Two researchers unveil springboard to new particle detection systems


By Hank Hogan

WEST LAFAYETTE, Ind.—Rashid Bashir, a Purdue University associate professor of electrical and biomedical engineering, recently unveiled a technique that could provide a springboard—literally—to new particle detection systems.

A similar scheme was just announced by Cornell University (Ithaca, N.Y.) engineering professor Harold Craighead. Other potential particle-counting methods related to these discoveries are being commercialized by such companies as Protiveris Inc. (Rockville, Md), whose products are partially based on established Oak Ridge National Laboratory (Oak Ridge, Tenn.) technology).

In the Purdue demonstration, researchers micromachined a cantilever out of silicon, and used it to find the weight (about 9 femtograms) of a single virus particle. The demonstration shows the cantilever detects particles a few tenths of a micron in size—the dimension of cleanroom contaminants that are monitored today.

"These cantilevers are fairly sensitive to particles of those dimensions," says Bashir.

Click here to enlarge image

This photo, taken with a scanning electron microscope, shows a miniature "cantilever"—a diving board-like beam of silicon that researchers at Purdue University have used to detect a single virus particle weighing about one-trillionth as much as a grain of rice. The work, funded by the National Institutes of Health, is aimed at developing advanced sensors capable of detecting airborne viruses, bacteria and other contaminants. Such sensors will have applications in areas including environmental health monitoring in hospitals and homeland security.

The Purdue approach measures the mass of the particle, which is done by using a laser to track the movement of cantilevers that are 4 to 5 µm long, 1 to 2 µm wide, and 20 to 30 µm thick. Thousands of cantilevers can be manufactured on a single wafer, and many can be built on an individual chip.

At room temperature, these cantilevers vibrate up and down like miniature diving boards. This natural resonant frequency changes when a particle lands on the cantilever.

Spotting that change, Bashir says, enabled the researchers to weigh a single virus.

Bashir plans to drive the springboard with an external oscillator, which will up the sensitivity. He also wants to detect the motion electrically, which will make the overall system more compact. But the main enhancement will be the addition of a coating, which will make the cantilever respond only to a particular type of particle—a given virus or a specific pathogen.

The Purdue approach, which measures the mass, isn't the only way to detect small particles with cantilevers. Years ago, Oak Ridge researchers produced microcantilevers that responded to stress, bending as film forms on their surface due to molecular interactions.

This approach is being commercialized by Protiveris, which is developing products that spot protein-to-protein interactions. While the market may eventually be in drug discovery, nanotechnology/biotechnology research, and homeland defense, that isn't the case today.

"Our initial customers are actually going to be in the government and university labs," says Robert Menzi, chief operating officer of Protiveris.

Stress-dependent microcantilevers can't detect an individual particle, but Oak Ridge scientist Thomas Thundat notes that the lower limit depends on the chemistry involved, the cantilever, and the detection scheme. Today, he says, "We can detect 1/1000th of a layer of molecular adsorption."

The technique could be used for particle detection of airborne contaminants. Either the stress or mass-driven cantilever technique could yield systems that cost less, are more compact, and are more specific than today's particle counters.

Before that happens, however, further research is needed. Problems such as how to deliver the particle-bearing air stream to the microcantilevers have to be solved. Indications are that it will be some time before these issues are worked out.

Purdue's Bashir notes that he hasn't done any research specifically looking at microcantilevers as a general particle counter. Furthermore, Bob Moss, vice president of engineering at laser particle counter maker Hach Ultra Analytics (Grants Pass, Ore.), says his company hasn't actively been monitoring the development of microcantilevers, which are a form of microelectromechanical systems (MEMS), for particle detection applications.