The photos depict the vibrant colors exhibited by a dispersion of magnetic nanoparticles when subjected to magnetic fields with varying chiral distributions, as observed through polarized lenses. Image: Yin lab, UC Riverside.Some molecules exist in two forms with the same chemical formula but mirror-image structures that are not superimposable, like our left and right hands. Such asymmetric molecules are termed chiral.
Chiral molecules tend to be optically active because they interact with light in different ways. Oftentimes, only one form of a chiral molecule exists in nature, as is the case with DNA. And if a specific chiral molecule works well as a drug, its mirror image can prove ineffective for therapy.
In trying to produce artificial chirality in the lab, a team led by chemists at the University of California, Riverside, has now found that the distribution of a magnetic field is itself chiral.
“We discovered that the magnetic field lines produced by any magnet, including a bar magnet, have chirality,” said Yadong Yin, a professor of chemistry, who led the team. “Further, we were also able to use the chiral distribution of the magnetic field to coax nanoparticles into forming chiral structures.” The team reports its findings in a paper in Science.
Traditionally, researchers have used a process called ‘templating’ to create a chiral molecule. This uses an existing chiral molecule as an initial template. Achiral (or non-chiral) nanoparticles are then assembled on this template, allowing them to mimic the structure of the chiral molecule. The drawback to this process is that it cannot be universally applied, being heavily dependent on the specific composition of the template molecule. Another shortcoming is that the newly formed chiral structure cannot be easily positioned at a specific location on, say, an electronic device.
“But to gain an optical effect, you need a chiral molecule to occupy a particular place on the device,” Yin said. “Our technique overcomes these drawbacks. We are able to rapidly form chiral structures by magnetically assembling materials of any chemical composition at scales ranging from molecules to nano- and microstructures.”
Yin explained that his team’s method uses permanent magnets that consistently rotate in space to generate chirality in magnetic nanoparticles. This chirality can then be transferred to achiral molecules by doping the magnetic nanoparticles with guest species such as metals, polymers, semiconductors and dyes.
Chiral materials acquire an optical effect when they interact with polarized light. The waves making up such light vibrate in a single plane, reducing its overall intensity. As a result, polarized lenses in sunglasses cut glare to our eyes, while non-polarized lenses do not.
“If we change the magnetic field that produces a material’s chiral structure, we can change the chirality, which then creates different colors that can be observed through the polarized lenses,” Yin said. “This color change is instantaneous. Chirality can also be made to disappear instantaneously with our method, allowing for rapid chirality tuning.”
The team’s findings could have applications in anti-counterfeit technology. A chiral pattern that signifies the authenticity of an object or document would be invisible to the naked eye but visible when seen through polarized lenses. Other applications of the findings are in sensing and optoelectronics.
“More sophisticated optoelectronic devices can be made by taking advantage of the tunability of chirality that our method allows,” said Zhiwei Li, first author of the paper and a former graduate student in Yin’s lab. “Where sensing is concerned, our method can be used to rapidly detect chiral or achiral molecules linked to certain diseases, such as cancer and viral infections.”
This story is adapted from material from the University of California, Riverside, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.