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Twisted By Design

Materials Synthesis: Polarized light helps nanoparticles self-assemble into ribbons of controllable chirality

by Matt Davenport
November 20, 2014 | A version of this story appeared in Volume 92, Issue 47

Micrographs of chiral ribbons made from self-assembled nanoparticles.
Credit: Nat. Mater.
Electron microscopy helped researchers render the surfaces of left-handed (left) and right-handed ribbons.

An international research team has coaxed semiconducting nanoparticles to self-assemble into helical ribbons using circularly polarized light (Nat. Mater. 2014, DOI: 10.1038/nmat4125). Such twisted inorganic nanostructures could be useful for chiral catalysis and optical data transmission, the researchers say, and their method’s high yield could make it suitable for mass production.

Scientists have investigated the light-driven assembly of chiral organic molecules in the past, but they usually got a nearly even mixture of left- and right-handed ones. The surplus of molecules with the desired chirality rarely exceeded 1%, says Nicholas A. Kotov of the University of Michigan. He was convinced researchers would fare better using inorganic nanomaterials.

To test that theory, his team started with aqueous suspensions of cadmium telluride nanoparticles. The roughly 5-nm particles are inherently chiral thanks to subtle geometric asymmetries and therefore respond differently to different light polarizations. Left-handed particles, for instance, are more likely to absorb photons with a left-handed circular polarization.

As the particles absorbed photons, they oxidized their thioglycolic acid coatings—coatings that kept the nanostructures dispersed. As more left-handed particles lost their coatings, they aggregated into left-handed helical ribbons up to 3 μm long. When the team used light with a right-handed circular polarization, right-handed ribbons preferentially self-assembled.

Using circularly polarized light, the team grew about 30% more structures whose chirality matched the light’s polarization than it did ribbons of the opposite chirality.

“I have never seen such clear evidence of the influence of circularly polarized light on the formation of helical nanostructures,” says Luis M. Liz-Marzán, a nanomaterials scientist at the Centre for Cooperative Research in Biomaterials (CIC biomaGUNE) in Spain, who was not involved in the study.

Kotov says he’s interested in extending this approach to other materials and seeing how finely researchers can tune their control over chirality to make increasingly complex nanostructures. “All semiconductors and metals have strong photochemical and optical activity,” he says. “There are a lot of possibilities here.”



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