Plasmonic properties of polycrystalline hollow Au nanoparticles: a surface roughness effect.

Hollow Au nanoparticles with a 25 nm polycrystalline shell and a 50 nm hollow core were produced in large amounts by using electrochemically evolved hydrogen nanobubbles as templates and reducing agents for electroless deposition from a Na3Au(SO3)2 electrolyte. The surface roughness of these nanoparticles can be tuned by adding NaSO3 into the electrolyte. Different surface roughnesses can be readily obtained for sub-100 nm particles with the same size. As surface roughness increases, surface plasmon resonance (SPR) peaks shift to longer wavelengths. Particles with an 8 nm roughness have a SPR peak centered at 750 nm, which is particularly attractive for in vivo diagnostic and therapeutic applications. A three-dimensional finite difference time domain (FDTD) simulation confirms that the red-shifts of SPR peaks are mainly caused by their surface roughness, and the hollow nature of these particles plays only a minor role. This unique plasmonic property of hollow Au nanoparticles opens up the possibility to maintain the desirable optical properties after loading other substances into the hollow core to form multifunctional core-shell nanoparticles.