Nanoceria exhibit redox state-dependent catalase mimetic activity (original) (raw)

In this study we have found that cerium oxide nanoparticles exhibit catalase mimetic activity. Surprisingly, the catalase mimetic activity correlates with a reduced level of cerium in the +3 state, in contrast to the relationship between surface charge and superoxide scavenging properties. Reactive oxygen species (ROS) are byproducts of normal oxygen metabolism that can cause oxidative damage to macromolecules in biological cells. Two dominant ROS, superoxide (O 2 •−) and hydrogen peroxide (H 2 O 2), are neutralized in cells by specific enzymes, primarily superoxide dismutase (SOD) and catalase, respectively. These enzymes thus act as antioxidants by protecting cells against the damaging effects of these two primary ROS. An intracellular imbalance between ROS and the enzymes that are involved in their detoxification can lead to a condition known as oxidative stress. 1 Oxidative stress has been implicated in several diseases and disorders, including Parkinson's and Alzheimer's disease, 2 cardiovascular disease 3 and even tumor development. 4 Additionally, the damage inflicted to cells by ROS and other reactive substances such as reactive nitrogen species (RNS) could be the root cause of aging and senescence, as per Harman's free radical theory of aging. 5 Cerium oxide nanoparticles (nanoceria) have been shown to protect cells against ROSinduced damage. 6-8 The small size of these nanoparticles gives them a high surface area to volume ratio, which is advantageous in that it provides a large surface for potential catalysis. Additionally, nanoceria are advantageous because of the ability of cerium to exist in either a +3 (reduced) or +4 (oxidized) state at the particle surface: Ce 3+ are associated with oxygen vacancies. The crystal structure of cerium oxide shows most of the cerium atoms are present in the +4 state in the crystalline lattice. However, a reduction in particle size of nanoceria results in the formation of surface oxygen vacancies, which results in higher levels of cerium atoms in the +3 state, 9 typically denoted as CeO 2−x. 10,11 Previous studies from our lab have shown that this characteristic of nanoceria affects their catalytic efficiency as a scavenger of superoxide anions. 12,13 Specifically, nanoceria were shown to exhibit SOD mimetic activity in a redox-state dependent manner, with the most efficient superoxide scavenging exhibited † Electronic supplementary information (ESI) available: Nanoparticle synthesis, characterization and details on spectrophotometric analysis of peroxides. See