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”).
If mirkin can be described as a nano scribe, IBM electrical engineer Peter Vettiger is a nano boxer, using AFM tips to punch at a soft polymer surface. Working in the same IBM Research lab in Zurich that helped invent the AFM in 1986, Vettiger and co-workers have built a data storage device that uses an array of 1,024 tiny AFM probes to make indentations in the polymer, each divot “writing” a bit of information no more than 50 nanometers in diameter. The scientists then use the same array of tips to rapidly read the indentations and erase them as needed.
For those pondering the future of information technology, the IBM work is exciting because storing a bit of data at that scale translates into the ability to pack immense amounts of data into a very small area. Today’s best storage products (based on magnetic memory) hold about two gigabits per square centimeter, and physicists believe the limit of magnetic memory is around 12 gigabits per square centimeter.
Results from Vettiger’s prototype, nicknamed “Millipede,” suggest the AFM-based memory could smash those limits. In tests done last year, the IBM scientists achieved a density of 35 gigabits per square centimeter (up to 80 gigabits per square centimeter using a single AFM tip), reading and writing the information at a speed that rivals existing magnetic devices. Such a density of information could make it possible, by integrating millions of tips together, to produce a hard drive with terabytes of memory-about 40 times greater than what is now commercially available. So far, says Vettiger, there aren’t any “show-stoppers” to achieving that vision.
Even more intriguing for those interested in pervasive computing, the technology could mean packing a few gigabytes (enough memory to hold a thousand high-resolution photographs or a thousand 200-page books) onto a device the size of a wristwatch. The advent of ubiquitous computing will create new markets for ultrasmall hard drives, particularly for mobile products such as cell phones and watches. Last summer, for example, IBM introduced a product called Microdrive that packs a gigabyte onto a miniaturized magnetic hard drive roughly the size of a matchbox. But, says Vettiger, the Millipede technology could go far beyond that, making gigabyte hard drives as small as a square centimeter. Equally important, he says, this AFM-based “nanodrive” will require less energy to operate than a magnetic hard drive-a critical factor in portable products.
The prospect of watching videos on his wristwatch, however, is not what drives Vettiger. Building the Millipede prototype proved the IBM technologists could integrate a large number of AFM tips with the electronics required to control them-and do it all on a small chip. Millipede is, in effect, a chip in which microelectronics are combined with micromechanics. And, says Vettiger, it could be possible to build a “smart” version of Millipede that intelligently searches its ultradense heap of data for patterns. “You now have millions of transistors on a chip. You can build the same number of mechanical devices on a simple chip, providing functions that electronics can’t do,” he says. “I’m very confident that in Millipede you’re just seeing the tip of the iceberg.”
Springs to Life
one reason for vettiger’s enthusiasm is that mechanical devices can do things electronics can’t. Electronics are great for moving information, but with mechanics you can detect physical forces and material properties-such as mass-possibly down to the level of individual molecules. Down the hall from Vettiger, Christoph Gerber, one of the inventors of the AFM, is turning loose his nanomechanical skills on biology in order to do just that.
An AFM’s tiny imaging tip is suspended from an ultrathin cantilever; as the tip rides over an atomic or molecular surface, minuscule deflections in the cantilever are measured optically with the help of ultrasmall lasers. These cantilevers are essentially small springs, sensitive enough to measure the nano force from individual atoms. Gerber’s idea is to use an array of these cantilevers as simple but extremely sensitive sensors. If you coat one of the cantilevers in the array with, say, a particular sequence of DNA, the complementary strand of DNA will selectively bind to that cantilever. You can then detect the deflection of that cantilever and use the information to measure the presence of that specific sequence of DNA-something that is of enormous value in medical research, disease detection and genetic screening.
Gerber and his co-workers have recently built such a sensor. Consisting of eight cantilevers that are each 500 micrometers long but less than one micrometer thick, the device is sensitive enough to measure deflections of only a few nanometers. In recent tests, the sensor differentiated DNA sequences differing by a single base pair (the smallest unit of DNA information); the ability to detect individual base-pair differences without radioactive or fluorescence tags is a remarkable accomplishment. Existing technology for DNA screening-DNA chips-has found wide applications in everything from disease diagnostics to biomedical research; but these commercial chips require the DNA to be fluorescently tagged and read by a bulky optical reader. Gerber believes his biosensors, which don’t require tagging of DNA, are potentially far simpler and easier to use.
The cantilever technology could also prove to be a simple way to detect specific proteins, a feat that Gerber says is difficult for current technology. “If we can fully develop this for proteins, we see a great potential,” says Gerber. For example, he says, the onset of a heart attack produces in the body a signature set of proteins. However, it often takes hours for physicians to sort out the welter of proteins and determine definitively whether a person is actually having a heart attack. Gerber believes his sensors could quickly and cheaply solve the problem. “We could have a device that says yes or no,” predicts Gerber.
Like Vettiger, however, Gerber gets most excited by the longer-term implications of his work. Demonstrating that DNA and protein molecules can actually move a tiny cantilever suggests it might be possible to build nanomachines that act independently. Imagine, Gerber suggests, implanted microcapsules for drug delivery that have a nanoscale valve able to detect a signature protein from a cancer cell; the binding of the protein to the cantilever would trigger the opening of the valve, releasing just the right amount of an anti-cancer drug from the microcapsule in the exact location needed.
At MIT’s Media Lab, Assistant Professor Scott Manalis is using some of the same tools-tiny cantilevers and AFM probes-to tackle similar biological problems. But Manalis is using a completely different strategy: probes that detect electrical signals. Many biological molecules, including DNA and proteins, are electrically charged. But from a materials point of view, the world of biomolecules, which normally exist in a watery environment, is largely incompatible with conventional microelectronics. (Spill water on your Palm Pilot, and you’ll get the point.) By altering the makeup of the electronic materials, however, Manalis has fabricated in essence a small transistor at the end of an AFM cantilever that works just fine under water.
The result is a microscopic detector that operates in the environment where DNA, proteins and cells flourish. So far, Manalis and his co-workers are using it as a sensitive probe that can be placed at the end of a microfluidic channel, for example, to detect the electrical signals of-and hence analyze–the DNA flowing out. Like the biosensors being developed at IBM, the tiny device detects DNA without tagging or bulky optical readers. Eventually, Manalis hopes the biosensor could help make possible one of biomedicine’s grander visions: a simple wireless device with a few electrodes that could be implanted in a patient with, say, kidney disease to act as an early warning signal detecting when a troublesome protein is being released.
the reliance on afm tips and cantilevers illustrates a decidedly mechanical bent in much of today’s nanotech research. Indeed, the strategy of using small silicon-based machines called MEMS (microelectromechanical systems) to manipulate nano devices is turning out to be an especially promising area. These micromachines are hundreds or thousands of times bigger than the nanoscale and are commercially used in everything from automobile air bags to switches in optical networks. But in the hands of skilled researchers, MEMS can offer a valuable way to control nano action.
In turn, the incorporation of nanoscale structures can greatly increase the utility of existing MEMS technology. “There are a number of situations with devices a few tens or even thousands of micrometers in size where one critical dimension needs to be smaller. Right at the heart of the device you may need a nanoscale feature,” says Michael Roukes, a physicist at the California Institute of Technology. Nanomachines are particularly useful in responding to “very feeble forces,” says Roukes, who has recently fabricated devices such as a nano resonator, which vibrates like the strings on a tiny guitar. Incorporating these nano devices into MEMS could, for example, yield signal processors that consume minuscule amounts of power.
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