The invention relates to microelectronic device (100) and a method that allow the controlled release of a reference substance (21, 22, 23) into the sample chamber (5) of a biosensor. In a particular embodiment, this is achieved by a supply unit (20) comprising a control wire (24) that can generate a magnetic field in a storage region of the sample chamber (5). Magnetic reference particles (22) can then enclose and retain reference target molecules (21) until the magnetic field is reduced or switched off, releasing the target molecules at a desired point in time. The controlled release of a reference substance (21, 22, 23) may for instance be used for the calibration of a magnetic biosensor.