Ceria nanocrystals (nanoceria) are well known for their excellent antioxidant activity providing effective scavenging of reactive oxygen species (ROS) both in water solutions and biological systems. The mechanism and dynamics of nanoceria action are determined by the features of its defect structure. Our experimental results confirm that Ce3+–Vo–Ce3+ surface defect complexes on the surface of nanoceria acting similar to active sites in enzyme molecules actually provide nanoceria with enzyme-like activity. In accordance with this supposition, the processes of hydrogen peroxide (HP) decomposition by nanoceria can be well described using the Michaelis–Menten equations usually used in the description of enzyme–substrate interaction. The maximum rate of HP decomposition shows a clear dependence on the size of nanoparticles, i.e., on the number of available surface sites for binding of HP molecules. At HP concentrations, for which almost all Ce3+–Vo–Ce3+ sites are involved in HP decomposition, the process of slow Ce3+ → Ce4+ oxidation turns into fast redox cycling and Ce3+/Ce4+ oscillations are observed. This effect can be explained by the combined action of fast synchronous Ce3+/Ce4+ and pH change in nanoceria water solutions, as well as by HP ability to act both as oxidative and reductive agents. ROS decomposition by nanoceria in the oscillatory regime is a rather unexpected effect that can provide deeper understanding of the redox mechanisms of nanoceria action as a whole.

The aggregation behavior of 1-methyl-1′-octadecyl-2,2′-cyanine perchlorate (amphi-PIC) and 3,3′-dimethyl-9-(2-thienyl)-thiacarbocyanine iodide (L-21) dyes in binary dimethylformamide (dimethylsulfoxide)/water (1:9) solution containing CeO2 nanoparticles (NPs) has been studied by optical absorption and fluorescent spectroscopy, as well as dynamic light-scattering method. Formation of stable hybrid complexes between NPs and dye J-aggregates due to Coulombic and stacking interactions has been revealed. The arrangement of dye molecules in aggregates (slipping angle θ, twisting angle α, the center-to-center distance R, and the exciton splitting energy ΔE) was analyzed within the Kasha exciton model based on oblique transition dipoles orientation. For amphi-PIC, formation of twisted face-to-face and oblique head-to-tail J-aggregates has been revealed depending on CeO2 NPs concentration, whereas L-21 dye forms aggregates of a twisted face-to-face fashion independently on NPs concentration.

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