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The New Nanofrontier

Despite recent concerns over the harm nanotechnology may ultimately do, a flurry of reports also show its promise
aging



J. W. Stewart
nanomotor
Source: SCIENCE

NANO-COPTER twirls its tiny nickel propellers when fueled with ATP, the molecule that powers cells. The biomotor is made of ATPase (top left), which is attached to the nickel propellers and mounted on a nickel post (bottom left). Put in a solution of ATP and other chemicals, the propellers spin at a rate of eight revolutions per second.

"It is a staggeringly small world that is below," Richard Feynman said in his famous 1959 speech about nanotechnology, There¿s Plenty of Room at the Bottom. "In the year 2000, when they look back at this age, they will wonder why it was not until the year 1960 that anybody began seriously to move in this direction." In fact, what may now inspire greater wonder is just how far nanotechnologists have come in 40 years. In the past week alone, a flurry of papers describing advances on many fronts have come forth, both in the November 23 issue of Nature and in the November 24 issue of Science, which contains a special section on nanotechnology.

The reports come fast after several calls for concern over the pace at which nanotechnology is progressing. In the April issue of Wired, Bill Joy, co-founder of Sun Microsystems, warned that research in nanotechnology should be stopped before it becomes the "gray goo" K. Eric Drexler described in his 1986 book Engines of Creation. The goo, as Drexler envisioned it, would consist of legions of miniature assemblers that replicated themselves ad infinitum, wiping out anything living or not in their path. In June, nanotechnologists from the Foresight Institute--a think tank where Drexler is chair--followed suit, issuing their own prophylactic guidelines to stop goo.

But other scientists--many of whom air their views in news items that accompany Science's special section--dismiss the grim predictions. And if nanotechnology doesn't devour humanity, it does appear to hold tremendous promise to help it. In the cover story from Nature, Galen Stucky and his colleagues at the University of California, Santa Barbara describe glassy materials with nanoscale pores, cages and channels that might be used, among others things, "to sense biotoxins and for the removal of toxic heavy metals from the environment." In that same issue, Virginia Tech chemist Harry Dorn describes a new breed of metal-containing fullerenes, which can be used as nanoscale building blocks.

In Science, Caltech physicists Stephen Quake and Axel Scherer review how soft materials--which are perhaps better suited than stiff silicon for nanoscale devices handling biological samples--are coming into their own. Cornell researcher Harold Craighead explains how new nanoelectromechanical systems (NEMS)--the smaller cousins of microelectromechanical systems (MEMS)--are paving the way for "a revolution in applications such as sensors, medical diagnostics, displays and data storage." And other papers lay out work being done to develop so-called nanonurses, which travel the body to find and fix problems within cells, and labs-on-chips, designed to carry out genetic analyses and other diagnostic tasks. The list goes on. Perhaps three of the more exciting results--twirling biomolecular nano-coptors, microrobots based on bending plastic actuators and dancing tin nanomotors--are described in greater depth below. --Kristin Leutwyler


Twirling Motors

Researchers from the Cornell Nanobiotechnology Center have devised a way to power virus-sized motors using the very same molecule that provides energy within cells, adenosine triphosphate (ATP). In Science, Carlo Montemagno and his colleagues explain how they built and tested the first such biomolecular motors, marrying inorganic nickel propellers to ATPase enzyme (see illustration). "With this demonstration, we believe we are defining a whole new technology," Montemagno says. "We have shown that hybrid nanodevices can be assembled, maintained and repaired using the physiology of life."

The team formed the nickel propellers¿each measuring some 750 nanometers long and 150 nanometers wide¿at the University¿s Nanofabrication Facility in a series of steps. First they used electron-beam lithography to define the structures on silicon wafers coated with polymethyl methacrylate (PMMA). Next they deposited the nickel using electron gun evaporation, and chemically removed any residual PMMA and nickel. They popped the propellers off the substrate by way of isotropic etching, and coated them with a thin layer of chemicals to help attach them to the rest of the device.

bead
Source: EDWIN JAGER et al.

NANO-ROBOT has an elbow, wrist and hand--with two to four fingers--that can grab, lift and move a 100-micrometer glass bead from one track to another. The joints, formed from bilayers of gold and conjugated polymer, bend when electrochemical oxidation or reduction makes the polymer undergo a volume change.

In fact, the propellers more or less self-assembled with the molecules of ATPase, which the group produced from genetically altered bacteria. The motor-propeller combination was then mounted on 200-nanometer high, 80-nanometer wide nickel posts, and immersed in a "fuel" solution of ATP and other chemicals. The ATPase broke atomic bonds in the ATP molecules, as it does in living organisms. But in this case, the reaction cranked a rotor-like protein inside ATPase, thereby spinning the propellers. When the scientists observed the hybrid motors using a charge-coupled device (CCD) video camera, they found that the propellers spun at a rate of roughly eight revolutions per second, in some cases continuing at that pace for two-and-a-half hours straight.

