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Microsensors Tackle Hazardous Waste

Photonics Spectra
Jan 2000
Kevin Robinson

Researchers from the University of Cincinnati have developed a highly selective microsensor that the US Department of Energy (DoE) may use in its efforts to monitor or clean up the Hanford nuclear weapons facility in Washington state. Besides being rugged enough for use in hazardous waste, the sensors have three levels of selectivity, which, according to Michael Clager, a senior graduate student on the project, ensures that a signal comes from the desired analyte.

The sensors are made from optically transparent electrodes that have been coated to attract only certain kinds of chemicals. Clager explained that this thin film collects chemicals with a specific electric charge out of the surrounding environment. When a repeatable cycle of voltage is applied to the electrodes, it changes the optical properties of the chemicals that are stuck inside of the thin film.

These optical properties are monitored using total internal reflection spectroscopy. As the analyte changes its electrochemical state, the amount of light transmitted through the total internal reflection element varies. The modulated optical response is the analytical signal.

The sensors are connected to fiber optic and electrical cable, enabling the transmission of optical and electrical signals. To produce an optical signal, the researchers use an array of illumination devices, including a HeCd laser from Kimmon Electric of Englewood, Colo., and a xenon light source from Sunnyvale, Calif.-based ILC Technology. They couple the light to the sensor with silica optical fiber from Fiberguide Industries Inc. of Sterling, N.J., and Schott prisms.

Clager said the thin-film coating is crucial to the sensor's success: Combining a specific polymer with sol-gel silica lays the groundwork for a selective and operational sensor. The group, which includes principal investigators William Heineman, Tom Ridgway and Carl Seliskar, will continue to develop new coating combinations.

Smaller is better

In an effort to miniaturize the sensor, graduate students Michael Stegemiller and Susan Ross are designing and testing new optical structures. Because this concept involves evanescent field spectroscopy, a miniature total internal reflection structure would produce a greater number of reflections at the sensing-area interface, resulting in lower detection limits and enhanced sensitivity.

The Environmental Management Sciences Program of the DoE's Office of Environmental Management funded the sensor's development. Tests have shown it to be sensitive to the 1-ppm level. The group hopes to put different versions of the device to use in biomedical and other sensing applications. Clager said that the decision to use it for the Hanford cleanup rests with the DoE.

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