In this report ceria nanoparticles are shown to act as catalysts that mimic superoxide dismutase (SOD) with the catalytic rate constant exceeding that determined for the enzyme SOD.

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.

The surface chemistry of biomaterials can have a significant impact on their performance in biological applications. Our recent work suggests that cerium oxide nanoparticles are potent antioxidants in cell culture models and we have evaluated several therapeutic applications of these nanoparticles in different biological systems. Knowledge of protein adsorption and cellular uptake will be very useful in improving the beneficial effects of cerium oxide nanoparticles in biology. In the present study, we determined the effect of zeta potential of cerium oxide nanoparticles on adsorption of bovine serum albumin (BSA) and cellular uptake in adenocarcinoma lung cells (A549). The zeta potential of the nanoparticles was varied by dispersing them in various acidic and basic pH solutions. UV-visible spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS) were used for the protein adsorption and cellular uptake studies, respectively. Nanoceria samples having positive zeta potential were found to adsorb more BSA while the samples with negative zeta potential showed little or no protein adsorption. The cellular uptake studies showed preferential uptake for the negatively charged nanoparticles. These results demonstrate that electrostatic interactions can play an important factor in protein adsorption and cellular uptake of nanoparticles.

Cerium oxide nanoparticles (nanoceria) have recently been shown to protect cells against oxidative stress in both cell culture and animal models. Nanoceria has been shown to exhibit superoxide dismutase (SOD) activity using a ferricytochrome C assay, and it is this mimetic activity that has been postulated to be responsible for cellular protection by nanoceria. The nature of nanoceria's antioxidant properties, specifically what physical characteristics make nanoceria effective at scavenging superoxide anion, is poorly understood. In this study electron paramagnetic resonance (EPR) analysis confirms the reactivity of nanoceria as an SOD mimetic. X-ray photoelectron spectroscopy (XPS) and UV-visible analysis of nanoceria treated with hydrogen peroxide demonstrate that a decrease in the Ce 3 + /4 + ratio correlates directly with a loss of SOD mimetic activity. These results strongly suggest that the surface oxidation state of nanoceria plays an integral role in the SOD mimetic activity of nanoceria and that ability of nanoceria to scavenge superoxide is directly related to cerium (III) concentrations at the surface of the particle. IntroductionCerium is a lanthanide series rare earth element, and is the most abundant of these rare earths, present at about 66 parts per million in the earth's crust. Cerium can exist in either the free metal or oxide form, and can cycle between the cerous, cerium (III), and ceric, cerium (IV), oxidation states [1]. Both oxidation states of cerium strongly absorb ultraviolet light and have two characteristic spectrophotometric absorbance peaks. The first peak is in the 230 to 260 nm range and corresponds to cerium (III) absorbance. The second peak absorbance occurs in the 300 to 400 nm range and corresponds to cerium (IV) absorbance [2]. Nanoceria has similar chemical and physical properties to bulk cerium, however, because of the increased surface area and oxygen vacancies present, nanoceria has potential as a unique catalyst [3]. Much of the unusual catalytic chemistry involved with nanoceria is believed to be due to oxygen vacancy sites at the surface of the nanoceria lattice. These oxygen vacancies are characterized by cerium (III) atoms in the center of the vacancy surrounded by adjacent cerium (IV) atoms [4]. The presence of cerium (III) at the surface of nanoceria is unique to the center * To whom correspondence is requested, W. T. Self wself@mail.ucf.edu; fax (407) 823-0956. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Current research using nanoceria as a catalyst is quite broad. Among other applications, nanoceria is being d...

Reactive oxygen and nitrogen species play a critical role in many degenerative diseases and in aging. Nanomaterials, especially modified fullerenes and cerium oxide nanoparticles, have been shown to effectively protect mammalian cells against damage caused by increased reactive oxygen or nitrogen species, likely through their direct reaction with superoxide radical, since each of these materials has been shown to act as effective superoxide dismutase mimetics in vitro. This critical review discusses the chemistry of these nanomaterials and the context in which their radical scavenging activities have been studied in biological model systems. Current studies are focused on determining the uptake, metabolism, distribution, toxicity and fate of these nanomaterials in cell and animal model systems. Ultimately if shown to be safe, these nanomaterials have the potential to be used to reduce the damaging effects of radicals in biological systems (101 references).

