Researchers at the University of Michigan in Ann Arbor have created synthetic microparticles that are more intricate than some of the most complicated ones found in nature. The findings could lead to more stable fluid-and-particle mixes, such as in paints, and new ways to twist light that could lead to holographic projectors.
The particles are composed of twisted spikes arranged into a ball a few microns, or millionths of a millimeter, across.
Some naturally occurring microscopic spiky structures include plant protein, immune cells, and some viruses. Among the most complex natural particles are coccolithophores, a type of algae known for building intricate limestone shells around themselves. Scientists and engineers try to make them in labs to better understand the rules that govern how the particles grow. Until now, there was no formalized way to measure the complexity of results.
“Numbers rule the world, and being able to rigorously describe spiky shapes and put a number on complexity enables us to use new tools like artificial intelligence and machine learning in designing nanoparticles,” says Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering at U-M, who led the project.
The team – which includes researchers at the Federal University of São Carlos and the University of São Paulo in Brazil, as well as the California Institute of Technology and the University of Pennsylvania – used the new framework to demonstrate that their particles were even more complicated than coccolithophores.
The computational arm of the team, led by André Farias de Moura, professor of chemistry at the Federal University, investigated the quantum properties of the particles and the forces acting on the nanoscale building blocks.
One of the key players in producing complexity can be chirality, or the tendency to follow a clockwise of counterclockwise twist. The team introduced chirality by coating nanoscale gold sulfide sheets, which served as their particle building blocks, with an amino acid called cysteine. Cysteine comes in two mirror-image forms, one driving the gold sheets to stack with a clockwise twist, and other tending toward a counterclockwise twist. In the most complex particle, a spiky ball with twisted spines, each gold sheet was coated with the same form of cysteine.
The team also used flat nanoparticles to create spikes that were flat rather than round. They also used electrically charged molecules to ensure that the nanoscale components built themselves into larger particles, bigger than a few hundred nanometers across, due to repulsion.
“These laws often conflict with each other, and the complexity emerges because these communities of nanoparticles have to satisfy all of them,” says Kotov, who is also professor of materials science and engineering and macromolecular science and engineering.
The complexity is useful. Nanoscale spikes on particles like pollen keep them from clumping together. Similarly, the spikes on the particles made by the research team help them disperse in virtually any liquid, a property that is useful for stabilizing sold/liquid mixtures.
The microparticles with twisted spikes also take in UV light and emit twisted, or circularly polarized, visible light in response. It appears that UV energy was absorbed into the hearts of the particles and transformed through quantum mechanical interactions, becoming circularly polarized visible light by the time it left through the curved spikes.
The researchers believe the findings can help scientists engineer particles that improve biosensors, electronics, and the efficiency of chemical reactions.
The study is titled “Emergence of Complexity in Hierarchically Organized Chiral Particles” and was published in the journal Science. The research was funded by the U.S. Department of Defense, the National Science Foundation, and Brazilian funding agencies CAPES, CNPq, and FAPESP.
