Performance of a highly sensitive optical waveguide Mach-Zehnder interferometer immunosensor

We describe a highly sensitive sensor which uses the evanescent field of a reusable planar optical waveguide as the sensing element. The waveguide used is optimized to obtain a steep dependence of the propagation velocity on the refractive-index profile near the surface. The adsorption of a layer of proteins thus results in a phase change, which is measured interferometrically using a Mach-Zehnder interferometer set-up. The stability of the interferometer is such that phase changes z( 1 x lo-*)Zn per hour can be measured. Immunoreactions have been monitored down to concentrations of 5 x IO-” M of a 40 kDa protein.