Contents I. Introduction
Magnetic particles have been widely used in bioscience and biotechnology. Magnetic techniques using small magnetic particles and magnetic gradient force have been developed for purifying biological materials and enhancing gene delivery [1], [2]. Compared with standard methods, magnetic techniques have the advantage of involving simple processes that can be applied in samples containing suspended solid and fouling components. The measurement of biological targets using nanoparticle magnetic markers also has been extensively studied for its applications in magnetic immunoassays (MIA) [3] and magnetic particle imaging [4], [5]. There are many types of immunoassays used for diagnosing diseases; for example, enzyme-linked immunosorbent assays and radio-immunoassays are currently used as highly sensitive clinical diagnosis tools. Compared with these immunoassay methods, MIA using magnetic labeled markers has great potential since it is simple to use and it has a low interference. Many types of MIAs based on various detection methods and the magnetic properties of the magnetic nanoparticles (MNPs) have been reported [6]–[16]. A magneto-resistive sensor such as a conventional magnetic sensor or a superconducting quantum interference device (SQUID) used as a highly sensitive sensor is normally utilized to detect a magnetic signal from MNPs that are bound to biological targets. Magnetization, magnetic relaxation, ac magnetic susceptibility, and magnetic remanence of MNPs magnetic properties are used for MIAs. There are two trends for the development of MIA. The main trend is an integrated miniature on-chip device for point-of-care diagnosis [17], [18], while the other one is a highly sensitive, multifunctional measuring system for experimental use. A liquid-phase MIA method that uses a high-temperature SQUID as the multifunctional measuring system has previously been deployed to obtain fundamental clinical information [19], [20]. This technique uses polymer-coated nanoparticles as magnetic markers, with an immobilized antigen or antibody on the surface. In a liquid-phase immunoassay reaction, the target proteins, magnetic markers, and polymer beads are observed to react to form a sandwich structure. The sandwich structure has a relatively large volume, which increases the Brownian relaxation time. When a sinusoidal magnetic field is applied to the sample solution, the magnetic signal is attenuated according to the biological-target concentration because of the difference in relaxation times between the free magnetic markers and the bound magnetic markers (Fig. 1). Therefore, the MIA method has the advantage of being a simple process that does not require the separation of the bound and free markers (B/F) in a measurement sample. However, the reaction times of magnetic and optical immunoassays are still the same. Many studies on reducing antibody-antigen reaction times have previously been conducted [21]–[26], and many methods identified such as rotating the capture substrate, mixing fluids under rotating magnetic field, among others. The purpose of this paper is to reduce the reaction time by applying a rotating magnetic field gradient and demonstrate its influence on quantitative measurement of MIA without B/F separation. The effect of changes in the number of antibodies conjugated onto the magnetic markers and polymer beads on the reaction time is also investigated. To verify the performance of the magnetic-shaking treatment, we measured the reaction time of C-reactive proteins (CRPs) that are used as markers for inflammation.
Fundamental response mechanism of signal separation between free and bound magnetic markers.
