Maximizing surface-enhanced Raman scattering sensitivity of surfactant-free Ag-Fe₃O₄ nanocomposites through optimization of silver nanoparticle density and magnetic self-assembly

Magnetic composite nanomaterials consisting of more than two functional constituents have been attracting much research interests due to the realization of multiple functionalities in a single entity. In particular, integration of ferromagnetic oxides and noble metal nanoparticles (NPs) in composites results in simultaneous magnetic activity and optical response where the optical property of the whole system could be modulated by application of an external magnetic field. In this work, we prepared Ag NPs-coated Fe₃O ₄ microspheres as a novel surfactant-free surface-enhanced Raman scattering (SERS) substrate through a solid-phase thermal decomposition reaction. The SERS sensitivity of the fabricated nanocomposites is maximized by adjusting the size and density of Ag NPs supported on the Fe₃O ₄ microspheres and further increased by magnetic-field-directed self-assembly of the composite substrates, with both effects attributed to the efficient generation of plasmonic near-field "hot" spots. At the optimal conditions, the prepared substrate is capable of detecting rhodamine 6G molecules at a concentration down to 10^-12 M, thus demonstrating the great potential of using bifunctional nanocomposites as an excellent candidate for ultra-high sensitive Raman spectroscopy and biosensors. We also reveal the underlying mechanisms responsible for the observed SERS enhancements through full-wave numerical simulations.