Fe(III)-Induced Satellite Structural Evolution of Ag@G Nanoparticles Toward Ultrasensitive Detection of Trace Drugs in Mice Serum.

The detection sensitivity of surface-enhanced Raman scattering (SERS) depends on high-density electromagnetic "hotspots" within metallic nanostructures. However, conventional salt-induced aggregation often leads to uncontrolled clustering, resulting in uneven hotspot distribution and occasional macroscopic precipitation. Here, we report a Fe3+-assisted spatially confined etching strategy for precise fabrication of silver-based core-satellite nanostructures (SGSI), using silver-graphene nanoparticles (Ag@G) as a template. Finite-difference time-domain (FDTD) simulations confirm that nanogaps within the satellite architecture induce strong plasmonic coupling, generating highly localized electromagnetic fields. Benefiting from uniformly distributed intra-cavity hotspots and excellent colloidal stability, the substrate exhibits high SERS activity and signal reproducibility. The superior performance arises from two synergistic effects: (i) carboxyl inherent to the graphene shell of Ag@G imparts a negative surface potential, promoting the electrostatic adsorption of Fe3+ ions and accelerating the etching process; and (ii) the graphene shell also serves as a permeable physical barrier that provides a spatial confinement effect, guiding Ag reorganization into stable satellite structures. This satellite platform enables highly sensitive detection of the anticancer drug methotrexate (55 nM) in mice serum. Overall, this work offers a new paradigm for constructing a high-performance and stable SERS sensing platform via the synergistic regulation of surface charge and spatial confinement effects.