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Posted: Sep 22, 2008

Nanotechnology escape artists squeezing through a hole in the wall

(Nanowerk Spotlight) The processes by which molecules pass through pores in thin films and biological membranes are essential for understanding various physical, chemical and biological phenomena. For instance, the fundamental behavior of molecules in porous solids and their transfer through cell membranes necessarily involves a process of molecules passing through pores – knowledge that, for instance, is crucial for the development of nanotechnology-based hydrogen storage materials for fuel tanks. So far, research methods have been based on studying the statistical average of the behavior of groups of molecules; there were no experimental approaches that examined the interaction of a single molecule with a pore.
There has been a lack of experimental methods that can obtain information on the structure and orientation of the molecules as they pass through a pore, and their interactions with the pore during passage. Researchers in Japan have now succeeded for the first time in observing a long chain of a fullerene-labeled hydrocarbon passing through a nanometer-size pore in the wall of a carbon nanotube as if it were alive.
"We used imaging with a transmission electron microscope to observe the structure and orientation of a hydrocarbon chain as it passes through nanoscale defects in the walls of a single-walled carbon nanotube and also to study the interactions between the chain and the nanopore" Dr. Eiichi Nakamura tells Nanowerk. "We bonded a long and thin hydrocarbon chain to a fullerene molecule (C60) that was a little larger than its molecular size as a marker so that the chain part could move freely. We then confined this molecule inside the nanotube. We were able to film images of the movement of the string-like molecule in various shapes in the space of the nanotube. 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 carbon nanotube."
Already last year, Nakamura and his team at the University of Tokyo reported in Science that when boron-atom-labeled organic molecules were confined in carbon nanotubes and observed by a transmission electron microscope (TEM), conformation and movement of each molecule could be observed as moving images ("Imaging of Single Organic Molecules in Motion").
This 2007 Science paper reported the first motion pictures of moving organic molecules that revealed the time-dependent structural change of a single molecule with resolution close to atomic resolution. This was also the first time that TEM's capability to image a single organic molecule without decomposition was demonstrated.
Organic molecules passing through a pore in the wall of carbon nanotube
Organic molecules passing through a pore in the wall of carbon nanotube. (a): Images of alkenyl fullerene on a sample stage at 293 K. (Image: Nakamura Lab, University of Tokyo)
In their latest work, Nakamura's team demonstrates that it is possible to visually observe the interaction of a single organic molecule – in this case alkyl and alkenyl chains – with other matter. They reported their findings in the September 14, 2008 online edition of Nature Nanotechnology ("Imaging the passage of a single hydrocarbon chain through a nanopore").
Interestingly, the movement of molecules observed by a microscope at room temperature was found to be far slower than expected. The reason the observation of molecules at room temperature has been considered very difficult is the ultrafast speed of the movement on a time scale of a trillionth of a second or less.
Possibly the most important finding in this work is the origin of the energy that causes the molecular motion of the hydrocarbon chain. "After observing hundreds of molecules, we clarified that 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" says Nakamura. "We also found that 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 the group's recently reported result that biomolecules having peptides bonded to the outside of carbon nanotubes can be stably observed ("Imaging of Conformational Changes of Biotinylated Triamide Molecules Covalently Bonded to a Carbon Nanotube Surface").
Nakamura expects that scientists will soon be able to freely observe, molecule by molecule, the movements of various molecules such as proteins and DNAs fixed outside of nanotubes. One practical application of this could be the study of the mechanism of gas absorption and separation using carbon nanotubes or graphitic materials.
Nakamura points out that the observations that he and his team have made generate a number of new, fundamental questions: "What force pushes the rotating chain out of a hole and then retracts it again into the tube interior? What is the molecular-level information that correlates to the kinetics of the transportation process as studied using bulk materials? Does TEM provide us with experimental tools to study the interactions between a molecule and a pore of the sidewall or a tube, and can we use this information to design functional membranes or functional porous materials such as zeolites?"
It appears likely that the dynamic structural analysis of organic molecules utilizing the spaces inside and outside of carbon nanotubes will develop as a new method in academic research on the chemical reactions and interactions of biomolecules. The results could find applications in key nanotechnology fields such as nanomedicine or fuel cells.
By . Copyright Nanowerk LLC

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