Optical properties of magneto-plasmonic nanoparticles chain of Fe3O4@SiO2@Ag and their sensing applications

In this paper, the optical properties of magneto-plasmonic core–shell nanoparticles composed of Fe₃O₄@SiO₂@Ag were investigated. Under an external static magnetic field, these nanoparticles self-assembled into linear chains. Using the finite element method, the absorption and extinction efficiencies as well as electric field enhancement of these chains were analyzed. Chains containing 1 to 12 nanoparticles were studied, revealing both longitudinal and transverse plasmonic modes. As the number of nanoparticles in a chain increases, the longitudinal mode exhibits a red shift, whereas the transverse mode shows a blue shift. The red shift grows rapidly with the addition of the first few nanoparticles, but its rate of change gradually decreases as more nanoparticles are added to the chain. Absorption efficiency peaks at the dimer stage and then gradually declines, while electric field enhancement reaches its maximum in a chain of four nanoparticles, over 11 times greater than that of a single nanoparticle, highlighting their potential for Surface-Enhanced Raman Scattering (SERS) applications. In addition, the results showed that sensitivity to changes in the surrounding refractive index increases with chain length, reaching 503 RIU⁻¹ for a 12-particles chain. These findings suggest that magneto-plasmonic nanoparticle chains, offering both spectral tunability and enhanced field localization, are promising candidates for advanced sensing applications.