杏吧原创

WHACKING high explosives with a hammer is not the safest occupation in the
world. Yet for Dana Dlott it鈥檚 all in a day鈥檚 work. He and his team routinely
pummel explosives and other materials hundreds of times every second. But Dlott
doesn鈥檛 plan to cash in his life insurance just yet, because these experiments
are on a seriously small scale. Instead of club hammers, the researchers are
using pinpoint laser battering rams. 鈥淲e use about 1 nanogram of material and
the shock duration is about a nanosecond,鈥� he says. 鈥淪o we call them
苍补苍辞蝉丑辞肠办蝉.鈥�

On an everyday scale, Dlott鈥檚 hammer blows may be puny, but to the molecules
caught beneath them the effect is cataclysmic. Each blow sends an intense shock
wave at supersonic speeds through a tiny chunk of sample just 0.2 millimetres
across. In less than 10 picoseconds (a hundred-billionth of a second) pressures
soar to 50 000 atmospheres and the temperature leaps to almost 400 掳C. Then,
almost as quickly as it appears, the nanoshock is gone. The molecules cool at a
phenomenal rate, says Dlott, equivalent to several hundred billion degrees per
second.

Dlott heads a team at the University of Illinois at Urbana-Champaign who are
using nanoshocks to squeeze and distort the shape and energy of a variety of
molecules. The researchers hope this will provide the solutions to a wide range
of problems, from better ways to design high-performance lubricants to revealing
exactly how skin or eye tissue is affected by laser surgery. Already the
technique has thrown up some surprises.

Exotic matter

It all began ten years ago. Dlott was at a workshop where researchers were
discussing the use of shock waves to make exotic types of 鈥渃ondensed matter鈥�:
new forms of iron, for example, or metallic hydrogen鈥攖he stuff that may
fill the core of Jupiter. These experiments required hugely powerful equipment
such as high-energy lasers like NOVA, or even nuclear weapons. This meant that a
single experiment could take weeks to prepare, and costs could easily run to
millions of dollars a shot. Clearly there were problems with creating shock
waves this way. 鈥淚 wanted to do something about it,鈥� Dlott recalls, 鈥渂ut I
couldn鈥檛 figure out how.鈥� Then, out of the blue, a friend phoned him asking for
advice. His company has developed a printing machine that uses pulses of laser
light to generate shock waves which throw ink onto paper. They had some problems
with the system and needed Dlott鈥檚 help, so he paid a visit to the company鈥檚
lab.

It was a revelation. 鈥淚 saw this thing and went `Whoa, these guys are making
ten million shock waves a second. That鈥檚 truly fantastic鈥�.鈥� So Dlott turned the
technology into a research tool. The rig he and his colleagues have now built
for themselves in the School of Chemical Sciences uses a commercially available
desktop laser to create short, powerful pulses of light, which are focused and
beamed onto the sample to produce localised shock waves.

The experimental set-up is simple. Dlott takes a glass slide and coats it
with a layer of sample about half a micrometre thick, sandwiched between two
buffer layers (see Diagram). On the very top he deposits a thin film
containing a dye mixed with a polymer binder.

The choice of dye is crucial: it must respond to the light from the laser to
generate the nanoshock鈥檚 explosive kick. When the light pulse strikes the dye
its molecules absorb almost all the light energy so that they heat up.
Straightaway they decompose explosively, blasting out molecular debris in all
directions and sending a shock wave down through the buffer layer towards the
sample. The buffer layer has two functions: it protects the sample from debris
thrown off the top layer, and it focuses this shock wave onto the sample
layer.

A few nanoseconds after the explosion鈥攚hen the shock wave has passed
and the debris has cleared鈥擠lott examines the sample using a 鈥減robe鈥� pulse
of white laser light that he beams down through the sample. The wavelengths that
the sample absorbs give Dlott a clue to what chemistry has taken place鈥攈ow
the molecules鈥� bonds have broken and re-formed in response to the shock. The
machine automatically records the sample鈥檚 spectrum. Then a motorised driver
moves the glass plate about half a millimetre sideways, exposing a fresh area of
sample ready for another blast. By changing the delay between the shock pulse
and the probe, Dlott can track the way the sample responds to the shock. Dlott鈥檚
equipment can deliver up to 1000 nanoshocks every second. At this rate, five
seconds鈥� worth gives him enough high-quality data to follow in detail the
changes that the nanoshock brings about in the sample鈥檚 molecules.

The nanoshock technique has parallels with flash photolysis, a system which
chemists can use to probe ultrafast changes that are initiated by light
(New 杏吧原创, 23 October 1999, p 16).
In flash photolysis, a 鈥減ump鈥� pulse of
light pushes a molecule鈥檚 electrons into a high-energy 鈥渆xcited鈥� state. This
distorts or even breaks a molecule鈥檚 bonds. As in Dlott鈥檚 nanoshock technique,
chemists find out what鈥檚 going on by illuminating the sample a moment later with
a flash of white light and recording which wavelengths it absorbs.

However, flash photolysis has its limitations鈥攏ot least that it only
works for reactions that are initiated by light. It is useless for studying
processes initiated by heating or physical deformation. Nanoshocks, by contrast,
can be used to distort molecules without the need to excite the electrons, so
they can be used to study virtually any material.

