Triggering microscale self-assembly using light and heat

Colloidal particles, which range in size from a few nanometers to a few micrometers, are often used to study self-assembly processes. Due to their small size, gravitational forces have minimal influence over their motion. As a result, these particles tend to move randomly while at the same time interacting with each other.

“Despite their small size, these colloidal particles can be imaged using conventional microscopy techniques,” notes Patrick Hage, former PhD researcher and now postdoc in the group Self-Organizing Soft Matter. “Arranging these materials on this length scale can result in materials with novel mechanical and optical properties. A natural example of a colloidal ‘superstructure’ with unique optical properties is an opal, which is composed of crystals of small silica spheres. Control over the superstructures could lead to new materials for photonic crystals, coatings, and sensors.”

Importance of control

To create responsive and reconfigurable colloidal materials, it is very important to have control over the interactions between particles and the ability to modulate these interactions using external prompts.

One way to help modulate the interactions is via surface functionalizing, where small single-DNA strands are attached to the surface of the particles. Just as you would find in the nucleus of a cell in the human body, these DNA strands link to each other to form a DNA helix.

“It’s the formation of these DNA helixes holds the particles together,” says Hage. “Particles with DNA on their surface can be modulated using temperature as a trigger. This controls how the particles interact with each other and leads to complicated structures such as colloidal crystals.”

Multiple triggers

The goal of Hage’s PhD research was to develop a system that responds to multiple triggers – light and temperature in this case. “Using multiple triggers allows for control over the growth of structures over both space and time.”

Hage achieved this by adding a light-responsive molecule to the DNA strands that are responsible for colloidal assembly. This resulted in particle interactions that were responsive to both light and temperature at the same time. Combining these particles with a fluorescent microscope, a heating chamber, and a digital micromirror device allowed for particle visualization while simultaneously giving precise temperature control and the ability to apply light with specific patterns onto the sample.

“I created a setup that allows for the imaging of the formation of superstructures (e.g., crystals) at specific temperatures, while gaining the ability to modify or remove undesired structures by applying local light patterns,” says Hage. “In future processes, this double control could be used to make self-assembled structures for a variety of applications such as advanced sensors or photonic crystals for photonic devices.”

Hage will now continue the work from his PhD as part of a 4-month postdoc position in the same group. “I’m looking forward to working further on optimizing the system further, and then transferring the knowledge to other members of the group.”

Title of PhD-thesis: Synthesis, functionalization, and self-assembly of isotropic and patchy microparticles. Supervisors: Ilja Voets (TU/e), Bert Meijer (TU/e), and Janne-Mieke Meijer (TU/e).