Last fall's anthrax attacks in the United States showed that current schemes for detecting the deadly bacterium carry an unwelcome trade-off: They're either fast but prone to mistakes, or highly accurate but slow (Science, 9 November 2001, p. 1266). Now researchers have unveiled simple electronic chips that can detect DNA from anthrax and other organisms in minutes. The chips appear to be vastly more sensitive than other high-speed techniques.
The wiring of the new chips, described in the 22 February issue of Science, consists of DNA linked to minute specks of metal, known as nanoparticles. A team led by Chad Mirkin, a chemist at Northwestern University in Evanston, Illinois, built the chips by placing a pair of electrodes 20 millionths of a meter apart atop a glass microscope slide. Then they anchored numerous identical snippets of single-stranded DNA to the glass surface between the electrodes. Each DNA fragment was designed to bind to one end of complementary DNA from the target organism: the anthrax bacterium. When the team immersed the setup in a beaker containing the target DNA, the chip-bound DNA yanked the target strands out of solution, filling the space between the electrodes with a patchy lawn of anthrax DNA.
To turn those DNA strands into a wire, Mirkin's group used a second set of single-stranded DNAs, called "probe" strands. One end of each probe was designed to bind to the free end of the target anthrax DNA; the other end carried a tiny gold particle. When the probes found their targets, the gold particles carried electric current between the two electrodes, Mirkin says. The chip could spot anthrax DNA in concentrations several orders of magnitude lower than the minimum for current high-speed detection schemes.
The new test is "a very clever idea that would lend itself to very inexpensive [diagnostic] devices," says Stephen Morse, a molecular biologist at Columbia University's Mailman School of Public Health and former program manager of the Advanced Diagnostics Program at the Defense Advanced Research Projects Agency. Mirkin and colleagues plan to pack their electrical DNA detectors into arrays that look for different target DNAs simultaneously. Such multitasking could pave the way for hand-held readers that scan for a battery of infectious agents.