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September 25, 2006

Carbon Nanotubes Yield Sensitive Detection Method for Cancer Markers

In an attempt to increase the sensitivity of cancer biomarker detection and to decrease the need for large samples from which to detect those molecules, a multi-institutional research team has shown that a “forest” of single-walled carbon nanotubes can be used to detect lower levels of the prostate specific antigen (PSA) than is possible using the current commercial assay. Moreover, this new system requires between 5 and 15 times less sample than does the commercial system.

James Rusling, Ph.D., at the University of Connecticut, led the research team that developed this new assay system. The investigators report their findings in the Journal of the American Chemical Society.

The investigators began by developing a method in which single-walled carbon nanotubes self-assemble into forests standing in upright bundles on a conductive surface. Chemically reactive groups on the ends of the nanotubes provide a place to which the researcher can attach enzymes or antibodies capable of reacting with or binding to specific biomolecules. As a demonstration in the current paper, the researchers use an antibody that binds to PSA. The researchers also create a second detection reagent by conjugating another PSA-binding antibody with individual carbon nanotubes and then attaching multiple molecules of an enzyme known as horseradish peroxidase to the carbon nanotubes.

The resulting system works as follows. A serum sample or fluid extracted from a tissue sample is added to the antibody-labeled nanotube forest. Any PSA in the sample binds to this antibody. After washing the nanotube forest, the second antibody, linked to the peroxidase-coated nanotube, is added and allowed to bind to the PSA molecules now captured on the nanotube forest. After a second round of washing, the researchers then apply a voltage to the device and add hydrogen peroxide. As the peroxidase enzyme converts hydrogen peroxide to water, the device produces an electrical signal proportional to the number of times this reaction occurs, which itself is a direct reflection of how much PSA is bound to the nanotube forest.

The investigators note that this system can detect PSA at levels as low as 4 picograms per milliliter (pg/mL) in a sample size of 10 microliters of serum. In comparison, the standard assay now used clinically has a detection limit of 10-100 pg/mL but requires a sample size of 50-150 microliters. In addition, this system was able to provide quantitative detection of PSA from laser microdissection tissue samples of 1,000 cancer cells, something that current technology cannot achieve.

This work, which was supported in part by the National Cancer Institute, is detailed in a paper titled, “Carbon nanotube amplification strategies for highly sensitive immunodetection of cancer biomarkers.” Investigators from Salve Regina University, the National Institute of Dental and Craniofacial Research, and the National Cancer Institute also participated in this study. An abstract of this paper is available through PubMed.
View abstract.