Nanotubes with added nitrogen are less toxic

Adding a dash of nitrogen to carbon nanotubes can make them substantially less toxic, according to researchers based in Mexico.

Multi-walled carbon nanotubes consist of nested cylinders of carbon atoms, and toxicology tests have previously found that these individual (single-walled) nanotubes can cause respiratory problems in rats - in some cases killing them off altogether. If nanotubes are to be used in medicine, scientists need to find a reliable way to cut out any such toxic effects.

So Juan Laclette and colleagues at the National Autonomous University of Mexico, Mexico City, compared how normal multi-walled carbon nanotubes, and their nitrogen-doped equivalents, affected mice.

The tubes were given to the mice either through their nose or mouth, or by injection into the windpipe or abdomen. All the mice survived and showed no signs of distress when nitrogen-doped tubes were used, in stark contrast to their fatal response to conventional multi- and single-walled nanotubes. Laclette claims that the nontoxic tubes could now be tested as drug delivery systems in mice and other animals.

Engineering out toxicity

The work could be important in assessing future uses of nanotechnology, according to Andrew Maynard, chief science advisor at the Project on Emerging Nanotechnologies, based at the Woodrow Wilson International Center for Scholars in Washington, DC, US. ’But you have to be a little bit careful. It depends whether this alteration of the carbon nanotubes changes their functionality,’ he said. 

Chiu-Wing Lam of Nasa’s Johnson Space Center, US, who has conducted previous nanotube toxicology studies on rats and mice, suspects that some of Laclette’s mice may have died because large doses of the test dust were used. ’If children ate large pieces of gummy candies and choked to death, we cannot say gummy candy is toxic,’ he told Chemistry World.

However, the work does show that you can take a multi-walled nanotube and change its toxicity - and that’s important, said Maynard. ’As soon as you begin to engineer a material to do what you want you also have the possibility of engineering it not to do what you don’t want - in other words engineering out toxicity.’

The tubes with added nitrogen may be less toxic because they have a segmented, bamboo-like structure that makes them less prone to sticking together into thick wads, suggests Laclette. He now hopes that further research may help to understand what causes some nanotubes to be more toxic than others. 

Katharine Sanderson