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Researchers have learned how to improve the performance of sensors that use tiny vibrating "microcantilevers," like the one pictured here, to detect chemical and biological agents for applications from national security to food processing. Image: Vijay Kumar, Birck Nanotechnology Center, Purdue University
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Researchers have learned how to improve the performance of
sensors that use tiny vibrating microcantilevers to detect chemical and
biological agents for applications from national security to food processing.
The microcantilevers—slivers of silicon shaped like small
diving boards—vibrate at their natural, or "resonant," frequency.
Analyzing the frequency change when a particle lands on the microcantilever
reveals the particle's presence and potentially its mass and composition.
The sensors are now used to research fundamental
scientific questions. However, recent advances may allow for reliable sensing
with portable devices, opening up a range of potential applications, said Jeffrey
Rhoads, an assistant professor of mechanical engineering at Purdue University.
Creating smaller sensors has been complicated by the fact
that measuring the change in frequency does not work as well when the sensors
are reduced in size. The researchers showed how to sidestep this obstacle by
measuring amplitude, or how far the diving board moves, instead of frequency.
"When you try to shrink these systems, the old way of
measuring does not work as well," Rhoads said. "We've made the signal
processing part easier, enabling small-scale, lower-power sensors, which are
more reliable and have the potential for higher sensitivities."
Findings are detailed in a paper appearing online in the Journal of Microelectromechanical Systems.
The paper was written by graduate research assistants Vijay Kumar and J.
William Boley, undergraduate student Yushi Yang, mechanical engineering
professor George Chiu, and Rhoads. An earlier paper was published in Applied Physics Letters.
The work is based at the Dynamic Analysis of Micro- and
Nanosystems Laboratory at Purdue's Birck
Nanotechnology Center.
The aim is to apply the new approach to build sensors
capable of reliably measuring particles that have a mass of less than one picogram
at room temperature and atmospheric pressure.
The microcantilever sensors have promise in detecting and
measuring constituents such as certain proteins or DNA for biological testing
in liquids, gases and the air. The devices might find applications in breath
analyzers, industrial and food processing, national security and defense, and
food and water quality monitoring.
"One question about these sensors is whether they
will continue to work in the field," Rhoads said. "We've been doing a
lot of blind, false-positive and false-negative tests to see how they perform
in a realistic environment. We've had only a few false positives and negatives
in months of testing."
The findings focus on detecting gases and show that the new
sensors should be capable of more reliably measuring smaller quantities of gas
than is possible with current sensors.
Measuring amplitude is far easier than measuring frequency
because the amplitude changes dramatically when a particle lands on the microcantilever,
whereas the change in frequency is minute.
"We haven't beaten the sensitivity of all other
sensors yet," Rhoads said. "But the difference is that we are trying
to do it with a compact device that is truly implementable at the microscale,
while many others use fairly large laboratory equipment."
The researchers tested the cantilevers in a chamber filled
with precisely controlled quantities of methanol to study their reliability. A
patent is pending on the invention.
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