Structure of the helical chiral supercrystal. Image: ITMO University.
Structure of the helical chiral supercrystal. Image: ITMO University.

Scientists from ITMO University in St Petersburg, Russia, and Trinity College Dublin in Ireland have designed an optically-active nanosized supercrystal with a novel architecture that should allow it to separate organic molecules, making it useful for drug synthesis. The study is published in Scientific Reports.

The new supercrystal has a structure similar to a spiral staircase, and is composed of numerous rod-shaped semiconductor nanocrystals known as quantum dots. Importantly, unlike individual quantum dots, this assembly possesses the property of chirality. Thanks to this distinctive feature, these supercrystals could be used in pharmacology to identify chiral biomolecules.

An object is chiral if it cannot be superimposed on its mirror image, with human hands being the most common example. The supercrystal is made up of two spiral staircases with quantum dots as steps: one staircase turns right, while the other turns left. This means the supercrystal can absorb left-polarized light but not right-polarized light, or other way round, depending on the precise architecture.

"As with any chiral nanostructure, the range of applications of our supercrystals is huge," says Ivan Rukhlenko, head of the Modeling and Design of Nanostructures Laboratory at ITMO University. "For example, we can use them in pharmacology to identify chiral drug molecules. Gathering in spirals around them, quantum dots can exhibit collective properties that enhance molecule absorptivity by hundreds of times. Thus, the molecules can be detected within solution with much more accuracy."

Chirality is inherent in almost all organic molecules, including proteins, nucleic acids and other substances in the human body. For this reason, two mirror forms (enantiomers) of one drug can have different biological activities: while one form may produce a therapeutic effect by interacting with chiral biomolecules, the other form may not have any effect at all or may even be toxic. This is why careful separation of enantiomers during drug synthesis is vitally important.

In addition to pharmacology, the optical activity of the supercrystals could find use in several technical applications where light polarization is required. The rod shape of each quantum dot causes them to interact with light along their longitudinal axis, which is why the positioning of the quantum dots is critical for the optical properties of the whole structure. Similarly, the optical effects of the supercrystal occur most strongly along its central axis. Therefore, by orienting the supercrystals in solution scientists can alter the optical activity of the system, in a similar manner to liquid crystals.

In order to study the properties of the supercrystals, the researchers modeled the effect of varying a number of structural characteristics. In particular, they stretched the supercrystal like a spring, and changed the distance between the quantum dots and their orientation relative to each other.

"For the first time, we could theoretically identify the parameters of chiral supercrystal that let us achieve maximum optical effect. Thanks to this approach, we avoided the fabrication of many unnecessary copies with unpredictable properties," says Anvar Baimuratov, research associate at the Centre of Information Optical Technologies (IOT) at ITMO University and lead author of the study. "Knowing the output parameters of optical properties, we can model a supercrystal to solve a specific problem. Conversely, having data on the supercrystal structure, we can accurately predict its optical activity".

Based on the results of this study, scientists at the Dresden University of Technology in Germany now plan to synthesize the supercrystal by means of DNA origami. This will involve using DNA molecules as a template for assembling the helical structure from quantum dots. "Experimental study of our supercrystals should confirm their theoretically predicted properties and identify new ones. But the main advantage of new semiconductor structure is already evident: varying its morphology in the synthesis process, we can change optical response of the supercrystal in a wide frequency range," says Rukhlenko.

"Assembling quantum dots in blocks, we get more degrees of freedom to change optical activity of supercrystal solutions," says Baimuratov. "The more complex the structure is, the stronger its properties depend on how we have put the elements together. Adding complexity to the structure will lead to the appearance of a number of new optical materials."

This story is adapted from material from ITMO University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.