DIVING BOARD Cantilevers are coated with antibodies to PSA, a marker for prostate cancer. When PSA binds to the antibodies, the cantilever is deflected, which is detected with a laser beam. KENNETH HSU/UC BERKELEY AND THE PROTEIN DATA BANK

Arun Majumdar, a professor of mechanical engineering at the University of California, Berkeley, and coworkers, including Thomas Thundat, Oak Ridge National Laboratory, use microcantilevers to detect prostate-specific antigen (PSA), which is a marker for early detection of prostate cancer and for monitoring progression of the disease [Nat. Biotechnol., 19, 856 (2001)].

To prepare the sensor, Majumdar and his coworkers attach PSA antibodies to a gold-coated silicon nitride microcantilever. Majumdar describes the cantilever as "a microscopic diving-board-shaped mechanical structure."

A sample containing PSA is then flowed over the microcantilever surface. The binding of PSA to the antibodies causes a deflection in the microcantilever that can be measured using a laser beam. The amount of the deflection is proportional to the concentration of PSA in the sample. Majumdar and his coworkers were able to detect PSA over a concentration range of 0.2 ng per mL to 60 µg per mL, even against a background of unrelated human serum proteins such as human plasminogen and human serum albumin. The detection limit of 0.2 ng per mL is significant, Majumdar says, because the threshold for cancer diagnosis is 4 ng per mL.

The microcantilever assay is as sensitive as the current method of detecting PSA, a type of immunoassay known as ELISA, which requires fluorescent labels. However, the microcantilever has not yet reached the limits of its sensitivity, Majumdar says. Its sensitivity could be improved by increasing its length--current lengths are 200 µm or 600 µm--or surface roughness.

Currently, Majumdar and his coworkers are working on making microcantilever arrays. "It's a nontrivial problem" going from one to many, he says. "That's what we're focused on right now, developing the optics and microfluidics to do that. Hopefully by the end of the year we'll have a platform that can do multiple analyte detection." This would allow the strategy to become a common platform for high-throughput studies of protein-protein interactions, DNA hybridization, or DNA-protein binding, as well as drug discovery.

David Klenerman, Matthew A. Cooper, and their colleagues in the department of chemistry at the University of Cambridge describe another biosensor--this one using a QCM to detect viruses [Nat. Biotechnol., 19, 833 (2001)]. They call their method "rupture event scanning."

Klenerman and coworkers attach antibodies to viruses on the surface of the QCM. Viruses in the sample then bind to the antibodies. The researchers make the QCM oscillate at increasing amplitude by applying an alternating electric field. When a critical amplitude is reached, the bond between the antibody and virus is broken. The QCM--a type of acoustic device--then acts as a "microphone," converting the acoustic emission from the bond rupture to an electrical signal.

"In essence, we separate and detect in a single process," Klenerman says. "Because we scan up, shaking off the things that are poorly attached first and then shaking off the things that are more strongly attached later, we discriminate between nonspecific and specific adsorption." They used the sensor to detect type 1 herpes simplex virus.

The QCM method is nearly as sensitive as PCR, the primary method for detecting viruses. "The advantages of our technique over PCR are that it's direct and rapid, with potentially very little sample preparation," Klenerman says. However, he notes, there is a big difference between experiments in the lab and real clinical samples from people.

There are plans to commercialize both sensors. The Berkeley team has licensed to a start-up company a patent on a way to analyze multiple cantilevers. The Cambridge researchers have spun out a start-up company, called Akubio Ltd., to develop the QCM sensor.

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BREAKING FREE An electric field applied to the quartz-crystal microbalance causes the device to oscillate, breaking the bond between the virus and the antibodies on the surface. ADAPTED BY PERMISSION FROM NATURE BIOTECHNOLOGY © 2001

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