Biochemists have long envied their molecular biology colleagues' ability to detect minute quantities of genetic material in a molecular sea. But now, biochemists may be on the verge of an equally revolutionary technique of their own thanks to a new scheme that can turn specks of iron and gold into biochemical bloodhounds that detect target proteins with up to 1 million times the sensitivity of the conventional approach.
For years, Chad Mirkin, a chemist at Northwestern University in Evanston, Illinois, and colleagues have used tiny metal particles to detect DNA sequences that can signal the presence of anything from cancer cells to the anthrax bacteria. But the approach couldn't unseat polymerase chain reaction (PCR). So Mirkin and graduate students Jwa-Min Nam and C. Shad Thaxton switched to a quarry PCR cannot detect: proteins.
Their target was prostate-specific antigen (PSA), a protein that can indicate prostate cancer in men and that is also being investigated as a possible marker for breast cancer in women. To detect PSA, Mirkin and his students started with two types of particles: 1-micrometer plastic spheres with magnetic iron cores, and much smaller nonmagnetic gold nanoparticles. The researchers linked the iron particles to genetically engineered proteins called monoclonal antibodies, designed to bind to PSA using the same molecular handle. They linked the gold nanoparticles to "polyclonal" antibodies designed to bind to PSA at different sites. They also tagged the gold particles with thousands of snippets of DNA.
For their experiments, Mirkin and his students added both sets of nanoparticles to solutions containing PSA. Both the monoclonal and polyclonal antibodies bound to the PSA, sandwiching the target proteins between the particles. The researchers then turned on a magnetic field to attract the magnetic particles to the side of the test tube. If PSA was present, both it and any attached DNA-toting nanoparticles were dragged along as well. The researchers then used another standard solution to make each DNA snippet release a short complementary strand that could be spotted with standard DNA detection schemes.
The approach worked beautifully, the researchers report in today's issue of Science. The thousands of DNA molecules on each gold nanoparticle amplified the signal, making it possible to detect proteins at concentrations of just 3 attomolar, or about 18 to 20 copies in 10 microliters of a solution. "You're getting the sensitivity similar to PCR, but with proteins," Mirkin says.
"The sensitivity is spectacular," says Charles Martin, a chemist at the University of Florida in Gainesville. By setting loose swarms of particles linked to assorted antibodies and corresponding DNA "bar codes," researchers could potentially detect hundreds of different targets at a time, he says, which would be a big boon not just for researchers, but eventually for physicians as well.