High-sensitivity SPR refractive index sensor modified with Fe3O4 nanospheres

This study develops a high-sensitivity surface plasmon resonance (SPR) refractive index sensor by integrating Fe₃O₄ nanospheres onto a gold film. Fe₃O₄ nanospheres (~ 80 nm diameter) were synthesized hydrothermally and deposited on a 50 nm Au-coated BK-7 prism via drop-casting. Finite element simulations revealed that a 10 nm Fe₃O₄ layer enhances sensitivity by 45.13% and extends penetration depth to 228.0 nm compared to bare Au. Experimentally, sensors with two drop-casting cycles achieved a refractive index sensitivity of 3638.68 nm/RIU—a 72.8% improvement over bare Au (2105.51 nm/RIU)—while maintaining a 90.08% resonance depth and a figure of merit (FOM) of 21.99 RIU⁻¹. This performance attributed to Fe₃O₄’s high refractive index, hierarchical stacking structure, large specific surface area (24.104 m²/g), and mesoporous distribution (7–25 nm), which amplify light-matter interactions. The sensor demonstrated excellent repeatability (resonance wavelength SD < 0.86 nm) and long-term stability. This work establishes a quantitative model for Fe₃O₄ coating optimization and highlights the potential of magnetic nanomaterials for advanced SPR sensing.