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 \(^{\mathrm {\mathbf {1/2}}}\) 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 \(N-{\mathrm {\mathbf {m }}} =5.6 \times 10^{\mathrm {\mathbf {5}}}\). 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 \(\mu \) m. 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 ( \(N-{\mathrm {\mathbf {B}}}\) ) of biotins. The minimum detectable number of biotins was as low as \(N-{\mathrm {\mathbf {B }}} =6.2\times 10^{\mathrm {\mathbf {5}}}\). Since the sample volume was \(60~\mu \) l, this value corresponds to a molecular number concentration of \(1.7\times 10~^{\mathrm {\mathbf {-17}}}\) mol/ml. This result indicated the high sensitivity of the method used.