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The findings come from the first toxicological
studies of water-soluble carbon nanotubes. The study, which is
available online, will be published in an upcoming issue of the
journal Toxicology Letters.
The research is a continuation of CBEN's pioneering
efforts to both identify and mitigate potential nanotechnology risks.
"Carbon nanotubes are high-profile nanoparticles
that are under consideration for dozens of applications in materials
science, electronics and medical imaging," said CBEN Director Vicki
Colvin, the lead researcher on the project. "For medical applications,
it is reassuring to see that the cytotoxicity of nanotubes is low and
can be further reduced with simple chemical changes."
Research has been conducted on the toxicity of
carbon nanotubes, but CBEN's is the first to examine the cytotoxicity
of water-soluble forms of the hollow carbon molecules. In their native
state, carbon nanotubes are insoluble, meaning they are incompatible
with the water-based environment of living systems. Solubility is a
key issue for medical applications, and researchers at Rice and
elsewhere have developed processing methods that render nanotubes
soluble. In particular, scientists are keen to exploit the fluorescent
properties of carbon nanotubes for medical diagnostics.
Nanotubes are long, hollow molecules of pure carbon
with walls just one atom thick. They are related to buckyballs, tiny
spheres of pure carbon that are about the same diameter.
In previous studies with buckyballs, CBEN found
that even minor surface modifications could dramatically reduce
cytotoxicity. The nanotube study found similar results. In both cases,
the researchers identified specific alterations that reduce toxicity.
Cytotoxicity refers to toxic effects on individual
cells. In cytotoxicological studies, identical cell cultures are
exposed to various forms and concentrations of toxins. In order to
compare the toxicity of different compounds, scientists look for the
concentration -- typically measured in parts per million or parts per
billion -- of materials that lead to the death of 50 percent of the
cells in a culture within 48 hours.
In the current study, CBEN researchers exposed skin
cell cultures to varying doses of four types of water-soluble
single-walled carbon nanotubes, or SWNTs. The four included pure,
undecorated SWNTs suspended in soapy solution and three forms of
nanotubes that were rendered soluble via the attachment of the
chemical subgroups hydrogen sulfite, sodium sulfite and carboxylic
acid.
The cytotoxicity of undecorated SWNTs was 200 parts
per billion, which compares to the level of 20 parts per billion
identified last year for undecorated buckyballs.
The modified nanotubes were non-cytotoxic. While
cell death did increase with dose concentration, cell death never
exceeded 50 percent for these compounds, which were each tested to a
level of 2,000 parts per million. Just as with buckyballs, CBEN found
that higher degrees of surface modification led to lower toxicity for
SWNTs.
"We now have two studies on carbon nanoparticles
that show us how to make them dramatically less cytotoxic," said CBEN
Executive Director Kevin Ausman, a co-author of the study. "In both
cases, it's the same answer: change the surfaces. This is an important
demonstration that there are general trends in biological responses to
nanoparticles."
Co-authors on the paper include graduate students
Christie Sayes, Feng Liang, Jared Hudson, Jonathan Beach and Condell
Doyle; undergraduate Joe Mendez; research scientists Wenhua Guo and
Valerie Moore; Professor of Chemistry Edward Billups; and Jennifer
West, the Isabel C. Cameron Professor of Bioengineering, professor of
chemical and biomolecular engineering, and director of the Institute
of Biosciences and Bioengineering. |
