Researchers at New York University have developed a method to direct the assembly of microscopic particles into crystals using light. This technique, detailed in the journal Chem, allows for real-time control over crystal growth and dissolution. The approach could enable new responsive materials for applications in optics and photonics.
Crystals form the basis of many natural and technological structures, from snowflakes to silicon in electronics. However, precisely controlling their formation has been challenging, as particles typically assemble on their own terms.
A team led by Stefano Sacanna, professor of chemistry at NYU, addressed this by introducing light-sensitive molecules called photoacids into a liquid suspension of colloidal particles. These tiny spheres mimic atomic arrangements in crystals and are used in sensors and lasers. When light hits the photoacids, they become more acidic, altering the particles' surface charges and thus their attraction or repulsion.
"Essentially, we used light as a remote control to program how matter organizes itself at the microscale," Sacanna said.
Experiments and simulations showed that varying light intensity, duration, or pattern enables precise manipulation. Researchers could trigger crystal formation, melt existing ones, reshape structures, or create uniform larger assemblies. Steven van Kesteren, a former postdoctoral researcher in Sacanna's lab now at ETH Zürich, noted: "Just turning the light up or down a little made the difference between the particle fully sticking or being fully free."
The method operates in a single "one-pot" setup, reversibly assembling and disassembling particles without altering other conditions. This simplicity stems from light's ease of control, allowing complex behaviors like selectively dissolving specific crystal regions.
The work, supported by the US Army Research Office, Swiss National Science Foundation, and NYU's Simons Center, points to light-programmable materials. Glen Hocky, associate professor of chemistry at NYU, said: "Our approach brings us closer to dynamic, programmable colloidal materials that can be reconfigured on demand."
Additional authors include Nicole Smina, Shihao Zang, and Cheuk Wai Leung, all from NYU.
