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We have developed a liquid-phase immunoassay technique for the detection of biological targets using magnetic markers. In this method, Brownian relaxation of magnetic markers in solution was used to p...erform the liquid-phase detection. Biological targets were fixed on the surface of large polymer beads whose size was typically a few 1m. When the magnetic markers were bound to the targets, their Brownian relaxation time was dominated by that of the polymer bead, becoming much longer than that of unbound (free) markers. The resulting difference between the magnetic properties of the bound and free markers was detected by relaxation measurements. Therefore, we can magnetically distinguish between the bound and free markers, i.e., we can omit a time consuming washing process called bound/free separation. We developed a detection system using magneto-resistive (MR) and flux gate sensors. We could detect 1.4 × 107 and 8.3 × 106 bound markers in 60 1l of solution for the cases of MR and flux gate sensors, respectively. If we assume that a single marker is bound to a single target, this sensitivity can be expressed as 3.8 × 10-16 mol/ml and 2.3×10-16 mol/ml in terms of the molecular-number concentration. We also demonstrated the detection of biological targets called biotins, which were conjugated on the surface of the polystyrene beads with a diameter of 3.3 1m. A strong relationship was obtained between the number of bound markers and the number of biotin-conjugated polymer beads, which confirmed the validity of the present method.show more
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