Some microscopes aren't just a window to a world invisible to the naked eye--they allow scientists to probe a bumpy molecular landscape by feel. Now a new kind of microscope, described in tomorrow's Nature, will expand these sensory horizons even further. The device allows scientists to sniff the molecules they are studying, and even alter them chemically, one at a time.
Today's best commercial atomic force microscopes have tips made of silicon or silicon nitride that run over the surface of a sample like the stylus of a record or CD player, recording all the bumps as they go along. But they are unable to tell anything about a molecule's chemistry--for example, whether it's acidic or basic. In 1994, Harvard University chemist Charles Lieber "functionalized" the tip of an atomic force microscope, outfitting it with a chemical that would be electrically attracted to acidic molecules. But this "chemical force microscope" was not as sensitive as Lieber wanted. It couldn't pick out individual molecules.
For this, Lieber needed a finer tip--which he found in newly discovered carbon nanotubes, rolled-up carbon sheets that are both thinner and sturdier than a silicon stylus. Lieber fashioned a tip from a carbon nanotube and affixed at the end of it a molecular fragment called a carboxyl group. With the new tip he was able to "sniff out" acid molecules from bases, with atomic level precision. And by affixing an enzyme to the tip, Lieber has done the work of a Lilliputian construction worker--attaching the enzyme to an individual protein.
"It's just a wonderful paper," says Daniel Colbert, a chemist at Rice University in Houston, Texas. He says the technique opens up the possibility of a new style of chemistry, based on delivering individual molecules to specific places. "I'd be surprised if this doesn't take off quickly," Colbert predicts.