Once over lightly with chemical microscope.

Like a record needle, the sharp probe of an atomic force microscope (AFM) scans the surface of a material, tracing out the hills and valleys of the molecular landscape. Now, a microscope developed by researchers in Germany takes the method one step further by mapping the chemical, as well as the topographical, features of a material.

The technique marries an AFM with infrared spectroscopy, says Fritz Keilmann of the Max Planck Institute for Biochemistry in Martinsreid. He and his coworker Bernhard Knoll shine infrared light of a single wavelength on an AFM probe tip as it moves. Different molecules scatter the light in characteristic patterns, which are affected by the tip.

By detecting the scattered photons, the researchers mapped out the two polymers or two elements making up each test surface. Meanwhile, the AFM tip recorded topography in the standard way. Keilmann and Knoll describe their new method in the May 13 NATURE.

The technique brings scientists closer to being able to identify unknown molecules simply by passing a tiny probe over them. Such probes could be used to explore the proteins on cell membranes or to monitor the quality of computer chips--"every problem you can apply regular microscopy to and then some," says Lori S. Goldner of the National Institute of Standards and Technology in Gaithersburg, Md.

To make a chemical microscope that could differentiate among a wide variety of compounds, scientists would need to shine a range of infrared wavelengths on each bit of the sample, Keilmann says. In the current study, he and Knoll only used two different wavelengths.

The new device grows out of a method known as near-field scanning optical microscopy (NSOM), which is widely used for imaging. NSOM has advantages over electron microscopes, which require difficult sample preparation and must operate at low pressures, says Goldner (SN: 10/24/98, p. 268). Until now, NSOM using infrared light, could not image objects smaller than about 1 micrometer.

Keilmann's technique is "very impressive," Goldner says. "This is the first convincing evidence that you can do chemical mapping with a scattering probe at 30-nanometer resolution."