Synthesis of varied oleic acid-coated Fe3O4 nanoparticles using the co-precipitation technique for biosensor applications

This study successfully synthesized oleic acid (OA)-coated Fe₃O₄ nanoparticles using the coprecipitation method for the needs of biosensor applications, as shown through the characterization of structure and morphology using XRD and TEM. This observation used 0.75 ml of OA (Fe₃O₄-OA(0.75)) and 1.25 ml of OA (Fe₃O₄-OA(1.25). A decrease in particle size distribution from 18.55 nm to 16.30 nm was observed, which means a reduction in agglomeration and increased dispersibility. Vibrating Sample Magnetometer (VSM) observations showed that the saturation magnetization of Fe₃O₄ particles modified with OA decreased from 47.71 emu/g to 45.90 emu/g at Fe₃O₄-OA(0.75) and decreased again to 42.29 emu/g at Fe₃O₄-OA(1.25). But they all show soft ferromagnetic properties with very low coercivity, making them suitable for Giant Magnetoresistance (GMR) biosensor applications. A series of concentrations in ethanol solution were performed to evaluate the detection sensitivity of GMR for these pure and oleic acid-modified Fe₃O₄ samples. Concentrations of 2, 5, 10, 15, 25, and 35 mg/ml were integrated at 2 μL each onto the surface of the GMR chip. A decrease in sensitivity was observed due to the change in saturation. Pure Fe₃O₄ with a sensitivity of 5.07 mV (mg/ml) decreased to 4.49 mV (mg/ml) and 3.35 mV (mg/ml) after oleic acid coating. Thus, although adding a proportional amount of oleic acid can slightly decrease the saturation magnetization, it can be a valuable method for applications such as GMR biosensors that demand high dispersibility and sensitivity that remains strong.