DNA-mediated colloidal interactions are limited by the steep and monotonic dependence of the interaction strength on temperature, which hinders their use in self-assembly applications and limits the complexity of the systems that can be realized. I show how to modify the dependence on temperature in a controlled way by incorporating DNA strand-displacement reactions. This method allows us to make multicomponent systems that self-assemble over a wide range of temperatures, melt upon cooling, transition between structures with different compositions, or have multiple melting transitions. I create this wide range of behaviors simply by adding a small number of DNA strands to the solution, demonstrating that the approach is modular and straightforward to implement.
These strand-displacement reactions enable systems to dynamically rearrange in time. We demonstrate a system where, by thermal ratcheting, a single particle (the dancer) can be driven to move through a programmed sequence of steps along a one-dimensional track composed of other particles. We lay out the requirements for a system to exhibit controlled motion on the mesoscale, and we demonstrate how these conditions can be realized experimentally. Specifically, we show how the non-monotonic phase behavior enabled by strand-displacement reactions allows us to turn on and off interactions between different pairs of particles, and thereby drive the motion of the dancer. We discuss the capabilities and limitations of using these interactions for applications in dynamic systems.
One such limitation is that a single variable---the temperature of the system---simultaneously controls interactions between several species of particles. We therefore aim to independently modulate interactions between many species. To this end, we present a new approach to dynamically control interactions between DNA-coated colloids using light. We infiltrate particles with dyes so that when we illuminate them with the appropriate wavelength, they heat. As a result, by uniformly illuminating samples with unfocused light, we can reversibly turn on and off the attractive interactions between particles. Although the light is incident on the entire sample, the heating is local to the particles. Therefore, by using multiple dyes, we can independently address the interactions between different sets of particles by using different wavelengths of light. This method of heating produces a short-range temperature gradient that builds up and dissipates on a time scale of milliseconds. Thus, the particles can be heated much more efficiently than by external heating, and the propensity of the entire system to aggregate can be modulated in less than 50 milliseconds. This rapid modulation opens the door to many applications, including non-equilibrium self-assembly.