"Carbon nanotubes are high-profile nanoparticles that are under consideration for dozens of applications in materials science, electronics and medical imaging," said Vicki Colvin, director of the Center for Biological and Environmental Nanotechnology (CBEN) at Rice. "For medical applications, it is reassuring to see that the cytotoxicity of nanotubes is low and can be further reduced with simple chemical changes."

Colvin and colleagues exposed human dermal fibroblasts in vitro to single-walled carbon nanotubes (SWNTs) with varying degrees of functionalization. These included SWNT-phenyl-SO3H and SWNT-phenyl-SO3Na - both with carbon/-phenyl-SO3X ratios of 18, 41 and 80 - and SWNT-phenyl-(COOH)2 with a carbon/-phenyl-(COOH)2 ratio of 23. The unfunctionalized SWNTs were stabilized in 1% Pluronic F108 surfactant.

The cytotoxicity of unfunctionalized SWNTs was 200 parts per billion (ppb), i.e. a concentration of 200 ppb would lead to the death of 50% of the cells in a culture within 48 hours. For comparison, undecorated fullerenes had a cytotoxicity of 20 ppb.

The researchers say that the modified nanotubes were non-cytotoxic. That is, while cell death did increase with dose concentration, it did not exceed 50% for concentrations of modified nanotubes up to 2000 parts per million. Nanotubes with greater sidewall functionalization were less toxic.

"We now have two studies on carbon nanoparticles that show us how to make them dramatically less cytotoxic," said Kevin Ausman, executive director of CBEN. "In both cases, it's the same answer: change the surfaces. This is an important demonstration that there are general trends in biological responses to nanoparticles."

The researchers reported their work in Toxicity Letters.