Prospect of core-shell Fe₃O₄@Ag label integrated with spin-valve giant magnetoresistance for future point-of-care biosensor

Magnetic-based biosensors are the analytical instruments that convert the biological recognition into the electrical signal through the generating of the stray-field of the magnetic nanoparticles (MNPs) attached to the biomolecule target. The magnetic biosensor feature relies on the transducer and the MNPs label selection. Recently, the biosensor with a point-of-care feature is the most expected device in the nowadays medical diagnostic field. So that, a review of the recent research related to the novel integration of magnetoresistance-based transducers with MNPs for biosensor application is vital for the point-of-care diagnostic development. Hence, the basic principle of biosensors and the giant magnetoresistance (GMR) with exchange bias phenomena are introduced. Furthermore, we provide a review of the cutting edge method in GMR biosensor with spin-valve structure (SV-GMR) which is integrated to MNPs for biomolecule labelling. As review results, among the nano-sized magnetoresistance transducer, the SV-GMR has some predominance, i.e. electrical robustness and moderate magnetoresistance ratio. Meanwhile, as compared to the other proposed MNPs such as pure Fe3O4, Fe2O3, and hybrid Fe3O4-graphene, the core-shell Fe3O4@Ag is potent to be used, which offers not only moderate saturation magnetisation but also good protein affinity, antimicrobial activity, and minimal cytotoxicity. According to the sensor performance comparison, the usage of Fe3O4@Ag for biomolecule labelling in synergy with SV-GMR transducer is prospective to be developed. The Ag shell espouses the protein immobilisation to the surface of the MNPs label that improves the sensor sensitivity. Furthermore, the SV-GMR possessed two modes of the Fe3O4@Ag rapid detection, which are through the moderate voltage change and the switching field shifting. Meanwhile, the concentration increase of Fe3O4@Ag can be well quantified. Moreover, the Fe3O4@Ag/SV-GMR system had a low operating magnetic field with rapid data collection. In conclusion, the Fe3O4@Ag/SV-GMR biosensor system is believed to be applied as a real-time, portable, and cost-effective biosensor.