Biomolecular detection by surface plasmon enhanced ellipsometry

We present an optical labelfree method which combines the advantages of ellipsometry and surface plasmon resonance (SPR). With this surface plasmon enhanced ellipsometry (SPEE) method we measured a giant ellipsometric effect of Δ cos Δ=0.1/nm of an antibody layer. The adequate thickness of the gold layer used for SPEE turned out to be about 25 nm which is significantly less than commonly used for SPR. As typical for SPR we also found a spectral shift of the ellipsometric tan ψ and cos Δ curves to larger wavelengths with increasing refractive index near the gold surface. Restricting to monochromatic light the maximum ellipsometric sensitivity is obtained near to the wavelength where cos Δ changes from negative to positive values. The method yields a sensitivity sufficient to detect changes in the effective thickness of biomolecular layers of less than 10 pm. In comparison to conventional SPR the simultaneously measurement of cos Δ and tan ψ yields more information which is useful for quantitative analysis based on fitting theoretical solutions of Fresnel’s equations to experimental results. In addition, SPEE is less sensitive than traditional SPR to extern stray light and intensity fluctuations of the incident light.