pH-Modulated Nanogaps in Self-Assembling Gold Nanoparticle Metasurface

The development of tunable metamaterials has attracted increasing attention. However, achieving real-time modulation of the optical properties remains a significant challenge. Stimuli-responsive polymers with their rapid response characteristics offer new opportunities for applications in optical modulation and sensing. Here, we propose a novel approach for controlling interparticle spacing and pH sensing by constructing a plasmonic metasurface via self-assembly of nanoparticles (NPs) at the liquid-liquid interface. Variations in the surface plasmon resonance peak position and reflectance in the reflection spectrum indicate changes in the interparticle spacing within the metasurface. Furthermore, pH variations in the aqueous phase modulate the intensity of the Raman characteristic peaks in surface-enhanced Raman scattering spectra. We demonstrate that AuNPs modified with poly(4-vinylpyridine) (P4VP) exhibit reversible pH-responsive behavior. The pH-induced coupling and decoupling of plasmonic modes enable tunable interparticle distances from 14 to 2.2 nm and linear pH sensing over a broad range of pH 3 to 11. The sensing performance has also been validated in practical samples.