 |
| Figure 1: Elastic electronic circuit made of ionic-liquid-based
Bucky gel. In cooperation with Takao Someya and Tsuyoshi Sekitani
at the University of Tokyo, Aida and his team are striving to
develop a new type of sensor based on elastic electronic circuits
made of ‘Bucky gel’. These circuits are worn on the
skin and can be used for sweat and temperature measurement. The
elastic electronic circuit has great potential for use as skin for
robots. |
| Copyright : RIKEN |
Takuzo Aida
group director
Emergent Materials Department
Responsive Matter Chemistry and Engineering Research Group
RIKEN Advanced Science Institute
“The appeal of chemistry is that you can create new molecules
that do not occur in nature,” says Takuzo Aida, Group
Director of the Responsive Matter Chemistry and Engineering
Research Group launched in October 2007. Aida has embarked on a
completely new research project. “An example is the
development of special molecular devices similar to living
organisms that can respond to different types of stimuli.”
What strategies can be used to create molecules with unconventional
functions like these? “Like movie actors, molecules are
carefully selected to play an important role in creating
interesting molecular devices. Thus, a chemist can be compared to a
movie director.”
The moment blessed with a discovery
One and a half years into the Aida Nanospace Project
(2000–2005), a research program supported by the Exploratory
Research for Advanced Technology (ERATO) program of the Japan
Science and Technology (JST) Agency, Takanori Fukushima (currently
team leader of the Responsive Matter Chemistry and Engineering
Research Group’s Functional Soft Matter Engineering Team)
placed some powdered carbon nanotubes into an ionic liquid and set
the mixture in an ultrasonicator to disperse them. He went out to
eat and came back about an hour later. To his surprise, the mixture
had turned into a viscous gel similar to mayonnaise. “It has
turned into a gel-like substance,” he reported to Aida.
Piqued with curiosity, Aida suggested, “Why don’t you
poke at it with a pair of tweezers.” The substance
immediately returned to its previous state of a powder in
suspension. “If Dr Fukushima had returned from eating 30
minutes later, the carbon nanotubes would likely have returned to
their original powdered state, and he would have missed the
phenomenon. So we believe that we were blessed just one time. Dr
Fukushima did not miss the moment,” says Aida.
However, so began a period of difficult work for Fukushima.
“I asked him to reproduce the phenomenon, but two months
passed without positive results. We thought the gel-forming
phenomenon had happened just that once. Dr Fukushima finally said,
‘Let’s stop investigating that phenomenon, it will
never happen again.’ However, I persuaded him to continue
investigating, since the successful reproduction of the phenomenon
would surely lead to the development of a new field of materials.
Then three months later, he finally succeeded in finding out how to
reproduce the phenomenon.” It was a bold method, involving
stirring the solution using a mortar.
Carbon nanotubes are of the order of one nanometer in diameter.
They are strong, and also excellent conductors of electricity. Many
researchers around the world have attempted in vain to disperse
carbon nanotubes uniformly to create new substances. Nanotubes tend
to intertwine with each other, preventing them from dispersing
uniformly. The problem, however, was solved when the solution was
simply mixed and stirred in a mortar. This caused the carbon
nanotubes to become disentangled and to disperse completely in the
ionic liquid, resulting in the formation of a gel-like substance.
Fukushima and his team called the new substance ‘Bucky
gel’ (Fig. 1).
A new type of conductive nanotube
Carbon nanotubes are a form of carbon with a graphite structure.
Graphite, found commonly in pencil ‘leads’, is composed
of stacked carbon sheets. A carbon nanotube is a cylindrical roll
of one of these sheets, and carbon nanotubes are the only type of
nanotube known to exhibit good electrical conductivity. “We
aimed to create a new type of nanotube that could also conduct
electricity.”
Aida and his team focused on the molecule graphene, which is a
small fragment of a single graphite sheet. They created a new type
of graphene molecule by adding hydrophilic and hydrophobic
moieties, then heated a solution of these molecules to 60 °C.
