A waveguide Mach-Zehnder Interferometer (MZI) immunosensor has been developed which can detect, in a direct way, a minimum average layer growth thickness of the analyte of 2×10−3 nm (bound mass, I′=1×10−4 δ cm−2). The design and fabrication of the sensor and the experimental set-up are aimed at decreasing the detection limit. A flow cell has been designed and incorporated into the set-up to allow a high mass-transport rate of the analyte molecules to the sensor surface, in this way decreasing the net response time. Results for the immunosystem α-hSA/hSA are presented. All improvements have been done as a first step towards the use of the MZI immunosensor in the direct detection of low coverages of pesticides with an expected layer growth of 2×10−4 nm.