A physics theory proposed by a University of East Anglia researcher 40 years ago has finally found its application.
Prof David Andrews published his theory in his first few weeks at UEA back in 1979. At the time, a real-world application for his work “could never have been imagined” he says.
But after four decades of pursuit by physicists around the world, researchers at the University of Bath have now have confirmed the physical effect of Prof Andrews’ theoretical predictions.
The technique precisely measures the twist, or chirality, of molecules using lasers. The experiments prove it to be 100,000 times more sensitive than current standard methods.
For the first time ever, the Bath research team used a physical effect – specifically the colour-changing of light scattered from chiral molecules – to measure the chirality present, confirming Prof Andrews’ predictions.
Chirality describes the orientation of molecules, which can exist in left or right ‘handed’ forms depending on how they twist in three dimensions. Many molecules essential to life, including DNA, amino acids and proteins, exhibit chirality and the handedness can totally change their function or properties. Therefore knowing the chirality of a substance is often critically important.
For decades scientists had sought to prove that you could accurately determine the chirality of molecules by measuring a colour-changing (nonlinear optical) effect upon illumination with circularly polarised light. In theory, such light at very high intensities should change colour, scattering differently from oppositely handed molecules – but this had never been demonstrated experimentally.
Prof Andrews, from UEA’s School of Chemistry, said: “Dr Valev’s pioneering work is a clever and highly significant achievement, for he has realised a kind of application that could never have been imagined when the theory was first laid, 40 years ago.
“His results serve as an encouragement to all pure theorists!”
Dr Ventsislav Valev, who leads the research group in the Department of Physics at the University of Bath, said: “We’ve demonstrated a new physical effect – you don’t get to say that every day. This is exactly why I got into science.
“We began thinking about the problem 13 years ago, together with Prof Thierry Verbiest, at KU Leuven, Belgium. Because the effect was so elusive, I knew half of the solution would be to develop a very sensitive experimental setup. This is what I did for many years. The other half was finding the right samples and I was really excited to discover the nanoscopic silver springs (nano-helices) fabricated by Prof Peer Fischer’s group, at the Max Planck Institute for Intelligent Systems, in Stuttgart, Germany.”
PhD student Joel Collins had an incredible moment when running a series of tests on these springs.
He said: “To be honest my attitude was almost ‘OK let’s get this out the way to make sure it doesn’t work and we can move onto something else’. Then, together with my colleague Dr Kristina Jones, we noticed that there did actually seem to be an effect, and I thought ‘Hmmmm, that’s interesting’.
“We kept repeating the experiment to make sure it was actually a real effect and we saw that not only is it there but it’s huge.
“For my part, I didn’t really recognise how important it is, and was expecting someone to come along and rip it to shreds, to say – ‘you haven’t thought of that’ or ‘you’ve missed this’. But over time it has dawned on me – this is actually amazing.”
The experiment sees nano springs dispersed in water within a glass box where they spread randomly. Then a laser is aimed at them. The twist (circular polarisation) of the laser is switched periodically and light scattered from the box at 90° is analysed to determine the chirality of the springs present.
Dr Valev added: “It’s taken 40 years, people have been looking for this without success, and not for lack of trying. It’s amazing. The theory was quite controversial, people thought that maybe the effect was impossible to observe, maybe something else was there, blocking it.
“For 200 years scientists have been using the same method to measure chirality. It’s not very sensitive, but it’s robust and simple, however precise measurements of chirality have become a major hurdle for human-made chiral nanotechnology because of false positives.
“Now we have a method 100,000 times more sensitive, free from false positives. There’s a new kind of manufacturing process currently emerging. It is called “lab-on-a-chip” and our effect fits very well with it.
“A more sensitive test means you can use lower quantities in quality control and reduce waste, there are applications in chemical and pharmaceutical manufacturing, as well as in microfluidics, in miniaturisation and for developing personal pharmaceutical technologies.”
Advanced laser sources, sensitive detection equipment and state-of-the-art nanofabrication techniques have all come together to enable the experimental observation of the new effect.
Next, the researchers will be using their findings to characterise chiral molecules and to develop its technological applications.
The paper “First observation of optical activity in hyper-Rayleigh scattering” is published in Physical Review on Wednesday, February 6, 2019.
The research was funded by The Royal Society, the Science and Technology Facilities Council (STFC) and the Engineering and Physical Science Research Council (EPSRC).