New design of a commercial chip-based GMR sensor with magnetite nanoparticles for biosensing applications

The availability of rapid and low-cost instruments to detect magnetic nanoparticles (MNPs) concentrations is vital in giant magnetoresistance (GMR)-based biosensors. This paper reports a new setup for a simple GMR sensor using the commercial chip AAL024 as a transducer. It was combined with a basic differential amplifier and microcontroller to acquire digital output voltages for the detection of green-synthesized (GS)-Fe3O4 MNPs as a label and streptavidin-coated MNPs in biosensor applications. As a characteristic feature of Fe3O4, the GS-Fe3O4 MNPs displayed a cubic inverse spinel structure. The average GS-Fe3O4 particle size was 11 nm and they exhibited soft ferromagnetic behavior with a saturation magnetization (MS) of 55.5 emu/g. Owing to the presence of phytochemical components in the Moringa oleifera (MO) extract, the MS of GS-Fe3O4 was lower than that of Fe3O4. To study sensor performance, the detection of the GS-Fe3O4 MNP labels and streptavidin-coated MNPs assay was investigated. Using the microcontroller as the supply voltage for the AAL024 and an analog-to-digital converter simplified data collection and made any additional measuring instruments unnecessary. The sensor showed promising performance with the GS-Fe3O4 MNP label and streptavidin assay owing to the linear correspondence between the signal and concentration of the MNP label. A small limit-of-detection of 4 mg/mL was achieved for GS-Fe3O4. The sensitivity of GS-Fe3O4 and streptavidin were 2.79 and 1.80 mV/(mg/mL), respectively. Moreover, the excellent stability and reproducibility of the sensor were confirmed by the stable signal for over 30 s with relative signal deviation (RSD) ranges of 2–20% and 2–10% for MNPs and streptavidin, respectively.