GROUNDBREAKING SCIENCE
Tiny Detectors

The microcantilever sensor invented by Thomas Thundat is the heart of numerous emerging physical, chemical, and biological sensors, including a compact airport explosives detector and a more sensitive detector of early-stage prostate cancer.

In 1991, Thomas Thundat observed an anomalous effect while conducting an experiment with an atomic force microscope (AFM) in his ORNL lab. He had performed the same experiment a few months earlier when he was a postdoctoral researcher at Arizona State University. Strangely, the AFM cantilever at Arizona State had been stable, whereas the one at ORNL kept drifting, ruining the experiment.

Microcantilevers coated with a material that attracts molecules of a target substance will bend more when near that substance, changing the angle at which incoming laser light will be deflected. Courtesy of Arun Majumdar and Kevin Lin, University of California at Berkeley.

"Eventually, I realized that because it rains a lot in Tennessee, my lab's humidity level often went up, so the cantilever drifted," Thundat says. "There was no variation in the cantilever in Arizona, where it is always dry. Then it occurred to me that the silicon nitride cantilever could be a sensor for humidity, because water vapor adsorbs on silicon nitride."

Thundat then developed a mercury sensor using a microcantilever, which resembles a tiny diving board. He deposited a layer of gold on a cantilever and exposed it to mercury vapors. Because mercury has an affinity for gold, mercury in the air binds with the gold layer, causing the cantilever to bend. Thundat says the cantilever is a sensitive detector because of mechanical forces induced by chemical binding between probe and target molecules. In 1996 Thundat and his colleagues extended the sensor concept to physical sensing and demonstrated an infrared detector and an infrared camera based on a bimetallic cantilever.

In the 1990s Thundat's group fabricated many different cantilever chemical and biological sensors by coating them with special chemicals that attracted and captured molecules of a target material. They made sensors that can detect not only trace amounts of environmental pollutants but also weapons of mass destruction.

Collaborating teams led by Thundat at ORNL and Jesse Adams at the University of Nevada recently devised a much more compact cantilever-based airport explosive sensor that could detect trace amounts of TNT. The innovation was reported in a paper in the October 2, 2003, issue of Nature; the paper's lead author, Lal Pinnaduwage, is a member of Thundat's team. The paper proposed distributing microsensors in airport ventilation systems to detect nitrogen-containing explosive molecules wafting through the air.

In each microsensor, a tiny voltage would be applied to an embedded piezoresistive heating element, which would heat the cantilever to 1000^oC for 0.1 sec. The adsorbed TNT molecules undergo nanoexplosions when the cantilever is heated very rapidly. Consequently, the cantilever would bend down, squeezing a zinc oxide crystal embedded below in a piezoelectric sensor-actuator. An electric charge would be produced, alerting airport officials to the location of an explosive. "This new device overcomes a number of limitations associated with conventional microcantilever sensors," Thundat says. "The process uses integrated elements on each cantilever, consumes 10,000 times less electricity, and allows for an array structure that simplifies the simultaneous use of many cantilevers."

Pinnaduwage and Thundat also developed a coating that, when applied on a cantilever, can very sensitively detect explosives, such as the PETN explosive used by the would-be "shoe bomber." "This sensor can detect PETN within 10 seconds at 1 part per trillion sensitivity," Thundat says.

Thundat found that a cantilever coated with double-stranded "probe" DNA will deflect when it hybridizes with a complementary DNA target, say, from a disease bacterium. He also showed that cantilevers can detect glucose levels in blood.

Recently, Thundat and Arun Majumdar of the University of California at Berkeley fabricated cantilevers coated with an antibody to the prostate-specific antigen (PSA). They showed that this technique in which PSA binds with the antibody coating on the cantilever, causing it to bend, detected early signs of prostate cancer in serum samples with 10 times higher sensitivity than conventional techniques.

Using ink-jet printing techniques, Thundat and Majumdar have developed a parallel array of 500 DNA- and antibody-coated cantilevers. Stacked charge-coupled device camera images show which cantilevers deflected after exposure to disease proteins. If this array could rapidly detect in body fluids any one of 100 different cancer markers, such as elevated or depressed levels of antibodies and enzymes, it could be the heart of a hand-held device for fast disease diagnosis.

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