Fe3O4@Au Magnetic–Plasmonic Nanoparticles Self-Assembled into Gammadion-Shaped Nanostructures in a Gradient Magnetic Field for High-Sensitivity Optical Sensing

Abstract

Most of the bottom-up approaches, such as light-induced, capillarity-assisted, and electric or magnetic field-assisted assembly methods, have been carried out in fabricating crystalline structures, which can exhibit unique optical, magnetic, or electrical properties. Among them, the magnetic field-driven self-assembly of magnetic–plasmonic nanoparticles (Fe3O4@Au) provides an adaptive approach for constructing metamaterials with peculiar optical and photothermal properties due to their localized surface plasmon resonance and magnetic drivability. Here, a microfluidic and magnetic template-assisted self-assembly protocol is proposed to form a gammadion-shaped nanostructure consisting of magnetic–plasmonic nanoparticles. This approach leverages strong magnetic field gradients and the mechanical constraints of specially designed traps to precisely control the assembly process of Fe3O4@Au nanoparticles. Lagrangian–Eulerian analysis and full-wave field theory are combined to investigate the self-assembly and optical behaviors of the nanostructure, respectively. These gammadion-shaped nanostructures can be transferred onto various substrates and exhibit remarkable tunability in optical responses. Experimentally, micronano fabrication techniques were employed to fabricate gammadion-shaped magnetic templates, and Fe3O4@Au nanoparticle assemblies were obtained under an external bias magnetic field, with SEM characterization preliminarily confirming the feasibility of the proposed strategy. Numerical simulations further reveal that, under external illumination, strong near-field coupling between adjacent nanoparticles generates significant localized electric field enhancement, making these structures promising candidates for high-sensitivity optical sensing and potential SERS-based molecular detection. Specifically, when transferred onto a glass substrate, the maximum field intensity at the interparticle “hot spots” reaches 8 × 109 V·m–1. Furthermore, when the structure is transferred onto a dielectric Al2O3 layer with an Au backplate, this metal–insulator–metal (MIM) structure achieves near-perfect absorption of light (absorptivity enhanced to 0.97) at a resonance wavelength of 2 μm, and the sensitivity at 2.34 μm reaches 1777.8 nm/RIU. This research provides a pathway for the efficient construction of asymmetric nanostructures and lays a solid theoretical foundation for the development of multifunctional metamaterials.