The devices and the tactics used to make them need improvement. Montemagno notes that of the first 400 assembled units, only five worked. In later batches, some of the motors lost their propellers and others, their test pedestals. Moreover, the team would like to make the devices friendlier for use among delicate organic molecules and living cells. They would like to strip away from the finished product more of the harsh chemicals used to make the propellers, and add computational and sensing functions. They would also like to find ways to fuel the biomolecular motors using light energy in place of ATP. Still, the work marks a major step.

Bending Plastics

Scientists from Link¿pings Universitet in Sweden and the University of Maryland have developed other potentially useful devices made from conjugated polymer actuators, which they review in the Science special section on nanotechnology. Edwin Jager, Elisabeth Smela and Olle Ingan¿s have worked with conjugated polymers for several reasons: First, these materials operate well in aqueous media¿and most biological samples are sampled and tested in liquids. Second, they can be patterned using standard photolithography. And they readily undergo volume changes (during oxidation and reduction reactions) that can be exploited to make them bend at a variety of angles: If you attach the polymer to, say, gold, only the polymer will change size, making the combined bilayer curl.

hand
Source: EDWIN JAGER et al

NANO-HAND, also made from bilayers of gold and conjugated polymer, can curl around a grain of sand.

Jager and his colleagues recently built and tested a tiny robotic arm, consisting of linked microactuators made from a bilayer of gold and polypyrrole, a particularly stable conjugated polymer. The arm, measuring a mere 670 micrometers long, had an elbow, wrist and hand, with two to four fingers. And by way of electrochemical oxidation and reduction, they were able to bend these separate "joints" so that the arm could grab, lift and move a 100-micrometer glass bead (see illustration). They envision such arms being used in tandem to transfer cells around on a lab-on-a-chip.

Other designs described in the Science paper, include self-assembling boxes with polymer-gold bilayer lids that can open or shut as needed (see illustration). Such microcavities might serve to deliver drugs more precisely, to carry out nano- and picoliter chemistry tests or to help scientists study cells in an application the authors call "cell clinics." Similarly, the microactuators could be used to block the flow of liquids through, say, a lab-on-a-chip. Jager and his colleagues are also developing with cell biologists an instrument dubbed a "cell tapper." This device would, as its name suggests, tap on an individual cell, thereby making it possible to study the forces the cell exerts under different circumstances.

Dancing Tin

Far more preliminary, but promising investigations from researchers at the Sandia National Laboratories appear in the same issue of Science. Andreas Schmid, Norm Bartelt and Robert Hwang report on the discovery of tin dancing on copper, a phenomenon that resembles the way camphor particles will shimmy across the surface of water. Their hope is that by finding ways to harness and choreograph the tin crystals¿ movements, they can force alloys into assuming useful nanoshapes and create super-efficient nanomotors.

hand
Source: EDWIN JAGER et al.

NANO-BOX opens and closes when the conjugated polymers from which it is fashioned bend the flaps together. Such microcavities could serve to deliver drugs more precisely and in studing and compartmentalizing cells within a lab-on-a-chip.

Schmid¿s team made the discovery by watching how tin and copper morph into bronze in real time using both scanning tunneling microscopy (STM) to obtain a topographical map of the material¿s surface and low-energy electron microscopy (LEEM) to watch the movements of the tin on that surface. They noticed that when tin is deposited onto copper, it clumps up into tiny two-dimensional crystalline islands within seconds. And these islands surf over the copper¿s surface, swapping tin atoms for copper atoms as they go. When the islands become bronze crystals, they eject the copper atoms they picked up. After a few moments, bronze clumps cover the surface and the tin islands dissolve.

The researchers provide an explanation for the dancing tin islands: tin already embedded within the copper repels the traveling tin islands, driving them on to find another surface spot. Only when they find such a spot do they make an atomic trade of tin for copper. The islands are so efficient in their search for a non-competitive spot that they will even force themselves into a corner of the copper surface to avoid any backtracking. Schmid explains that this "completely unanticipated cooperative process" takes place because the islands "lower the surface free energy by moving toward unalloyed regions of the surface."

In an accompanying essay, Flemming Besenbacher of the University of Aarhus and Jens Noskov of the Technical University of Denmark note that the discovery "can be viewed as a direct observation of a nanomotor" because the tin islands translate chemical energy into forward motion. This sort of naturally-occuring motor is surely efficient¿and it appears to be remarkably powerful as well: Whereas a car delivers some 0.1 horsepower for each kilogram of its weight, the tin islands offer a power-to-weight ratio closer to 0.3 hp/kg. In conclusion, Besenbacher and Noskov note, "the challenge is to devise nanomotors whose motion can be controlled externally (so that they can be used to move things around at will) and that can be refueled."

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