Handheld DNA detector near

By Kimberly Patch, 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 the electrodes.

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
Funding:   Government
TRN Categories:   Biology; Biotechnology
Story Type:   News
Related Elements:  Technical paper, "Array-Based Electrical Detection of DNA with Nanoparticle Probes," Science, February 22, 2002.


February 27, 2002

Page One

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