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New tools are catalysts for proactive chemistry
New research tools are catalysts for
chemists' proactive approach
03/30/98
By Alexandra Witze / Science Writer of The
Dallas Morning News
Memories of high school chemistry conjure
up images of test tubes, flasks and other
laboratory glassware. But many of today's
chemists haven't touched a beaker in
years.
These days, you're more likely find
supercomputers and lasers in a chemistry
lab. Scientists are using these powerful
new technologies to scrutinize how atoms
and molecules fit together. And as
researchers develop these tools, they're
learning that they aren't limited to a
passive science of observation.
Once, people were at the mercy of the
molecules: Scientists had to wait for
chemical reactions to occur, then do their
best to understand what had happened.
Chemists today still use this traditional
approach of wait-and-watch, and it helps
them learn the detailed mechanics of
chemical reactions.
But some scientists are trying to go
beyond this: They are controlling how
chemistry happens. By moving atoms around,
or manipulating the outcome of chemical
reactions, researchers are discovering how
to drive chemistry. Instead of being
observers on the sidelines, chemists are
now out on the playing field.
"We can use chemistry for more than just
making molecules interact with each
other," said Harvard University chemist
George Whitesides.
This new, active science is on display
this week in Dallas, where some 10,000
chemists are gathered for the annual
meeting of the world's largest scientific
society, the American Chemical Society.
From making better plastics to
understanding how molecules in the body
interact, the latest discoveries rely on
the proactive approach to chemistry.
* The quest to understand and drive
chemistry begins at the science's smallest
level - the atom.
Researchers are now able to manipulate
atoms one at a time. In 1990, a group from
IBM's Almaden Research Center in San Jose,
Calif., described the world's smallest
advertisement - the letters "IBM" spelled
out in xenon atoms.
By using a special tool called a scanning
tunneling microscope, the scientists had
figured out how to nudge atoms around.
From the metallic tip of the microscope
flows a small electric current that
repulses atoms, allowing the scientists to
push the atoms exactly where they want.
The experiment has to be done in a
near-perfect vacuum, at very low
temperatures to keep the atoms from
wiggling around with heat.
"Otherwise they won't stand still to have
their picture taken," said IBM team leader
Donald Eigler.
The research group has since drawn more
elaborate patterns of atoms on surfaces,
including the Japanese kanji characters
for "atom." They've devised "quantum
corrals," in which a ring of atoms traps
electrons on a surface. And they figured
out how to attach another atom to the tip
of the microscope and drag it across the
surface.
Such work might one day help chemists
build novel structures, designed atom by
atom.
"We've never built molecules this way
before, so let's try," said Dr. Eigler.
* Occasionally the scientists can trigger
chemical reactions by placing two atoms
close enough to each other. Sometimes the
researchers have to zap the two atoms with
heat, electrons, or particles of light to
make the reaction happen, Dr. Eigler
reported recently in Philadelphia at a
meeting of the American Association for
the Advancement of Science.
In 1991, his group built a tiny atomic
"switch": The microscope tugged a single
xenon atom off a nickel surface, then
dropped it again, to represent "on" and
"off" positions. Expanding on this idea,
other researchers have found ways to
control single molecules as they rotate on
a surface.
Wilson Ho of Cornell University and
colleagues have sent an electric current
through the tip of a scanning tunneling
microscope to change how an oxygen
molecule spins above a platinum surface.
The idea is to learn more about what it
takes to rotate a molecule from one
position to another; after all,
researchers won't be able to build tiny
machines out of single molecules, as they
dream, if they can't control the
molecules' orientation.
Dr. Ho's team found that the oxygen
molecule would flip its position as they
varied the voltage and current coming from
the microscope's tip. The molecule's
rotation was apparently caused by the
motion of low-energy electrons within it,
the researchers reported this month in
Science.
Such studies, they wrote, add to the
growing knowledge of what it takes to
control individual molecules in different
environments.
* More insight into the control problem
comes from lasers, which can help
scientists see what happens when atoms and
molecules combine and recombine.
For example, chemist Ahmed Zewail of the
California Institute of Technology uses
very fast lasers to take snapshots of
chemical reactions as they occur. Atoms
separate and rejoin as molecules on the
timescale of "femtoseconds," or
quadrillionths of a second. So in order to
watch the reactions happening, Dr.
Zewail's group has to flash laser light
every few femtoseconds, like a strobe
light blinking in a disco.
In such rapid flashes, "any molecule will
look to you as if it is frozen," said Dr.
Zewail.
