A single molecule has been filmed wriggling its way out of a miniscule pore in the wall of a hollow tube of carbon atoms just a few billionths of a meter in diameter.

These results suggests that similar biological or catalytic processes could be observed using the same technique, known as transmission electron microscopy (TEM). Electrons from the TEM produce a molecular-scale image of a sample as they pass through it.

To visually reorder the molecular escapology performance, Hiroyuki Isobe and Eiichi Nakamura of the University of Tokyo and colleagues1 first treated carbon nanotubes with hot oxygen gas, to remove the end caps and create holes in their walls. Then they filled the tubes with tadpole-shaped molecules, each made from a ball of carbon atoms called a fullerene, attached to a long chain of carbon atoms.

Using TEM, they could see individual molecules trapped inside the nanotubes, rotating slowly in a volume of just 1.5 nm3. By making the observations at room temperature, and then again at minus 269°C—a mere four degrees above the coldest temperature possible—they discovered that the molecules moved at roughly the same speed under both conditions.

Fig. 1: The molecule's tail wriggles out of a pore in the nanotube's wall, as shown by TEM (left); simulation (middle); molecular model (right).

They also saw that the ‘tail’ of a molecule could wriggle through one of the holes created in the wall of the nanotube, staying outside the tube for up to a minute or so before retracting (Fig. 1).

“We were surprised to see the tail of the molecule coming out from the nanopore,” says Nakamura, adding that the serendipitous discovery was only possible thanks to the careful observations made by the group leader in the project, Masanori Koshino at the Japan Science and Technology Agency. The team do not yet know why the molecule creeps through the nanopore in this way.

Being able to watch these processes in action could help scientists trying to design functional membranes to separate different types of molecules, or porous catalysts that speed up important industrial processes. Nakamura adds that the team are now hoping to use the TEM technique to study the molecular interaction of a drug with a protein, a proof of principle that may aid in the design of new medicines.