SERS Activity of Reporter-Particle-Loaded Single Plasmonic Nanovoids

Signal enhancement related to indentations in a gold surface layer during micro-Raman scattering experiments was investigated. The indentations were prepared based on colloidal templating and the voids filled with 4-mercaptobenzoic acid (MBA)-loaded gold nanospheres. The periodic void structure has been designed to allow selective excitation of a single void in such a way that at the laser wavelength of the micro-Raman setup the cavity-type plasmon modes localized at the metallic void interface can be effectively excited. The surface modification of the gold particles by MBA was studied in detail, and the number of MBA molecules present on a single gold nanoparticle inferred from optical and electrophoretic-mobility measurements was found to be ca. 210. Correlative scanning electron microscopy and micro-Raman measurements allowed the investigations at the single void level. The Raman signal from a single MBA-loaded gold nanoparticle in the cavity was already detectable. The number of particles present at a single void site provided a straightforward way to limit the number of molecules excited during the experiments. By measuring the signal strength as a function of particle number trapped inside a single nanovoid and comparing it with a reference sample (clusters of given number of particles on a flat gold surface), a 25-fold experimental signal enhancement attributed to the nanostructured nature of the interface could be inferred.