Over the past few years, his group has
gone from "freezing" simple reactions
between inorganic molecules to much more
complex interactions involving organic
molecules. For instance, his group is
currently studying the structure of DNA
and how it is affected by electrons moving
through it.
"We want to see the architecture of the
molecule itself changing over time," said
Dr. Zewail.
Other groups are studying a protein known
as bacteriorhodopsin, which is involved in
sight. A twist of this protein in the eye
triggers vision. By studying
bacteriorhodopsin with very fast laser
pulses, chemists have found that it takes
about 200 femtoseconds for the protein to
twist when light strikes the eye. One day,
scientists might be able to use
femtosecond laser pulses to drive chemical
reactions.
This month in Science, chemist Richard
Zare of Stanford University also described
molecular control.
"We may actively intervene during the
course of the reaction," wrote Dr. Zare -
meaning that scientists could drive the
reactants to follow a chosen path out of
many different reaction possibilities.
Scientists had already known they could
send a chemical reaction somewhat in the
direction they wanted - by changing the
temperature or pressure at which the
reaction happened, for instance, or by
adding different catalysts to trigger the
reaction. Lasers, however, come in handy
when chemists want to set up the right
reaction, or even guide it as it occurs.
By carefully pumping the right kind of
laser light into a mixture of chemicals,
researchers can nudge molecules to be in
specific orientations or energy states. In
turn, that makes some molecules react in
different ways with others.
For example, scientists have slammed
charged particles, or ions, of nitric
oxide into a crystal of silver to produce
oxygen ions (formed when the bonds between
nitrogen and oxygen atoms break in the
nitric oxide). In 1995, chemists reported
that they got many more oxygen ions if
they used laser pulses to force the nitric
oxide to collide end-on, rather than
side-on, with the silver.
Another way to influence reactions is to
continually flash laser light into a
mixture of chemicals while the reaction is
going on. So far, only a few experimental
groups have been able to demonstrate this
type of control in the laboratory.
One scientific team recently used a laser
to alter the way in which an iodine
molecule absorbed energy. The regular
flashes of light struck the iodine at just
the right intervals to change the
molecule's energy states.
Chemists are just beginning to learn how
well they can control reactions by using
lasers. Any practical applications in
industry or other areas lie decades away,
said Dr. Zewail.
"Nevertheless, the real gain in pursuing
laser-based collision control is likely to
be the increased understanding of how
chemical reactions occur, which in time
will no doubt lead to important
applications," Dr. Zare wrote in the
Science article.
*
Yet even when scientists think they are
beginning to understand some chemical
processes, they discover behaviors that
are completely surprising.
For example, Stanford University
researchers have found that identical
polymer molecules can act completely
differently when asked to flow through a
fluid.
Polymers, which are long, chainlike
molecules made up of repeating groups of
atoms, can actually help push fluid
through pipes when they're dissolved in
very small amounts. (Firetrucks use
polymers to force water out of their hoses
faster.) Nobel-laureate physicist Steven
Chu and colleagues have been studying how
long strands of the polymer DNA uncoil in
tiny currents.
Each strand of DNA is made up of similar
repeating sets of chemicals, so
theoretically the strands should behave
alike. But Dr. Chu's team has found that
the DNA molecules suffer from stubborn
individualism.
The researchers stained the DNA strands
with fluorescent dye, then photographed
the strands unraveling within a flow. They
found that some of the DNA strands
remained coiled within the current like
little snakes. Others untangled into long,
straight strands with knots at each end,
like dumbbells. Still others folded in
half, or they remained kinked in the
middle, like knotted pieces of string.
Even though the strands were otherwise
identical, they behaved very differently
from each other - something that couldn't
have been learned by studying millions of
polymer molecules at the same time, Dr.
Chu reported at the Philadelphia meeting.
"It's amazing that you can find this very
diverse behavior by studying single
molecules," he said.
The experiments show how unpredictable
single molecules can be. Trying to learn
about them by studying their behavior in a
crowd is like trying to figure out what
kinds of animals a zoo contains by
studying only a creature made of a blend
of all the giraffes, snakes and bears in
the place, Dr. Chu said.
Scientists are finally figuring out ways
to peer into matter at its most basic
levels, as single molecules or even atoms.
Such new methods are unprecedented in the
history of chemistry. If the American
Chemical Society meets in Dallas in the
future, one prediction is certain:
Chemists will have gone even smaller in
their quest, and there probably still
won't be many test tubes in sight.
© 1998 The Dallas Morning News