NYU Scientists Use Light as a Remote Control to Assemble, Sculpt, and Erase Colloidal Crystals

Crystals do not usually cooperate. Whether you are growing salt crystals in a kitchen experiment or engineering photonic structures for a laser, the basic challenge is the same: crystals tend to nucleate where and when they want, and once experimental conditions are set, options for real-time adjustment are limited. A research team at New York University has now demonstrated a way to change that - using light as a tunable switch to dictate exactly when crystals form, where they form, and how large and ordered they become.

The approach, published in the Cell Press journal Chem, exploits light-sensitive molecules called photoacids to mediate the electrical interactions between microscopic particles suspended in liquid. By adjusting light intensity - simply turning illumination up or down - the researchers could trigger crystal formation, melt crystals, sculpt their boundaries, and improve their order. All of this happened in the same container, without modifying the particles themselves or adding new chemicals between experiments.

Why controlling colloidal crystals is difficult

Colloidal particles - microscopic spheres suspended in a liquid - are widely used as model systems for studying crystallization because they can be observed directly under a microscope as they self-assemble. They also have practical applications as building blocks for photonic materials: sensors, lasers, optical coatings, and display technologies that depend on regular arrangements of particles to manipulate light.

The challenge is that particle interactions - whether particles attract or repel each other - are determined by their surface chemistry and the ionic environment of the surrounding solution. Changing those interactions typically requires changing the particles' surface composition, the solution's salt concentration, or the temperature. None of these adjustments is fast, reversible, or spatially targeted in the way that light can be.

"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 and a senior author of the study.

Photoacids as a light-activated charge controller

The solution the team developed uses photoacids - molecules that temporarily become more acidic when illuminated. When added to a colloidal suspension, photoacids alter the proton concentration in the liquid near particle surfaces when light is applied. That change in local acidity modifies the electric charge on the particle surfaces, which in turn controls whether particles attract or repel each other.

The relationship proved exquisitely sensitive. "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 Zurich, who conducted this work at NYU as a postdoctoral researcher.

Because the photoacid effect is temporary - the molecules return to their normal state when the light is turned off - the process is reversible. Particles that crystallized under illumination dispersed when the light was removed. This reversibility makes the system suitable for dynamic materials, where the same material might need to assume different structural configurations at different times.

What the experiments demonstrated

In a series of experiments and computer simulations, the researchers showed four distinct capabilities. They could trigger crystal formation on demand by turning on the light. They could melt existing crystals by removing it. They could target specific locations - shining a focused beam on a cluster of particles caused that cluster to crystallize while particles elsewhere remained dispersed. And they could improve crystal quality over time by cycling illumination, allowing poorly ordered structures to dissolve and reform with better arrangement.

"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," van Kesteren said. "We could also remove specific crystals quite easily by simply unsticking the particles at that spot."

Potential applications and limitations

The photonic applications the team identifies are among the most compelling: materials whose optical properties - color, reflectance, transparency - depend on the crystalline arrangement of colloidal particles, and which could be written, erased, and rewritten by illumination rather than by fabrication. Such systems could form the basis for reconfigurable optical coatings, adaptive sensors, or information storage technologies where the physical structure of the material encodes data.

The current work was conducted with spherical particles in a controlled laboratory solution. Extending the approach to particles of different shapes, to non-aqueous solvents, or to the thin-film configurations needed for actual photonic devices will require additional optimization. The photoacid's effect is also limited by how much acidity change it can produce, which sets a maximum on the charge modification and therefore on the strength of attraction achievable. More powerful photoacids or alternative light-responsive surface chemistries may be needed for applications requiring stronger particle bonding.