"Based on our latest results, I personally believe that all kinds of CNTs have a threshold dose," Yao-dong Dai of Nanjing University of Aeronautics and Astronautics told nanotechweb.org. "Below this value, CNTs are low toxic or non-toxic to living systems. Yet when the concentration is higher than this threshold, the toxic effect is obvious."

We must be very cautious about nanomaterials, but not scared of themBin Kang

Using fluorescence labeling and a confocal microscope, the team, which also includes researchers from the affiliated Drum Tower Hospital of Nanjing University, has monitored the trafficking and distribution of single-walled carbon nanotubes (SWNTs) within living cells. The SWNTs under observation were highly soluble in water following functionalization with a biocompatible polymer (chitosan). Over a three-hour period, the SWNTs first bonded with the outside of the cell and then entered the structure only to be redistributed and discharged two hours later.

Threshold effect
By performing microtiter tetrazolium (MTT) assays, the researchers were able to look at the impact of SWNT (in this case, both functionalized and unfunctionalized) on cell proliferation. At doses below 50 µg ml–1, the cells showed no negative effect for all types of carbon nanotubes. Closer examination revealed an increase in the rate of apotosis (cell death) from 9.9% to 19.3% to 28.9% when the SWNT concentration was raised from 10 to 20 to 50 µg ml–1. This suggests that some damage to the cell is taking place at low concentrations, but a significant reduction in cell survival didn't occur until the dose reached a level of 100 µg ml–1.

"We must be very cautious about nanomaterials, but not scared of them," reasoned Bin Kang, who also took part in the work. "We can understand them and we can control them."

Over the next three years, the group plans to focus on the direct detection and quantitative determination of CNTs inside cells. The researchers intend to examine the dependence of tube length, diameter, functional condition and further chirality on the uptake pathway, sub-cellular distribution and cytoxicity.

The researchers presented their work in Nanotechnology.