Hisao Kanzaki

Size Distribution of Magnetic Marker Estimated from AC Susceptibility in Solution for Biosensor Application

The ac susceptibility of magnetic markers in solution was studied for biosensor application, where the marker consisted of magnetic nanoparticles and a coating material. From the frequency dependence of the susceptibility caused by the Brownian rotation of the marker, we estimated the distribution of marker size, which is an important parameter for biosensor application. For this purpose, we analyzed the experimental data by the singular value decomposition (SVD) method. Using this method, we can directly estimate the size distribution without assuming any distribution function. The estimated distributions were also compared with those obtained from optical dynamic light scattering (DLS) measurements. It was shown that the size distribution estimated by magnetic measurement (SVD) slightly shifts to a size lower than that estimated by the optical measurement (DLS). It was also shown that the frequency dependence of the susceptibility can be better explained by the size distribution estimated by the SVD method than by the distribution estimated with DLS. The difference between the magnetic and optical measurement results was discussed in terms of aggregation of the markers.

Fast Detection of Biological Targets With Magnetic Marker and SQUID

We have been developing a SQUID system for the detection of biological targets. In this system, magnetic markers are bound to the targets, and the magnetic signal from the bound markers is detected with the SQUID. In order to realize fast detection of the targets, we developed a liquid-phase detection method. First, we used large polymer beads as material to capture the targets. Since the polymer beads are uniformly dispersed in liquid, biological targets on the surface of the polymer bead can be easily coupled to the markers, which results in the fast reaction time. Next, we detected the bound markers without using the washing process to separate the bound and unbound markers, which was realized by using the difference in the Brownian relaxation time between them. Using this procedure, we demonstrated the detection of the target called IgE, as well as biotin-coated polymer beads. We obtained a good relationship between the amount of IgE and the magnetic signal. The result was the same as that obtained using the conventional procedure. The reaction time for the coupling between the magnetic marker and the target was 4 min, which was much shorter than the conventional method. These results show the usefulness of the present method.

AC Susceptibility of Magnetic Fluid in Nonlinear Brownian Relaxation Region: Experiment and Comparison with Numerical Simulation

This study investigated the AC susceptibility of magnetic fluids in the nonlinear Brownian relaxation region. The nonlinear properties of the susceptibility in high excitation fields were measured comprehensively, including the decrease in susceptibility, field-dependent Brownian relaxation time, and occurrence of the third harmonic for the susceptibility. These experimental results were compared with numerical simulations based on the Fokker–Planck equation, which describes nonlinear Brownian relaxation. We first performed the numerical simulation by assuming mono-dispersed single-domain nanoparticles. The observed nonlinear properties were shown to be roughly explained by the simulation. To compare the experiment and simulation more accurately, we then considered the size distribution of the magnetic nanoparticles existing in practical samples; this was obtained by analyzing the frequency dependence of the susceptibility in weak fields. Quantitative agreements were obtained between the experiment and simulation for the frequency and field dependences of the nonlinear susceptibility.

Characterization of magnetic markers for liquid-phase immunoassays using brownian relaxation

We characterized the magnetic markers used in biological immunoassays based on Brownian relaxation. Because the markers are composed of aggregated nanoparticles, i.e., magnetic nanoclusters, we first clarified their magnetic properties using AC susceptibility measurements, magnetization (M–H) curves, and magnetic relaxation properties. Analyzing the experimental results, we obtained the key parameters for the immunoassay, i.e., hydrodynamic diameter dh, magnetic moment mB, and anisotropy energy EB of the markers. Because these parameters were distributed in practical samples, we took their distribution into account in the analysis. Next, we showed the relationship between these parameters obtained from different samples. It was shown that mB increased approximately in proportion to dh. On the other hand, no clear correlation between mB and EB was obtained. These results were very different from those expected from single-domain nanoparticles and must be taken into account when magnetic markers are used in immunoassays based on Brownian relaxation.

Development of a SQUID System Using Field Reversal for Rapidly Detecting Bacteria

Pathogen identification usually requires growth of the pathogen by culture, which requires considerable time and manipulation by an experienced operator, leading to delays in diagnosis and treatment. We have investigated pathogen detection using a highly sensitive HTS-SQUID and magnetic markers and have developed a rapid and simple pathogen detection method. The magnetic markers, magnetic nanoparticles coated with detecting antibodies, bind to the target substance (antigen). The magnetic signal of the bound markers is measured with the highly sensitive SQUID. A remarkable feature of the magnetic assay is the disappearance of the magnetic signal from the unbound markers due to Brownian rotation. This makes it possible to detect the magnetic signal of the bound magnetic markers without removal of the unbound markers. In practice, however, the residual field around the SQUID generates an undesired magnetic signal from the unbound markers as a result of biased Brownian rotation. We developed a field reversal method - a measurement scheme - that eliminates the magnetic signal from the unbound markers. A difference signal is obtained by subtracting the magnetic signals measured by applying a magnetization field in two directions. The validity of this method was demonstrated experimentally using polymer beads as simulated bacteria. Its feasibility was demonstrated by the detection of Candida albicans, a pathogenic fungus. A magnetic signal of 3 mPhi0 was detected from a sample containing 300 cells of Candida albicans. The detection limit was estimated from the system noise level of 0.5 mPhi0 to be about one hundred cells of Candida albicans, indicating that this method has high sensitivity. These results show that magnetic assay using a highly sensitive SQUID can provide rapid and simple pathogen testing without culture.