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How 'toxic' nanotubes were faking it
[June 02, 2006]

How 'toxic' nanotubes were faking it

(New Scientist Via Thomson Dialog NewsEdge)CARBON nanotubes might not be as nasty as they are made out to be. It turns out they are tricking a commonly used toxicity test into producing false positive results.

Researchers had hoped that single-walled carbon nanotubes rolled-up sheets of carbon atoms could be used in a wide range of medical applications, including drug delivery systems and molecular sensors. This plan was dealt a blow when some forms of nanotubes were shown to be toxic, killing cells they came into contact with.

Now it appears some researchers may have been hoaxed. Jrg Wrle-Knirsch and colleagues at the Institute of Toxicology and Genetics at the Karlsruhe Research Centre in Germany analysed a test called the MTT assay, which is used to measure toxicity in lab studies of cell cultures. Living cells release enzymes that react with a salt called methylthiazol tetrazolium (MTT), producing a purple dye. This gives an indirect measure of how many cells have survived treatment with a chemical.

When Wrle-Knirsch and colleagues used the MTT assay on cells exposed to carbon nanotubes, the test showed the same degree of toxicity regardless of the concentration of nanotubes. The length of exposure to the nanotubes also had no effect on the result. Suspecting something was wrong, the researchers used another salt test, the WST assay. This time they found no toxicity at all.

When the team examined the MTT test under an electron microscope, they discovered that the salt had latched onto the nanotubes, forming insoluble pellets. The researchers think this prevented the salt from reacting with the cell enzymes and changing colour. "The carbon nanotubes faked a cytotoxic effect," Wrle-Knirsch says. "Depending on the assay used, the cells appear either dead or alive." The results will be published in the journal Nano Letters

Andrew Maynard, science adviser to the Project on Emerging Nanotechnologies in Washington DC, says the finding suggests that in future multiple tests should be used on nanomaterials to prevent such interferences confounding results.

More research is also needed to formally characterise nanotube materials, as they can have different structures or levels of impurity depending on how they are made, Maynard says.

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