Fig. 1. Schematic illustration of the preparation of detecting antibody conjugated magnetic iron oxide nanoparticles (IONPs).

Fig. 2. Schematic illustration of the preparation of capturing antibodies biofunctionalized MTJ sensor. (a) One droplet of antibody solution was introduced onto the sensor sensing area surface. (b) Capturing antibodies were passively attached to the surface of MTJ sensor by incubation in buffer. (c) Blocking buffer was added to eliminate non-specific binding effect and block the remaining active area (blocking areas are represented by red color). (d) A biofunctionalized MTJ sensor was achieved.

Fig. 3. Schematic illustration of magnetic sandwich immunoassay based on MTJ sensor and magnetic iron oxide nanoparticles (IONPs). This system exploits the capturing antibody functionalized MTJ sensor to capture biomarkers (antigens). The IONPs were labeled with detecting antibody. The target antigens were conjugated with both the capturing and detecting antibodies at different epitopes.

Fig. 4. (a) Schematic drawing of the MTJ array sensor, consisting of 76 tunneling junctions indicated by ellipses. (b) MR loop of the MTJ array sensor with 10 Oe of hard-axis bias field along the hard-axis.

Fig. 5. Schematic drawings of p24 detection with a sandwich-assay configuration. The stray magnetic field produced by IONPs magnetized by the magnetic field in the parallel direction to the MTJ array sensor (magnetoresistive sensor) surface.

Fig. 6. (a) Diagram of the MTJ sensor resistance variation ΔR after binding with the target p24 antigens with a concentration of 1 μg/ml. (b) Relation between sensor maximum ΔR and p24 concentration. Generally, the maximum ΔR increases with the logarithm of p24 concentration.