Janus spheres have two-faced function

U. ILLINOIS (US) — When placed in saltwater, tiny latex colloids called Janus spheres exhibit a water repellant effect that allows for self-assembly into useful structures.

Named for the dual-natured Roman god, Janus sphere surfaces are hydrophobic (water repellent) on one hemisphere and hydrophilic (water attracting) on the other.

Work involving Janus spheres has progressed to the point where Steve Granick, professor of chemistry, chemical and biomolecular engineering, physics, and materials, science, and engineering at the University of Illinois recently used them to create helical supermolecules that self-assemble into supramolecules with intriguing functionalities and also reported on the development of triblock Janus spheres for creating smart materials.

Janus spheres are simple colloidal balls, (particles that are less than a few micrometers but larger than molecules) that in a saltwater solution, form supermolecules that have the functionality of complex colloidal structures, such as those found in man-made materials like soap or in nature in the biological phospholipids that help make up cellular tissue.

In saltwater the Janus spheres attract each other on their hydrophobic halves and begin the process of self-assembly into larger structures.

Granick and colleagues study the spheres for what they can reveal about molecular interactions, for their experimental use, and for their potential applications, such as in bottom-up manufacturing methods, or for use in items such as non-allergenic soap, filters, or smart materials like targeted drug delivery systems.

“We can now make a whole new class of smart materials, which opens the door to new functionality that we couldn’t imagine before,” Granick says.

In an earlier paper, Granick and co-authors Shan Jiang and Qian Chen compared soap molecules and phosopholipid molecules to Janus’ dual personality. “Soap molecules and phospholipid molecules possess the awkward ambivalence symbolized by Janus,” they wrote, comparing those characteristics to longer range magnetic interactions, adding they are taking advantage of the properties of the Janus spheres: “With particles whose interactions are softer—shorter in range than the particle size—a whole new spectrum of behavior may be observed.”

These interactions occur when the repelling effect is softened by the ions in salt water and their orientation changes, permitting the hydrophobic ends of the spheres to attract one another; allowing the spheres to form clusters and, with higher salt concentrations, kinetically self-assemble into chains with helical structures and then into larger clusters called supramolecules.

The group reported on the process this year in Science in an article titled Supracolloidal Reaction Kinetics of Janus Spheres. In it, they compared Janus spheres to atoms or molecules in their ability to serve as self-assembling building blocks for forming clusters that are an intermediate level of matter between particles and bulk phase materials.

Another paper this year in Nature reported on the group’s development of triblock Janus spheres that can form intricate lattice structures that could potentially be used as filters. The triblock Janus spheres have three stripes of reactivity, with a charged center band and hydrophobic poles that, in saltwater, are drawn together to form a lattice.

When the polar caps of the triblock Janus spheres come into contact with two other spheres, they can self-assemble into a lacey six-pointed star with simultaneous hydrophobic and hydrophilic properties, making it ideal for use as a filter for both water-soluble and oil-soluble materials.

“This is a big step forward in showing how to make non-trivial, non-obvious structures from a very simple thing,” says Granick. “People know a lot about how to do it with molecules—soaps for example—but scientists and engineers know very little about how to make it happen with particles.”

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