Breaking the Spatial‐Sensitivity Trade‐Off: Dual‐Field Coupling in OPC@Ag SERS Nanostructures for Ultrasensitive Large‐Area Biosensing

ABSTRACT Surface‐enhanced Raman scattering (SERS) faces limitations in its spatial detection range (the “SERS skin depth”) and a trade‐off between performance and fabrication simplicity. Here, we propose an opal photonic crystal (OPC)‐guided spatial‐range SERS detection strategy. This approach synergistically integrates the electromagnetic enhancement of plasmonic nanostructures with the wide‐range localized field regulation capability of an OPC to construct a novel hybrid substrate (OPC@Ag NL). The substrate is fabricated by depositing an adjustable silver nanolayer (Ag NL) onto a water‐stable, self‐assembled OPC template with a tunable PBG. This design synergizes plasmonic enhancement with the OPC's ability to modulate excitation laser, generating an expanded localized field that suppresses electric field decay. Consequently, significant SERS enhancement is maintained up to 300 nm from the surface, enabling 3D Raman imaging of macrophages. The substrate demonstrates high sensitivity (detection limit: 10 − 1 1 m for 4‐MBA, EF: 1.7 × 10 7 ) and uniformity (RSD < 5.8%), achieving bacterial detection limits of 10 CFU/mL (labeled) and 10 2 CFU/mL (label‐free). This work marks a paradigm shift from hotspot engineering to macroscopic field modulation for overcoming SERS spatial constraints.