We report experimental and theoretical results on the effect of electromagnetic coupling between metal particles in surfaceenhanced Raman scattering (SERS). Model calculations of the near-field optical properties of Ag and Au nanoparticleaggregates show that the electromagnetic surface-enhancement factor can reach 11 orders-of-magnitude in gaps between nearly touching particles. Single particles exhibit a much weaker enhancement, unless the particles contain extremely sharp surface protrusions. Data on spectral fluctuations in single-molecule SERS and measurements on the efficiency of nanofabricated SERS substrates give experimental support for the idea that an efficient interparticle coupling is a necessary requirement for an ultra-high surface-enhancement. We suggest a route for biorecognition induced coupling of metal particles for use in biosensing applications.
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