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.
It’s called “dip-pen nanolithography.” But don’t think of a fountain pen or even of an antique quill pen–this nano pen isn’t for writing, at least not in the familiar sense.
Using hundreds or even thousands of the probes in parallel, dip-pen lithography could be a quick way to manufacture nano components for everything from microelectronics to faster and denser DNA chips used in genetic screening (see “DNA Chips”). “This could be much more than a research tool,” says Mirkin. “It could be a way to mass-produce nanostructures.”
In 1989, physicists at IBM’s Almaden Research Center in San Jose, Calif., dazzled the scientific world when they used a microscopic probe to painstakingly move a series of xenon atoms on a nickel surface to form a Lilliputian version of the three letters in Big Blue’s logo. While the experiment suggested that it might be possible to build things on the nanoscale, it remained an exotic, one-off trick, requiring a custom-built microscope that filled a small, vibration-damped room and temperatures around 270 C, just a few degrees above absolute zero.
A decade later, Mirkin is turning nano writing into a practical fabrication tool. By incorporating an array of eight tips into a desktop AFM process, Seunghun Hong, a postdoc in Mirkin’s lab, recently wrote out a section of a famous 1960 paper by the physicist Richard Feynman predicting the future of nanotechnology. It took Hong less than 10 minutes, and he did it at room temperature. Equally impressive, with Mirkin’s technique, one can write using various types of molecules, including biological ones such as DNA, and can readily switch from one “ink” to another. This versatility allows Mirkin to create complex structures. He could, for example, craft an array of thousands of nanostructures, each one consisting of a different type of biological molecule. Such ultradense nano arrays could prove invaluable in discovering new drugs or diagnosing disease.
Mirkin’s invention illustrates the rapid progress academic and industrial labs are making in transforming nano doodling into real technology. For many purists, true nanotechnology means building atom-by-atom to make nano machines that operate independently. It’s a powerful vision, but it’s one that likely remains years away from reality. Meanwhile, a growing number of physicists, chemists and electrical engineers are on the verge of realizing a more practical version of nanotech.
Their ultrasmall structures are a far cry from the “nanobots” (nanoscale robots) and microscopic computers envisioned by some enthusiasts. Today’s nano devices often consist of hundreds or even millions of atoms and molecules-and they lack the atomic precision that could eventually be possible in nanotechnology. What’s more, current nanostructures are frequently just one component in much larger devices. But they have one big advantage over the purists’ version: They are real. And though even the most well developed of these nano machines are still probably several years away from being commercially useful, prototypes are already demonstrating the potential role of nanotech in making possible many of tomorrow’s most alluring technologies, from pervasive computing (see “Computing Goes Everywhere”) to personalized medicine (see “Medicine Gets Personal”).