Our research group has succeeded for the first time in the world in direct TEM observation of shape changes and appearances in the space migration of a single molecule passing through a nanometer-size pore.
The processes by which molecules pass through pores in thin films and bio-membranes are basic phenomena for understanding various physical, chemical and biological phenomena. For instance, the fundamental behavior of molecules such as storage in porous solids (storage of hydrogen, methane, etc.), separation by membranes (separation of methane from methane hydrate, etc.) and transfer of molecules through cell membranes necessarily involves a process of molecules passing through pores. Previous research methods, however, were based on studying the statistical average of the behavior of plural molecules; there were no experimental approaches that examined the interaction of a single molecule with a pore. In other words, there was no research on the shape of a molecule going through a pore, and the interaction between molecules and pores. We reported last year that when boron-atom-labeled organic molecules were confined in carbon nanotubes and observed by a transmission electron microscope, conformation and movement to and fro of each molecule could be observed as moving images (Science, 2007, 316, 853).
We have now succeeded in observing a long chain of a fullerene-labeled hydrocarbon passing through a nanometer-size pore in the wall of a nanotube as if it were alive. We focused on a long and thin hydrocarbon chain, which was bonded to a soccer-ball-shaped molecule (C60 fullerene) that was a little larger than its molecular size as a marker so that the chain part could move freely, and which was confined in a nanotube (Fig.1). Images of the molecule in various shapes in the space of the nanotube, and movement of the string-like molecule could then be filmed. By further close observation of each molecule, we also succeeded in observing the phenomenon of the string-like molecule passing through the pore in the wall of a nanotube (Fig.2).
A C60 molecule having a hydrocarbon chain confined in a carbon nanotube of about 1.4-nanometer diameter was observed transforming into a straight configuration and passing through the pore in the carbon nanotube at room temperature. This is the first such observation of the moment when a molecule moves in a tube and its structure changes according to the environment.
Although observation of molecules at room temperature has been considered very difficult since they move very rapidly with thermal energy (on a time scale of a trillionth of a second or less), the movement of molecules observed by a microscope at room temperature was found to be far slower than expected. Further, it was clarified that (1) the energy source of molecular movement is the energy of the electron beam used for observation since the velocity of molecular movement is not so different even at extremely low temperature, and (2) the velocity of movement and the life of observed molecules are not greatly different, whether they are inside or outside of the carbon nanotube.
The latter conclusion coincides with our recently reported result that biomolecules having peptide bonding bonded to the outside of carbon nanotubes can be stably observed (Nakamura et al.，Journal of American Chemical Society , 2008, 130, 7808). The fact that changes of conformation of molecules placed outside of tubes can be researched, as reported in that paper and the present paper, will solve the fundamental problem of the former report of confinement inside nanotubes, namely that the size and form of the observation target are restricted. We expect that it will soon be possible to freely observe, molecule and molecule, the movements of various molecules such as proteins and DNAs fixed outside of nanotubes.
The dynamic structural analysis of organic molecules utilizing the spaces inside and outside of carbon nanotubes is expected to develop as a new method in academic research on the chemical reactions and interactions of biomolecules. The results are also expected to be applied in key nanotechnology fields of Japanese industry and medical fields such as the development of new medicines.
Information about publication
This study was performed in research collaboration between the Department of Chemistry, The University of Tokyo, and the Exploratory Research for Advanced Technology (ERATO), Nakamura Functional Carbon Cluster Project, Japan Science and Technology Agency (JST).
A part of this study was supported by MEXT (KAKENHI, No. 18655012). Electron microscopy experiments were carried out in collaboration with the Nanotube Research Center, the National Institute of Advanced Industrial Science and Technology (AIST). This work has been published in Nature Nanotechnology:
http://www.nature.com/nnano/index.html
* This article was appeared on the front page of the Nature Nanotechnology as a highlight article.
“Imaging the passage of a single hydrocarbon chain through a nanopore”,
Nature Nanotechnology,
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