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

Developing plasmonic nanogaps via 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 (∼10⁸) 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 As³⁺ and As⁵⁺, 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.