Dendrimers are three-dimensional
polymers characterized by a regular tree-like array of
branched units. The name originates from Dendron which
means trees in Greek. In 1991, Prof. Aida, who had been
studying plastics in his laboratory, decided to start studying
dendrimers as a new research subject. Although most of
the research involved the attachment of some substance
to the periphery of a dendrimer to give it new functions,
Prof. Aida focused on the interior of the dendrimer. Prof.
Aida says, "A researcher is also a scriptwriter.
It is meaningless to write the same story as others. I
thought that surely there was something interesting to
do with the interior that nobody had done, yet."
"How can I make other researchers understand that
the interior of the dendrimer is important?" Prof.
Aida then decided to investigate the behavior of the iron
porphyrin complex, or heme, encapsulated in the center
of the cavity created by a dendrimer molecule. The heme
moiety carries oxygen in your blood, and because it is
surrounded by a protein, heme stably binds oxygen. Prof.
Aida found that, even if heme is surrounded by a dendrimer,
instead of a protein, it still functions as a carrier.
This phenomenon may lead to the development of artificial
blood.
While investigating what happens in the interior of a
dendrimer, Prof.
Aida found that when azobenzene, which is encapsulated
in the center of the dendrimer cavity, is irradiated with
low-energy light, the azobenzene isomerizes. Normally,
isomerization does not occur when low -energy light is
used, and thus, Prof. Aida discovered the light- harvesting
ability of dendrimers. Light-harvesting also occurs in
chlorophyll, which is ring-shaped, during photosynthesis.
He prepared dendrimers with a diameter of 15 nm and with
light-harvesting units similar to chlorophyll. When they
were exposed to light, 70% of the energy of the irradiated
light was concentrated at the center of the dendrimers.
Recently, research involving hydrogen extraction from water
is progressing using light-harvesting. Because of his research
on dendrimers, he has become more interested in the nanospace
within molecules. He is now the leader of the Aida Nanospace
Project of the Exploratory Research for Advanced Technology
(ERATO) of the Japan Science and Technology Agency (JST).
The aim of his project is to research the potential functions
of the nanospace of various large molecules which is covered
by the periphery of the molecule.
Prof. Aida regards the gelation of carbon nanotubes as
an important result from his project. Dispersing carbon
nanotubes within polymers improves the properties of the
polymer, such as mechanical strength and electrical conductivity,
but so far no effective method has been found to uniformly
mix polymer materials and carbon nanotubes. However, from
Prof. Aida's project, it was discovered that, when nanotubes
are put into an ionic liquid, gelation occurs to form a
paste, which is more easily handled. Therefore, if a polymerizable
component is introduced into an ionic liquid, it can be
molded like conventional polymers. "This was a totally
unexpected result from the project.
Interesting things are often found unexpectedly, outside
the original script," Prof. Aida says. Polymers containing
carbon nanotubes were named "Bucky plastic," and
it has a mechanical strength 4 to 10 times higher than
polymers without nanotubes and is electrically conductive.
Prof. Aida has had some remarkable results through his
study of various types of nanospace. A particular type
of silicate material with a honeycomb-like porous framework
was utilized as a nanoflask for the polymerization of ethylene.
The polymer chains were extruded from mesopores with a
diameter of 2 nm and then assembled to form extended- chain
crystalline polyethylene nanofibers with a diameter of
50 nm and excellent mechanical properties. As well, Prof.
Aida showed that a chaperon, which is a cylindrical protein
aggregate, absorbed nano- sized cadmium sulfide particles,
which are semiconductors, into its 4.5 nm sized holes.
The nanoparticles were stably held within the holes until
the chaperon was activated by ATP to release the particles.
His findings may lead to the development of a new drug
delivery system and switches for electronic circuits. As
for Prof. Aida's research interests, both artificial and
natural materials are included in his concept of "nanospace".
He says, "Among physicists, chemists and biologists,
chemists tend to adhere to a substance most. However, if
you are obsessed with a substance, you may end up becoming
stuck and unable to progress. Although I am a chemist,
I want to adhere to phenomena and concepts."
(Interviewer: Yu Tatsukawa, Cosmopia Inc.)
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/056a.html
