Core-shell Fe3O4@Ag magnetic nanoparticles detection using spin-valve GMR sensing element in the wheatstone bridge circuit

Core–shell Fe3O4@Ag magnetic nanoparticles (MNPs) integrated with a Wheatstone bridge-giant magnetoresistance (GMR) sensor provide access to GMR-based biosensors. The Fe3O4 nanoparticles synthesized using the coprecipitation method demonstrated 77 emu g−1 of magnetization saturation (MS), 51 Oe of coercivity (HC), and particle size of 11 nm. Furthermore, core–shell Fe3O4@Ag MNPs prepared by the aqua-solution method possessed 53 emu g−1 of MS, 145 Oe of HC, and 17 nm of particle size. This high MS of nanoparticles not only offer a large induced magnetic field but is sufficient for particle penetration within the biofilms. It was discovered that the sensor can distinguish between the bare Fe3O4 with the Fe3O4@Ag nanoparticles through an output voltage increase corresponding to a decrease in MS. The output signal of the sensor responds linearly to an increase in the core–shell Fe3O4@Ag nanoparticle concentration, owing to an increase in the induced-field. The sensor exhibits better sensitivity when applied in detecting less than 2 g L−1 of nanoparticle concentration, that is, 0.76 mV per unit of concentration (g/L).