Biosensor using magnetically-detected label

This method and apparatus for detecting target molecules in a liquid phase. The apparatus monitors whether the target molecule has selectively bound to recognition agents on the surface of a magnetic field sensor by monitoring the output of the sensor. The recognition agents which selectively bind target molecules are covalently bound to microfabricated magnetic field sensors. These sensors are then exposed to a sample suspected of containing the target molecules, whereupon the recognition agents bind to and immobilize any target molecules present. Depending on the embodiment, recognition agents that selectively bind the target molecule, or recognition agents that selectively bind the sensor-bound recognition agents, are covalently bound to magnetizable particles. These particles are then added to the sensors and, again depending on the embodiment, attach either to any immobilized target molecules or to sensor-bound recognition agents. Unattached particles are removed, and the magnetic particles are then magnetized. A change in the output of the magnetic field sensors indicates the presence of magnetic particles bound to the sensors, and thereby indicates the presence and concentration of target molecule in the sample.