Nanorod-mediated surface plasmon resonance sensor based on effective medium theory.

We investigate a nanorod-mediated surface plasmon resonance (SPR) sensor for sensitivity enhancement. The theoretical model containing an anisotropic layer of nanorod is investigated using four-layer Fresnel equations and the effective medium theory. The properties of the nanorod-mediated SPR curves versus the metal thin film thickness d(f), length l, and diameter D of the nanorod are studied in the environment with refractive indices of 1.00 and 1.33. Compared to the conventional thin metal film SPR configuration, the nanorod-mediated SPR sensor presents a larger resonance angle shift and the sensitivity increases with increasing refractive index of the target analyte. Besides the theoretical analysis, we fabricate different Ag nanorod array/Ag film substrates by oblique angle deposition and characterize their SPR responses using a laboratory-made SPR setup in air and in deionized (DI) water. Compared with the Ag film sample, the SPR angles observed for Ag nanorods/Ag film samples shift to larger angles in air (for shorter nanorods), while it is hard to observe the SPR angle in DI water, which is qualitatively consistent with theoretical results. We believe that the nanorod-mediated SPR sensor is able to improve the sensitivity and the theoretical discussion is helpful for sensor fabrication.