Multifunctional Fe<sub>3</sub>O<sub>4</sub>@Ag/SiO<sub>2</sub>/Au Core–Shell Microspheres as a Novel SERS-Activity Label via Long-Range Plasmon Coupling

Noble metallic nanostructures exhibit a phenomenon known as surface-enhanced Raman scattering (SERS) in which the Raman scattering cross sections are dramatically enhanced for the molecules adsorbed thereon. Due to their wide accessible potential range in aqueous solutions and the high biocompatibility, Au supports are preferred for spectro-electrochemical investigations. However, the optical range in SERS spectroscopy is restricted to excitation lines above 600 nm, which is shorter than the Ag supports. In addition, these SERS-activity materials are not easy to separate and reused. Herein, the present article reports the novel multifunctional Fe<sub>3</sub>O<sub>4</sub>@Ag/SiO<sub>2</sub>/Au core–shell microspheres that display long-range plasmon transfer of Ag to Au leading to enhanced Raman scattering. The well-designed microspheres have high magnetization and uniform sphere size. As a result, Fe<sub>3</sub>O<sub>4</sub>@Ag/SiO<sub>2</sub>/Au microspheres have the best enhancement effect in the Raman active research by using Rhodamine-b (RdB) as a probe molecule. The enhancement factor is estimated to be 2.2 × 10<sup>4</sup> for RdB from the long-range plasmon transfer of Ag to Au, corresponding to an attenuation of the enhancement by a factor of only 0.672 × 10<sup>4</sup> compared to RdB adsorbed directly on the Fe<sub>3</sub>O<sub>4</sub>@Ag microspheres. RdB can be detected down to 10<sup>–9</sup> M even without the resonance SERS effect. The unique nanostructure makes the microspheres novel stable and a high-enhancement effect for Raman detection.