Promising results have been obtained using cerium (Ce) oxide nanoparticles (CNPs) as antioxidants in biological systems. CNPs have unique regenerative properties owing to their low reduction potential and the coexistence of both Ce(3+)/Ce(4+) on their surfaces. Defects in the crystal lattice due to the presence of Ce(3+) play an important role in tuning the redox activity of CNPs. The surface Ce(3+):Ce(4+) ratio is influenced by the microenvironment. Therefore, the microenvironment and synthesis method adopted also plays an important role in determining the biological activity and toxicity of CNPs. The presence of a mixed valance state plays an important role in scavenging reactive oxygen and nitrogen species. CNPs are found to be effective against pathologies associated with chronic oxidative stress and inflammation. CNPs are well tolerated in both in vitro and in vivo biological models, which makes CNPs well suited for applications in nanobiology and regenerative medicine.

Cerium oxide nanoparticles (CNPs) have been demonstrated to protect biological tissues against radiation induced damage and scavenging of superoxide anions, prevent laser induced retinal damage, reduce spinal injury, possess pH dependent antioxidant properties, prevent cardiovascular myopathy, and as a tool for immunoassays and other inflammatory diseases.1a-j It is speculated that nanoceria is a regenerative radical scavenger with the ability to regenerate the active Ce 3+ oxidation state for radical scavenging which separates it from other nanomaterials based antioxidant systems such as hydroxylated and water-soluble C-60 and SWCNTs.1k, l Thus far there are no reports on controlling the regeneration of the Ce 3+ oxidation state which is the most important parameter in the application of CNPs as a reliable, regenerative radical scavenger. There is an imminent need to increase the residence time of CNPs in the body and to control the regeneration of the Ce 3+ oxidation state. PEG has been reported to increase the residence time of NPs and proteins inside cells and provide biocom-patibility.2 PEGylated counterparts of the Superoxide Dismutase (SOD) enzymes have shown improved performance over non-PEGylated enzymes. 2 Herein, we report our efforts to synthesize CNPs directly in PEG (600 MW) solution and determine the effect of increasing [PEG] (PEG vol % as 5, 10, 20, 40, 60, and 80) on the SOD mimetic properties exhibited by nanoceria. We also report how the active Ce 3+ oxidation state can be regenerated and demonstrate the role of PEG on the redox chemistry of CNPs catalyzed by H 2 O 2 . Several complexes of PEGs with lanthanides have been reported and characterized.3 To evaluate the effect of [PEG] on the complexation of cerium, UV-vis spectra of the precursor salt of cerium (cerium nitrate hexahydrate) in different solutions of PEG were obtained (SI-1). All PEG solutions show higher absorption relative to the water based solution of cerium nitrate, but the observed nonspecific trend could not be ascribed to a systematic decrease in the solvent polarity or dielectric constant. This observation indicates the complexation of cerium ions with PEG. In contrast to this Uekawa et al.4a, b reported a red shift upon addition of cerium nitrate in PEG and ascribed the red shift to the complexation of PEG with cerium ions. The CNPs were synthesized as described in the experimental details (SI-2). A high resolution transmission electron micrograph (Figure 1a NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscriptdemonstrates that PEG is present as an amorphous layer on CNPs confirmed by an amorphous background around the crystalline CNPs. To confirm further, CNPs synthesized in PEG were dialyzed using a 3500 MWCO cellulose membrane and the FTIR spectrum was collected from the dried powder. Figure 1b confirms the presence of PEG on the nanoceria particles from FTIR of 20% PEG CNPs. Biocompatibility and SOD Mimetic Activity of CNPs in PEGCell viability analysis was performed for CNPs in PEG solution u...

Cerium oxide nanoparticles (CeNPs) have shown promise as catalytic antioxidants in cell culture and animal models as both superoxide dismutase and catalase mimetics. The reactivity of the cerium (Ce) atoms at the surface of its oxide particle is critical to such therapeutic properties, yet little is known about the potential for a protein or small molecule corona to form on these materials in vivo. Moreover Ce atoms in these active sites have the potential to interact with small molecule anions, peptides, or sugars when administered in culture or animal models. Several nanomaterials have been shown to alter or aggregate under these conditions, rendering them less useful for biomedical applications. In this work we have studied the change in catalytic properties of CeNPs when exposed to various biologically relevant conditions in vitro. We have found that CeNPs are resistant to broad changes in pH and also not altered by incubation in cell culture medium. However to our surprise phosphate anions significantly altered the characteristics of these nanomaterials and shifted the catalytic behavior due to the binding of phosphate anions to cerium. Given the abundance of phosphate in biological systems in an inorganic form, it is likely that the action of CeNPs as a catalyst may be strongly influenced by the local concentration of phosphate in the cells and/or tissues in which it has been introduced.