Ligand-directed Janus self-assembly with dual-aggregation enabling magneto-optical switching for enhanced lateral flow immunoassay.

The spatial organization of disparate functionalities within a single nanoscale construct critically governs the performance of multifunctional probes, yet achieving precise structural control and exploiting emergent properties remains challenging. Here, we report an oleic acid (OA) ligand-density-directed self-assembly strategy that enables a controlled morphological transition from isotropic core-shell to fully segregated Janus multifunctional AIEgen-Fe3O4 nanoparticles (MAFNPs) comprising aggregation-induced emission luminogens (AIEgens) and superparamagnetic Fe3O4 clusters. This structural evolution is systematically correlated with optical and magnetic properties: Janus-segregated OAL-MAFNPs with unique "dual-aggregation" architecture effectively preserve AIEgen fluorescence (71.4% retention) while maximizing saturation magnetization (23.7 emu g-1) through dense magnetic clustering, in stark contrast to core-shell counterparts where mutual interference attenuates both functionalities. Leveraging this Janus geometry, we further uncover a unique magnetic-field-directed fluorescence modulation-orientation of the Fe3O4 hemisphere away from the optical path enhances emission by ∼22%-and integrate it into a lateral flow immunoassay (LFIA) for staphylococcal enterotoxin B (SEB). The resulting OAL-MAFNPs-LFIA achieves a stepwise sensitivity improvement from 1.39 ng mL-1 (probe alone), to 0.32 ng mL-1 (with magnetic enrichment), and finally to 0.23 ng mL-1 (with combined enrichment and orientation) in complex food matrices, representing a 26-fold improvement over conventional gold nanoparticle-based LFIA. This work establishes a ligand-regulated assembly paradigm for designing anisotropic Janus architectures, wherein spatial segregation preserves individual functionalities while enabling dynamic optical control, thereby opening new avenues for point-of-care diagnostics and biosensing.