Magnetic biosensing techniques that are based on the use of bio-functionalized magnetic nanoparticles (magnetic markers) and superconducting quantum interference devices (SQUIDs) are expected to have various advantages when compared with conventional biosensing methods. In this paper, we review the recent progress made in magnetic biosensing techniques. First, we describe the most important parameters of magnetic markers that are intended for use in biosensing, i.e., the magnetic signal and the relaxation time that are determined by the Brownian and/or Néel relaxation mechanisms. We note that these parameters are significantly dependent on the marker size, and as a result, commercial markers exhibit a wide variety of values for these key parameters. Next, we describe three measurement methods that have been developed based on the magnetic properties of these markers, i.e., AC susceptibility, relaxation and remanence-based measurement methods. The weak (picotesla-range) signals emitted by the markers can be measured precisely with a SQUID system using these methods. Finally, we give examples of biosensing for in vitro and in vivo medical diagnosis applications. For in vitro diagnosis, high-sensitivity detection of various biological targets has been demonstrated without use of any washing process to separate the bound and free markers. For in vivo applications, detection of the quantities and the three-dimensional positions of the markers that have been injected into the test subject are demonstrated. These results confirm the effectiveness of magnetic biosensing techniques.

SUMMARYIn this paper, we propose improved methods of liquidphase detection of biological targets utilizing magnetic markers and a high-critical-temperature superconducting quantum interference device (SQUID). For liquid-phase detection, the bound and unbound (free) markers are magnetically distinguished by using Brownian relaxation of free markers. Although a signal from the free markers is zero in an ideal case, it exists in a real sample on account of the aggregation and precipitation of free markers. This signal is called a blank signal, and it degrades the sensitivity of target detection. To solve this problem, we propose improved detection methods. First, we introduce a reaction field, B re , during the binding reaction between the markers and targets. We additionally introduce a dispersion process after magnetization of the bound markers. Using these methods, we can obtain a strong signal from the bound markers without increasing the aggregation of the free markers. Next, we introduce a field-reversal method in the measurement procedure to differentiate the signal from the markers in suspension from that of the precipitated markers. Using this procedure, we can eliminate the signal from the precipitated markers. Then, we detect biotin molecules by using these methods. In an experiment, the biotins were immobilized on the surfaces of large polymer beads with diameters of 3.3 μm. They were detected with streptavidinconjugated magnetic markers. The minimum detectable molecular number concentration was 1.8 × 10 −19 mol/ml, which indicates the high sensitivity of the proposed method. key words: high-critical-temperature superconducting quantum interference device (SQUID), magnetic marker, immunoassays, liquid-phase detection

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