Iron bioavailability and redox activity in diverse carbon nanotube samples

The ultimate success of many nanotechnologies will depend on our ability to understand and manage nanomaterial health risks. Carbon nanotubes are now primarily fabricated by catalytic routes and typically contain significant quantities of transition metal catalyst residues. Iron-catalyzed free-radical generation has been hypothesized to contribute to oxidative stress and toxicity upon exposure to ambient particulate, amphibole asbestos fibers, and single-wall carbon nanotubes. A key issue surrounding nanotube iron is bioavailability, which has not been systematically characterized, but is widely thought to be low on the basis of electron microscope observations of metal encapsulation by carbon shells. Here, we validate and apply simple acellular assays to show that toxicologically significant amounts of iron can be mobilized from a diverse set of commercial nanotube samples in the presence of ascorbate and the chelating agent ferrozine. This mobilized iron is redox active and induces single-strand breaks in plasmid DNA in the presence of ascorbate. Iron bioavailability varies greatly from sample to sample and cannot be predicted from total iron content. Iron bioavailability is not fully suppressed by vendor "purification" and is sensitive to partial oxidation, mechanical stress, sample age, and intentional chelation. The results suggest practical materials chemistry approaches for anticipating and managing bioavailable iron to minimize carbon nanotube toxicity.