In a new study where they use nanoscale structures to "twist" light so it interacts uniquely with matter, scientists show they can distinguish between left- and right-handed versions of the same molecule. Such a sensing mechanism could help ensure greater purity and safety of pharmaceuticals, they say.
With colleagues from research centers in Belgium and Italy, Dr. Ventsislav Valev and Professor Jeremy Baumberg from the Cavendish Laboratory at the University of Cambridge in the UK designed the sensing mechanism using powerful lasers combined with a nanopatterned gold surface.
They report their work in the journal Advanced Materials.
"Together, these technologies could help ensure that new drugs are safe and pure," says Dr. Valev.
With some shapes, the mirror image is symmetrical - if you mix them together, you cannot tell the difference between a structure and its mirror "twin." But with other shapes, for example our hands, the left is a mirror image of the right, but it does not match it - your right hand does not fit in your left-hand glove.
In chemistry, this "handedness" is called chirality. The chirality of a molecule affects how it interacts with its environment, and different chiral forms of the same molecule can have completely different effects.
Chirality is important in drug development because it may be only the right- or left-handed version that has the desired effect.
Perhaps the best-known example of chirality that highlights unwanted side effects is that of Thalidomide, a drug that was prescribed to pregnant women in the 1950s and 1960s.
One chiral form of Thalidomide was effective in treating morning sickness in early pregnancy, but the other form prevented healthy growth of the fetus. But the drug that patients received contained both forms, and over 10,000 children around the world were born with shortened or missing limbs and other defects.
Thus, identifying the correct chiral form of a drug could be crucial. Specific forms bind to specific cell receptors, so ensuring the correct chiral version is present determines the purity and effectiveness of the final product. But the problem is, when a drug is synthesized, both chiral forms are made in equal quantities.
Lasers, gold nanostructures and twisted light to select molecule chirality
Using beams of polarized light is one way that chemists can detect different chiral forms of molecules. Different chiral forms of the same molecule twist the light in different ways - producing "chiroptical effects." However, these are typically very weak.
The researchers say that when twisted light matches the twist of nanostructures, strong interactions with chiral molecules could arise.
Image credit: Credit: Ventsislav Valev
But by using powerful lasers, scientists can produce second harmonic generation (SHG) chiroptical effects, which are typically three orders of magnitude stronger. And more recently, scientists have also developed a super-twisted form of light called "superchiral light" that increases the different chiroptical effects that left- and right-handed versions of chiral molecules produce.
Dr. Valev and colleagues took another step toward even stronger chiroptical effects by finding a direct link between the fundamental equations for superchiral light and SHG - a combination that they believe can yield record-breaking effects, resulting in very high sensitivity for measuring the chiral purity of drugs.
They also further enhanced the sensitivity by using tiny gold structures - known as plasmonic nanostructures - that focus the light on hotspots on their surfaces, where superchiral light and SHG combine their effects.
In their study report, they "highlight four different strategies which have been used to achieve giant chiroptical effects in chiral nanostructures."
They also present two examples of what they describe as "chiral switches":
"Whereas in the first one, switching the chirality of incoming light causes a reversal of the handedness in the nanostructures, in the second one, switching the handedness of the nanostructures causes a reversal in the chirality of outgoing light."
Dr. Valev says, "By using nanostructures, lasers and this unique twisting property of light, we could selectively destroy the unwanted form of the molecule, while leaving the desired form unaffected."
In July 2013, Medical News Today reported how researchers in Switzerland developed a super-quick test for bacteria using nano-sized tuning forks. At present, some tests for identifying bacteria can take days. With a test based on the nano tuning forks, the researchers believe the timescale for results would reduce to minutes, potentially saving the lives of patients whose infections are so severe they will die if they do not get the right antibiotic straight away.
Written by Catharine Paddock PhD