Synthesis of hybrid Fe₃O₄@Ag nanoparticles for magnetic preconcentration and SERS sensing

Magneto-plasmonic nanoparticles (MPNPs) offer a synergistic approach to surface-enhanced Raman scattering (SERS) by integrating magnetic preconcentration with plasmonic field enhancement. In this study, we report the controlled synthesis of hybrid Fe₃O₄@Ag nanostructures featuring diverse silver morphologies (spherical, oval, cubic, and urchin-like). Advanced structural characterization, including high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD), confirmed the phase purity and revealed the distinct hierarchical architecture of the urchin-like Ag nanoparticles. Structural and optical characterization revealed that the urchin-like MPNP architecture provides superior SERS performance, attributed to high-efficiency electromagnetic enhancement formed by surface protuberances. Multiwavelength Raman spectroscopy analysis demonstrated that these platforms are specifically optimized for the near-infrared (NIR) region, achieving a maximum enhancement factor of 8.3 × 10⁶ at 785 nm excitation. This NIR preference is critical for biological applications as it minimizes background fluorescence and prevents thermal degradation of sensitive analytes. The practical utility of the platform was demonstrated through the trace detection of ergothioneine, a significant biomarker for oxidative stress and cancer. By applying magnetic-assisted collection, we successfully lowered the limit of detection to 2.6 nM, by an order-of-magnitude improvement over the conventional drop-casting technique. Furthermore, the MPNPs maintained high selectivity and sensitivity within a complex fetal bovine serum matrix, despite the presence of competitive protein adsorption. These results establish urchin-like MPNPs as a robust, highly sensitive tool for clinical diagnostics and liquid biopsies.