Au@Ag Nanocube-Coated Polystyrene Microspheres as Hierarchical SERS Substrates for Trace Detection of As⁵⁺

The plasmon coupling properties of assembled metal nanogap structures are highly sensitive to the distance, shape, orientation, and configuration of faceted nanoparticles. However, the orientation control of nanoparticles is usually limited to in-plane configurations. In this paper, Au@Ag nanocubes (NCs) with sharp edges and corners were assembled on a polystyrene (PS) microsphere array as a hierarchical surface-enhanced Raman spectroscopy (SERS) substrate via a facile self-assembly technique. The Au@Ag NC monolayer undergoes an out-of-plane reconstruction and forms V-shaped nanogaps under the modulation of the PS microsphere array curved surface. This unique V-shaped nanogap can generate electromagnetic fields stronger than those of traditional parallel gaps and make it easier for target molecules to diffuse into the nanogaps. The three-dimensional (3D) SERS substrates have a wider spectral response and 20 times higher SERS activity than those of the Au@Ag NC monolayer films on flat substrates. The high sensitivity was demonstrated by the detection limit down to 10^–9 M for 4-MBA with a substrate enhancement factor up to 3.1 × 10⁷. Furthermore, the optimized 3D SERS substrate was applied to determine As⁵⁺ in water with detection limits as low as 10^–8 M assisted by machine learning, indicating that the 3D SERS substrates have promising applications in food safety, environmental monitoring, and biological diagnosis.