AMES, IA - Scientists at the U.S. Department of Energy's Ames Laboratory, in collaboration with scientists at the University of Michigan, Ann Arbor, have developed and demonstrated a novel, fluorescence-based chemical sensor that is more compact, versatile and less expensive than existing technology of its kind. The new sensor holds promise for myriad potential applications, such as 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.
The new sensor grew out of a basic research effort by Ames Laboratory senior physicist Joseph Shinar and members of his group to study the photophysics of luminescent organic thin films and organic light-emitting devices, or OLEDs, i.e., those that luminesce, or emit light, when a voltage is applied. The University of Michigan researchers, led by chemist Raoul Kopelman, were interested in developing fluorescent sensors. The collaboration resulted in the creation of an integrated OLED/optical chemical sensor.
"Integration and miniaturization of fluorescence-based chemical sensors is highly desirable, as 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 compounds," said Shinar. He explained that fluorescence-based chemical sensing devices, in general, 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.
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," said Shinar. "With this kind of geometry, called 'back detection,' we should be able to use the sensor for in vivo biological applications." (In vivo refers to occurring within the living organism.)
Explaining the back-detection design, Shinar said, "Let's say you have some solution on a glass substrate - blood, urine, whatever - that 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. As the OLED light source excites the sensor, the sensor fluoresces. When the sensor detects the compound of interest in the sample solution, its fluorescence changes, and the change is picked up by a photodetector positioned behind the OLED light source."
Early in 2001, Shinar and his collaborators successfully demonstrated an oxygen-sensor prototype in which the OLED was integrated with the oxygen-sensor film. "We got really good results on the oxygen sensor," he said. ?The response of the sensor to oxygen was very fast."
Shinar said they had used front detection instead of back detection with the oxygen-sensor prototype. Now they're trying to demonstrate the back-detection capability. In addition, he anticipates that the Ames Lab research team and the University of Michigan collaborators 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 said.
The versatility, flexibility and cost-effectiveness of OLEDs offer excellent opportunities for developing OLED/optical chemical sensor arrays and high-density microarrays. Such systems, in principle, could contain up to 16x16 = 256 sensors on a single, one-square-millimeter chip. They would be able to discriminate between multiple compounds in complex biological samples, such as blood, urine, saliva or airborne particles, creating what Shinar described as a very low-cost, compact and versatile 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," said Shinar. "The integration is really the big step forward."
Ames Laboratory is operated for the DOE by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.
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