Chemists have constructed a sensor, made from a web of DNA and gold particles, that turns from red to blue when it detects a precise strand of DNA. This easy-to-read color change, described in tomorrow's Science, could lead to simple and cheap detectors of pathogens for use everywhere from doctors' offices to the battlefield.
To create the new sensors, a team led by chemists Chad Mirkin and Robert Letsinger of Northwestern University in Evanston, Illinois, started with two batches of DNA strands, which act as probes. Each probe sequence was complementary to half of the sequence of a third strand, the "target" strand that the sensor is designed to detect. The researchers then attached small gold-binding organic groups called thiols to the two sets of DNA probes and mixed each set with tiny gold nanoparticles, resulting in fuzzy gold particles coated with dozens of DNA strands each.
Next, the researchers combined the two sets of DNA-coated particles with the target DNA. The result: the first probe linked to half of the target DNA strand, the second probe to the other half, causing the target strand to bridge the two probes. Repeated millions of times, the process glued the nanoparticles together in a three-dimensional web. And the formation of this web changed the electronic behavior of the particles, producing a color change from red to blue.
"It's really marvelous," says Paul Alivisatos, a University of California, Berkeley, chemist, who works in this field. Because the probe strands can be tailor-made, the sensor can be designed to detect any DNA sequence. Mirkin believes that such a cheap and simple DNA detector system could prove useful for everyone from doctors quickly testing patients for infections at the bedside to soldiers scanning for biological warfare agents on the battlefield.
Equally important, Mirkin and others say, the nanoparticle sensor is a proof-of-principle for a strategy for exploiting the talents of DNA to organize nanoparticles into precisely structured devices. Because of DNA's ability to recognize and bind specific sequences, it could direct the assembly of various inorganic nanoparticles into ultra-small electronic circuits. Such circuits would be many times smaller than those housed by the millions on semiconductor chips, which are reaching a practical limit of miniaturization.