Hierarchical hotspot engineering for ultrasensitive antibiotics sensing in human body fluids

Simultaneously achieving high sensitivity, long-term stability, and recyclability in surface-enhanced Raman scattering (SERS) substrates remains a major challenge that limits their practical deployment in biochemical sensing. Here, we present a fully solid-state SERS substrate that realizes hierarchical hotspot engineering within an Au@TiO 2 @Ag multilayered nanoarchitecture. An ultrathin TiO 2 spacer layer deposited by atomic layer deposition conformally separates the Ag nanorod array and sharp-tipped Au nanoparticles, generating a cascade of plasmonic hotspots from the atomic scale to the microscale. The multilevel electromagnetic couplings, including inter-rod gap coupling, mirror-coupling, and tip-induced field concentration, act cooperatively to provide strong and uniform Raman enhancement while maintaining surface integrity under complex environments. The Au@TiO 2 @Ag substrate exhibits excellent reproducibility and long-term durability, enabling ultrasensitive antibiotic detection down to 10 −12  M directly in various human body fluids. This solid-state and hierarchically engineered SERS platform provides a general and robust pathway toward practical, high-performance biochemical sensing applications.