Miniature sleuth searches for toxins

A portable, chip-size version of a detection system that is commonly used by industry and law enforcement to identify everything from agricultural toxins to DNA has been created by researchers at Purdue University, West Lafayette, Ind. The miniature detector could move certain types of testing from the lab into the field, saving time and money while increasing security.

"Now we have a way of putting all of the critical components on one wafer," says Timothy D. Sands, the Basil S. Turner Professor of Engineering in the School of Materials Engineering and the School of Electrical and Computer Engineering. "It's much the same in concept as going from separate transistors to an integrated circuit that includes many transistors on a single chip."

The traditional fluorescence detection system works by attaching a fluorescent dye to specific molecules in a substance and then shining a laser onto the substance. The laser light is absorbed by the dyed molecules, causing them to emit a certain color, which is picked up by a sensor. The detection work normally is done using bulky, stationary equipment in a laboratory. The new device, however, fits on a centimeter-wide chip, promising the development of miniature detectors. Such portable instruments would be useful for a wide range of applications, from biologists doing basic research to farmers testing crops for toxins.

To create the chips, Sands used a technique known as "laser liftoff," which employs a powerful laser to separate selectively and transfer thin-film components from one substrate to another to build up the successive layers of a "system-on-a-chip."

Moreover, "something new that we recently reported was to put two colors, a blue and green LED [light-emitting diode] on one chip." At least two colors are considered critical for the analysis of biological and chemical materials.

"If you wanted to do biochemical detection of anthrax or some other substance, you almost always have to have two colors and your sample is tagged with two dyes," Sands explains. "One serves as a control--to precisely calibrate the measurement--and the other color is for actually detecting the molecule you are after."

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