Credit: © 2006 National Academy of Sciences, USA

With an ever-increasing number of applications using carbon nanotubes, concerns over their safety have come to the fore. Previous studies in mice showed that covalently modified single-walled carbon nanotubes (SWNT) were rapidly cleared through the kidneys and not retained in any organs, including the liver and spleen. Now, using a rabbit model, researchers at the University of Texas and Rice University in the USA show that pristine (unmodified) SWNTs have a longer blood-circulation time than previously thought and are retained in the liver1.

To study the biodistribution of non-aggregated nanotubes, Bruce Weisman and colleagues dispersed SWNTs in a surfactant and injected this suspension into rabbits at a dose of 20 mg per kilogram of body mass. Blood samples were monitored over time by measuring the characteristic near-infrared fluorescence signature of individual nanotubes. The spectral changes showed that blood proteins displaced the surfactant coating within seconds of injection. Further examination of the organs revealed that 24 h after injection, SWNTs were detectable only in the liver. Moreover, no gross organ abnormalities nor pathological differences between the treated and control animals were seen. The animals also exhibited normal behaviour at this dosage.

Weisman and colleagues are confident that the fluorescence-based method is a reliable technique because it gives an unambiguous observation of the unmodified SWNTs in the animals.