NANO: Cantilever Tales

Date: Sat Nov 25 2000 - 06:05:03 MST

Cantilever Tales

Alexander Hellemans*

Building even the simplest nanomachines is a daunting challenge, but
working models serve as springboards to grander designs. A classic
example is the cantilever, an indispensable cog in the nanoworld that
ushered in the scanning probe microscopy revolution. Today
cantilevers, which resemble tiny diving boards, are the operating
principle behind a host of experimental devices that could debut in
the next decade.

Nanosized cantilevers earned their claim to fame in the mid-1980s with
the invention of atomic force microscopes (AFMs). To chart the
surfaces of molecules, AFMs run the tip of a cantilever across an
object under investigation; intermolecular forces between probe and
object tug the cantilever tip up and down over the surface, like the
stylus of a record player. A reflected laser beam records this motion,
and the signal can be converted into an image of the surface.

Cantilevers are proving to be more versatile than anyone
imagined. Perhaps it's no surprise that a pioneering center for
manipulating these tiny tools is the IBM Zurich Research Laboratory in
Switzerland, the birthplace of scanning probe microscopy. "The whole
field was started right there," says Naomi Halas, a specialist in
applying nanotechnology to chemistry at Rice University in
Houston. One master cantilever builder at IBM Zurich is James
Gimzewski, leader of the Nanoscale Science group. He and his team set
the pace for the rest of the field, Halas says: "When they publish
something, it is usually the first and best for a long time."

The key to the next generation of cantilever devices is being able to
make the miniature planks bend on demand. One approach is to coat the
top surface of an AFM cantilever, a blade of silicon about 500
nanometers long and 100 nanometers wide, with short DNA chains called
oligonucleotides. The researchers next expose these coated
cantilevers, which are in solution, to oligonucleotides with a
complementary sequence of base pairs. When the matched pairs bind,
they exert an intermolecular force that expands the coating, bending
the cantilever downward much as the bimetallic strip in a thermostat
curls in response to temperature changes.

A scanning laser can measure the extent to which the oligonucleotide
pairs bend the cantilever; the more base pairs that match, the more
the cantilever bends. Thus coated, cantilevers might serve as
sensitive probes for specific DNA sequences. "We were able to detect a
single [base-pair] mismatch," says Gimzewski, whose team described its
advance in Science (14 April, p. 316). This proof of principle has
attracted attention from biotech companies, which view cantilever
setups as potential rivals to DNA arrays for searching for genetic
sequences of interest, including disease genes. "We are now trying to
make [coated cantilevers] into a general-purpose diagnostic
technique," Gimzewski says. "This is a new area. There aren't many
sensitive tools around."

But cantilever-based devices need not be constrained to having the
action--or molecules--come to them. They might be used as smart gates
that release drugs or other chemicals in response to precise molecular
signals. For instance, an anticancer pill equipped with cantilever
gates might unleash a powerful drug at the site of a tumor only when a
tumor-specific protein gloms onto a specially tailored molecular
adhesive coating the cantilevers. Or a chemical for cleaning up a
hazardous spill might be stored in pellets and released only when the
target pollutant tugs at a cantilever gate, "rather than putting
chemicals all over the place," Gimzewski says.

Closer to reaching the market, however, are cantilevers for computer
data storage. In a project called "Millipede" spanning several labs at
IBM Zurich, scientists are testing an array of about 1000 cantilevers
as a new way of building nanoscale memory devices. Piezoelectric
signals would tell the cantilevers when to jab their hot tips at a
polymer film. The impressions in the film would record the data in a
much denser format than current media do. "That has the potential to
displace magnetic technologies," Gimzewski says. That bold prediction,
no doubt, will be heeded in the nano community: Gimzewski's group has
a track record for coming through. "I always aim very high and fail a
lot of the time," he says, "but the few things in which I succeed make
an impact and are extremely enjoyable."

Alexander Hellemans writes from Naples, Italy.

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