An optochemical sensor unit including: an optical waveguide; a transmitting unit for emitting a first transmission signal for exciting a luminophore; a receiving unit for receiving a received signal comprising a signal component emitted by the excited luminophore; a measuring chamber for receiving a fluid, wherein the fluid includes magnetic microspheres; a membrane arranged between the measuring chamber and a measuring medium for exchanging an analyte between the measuring medium and the fluid in the measuring chamber, wherein the measuring diaphragm is impermeable to the magnetic microspheres; and an electromagnet for attracting magnetic microspheres to a sensor membrane with a fluid-contacting surface and/or to a fluid-contacting surface of the optical waveguide, or to a surface of a transparent substrate layer of the optical sensor unit that is connected to the optical waveguide.