Using two thin, tiny gold nanorods 10,000 times thinner than a human hair, researchers from Stuttgart and Munich have succeeded in creating an adjustable filter for so-called circularly polarised light. The crucial factor for how the system absorbs light is the angle between the two gold rods.
In circularly polarised light, the oscillating light wave rotates about the axis along which the light beam propagates. A distinction can be made between left-handed and right-handed circular polarisations, depending on the direction of rotation. Many molecules change their absorption properties in respect of this kind of light when their internal spatial arrangement is modified.
The angle between two gold rods determines their light absorption
Physicists from the Max Planck Institute for Intelligent Systems in Stuttgart, Ludwig-Maximilian-Universität in Munich, and Ohio University in Athens, USA, have now exploited this fact. Depending on the angle formed between the gold rods, they exhibit a preference for absorbing either left-handed or right-handed circularly polarised light. The experts call this behaviour circular dichroism. The absorption, which also depends on the incident wavelength, causes collective electronic oscillations to be excited in the metal lattice, so-called plasmons. The resonance conditions which have to be fulfilled here for the absorption of left-handed or right-handed circularly polarising light are also affected by how the gold rods are arranged with respect to each other.
When choosing the metal, it was important for the researchers that their arrangement exhibited the circular dichroism in the range of visible light. “This is only the case with gold,” explains Laura Na Liu, who headed the project for the Max Planck Institute for Intelligent Systems. There was a further issue of how the angle between the rods could be regulated in a controlled way from the outside. The scientists needed a kind of flexible hinge between the gold nanorods. A switch.
To this end they initially fixed each nanorod on a so-called DNA origami bundle. These are multiply folded DNA structures with an overall elongated orientation. “Hinges are extremely difficult to realise on the nanoscale,” says Laura Na Liu. “The use of DNA is quite obvious, especially after the introduction of DNA origami.”
DNA fragments work like hook-and-loop fasteners on the gold cross
The chemical bond between one DNA bundle and one gold rod causes them to line up absolutely parallel to each other. Two DNA bundles – and thus the associated gold rods as well – are initially more or less at right angles to each other, which is very similar to the arrangement which would be created if one were to cross two branches in the woods.
The actual trick with this arrangement was that the two DNA bundles and thus the gold rods affixed to them can be rotated with respect to each other. The researchers made use of a special property of DNA molecules here, i.e. that two DNA chains join up to form a double strand when the sequence of the bases along the two individual strands is complementary.
In order to be able to exploit this, the researchers allowed remnants of DNA molecules with a particular base sequence to protrude at very specific locations of their bundle arrangement. These remnants can be thought of as one side of a hook-and-loop fastener. Initially these fasteners are still partially blocked. The blockage can be lifted by adding specific DNA molecular fragments – so that they are ready to link up with their matching counterpart. The researchers let this counterpart protrude from the other DNA bundle in each case. The bottom end of the vertical bundle thus approaches either the right or the left end of the horizontal bundle, depending on the type of DNA fragment added. The consequence is: in both cases the crossed arrangement at almost right angles was transformed into a kind of St. Andrew’s cross with bundles crossing each other at an oblique angle. Since this also caused the gold rods to change their alignment with respect to each other, their absorption behaviour for circularly polarised light changed as well.
A sensor for biochemical reactions
“In order to be able to use this kind of switch for practical applications as well, it is naturally important that this process is reversible,” explains Liu. And the researchers were also successful here: adding a further DNA fragment breaks open the bond between the horizontal and the vertical bundle again – and thus recreates the original situation. The process can be restarted by adding DNA again. And so on and so forth.
The physicists have thus created a nanostructure which can be reversibly switched with the aid of DNA molecules. This provides the researchers with a number of possible applications – not only as a switching element in nano-optics or photonic information processing. They can imagine using such a system as a nanosensor for biochemical reactions, for example. If one were to bind one of the reaction components to the free DNA remnants, i.e. to the end of the hook-and-loop fastener, the subsequent chemical reaction with a different component could change the conformation of the whole system in such a way that this change could be observed by measuring the absorption behaviour in real time. The design of so-called optical super-fluids is also conceivable, say the researchers. These fluids allow the diffraction index to be set at the push of a button, so to speak.