The Distance-Dependence of Colloidal Au-Amplified Surface Plasmon Resonance

Au nanoparticle-amplified surface plasmon resonance (PA-SPR), like traditional SPR, is typically used to detect binding events at a thin noble metal film. Here we describe the use of SiO2-overcoated Au films as a substrate for PA-SPR. Changes in SPR angle shift upon Au nanosphere binding were investigated as a function of the SiO2 thickness. In these experiments, a submonolayer of Au nanoparticles was immobilized atop a thin Au film separated by an intervening SiO2 layer. Although SPR is an evanescent wave technique, the observed SPR angle shift does not decrease monotonically with increasing Au nanoparticle−Au surface separation, but rather increases to a maximum at ∼32 nm separation before decreasing. In contrast to the previously proposed electromagnetic coupling between the metal particles and the surface as the main cause for the observed distance-dependent SPR response, we propose and have demonstrated qualitatively that the energy modulating capability of the SiO2 spacing layer is the main contribu...