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The synthesis, structural and optical characterization, and application of superparamagnetic and water‐dispersed Fe3O4‐Au core‐shell nanoparticles for surface enhanced Raman scattering (SERS) is reported. The structure of the nanoparticles was determined by scanning transmission electron microscopy (STEM) and high‐resolution transmission electron microscopy (HRTEM). STEM images of the Fe3O4‐Au core‐shell nanoparticles reveal an average diameter of 120 nm and a high degree of surface roughness. The nanoparticles, which display superparamagnetic properties due to the core Fe3O4 material, exhibit a visible surface plasmon resonance (SPR) peaked at 580 nm due to the outer gold shell. The nanoparticles are used as a substrate for surface enhanced Raman scattering (SERS) with rhodamine 6G (R6G) as a Raman reporter molecule. The SERS enhancement factor is estimated to be on the order of 106, which is ∼ 2 times larger than that of conventional gold nanoparticles (AuNPs) under similar conditions. Significantly, magnetically‐induced aggregation of the Fe3O4‐Au core‐shell nanoparticles substantially enhanced SERS activity compared to non‐magnetically‐aggregated Fe3O4‐Au nanoparticles. This is attributed to both increased scattering from the aggregates as well as “hot spots” due to more junction sites in the magnetically‐induced aggregates. The magnetic properties of the Fe3O4 core, coupled with the optical properties of the Au shell, make the Fe3O4‐Au nanoparticles unique for various potential applications including biological sensing and therapy.

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