We propose a novel structure with two input and output silicon waveguide ports separated by the Insulator-Metal- Insulator channel deposited on silicon nitride base. In principle, both the top surface insulator/metal interface and bottom surface can support SPP a decoupled modes. Once the SPP modes excited input silicon waveguide, the SPP signals from the two optical branches (the top and bottom interfaces) propagate to the output silicon waveguide. At the output waveguide both branches interfere with each other and modulate the far-field scattering. The top surface is considered as the sensing arm of this plasmonic Mach-Zehnder interferometer (MZI). The bottom surface is considered as the reference arm of the sensor. High sensitivity and small foot print is achieved using this integrated simple plasmonic design. The combination of sensitive interferometric techniques and the optimization process of the design and the material yields to enhanced sensitivities up to 3000 nm/RIU.