"Functionalized carbon nanotubes were shown to be cleared from blood circulation and into the urine quite fast as intact nanotubes, in the absence of any appreciable tissue accumulation or organ function complication," Kostas Kostarelos of the University of London School of Pharmacy told nanotechweb.org. "This is significant for the toxicity and safety profile of such carbon nanomaterials."

Kostarelos and colleagues used single-walled carbon nanotubes functionalized with the chelating molecule diethylentriaminepentaacetic (DTPA). They labelled the nanotubes with a radioactive indium isotope to enable imaging.

The team injected the nanotubes intravenously into mice and used gamma scintigraphy to trace the tubes. This revealed that the functionalized nanotubes were not retained in the liver or spleen and were rapidly cleared from the blood system through processing in the kidneys and excretion in urine. The nanotubes had a half-life of roughly three hours.

Subsequent examination of urine samples in an electron microscope revealed that the nanotubes were excreted intact. Multiwalled carbon nanotubes also passed through the body intact.

"We hope that this work will form the basis of any future development of functionalized carbon nanotubes in biomedical applications - therapeutic or diagnostic - and even more so, carbon nanotube pharmaceuticals," said Kostarelos. "We need to figure out ways to prolong the blood circulation half-life of carbon nanotubes in order to give them any possible chance to reach their target tissue. The good news is that, unless their surface is dramatically altered, we know now that eventually the nanotubes will be cleared from the body intact without any major organ complication."

The researchers reported their work in PNAS.