Upon cooling, the molecules combined to form nanotubes of 20 nm in
diameter with the hydrophobic moieties on the inside (Fig. 2).
“We took advantage of molecular ‘self-organizing’
behavior to create a new type of nanotube.” Self-organization
is a mechanism by which molecules combine together naturally to
form a complex structure.
Today’s computer circuits are fabricated using light to print
very fine structures on hard semiconductor materials such as
silicon. This method of forming microscopic structures, based on
printing techniques, is generally referred to as a
‘top-down’ method. Improvements in computer performance
have relied on continuous enhancement of the top-down method, and
efforts have been focused on drawing electrical circuit patterns
with ever-finer feature sizes. The top-down method, however, is now
reaching its practical limit because the width between wire traces
is now less than 100 nm, which is far shorter than the wavelength
of the light beam used for processing. Thus,
‘bottom-up’ methods are attracting more and more
attention as a means of building up molecules to create fine
structures. “The keyword for the bottom-up method is
‘self-organization’. This method allows the fabrication
of superfine patterns of several nanometers in width. In other
words, it can be used to build up molecules, potential allowing the
perfect design of devices.”
The new graphite nanotube is inherently an electrical insulator,
but it becomes a conductor when subjected to a simple chemical
treatment. Thus, a new electrically conductive nanotube was
realized.
“Later we found that the graphite nanotube can still be
formed even when the structural design of the original molecule
including graphene is modified. In the case of conventional carbon
nanotubes, the addition of moieties to the surface causes the tube
structure to collapse, and in most cases, also results in degraded
electrical conductivity. One of the advantages of graphite
nanotubes is the ability to change the structural design of the
original molecules, which allows us to specify what new functions
are being created.”
For example, Aida and his team, by changing the structure of the
original molecule, have successfully created a graphite nanotube
that conducts electricity when exposed to light. The expansion of
this technique could lead to the construction of highly efficient
solar batteries based on organic materials. Conventional solar
batteries are mainly made of inorganic silicon material. The use of
organic materials will contribute to cost and weight reductions,
and will expand the range of application because they are flexible.
For these reasons, there is broad interest in the research and
development of organic solar batteries. “The energy
conversion efficiency and durability of organic solar batteries
will not be better than for inorganic solar batteries such as those
made of silicon material. Organic materials, however, can be
obtained from plants even when oil reserves become depleted. Thus,
it is very important for us to establish the basic technology for
organic solar batteries because they can be obtained at a low price
and resources are inexhaustible.”
Molecular devices activated by two stimuli
Aida launched the Responsive Matter Chemistry and Engineering
Research Group at RIKEN in October 2007, and Fukushima assumed the
position of team leader of the group’s Functional Soft Matter
Engineering Team. “At RIKEN, I do not adopt research themes
that I pursued in other research institutes. Instead, I am
proceeding with independent research based on new themes. One
example is the development of molecular devices that can respond to
various types of stimuli, similar to living organisms.”
Receptors on the membranes of cells in living organisms
differentiate among various signals and open ‘gates’ in
response to specific signals in order to take in the necessary
materials. “We have also created useful molecular devices
that can perform a single function in response to a specific
stimulus.” For example, a research laboratory at the
University of Tokyo lead by Aida created a pair of ‘molecular
pliers’ that are activated by light (Fig. 3). “You
cannot hold them of course because the pliers are nano-sized. They
have a special moiety, mounted on the ‘grip’, that
contracts when exposed to visible light. Using this pair of
molecular pliers, we succeeded in clamping and twisting an
object.
“We can expand the concept of molecular pliers even further:
for example, it could twist when additionally exposed to
ultraviolet light, allowing it to twist a clamped object when
exposed to ultraviolet light and visible light at the same time, or
to twist a clamped object when exposed to ultraviolet light and
then to chop if off when exposed to visible light. The device
effectively discriminates among various types of stimuli and
performs tasks autonomously, similar to a living organism. This is
one of the major goals of our project at RIKEN.”
