Single-walled carbon nanotubes (SWNTs) show great promise for applications in biomedicine. However, researchers are still unsure as to the toxicity of these nanomaterials. Previous studies showed nanotubes to be acutely toxic to cells but direct images of the nanomaterials have never been taken before because of the difficulty in distinguishing carbon-based nanotubes from carbon-rich cell structures.

Now, Alexandra Porter of the University of Cambridge in the UK and colleagues have directly imaged SWNTs within cells. The researchers obtained clear evidence that the nanotubes enter human cells and accumulate in the cell cytoplasm and nucleus, where they cause cell death.

The team, which includes scientists from the Daresbury Lab in Cheshire, imaged unlabelled SWNTs within macrophage cells using a combination of transmission electron microscopy (TEM) and confocal microscopy techniques. "Specifically, we mapped SWNTs using energy filtered TEM (EFTEM) and electron energy loss (EEL) spectrum imaging - methods that can differentiate between the graphitic SWNTs and the carbon rich amorphous cell by yielding maps of the characteristic energy loss information as electrons pass through the sample," Porter told

The researchers were able to image light reflected from the intracellular SWNTs in the confocal microscope by filling the nanotubes with metal halides.

Porter and co-workers found that cells exposed to high nanotubes concentrations (of 5 µg/ml) remained relatively healthy even after two days. However, after four days, lower concentrations (of 0 to 19 µg/ml) of nanotubes led to a significant decrease in the cells' viability. This was because some of the nanotubes had entered the cells' cytoplasm and crossed into the nucleus.

The researchers chose to look at the behaviour of macrophages because they are the first line of defence against foreign materials in many tissues, including the lung. Inhaled nanoparticles should be ingested by macrophages and stop them from entering the body any further.

Porter and colleagues stress that their results do not necessarily mean that carbon nanotubes are inherently toxic and so should not be used in any kind of therapy. "There are many other factors that affect exposure, including bio-persistence and the routes of exposure of the nanoparticles into the body, all of which all must be considered before we can confirm they are toxic," stated Porter. "Our study must be considered alongside a significant body of previous scientific research."

The team says it will now continue developing sophisticated imaging techniques to deepen its understanding of how toxic a range of nanoparticles are to a variety of cell types.

The results were published in Nature Nanotechnology.

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