeO2-ZrO2 nanocrystals as well as for ceria nanocrystals, Ce3+ - Vo -Ce3+ complexes, and F0 centers are the most common defects, while for CeO2 - Re2O3 (Re = Y, Eu, Tb) nanocrystals at high concentrations of doped ions the Re3+-Vo-Ce3+ complexes prevail. The incorporation of doped ions and decrease of the nanocrystal size allow obtaining the concentrations of Ce3+ ions as high as 30-35 at. %.
5d→4f luminescence of Ce3+ ions in CeO2-x nanocrystals opens the possibility of controlling the interaction of reactive oxygen species with ceria nanocrystals. Using this approach was found that the ability of CeO2-x nanoparticles to recover their antioxidant characteristics after interaction with reactive oxygen species is associated with a process of accumulation and release of oxygen by ceria nanoparticles. ROS-nanoceria interaction leads to Ce3+ → Ce4+ oxidation accompanied by quenching of Ce3+ luminescence of nanoceria, and after ROS decomposition the recovery of initial Ce3+ luminescence intensity occurs with sufficient time delay (up to few days). The role of oxygen transport within ceria nanoparticles in regeneration of antioxidant properties of nanoceria after interaction with an oxidant was determined. Involvement of oxygen diffusion into recovery of nanoceria antioxidant properties hampers the redox activity of ceria nanoparticles making it size- and temperature-dependent. The accumulation of oxygen for some nanocrystals proceeds via single-file diffusion, and at high temperatures and high oxidant concentrations the long-lasted oscillations of Ce3+/ Ce4+ ratio were observed. The antioxidant activity of ceria and ceria-based mixed (CeO2 - Re2O3 (Re = Y, Eu, Tb), CeO2-ZrO2) colloidal nanoparticles depends strongly on the temperature, size of nanoparticle, type and concentration of doped ions, and laser irradiation. The key role of Ce3+-Vo-Ce3+ complexes in H2O2 decomposition by ceria-based nanoparticles was revealed. While for CeO2-ZrO2 nanoparticles the rate of H2O2 decomposition was higher than in CeO2-x nanoparticles, for other mixed nanocrystals the sufficient slowing-down of H2O2 decomposition was observed. This effect was explained by the main role of Ce3+-Vo-Ce3+ complexes in the processes of hydrogen peroxide decomposition, which role is similar to the role of active sites of enzymes. The number of Ce3+-Vo-Ce3+ complexes increases at Zr4+ incorporation due to increase of the content of oxygen vacancies, but decreases at incorporation of isovalent ions (Y3+, Eu3+, Tb3+) leading thereby to higher or lower rate of H2O2 decomposition, respectively. On the contrary, for hydroxyl radicals (˙OH) and superoxide anions (O2-) the antiradical activity of ceria nanoparticles is determined not by the content Ce3+-Vo-Ce3+ complexes, but merely by Ce3+/Ce4+ ratio on the nanoceria surface. The continuous laser irradiation with specific wavelengths leads to formation of additional oxygen vacancies, and so, of additional Ce3+-Vo-Ce3+ complexes providing better antioxidant activity of pre-irradiated ceria nanoparticles.
Finally, a new application of nanoceria as a luminescent sensor of hydrogen peroxide concentration in water solutions was found. Hydrogen peroxide sensing was based on the reversible shift of Ce3+↔Ce4+ balance in ceria nanoparticles as a result of nanoceria-oxidant interaction. Intensity of 5d→4f luminescence of Ce3+ ions in nanoceria decreases at increase of hydrogen peroxide concentration in water solutions and, the dependence of Ce3+ luminescence intensity on HP concentration is linear in semi-log coordinates. So, this nanoparticles presents a new type of antioxidants with ability both to scavenge reactive oxygen species and to visualize the change in ROS concentration during this process.
