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 SiO ₂-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 SiO ₂ thickness. In these experiments, a submonolayer of Au nanoparticles was immobilized atop a thin Au film separated by an intervening SiO ₂ 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 SiO ₂ spacing layer is the main contributor. The impact of the particle size, composition, and surface densities on this maximum in SPR angle shifts was also examined. The results indicate that the optimal SiO ₂ thickness was independent of Au nanoparticle surface coverage for coverage less than 10%. This work extends the use of PA-SPR to monitoring binding events at glass interfaces and points to a route for increased sensitivity, as evidenced in an PA-SPR assay for human immunoglobin G in a sandwich immunoassay format.