Mesostructured Arrays of Nanometer‐spaced Gold Nanoparticles for Ultrahigh Number Density of SERS Hot Spots

Abstract

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.

Multilayered Materials Comprising Mesoporous Thin Films and Metal Nanoparticles

“…Many of these complex architectures based on MPTF alone or combined with metallic NP have been tested for applications that take advantage of their special properties. In particular, these materials have been used as optical sensors, Surface Enhanced Raman Spectroscopy (SERS) based sensors, catalysts, photocatalysts, photochromic materials, and for nonlinear optics …”

Simultaneous Thermal Stability and Ultrahigh Sensitivity of Heterojunction SERS Substrates

“…As conceptually illustrated in Figure 1, we first deposit AgNRs and cap them with Al 2 O 3 layers, and then deposit additional Ag of optimal thickness. The AgNRs-Al 2 O 3 -Ag heterojunctions give rise to giant electromagnetic (EM) enhancement [20], which in turn leads to ultrahigh SERS sensitivity [21,22]. In this paper, we demonstrate the feasibility of this proposal using the glancing angle deposition (GLAD) technique [10,12].…”

Effect of chemical surface modification on dendritic gold in surface‐enhanced Raman scattering‐active substrates

“…Nanometer‐sized plasmonic materials possess attractive chemical and physical properties for applications in a wide variety of fields, such as trace molecular sensing, photocatalysis, and optoelectronics . In molecular sensing, the spacing and packing order of metallic nanoparticles have been found to have an intriguing hotspot effect that dramatically improves surface‐enhanced Raman scattering (SERS) . The most common SERS‐active substrates are fabricated from coinage metals, that is, copper, silver, and gold.…”