Cassie-state superhydrophobic surface-induced 3D hotspots construction for ultrasensitive SERS detection.

The sensitivity of surface-enhanced Raman scattering (SERS) substrates critically depends on the efficient spatial coupling between high-density electromagnetic "hotspots" and target molecules. Traditional SERS substrates confine hotspots to zero-dimensional points, one-dimensional lines, or two-dimensional planes, which limits their overlap with the three-dimensional (3D) laser focal volume. In this work, we demonstrate a Cassie-state superhydrophobic Si nanorod platform fabricated on a 4-in. wafer via reactive ion etching and self-assembly of 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane. By optimizing the bullet-like nanorod morphologies and employing a heating-assisted evaporation process, our platform enables uniform vertical assembly of urchin-like Au nanoparticles into 3D architectures, leading to enhanced spatial overlap between the hotspots and the laser depth of focus. Our platform supports stable contact angle evaporation and uniform vertical nanoparticle aggregation. The resulting SERS substrate achieves detection limits of 8.9 × 10-14 M for rhodamine 6G and 8.9 × 10-12 M for malachite green, while effectively suppressing coffee-ring effects. This scalable strategy offers a pathway toward reproducible and ultrasensitive SERS detection through 3D hotspot engineering.