Scientists at New York University have developed a method that uses light to guide how microparticles are arranged in crystals. The study was reported in the journal Cell Press chemistrydescribes a simple and reversible technique for constructing crystals that can support the creation of new classes of responsive and adaptable materials.
Crystals are found everywhere in nature and technology, from snowflakes and diamonds to the silicon in electronic devices. The core of the crystal is made up of particles organized in a precise repeating pattern. To better understand how these structures emerge, researchers often study colloidal particles. Colloidal particles are small spheres suspended in a liquid that spontaneously aggregate to form regular arrays known as colloidal crystals. These particles also serve as key components in advanced materials used in optical and photonic applications such as sensors and lasers.
Although crystals are common and extremely useful, controlling precisely when and how they form remains a major obstacle.
“The challenge in this field has been control. Crystals typically form whenever and wherever they want, but once the conditions are set, there is limited ability to adjust the process in real time,” said study author Stefano Sacanna, professor of chemistry at New York University.
Controlling particle interactions using photoacids
In their chemistry work, the researchers identified a surprisingly simple way to direct crystal formation: by shining light on the system.
The researchers introduced a photosensitive molecule known as a photoacid into a liquid containing colloidal particles. When exposed to light, these photoacids temporarily become more acidic. That change affects the particle’s interaction with its surface, changing its charge. By changing the charge, scientists can control whether the particles are attracted and stuck together or pushed apart and separated.
“Essentially, we used light as a remote control to program how matter is organized at the microscopic scale,” Sakanna said.
Real-time crystal growth and melting
Using a combination of experiments and computer simulations, the researchers demonstrated that by adjusting the brightness, duration, and pattern of the light, the behavior of the crystals could be controlled with surprising precision. They can start growing crystals or dissolve crystals at any time. We can pinpoint where crystallization occurs and reshape and “sculpt” the structure, enhancing its uniformity and size to create larger, more complex colloidal assemblies.
“Using photoacid, we were able to control the attraction between the particles to an amazing degree. Just a little bit of raising or lowering the light made the difference between the particles being completely attached or completely free,” said study author Stephen van Kesteren of ETH Zurich, who conducted the work at New York University as a postdoctoral fellow in the Sakanna lab.
“Light can be controlled very easily, so we can make the system do very complex things. We can shine light on a clump of particles and watch them melt under a microscope, or we can shine light and make random clumps of particles align into crystals. We can also very easily remove specific crystals by just peeling the particles in place,” van Kesteren added.
One-pot setup with reversible assembly
A notable advantage of this approach is that it functions as a “one-pot” experiment. The team did not have to redesign the particles or repeatedly adjust the salt concentration in separate tests. Simply changing the level of illumination can cause particles to clump together into crystals or break apart again.
Aiming for optically programmable materials
This advance points toward materials whose internal structure, and thus their properties, can be tuned using light. For example, photonic materials can have their color or optical response written, erased, and rewritten on demand. Optically programmable colloidal crystals could ultimately enable reconfigurable optical coatings, adaptive sensors, and next-generation display and data storage technologies, where patterns and features are dynamically defined by illumination rather than being fixed during manufacturing.
“Our approach brings us closer to dynamic, programmable colloidal materials that can be reconfigured on demand,” said study author Glenn Hockey, associate professor of chemistry and faculty member at New York University’s Simons Center for Computational Physical Chemistry. “This system also allows us to test many predictions about how self-assembly behaves when the interactions between particles or molecules vary across space and time.”
Other study authors include Nicole Smina, Shihao Zang and Cheuk Wai Leung of New York University. This research was supported by the U.S. Army Research Office (grant W911NF-21-1-0011), the Swiss National Science Foundation (grant 217966), and the New York University Simmons Center for Computational Physical Chemistry (grant 839534).
