Plasmonic Silver Supercrystals with Ultrasmall Nanogaps for Ultrasensitive SERS‐Based Molecule Detection

Abstract

surface plasmon resonance. The plasmonic coupling effect at the nanometer gaps between metallic nanoparticles, i.e., the plasmonic hot spots, has also been utilized to further increase the SERS signals. [ 3 ] Extensive studies have been focused on creating hot spots spaced by small dielectric gaps of less than 10 nm. In addition to the original SERS platform made from electrochemically roughened surface, [ 4 ] a diverse novel SERS substrates have been developed, which include the well-ordered patterned arrays fabricated via lithographic techniques. [ 5 ] However, these techniques are limited by their low throughput, expensive operating cost, and small SERS active area. Importantly, it is still diffi cult to process small nanogaps down to sub-5 nm over large area. As such, size/shape controlled colloidal nanoparticles and their assemblies such as dimers, trimers, liner chains, core-satellite structures are attractive options due to the ability to synthesize through wet chemical approaches in bulk scale without the need of expensive instrumentation. [ 6 ] However, owing to the "coffee ring" effect, regulating the geometrical dimension, i.e., uniform nanoparticle gaps between colloidal nanoparticles via spontaneous aggregation or assembly are challenging. [ 7 ] With the assistance of surface ligands, nanoparticles can be assembled into superlattices, [ 8 ] but the introduction of polymer ligands unavoidably sacrifi ces the sensitivity of signals in a variety of applications, such as SERS, biosensors, plasmon-enhanced fl uorescence. Therefore, the central problem is to homogeneously produce well-defi ned plasmonic structures that can reproducibly fabricate sub-5 nm nanogaps with controllable gap distance and enhanced hot spot density within the plasmonic structure.In comparison with the nanoparticle superlattices obtained via self-assembly of metallic nanocrystals by means of the polymer ligands, [ 8 ] the ordered nanoparticle supercrystals may also be synthesized via replicating 3D colloidal (e.g., polymer or SiO 2 spheres) template [ 9 ] or ordered mesoporous silica (e.g., KIT-6, SBA-15, MCM-48) templates. [ 10 ] For example, 2D dimpled gold nanoplates were fabricated by vapor reduction using a 3D lattice of silica nanospheres (≈40 nm) as a template. [ 9 ] Unfortunately, the dimpled-type array is not suitable for the applications in SERS. Using ordered mesoporous silica (KIT-6 and SBA-15) template, Yamauchi's group has successfully fabricated polyhedral mesoporous Pt nanoparticles through a chemical reduction route. [ 10 ] However, the synthesis of highly Metallic periodic structures provide unique optical and photonic coupling effects. However, geometrically precise control of periodic structure is particularly challenging when studying scale at few nanometer. Here, a new class of highly ordered silver plasmonic supercrystals is successfully synthesized by means of a nanocasting process using ordered mesoporous silica as template. During the nanocasting via a chemical reduction, the diffusibility, viscosit...

Hedgehog Inspired CuO Nanowires/Cu2O Composites for Broadband Visible‐Light‐Driven Recyclable Surface Enhanced Raman Scattering

“…In particular, reusable biosensors have recently drawn increasing research attention because of their advantages to be applied in remote and resource‐limited environments for point‐of‐care (POC) diagnostics . Among a variety of biosensors, surface‐enhanced Raman scattering (SERS) is emerging as one of the most cutting‐edge techniques for its intriguing role to demonstrate sensitivity down to the single molecule level . Such accurate label‐free and fingerprint detection technique has been considered as a promising and noninvasive POC diagnostic tool in various applications, such as environmental monitoring, drug detection, explosives analyses, as well as healthcare inspection .…”

Highly sensitive, reproducible and uniform SERS substrates with a high density of three-dimensionally distributed hotspots: gyroid-structured Au periodic metallic materials

“…14 Gyroid-structured motifs self-assembled from block polymer systems have frequently been adopted as hard templates to fabricate Au plasmonic materials and the optical properties of such systems have been thoroughly investigated. 4,[15][16][17][18][19][20][21] Three-dimensional (3D) gyroid-structured Au nanoparticles with a unit cell size below 100 nm have been reported to show excellent SPR performance originating from the abundant exposed nanotips on the external surface. 15,17 However, the hotspots were still confined to a two-dimensional distribution of limited density and enhancement, and the merits of the gyroid morphology remain unexploited.…”

From colloidal CdSe quantum dots to microscale optically anisotropic supercrystals through bottom-up self-assembly

“…As these applications are based on the properties arising from the morphology of the material, the development of protocols for the production of composite materials with controlled morphologies, including strictly determined interparticle distance and mutual orientation of the nanoparticles in the superstructures, is a critical emerging goal. Currently, superstructures can be formed by a number of different methods, which can be divided into top-down (template-assistant, [4][5][6] focused ion beam milling, 7 electron-beam and interference lithography, 8 and laser beam printing 9 ) and bottom-up (self-assembly) 10 processes. Top-down methods of obtaining nanostructured materials with a given morphology allow only sequential manipulations with a two-dimensional structure on a solid substrate.…”

“…Selfassembly, however, makes it possible to obtain three-dimensional structures of a specified shape with a high accuracy, in one step, with a wide range of substrates available, even including flexible substrates. 11 These composite materials can be used in various areas, including photodetectors, 12 surface-enhanced Raman spectroscopy sensors, 4,5,13 light emitters, 2 and solar cells. 14 Despite the advantages of this method, it is currently both less controllable and less predictable than ''classical'' epitaxial approaches.…”