New class of sensors fashioned from LEDs

Already glowing away on thousands of consumer electronics products, the light-emitting diode (LED) is proving to be a remarkably versatile material.

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 new super-efficient traffic lights. Now a group of UW–Madison scientists have shed light on a valuable new use for LEDs by demonstrating their usefulness as chemical sensors.

In research published in the Jan. 25 issue of the journal 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. That 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, inexpensive and mass-producible method to integrate sensors on to computer chips.

“There’s a big movement to make sensors smaller, more versatile and to use the economy of scale you get from the semiconductor industry,” says co-author Thomas Kuech, a professor of chemical engineering and materials science. “What’s 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, both indoors and outdoors, and monitor problems in car engine performance. But the existing technology is primitive compared to the “smart environments” imagined by scientists.

Arthur Ellis, professor of chemistry and co-author of the paper, says this project is funded through a National Science Foundation initiative called “XYZ on a Chip.” The essential challenge is to demonstrate how a wide range of non-electrical 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.

A light-emitting diode, a tiny chip made of semiconducting materials, converts electrical energy into visible light. The chips also can convert light into electricity when used in a solar cell or photo cell. In past research, Ellis demonstrated that light emitted from these materials could be altered by exposure to chemicals.

Ellis teamed with Kuech and electrical and computer engineer Luke Mawst to apply this discovery to a 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 they placed the system in a chemical environment, the chemicals that interacted with the semiconductor surface changed the amount of light emitted – and thus detected. But rather than just indicating the presence of that chemical, the system was also sensitive to the amount of that chemical in the air.

Mawst, co-author of the paper, says the technology’s most attractive commercial potential is its simplicity. Conventional sensors are much 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. These very flexible sensors 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, Kuech says.

Mawst says industry has shown early interest in the technology. In the next step, researchers will try to better understand the basic chemical reactions that are taking place on the surfaces of LEDs in order to optimize the process.