Optical Microscopy Shines Light On Biology

December 06, 1996

LAWRENCE, Kans. -- A chance to study individual biological molecules in a liquid environment has been a goal of scientists for years.

At the University of Kansas, Robert C. Dunn, assistant professor of chemistry, has reached that goal by improving on near-field optical scanning microscopy, a technique that can be used to look at individual molecules.

In instruments that use this process, the object to be viewed is placed close to a special fiber-optic probe. Laser light emitted from the probe is used to view and study samples at extremely high magnification.

Dunn, along with Chad Talley, Lawrence doctoral student, and Gregory Cooksey, Quinter junior, have designed a new "cantilevered" probe, which features a near-90-degree curve.

"With these new probes, we are one of the few sites successfully using near-field optical microscopy to study biological samples," he said. "People thought it couldn't be done."

Dunn said the softness and fragility of biological samples have been a problem for near-field microscopy because of the way the probe is positioned. The new tip design avoids many of those problems and opens up significant applications in the biological sciences.

"The cantilevered near-field tip brings less force onto the sample, making it much gentler," Dunn said. "The way we sense when the probe is near the sample is by vibrating the probe back and forth and watching the amplitude of that movement."

When the probe gets close to the sample's surface, the movement of the probe's tip is reduced by the sample, he said. This is used to regulate the tip position above a specimen and to follow contours in its surface.

"With the straight tip, the problem is that when you start going through water, the water itself reduces the vibration of the tip," Dunn said. "With our tips, we vibrate vertically with respect to the sample so you don't get the large damping caused by the water."

With the high sensitivity of near-field scanning optical microscopy and the newly developed cantilevered probe, researchers can examine individual biological molecules.

"It's important to see what single proteins do and map their distribution in a system," he said. "If you can understand that, it can lead to a better understanding of how protein changes affect biological processes."

By studying a single molecule, "we can examine how the molecule moves very precisely," he said. "For us, we plan to use this to watch a single protein ion channel open and close."

The near-field scanning optical microscopy technique is relatively young. Although the fundamentals were mentioned in a series of letters between Edward A. Synge and Albert Einstein, near-field optical microscopy didn't become practical until 1991.

Dunn has been active in the field since 1992, when he carried out a postdoctoral fellowship with X. Sunney Xie at the Pacific Northwest Laboratory, Richland, Wash.

"It's great getting in on the ground floor," Dunn said. "Everything you see is new."

University of Kansas

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