"Our goal was both to learn how the biological function of the cells was affected by the nanotubes and to see if the fluorescent properties of the nanotubes would change inside a living cell," said Bruce Weisman of Rice University. "On the first point, we found no adverse effects on the cells, and on the second, we found that the nanotubes retained their unique optical properties, which allowed us to use a specialized microscope tuned to the near-infrared to pinpoint their locations within the cells."
Weisman and colleagues incubated macrophage-like cells in a growth medium containing single-walled carbon nanotubes in a surfactant. The nanotubes had an average diameter of 1 nm and an average length of 1 micron.
The cells showed the same population growth as cell cultures containing surfactant but no nanotubes, and normal adhesion, morphology and confluence. That indicates that the nanotubes didn't adversely affect the cells, at least in the short term.
Examining the cells with a fluorescence microscope adapted for imaging between 1125 and 1600 nm showed that they had taken up the nanotubes. The team believes the cells actively ingested the nanotubes. That's because the uptake rate was slower at lower temperatures, as would be found for phagocytosis - the process by which white blood cells ingest particles such as bacteria and viruses.
The imaging process revealed many localized regions of near-infrared emission inside the cell. The researchers think these resulted from nanotubes contained in phagosomes - small chambers in which the cell sealed off the tubes as it ingested them.
What's more, the nanotubes retained their emissive properties despite the harsh oxidizing environment inside the cell. They were photostable for at least 100 minutes.
The nanotube ingestion rate increased with nanotube concentration. For the highest concentration of nanotubes (7 µg/mL) the average ingestion rate was one nanotube per second per cell. The researchers incubated samples so that they contained around 70,000 nanotubes per cell.
The team says future applications for their work could include studying nanotube distributions in organisms and the development of new families of bioconjugated fluorescent markers and contrast agents for use in cell biology research and medical diagnosis.
Although use of nanotubes in the body would require long-term studies on toxicity and biodistributions, the tubes could be used as imaging markers in laboratory in vitro studies at an earlier stage. Unlike fluorescent quantum dots, nanotubes don't contain heavy metals, reducing the risk of heavy metal toxicity.
The researchers reported their work in the Journal of the American Chemical Society.