Highly efficient SERS substrates with different Ag interparticle nanogaps based on hyperbolic metamaterials

Surface-enhanced Raman scattering (SERS) substrates based on Hyperbolic metamaterials (HMM) have received tremendous attention due to the presence of confined optical modes, i.e., the surface plasmon polaritons (SPP) and bulk plasmon polaritons (BPP). In the present study, a multilayer HMM SERS substrate was fabricated, comprising a monolayer of silver nanoparticles (Ag NPs) and Au-Al₂O₃ films. Our study demonstrates that the Ag NPs layer excited SPP and BPP, and the localized surface plasmon (LSP) resonance of the Ag NPs layer coupled with BPP, creating a strong electromagnetic field at the interparticle gaps of the Ag NPs. Interestingly, increasing the multilayer stacking of HMM will strengthen the SERS activity until a certain threshold. On the top of HMM, the interparticle gaps of the Ag NPs were adjusted to enhance the SERS characteristics and the maximum EF appeared at the gap of 10 nm. Performing theoretical simulations as well as experiments, the ultra-sensitivity, good reproducibility, reusability, and high enhancement factor of the proposed SERS substrate were assessed. Lastly, the ultralow limits of detection for melamine in milk were 10⁻⁷ M, satisfying the needs for successful implementation in the food safety field.