In a laboratory at the University of Tokyo, researchers have
already succeeded in creating a new molecular device that can be
activated by two stimuli through the modification of a biological
molecule called a ‘chaperone’. The functions of
proteins are derived from their three-dimensional structure. Thus,
they do not function well when their structure collapses. The
cylindrical chaperone molecule takes in collapsed proteins and
restores them to their original state. It opens its gate and
releases the proteins when exposed to an energy carrier called
adenosine triphosphate (ATP). “We added to the outside of the
chaperone’s gate an artificial gate that opens when exposed
to ultraviolet light so that it had a double gate function (Fig.
4). The modified chaperone molecule only opens the double gate
completely when exposed to both ultraviolet light and ATP,
releasing the restored proteins. By analogy, cars are equipped with
a safety system that requires two tasks to be carried out at the
same time in order to perform a particular operation. We succeeded
in creating a molecular device with this mechanism using the
biological chaperone molecule. At RIKEN, we want to use a molecule
designed entirely by ourselves for the creation of a device that
can be activated by various types of stimuli.”
How can such devices be created? “When you knock over the
first in a line of dominoes, it brings down all the other domino
pieces in turn after it. This is a good example of an event source
affecting the whole system. In an organic material, molecules are
arranged in an orderly manner. Thus, we can design the structure of
an organic device such that, for example, when a molecule at a
certain position is tilted by a certain stimulus, all the other
molecules can also be tilted. Furthermore, it is also possible to
design the structure of the organic molecular device by controlling
the interaction between molecules so that all molecules can be
tilted only when the organic material is exposed to two stimuli at
the same time. Based on this design concept, we are making efforts
to create new molecular devices.”
Focus and patience—creating new molecular devices
“The appeal of chemistry is that you can create new molecular
structures that do not exist in nature,” says Aida.
“You need to be very focused when creating new molecular
devices. Even very smart researchers who are very careful in
experiments are sometimes unable to create new structures, while
particularly focused researchers are often quite successful, even
if they are not refined researchers. What is the difference? I
don’t know.”
Aida points out that patience is also a must in creating new
molecular devices. “We can handle failure many times. In
fact, it was not until the fifth attempt in the design of the new
graphite nanotube that we finally succeeded in creating it. I was,
however, on the verge of losing patience. I think if you have about
seven failures in succession, you being to think that your basic
idea is flawed. Then, you lose your sense of anticipation, and fail
to notice positive signs even if they could be the ones leading to
the discovery of new functions. I have face-to-face discussions
with our researchers, and I sometimes give up on a research theme
if the researcher seems to be unable to cope with any more
failures. A little more effort may have led to a successful result,
but it cannot be helped, because these new devices are being
created by real people.”
At the Responsive Matter Chemistry and Engineering Research Group,
researchers deal in difficult, painstaking work in the creation of
new molecular devices with specific functions. “We have
produced very exciting results, including a new molecule that
exhibits an interesting phenomenon that was never expected.
Unfortunately, however, we cannot speak about the phenomenon yet
because the results have not been published.” Look for the
results to be published by the Responsive Matter Chemistry and
Engineering Research Group in the near future.
-------------------------------------------
About the Researcher
Takuzo Aida
Takuzo Aida was born in Oita Prefecture, Japan, in 1956. He
received a bachelor degree in physical chemistry from Yokohama
National University in 1979, and then earned his PhD in polymer
chemistry in 1984 at the Graduate School of Engineering, University
of Tokyo. After serving as a researcher and lecturer, he was
appointed associate professor of the Department of Chemistry and
Biotechnology at the same university in 1991, and was later
promoted to professor in 1996. He joined RIKEN in 2007, where he
pursues his research interests in macromolecular chemistry,
supramolecular chemistry and bio-science-related chemistry. |