According to scientists at the US Department of Energy's Ames Laboratory in Ames, Iowa, back-detection geometry may provide the key to opening up in vivo biological applications for a fluorescence-based chemical sensor. Developed in collaboration with researchers at the University of Michigan in Ann Arbor, the sensor platform integrates an organic LED with a fluorescent probe/sensor and detector.Because low-voltage organic LEDs, such as this brilliant blue light source, produce little joule heat, the fluorescent layer of integrated organic LED/optical chemical sensors under development by researchers at Ames Laboratory and the University of Michigan can safely incorporate biological molecules such as enzymes, oligonucleotides and proteins.Ames senior physicist Joseph Shinar said it may be possible to package up to 256 of these sensors on a 1-mm2 chip to produce a flexible, low-cost, high-density sensor microarray. Such a device could, in principle, discriminate among multiple compounds in complex biological samples such as blood, saliva or even airborne particles.Exploring back detectionAn earlier prototype of the system, introduced last year for oxygen monitoring, relied on front detection. The scientists are now exploring back detection, with the device integration strategy based on depositing the sensor film -- a fluorescent dye embedded in a matrix -- on one side of the transparent substrate, where it would be in contact with the sample, and the organic LED on the other side, where it would be shielded from the sample. Shinar equated this with a sandwichlike effect: sample solution, sensor, substrate, organic LED. The light source excites the sensor, making it fluoresce. The fluorescence changes when the sensor detects a specific chemical in a sample and is picked up by a photodetector behind the LED; hence, the use of the term back detection. Low cost is not the only benefit organic LEDs bring to the project. First and foremost, their low voltage produces little joule heat, which is why the fluorescent layer can safely incorporate biological molecules such as enzymes, oligonucleotides and proteins. In addition, Shinar pointed to recent developments in manufacturing, such as ink-jet printing, that allow the production of low-cost multidevice arrays in large quantities with diverse shapes and sizes. If necessary, each sensor could have its own light source. And because organic LEDs cover the entire visible spectrum, sensor platforms could easily incorporate mixed sensor types that respond to different chemical species.Shinar and colleagues, including a University of Michigan project team headed by chemist Raoul Kopelman, detail preliminary results with the fluorescence-based chemical sensor and the back-detection strategy in a report recently submitted for publication.