Biological Immunoassay with a SQUID Magnetometer

A SQUID system for application to the biological immunoassay process is shown. In this system, the biological binding-reaction between an antigen and its antibody is detected using a magnetic marker and a SQUID magnetometer; that is, the binding reaction is detected by measuring the magnetic field from the marker. A so-called SQUID microscope was used in order to achieve a close distance between the cooled SQUID and the room-temperature sample. Three methods have so far been developed for measurement: susceptibility, relaxation and remanence. The measurement method is chosen by the properties of the magnetic marker. It is pointed out that a marker that is optimized for the immunoassay should be developed. For this purpose, we have developed a new marker made of an Fe3O4 particle having a diameter of 25 nm. Since the new marker can keep a remanence after a field of 0.1 T is applied, we use the remanent field of the marker to detect the binding reaction. We conducted an experiment to detect an antigen called Interleukin 8 (IL8). It was shown that the present system can detect IL8 at a weight of 0.1 pg.