Scientists at NYU have developed a way to use light to guide how microscopic particles arrange themselves into crystals. The work, reported in the Cell Press journal Chem, describes a straightforward and reversible technique for building crystals that could support the creation of a new class of responsive, adaptable materials.
Crystals appear everywhere in nature and technology, from snowflakes and diamonds to the silicon inside electronic devices. At their core, crystals consist of particles organized in precise, repeating patterns. To better understand how these structures emerge, researchers often study colloidal particles, which are tiny spheres suspended in liquid that naturally assemble into ordered arrangements 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 highly useful, controlling exactly how and when they form has remained a major obstacle.
“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 study author Stefano Sacanna, professor of chemistry at NYU.
Using Photoacids to Control Particle Interactions
In their Chem study, the team identified a surprisingly simple method for directing crystal formation: shining light on the system.
The researchers introduced light sensitive molecules known as photoacids into a liquid containing colloidal particles. When exposed to light, these photoacids briefly become more acidic. That change affects how they interact with the surfaces of the particles, altering the particles’ electric charge. By modifying the charge, the scientists can control whether the particles pull together and stick or push apart and separate.
“Essentially, we used light as a remote control to program how matter organizes itself at the microscale,” said Sacanna.
Real Time Crystal Growth and Melting
Through a combination of experiments and computer simulations, the researchers demonstrated that adjusting the brightness, duration, and pattern of light allows them to direct crystal behavior with remarkable precision. They can initiate crystal growth or cause crystals to dissolve whenever they choose. They can determine where crystallization occurs, reshape and “sculpt” the structures, and enhance their uniformity and size to create larger and more intricate colloidal assemblies.
“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 study author Steven van Kesteren of ETH Zürich, who conducted this work at NYU as a postdoctoral researcher in Sacanna’s lab.
“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.
One Pot Setup With Reversible Assembly
A notable advantage of this approach is that it works as a “one pot” experiment. The team did not need to redesign particles or repeatedly adjust salt concentrations in separate trials. By simply changing the level of illumination, they could prompt particles to assemble into crystals or break apart again.
Toward Light Programmable Materials
This advance points toward materials whose internal structure, and therefore their properties, can be tuned using light. For example, photonic materials could have their color or optical response written, erased, and rewritten on demand. Light programmable colloidal crystals may eventually enable reconfigurable optical coatings, adaptive sensors, and next generation display and data storage technologies, where patterns and functions are defined dynamically by illumination rather than fixed during manufacturing.
“Our approach brings us closer to dynamic, programmable colloidal materials that can be reconfigured on demand,” said study author Glen Hocky, associate professor of chemistry and a faculty member at the Simons Center for Computational Physical Chemistry at NYU. “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.”
Additional study authors include Nicole Smina, Shihao Zang, and Cheuk Wai Leung of NYU. The research was supported by the US Army Research Office (award W911NF-21-1-0011), the Swiss National Science Foundation (grant 217966), and the NYU Simons Center for Computational Physical Chemistry (grant 839534).