Ann Arbor Times

Researchers at the University of Michigan and Indiana University have developed a new approach that enables nanoparticles to reconfigure themselves into different organized structures on command. The technique, which combines an electron microscope, a small sample holder with microscopic channels, and computer simulations, was detailed in a study published in Nature Chemical Engineering.

This method could potentially lead to smart materials and coatings capable of switching between various optical, mechanical, and electronic properties. Tobias Dwyer, a doctoral student in chemical engineering at U-M and co-first author of the study, likened this phenomenon to how chameleons change color by altering the spacing between nanocrystals in their skin. "The dream is to design a dynamic and multifunctional system that can be as good as some of the examples that we see in biology," Dwyer said.

The imaging technique allows researchers to observe how nanoparticles react to environmental changes in real time. The Indiana team first suspended nanoparticles—materials smaller than bacteria cells—in tiny liquid channels on a microfluidic flow cell. This setup enabled them to introduce different fluids while viewing the mixture under an electron microscope. They found that electrostatic repulsion provided by the instrument allowed the nanoparticles to assemble into ordered arrangements.

Gold nanocubes either aligned perfectly or formed more disordered clusters based on the properties of the suspending liquid. Flushing new liquids into the flow cell caused these nanoblocks to switch between different arrangements. This experiment demonstrated how changing environments could steer nanoparticles into desired structures.

"You might have been able to move the particles into new liquids before, but you wouldn’t have been able to watch how they respond to their new environment in real-time," said Xingchen Ye, IU associate professor of chemistry and lead corresponding author of the study.

Ye also noted potential applications for pharmaceutical companies: "We can use this tool to image many types of nanoscale objects... Pharmaceutical companies could use this technique to learn how viruses interact with cells in different conditions."

Although an electron microscope isn't necessary for activating particles in practical morphable materials—changes in light and pH could suffice—the researchers need further understanding of adjusting liquids and microscope settings for other nanoparticle types. Computer simulations run by U-M researchers identified forces causing particle interactions and assembly.

Tim Moore, U-M assistant research scientist of chemical engineering and co-first author who designed these simulations alongside Dwyer and Sharon Glotzer, expressed optimism about future work: "We think we now have a good enough understanding of all the physics at play."

Sharon Glotzer added: "The combination of experiments and simulations is exciting because we now have a platform to design, predict, make and observe in real time new morphable materials together with our IU partners."

The research received funding from the National Science Foundation.