Tiny, self-assembled bow ties made of nanoparticles form a variety of different curl shapes that can be precisely controlled, a research team led by the University of Michigan has shown.
The development paves the way for the easy production of materials that interact with refracted light, providing new tools for machine vision and drug production.
While biology is full of twisted structures like DNA, known as chiral structures, the degree of twist is locked — trying to change it breaks the structure. Now, researchers can plot the degree of twist.
Such materials could allow robots to precisely navigate complex human environments. The twisted structures would encode information in the shapes of light waves reflecting off the surface, rather than in the two-dimensional array of symbols that comprise most human-readable signs. This would take advantage of an aspect of light that humans can barely perceive, known as polarization. Twisted nanostructures preferentially reflect certain kinds of circularly polarized light, a shape that twists as it moves through space.
“It’s basically like polarization vision in crustaceans,” said Nicholas Kotov, Irving Langmuir University Distinguished Professor of Chemical Sciences and Engineering, who led the study. “They get a lot of information despite the blurry environments.”
The robots could read markings that look like white dots on human eyes. the information would be encoded in the combination of the reflected frequencies, the tightness of the twist, and whether the twist was left-handed or right-handed.
By avoiding the use of natural and ambient light, relying instead on circularly polarized light produced by the robot, robots are less likely to miss or misinterpret a cue, whether in bright or dark environments. Materials that can selectively reflect twisted light, known as chiral metamaterials, are usually difficult to make — but bow ties are not.
“Previously, chiral metasurfaces were made with great difficulty using multimillion-dollar equipment. Now, these complex surfaces with multiple attractive uses can be printed like a photo,” said Kotov.
Twisted nanostructures can also help create the right conditions for producing chiral drugs, which are difficult to make with the right molecular twist.
“What has not been observed in any chiral system before is that we can control the twist from a fully twisted left-handed structure in a flat pancake to a fully twisted right-handed structure. We call this the chirality continuum,” said Prashant Kumar, a postdoctoral researcher at UM in chemical engineering and first author of the study at Nature.
Kumar experimented with bow ties as a type of paint, mixing them with polyacrylic acid and dabbing them onto glass, fabric, plastic and other materials. Laser experiments showed that this color reflected twisted light only when the twist in the light matched the twist in the bowtie shape.
Bow ties are made by mixing cadmium metal and cystine, a protein fragment that comes in left-handed and right-handed versions, in water spiked with lye. If the cystine was entirely left-handed, left-handed bowties were formed, and the right-handed cystine produced right-handed bowties — each with a candy wrapper.
But with different proportions of left-handed and right-handed cystine, the team made intermediate flips, including the flat pancake in a 50-50 ratio. The pitch of the narrowest bow ties, basically the length of a 360 degree turn, is about 4 microns – within the wavelength range of infrared light.
“Not only do we know the evolution from the atomic scale to the micron scale of bow ties, we also have theory and experiments that show us the guiding forces. With this fundamental understanding, you can design a bunch of other particles,” said Thi Vo, a former UM postdoctoral researcher in chemical engineering.
He worked with Sharon Glotzer, co-corresponding author of the study and Anthony C. Lembke Department of Chemical Engineering at UM.
Unlike other chiral nanostructures, which can take days to self-assemble, the bow ties formed in just 90 seconds. The team produced 5,000 different shapes across the bow tie spectrum. They studied the shapes in atomic detail using X-rays at Argonne National Laboratory before simulation analysis.
Additional material analysis and theory input were provided by collaborators at UM, the University of Pennsylvania, the University of Palermo in Italy, and Pro Vitam Ltd, Romania. The study was supported by the Office of Naval Research, the National Science Foundation and the Army Research Office.
Kotov is also the Joseph B. and Florence V. Cejka Professor of Engineering and Professor of Chemical Engineering and Macromolecular Science and Engineering. Vo is now a professor of chemical and biomolecular engineering at Johns Hopkins University. Glotzer is also the John Werner Cahn University Distinguished Professor of Engineering, Stuart W. Churchill Associate Professor of Chemical Engineering, and professor of materials science and engineering, macromolecular science and engineering, and physics.