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Chemists make molecular maps

By modifying a microscopic needle ordinarily used to study surface topography, MIT chemists and colleagues have been able to map the distribution of different molecules lying on a surface.

The work, which is based on chemical interactions between molecules on the tip of the needle and molecules on the surface, has launched a new form of microscopy that could allow scientists to probe the molecular basis for why nothing sticks to Teflon, for example.

The researchers involved in the work are Provost Mark S. Wrighton, CIBA-GEIGY Professor of Chemistry; Lawrence F. Rozsnyai, a graduate student in chemistry, and Harvard chemists C. Daniel Frisbie, Aleksandr Noy and Charles M. Lieber. The five published the work in Science last month.

To create their molecular maps, the chemists modified an existing technique known as atomic force microscopy. "AFM is like a microscopic record player," said Mr. Rozsnyai. "As a [microscopic] needle or probe tip moves over a surface, it bounces up and down. If the tip is scanned across the entire surface, you can get a topographical map of the surface."

That map, however, gives no information about the chemical composition of the surface or specific chemical interactions between the tip and the surface (conventional tips are essentially inert). Such information would be useful for studies of adhesion and lubrication, or why one surface sticks to or slides past another (e.g., the molecular basis for nonstick cookware).

But what if the tip was modified by attaching specific molecules to its end? The MIT and Harvard scientists reasoned that a modified tip would interact with molecules on a surface and that forces between the two could be measured and converted to an image.

To test this hypothesis the researchers modified the tip with either CH3 molecules (hereafter referred to as X) or COOH molecules (Y). The tip is so small that only about 50 molecules can fit on it.

They also created a surface with a geometrical pattern of X and Y molecules that is invisible under most circumstances. But after scanning the modified tip over the patterned surface, they found that they could measure the friction between the molecules on the tip and those on the surface. And those data could indeed be converted to a friction image of the surface, showing the distinct X and Y areas.

This works because of a simple physical fact: like likes like. In other words, a tip modified with X molecules will "catch" on surface areas patterned with X, while essentially gliding over Y areas. Areas of higher friction appear brighter in the resulting images.

The chemists created two friction images of the patterned surface: one was created with an X tip, the other with a Y tip. The result? While geometrically identical, the contrast in the two images is reversed. With an X tip, areas on the surface patterned with X molecules appear brighter than those patterned with Y and vice versa for a Y tip.

"So this pair of images shows that we have indeed modified the tip, and that we are actually measuring intermolecular frictional forces between molecules on the tip and sample," Mr. Rozsnyai said.

Mr. Rozsnyai noted that the friction forces measured over the surface for X and Y groups correlated well with the adhesion force for different tips and surfaces-the force required to pull the tip from the surface. "These should be related, and we want to understand how in a chemically specific way," he said.

The new technique, which the researchers call chemical force microscopy, could have a number of applications in addition to its usefulness in studying adhesion and lubrication. For example, it could be an aid to biologists searching for new drug compounds. By putting an array of drug molecules on a surface, then scanning that surface with a tip coated with a receptor molecule, "the scanning could quickly reveal the strongest interaction, and thus the molecule best suited to bind to that receptor," wrote Jocelyn Kaiser in a Science story that accompanied the researchers' technical article.

Professor Wrighton commented that "the work we have done collaboratively with Professor Lieber and his group at Harvard represents a step forward in the understanding of surface properties as well as [a new] methodology for characterizing surfaces.

"Interdisciplinary efforts and partnerships like this bring important educational benefits to our students and contribute to advancing scientific understanding of complex systems."

The work was supported by the Office of Naval Research, the Air Force Office of Scientific Research and the National Science Foundation.

A version of this article appeared in the October 26, 1994 issue of MIT Tech Talk (Volume 39, Number 9).

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