Ultralow detection limit of giant magnetoresistance biosensor using Fe3O4–graphene composite nanoparticle label

A special Fe3O4nanoparticles–graphene（Fe3O4–GN） composite as a magnetic label was employed for biodetection using giant magnetoresistance（GMR） sensors with a Wheatstone bridge. The Fe3O4–GN composite exhibits a strong ferromagnetic behavior with the saturation magnetization MS of approximately 48 emu/g, coercivity HC of 200 Oe, and remanence Mr of 8.3 emu/g, leading to a large magnetic fringing field. However, the Fe3O4 nanoparticles do not aggregate together, which can be attributed to the pinning and separating effects of graphene sheet to the magnetic particles. The Fe3O4–GN composite is especially suitable for biodetection as a promising magnetic label since it combines two advantages of large fringing field and no aggregation. As a result, the concentration x dependence of voltage difference ｜？V｜ between detecting and reference sensors undergoes the relationship of ｜？V｜ = 240.5 lgx ＋ 515.2 with an ultralow detection limit of 10 ng/mL（very close to the calculated limit of 7 ng/mL） and a wide detection range of 4 orders.