Morphological tunability of the plasmonic response: From hollow gold nanoparticles to gold nanorings

The optical and morphological properties of hollow gold nanoparticles (HGNPs) can be finely modified by employing poly-l-lysine hydrobromide (PLL), a homopoly amino acid of the l-lysine, used as reducer and stabilizer. We demonstrate that the reshaping of these plasmonic hollow nanostructures responds to an Ostwald ripening process in which the initial HGNPs dissolve under the acidic conditions provided by the PLL generating chloroaurate ions. These ions are then reduced by the primary amines of the PLL itself to produce metallic gold, which is then redeposited on the surface of the HGNPs, rendering the final shape: either nanorings or even spheres depending on the contact time. We investigate locally the plasmonic response of these nanostructures by electron energy loss spectroscopy (EELS). The plasmon excitations are interpreted by discrete dipole approximation (DDA) simulations. We demonstrate that, thanks to this controlled top-down morphological modification, a fine-tuning of the optical response is possible. Unlike the traditional lithographic techniques, this has been achieved in a controlled manner using wet chemistry, enabling the potential use of these nanostructures for a broad range of plasmonic applications, including biomedicine, catalysis, and quantum communications.