We describe the realization of a symmetric integrated channel waveguide Mach-Zehnder sensor which uses the evanescent field to detect small refractive-index changes (Δnmin ≈ 1 × 10−4) near the guiding-layer surface. This guiding layer consists of ridge structures with a height of 3 nm and a width of 4 μm made in Si3N4. This layer has a thickness of 100 nm. The sensor device has been tested with glucose solutions of different bulk refractive indices. Results of a slab-model calculation are in good agreement with obtained experimental results. The feasibility of applying this sensor for immunosensing, detecting directly the binding of antigen to an antibody receptor surface, is shown with antibody-antigen binding experiments.