Mini crystals can look messy on the outside, but perfect on the inside

Researchers at Utrecht University have quantitatively mapped the three-dimensional structure of photonic supraparticles for the first time. Supraparticles are microscopic spheres composed of thousands of smaller colloidal particles. Until now, researchers could only examine the outer surface of these structures. Using a combination of super-resolution microscopy and machine learning, the team shows that particles that appear disorganised on the outside are often almost perfectly crystalline on the inside. 

Blue morpho butterflies owe their vibrant color to the internal structure of their wings, rather than pigment. The arrangement of particles on a microscopic scale causes light to be reflected in such a way that the butterflies appear intensely blue, and that the colour looks the same from every viewing angle.

This principle fascinates PhD candidate Jesse Bückmann. How does the arrangement of particles determine the properties of a material? And what happens if that arrangement is changed just slightly?

Mini crystals

Butterflies do not actually have a role in the experiments of Bückmann and his colleagues. Instead, they work with so-called photonic supraparticles: microscopic spheres made up of thousands of smaller silica particles that together form a crystal lattice and, much like the morpho butterfly, produce the same angle-independent colour.

The team produces these mini crystals themselves in one of the laboratories of the Ornstein building at Utrecht University. “You always hope they turned out well,” Bückmann says. “But you only know once you look at them under the microscope.”

Undercover

Until now, supraparticles were often studied using scanning electron microscopy. This technique provides sharp images of the outer surface, but it cannot show what is happening inside the mini crystals, Bückmann explains. "The interesting material properties often arise from the internal structure, so you need different microscopy techniques to see that."

And the interior can still hold surprises. Structures that appear messy under an electron microscope sometimes turn out to be almost perfectly ordered inside. Simulations had already predicted that such ‘undercover’ crystals could exist, but now experiments confirm that this indeed happens.

Nobel Prize technique

To make this possible, the team used a combination of 3D confocal microscopy and STED microscopy, a technique that was awarded the 2014 Nobel Prize in Chemistry because of its greatly improved resolution. In this study, the resolution was improved by roughly a factor of four compared with conventional confocal microscopy. That difference is crucial for the results: the particles studied are smaller than 500 nanometres, which is about the same size as the wavelength of visible light. Only with this increased resolution can individual particles be clearly identified in three dimensions.

Alfons van Blaaderen, professor of Soft Condensed Matter, developed a method to incorporate fluorescent cores into the supraparticles. Using this approach, Bückmann could locate individual particles and determine their exact coordinates. A machine-learning model developed by researchers Alptuğ Ulugöl and Laura Filion was then used to distinguish between different crystal classes, reflecting subtle differences in how the particles are packed together.

From fundamental to applicable

The research is fundamentally oriented, but the method is broadly applicable to other colloidal systems. Photonic supraparticles could, for example, potentially be used as a more durable alternative to paint, where colour remains stable for longer. The team is also working on ways to make the structures more stable, so they can more easily be used for such applications.