Biological sciences is a broad field of study encompassing many areas, including chiral structures such as DNA. Chirality is a mathematical notion that explains a structure's geometrical quality. Chirality is often reduced in chemical disciplines as a binary right or left feature of compounds, which does not completely convey the complexity of chiral forms.
Micron-scale bowties with candy-wrapper twists in a colorized electron microscope image. The ability to control the degree of twist in a curling, nanostructured material could be a useful new tool in chemistry and machine vision. Credit: Prashant Kumar, Kotov Lab, University of Michigan.
A recent article published in the journal Nature focuses on developing nanostructured particles with an anisotropic bowtie shape that displays a continuous range of chirality. These microparticles can be precisely tuned in various dimensions such as twist angle, width, pitch, length, and thickness.
Chirality is an intriguing property that is present in various forms, from the macro to the molecular level. At the macro level, chiral geometries can be observed in the form of helical springs, which are often stretched to create coils of different lengths, also known as pitch.
On a smaller scale, such as in origami sheets, polymeric solids, and nanocomposites, chirality is more complex and continuously variable. On the molecular level, however, chirality is often regarded as a binary feature. Chiral compounds are either left- or right-handed, with binary stereochemical structures.
Creating continuously controllable chirality in chemical compounds would have far-reaching implications in domains such as chiral optoelectronics, chiral nanocomposites, biological separations, and chiral catalysts.
The accessibility of continuously varying chiral substances would aid in developing fundamental relationships between chirality measurements and chemical characteristics, which could lead to new materials with unique optical properties, more efficient separation processes, and improved catalytic processes.
Chiral Nanomaterials: A New Tool for Chemistry and Machine Vision
Controlling the degree of twist in nanostructured materials is an emerging technique that has the potential to revolutionize chemistry and machine vision. Chiral nanomaterials, which selectively reflect twisted light, have been notoriously difficult to produce.
Efforts to correlate optical activity with various chirality measures in traditional molecules have largely failed. Still, recent research suggests that chiral nanostructures and their assemblies, such as bowtie-shaped nanostructures, may hold promise because of changes in the physics of chiroptical activity between them and binary chiral molecules.
The development of chiral nanomaterials could enable robots to navigate complex human environments with greater accuracy. Twisted structures encode information in the shapes of reflected light waves rather than in the two-dimensional arrangement of symbols found in most human-readable signs.
This technique takes advantage of an aspect of light known as polarization, which humans can barely sense. Twisted nanostructures preferentially reflect certain kinds of circularly polarized light, a shape that twists as it moves through space. This makes chiral metamaterials an exciting prospect for the future of machine vision and navigation.
Highlights of the Current Study
In this study, the researchers employed a hierarchical assembly process to create the micron-sized "bow ties" that involved interconnecting nanoribbons containing helical chains of cystine with cadmium ions.
The chirality of the cystine determined the handedness of the resulting bow ties, with left-handed cystine yielding left-handed bow ties and right-handed cystine yielding right-handed bow ties. Each bow tie had a candy-wrapper twist due to its unique shape.
By electrostatically restricting this assembly process, the researchers could synthesize bow ties with precise control over their pitch, width, thickness, and length. These bow ties were then mixed with polyacrylic acid and used as a sort of paint on various materials such as glass, fabric, plastic, and more.
To test the properties of the bow ties, the researchers conducted experiments using lasers to determine the reflection of twisted light from the bow ties, depending on the twist in the bow tie shape.
Important Findings and Prospects of the Research
Unlike other chiral nanostructures, which can take days to self-assemble, the bow ties formed in just 90 seconds. The team was able to produce an impressive 5,000 different shapes within the bow tie spectrum, indicating the high degree of control and reproducibility achievable with this assembly process.
The self-limited assembly of the bowties has several benefits, including high synthetic reproducibility, computational predictability, and size monodispersity of their geometries for different assembly conditions.
“Not only do we know the progression from the atomic scale up to the micron-scale of the bow ties, but we also have theory and experiments that show us the guiding forces. With that fundamental understanding, you can design a bunch of other particles,” said Thi Vo, a co-author of the study.
In addition, the bow tie particles were found to have variable polarization rotation, making them useful for printing photonically active metasurfaces with spectrally tunable positive or negative polarization signatures. This property could have important applications in light detection and ranging (LIDAR) devices, among other areas.
Kumar, P. et al. (2023). Photonically active bowtie nanoassemblies with chirality continuum. Nature. https://doi.org/10.1038/s41586-023-05733-1
Source: University of Michigan