Fabrication of Fe3O4@Ag magnetic nanoparticles for highly active SERS enhancement and paraquat detection

Fe3O4 magnetic nanoparticles with a particle size of ∼ 250 nm were synthesized by the thermal solvent method, and then modified with polyethyleneimine (PEI) as the intermediate layer on the Fe3O4 surface to form ultra-thin polymer interlayer to replace the traditional carbon shell and silicon shell interlayer. The seeding of Fe3O4 magnetic particles was completed thanks to the strong positive charge of the amino group of PEI-adsorbed 3–5 nm colloidal gold on the surface of magnetic beads, and the addition of silver nitrate and reducing agent to the magnetic microsphere enabled the silver shell to be coated on the surface of the magnetic microspheres almost without affecting their magnetism. Moreover, the effects of the additive amount of colloidal gold, silver nitrate and ultrasonic time on the magnetic nanoparticles were systematically studied. Morphology and characterization analysis showed that the prepared Fe3O4@Ag magnetic nanoparticles (MNPs) had uniform particle size, good dispersibility, strong magnetic response, and powerful SERS performance. Furthermore, the SERS performance of the composite nanometer material was investigated by using 4-aminophenol and pesticide paraquat as active targets, with the molecular concentration down to 10-8 M and 10-10 M, respectively, confirming that the functional composite had a good SERS activity and Fe3O4 magnetic core.