An uncommon sensor

A new spectroelectrochemical sensor for monitoring hazardous waste uses three modes of selectivity instead of the usual one or two of existing sensors.

The three modes, from which the sensor derives its name, are spectroscopy, electrochemistry, and selective coating. The new sensor, developed by the University of Cincinnati and the Pacific Northwest National Laboratory, is based on waveguide technology in which a light beam is propagated along the sensor core.

‘This combines the best of chemistry with waveguide technology, which is traditionally the ballgame of electrical engineering,’ said Carl Seliskar, Professor of Chemistry at the University of Cincinnati.

The multidisciplinary team included project leader William Heineman, Thomas Ridgway, Seliskar, and electrical engineer Joseph Nevin, all professors at the University of Cincinnati. Chemist Sam Bryan from the Pacific Northwest National Laboratory also collaborated on the three-year project that was funded by US DOE’s Environmental Management Science Program (EMSP).

‘The Department of Energy wanted a sensor they could put in a waste tank and make lots of measurements more quickly, or leave it in there, and monitor what was going on over months or a year,’ explained Heineman.

Heineman and Seliskar developed a sensor coating that is selective for ferrocyanide, the target compound within the jumbled mix of wastes such as those found in the 117 Hanford underground waste tanks in southeastern Washington State that were built to hold radioactive waste between 1943 and 1985.

‘I call these smart materials,’ said Seliskar. ‘You can exclude all the things you don’t want to measure and include what you do want to measure.’

The selective coating only allows certain compounds to enter. For example, all negatively charged ions might be able to enter the sensor while all positively charged ions are excluded. Then the electrochemistry comes into play when an electrical potential is applied, and an even smaller group of compounds are electrolyzed. Finally, a very specific wavelength of light is used to detect the actual compound of interest – in this case ferrocyanide.

Pacific Northwest chemist Bryan assisted the UC team in testing the spectroelectrochemical sensor using actual ferrocyanide-containing waste. The results from the test on the tank waste material were nearly identical to those predicted from tests using a simulant in the university’s lab. This result meant that the sensor works well with actual contaminated materials.

Bryan and others have been involved in ferrocyanide characterization at Hanford since the 1980s. Initially, ferrocyanide was thought to pose the threat of explosion within the underground tanks, but that fear has since been relieved by the discovery that ferrocyanide completely decays within the tanks. However, because of the knowledge accumulated about this chemical, it made a good test for the new chemical sensor.

‘We’ve demonstrated that the novel concept works on a number of systems,’ said Heineman. ‘Now, we can move forward with specific applications.’

To that end, the EMSP has awarded a three-year renewal of the project to the University of Cincinnati/PNNL team to apply the spectroelectrochemical sensor in remote detection of pertechnetate, a soluble form of the radioactive element technetium that is currently a threat to groundwater near the Hanford Site.

Graduate students at UC are also adapting the basic design and concept to monitor other compounds as well. These include glucose monitoring, which would benefit diabetics and offer a less invasive way to monitor premature infants, as well as detection of the toxic pesticide paraquat.

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