Water-Wettable Open Plasmonic Nanocavities for Ultrasensitive Molecular Detections in Multiple Phases.

Plasmonic nanocavities between metal nanoparticles on metal films are either hydrophobic or fully occupied by nonmetallic spacers, preventing molecular diffusion into electromagnetic hotspots. Here we realize water-wettable open plasmonic cavities by devising gold nanoparticle with site-selectively grown ultrathin dielectric layer-on-gold film structures. We directly confirm that hydrophilic dielectric layers of SiO2 or TiO2, which are formed only at the tips of gold nanorod via precise temperature control, render sub-10 nm cavities open to the surroundings and completely water-wettable. Simulations reveal that spontaneous wetting in our cavities is driven by the presence of tip-selective hydrophilic layer and tendency of minimizing high energy air/water interface inside the cavities. Our plasmonic cavities show significant Raman enhancement of up to 4 orders of magnitude higher than those of conventional ones for molecules in various media. Our findings will offer new opportunities for sensing applications of plasmonic nanocavities and have huge impacts on cavity plasmonics.