Development of ultrasensitive biological immunoassay system using Fe3O4 nanoparticles and magnetic sensor in the liquid phase

Abstract

In this paper, we first describe details of the measurement system (MR and Flux Gate sensor). The measurement system using the MR sensor showed a sensitivity to detect 1.4 × 10⁷ of the markers in 60 µl of solution. The sensitivity was improved as 8.3 × 10⁶ when the flux gate sensor was used.  The sensitivity of the present method was estimated as 3.8 × 10^-16 and 2.3×10^-16 mol/ml in terms of the molecular-number concentration for the MR and the flux gate sensor, respectively. We next demonstrate the detection of biological targets known as biotins, which were conjugated on the surface of the polystyrene beads with a diameter of 3.3 µm. The minimum detectable number of beads was N_p = 10,000 and 5,000 for the case of the MR and the flux gate sensor, respectively. Since about 700 biotins were fixed on the single polymer bead, the minimum detectable number of biotins was estimated as N_b = 7 × 10⁶ and 3.5× 10⁶ for the case of the MR and the flux gate senor, respectively.  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 method. The detection sensitivity can be estimated as 1.9× 10^-16 and 0.8 × 10^-16 mol/ml in terms of the molecular-number concentration of biotin for the MR and the flux gate sensor, respectively. These results are consistent with the estimated sensitivity of the measurement system.