Optimizing the Refractive Index Sensitivity of Plasmonically Coupled Gold Nanoparticles

The possibility to enhance the local refractive index sensitivity using plasmonic coupling between spherical gold nanoparticles (Au-NPs) has been investigated. A strong and distinct optical coupling between Au-NPs of various sizes was achieved by controlling the interparticle separation using a layer-by-layer assembly of polyelectrolytes. The frequency of the coupled plasmon peak could be tuned by varying either the particle size or the interparticle separation, shown both experimentally and by theoretical simulations. The bulk refractive index (RI) sensitivity for the plasmonic coupling modes was investigated and compared to the RI sensitivity of monolayers of well-separated Au-NPs, and the results clearly demonstrates that the RI sensitivity can be significantly enhanced in plasmonically coupled Au-NPs. The proposed approach is simple and scalable and improves the rather modest RI sensitivity of spherical gold nanoparticles with a factor of 3, providing a new route for fabrication of inexpensive sensors based on plasmonic nanostructures.