DNA detector near
Technology Research News
Pathogens like anthrax and botulism, as
well as garden variety bacteria, can be identified by their DNA signatures.
Finding a way to quickly and easily read telltale DNA sequences could
lead to a convenient and lifesaving tool for doctors' offices and emergency
workers in the field.
Researchers at Northwestern University have identified tiny amounts of
DNA in a sample by catching a particular pathogen's DNA between tiny gold
electrodes, then using electric current to identify whether the electrodes
have picked up the target DNA.
The method could eventually be used in a hand-held device sensitive enough
to quickly identify pathogens in the field, said Chad Mirkin, a chemistry
professor at Northwestern University. "It addresses a very important need
in the detection arena, a hand-held device which offers the sensitivity
and selectivity necessary for point-of-care applications," he said.
The researchers' device contains single-stranded DNA stretched between
tiny gold particles that act as microelectrodes.
The familiar double helix of biological DNA contains long strings of paired
bases attached to sugar-phosphate backbones. The single strand of DNA
contains sequences of bases that can pair up with the target DNA. If the
target DNA is present it is essentially caught by the DNA strand spanning
Once this happens, a strand of probe DNA that has a metal nanoparticle
in tow binds to another portion of the captured DNA. The particles are
13 nanometers in diameter, or about one millionth of the thickness of
a dime. "If enough binding events take place, an electronic bridge [of
nanoparticles] is formed between the two electrodes," said Mirkin.
To make the device more sensitive, the electrical signal can be strengthened
by treating the device with photographic developing solution, Mirkin said.
"When the gaps with particles are exposed to the solution, silver is played
out on the particles, increasing the conductivity between the microelectrodes,
and therefore the signal associated with the detection process."
The researchers were able to detect DNA molecules in concentrations as
low as 500 femtomolars, which is equivalent to about 15 million DNA molecules
in a sample the size of the drop of water. A water drop contains more
than one billion trillion water molecules.
The researchers also developed a new method for differentiating molecules
that nearly match, but do not bind fully with the captured DNA string,
from perfect matches. Instead of using the usual heating method to differentiate
mismatched strands from perfectly matched samples, the researchers found
a way to use a salt solution, according to Mirkin.
The tricky part of developing the method was making the tiny probes, said
Mirkin. "The development of the nanoparticle probes was not trivial. They
must be stabilized and made highly specific for the DNA targets," he said.
The researchers' next step is to make the device smaller, which should
make it more sensitive, according to Mirkin. "The sensitivity of the device
should be inversely proportional to gap size," he said.
The researchers' prototype device contained four electrode pairs. Because
the device is essentially a computer chip, this number could be greatly
expanded, according to Mirkin. DNA identification chips can eventually
be designed with thousands of electrode pairs with different DNA strands
between them, each designed to detect different types of DNA, he said.
The technology is currently being commercialized by Nanosphere, Inc. "They
expect to have a product within two years," said Mirkin.
Mirkin's research colleagues were So-Jung Park and T. Andrew Taton. Teton
is now at the University of Minnesota. They published the research in
the February 22, 2002 issue of Science. The research was funded by the
Defense Advanced Research Projects Agency (DARPA), the Army Research Office
(ARO), Air Force Office of Scientific Research (AFOSR) and the National
Science Foundation (NSF).
Timeline: 2 years
TRN Categories: Biology; Biotechnology
Story Type: News
Related Elements: Technical paper, "Array-Based Electrical
Detection of DNA with Nanoparticle Probes," Science, February 22, 2002.
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