This Microscope Can Poke At Atoms

SINCE THEIR INVENTION A DECADE AGO AT IBM AND STANFORD UNIVERSITY, atomic force microscopes (AFMS) have produced startling images of exotic atomic landscapes. The scopes perform their magic by dragging a superfine needle across a sample of material. The needle bumps up and down as the atoms that form its tip are repelled by atoms on the sample. A computer then translates the movement into dazzling, multicolored topographic drawings.

Early AFMs used silicon tips. But in 1996, Nobel prize-winning chemist Richard E. Smalley replaced the silicon with a tough new carbon structure known as a "nanotube," which proved more flexible. Now, in the July 2 Nature, Harvard University chemistry professor Charles M. Lieber offers a glimpse of what nanotube tips will do for biology. Lieber and his team chemically modified a nanotube so that the AFM needle itself can perform chemical analyses as it travels over the sample. Since the tube is made of carbon, Lieber explains, "we can exploit well-developed organic reactions to attach various molecules that interact differently with the sample." That opens great possibilities. For example, it will be possible to explore features on a cell by attaching something to the nanotube that binds to a particular receptor. Or one could test whether a new drug molecule binds to a protein.

AFMs still can't deliver 3-D images of molecules. That requires x-ray crystallography, which constructs images out of patterns of x-rays diffracted from crystals. "But AFMs allow you to get functional information about the molecule you are studying," Lieber says.