Having perfected their setup, Dlott and his colleagues are busy exploring the
analytical power of nanoshocks. A natural starting point is the study of
explosives. Most powerful explosives can only be set off by a shock wave created
by a device containing a more sensitive explosive, known as a detonator.
Commonplace as this is, no one really understands exactly what happens in the
instant before a lump of explosive detonates. 鈥淥ne of the holy grails of shock
wave spectroscopy has been to understand the complicated processes of shock
initiation and detonation,鈥� says Dlott.

One of the first materials the researchers looked at was a powerful explosive
called nitrotriazolone or NTO. For something so energetic, NTO is pretty stable:
it won鈥檛 explode without the help of a more sensitive initiator compound. So the
researchers bashed thin samples of NTO with nanoshocks of 50 000
atmospheres鈥攖he equivalent of striking the sample with a hammer moving at
almost 1 kilometre per second. This is not violent enough to cause an explosion,
but it is enough to probe the changes that take place in the material just
before detonation.

In its unshocked state, NTO contains millions of tiny
nanocrystallites鈥攕mall regions of crystalline material that are aligned at
random. However, Dlott discovered that nanoseconds after the shock waves had
dissipated, there was a permanent transformation in the NTO sample. The NTO
crystals had become aligned, says Dlott. Chemists already knew that an
explosive鈥檚 sensitivity depends on the direction in which its crystals are
aligned, so Dlott suggests that shock-induced orientation could be used to tune
the sensitivity of an explosive. It鈥檚 speculative, admits Dlott, but one day
chemists might use tools such as nanoshocks to control the behaviour of
explosives. 鈥淚n the long run it might be possible to make materials oriented to
be safer, or more sensitive, according to the application,鈥� he says.

It鈥檚 not just explosives that Dlott is investigating. 鈥淢y work is devoted to
moving shock techniques out of the weapons arena and into studying other
interesting problems in chemistry,鈥� he says. So Dlott and his colleagues have
turned their attention to proteins, the molecular workhorses of living cells.
Proteins play a huge variety of roles, ranging from enzymes to structural
molecules such as keratin, one of the major components of skin, nails and hair.
Any protein molecule can, in principle, fold itself up into an astronomical
number of different three-dimensional shapes, or 鈥渃onformations鈥�. Yet for most
proteins, only one specific conformation can do the job. Somehow, a newly formed
protein molecule always manages to rapidly fold itself into that exact
conformation and no other. How this folding process takes place is one of the
big questions facing biologists.

Dlott has begun by looking at proteins such as bovine serum albumin (BSA) and
myoglobin, huge molecules that are representative of a wide range of other
proteins. Dlott has found that nanoshocks will compress some protein molecules
by 25 per cent in less than a nanosecond. This creates such high temperatures
and pressures that it ought to make protein molecules unfold completely. Yet
Dlott鈥檚 experiments on BSA told a different story.

The nanoshocks didn鈥檛 seem to unfold the BSA, but subtle changes revealed
that a slow relaxation process was occurring after the shock wave passed. Even
with longer shock waves that lasted for 1 microsecond, says Dlott, they couldn鈥檛
pull the protein apart. So what was happening?

Dlott believes he has the answer. Somehow, he says, the protein molecules are
being shocked into partially unfolded conformations which are impossible to
create any other way. These states are relatively stable so they relax back into
their natural conformation only slowly. This should give the researchers plenty
of time to follow the structural changes in detail.

Work like this has important implications. Many proteins change shape as they
do their work: enzymes, for example, rearrange as they bind to their molecular
target. Dlott hopes that nanoshocks will allow him to map out the motions and
energy changes involved in these transitions. He also thinks his research may
have more practical consequences.

Laser surgery, as used to correct defective vision, for example, uses
powerful pulses of laser light to cut through or vaporise tissue. But it also
produces violent shock-wave effects in surrounding tissues. 鈥淐learly there will
be changes in your eye because the cells were exposed to really high pressures
for short times,鈥� Dlott says. 鈥淭here are all kinds of things in there,
organelles, membranes, nucleic acids,鈥� he adds. 鈥淲e don鈥檛 know what it is that
gets hurt.鈥� The researchers plan to model these processes by applying nanoshocks
to tissue samples, in the hope that this will lead to the development of better
medical treatments. This research will also be relevant to more immediate
problems, such as wound trauma induced by the shock waves from bullets and
bombs, he says.

Another problem the researchers have in their sights is to understand the way
lubricants work. 鈥淭here is absolutely no data on the response of lubricant
molecules to large-amplitude forces,鈥� says Dlott. Such forces are typical of
real-life applications: for example, when an engine lubricant is trapped between
a bearing and a shaft turning at high speed. The group is now developing
experiments to understand the way lubricant molecules respond to these extreme
forces.

Dlott is confident that this is just the start, and that there are many more
applications for nanoshocks out there. 鈥淥ur techniques open up the possibility
for any research group equipped with lasers to study the effects of shock waves
on complex systems in chemistry, biology and medicine,鈥� he says. These tiny
blows could be used to drive particular kinds of chemical reactions鈥攖o
kick electrons from one molecule to another鈥攈e predicts. And while
researchers already know a great deal about the effects that high pressures have
on certain systems鈥攑hotosynthetic reaction centres and light emitting
polymers, for example鈥攖his pressure is usually static. 鈥淲hat happens when
you change the pressure very fast?鈥� he asks.

With such a variety of possibilities beckoning, nanoshocks can truly claim to
be the wave of the future.