Liquid Phase Immunoassay Using AC Susceptibility Measurement of Magnetic Markers

AC susceptibility measurement of magnetic markers in solution was applied to biological immunoassays. In this method, the markers that bound to biological target can be detected in the presence of unbound markers. The bound markers were distinguished from the unbound ones by using their different frequency dependence of susceptibility. We detected biotin-conjugated polymer beads with avidin-coated Fe 3 O 4 markers. Changes of the susceptibility caused by the binding reaction between them were shown. Good relationship was obtained between the decrease of susceptibility and the number of polymer beads. Sensitivity of the system was estimated as 3×10 -16 mol/ml in terms of molecular-number concentration.