Surface Chemistry of Combustion-Synthesized Iron Oxide Nanoparticles Determined by Electron Energy-Loss Spectroscopy
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Surface Chemistry of Combustion-Synthesized Iron Oxide Nanoparticles Determined by Electron Energy-Loss Spectroscopy V.J. Leppert1, K. E. Pinkerton2 and J. Jasinski1 1 2
School of Engineering, University of California, Merced, CA 95344, USA Center for Health and the Environment, University of California, Davis, CA 95616, USA
ABSTRACT Particulate matter (PM) that is composed of a complex mixture of organic compounds, soot, transition metals, sulfates, nitrates, and other trace elements has been associated with a range of adverse health effects. The effect of the separate components of PM and their interaction with each other is important in order to understand the origins of toxicity in these materials. Iron and soot are common environmental contaminants occurring in ultrafine particulate matter in the air, and have been experimentally generated, in a manner simulating high-temperature, industrial processes, through the combustion of iron pentacarbonyl in a mixture of acetylene and ethylene. Nanoprobe electron energy-loss spectroscopy (EELS) in the transmission electron microscope was used to collect Fe L3:L2 white line-intensity ratios from the interiors and surfaces of iron oxide nanoparticles present in samples generated under low and high soot conditions. This ratio is sensitive to any change in the iron oxidation state, and indicated that iron oxide nanoparticles co-generated with large quantities of soot during combustion processes show a subsurface layer (of thickness 2-3 nm) of decreased iron oxidation state. A quantitative analysis of the iron L32 EELS ionization edge measured in nanoprobe mode indicated that in these particles, the oxidation state of iron at the surface is decreased from Fe3+ to Fe2+, possibly explaining the toxicity of these materials in the respiratory tract. The application of EELS to understanding the nanoscale distribution of the various components and their chemical interactions in these samples, and correlation of these results with acute respiratory toxicity studies of the laboratorygenerated particulate matter, are discussed. INTRODUCTION Inhalation into the respiratory tract of particulate matter (PM) with a mass median aerodynamic diameter of 10 µm or less (PM10) has been associated with a range of adverse health effects in humans. Short-term exposure to ambient PM at or above the National Ambient Air Quality Standard of 150 µg/m3 (averaged over 24 h) has been shown to be associated with increased cardiopulmonary symptoms, as well as increased morbidity and mortality [1,2]. PM is composed of organic compounds, soot, transition metals, sulfates, nitrates, and other trace elements [3]. Identification of the toxic components in PM is an active area of investigation and requires study of respiratory effects on animals under well-characterized and controlled conditions. Since soot is an important component of PM, being mainly composed of elemental carbon and ranging from 3.5 to 17.5% of total particle mass, and iron is the predominant transition metal found in PM [3,4], a st
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