This was the first study of buckyballs' toxicity to individual human cells. The results could help scientists tailor the toxicity of the material for a given application.

"There are many cases where toxicity is desirable," said Vicki Colvin, director of Rice's Center for Biological and Environmental Nanotechnology (CBEN). "For example, we might want particles that kill cancer cells or harmful bacteria. In other cases - like applications where particles may make their way into the environment - toxicity is undesirable."

The study looked at nano-C60 - a nanoscale aggregate of C60 molecules that generally forms in water - as well as fullerene molecules derivatized with three sets of chemical groups. The researchers tested the materials on two types of human cells - liver carcinoma cells and dermal fibroblasts.

The nano-C60 proved massively more toxic to the human skin cells than C60(OH)24 - the fullerene molecule that contained most additional chemical groups on its surface.

And, in general, the molecules with a higher degree of surface modification were less toxic.

"We're encouraged to see that controlling the surface properties of buckyballs allows us to dial the level of toxicity up or down," said Colvin. "Moreover, we believe the technique can prove useful in tuning the toxicity of other nanoparticles."

The researchers reckon the fullerene molecules killed the cells by generating oxygen radicals that oxidized the cell lipids, damaging the cell membrane. They believe that more highly derivatized C60 systems are less toxic to cells as they do not generate oxygen radicals so easily.

"Cytotoxicity [the study of toxic effects on individual cells] should not be confused with a fully fledged toxicological risk assessment," said Kevin Ausman, executive director of CBEN. "Risk assessments take into account exposure rates, uptake mechanisms, transport within the body and much more. Based on our results, we think buckyballs should be studied in more detail and we're already working to arrange additional studies."

The nano-C60 caused half of the skin cells to die at a concentration of 20 parts per billion (ppb). For C3 the equivalent figure was 10,000 ppb. Meanwhile, Na+2-3[C60O7-9(OH)12-15](2-3)- had an LC50 value of 40,000 ppb and that for C60(OH)24 was >5,000,000 ppb.

The researchers reported their work in Nano Letters.