NEWS OF THE WEEK NANOTECHNOLOGY
DNA-BASED ASSAY FOR PROTEINS
PROTEIN S A N D W I C H Target proteins are captured between magnetic microparticles and DNA-coated gold nanoparticles. After separation using a magnetic field, the DNA is released from the gold nanoparticles and quantified using DNA detection.
Method provides orders of magnitude better sensitivity than current methods
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NEW METHOD DEVELOPED
by Northwestern University chemistry professor Chad A. Mirkin and coworkers provides ultrasensitive protein detection and maybe useful in diagnosing several types of cancer [Science, 301,1884 (2003)]. In the analysis of prostate-spe-
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sitive than current clinical methods. PSA is an extremely useful marker for detecting prostate cancer. It also is being explored as a target for detecting breast cancer because it is found in the serum of breast cancer patients at levels too low to be picked up by current diagnostic tests.
w
Target (PSA)
cific antigen (PSA), for example, the assay is three orders of magnitude more sensitive than the best methods in the literature and six orders of magnitude more sen-
GLOBAL
Mirkin and graduate students Jwa-Min Nam and C. ShadThaxtonuse two different types of particles to capture the PSA from a serum sample. Monoclonal anti-
ECONOMY
G-7 Currency Policy Could Help U.S. Chemicals
A
call for more flexible currency exchange rates issued at the end of a meeting of the finance ministers and central bankers of the Group of Seven industrialized countries earlier this month sent stock markets around the world reeling and weakened the value of the U.S. dollar. The dollar hit an almost three-year low against the Japanese yen and weakened against the British pound, the Swiss franc, and the euro. The immediate effect on currencies is exactly what the G-7 ministers were aiming for. However, subsequent action will have to be taken by some countries for the effect to be long-lived. Observers say the statement was aimed primarily at Asian countries—especially Japan and China. Japan, they say, has intentionally kept the yen weak. And China's currency is pegged to the dollar rather than being al6
C&EN
/ SEPTEMBER
29,
2003
lowed to float according to market conditions. In the long run, a weaker dollar, which many economists say has been too strong too long, would have a beneficial effect on U.S. industries and especially the chemical industry, with its large export position and overseas asset base. T. Kevin Swift, senior director for economics and statistics at the American Chemistry Council, notes that imports to the U.S. would become more expensive while exports would become cheaper and, thus, more competitive. Also, more favorable currency translations from foreign operations would add to company financials. A recent survey of 40 U.S. chemical companies by C&EN showed that an average of 46.4% of their sales in 2002 were overseas, up from 45.1% the year before (C&EN, July 7, page 3U.-WILLIAMST0RCK
bodies on the surface ofa magnetic microparticle bind the PSA. Then a gold nanoparticle coated with double-stranded D N A and PSA antibodies sandwiches the protein. Because each sandwich contains a magnetic component, a magnetic field is used to pull the complexed particles out of the sample. Then the D N A in the magnetic complex is dehybridized by rinsing the gold particles in purified water. T h e sequence of these single D N A strands is specific to the analyte, so it serves as a biological bar code. The code can be analyzed by standard DNA detection methods. Depending on the size of the gold nanoparticle, hundreds to thousands of DNA strands are released into solution for each protein molecule captured. By using a different bar code for each protein, researchers can analyze multiple proteins simultaneously If desired, a second magnetic field can remove the magnetic particles, the gold nanoparticles, and even the protein target. 'All we're left with is a short strand of D N A that we synthesized ahead of time," Mirkin says. "It dramatically simplifies and cleans up the soup involved in a normal proteindetection experiment. "The beauty of this is that one can drive the equilibrium to the captured state very efficiently sequester all of the PSA in solution, and not waste any target," Mirkin says. "With high-sensitivity detection, that's critical, because any target that is wasted is going to be recorded in the form of lower sensitivity" "There are established and very good D N A assays, but similar, generally applicable protein assays are not available," says Charles R. Martin, chemistry professor at the University of Florida. "Mirkin takes what works well—DNA assays—and uses them to detect proteins. Very elegant and innovative." Mirkin is more enthusiastic. This method, he claims, could "change diagnostics forever."-CELIA HENRY HTTP://WWW.CEN-ONLINE.ORG
