In recent years chemists have tried to tiptoe rather than stomp through the material world, by increasingly refining their methods of manipulating chemical reactions. Last April chemist Peter Schultz and physicist Paul McEuen of the University of California at Berkeley took that quest to its extreme. They announced that they had come up with a way of triggering chemical reactions molecule by molecule.
The key to the technique is to put a dab of platinum on the microscopic tip of an atomic force microscope. (The tip of such a microscope is a tiny cantilever that rides like a phonograph needle just above the surface of a sample and reacts to forces exerted by the electrons beneath it.) The scientists then prepared a surface of azide, a compound containing three nitrogen atoms, and bathed it in alcohol that had been spiked with hydrogen. Then they tapped an azide molecule with the microscope’s platinum-coated tip. In this situation the platinum acted like a catalyst, stimulating the azide and the alcohol to react with each other: specifically, two hydrogen atoms from an alcohol molecule replaced two of the three nitrogen atoms in the azide to form amine, an organic compound chemically similar to ammonia. By repeating the process on one molecule after another, Schultz and McEuen drew tiny patterns of amine molecules in the azide surface. There are any number of techniques to draw patterns on a small scale, says Schultz, but not many to do chemistry on a small scale.
The breakthrough opens new possibilities for nanoengineering and materials science. Of more immediate value, the technique lets researchers see what happens as they go about changing the world, one molecule at a time.