Univ. of Tokyo, Others Capture Video Images of Changes in Molecular Structure

Feb 26, 2007
Atsushi Takano, Nikkei Monozukuri

A joint research group of the University of Tokyo, Japan Science & Technology Agency (JST) and Japan's Institute of Advanced Industrial Science and Technology (AIST) has succeeded in recording video images showing temporal changes in organic molecule structures.

The group first synthesized a compound resembling a lipid molecule, which is a major component of the cell membrane, and volatilized it in a vacuum, and then contained the resultant compound in a carbon nanotube (CNT) for observation with a high-resolution electron microscope. The group reported it observed movements of saturated hydrocarbon chains as well as reciprocating motion of the compound in the CNT on the second time scale.

In addition to this experiment, the group has found that, when the same molecules are trapped in the CNT with a diameter of 1.2 nm, video images of molecule movements can be captured on the second time scale. In specific, the group observed molecules with two hydrocarbon chains and succeeded in recording video images of rotational movements of the two chains in the CNT. In the video images, it was observed that the chains were not changing in a sequential manner, but rather in a manner of jumping from one form to another. Such a phenomenon has never been experimentally observed on the molecule level, the group claimed.

The object of the research was to "observe changes in an organic molecule as if viewing a molecular model." Since the target molecules had a problem of jumping around quickly in a vacuum and being vulnerable to electron beam emission, the group placed the organic molecules in a CNT to decelerate the molecule movements, and reduced the heat generation caused by electron beams, thereby eliminating the possibility of chemical reaction between the molecules. In this way, the group has succeeded in the direct observation of single molecules. A transmission electron microscope (TEM) with a resolution of 2.1 angstroms (0.21 nm) was used at an accelerating voltage of 120 kV for the observation experiment.

Electron microscope image of an organic molecule having a single-stranded chain with a carbon chain length of 12 (above) and its molecular model (below): The bar in the image is 1 nm long. In the model, pink, gray, and white portions indicate boron, carbon and hydrogen atoms, respectively.

In this research, it is important to easily discriminate the molecule structure of the target. Thus, a molecule with a peculiar structure resembling that of a lipid molecule was designed and synthesized by bonding a boron cluster as a marker and a flexible chain molecule (hydrocarbon). The resultant molecule was used as the observation target. The molecule was then placed in a CNT with a diameter of 0.9 nm. Under the observation with a microscope, the existence of boron atom was confirmed by measuring the electron beam energy loss spectrum. Then, by confirming the aforementioned "peculiar" molecular structure, it was proved that a single molecule can be observed with an electron microscope. Although each carbon atom cannot be observed on a one-by-one basis since the resolution of the microscope is lower than the length of carbon-carbon bond (1.5 angstroms), the group allegedly confirmed that a spherical portion composed of carbon chains and boron can be seen clearly.

The experiment using the 1.2 nm diameter CNT showed molecules moving back and forth at a speed of approximately 10 nm/sec. It was also found out that the speed of molecules frequently changes and that the molecules are sometimes caught on the CNT surface and stop their motions.

Group of photos A are sequentially observed images of a molecule having a double-stranded chain with a carbon length of 22. Reciprocating and rotational motions of molecule in the CNT can be observed. The numbers in the photos are time elapsed from the start of observation (in unit of sec). The bar in the photo is 1 nm long. Photos B and C are molecular models showing the states when 4.2 and 6.3 seconds elapsed, respectively. Click here to watch a streaming video of WMV format (2x). (Click image to enlarge in separate window.)

Sequentially observed images of a molecule having a double-stranded chain with a carbon length of 22: Again, reciprocating and rotational motions of molecule in the CNT can be observed. The numbers in the photos are time elapsed from the start of observation (in unit of sec). The bar in the photo is 1 nm long. Click here to watch a streaming video of WMV format (2x). (Click image to enlarge in separate window.)

Electron microscope image and molecular model of a molecule having a double-stranded chain with a carbon length of 12: Reciprocating motion of molecule in the CNT can be observed. The numbers in the photos are time elapsed from the start of observation (in unit of sec). The bar in the photo is 1 nm long. Click here to watch a streaming video of WMV format (2x).

What is predicted from the experiment is that a lipid molecule, which was the model of the target in the observation, will also move in a stepwise fashion in the cell membrane as with the case of the experiment. The hydrocarbon molecules are widely used in lubricant oils. The experiment indicates that individual molecules do not move smoothly at the same time even though lubrication is caused by the sum of interactions between numerous molecules.

A TEM (available from JEOL Ltd.) used in Nakamura Functional Carbon Cluster Project. In order to improve observation accuracy, a target can be cooled down to -269属C (LHe temperature).