Programmable and Reconfigurable Self-assembly of Light-responsive Microparticles

We study the self-organization of microscopic colloidal particles into programmable and reconfigurable shapes. In recent years, self-assembly has received attention as a promising bottomup method of designing materials with tunable properties. Self-assembly that is responsive to external stimuli, such as light, can give rise to the formation of specific patterns that can be reconfigured on demand. We employ a binary mixture of gold nanoparticles of two different sizes – 100 nm and 500-1000 nm, to drive light-responsive self-assembly. Due to the surface plasmon resonance, the smaller 100 nm gold particles absorb light and generate a temperature gradient, which creates a convection current surrounding the region illuminated by an LED light. The larger 500 nm/1000 nm particles assemble at the illuminated region thanks to this convection current. We examined the self-assembled structures under different projected light patterns (horizontally and vertically elongated rectangles) and observed that the assembled structures conform to the shapes of the patterns. We measured the growth rates of the assembly to determine if the assembly can be self-limited to a programmed size. Additionally, we examined the assembly of larger polystyrene microspheres (2 µm) and observed reconfiguration from one assembled shape to another. The fundamental insights gained from our experiments can be applied to develop selfassembled robots at the micro- and nanoscale.