Angewandte Chemie International Edition 2007, 46,
Swirled to the Left or Right?
Nanofibers align in stirred liquid
Contact: Takuzo Aida, University of Tokyo (Japan)
Registered journalists may download the original article here:
Spectroscopic Visualization of Vortex Flows Using Dye-Containing Nanofibers
Is the vortex in a stirred liquid swirling clockwise or
counterclockwise? A zinc porphyrin dendrimer—a branched molecule with a
central zinc atom—can answer this question. As Japanese researchers
report in the journal Angewandte Chemie, the optical activity of
a solution containing this substance changes rapidly when the direction
of stirring is changed.
It is possible that vortexes in the distant past were responsible for
breaking the symmetry in nature to give us the “handed” life we see
today, which has clear preferences for “left-” or “right-handed”
molecular building blocks like sugars and amino acids. Vortexes in
liquids clearly twist either one way or the other, as do screws, our
hair, or snail shells. They can be related to each other like mirror
images or left and right hands. This is called “handedness” (chirality).
Vortexes are very complex structures, containing many regions with
currents moving in completely different directions. For example, if a
liquid is stirred in a cuvette, a dense circular current forms at the
center while a loose spiral-shaped flow is present in the outer regions
of the vortex.
A research team headed by Takuzo Aida and Akihiko Tsuda has now
synthesized a zinc porphyrin dendrimer that makes these individual local
currents observable by spectroscopy. The highly branched zinc-containing
molecules aggregate in solution to form long nanofibers. If the solution
is not stirred, it is not optically active. As soon as it is stirred, it
becomes optically active: The stirred solution rotates right- and
left-circularly polarized light to different degrees. This difference
(circular dichroism), when measured over all wavelengths, results in a
characteristic spectrum. If the direction of stirring is changed, the
sign of the circular dichroism switches. In addition, the magnitude of
the circular dichroism increases with increased stirring.
This phenomenon does not stem, as first thought, from the twisting of
individual nanofibers. It is evidently caused by a special macroscopic
spatial arrangement of the fibers within the sample cuvette: Like a flag
waving in the breeze, the individual fibers are directed by the current.
Along the beam of light shining through the cuvette, the different
currents within the vortex drive the fibers into a helical arrangement—a
structure reminiscent of certain liquid-crystalline phases. When the
direction of stirring is changed, the helical structure also changes the
direction it twists.