A three-year effort to find a safe way to monitor the radioactive and hazardous wastes inside the giant tanks at the Hanford nuclear weapons facility in Washington has resulted in a novel idea and prototype for remote sensing. The results will be presented in a series of talks and poster presentations during the annual meeting of the American Chemical Society in New Orleans Aug. 22-26.
"They (the Department of Energy) want a sensor they can put in a tank and make lots of measurements more quickly, or leave it in there, and monitor what's going on over months or a year," explained University of Cincinnati Distinguished Research Professor William Heineman, one of the principal investigators on the DOE-funded project. The others are UC chemistry professors Tom Ridgway and Carl Seliskar.
The sensor is an improvement over existing sensors, because it contains an extra "dose" of selectivity. "Most sensors only have one or two modes of selectivity," said chemistry graduate student Susan Ross. "Our concept is new, because it actually has three."
In practical terms, that means the sensor has three different ways to find and identify the compound of interest. That's important, because the Hanford tanks are a jumbled mix of chemical and radioactive wastes.
"It's a really harsh, harsh mixture as far as sensors are concerned," said Ross who noted UC's experimental work demonstrated the sensor can hold up under conditions which simulate those in the Hanford tanks.
The target compound for the experiments was ferrocyanide, which was of high interest to the Department of Energy managers at the Hanford site because of the potential of ferrocyanide and nitrates reacting in a violent manner within the tanks. However, it has since been found that all the ferrocyanide was destroyed in the tanks, eliminating that threat.
However, other graduate students are 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. Other applications involve the detection of the toxic pesticide paraquat and the neurotransmitter dopamine.
"We've demonstrated that the novel concept works on a number of systems," said Heineman. "Now, we can move forward with specific applications."
The three-way selectivity comes from the use of selective coatings, electrochemistry, and spectroscopy. 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. Next comes the electrochemistry. A 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.
The following is one example of how the system works: A mix of negatively charged ions is pulled into the sensor by the selective coating. The applied potential turns the compound of interest yellow. A blue light is shined through the detector portion of the sensor, and the compound is detected and measured.
"The color of the light coming in here is blue," explained Ross, pointing out a model version of the sensor. "Yellow will absorb the blue wavelengths, so it will decrease the amount of light going through here. That's what we measure."
So, the sensor gives both qualitative and quantitative information. The color of the light can be any visible color, depending on what compound needs to be detected.
"What we actually do is modulate it," said Ross. "We cycle the potentials to reduce it and to oxidize it...so here it might be yellow...here it's colorless. Every time it goes to yellow, we can measure it."
The first prototype sensor relies on a well-known design called a "multiple internal reflection device" (MIRD). That allows the light waves to bounce up and down through the sensor where they can be detected. Unfortunately, the waves typically only bounce two to three times in a microsensor. That only gives you two to three chances to measure the drop in the light's intensity.
To improve detection, the UC researchers are also developing another model using waveguide technology with the assistance of electrical engineering professor Joseph Nevin and graduate student Saroj Aryal. The physics and engineering are rather complex, but it really boils down to more bounces and more opportunity to detect the compound of interest.
"If we can build a device where these bounces are continuous all along this, then that greatly enhances the detection capabilities," said Ross.
The MIRD can detect compounds in concentrations as low as 10 micromolar (one molecule in a million). In lab tests, the newer design was 1,000 times more sensitive, detecting compounds at the nanomolar level. The goal is to reach femto- or zepto-molar levels which is essentially the same as counting individual molecules.
There are two versions of the waveguide sensor using gold and indium tin oxide as electrodes. The waveguide itself is made from germanium-doped silica or from ion-exchanged glasses, and the UC researchers were able to demonstrate for the first time that it will work with wavelengths of light as low as 440 nanometers. That means it can detect yellowish substances which are quite common.
Chemistry graduate student Mike Stegemiller will take some of the prototypes to the Hanford site this September, but sensitivity won't be as important there. The wastes are quite concentrated there. But the new approach should be valuable in medical monitoring and other applications requiring high sensitivity.
The group of UC researchers presenting results from the project at the American Chemical Society in New Orleans include Heineman and Ross as well as graduate students Michael Stegemiller and Michael Clager, and chemistry professor Carl Seliskar. Other graduate students on the project include Jennifer DiVirgilio-Thomas, Mila Maizels, Tanya Rarog, Mark Wanamaker, and Imants Zudans.
Additionally, Heineman and Gary Eller of Los Alamos organized a five-day symposium at the ACS meeting featuring research results from DOE's Environmental Management Science Program which funds the sensor research.
The work has been so promising, DOE recently awarded the team a three-year renewal to develop a sensor which can remotely detect pertechnetate, a soluble form of the radioactive element technetium which is a threat to groundwater sources near the Hanford site.
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