Development of an AC Magnetic Characterization System and Its Application to Magnetic Immunoassay

Magnetic nanoparticles (MNPs) are nanometer-sized magnetic particles that are expected to have various medical applications. To achieve highly sensitive testing, it is necessary to evaluate the AC magnetic properties of MNPs under various conditions. Commercially available systems use SQUID sensors and have excellent detection sensitivity. However, they have problems such as the need for expensive refrigerants and the large equipment to maintain the superconducting state. Therefore, in this research, we aimed to develop a low-cost and compact AC magnetic characterization system consisting of two lock-in amplifier circuits. Additionally, we conducted an experiment to detect biotin-avidin binding reaction, which is the target of conventional optical methods, and investigated the effect of immunoreaction on the AC magnetic properties of MNPs. As a result of the experiment, we were able to measure the AC magnetic properties of MNPs in the liquid phase using the developed system. Furthermore, in immunoreaction experiments, it was confirmed that the signal intensity of MNPs decreased as the biotin concentration increased. This is thought to be due to the antigen-antibody reaction between biotin and the streptavidin antibody modified on the MNPs surface, which caused the MNPs to aggregate and change the AC magnetic properties. These results suggest that the developed system is applicable to quantitative liquid-phase immunoassays.