LEDs give birth to a new class of sensors

Feb. 6, 2001
A group of scientists at the University of Wisconsin-Madison (Madison, WI) have shed light on a valuable new use for LEDs by demonstrating their usefulness as chemical sensors.

On almost a daily basis now, the light-emitting diode (LED) continues to prove its versatility. The same technology behind the glowing lights reminding people to turn off VCRs and stereos is being applied to new treatments for hard-to-heal wounds and super-efficient traffic lights. Now a group of scientists at the University of Wisconsin-Madison (Madison, WI) have shed light on another application for LEDs by demonstrating their usefulness as chemical sensors.

In research published in the Jan. 25 issue of Nature, the UW-Madison researchers illustrate how chemical exposure can alter the surface structure of LED materials, causing the intensity of the light to fluctuate. The resulting light change can be put to use in simple, highly sensitive systems that warn of chemicals in the air or water. The finding may have a big impact on the national campaign to develop laboratories on a chip by offering an accurate, low-cost, mass-producible method to integrate sensors on to computer chips.

�There is a big movement to make sensors smaller and more versatile and to use the economy of scale you get from the semiconductor industry,� adds UW-Madison professor of chemical engineering and materials scienceThomas Kuech, . �What is nice about this effort is the prospect of making very small optical emitters and detectors that are chemically sensitive to a wide range of substances you would care about in the environment.�

Perhaps the most ubiquitous chemical sensors are at work in home safety systems used to detect smoke, radon, or carbon monoxide. They are also used to monitor air and water pollution indoors and outdoors, as well as problems in car engine performance. The existing technology is still relatively primitive, though, compared to the smart environments envisioned by scientists.

According to professor of chemistry Arthur Ellis, the project, which is funded through a National Science Foundation initiative called XYZ on a Chip, has the goal of demonstrating how a wide range of nonelectrical processes can exploit the power and sophistication of integrated chip technology. In addition to chemical sensors, the effort is being applied to genomics, chemistry, mechanics, and software development.

In past research, Ellis demonstrated that light emitted from certain LED materials could be altered by exposure to chemicals. Ellis teamed with Kuech and electrical and computer engineer Luke Mawst to apply this discovery to the new class of sensors. The group began by changing the surface of the light-emitting structure to enhance its chemical sensitivity. Then they integrated it onto a chip with a nearby detector system, where both the emitter and detector can communicate.

When the chip was placed in a chemical environment, the chemicals that interacted with the semiconductor surface changed the amount of light emitted and thus detected. The resulting device not only indicated the presence of that chemical, it also was sensitive to the amount of that chemical in the air.

Mawst believes the technology's most-attractive commercial potential lies in its simplicity. Conventional sensors are more-complex devices made from a variety of materials, whereas these are modeled from the same chunk of material and can be built with the cost-effectiveness of computer chips. The very flexible sensors also could be adjusted to detect everything from ammonia in a factory environment to biological molecules in a war zone.

One ultimate goal of the lab-on-a-chip research effort is to create a real-time response to environmental dangers, whether it be a chemical spill in a river or the threat of chemical warfare or bioterrorism. The current technology is nowhere near meeting that challenge, though. According to Kuech, the next step in the research will involve efforts to better understand the basic chemical reactions taking place on the surfaces of LEDs in order to optimize the process.

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