Many research groups are studying iron-containing nanoparticles for use in gene therapy, magnetic resonance imaging and other medical applications. Iron oxide nanoparticles are also important candidates for drug delivery and targeting tumours. Since iron is an essential nutrient for mammals, iron-containing nanoparticles are generally thought of as being safe.

Now, however, Jin and co-workers have observed toxic dose-dependent effects in nerve cells that are laden with coated iron nanoparticles. When exposed to the nanoparticles, some cells die and other show a diminished ability to produce neurites – thin, sprout-like cellular extensions vital for transmitting neuronal signals (see figure 1). The discovery of this nano-toxicity, which will be published in Biomaterials, suggests that these particles may not be as safe as we had once thought, says Jin.

Meanwhile, Becker and colleagues have found that human lung fibroblasts readily absorb DNA-wrapped single-walled carbon nanotubes (SWCNTs) that are shorter than around 200 nm. Depending on the concentration levels of the nanomaterials, the cells either die or show other signs of toxicity. In contrast, nanotubes longer than around 200 nm do not enter the cells (see figure 2).

SWCNTs are tiny rolled up sheets of carbon atoms with excellent mechanical and electrical properties. They could be used for a variety of technological and industrial applications but their biocompatibility is still little understood. "Our results demonstrate that cellular uptake in these lung cells depends significantly on the length of the nanotubes," explains Becker. "This is the first of many steps in the critical goal of reducing health risk by de novo engineering of the nanotubes themselves."

The NIST researchers published their results in Advanced Materials.