Tracking and Controlling Monolayer Water in Gold Nanogaps using Extreme Plasmonic Spectroscopy.

Nanogaps are ubiquitous across science and technology, confining molecules and thus changing chemistries that influence many areas such as catalysis, corrosion, photochemistry, and sensing. However, in ambient conditions, it is unclear how water solvates nanogaps and even if nominally dry, what water structure persists. Despite its low Raman cross-section, surface-enhanced Raman spectroscopy (SERS) enables the study of water at coinage metal surfaces. Using multi-layer aggregates of close-packed gold nanoparticles with sub-nanometer gaps precisely defined by organic spacer molecules, consistent and large SERS enhancements are achieved, enabling a systematic study of water within these confined spaces. Ostensibly dry facets in air evidence water monolayer coatings, with hydrogen-bonding only reappearing upon immersion in solution. Under negative applied potentials, surface water is seen to re-orient at the metal facets with distinct spectral shifts among the quartet of vibrational peaks that correspond to those expected from interacting water dimers. Comparing nanogaps in deuterated water also reveals how individual water molecules bind onto organic spacer molecules in such nanogaps. Realistic models of water dressing will enable a better understanding of catalytic and contact chemistries.