Magnetic immunoassays utilizing magnetic markers and a high-Tc SQUID

Magnetic immunoassays utilizing magnetic markers and a high-T_c SQUID have been performed. We first showed a design of the SQUID for the sensitive detection of the magnetic signal from the marker, where the spatial distribution of the signal field was taken into account. Using the design, we can obtain the relationship between the magnetization of the maker and the signal flux detected with the SQUID. This relationship is important for quantitative evaluation of the immunoassay. Next, we detected biological materials called IgE with a remanence measurement method, where a marker made of Fe₃O₄ particles with diameter of 25 nm was used. We showed that 2 atto-mol of IgE can be detected, which shows high sensitivity of the present method.