How to avoid a negative shift in reflection-type surface plasmon resonance biosensors with metallic nanostructures.

We experimentally demonstrate that introduction of a dielectric film can prevent the surface plasmon resonance (SPR) curve from being shifted to a smaller angle, called negative shift, which occurs unpredictably when metallic nanostructures deposited on a metal film are exposed to an adsorption of binding analytes. From parylene coating experiments, we find that the proposed reflection-type SPR system with a low refractive index MgF2 film and gold nanorods can provide an enhanced sensitivity by more than 6 times as well as a reliable positive shift. It is due to the fact that use of a dielectric film can contribute to the compensation of an anomalous dispersion relation and the prevention of a destructive interaction of propagating surface plasmons with multiple localized plasmon modes. Our approach is intended to show the feasibility and extend the applicability of the proposed SPR system to diverse biomolecular reactions.