Liquid-Phase Detection of Biological Targets Using Magnetoresistive Sensor and Magnetic Markers

A magnetic immunoassay technique utilizing magnetoresistive sensor and Brownian relaxation of magnetic markers was developed for use in the liquid-phase detection of biological targets. The magnetic field noise spectrum of a detection system was 25 pT/Hz^1/2 at the measurement frequency, and the peak-to-peak field noise was about 35 pT when 72 measurements were averaged. We detected 10 ng of markers, and the minimum detectable number of the markers was estimated to be N_m = 5.6 × 10⁵. We demonstrated the detection of biological targets called biotins. In the experiment, biotins were conjugated on the surface of the polystyrene beads with a diameter of 3.3 μm. Streptavidin-coated magnetic markers were bound to the biotins. The bound and unbound (free) markers were magnetically distinguished by the Brownian relaxation of the free markers without using a washing process for maker separation. A clear correlation was obtained between the detected signal and the number (N_B) of biotins. The minimum detectable number of biotins was as low as N_B = 6.2 × 10⁵. Since the sample volume was 60 μl, this value corresponds to a molecular number concentration of 1.7 × 10^-17 mol/ml. This result indicated the high sensitivity of the method used.