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Biomimetic self-assembly based on particle symmetry yields highly complex microparticles

By Judy Meiksin July 14, 2020
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Scanning electron micrograph image of gold thiolate hierarchically organized particle with cysteine surface ligands. Made from nanosheets that all twist in the same direction, the spiky nanoparticle achieved the highest complexity measured. It absorbs UV light and emits twisted light in the visible part of the spectrum. Credit: Wenfeng Jiang, Kotov Lab, University of Michigan.

In biomineralization, hierarchically organized nanoparticles over multiple scales are assembled with twisted spikes and other morphologies. Researchers want to uncover how they are assembled in order to engineer particles with structural complexity. Such particles could find applications in sensing, electronics, and efficient chemical reactions. One of the key players in producing complexity can be chirality.

“In principle, any known chalcogenide may be viewed as a candidate material to build a 2D chiral sheet, since any chiral thiol [like an amino acid called cysteine] may replace the chalcogenide atoms and inhibit growth in one crystalline direction while imprinting chiral information at the same time,” says André F. de Moura of the Brazilian Center for Research in Energy and Materials.

By investigating a type of algae called spiky coccolithophores that build intricate limestone shells around themselves and using computational methods incorporating artificial intelligence and machine learning, de Moura together with an international research team led by Nicholas Kotov of the University of Michigan came up with synthetic particles that were even more complicated than the coccolithophores.

The researchers determined how such intricacy arises. They devised a way to measure this and put this to use to design synthetic microparticles with predetermined complexity and new property sets. Their work appears in a recent issue of Science.

The research team introduced chirality by synthesizing nanoscale gold sulfide sheets with cysteine, which served as their particle building blocks. Cysteine comes in two mirror-image forms, one driving the gold sheets to stack with a clockwise twist, and the other tending toward a counterclockwise twist. In the case of the most complex particle—a spiky ball with twisted spines—each gold sheet was coated with the same form of cysteine. 

To measure complexity in their synthetic particles, the researchers used a newly developed structural description of self-assembled particles and graph theory. Computer simulation enabled them to accurately describe the forces acting on the nanoscale building blocks. “Their intricate organization emerges from competing chirality-dependent assembly restrictions that render assembly pathways primarily dependent on nanoparticle symmetry rather than size,” the researchers write in their article.

The researchers can also tune the competitive interactions with other metals. Kotov told MRS Bulletin, “Doping of gold sulfide nanosheets with silver and copper changes their nanoscale mechanics. The elementary nanosheets become thicker and stiffer, which in turn, increases the energy required for their twisting, which makes it a dominant factor determining the shape of the assembling particles.”

The spikes on these particles made by the research team help them disperse in virtually any liquid, a property that is useful for stabilizing solid/liquid mixtures such as paints, plastics, and hydrophobic solvents. This property can be particularly useful in their implementation in flexible photonic devices.

“Their strong chiroptical activity that can be ‘tuned’ using the chirality as a property-defining parameter enables their applications in chiral photonics including implementations of optical computers and information technologies,” Kotov says.

“We are at the verge of a whole new field of investigation, combining the amazing 2D materials with chiral information,” says de Moura. “Combined, these properties allow unprecedented control of many properties, especially the interaction with circularly polarized light.”

Indeed, in another example, the research team found that the microparticles with twisted spikes also absorb UV light and emit circularly polarized visible light in response. It appears that UV energy was absorbed into the gold sulfide nanosheets and transformed through quantum mechanical interactions, becoming circularly polarized visible light as it is being transmitted through the curved spikes.

Kotov says, “Successful synthesis of these particles opened multiple new directions for future work because they have unique colloidal, optical, chemical, and topological characteristics. Their unusually high stability in unfriendly solvents due to the presence of stiff spikes opens a possibility to use them in catalysis and biosensing.”

“We envision a number of exciting applications, particularly in the production of materials with tailored interactions with biological systems,” de Moura agrees.

Read the abstract in Science.