Ac susceptibility measurement of magnetic markers in suspension for liquid phase immunoassay

ac susceptibility measurement of magnetic markers in solution was performed for the liquid phase detection of biological targets. First, the properties of the magnetic markers were clarified, such as size distribution, frequency dependence of susceptibility, and field dependence of magnetization. Next, we demonstrated a detection method in which we used large polymer beads to immobilize and prolong the Brownian relaxation time of the bound markers. In this method, we could detect the bound markers by the decrease in the susceptibility that they displayed. Using this method, we detected biotin-conjugated polymer beads with avidin-coated Fe₃O₄ markers. Changes in susceptibility caused by the binding reaction between them were measured with a magnetoresistive sensor. A strong relationship was observed between the decrease in susceptibility and the number of polymer beads, which indicates that the bound markers were detected correctly without the use of the washing process called bound/free separation. The sensitivity of the system was estimated to be as high as 1.3× 10^-16 mol/ml in terms of the molecular-number concentration of the markers. The binding process was also discussed from the dependence of the signal on the incubation time.