Researchers have observed hyper-Raman optical activity, an optical phenomenon scientists have so far struggled to observe without damaging the molecules being studied.
The study, by an international team of scientists led by physicists at the University of Bath in the UK, has significant implications for pharmaceutical science, security, forensics, environmental science, art conservation and medicine.
The hyper-Raman effect
The Raman effect, where for every million light particles (photons), a single one changes colour, is often used to help paint a picture of the energy states of molecules and identifies them.
Yet some molecular features (energy states) are invisible to the Raman effect. To reveal them and paint a more complete picture, ‘hyper-Raman’ is needed.
The hyper-Raman effect is a more advanced phenomenon than simple Raman. It occurs when two photons impact the molecule simultaneously and then combine to create a single scattered photon that exhibits a Raman colour change.
Hyper-Raman can penetrate deeper into living tissue, it is less likely to damage molecules and it yields images with better contrast (less noise from autofluorescence). Importantly, while the hyper-Raman photons are even fewer than those in the case of Raman, their number can be greatly increased by the presence of tiny metal pieces (nanoparticles) close to the molecule.
Despite its significant advantages, so far hyper-Raman has not been able to study a key enabling property of life – chirality.
Subtle and impossible to measure
In molecules, chirality refers to their sense of twist – in many ways similar to the helical structure of DNA. Many bio-molecules exhibit chirality, including proteins, RNA, sugars, amino acids, some vitamins, some steroids and several alkaloids.
Light too can be chiral and in 1979, the researchers David L Andrews and Thiruiappah Thirunamachandran theorised that chiral light used for the hyper-Raman effect could deliver three-dimensional information about the molecules, to reveal their chirality.
However, this new effect – known as hyper-Raman optical activity – was expected to be very subtle, perhaps even impossible to measure. Experimentalists who failed to observe it struggled with the purity of their chiral light. Moreover, as the effect is very subtle, they tried using large laser powers, but this ended up damaging the molecules being studied.
Measuring chirality with an indirect approach
While previous attempts aimed to measure the effect directly from chiral molecules, the Bath team took an indirect approach.
“We employed molecules that are not chiral by themselves, but we made them chiral by assembling them on a chiral scaffold,” said Explaining, Professor Ventsislav Valev who led both the Bath team and a study recently published in Nature. “Specifically, we deposited molecules on tiny gold nanohelices that effectively conferred their twist (chirality) to the molecules.”
The gold nanohelices have another very significant benefit – they serve as tiny antennas and focus light onto the molecules. “This process augments the hyper-Raman signal and helped us to detect it,” Valev added.
“Such nanohelices were not featured on the 1979 theory paper and in order to account for them we turned to none other than one of the original authors and pioneer of this research field.”
Confirming a 45-year-old theory
Emeritus Professor Andrews from the University of East Anglia and co-author of the paper said: “It is very gratifying to see this work, the experiment finally confirms our theoretical prediction, after all these years.”
This new effect could serve to analyse the composition of pharmaceuticals and to control their quality. It can help identify the authenticity of products and reveal fakes. It could also serve to identify illegal drugs and explosives at customs or crime scenes.
It will aid detecting pollutants in environmental samples from air, water and soil. It could reveal the composition of pigments in art for conservation and restoration purposes, and it will likely find clinical applications for medical diagnosis by detecting disease-induced molecular changes.
Professor Valev said: “This research work has been a collaboration between chemical theory and experimental physics across many decades and across academics of all stages – from PhD student to Emeritus Professor.
“We hope it will inspire other scientists and that it will raise awareness that scientific progress often takes many decades.”
