- Carbon-based nanomaterials display size-dependent toxicity
Lauren I. Rugani
Carbon-based nanomaterials are becoming more prominent in a variety of nanotechnology applications;
for example, as contrast agent transporters for imaging. The growing use of these
materials will result in greater exposure by both professionals and the public —
and in mounting debate over the toxicity of such materials.
A team of researchers at Ecole Polytechnique
Fédérale de Lausanne, the Universities of Fribourg and Lausanne, and Laboratoire
Cytopath, all in Switzerland, has found that exposure to carbon-based nanomaterials
has a lethal effect on cell proliferation.
Cells of the human lung-tumor line
H596 were exposed in vitro to three types of carbon-based nanomaterials: nanotubes,
with an average diameter of 20 nm and aspect ratios ranging from 80 to 90; nanofibers,
measuring an average of 150 nm across and with aspect ratios ranging from 30 to
40; and submicron particles, with an aspect ratio of 1. Cells were exposed to each
of the materials with concentrations of 0.002, 0.02 and 0.2 μg/ml for four
The effect of the materials was evaluated
by an MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay,
which is based on the accumulation of dark blue formazan crystals inside the living
cells. The researchers quantified the concentration of the crystals with a spectrophotometer
from Dynex Technologies of Chantilly, Va., to measure the absorbance at 570 nm.
A linear relationship was established between the number of living cells and optical
density, which enabled the quantification of changes in the rate of cell proliferation.
In all cases, carbon nanoparticles
proved to be the most toxic, followed by nanofibers and nanotubes. Although dose-dependent
toxicity was predictable, the size-dependent toxicity was contrary to the researchers’
expectations, given that nanotubes have thus far been of primary concern within
the science community. It is possible that the toxicity — apparent after only
24 hours — correlates to the presence of dangling bonds, which appear in high
density in carbon nanoparticles as compared with nanotubes.
Researchers used three types of carbon-based nanomaterials to determine
their toxic effect on cells: (from left to right) multiwalled carbon nanotubes,
carbon nanofibers and carbon black nanoparticles. Scale bars correspond to 2 μm.
Images reprinted with permission from Nano Letters.
Although the experiment demonstrates
inhibition of cell proliferation, other studies find carbon-based nanomaterials
to be less toxic than previously used materials such as metals and silicon. Researcher
Arnaud Magrez speculates that the findings may differ because of the specific processing
method of the experiments or because the dose limit under which the nanomaterials
are not toxic is much lower than the concentrations used by the research team.
For safe use in biological applications,
the scientists believe that the amount of carbon-based nanomaterials must be extremely
low. However, researcher László Forró warned, “Our observations
point out that carbon-based nanomaterials can be toxic. This means, before using
them as contrast agent transporters, we have to make sure that they do not act as
After a one-day treatment with 0.02 μg/ml of multiwalled carbon nanotubes, cells
have lost their mutual attachments and retracted their cytoplasm. The nuclei appear
smaller and more condensed.
It is possible for the toxicity to
be controlled with specific functionalization of the materials’ surfaces.
Another important aspect is determining their stability inside the body and how
they would be eliminated.
Much higher concentrations are used
in composites, and their use in construction or textiles would bring people in contact
with them on a daily basis. Thus, it is not enough to know that these nanomaterials
are toxic, which is why this group hopes to determine the origin of the cell death
and to understand the cell death pathways.
Forró and his team may extend
their research to carbon-based nanomaterials with modified surfaces to find out
which surfaces are the most toxic and what can be done to avoid them.
Nano Letters, ASAP, May 20, 2006.
MORE FROM PHOTONICS MEDIA