Properties of magnetic nanoparticles for magnetic immunoassays utilizing a superconducting quantum interference device

Properties of magnetic nanoparticles are studied, for application to magnetic immunoassays utilizing a superconducting quantum interference device (SQUID). In this application, a magnetic marker is made of magnetic nanoparticles, and the binding reaction between an antigen and its antibody is detected by measuring the magnetic field from the marker. Magnetization of an assembly of the particles is simulated when the effects of the thermal noise and the distribution of the particle size are taken into account. Magnetic signals from the assembly of the particles are clarified for three detection methods, i.e., susceptibility, relaxation and remanence. From the comparison with experimental results, it was shown that the demagnetization effect also significantly influences the magnetic signal from the particles. When these effects are taken into account, the simulation results agree semi-quantitatively with the experimental ones.