Hybrid nanoparticle and thin film SPR biosensor with a high figure of merit

Due to their extreme sensitivity to refractive index changes, surface plasmon resonance (SPR) sensors have long been established as extremely valuable tools for biosensing. In the past few years researchers have begun investigating various other metallic nanostructures as candidates for localized SPR (LSPR) sensing. Although LSPR is not nearly as sensitive to bulk refractive index changes as standard SPR, is has the advantage of being extremely sensitive to local refractive index changes, thereby providing detection on the level of a single molecule. In practice such sensitivity criterion is of paramount importance since the analyte layer under investigation is often only a few nanometers thick and deposited directly on the surface of the metal. Most desirable, however, is a sensor that retains the total integrated sensitivity of a traditional SPR sensor and at the same time localizes this sensitivity right at the sensor surface. For this reason, we have investigated a hybrid structure composed of a 2D Au nanoparticle array coupled to a Au film. We show that this structure, when excited in the Kretschmann configuration, retains to a surprising degree the total integrated sensitivity of an ideal SPR sensor and is able to concentrate that sensitivity within a few nanometers of the sensor surface, thereby yielding a hybrid sensor with the advantages of both LSPR and SPR sensing, i.e. both a high local sensitivity and a high figure of merit (FOM).