Professor, Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo
A new world expanding from nanospace
—Dendrimer and Bucky plastics—
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.”
