Magnetic immunoassay with SQUID and magnetic marker

In this study, a magnetic immunoassay utilizing SQUID and magnetic marker is developed to detect antigen IgE (or IL8). The antigen is detected by using its antibody that is labeled with a magnetic marker made of Fe₃O₄ particle with nominal diameter of 25 nm. The particle is coated with polymer, and the antibody is immobilized on the surface of the polymer. Since the marker can keep remanence μ_oM_r = 49 mT after magnetization, we use the remanent field for the detection of the antigen-antibody binding. A high T_c directly-coupled SQUID is used to detect the magnetic field from the marker. Since the magnetic field from the marker rapidly decreases with distance, it is important to realize close distance between the cooled SQUID and the room temperature sample. The results show the relationship between the signal flux Φ_s and the molecular weight of IgE (or IL8) is shown. The horizontal and vertical axes represent the molecular weight and the signal flux Φ_s, respectively. As shown, good relationship between Φ_s and the molecular weight was obtained. We can detect as small as 2 att-mol of IgE. This result demonstrates the high sensitivity of the present method. Here, it should be noted that the signal flux Φ_s = 1 mΦ₀ roughly corresponds to the magnetic field of 10 pT. We also note that the improvement of the sensitivity of the system is possible since the SQUID can detect the signal flux much less than 1 mΦ₀.