Chip senses trace DNA
Technology Research News
The tricorders in science fiction's Star
Trek instantly read minuscule amounts of DNA left in a room days after
a person passes through.
In reality, however, DNA detection happens in the laboratory and
involves replicating the molecules in order to make samples large enough
to sense, then adding fluorescent dyes and reading each sample with the
help of lasers.
Research aimed at building handheld DNA detectors could dramatically
narrow the gap between fiction and reality within just a few years. The
key to making a handheld DNA detector is sidestepping replication by finding
a way to detect extremely small amounts of DNA.
Scientists at NASA Ames Research Center have come a step closer
to the sensor used by the agency's fictional descendant by developing
an ultrasensitive electronic DNA detector that employs a forest of carbon
nanotubes to sense meager amounts of DNA. Carbon nanotubes are rolled-up
sheets of carbon atoms.
Unlike DNA detectors that are based on conventional fluorescence,
the electronic DNA chip can be readily miniaturized and automated, and
can test for a wide range DNA at once, said Jun Li, a researcher at NASA.
The detector can also sense other substances, including proteins, chemicals
and pathogens. "It is a very versatile platform that can be used for many
purposes," said Li.
Previous attempts at electronic DNA chips have produced devices
that were still orders of magnitude less sensitive than fluorescence-based
DNA chips, according to Li. The NASA device, however, has the potential
to reach the sensitivity of laser-based fluorescence techniques, according
The key to this sensitivity is the small size and electronic properties
of carbon nanotubes.
The researchers' prototypes consist of arrays of 2- to 200-square-micron
chromium electrodes on a silicon wafer. Multi-walled nanotubes ranging
from 30 to 50 nanometers in diameter -- about two orders of magnitude
smaller than a red blood cell -- cover the electrodes and are encased
in a layer of silicon oxide.
The nanotubes are packed onto the electrodes at densities of anywhere
from 100 million to 3 billion nanotubes per square centimeter.
The bottoms of the nanotubes are in contact with the electrode
and their tops are exposed at the surface of the silicon oxide layer.
Strands of probe DNA are attached to the ends of the nanotubes.
When a liquid sample containing target DNA molecules comes into
contact with the detector, the target DNA attaches to the probe DNA, and
this increases the flow of electrons through the nanotubes to the electrode.
The fewer and more widely spaced the nanotubes, the more sensitive the
The device is at least sensitive enough to detect DNA in samples
containing as few as 3.5 million molecules, and is probably capable of
detecting DNA in samples with only a few thousand molecules, according
to the researchers. A drop of water, in contrast, contains trillions of
The carbon nanotube nanoelectrode array could be used as a portable
sensor for cancer cells and environmental contaminants, according to Li.
"The main applications will be for handheld devices for quick and simple
testing, such as personal care for early cancer detection, environmental
monitoring, bio defense and space exploration," he said.
The main challenges to using the device for real-world applications
are integrating the system into a practical package, and developing appropriate
bioassays for each application, according to Li.
The array could be used in practical applications within two years,
said Li. A NASA spinoff company, Integrated Nanosystems Inc., is commercializing
the system, he said.
Li's research colleagues were Hou Tee Ng, Alan Cassell, Wendy
Fan, Hua Chen, Qi Ye, Jessica Koehne, Jie Han and M. Meyyappan. The work
appeared in the May 14, 2003 issue of Nano Letters. The research was funded
by NASA and the National Cancer Institute.
Timeline: 2 years
Funding: Government, Institute
TRN Categories: Nanotechnology; Biotechnology
Story Type: News
Related Elements: Technical paper, "Carbon Nanotube Nanoelectrode
Array for Ultrasensitive DNA Detection," Nano Letters, May 14, 2003.
July 30/August 6, 2003
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