Application of high T/sub c/ SQUID magnetometer to biological immunoassays

A high T/sub c/ SQUID system is developed for the application to biological immunoassay. In this application, magnetic nanoparticles are used as magnetic markers to perform immunoassay, i.e., to detect binding reaction between an antigen and its antibody. Design and set up of the system is described. Minimum detectable amplitude of the magnetic flux is 0.6 m/spl Phi//sub 0/ for the measurement bandwidth from 0.2 Hz to 5 Hz when we use a magnetometer. The system noise does not increase when the magnetic field of 0.8 mT is applied in parallel to the SQUID. An experiment to measure the antigen-antibody reaction shows that the sensitivity of the present system is 10 times better than that of the conventional method using an optical marker. When a gradiometer is used, the system noise decreased by a factor of 5, compared to the case of the magnetometer. This improvement indicates the usefulness of the gradiometer to suppress the residual environmental noise in the present system. Magnetic markers that have remanent magnetic moment are also studied in order to increase the signal.