Assembly of gold nanoparticles directed by light polarization*

When polarized light is incident on a small metal nanoparticle, the light intensity at the particle surface is enhanced in two opposite polar regions defined by the polarization direction. By functionalizing the nanoparticles with photosensitive ligands, this intensity distribution can be transferred into permanent surface properties, forming two opposite patches on the particle. Using a ligand that photocleaves to reveal a positively charged amine group, we assemble smaller (15 nm diameter) gold nanoparticles onto the larger (80 nm diameter) functionalized patchy particle. Since all areas of the 80 nm particle are exposed to light, binding can take place in any orientation, but is strongly preferential to the direction set by the polarization of the exposing light, resulting in arrays of oriented nanoparticle assemblies. Interestingly, we also see increased particle binding around the equator of the particle, where light exposure was at a minimum. Possibly, this is due to electrostatic bias in the random walk toward the polar regions being absent around the equator, leading to a relative accumulation of the 15 nm particles in this region.