It's an odd way to do chemistry. In a small room off his main lab at Northwestern University, Chad Mirkin sits at a personal computer and types. Next to him on the desktop is a plain-looking analytic instrument. Only this is no ordinary piece of lab equipment. It's an atomic force microscope, or AFM, and it's changing the way scientists interact with matter on the very small scale. This particular version of the AFM, specially modified by Mirkin and his co-workers, is about to perform a feat that just a few years ago would have been unthinkable.

Inside a chamber of the AFM, invisible to the naked eye, the tips of tiny probes dip into a well of organic molecules. The microscopic tips, sharpened to a point only a few atoms wide, then "write" the words typed by Mirkin in letters tens to hundreds of nanometers wide (a nanometer is a billionth of a meter). The process works because the organic molecules flow off the probe-just like ink from the point of a fountain pen-via a water droplet that forms on the end of the tip; the molecules then bind to the gold writing surface in orderly fashion. By automating the procedure and rigging up a number of tips in parallel, Mirkin has learned how to use the AFM to rapidly and directly create structures at the nanometer scale. At the magnification required to read the letters, a line from a ballpoint pen would be over a kilometer wide.