A functionalization-free plasmonic hole-sphere nanogap SERS platform for reliable on-site analysis and oxide-state classification.

Developing plasmonic nanogaps simple, lithography-free methods is essential for advancing practical surface-enhanced Raman spectroscopy (SERS) sensors. Here, we present a functionalization-free, lithography-free plasmonic hole-sphere nanogap (HSNG) platform, enabling sensitive differentiation of oxidation states at trace levels. The HSNG structure, fabricated by straightforward thermal annealing and metal deposition, achieves highly uniform nanogaps (signal deviation <15%), resulting in a strong (∼10) and uniform Raman enhancement. The fully metal-coated nanocavity structure eliminates background interference, significantly enhancing the analytical reliability in complex environmental samples. Using this platform, we demonstrated the ability to distinguish trace concentrations of As and As, which were difficult to distinguish with conventional gold nanoparticles due to low signal intensity, at the on-site analysis level. Remarkably, reliable oxidation state identification remains possible even under reduced spectral resolution, ensuring compatibility with simplified detection setups such as bandpass filters or smartphone-based spectrometers. This HSNG-based SERS platform provides a scalable, accessible, and field-applicable approach to chemical sensing, readily extendable to the detection of diverse environmental contaminants.