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Method provides orders of magnitude better sensitivity than current methods CELIA HENRY A new method developed by Northwestern University chemistry professor Chad A. Mirkin and coworkers provides ultrasensitive protein detection and may be useful in diagnosing several types of cancer [Science, 301, 1884 (2003)]. PROTEIN SANDWICH 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. In the analysis of prostate-specific 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 sensitive 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. Mirkin and graduate students Jwa-Min Nam and C. Shad Thaxton use two different types of particles to capture the PSA from a serum sample. Monoclonal antibodies on the surface of a magnetic microparticle bind the PSA. Then a gold nanoparticle coated with double-stranded DNA 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 DNA in the magnetic complex is dehybridized by rinsing the gold particles in purified water. The sequence of these single DNA 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 DNA that we synthesized ahead of time," Mirkin says. "It dramatically simplifies and cleans up the soup involved in a normal protein-detection 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 DNA 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." |
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