Nanoscopic golden springs 5,000 times thinner than a human hair have been used to unravel twisted molecules and reveal the secrets of their chirality.
The chirality – or ‘handedness’ – of a molecule can determine its characteristics. The molecule which makes oranges smell like oranges and lemons smell like lemons are the same, except one twists one way and one twists the other.
More importantly, the chirality of a molecule could have significant implications in pharmaceuticals, as seen with the thalidomide tragedy in the 1960s. Thalidomide was prescribed as a mild sleeping pill safe even for pregnant women, however what scientists didn’t know was that while one orientation of the molecule was safe, its mirror image was not and led to many babies being born with deformed limbs.
Now scientists from the University of Bath, together with colleagues from the Max Planck Institute for Intelligent Systems have used nanoscopic golden springs – dubbed chiral nanostructures – and powerful lasers to better detect chiral molecules. The applications could improve not only pharmaceutical design but telecommunications and nanorobotics.
Chirality (the twist in objects) is a fundamental property of life, as all amino acids and sugars within living organisms are chiral and all have the same chirality. However, while chirality is very useful to understand molecules, molecules are not ideal for understanding chirality. The reason is, we cannot tune the geometry of molecules, and we cannot chose the lengths or orientation of molecular bonds – those are fixed by nature. This is why we create 'meta-molecules', artificial metal structures whose dimensions are able to be tuned at the nanoscale. In this case, we used chiral nanohelices made of a gold-copper alloy, a work somewhat akin to doing nano-jewellery.
Ventsi Valev, Research Team Leader at Bath
Scientists can study chiral molecules using a particular laser light which twists as it travels. The nanosprings help twist this light to better fit the chiral molecules, making it easier to detect very small amounts.
This study examined how effective the springs were at enhancing interactions between light and chiral molecules. It was based on a color-conversion method for light, known as Second Harmonic Generation (SHG), where the better the performance of the spring, the more red laser light converts into blue laser light.
It is well known that we can mix colors of light to get other colors. For instance, mixing red and green light produces yellow light. Here, we used a very unusual method of color mixing – we combined red and red to produce… blue. This process is called Second Harmonic Generation, and for it to occur, we need a LOT of red light. In practice, we use very powerful laser pulses on the nanostructures. The amount of blue light we can produce is roughly proportional to the 'twist' of the chiral nanostructures.
The team used light in the near infrared region at 800 nm, i.e. red, mixing two photons to produce a single photon at 400 nm, strictly speaking ultraviolet, or blue.
We found out that, unfortunately, in addition to the blue light produced by chirality, there was a lot of unwanted blue light from other geometric features of the helices. How the helices are oriented, the position of the wire end, the ordering of the helices with respect to one-another, all of these yield blue light that can mask the signal from chirality.
The results – published in Advanced Materials – show that while the springs were indeed very promising, how well they performed depended on the way they were facing.
It is like using a kaleidoscope to look at a picture; the picture becomes distorted when you rotate the kaleidoscope. We need to minimize the distortion.
David Hooper, Physics PhD student and author
The team are now working on ways to optimize the springs by developing methods to isolate the signal from the chirality, independently of other geometric features. Valev says they are pursuing this goal, both mathematically through a comprehensive analysis of all possible signal sources, and experimentally by looking for conditions where only the chirality would be visible.
University of Bath
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