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MADISON - In this time of the chronic threat of terrorism and the possibility of war with an adversary who may be armed with biological weapons, high on the wish list of security agencies and battlefield commanders is a quick and easy way to detect the presence of dangerous biological agents.
Now, with the help of a novel diamond film developed by chemists at the University of Wisconsin-Madison, the age of the inexpensive, compact sensor that can continuously scan airports, subways and battlefields for the slightest trace of biological weapons may be at hand. Coupled with modern electronics, the new sensors would not only be able to detect nearby biological agents, but also sound alarms and even call for help.
The new technology, which has been reported in a series of articles in scientific journals and at scientific meetings, is centered on a newfound ability to make highly stable, DNA-modified diamond films. The ability to build a stable platform that can "constantly sniff" for anything unusual - and that can be integrated with microelectronic devices - has long been a problem of surface chemistry.
"The real advance is getting the needed chemical stability and then combining that with electronic sensing," says Robert J. Hamers, a professor of chemistry at UW-Madison.
Hamers worked in collaboration with Lloyd Smith, also a UW-Madison professor of chemistry, to develop the chemistry for the new diamond surfaces, and with Dan van der Weide, a UW-Madison professor of electrical and computer engineering, to achieve the electronic sensing.
"Although there have been many advances in 'bio-chip' technologies, getting a stable platform that can be used for continuous monitoring - not just one-shot analysis - has been a long-standing problem," Hamers says. "And diamond solves it."
Biological sensors of the future will need to operate at the interface of biology and modern microelectronics. Not only must those sensors possess the ability to detect biological molecules of interest, they will also need to take advantage of the signal amplification and processing properties of microelectronics. Because diamond films can be deposited on silicon, the stuff of which computer chips and other microelectronic devices are made, it provides a bridge between the world of miniature electronics and biology, which requires a chemically stable platform for biosensing.
Such sensors, according to Hamers, would be about the size of a postage stamp and could be sprinkled in public places such as airports, bus depots, subways, stadiums and other places where large numbers of people gather. They could act, he says, like a "bio cell phone, where they just sit in place and sniff, and when they detect something of interest, send a signal" to alert security or sound an alarm.
"This is where we are going and we are almost there. The science is there. We've proven we can make surfaces that are much more stable than anything that existed before," he says. "And we've proven that we can detect the electrical response when biomolecules bind to the diamond surface."
Hamers acknowledges that before the new biosensors become practical, significant engineering for packaging and fluid-handling systems for sample introduction must be completed. But while some work remains, he says, "the hardest part appears to be over."
In the past, scientists tried in vain to develop surfaces with long-term stability for use as biosensors. But silicon, the material upon which computer-chip technology rests, tended to defy efforts to harness it as a stable surface for sensing biological molecules.
"A widely recognized problem was that silicon oxide proved not to be a good material to do sensing on," says Hamers. "In the case of silicon, the best available technology did not permit leaving a surface in contact with water for any period of time. It eventually degrades. That was an obstacle to the merging of the microelectronic and biotechnology communities."
Other materials such as gold, glass and glassy carbon proved either unstable or difficult to integrate with silicon.
The biologically modified diamond films, on the other hand, have proved to be remarkably durable, able to withstand multiple cycles of processing DNA, genetic material that can be diagnostic of such things as anthrax, ricin, bubonic plague, smallpox and other molecules that can potentially be used as biological weapons or agents of terror.
"You can really abuse it and it doesn't care," Hamers says. "The diamond films are chemically durable and they are electrochemically durable."
In a battlefield environment such as the ones U.S. troops may encounter in a war with Iraq, a country accused of developing biological weapons, such sensors could be deployed on vehicles or scattered across the landscape to warn of the presence of such agents. Early warning could save lives and enable soldiers to prepare to operate in a contaminated environment.
Chips made with diamond films may also have important economic implications in research. One of the most important new technologies in biology are 'bio-chips' or 'gene chips,' a technology that permits scientists to, among other things, scan biological molecules to assess gene activity. Current technology relies on expensive chips that are used once and discarded.
"People are putting a lot of time and energy into building 'bio-chips,'" says Hamers. "You use them once and throw them away. There can be a lot of money invested in building a single chip. Although probably not useful for use in clinical applications, if you can reuse a chip in a research environment, it may have important economic implications."
The work of the Hamers and Smith groups on biological modification of diamond has been reported in the journal Nature Materials, and at meetings of the Materials Research Society and the American Vacuum Society. The technology's extension to electronic detection will be reported in March at a meeting of the American Chemical Society.
In addition to Hamers, Smith and van der Weide, Wensha Yang a UW-Madison graduate student in chemistry, played a key role in the research. Other UW-Madison co-authors include Wei Cai, Tami Lasseter and Tanya Knickerbocker. The diamond films chemically and biologically modified by the Wisconsin team were provided by John Carlisle and Dieter Gruen of Argonne National Labs, and by John Russell and James Butler of the Naval Research Labs.
The work was supported by the Office of Naval Research, the Wisconsin Alumni Research Foundation, the National Institutes of Health, the National Science Foundation and the Department of Energy.
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