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).
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