Long-Range Refractive Index Sensing Using Plasmonic Nanostructures

We study the variation in localized surface plasmon resonance (LSPR) spectra (LSPR600 nm) as a function of dielectric coating thickness. Langmuir−Blodgett multilayers composed of 22-tricosenoic acid (n1.53) were deposited onto short-range-ordered Au nanodisks and nanoholes on glass in air. For large dielectric coating thicknesses (d100−340 nm), the LSPRs exhibit a pronounced oscillatory behavior with a periodicity of ∼190 nm. This is in agreement with a simple image-dipole model, which yields a periodicity of ∼LSPR/2n. However, the amplitude of the dipolar plasmon resonance wavelength oscillation LSPR(d) is surprisingly large, of the order 25−45 nm for d300 nm, indicating the importance of finite size effects. The large LSPR shifts observed at such a large distance from the actual metal surface suggest the possibility of using comparatively thick dielectric films as spacer layers in bio/chemo LSPR sensor applications.