Optimization of Long-Range Surface Plasmon Waveguides for Attenuation-Based Biosensing

The design and optimization of straight long-range surface plasmon waveguides to maximize attenuation surface sensitivity in biochemical sensing applications are discussed. The sensor consists of a Au stripe embedded in CYTOP, with a microfluidic channel etched into the top cladding to expose the surface of the Au stripe and define the sensing channel. The attenuation α_s of the structure changes as a biological adlayer grows on the Au surface. The dimensions of the stripe (thickness, width), the sensing length and the refractive index of the sensing buffer were varied in order to understand their impact on sensor performance. The attenuation sensitivity ∂α_s/∂a dominates over a wide range of waveguide designs, so we define a parameter K = (∂α_s/∂a)/αs where maximizing |K| and selecting the optimal sensing length as L_opt = 1/(2α_s) maximizes the overall sensitivity of the structure. Experimental results based on observing the physisorption of bovine serum albumin (BSA) on bare Au waveguides agree qualitatively and quantitatively with theory. Detection limits of ΔΓ_min <; 0.1 pg·mm^-2 are predicted for optimal designs, and a detection limit of ΔΓ_min = 4.1pg/mm² (SNR = 1) is demonstrated experimentally for a sub-optimal structure.