Fiber Sensor Detects Lithium

Brent D. Johnson

With its ability to readily accept and discharge electron carriers, lithium is one of the most common species of elements in rechargeable power cells. When it comes in contact with water, however, it is highly volatile. Lithium batteries, therefore, are composed of lithium salts that are dissolved with nonaqueous electrolytes, such as propylene carbonate, that stabilize the material.

But the processes that take place in nonaqueous materials are more complex than those in aqueous ones and resist analysis with conventional chemical sensors. Now researchers at the University of South Carolina have developed a fiber optic fluorescence sensor for measuring the concentration of lithium ions in a nonaqueous solution.

Lithium is a difficult metal to detect because it does not offer an optical signal, explained Catherine J. Murphy, a researcher on the project. The team resolved the problem by turning to a molecule called 2-(2-hydroxyphenyl) benzoxazole (HPBO), which binds to lithium and causes it to fluoresce in the blue. To construct the sensor, S. Michael Angel and his colleagues immobilized the HPBO at the tip of an optical fiber behind a polyvinyl chloride slow-release membrane, where it was excited by the light from a 150-W xenon lamp sent through the fiber.

When the sensor is in a solution containing lithium, an HPBO-Li⁺ complex forms and fluoresces. The response is transmitted back through the fiber to a photodetector, which produces an electrical signal proportional to the lithium concentration. The use of a slow release-membrane enables the sensor to respond continuously to changes in concentration.

The advantages of the sensor are twofold. First, because it is an optical system, it does not suffer from electrical interference. Second, because the fluorescence is limited to 420 to 440 nm, the background noise is reduced, enabling a measurement range of 1 x 10^-2 to 1 x 10^-6 mol/l and a sensitivity as low as 3 x 10^-7 mol/l.

Ultimately, the group would like to use the technique in an optical charge indicator for lithium batteries. Anoth-er potential application is with the lithium carbonate medication taken to treat disorders such as manic depression and schizophrenia.