Researchers trick nanoparticles into reconfiguring themselves

An approach combining an electron microscope, a small sample holder with microscopic channels and computer simulations now makes it possible to see how nanoscale building blocks can rearrange themselves into different, ordered structures, according to a new study by researchers at the University of Michigan and Indiana University.

This approach could eventually make it possible to create smart materials and coatings capable of switching between different optical, mechanical and electronic properties.

“One of my favorite examples of this phenomenon in nature is chameleons,” said Tobias Dwyer, a U-M chemical engineering doctoral student and co-first author of the study published in Nature Chemical Engineering.

“Chameleons change color by changing the spacing between nanocrystals in their skin. The dream is to design a dynamic, multifunctional system that can perform as well as some of the examples we see in biology.”

The imaging technique allows researchers to observe in real time how nanoparticles respond to changes in their environment, providing an unprecedented window into their assembly behavior.

In the study, the Indiana team first suspended nanoparticles, a class of materials smaller than the average bacterial cell, in tiny fluid channels on a microfluidic flow cell. This type of device allowed the researchers to inject different types of fluids into the cell on the fly while observing the mixture under an electron microscope.

The researchers found that the instrument gave the nanoparticles, which are normally attracted to each other, just enough electrostatic repulsion to push them apart and allow them to assemble into orderly arrangements.

The nanoparticles, which are cubes of gold, either neatly align their faces in a well-ordered cluster or form a more disordered arrangement. The final arrangement of the material depends on the properties of the liquid in which the blocks are suspended, and injecting new liquids into the flow cell causes the nanoblocks to switch between the two arrangements.

This experiment demonstrated how to steer nanoparticles toward desired structures. Nanoparticles are too small to manipulate manually, but this approach could help engineers learn to reconfigure other nanoparticles by changing their environment.

“You might have been able to move the particles into new liquids before, but you wouldn’t have been able to observe how they respond to their new environment in real time,” said Xingchen Ye, an associate professor of chemistry at IU who developed the experimental technique and is the study’s corresponding senior author.

“We can use this tool to image many types of objects at the nanoscale, such as chains of molecules, viruses, lipids and composite particles. Pharmaceutical companies could use this technique to understand how viruses interact with cells under different conditions, which could impact drug development.”

According to the researchers, it is not necessary to use an electron microscope to activate the particles in morphable materials. Changes in light and pH could also serve this purpose.

But to extend the technique to other types of nanoparticles, researchers will need to know how to modify liquids and microscope settings to organize the particles. The computer simulations performed by the UM team pave the way for this future work by identifying the forces that caused the particles to interact and assemble.

“We think we now have enough understanding of the full range of physics involved to predict what would happen if we used particles of different shapes or materials,” said Tim Moore, a U-M assistant professor of chemical engineering and co-first author of the study. He designed the computer simulations in collaboration with Dwyer and Sharon Glotzer, chair of the Anthony C. Lembke Department of Chemical Engineering at U-M and corresponding author of the study.

“The combination of experiments and simulations is exciting because we now have a platform to design, predict, fabricate and observe new morphable materials in real time with our partners at IU,” said Glotzer, who is also the John Werner Cahn University Distinguished Professor and the Stuart W. Churchill University Professor of Chemical Engineering.