Study on Magnetic and Plasmonic Properties of Fe₃O₄-PEI-Au and Fe₃O₄-PEI-Ag Nanoparticles

Magnetic–plasmonic nanoparticles (NPs) have attracted great interest in many fields because they can exhibit more physical and chemical properties than individual magnetic or plasmonic NPs. In this work, we synthesized Au- or Ag-decorated Fe₃O₄ nanoparticles coated with PEI (Fe₃O₄-PEI-M (M = Au or Ag) NPs) using a simple method. The influences of the plasmonic metal NPs’ (Au or Ag) coating density on the magnetic and plasmonic properties of the Fe₃O₄-PEI-M (M = Au or Ag) NPs were investigated, and the density of the plasmonic metal NPs coated on the Fe₃O₄ NPs surfaces could be adjusted by controlling the polyethyleneimine (PEI) concentration. It showed that the Fe₃O₄-PEI-M (M = Au or Ag) NPs exhibited both magnetic and plasmonic properties. When the PEI concentration increased from 5 to 35 mg/mL, the coating density of the Au or Ag NPs on the Fe₃O₄ NPs surfaces increased, the corresponding magnetic intensity became weaker, and the plasmonic intensity was stronger. At the same time, the plasmonic resonance peak of the Fe₃O₄-PEI-M (M = Au or Ag) NPs was red shifted. Therefore, there was an optimal coverage of the plasmonic metal NPs on the Fe₃O₄ NPs surfaces to balance the magnetic and plasmonic properties when the PEI concentration was between 15 and 25 mg/mL. This result can guide the application of the Fe₃O₄-M (M = Au or Ag) NPs in the biomedical field.