The optical activity of a solution containing nanofibres changes rapidly when the direction in which the solution is stirred is changed. This is according to researchers in Japan, who analysed the vortices in a stirred liquid containing dye-functionalized zinc porphyrin dendrimers. The result could be used to make vortex flows optically active.
A vortex is a spinning flow of fluid that moves in a spiral. However, a vortex can be a very complex structure containing many regions, with currents moving in completely different directions. For example, if a liquid is stirred in a container, a dense circular current forms at the centre while a loose spiral-shaped flow appears at the outer regions of the vortex.
Scientists believe that vortices were responsible for breaking the chiral symmetry in nature to give us the "handed" life that we see today, which has clear preferences for either left or right-handed molecular building blocks, such as sugars and amino acids. Vortices clearly twist in either one direction or another and the different directions can be related to each other like mirror images.
Now, Takuzo Aida of the University of Tokyo and Akihiko Tsuda at Japan Science Technology Agency have synthesized a zinc porphyrin dendrimer – a branched molecule with a central zinc atom – that allows them to observe these individual currents using spectroscopy. The highly branched zinc-containing molecules aggregate in solution to form long nanofibres.
The researchers found that if the solution is not stirred, it is not optically active. However, it becomes so as soon as it is stirred. The stirred solution rotates right and left-circularly polarized light to different degrees. When measured over all wavelengths, this difference, known as circular dichroism, results in a characteristic optical spectrum.
When the researchers then changed the direction of stirring, the sign of the circular dichroism switched. What is more, the magnitude of the circular dichroism increases with increased stirring.
According to the team, the effect does not come from the twisting of individual nanofibres – as first thought – but is caused by a special macroscopic spatial arrangement of the fibres within the sample. "Like a flag waving in the breeze, the individual fibres are directed by the current," say Aida and colleagues.
By shining a light through the cuvette containing the liquid, the researchers were clearly able to observe that the different currents within the vortex drive the fibres into a helix shape. When the direction of stirring is changed, the helical structure changes along the direction in which it twists.
A vortex is a spinning flow of fluid that moves in a spiral. However, a vortex can be a very complex structure containing many regions, with currents moving in completely different directions. For example, if a liquid is stirred in a container, a dense circular current forms at the centre while a loose spiral-shaped flow appears at the outer regions of the vortex.
Scientists believe that vortices were responsible for breaking the chiral symmetry in nature to give us the "handed" life that we see today, which has clear preferences for either left or right-handed molecular building blocks, such as sugars and amino acids. Vortices clearly twist in either one direction or another and the different directions can be related to each other like mirror images.
Now, Takuzo Aida of the University of Tokyo and Akihiko Tsuda at Japan Science Technology Agency have synthesized a zinc porphyrin dendrimer – a branched molecule with a central zinc atom – that allows them to observe these individual currents using spectroscopy. The highly branched zinc-containing molecules aggregate in solution to form long nanofibres.
The researchers found that if the solution is not stirred, it is not optically active. However, it becomes so as soon as it is stirred. The stirred solution rotates right and left-circularly polarized light to different degrees. When measured over all wavelengths, this difference, known as circular dichroism, results in a characteristic optical spectrum.
When the researchers then changed the direction of stirring, the sign of the circular dichroism switched. What is more, the magnitude of the circular dichroism increases with increased stirring.
According to the team, the effect does not come from the twisting of individual nanofibres – as first thought – but is caused by a special macroscopic spatial arrangement of the fibres within the sample. "Like a flag waving in the breeze, the individual fibres are directed by the current," say Aida and colleagues.
By shining a light through the cuvette containing the liquid, the researchers were clearly able to observe that the different currents within the vortex drive the fibres into a helix shape. When the direction of stirring is changed, the helical structure changes along the direction in which it twists.
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