Kenneth J. Reed

Exploring the properties and applications of nanoceria: is there still plenty of room at the bottom?

Nanoceria is an exceptionally versatile, commercially valuable catalytic material whose properties vary dramatically from that of the bulk material. Nanocerias redox properties can be tuned by choice of method of preparation, particle size, nature and level of dopant, particle shape and surface chemistry. The two oxidation states of the cerium element in the lattice make possible the formation of oxygen vacancies which are essential to the high reactivity of the material, its oxygen buffering capability and thus its ability to act as a catalyst for both oxidation and reduction reactions. Ceria has important commercial utility in the areas of chemical mechanical polishing and planarization, catalytic converters and diesel oxidation catalysts, intermediate temperature solid oxide fuel cells and sensors. Its potential future uses include chemical looping combustion, photolytic and thermolytic water splitting for hydrogen production and as a therapeutic agent for the treatment of certain human diseases. We have seen that the method of synthesis, particle size, stabilizing corona, and purity dictate where it is used commercially. Finally, in regards to the prescient words of Dr. Feynman, we note that while there is indeed “plenty of room at the bottom”, there quite possibly exists an optimal nanoceria size of between 2–3 nm that provides maximal reactivity and thermodynamic stability.

Nanoceria: factors affecting its pro- and anti-oxidant properties

Nanoceria redox properties are affected by particle size, particle shape, surface chemistry, and other factors, such as additives that coat the surface, local pH, and ligands that can participate in redox reactions. Each CeO2 crystal facet has a different chemistry, surface energy, and surface reactivity. Unlike nanocerias industrial catalytic applications, biological and environment exposures are characterized by high water activity values and relatively high oxygen activity values. Electrochemical data show that oxygen levels, pH, and redox species affect its phase equilibria for solution and dissolution. However, not much is known about how the many and varied redox ligands in environmental and biological systems might affect nanocerias redox behaviour, the effects of coated surfaces on redox rates and mechanisms, and whether the ceria solid phase undergoes dissolution at physiologically relevant pH and oxygen levels. Research that could answer these questions would improve our understanding of the links between nanocerias redox performance and its morphology and environmental conditions in the local milieu.

Cerium oxide nanoparticles with antioxidant properties ameliorate strength and prolong life in mouse model of amyotrophic lateral sclerosis

Cerium oxide nanoparticles (CeNPs) neutralize reactive oxygen and nitrogen species. Since oxidative stress plays a role in amyotrophic lateral sclerosis (ALS) in humans and in the SOD1G93A mouse model of ALS, we tested whether administration of CeNPs would improve survival and reduce disease severity in SOD1G93A transgenic mice. Twice a week intravenous treatment of SOD1G93A mice with CeNPs started at the onset of muscle weakness preserved muscle function and increased longevity in males and females. Median survival after the onset of CeNP treatment was 33.0±3.7days (N=20), and only 22.0±2.5days in mice treated with vehicle, control injections (N=27; P=0.022). Since these citrate-EDTA stabilized CeNPs exhibited catalase and oxidase activity in cell-free systems and in in vitro models of ischemic oxidative stress, we hypothesize that antioxidant activity is the protective mechanism prolonging survival in the SOD1G93A mice.

Simulations of ceria nanoparticles

Atomistic computer simulations, using classical potential models, have been used to model ceria nanoparticles (NPs) with diameters of approximately 1 and 2 nm. Lattice expansion is observed in the stoichiometric 1 nm NP, consistent with experiment, indicating that reduction is not the primary driver for such expansion. Furthermore, on reduction, the 1 nm NP is found to distort significantly, offering a possible explanation for its reduced oxygen storage capacity compared to the 2 nm NP. Point defect calculations on the 2 nm NP indicate that while doping with La is energetically favourable, Fe incorporation is not.

One-Vessel synthesis of iron oxide nanoparticles prepared in non-polar solvent

This paper presents a simple, reliable, and efficient method to produce a stable suspension of monodisperse maghemite (γ-Fe2O3) nanoparticles that are highly crystalline and 3 nm to 3.5 nm in diameter. Particles were characterized by X-ray diffraction to determine the crystal phase of the particles and transmission electron microscopy with images used to size the particles. Iron oxide nanoparticles were synthesized through the thermal decomposition of an organic mixture of iron(II) acetate, oleic acid, and 1-octadecene, with particular emphasis on the molar ratio of oleic acid to iron content. These unique conditions produce nanoparticles with physical properties ideal for application as an automotive combustion catalyst in diesel engines and numerous other applications. The one-vessel synthesis method creates an efficient procedure to synthesize iron oxide nanoparticles readily miscible, as made, with Kensol-50H for example, or other petroleum products; producing a ready-made material for mass production with no waste in materials nor subsequent chemical work-up.

Application of mass spectrometry to characterize localization and efficacy of nanoceria in vivo.

In vivo study of nanomaterials is complicated by the physical and chemical changes induced in the nanomaterial by exposure to biological compartments. A diverse array of proteins can bind to the nanomaterial, forming a protein corona which may alter the dispersion, surface charge, distribution, and biological activity of the material. Evidence suggests that unique synthesis and stabilization strategies can greatly affect the composition of the corona, and thus, the in vivo properties of the nanomaterial. Protein and elemental analyses techniques are critical to characterizing the nature of the protein corona in order to best predict the in vivo behavior of the nanomaterial. Further, as described here, inductively coupled mass spectroscopy (ICP-MS) can also be used to quantify nanomaterial deposition in tissues harvested from exposed animals. Elemental analysis of ceria content demonstrated deposition of cerium oxide nanoparticles (CeNPs) in various tissues of healthy mice and in the brains of mice with a model of multiple sclerosis. Thus, ICP-MS analysis of nanomaterial tissue distribution can complement data illustrating the biological, and in this case, therapeutic efficacy of nanoparticles delivered in vivo.

Reactive CeO2 nanofluids for UV protective films

We investigate surface modification by organo-trimethoxysilanes of nano-ceria and if such surface-modified nano-ceria can be transformed into solvent-free nanofluids. We also examine whether simultaneous modification with ionic liquid salts and with acrylate groups yields nanofluids suitable for forming UV-protective films and clear coatings by UV-initiated polymerization. Nominally 3nm diameter CeO2 was successfully synthesized and surface decorated with an ionic liquid salt and with acrylate groups to produce a core/shell structured solvent-free nanofluid after ion exchange of chloride for a soft polyoxyethylene sulfonate anion. This room temperature nanofluid melts at about -10°C and exhibits a glass transition at about -71°C. The melting enthalpy, about 19J/g, corresponds approximately to the gain in surface free energy of such nanofluid particles upon transforming from the solid state to liquid state. Robust films were made by UV photoinitiation of this nanofluid in combination with ethylene glycol dimethacrylate and with a polyoxyethylene diacrylate to yield cross-linked films with absorption coefficients α350nm=6.6±0.8cm2/mg and α300nm=24.5±3.5cm2/mg. Average near UV protection over 300-350nm of 1-3 optical density units can be obtained with 0.065-0.19mg/cm2 of CeO2. These materials appear almost three-fold more effective, per unit ceria, than previously reported clearcoats of nanoceria.