Facile Fabrication of High‐Density Sub‐1‐nm Gaps from Au Nanoparticle Monolayers as Reproducible SERS Substrates

Abstract

The fabrication of ultrasmall nanogaps (sub‐1 nm) with high density is of significant interest and importance in physics, chemistry, life science, materials science, surface science, nanotechnology, and environmental engineering. However, it remains a challenge to generate uncovered and clean sub‐1‐nm gaps with high density and uniform reproducibility. Here, a facile and low‐cost approach is demonstrated for the fabrication of high‐density sub‐1‐nm gaps from Au nanoparticle monolayers as reproducible surface‐enhanced Raman scattering (SERS) substrates. Au nanoparticles with larger diameters possess lower surface charge, thus the obtained large‐area nanoparticle monolayer generates a high‐density of sub‐1‐nm gaps. In addition, a remarkable SERS performance with a 1011 magnitude for the Raman enhancement is achieved for 120 nm Au nanoparticle monolayers due to the dramatic increase in the electromagnetic field enhancement when the obtained gap is smaller than 0.5 nm. The Au nanoparticle monolayer is also transferred onto a stretchable PDMS substrate and the structural stability and reproducibility of the high‐density sub‐1‐nm gaps in Au monolayer films are illustrated. The resultant Au nanoparticle monolayer substrates with an increasing particle diameter exhibit tunable plasmonic properties, which control the plasmon‐enhanced photocatalytic efficiency for the dimerization of p‐aminothiophenol. The findings reported here offer a new opportunity for expanding the SERS application.

Sub‐5 nm Metal Nanogaps: Physical Properties, Fabrication Methods, and Device Applications

“…In these studies, the size of the gold nanoparticle is small because of the requirement of high surface activity of the Au nanoparticles. Recently, Si et al demonstrated that high‐density sub‐1 nm gaps can be easily fabricated from Au nanoparticle monolayers by taking advantage of the oil–water interfacial self‐assembly of large‐sized Au nanoparticles (30–120 nm) without any assistance from molecular ligands . Larger Au nanoparticles possess a lower surface charge that decreases the repulsive force between the adjacent nanoparticles at the oil–water interface, thus, the obtained large‐area nanoparticle monolayer generates a high density of sub‐1 nm gaps, as shown in Figure b.…”

“…b) TEM image of Au nanoparticle monolayers with an interparticle distance of 0.5 nm for Au nanoparticles self‐assembled at the oil–water interface. Reproduced with permission . Copyright 2016, WILEY‐VCH Verlag GmbH & Co. c) A scheme for stepwise dimer assembly using masked desilanization (top).…”

Engineering State‐of‐the‐Art Plasmonic Nanomaterials for SERS‐Based Clinical Liquid Biopsy Applications

“…[55] For example, Yang et al achieved a 3D hotspot matrix by evaporating a droplet of plasmonic nanoparticles on a hydrophobic substrate (Figure 6a). To generate SERS substrates with a uniform distribution of hot spots, Si et al proposed oil-water interfacial self-assembly of gold nanoparticles on stretchable poly(dimethylsiloxane) substrates (Figure 6b), [56] which created sub 1 nm gaps of higher density and reproducibility as compared to by using evaporation-based synthesis methods. [55b] However, this platform might suffer from relatively poor signal reproducibility due to the random distribution of hot spots.…”

“…[64] These structures were created through the facile solvent wetting (Wenzel state) method, which provides attractive capillary forces among nanopillars (Figure 7a,b). [56] Copyright 2016, Wiley-VCH. [65] Although these close-packed nanoparticles or nanopillars (usually at sub 10 nm) dramatically enhance SERS signaling, they are often subjected to poor reproducibility due to uncontrollable aggregation.…”

Ultrasensitive and reproducible SERS platform of coupled Ag grating with multibranched Au nanoparticles

“…13,14 However, the last requirement of reproducibility remains one of the biggest challenges in the SERS field. There are various research projects that achieved the creation of a reproducible substrate, 15,16 but the achievement of high detection limit and reproducibility of results (desirably using the portable Raman spectrophotometer) remains a problem. More common methods to realize the uniformity of SERS response (i.e.…”