Controlled synthesis of metal–insulator–metal nanoparticles for enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a powerful spectroscopic technique offering non-destructive and ultra-sensitive molecular characterization. Reliable SERS signals are typically generated using patterned metal surfaces or metal nanoparticles, primarily composed of gold (Au) and silver (Ag). However, the extreme spatial confinement required to form these hotspots (less than 5 nm) necessitates a complex fabrication process. Here, we present a novel approach, termed metal-insulator-metal (MIM) nanoparticle-enhanced Raman spectroscopy, which achieves robust Raman signal amplification through nanoparticles comprising a gold core, an ultrathin silica insulating layer, and surface gold satellites. The ultrathin silica layer prevents aggregation, enabling the formation of active SERS regions both between gold nanoparticles and on the outer surface. This configuration provides high-quality SERS spectra for liquid samples without requiring patterned substrates or stringent nanoparticle manipulation. The optimal MIM structure is determined by Finite Element Analysis (FEA) with a 120 nm gold core, a 3 nm silica layer, and a 13 nm surface gold nanoparticle. The detection limit of ciprofloxacin using the MIM nanoprobe is 1 pg mL^-1. By incorporating theory-guided structure-controlled synthesis, our method significantly enhances the versatility and accessibility of SERS, paving the way for transformative applications in materials science, life sciences, and environmental monitoring.