Two spin-valve GMR thin films on half wheatstone bridge circuit for detecting green-synthesized Fe3O4@Ag nanoparticles-labeled biomolecule

The availability of low-cost, rapid, and easy-to-use instruments for detecting biomolecules is critical in giant magnetoresistance (GMR) based biosensors. This research reports the performance of two spin-valve (SV) GMR thin films on half Wheatstone bridge circuits for detecting biomolecules. The SV thin films with the structure of Ta(2 nm)/Ir20Mn80(10 nm)/Co90Fe10(3 nm)/Cu(2.2 nm)/Co84Fe10B4(10 nm)/Ta(5 nm) used as sensing element was fabricated using RF magnetron sputtering on Si/SiO2 substrate. The Fe3O4 magnetic nanoparticles as a label were synthesized by coprecipitation method based on the green synthesis route using Moringa oleifera (MO) extract. Furthermore, the utilization of Ag nanoparticles to form Fe3O4@Ag was also green-synthesized by the aqua-solution method using MO extract. The nanoparticles exhibited soft ferromagnetic behavior with the saturation magnetization of 55.0 emu/g for Fe3O4 and 43.4 emu/g for Fe3O4@Ag. A small amount of magnetic nanoparticles-ethanol solution was dropped onto each sensing element surface on half Wheatstone bridge. For comparison, one thin film was also investigated as a quarter Wheatstone bridge circuit. Based on this research, two sensors have faster magnetic switching and saturation than one sensor, confirmed by the left-shifting of Hs(sv) from 12.4 Oe down to 10.5 Oe for two sensors of the Fe3O4 sample. BSA is used as a biomarker to observe the performance of biosensors. The sensor on the half-bridge circuit in detecting Fe3O4@Ag/BSA achieved a better sensitivity of 0.51 mV/mg/mL than that of the quarter bridge of 0.46 mV/mg/mL. A small limit-of-detection of 0.5 mg/mL was achieved for Fe3O4@Ag/BSA. Moreover, the excellent reproducibility and stability of the sensor were confirmed by relative signal deviation ranges of 2-13 % for 60 s. Therefore, the half-bridge configuration is reliable for achieving competitive performance with green-synthesized magnetic labels in GMR as biosensors in encouraging more responsive detection.