Programmable Self-limiting Growth of Self-assembled Microparticles

We study the self-organization of microscopic colloidal particles into programmable shapes. In recent years, self-assembly has received attention as a promising bottom-up method of designing materials with programmable 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 and compared the growth rates of the gold particles (500 nm/1000 nm) and the polystyrene microspheres (2 µm), discovering that the assembly can be self-limited to a programmed size. The fundamental insights gained from our experiments can be applied to develop self-assembled robots at the micro- and nanoscale.