A nanomaterial that is both twisted and not at the same time

Materials World magazine
29 Oct 2019

A recently developed nanomaterial is proving highly effective at producing more complex and engineered pharmaceuticals. Idha Valeur reports.

A new nanomaterial incorporating molecules that can twist in different directions at the same time, which could prove useful for scientists working on several types of pharmaceuticals, has been developed by researchers from the University of Bath, UK. The development of the material will support a new technique that makes it easier to establish the directions of the twists on a molecular level.

Identifying and detecting how molecules twist, known as chirality, and the property of a molecule’s mirror image is extremely important as the mirror image of a specific molecule may have very different properties and characteristics. For industries such as pharmaceuticals, this knowledge is essential.

University of Bath Department of Physics Professor, Ventsislav Valev, told Materials World that in pharmaceuticals, being able to distinguish the direction of molecular twist can be a matter of life or death. ‘For instance, naproxen is an anti-inflammatory medical drug whose mirror-image twists the opposite way and causes liver poisoning. Similarly, ethambutol can treat tuberculosis, yet the exact same molecule, with a different direction of twist, causes blindness,’ he explained.

‘Usually, light is used to detect the direction of molecular twist. Recent and very promising research suggests that metal nanostructures could improve upon the current detection methods. These nanostructures are twisted themselves, so that, in a sense, they amplify the molecular twist, making it easier to detect for light.’ A material previously made to help establish a molecule’s chirality also contained small twisted metal wires. This structure made it challenging to distinguish between the chirality of the nanomaterial and that of the molecule.

‘Upon using twisted metal nanostructures to detect molecular twist, the question becomes, how can we distinguish between the two? The nanomaterials are therefore subject to contradicting requirements – on the one hand, they should be twisted, so they can match and amplify the molecular twist, on the other, they should have no twist of their own, so that they do not lead to false positives,’ Valev explained.

The team researched a suitable material for this purpose, their nanomaterial is simultaneously twisted and not twisted to eliminate the contradictory requirements.

‘We produced an array of metal nanostructures, whereby each twisted unit (or cell) is surrounded by units with the opposite twist. In this manner, although all individual nanostructures are twisted, on a larger scale, the opposite twists cancel out, resulting in an untwisted material,’ Valev said.

According to Valev, the nanostructures are made of pure gold, which is deposited on a thin layer of chromium. ‘At the nano scale, gold, and other coinage metals have a fantastic property whereby, depending on the size of the nanoparticle, the surface electrons resonate with different colours of light. In practice, these metal nanoparticles serve as tiny lenses, efficiently focusing light on molecules. They also serve as miniature antennae, manipulating the electromagnetic fields of light to fit the twisted shape of molecules.’

Further, he explained that they used group theory analysis to show that their dual-spiralled material is different from one with no twists at all. ‘We then used diffraction spectroscopy to exploit the difference and offer a way to optimise the light-matter interactions. We are now working on applying our technology to determine the direction of molecular twist,’ he said. ‘Ultimately, this is where the main applications will be – in developing an ultra-sensitive method to distinguish between useful pharmaceuticals and their dangerous counterparts’.