NYU researchers have developed a method to use light to control how microscopic particles assemble into crystals, according to a study published in the journal Chem. This technique offers a simple and reversible way to form crystals, which could lead to new adaptable materials.
Crystals are structures where particles arrange themselves in repeating patterns. They are found in natural and synthetic materials, including snowflakes, diamonds, and silicon used in electronics. Scientists often study crystal formation using colloidal particles—tiny spheres suspended in liquid that can self-assemble into larger structures. These colloidal crystals are important for developing advanced optical and photonic technologies such as sensors and lasers.
Controlling when and where these crystals form has been challenging. “The challenge in the field has been control: crystals usually form where and when they want, and once conditions are set, you have limited ability to adjust the process in real time,” said Stefano Sacanna, professor of chemistry at NYU.
The team’s approach involves adding light-sensitive molecules called photoacids to a solution containing colloidal particles. When exposed to light, these molecules become temporarily more acidic, affecting their interaction with particle surfaces. This change alters the electric charge on the particles, determining whether they attract or repel each other.
“Essentially, we used light as a remote control to program how matter organizes itself at the microscale,” Sacanna explained.
Through experiments and simulations, the researchers demonstrated that by changing light intensity, timing, and spatial patterns, they could cause crystals to form or melt on demand. They were able to direct where crystallization occurred and modify existing crystal shapes under controlled conditions.
“Using our photoacid gave us a surprising level of control over the attraction between particles. Just turning the light up or down a little made the difference between the particle fully sticking or being fully free,” said Steven van Kesteren of ETH Zürich, who conducted this research at NYU as a postdoctoral researcher in Sacanna’s lab.
Because controlling light is straightforward, researchers observed various effects under microscopes: melting clusters of particles with focused laser spots or causing random groups of particles to organize into ordered crystals with broad illumination. “Because light is so easy to control, we could make our system do quite complex things. We could shoot light at particle blobs and see them melt under the microscope, or shine a light so that random blobs of particles ordered themselves into crystals. We could also remove specific crystals quite easily by simply unsticking the particles at that spot,” added van Kesteren.
This method allows manipulation within one experiment without needing multiple redesigned solutions or varying salt concentrations; simply adjusting lighting was enough for assembly or disassembly of structures.
Potential applications include creating materials whose structure—and thus properties—can be adjusted using light. For example, photonic materials could have colors or optical responses written and erased dynamically instead of during manufacturing. Such systems may be useful for reconfigurable coatings, adaptive sensors, next-generation displays or information storage technologies defined by illumination rather than fixed design.
“Our approach brings us closer to dynamic, programmable colloidal materials that can be reconfigured on demand,” said Glen Hocky, associate professor of chemistry at NYU’s Simons Center for Computational Physical Chemistry. “This system also allows us to test a number of predictions on how self-assembly should behave when interactions between particles or molecules are changing across space or time.”
Other contributors included Nicole Smina, Shihao Zang and Cheuk Wai Leung from NYU. The project received support from agencies including the US Army Research Office (award W911NF-21-1-0011), Swiss National Science Foundation (grant 217966), and NYU Simons Center for Computational Physical Chemistry (grant 839534).
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