Modular plasmonic antennas built of ultrathin silica-shell silver-core nanoparticles

Dimers of metallic nanoparticles can serve as antennas to locally enhance optical fields through plasmonic excitations. Such dimeric structures can be particularly useful for sensing applications using surface-enhanced Raman scattering (SERS). It has been challenging to devise a synthetic scheme that facilitates creating dimer antennas from different types of particles, at the same time allowing control over the size of the gap between the particles and enabling the introduction of any molecule into the gap. We describe here a method that answers to this challenge. We first introduce a recipe for the creation of a silica shell as thin as 1 nm on silver particles. Analyte molecules are attached to the silica shell, and finally, the silica-shell silver-core particles, whose surface is negatively charged, are mixed with positively charged bare silver particles to create dimers. A demonstration of SERS from individual dimers with gaps of 1.4 and 3.7 nm paves the way to systematic studies of the effect of gap size and composition on plasmonic enhancement.