Chemical Treatment Alleviates Nanotube Toxicity

Chemically modifying the surface of certain carbon nanotubes can reduce their asbestos-like toxicity, scientists report (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201207664). The finding could lead to safer nanotube-based products.

Nanoscientists were alarmed to learn in 2008 that multiwalled carbon nanotube fibers that are at least 20 μm in length trigger a toxic response in mice akin to what asbestos does. “The report scared the entire scientific community and still prompts skeptical attitudes whenever we talk about nanotubes for biological applications,” says Maurizio Prato, a nanotube expert at Italy’s University of Trieste.

So Prato, along with Alberto Bianco of France’s National Center for Scientific Research and Kostas Kostarelos of University College London, set out to see whether potentially dangerous nanotubes could be transformed into safer material. They took the same nanotubes that researchers had previously shown to be dangerous and then covalently modified their surfaces.

To one group of nanotubes they added long alkyl chains and to another they added hydrophilic chains containing oxygen atoms and an amino group on their ends. In mice, the tubes with alkyl chains had the same type of asbestos-like toxicity as pristine nanotube fibers. The nanotubes with hydrophilic chains, however, appeared to be harmless.

“This type of chemical functionalization reaction led to better dispersion and broke bundled nanotubes apart, thereby reducing their effective length so they did not prompt a reaction like asbestos fibers,” Bianco explains.

Jun Kanno, a researcher at Japan’s National Institute of Health Sciences who led one of the 2008 studies, is skeptical of the group’s conclusions. He wonders whether the chemical functionalization process skims off the outer layers of the nanotubes and introduces structural defects that make them thinner and shorter, and therefore less toxic.

Prato and coworkers say only the surface of the nanotube is functionalized and that the dispersion of nanotube bundles is what reduces toxicity.