Detection of green-synthesized magnetite nanoparticles using spin-valve GMR-based sensor and their potential as magnetic labels

Abstract

Exploration of magnetic nanoparticle labels for giant magnetoresistance (GMR) biosensing technology is crucial because their characteristics determine GMR-based sensor performance, related to their stray field and compatibility to biomolecule. We investigated the potency of the green-synthesized magnetite (Fe3O4) nanoparticle as a green-magnetic label on a spin-valve (SV) GMR sensor based on a [Ta (2 nm)/Ir20Mn80 (10 nm)/Co90Fe10 (3 nm)/Cu (2.2 nm)/(Co84Fe10B4 (10 nm)/Ta (5 nm)] structure that generated a 7.3 Oe exchange-bias field and 5.73% magnetoresistance. Fe3O4 nanoparticles were synthesized by a chemical coprecipitation method utilizing Moringa oleifera (MO) leaf extract as a natural reducing agent. The green-synthesized Fe3O4 nanoparticles exhibited a cubic inverse spinel structure as a pristine Fe3O4 characteristic. The crystallite size of green-synthesized Fe3O4 was larger than that of Fe3O4, reaching ∼ 14 nm, and formed a particle size of 29.1 nm. Its magnetic properties included soft ferromagnetic behavior with 55.5 emu/g saturation magnetization and 54 Oe coercivity; its saturation was lower than that of Fe3O4 due to the presence of the phytochemical compound of MO extract. In addition, to observe its potency as a magnetic label for biosensing, conventionally synthesized nanoparticles, surface modification using polyethylene glycol (PEG), and attachment of an α-amylase enzyme as a biomarker sample on green nanoparticle labels were also investigated. When measuring various concentration labels using the SV-GMR thin film, the green-synthesized Fe3O4 achieved a linear relationship and a better sensitivity of 0.098 mV/mg/mL than that of the conventional compound. The sensor could detect Fe3O4-labelled α-amylase enzyme as indicated by the decreasing output voltage, a consequence of altering the surface conditions of the nanoparticles. Therefore, green-synthesized nanoparticles are reliable and achieve competitive performance as magnetic labels in GMR sensors.