Abstract: An improved biosensor is disclosed comprising an immobilised membrane adhering to a pH sensitive surface of an ion-sensitive field effect transistor by a polysiloxane matrix and comprising an analyte detection agent for detecting and/or quantifying a target analyte. The improvement resides in the immobilised membrane having a thickness of less than about 100 nm which, when compared to conventional immunochemical membranes having a thickness of between 500 nm and 2.0 &mgr;m, has reduced propensity for antibody aggregation with improved antibody affinity and sensitivity of the sensor.