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 fi rst self-assembled at the air-water interface; upon UV irradiation, anion exchange cycles enable the free-fl oating PSS fi lm to continuously uptake gold precursors from the solution subphase for diffusion-controlled and POM-site-directed photoreduction inside the silica channels. NPs ≈ 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 (≈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 n M , with corresponding SERS enhancement factors above 10 8 .) into the PSS fi lm via anion exchange (middle), then POM-site-directed reduction and NP formation inside the silicate-surfactant channels upon UV irradiation (right). Given at bottom is a schematic illustration of the anion exchange cycle activated by UV irradiation for a continuous supply of Au atoms for uniform NP formation inside the silica channels.

Kinetic details of mesostructured silicate-surfactant films formed at the air-water interface of acidic solutions of cetyltrimethylammoium bromide and tetraethyl orthosilicate were systematically studied. Time-resolved grazing-incident small-angle X-ray scattering was adopted to capture the formation kinetics of the free-standing films comprising mesostructured silicate channel domains highly oriented to the airwater interface. Evolutions of the ordered domain size and phase volume are interpreted on the basis of the Avrami analysis, from which intermediate phases and corresponding phase transitions (as modulated by temperature, pH level, and/or composition) during film formation were quantitatively identified.Extracted kinetic parameters, complemented with the rate constant of silicate hydrolysis obtained via Raman spectroscopy, revealed details of the dynamic interplay between silicate polymerization and inorganic-organic self-assembling. Effects of the air-water interface on the formation of the silicatesurfactant films were illustrated in terms of the highly oriented mesostructure, the greatly enhanced kinetics characterized by sporadic nucleation and diffusion-controlled growth, and the reduced activation energy for silicate polymerization, in sharp contrast to solutions without the air-water interface. A schematic diagram of the free energy vs. surfactant headgroup area is constructed to correlate the observed kinetics pathways for mesostructure formation during film fabrication at the air-water interface.Advantages of the interface coupling with either air-water or solution-substrate on film formation are discussed.