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Carbon nanotubes stretch the boundaries of biomarker detection
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ANALYTICAL CHEMISTRY /
MAY 1, 2010
In this strategy, the researchers assembled upright, densely packed “forests” of single-walled carbon nanotubes (SWNTs) on a solid surface, with capture antibodies attached to the nanotube ends. When the investigators added serum from JAMES RUSLING
In recent years, researchers have tried to develop portable instruments for on-thespot cancer detection. But so far, clinicians’ many requirements for point-of-care devices, including sensitivity, accuracy, simplicity, and cost-effectiveness, have not been fulfilled by a single technique. To move devices for cancer biomarker detection one step closer to the clinic, James Rusling and colleagues at the University of Connecticut, the University of Connecticut Health Center, and the U.S. National Institutes of Health developed an ultrasensitive electrochemical immunosensor for the oral cancer biomarker interleukin-6 (IL-6). They report their results in a new AC paper (DOI 10.1021/ac902802b). Although existing methods can detect cancer-related biomarkers, various limitations make them largely unsuitable for point-of-care applications. For example, LC/MS proteomics analyses are laborintensive and require expensive instrumentation, and ELISAs are not amenable to multiplexing. Usually, health care providers must send samples to off-site labs for costly and time-consuming biomarker testing, which contributes to patient anxiety. If, on the other hand, doctors could simply place a drop of a patient’s serum or saliva in a portable, automated detection device, test results could be obtained in mere minutes and at a fraction of the cost. “What we’re aiming for is a device that could be used in a doctor’s office or a clinic to routinely screen people for various types of cancer,” says Rusling. As a proof of principle, Rusling and co-workers developed a method to sensitively detect IL-6 in conditioned medium from various cancer cell lines. The serum concentration of IL-6 is elevated in several cancers such as head and neck squamous cell carcinoma (HNSCC), but normal IL-6 levels are vanishingly low (ⱕ6 pg/ mL). To accurately measure a wide range of IL-6 concentrations, Rusling’s team built on previous work in which they used nanostructured immunosensors to detect four prostate cancer biomarkers (DOI 10.1021/ac9018022).
Detecting biomarkers with carbon nanotubes. (Top) (A) Moderately sensitive labeling approach provides 14-16 HRP labels per secondary antibody. (B) Ultrasensitive technique adds 106 HRPs per 100 nm of MWNT attached to the secondary antibody. (Bottom) AFM images of (a) SWNT forest on silicon and (b) antibody-derivatized SWNT forest.
prostate cancer patients to the immunosensor, the cancer biomarkers bound to their respective antibodies on the SWNTs. Biomarkers were detected electrochemically by adding a biotinylated secondary antibody and a streptavidin⫺horseradish peroxidase (HRP) conjugate and then performing rotating disk amperometry. In the presence of a hydroxyquinone mediator and hydrogen peroxide, HRP gave a strong catalytic reduction signal that was proportional to the amount of biomarker in the sample. Although this method, which introduces 14⫺16 HRP labels per antigen, can detect the elevated IL-6 levels (ⱖ30 pg/ mL) that are observed in some cancers, Rusling and colleagues needed to improve the technique’s sensitivity to detect nor-
mal IL-6 levels. To this end, the researchers synthesized a bioconjugate consisting of a secondary IL-6 antibody attached to an HRP-labeled multiwall carbon nanotube (MWNT). Each bioconjugate carried 106 HRP labels per 100-nm MWNT length, which enabled the detection of as little as 0.5 pg/mL human IL-6 in calf serum. With the moderately sensitive and ultrasensitive detection approaches in hand, the investigators measured a broad range of IL-6 levels (10⫺1005 pg/mL) in culture medium from 10 HNSCC cell lines. “Rusling and co-workers have developed an electrochemical immunosensor for IL-6 that has a very low limit of detection, a wide dynamic range, and excellent sensitivity,” says Robert Forster at Dublin City University (Ireland). “What is particularly impressive is the care taken to minimize non-specific binding, which is absolutely essential if the sensors are to be employed under real-world conditions.” “The excellent sensitivity of the device opens the door for early diagnosis of cancer, which is essential for a timely therapeutic intervention,” comments Joseph Wang at the University of California San Diego. “Although various groups have reported on amplified bioelectronic strategies in buffer solutions, Rusling and co-workers have successfully demonstrated practical clinical applications of their elegant detection scheme.” Rusling emphasizes that the current work is “just a step along the pathway” to point-of-care cancer detection devices. “We’ve now put this type of technology into a microfluidic system with an array of electrodes so that we can measure up to eight proteins at a time, and we’re working on automation to make the system as trouble-free as possible,” he says. “In the long run, the dream is to be able to measure a panel of biomarkers for all of the major cancers, which would probably be at least 25 proteins.” —Laura Cassiday
10.1021/AC100643A 2010 AMERICAN CHEMICAL SOCIETY
Published on Web 03/22/2010
