A novel one-trough synthesis via an air-water interface is demonstrated to provide hexagonally packed arrays of densely spaced metallic nanoparticles (NPs). In the synthesis, a mesostructured polyoxometalate (POM)-silicatropic template (PSS) is first self-assembled at the air-water interface; upon UV irradiation, anion exchange cycles enable the free-floating PSS film to continuously uptake gold precursors from the solution subphase for diffusion-controlled and POM-site-directed photoreduction inside the silica channels. NPs approximate to 2 nm can hence be homogeneously formed inside the silica-surfactant channels until saturation. As revealed via X-ray diffraction, small-angle X-ray scattering (SAXS), grazing incidence SAXS, and transmission electron microscopy, the Au NPs directed by the PSS template are arrayed into a 2D hexagonal lattice with inter-channel spacing of 3.2 nm and a mean along-channel NP spacing of 2.8 nm. This corresponds to an ultra-high number density (approximate to 10(19) NPs cm(-3)) of narrowly spaced Au NPs in the Au-NP@PSS composite, leading to 3D densely deployed hot-spots along and across the mesostructured POM-silica channels for surface-enhanced Raman scattering (SERS). Consequently, the Au-NP@PSS composite exhibits prominent SERS with 4-mercaptobenzoic acid (4-MBA) adsorbed onto Au NPs. The best 4-MBA detection limit is 5 nm, with corresponding SERS enhancement factors above 10(8).