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Sensible sensors

The beauty of a new chemical sensor lies in its simplicity



Senior physicist Joseph Shinar (right) and graduate assistant Lijuan Zou watch as graduate assistant Kwang-Ohk Cheon adjusts an organic light-emitting device, or OLED, prior to testing its brilliance.

"We can envision the whole thing to be about the size of a silver dollar. Ultimately, it should have extremely wide-ranging applications," says Joseph Shinar.

The Ames Laboratory senior physicist is talking about the novel, fluorescence-based chemical sensor he developed in collaboration with chemist Raoul Kopelman from the University of Michigan, Ann Arbor. The new sensor is smaller, less expensive and more versatile than existing technology of its kind, making it a potentially perfect fit for monitoring oxygen, inorganic gases, volatile organic compounds, biochemical compounds and biological organisms. Within the field of molecular diagnostics for biomedical and biochemical research, the sensor could be used for point-of-care medical testing, high-throughput drug discovery, and detection of pathogens and other warfare agents.

Ideally Integrated The new sensor grew out of a basic research effort by Shinar and his Ames Lab research group to study the photophysics of luminescent organic thin films and organic light-emitting devices, or OLEDs (those that luminesce, or emit light, when a voltage is applied). The University of Michigan researchers, led by Kopelman, were interested in developing fluorescent sensors. So the two groups joined forces to drive the invention of an integrated OLED/optical chemical sensor.

Shinar says the sensor had its beginning with a simple idea put forth by postdoc Jon Aylott working with Kopelman. "The invention started with just some scribbled notes about using organic light-emitting devices as light sources for fluorescent sensors," Shinar recalls. "When the idea was first brought up, my own response was, ‘Well, there's nothing here. It's so obvious — of course you can excite a fluorescent sensor with an OLED.' You can, but it hadn't been done yet. And the beautiful thing is that you can integrate the two."

Shinar explains that the integration and miniaturization of fluorescence-based sensors is highly desirable because it is the first step towards the development of fluorescence-based sensor arrays that could be used for analysis of living cells and organisms, and biochemical com-pounds. In general, fluorescence-based chemical sensing devices include three components: a light source that excites the sensing element, the sensing element that produces the fluorescence (usually a fluorescent dye that is used to tag the sample under investigation), and a photodetector that responds to the fluorescence of the sensor. Conventional sensors use lasers or inorganic light-emitting devices as light sources, but they present problems. Not only are they expensive, they are also bulky and cannot be integrated with the other sensor components.

Stylishly simple

Shinar's and Kopelman's OLED/optical chemical sensor is unique in the simplicity of integration of the detector and the OLED light source that excites the fluorescence. "This is a real advantage," says Shinar. "With this kind of geometry, called ‘back detection,' we should be able to use the sensor for in vivo biology applications." (In vivo refers to occurring within the living organism.)

Explaining the back-detection design, Shinar says, "Let's say you have some solution — blood, urine, whatever — on a glass substrate. The solution has lots of compounds in it that you want to detect. Your sensor is in contact with the biological solution on the substrate, and your OLED light source is behind the substrate. It's like a sandwich: sample solution, sensor, substrate, OLED." Powered by a miniature battery, the OLED light source excites the sensor, which fluoresces. When the sensor detects the compound of interest in the sample solution, its fluorescence changes, and the change is picked up by a photo-detector positioned behind the OLED.

Shinar says the integration of the OLED light source, fluorescent sensor and detector makes the whole device a lot more compact and should also permit the development of an array of fluorescent sensors to be driven by an array of OLEDs. Describing an envisioned array, he says, "You could have a square with the corners formed from four OLED pixels that would be exciting the sensor in front of them. The light from the fluorescent sensor would come back through the area between the OLED pixels. Each pixel could be 10 microns, so this whole thing could easily be a square of 40 microns by 40 microns — that's your sensor."

Prototype and promising product

Early in 2001, Shinar and his collaborators successfully demonstrated an oxygen sensor prototype in which the OLED was integrated with the oxygen sensor film. "The results were excellent," he says. "The response of the sensor to oxygen was very fast."

Shinar says they used front detection instead of back detection with the oxygen sensor prototype. Now their goal is to demonstrate the back-detection capability. In addition, he anticipates that the Ames Lab and University of Michigan research teams will be able to develop a prototype for a glucose sensor in the near future. "The recipe is there, but we're wrestling with the stability of the glucose enzyme, which has a drastic effect on the uptake of oxygen by glucose," he says.

The integrated OLED/optical chemical sensor has tested out so well that Shinar is investigating the possibility of starting up a business. "We'd be making the whole sensor device," he says. "It would include packaging the OLED with the sensor, filters, photodetector, power supply and the readout mechanism, which would be digital, either on a liquid crystal display or a regular light-emitting diode — or, it could be an OLED readout."

Because most of the components for the sensor are so cheap, Shinar says almost the whole package could be disposable. The only component that wouldn't be disposable, at least in the foreseeable future, would be the photodetector. "If the sensor is produced in mass volume, I could easily envision its price to be less than $50 for the whole thing," he says. "That's just a guess based on having the photodetector be less expensive, because everything else should be really in the pennies."

The versatility, flexibility and cost-effectiveness of OLEDs offer excellent opportunities for developing OLED/optical chemical sensor arrays and high-density microarrays. Such systems would be able to discriminate between multiple compounds in complex biological samples, such as blood, urine, saliva or airborne particles, creating what Shinar describes as a very economical, compact and practical optoelectronic "nose" or "tongue," suitable for in vivo measurements.

"The big excitement is that a whole new paradigm in sensor technology could emerge from this work — from this basic idea of just integrating the very low-cost OLED light source with the fluorescent sensor," says Shinar. "The integration is really the big step forward."

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