Air pollution toxicity model studies typically use industrial carbon back as a model system for ultrafine carbon nanoparticles (CNP). Atmospheric processing such as oxidation, metal deposition, and small molecule adsorption are overlooked in these representative systems. Changes in surface and physicochemical properties from atmospheric processes influence biochemical reactions and subsequent health impacts. Here we investigate chemical oxidation and iron deposition on surfaces to account for environmental processing of carbon nanoparticles. As a result of surface oxidation, CNP formed stable dispersions in both water and cell culture media. Aggregate size of oxidized particles was below 500 nm in all solutions, whereas untreated particles were above 3 µm (by TEM) and had poor dispersity in water. Carbon K-edge spectroscopy using scanning transmission X-ray microscopy (STXM) and electron energy loss spectroscopy (EELS) suggest that the carbon surface is enriched with carboxylic groups after oxidation, which are responsible for the colloidal stability due to electronic and steric repulsions. Oxidized CNP have high affinity for Fe3+ ions in solution. The local structure of Fe3+ on carbon surfaces after slow drying (at 40°C) was examined using Fe L- and K- edge spectroscopy. Both STXM and EELS Fe L-edge spectra showed that adsorbed iron maintained the 3+ oxidation state. Synchrotron X-ray absorption spectroscopy (XANES and EXAFS) at the Fe K-edge confirmed the 3+ oxidation state and showed that the majority of Fe3+ on oxidized carbon surfaces was coordinated with carboxylic groups rather than precipitated as iron oxides or (oxy)hydroxides. The labile nature of coordinated Fe3+ implies possible delivery of metal ions inside the cells by CNP. The increased flux of Fe3+ in cells could affect its homeostasis and also could trigger inflammatory and/or anti-oxidant responses.

Surface oxidation and iron speciation on carbon nanoparticles for precise air pollution models

255th National Meeting (2018)

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