Bug-popping nanotubes promise clean surfaces

Technology 22 August 2007

Coating surfaces with carbon nanotubes could keep them microbe-free, according to a study that shows how they pop and kill bacteria upon contact.

Several previous studies have shown that carbon nanotubes can be toxic to human cells in the lab, and to some animals, although results have sometimes conflicted and often been controversial (see The great nanotech gamble).

However, until now no-one had checked to see if carbon nanotubes could kill microbes. “We thought, why not see if we can use this toxic effect in a beneficial way,” says Menachem Elimelech of Yale University, who led the new study.

Elimelech and colleagues tested single-walled carbon nanotubes, the simplest kind of nanoscopic roll-up carbon. The nanotubes were each about 1 nanometre across – a small fraction of the size of a bacterial cell.


Purified tubes

Earlier studies looking at the toxicity of nanotubes often used ones that had not been cleaned properly beforehand, Elimlech says, which may explain why results often conflicted.

Elimelech’s team cleaned their nanotubes extensively to get rid of nearly all impurities, such as toxic metals left over as by-products of manufacturing. This way they could properly study the effects of the nanotubes alone.

They presented the results at the American Chemical Society meeting in Boston, US, on 20 August 2007.

The group first exposed the common bacteria E. coli to the nanotubes, presented in two different forms: floating in solution, where they tended to clump together, and stuck to filter paper. Either way, contact with the nanotubes always led to the bacteria’s demise.

The team measured the metabolism of the cells, and found very little after contact with the nanotubes. After an hour, roughly 80% of the bacteria were dead.

They also found a lot of loose DNA and RNA floating around after exposure, suggesting that the nanotubes had punctured the cells, causing this genetic material to float out. Furthermore, in high-resolution scanning electron micrograph (SEM) images, the nanotube-treated bacteria “look flattened,” Elimelech says. “It looks like they had some really severe cell damage.”

Size matters

In further, as-yet unpublished studies, Elimelech and colleagues tested multi-walled carbon nanotubes, which can be 10 times as wide as single-walled nanotubes or more.

These thicker nanotubes were much less toxic to bacteria, they found. “Size is very important,” Elimelech says, probably because the more slender nanotubes are “sharper”, and so better at puncturing the cells.

Elimelech envisages materials covered with carbon nanotubes to keep them microbe-free. The nanotubes could either be applied to the surface or incorporated during the manufacturing process. But before anything like this should go into production, Elimelech stresses that researchers need to be sure that the nanotubes would not come free, posing a potential hazard to people.

This is “good and important work,” says Joseph Hughes of Georgia Institute of Technology, but he warns that, since nanotubes can kill bacteria, they could have a major impact on ecosystems.

“Microbial function is critical in ecosystem sustainability and we rely on microbes to detoxify wastes in environmental systems,” he says. “If they are impaired by nanotubes, or other materials, it is the cause for significant concern.”

Journal reference: Langmuir (DOI: 10.1021/la701067r)

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