A sensor device for measuring the concentration of an analyte in solution is described. The device includes a composite membrane which incorporates a porous membrane containing an immobilized biologically-active protein and at least one other membrane. The other membrane may optionally be a blocking membrane, partly embedded in the porous membrane, which is adapted to exclude low molecular weight interfering species such as ascorbic acid. The other membrane may optionally be a protecting membrane which is useful for preventing high molecular species from fouling the porous membrane. Both a blocking membrane and a protecting membrane may be included in the composite membrane, simultaneously. The composite membrane is attached and reliably sealed to a housing by means of ultrasonic welding.
These features operate synergistically so that the device exhibits relatively fast response and recovery times, is reliably leak-proof, and may be constructed in relatively small sizes. A method of manufacturing the composite membrane is also provided.