Highly Surface‐roughened “Flower‐like” Silver Nanoparticles for Extremely Sensitive Substrates of Surface‐enhanced Raman Scattering

Surface‐enhanced Raman scattering (SERS) is a new optical spectroscopic analysis technique with potential for highly sensitive detection of molecules. Recently, many efforts have been made to find SERS substrates with high sensitivity and reproducibility. In this Research News article, we provide a focused review on the synthesis of monodispersed silver particles with a novel, highly roughened, “flower‐like” morphology by reducing silver nitrate with ascorbic acid in aqueous solutions. The nanometer‐scale surface roughness of the particles can provide several hot spots on a single particle, which significantly increases SERS enhancement. The incident polarization‐dependent SERS of individual particles is also studied. Although the different “hot spots” on a single particle can have a strong polarization dependency, the total Raman signals from an individual particle usually have no obvious polarization dependency. Moreover, these flower‐like silver particles can be measured by SERS with high enhancement several times, which indicates the high stability of the hot spots. Hence, the flower‐like silver particles here can serve as highly sensitive and reproducible SERS substrates.     

Plasmonic nanogap-enhanced Raman scattering using a resonant nanodome array

The optical properties and surface-enhanced Raman scattering (SERS) of plasmonic nanodome array (PNA) substrates in air and aqueous solution are investigated. PNA substrates are inexpensively and uniformly fabricated with a hot spot density of 6.25 × 10(6) mm(-2) using a large-area nanoreplica moulding technique on a flexible plastic substrate. Both experimental measurement and numerical simulation results show that PNAs exhibit a radiative localized surface plasmon resonance (LSPR) due to dipolar coupling between neighboring nanodomes and a non-radiative surface plasmon resonance (SPR) resulting from the periodic array structure. The high spatial localization of electromagnetic field within the ～10 nm nanogap together with the spectral alignment between the LSPR and excited and scattered light results in a reliable and reproducible spatially averaged SERS enhancement factor (EF) of 8.51 × 10(7) for Au-coated PNAs. The SERS enhancement is sufficient for a wide variety of biological and chemical sensing applications, including detection of common metabolites at physiologically relevant concentrations.     

Surface-Enhanced Raman Scattering on Template-Embedded Gold Nanorod Substrates

Surface-enhanced Raman scattering (SERS) of rhodamine 6G was investigated on template-embedded gold nanorods produced by anodic aluminum oxide template-assisted nanofabrication. A signal enhancement of about 10⁶ was obtained. Two-dimensional arrays of gold nanospheres with different diameters and gap sizes were used as simplified model systems. SERS substrate design principles were investigated in order to achieve maximum electromagnetic enhancement of both the incident and Raman scattered fields.     

Split‐GFP: SERS Enhancers in Plasmonic Nanocluster Probes

The assembly of plasmonic metal nanoparticles into hot spot surface‐enhanced Raman scattering (SERS) nanocluster probes is a powerful, yet challenging approach for ultrasensitive biosensing. Scaffolding strategies based on self‐complementary peptides and proteins are of increasing interest for these assemblies, but the electronic and the photonic properties of such hybrid nanoclusters remain difficult to predict and optimize. Here, split‐green fluorescence protein (sGFP) fragments are used as molecular glue and the GFP chromophore is used as a Raman reporter to assemble a variety of gold nanoparticle (AuNP) clusters and explore their plasmonic properties by numerical modeling. It is shown that GFP seeding of plasmonic nanogaps in AuNP/GFP hybrid nanoclusters increases near‐field dipolar couplings between AuNPs and provides SERS enhancement factors above 10⁸. Among the different nanoclusters studied, AuNP/GFP chains allow near‐infrared SERS detection of the GFP chromophore imidazolinone/exocyclic C?C vibrational mode with theoretical enhancement factors of 10⁸–10⁹. For larger AuNP/GFP assemblies, the presence of non‐GFP seeded nanogaps between tightly packed nanoparticles reduces near‐field enhancements at Raman active hot spots, indicating that excessive clustering can decrease SERS amplifications. This study provides rationales to optimize the controlled assembly of hot spot SERS nanoprobes for remote biosensing using Raman reporters that act as molecular glue between plasmonic nanoparticles.     

Competitive Reaction Pathway for Site‐Selective Conjugation of Raman Dyes to Hotspots on Gold Nanorods for Greatly Enhanced SERS Performance

Common methods to prepare SERS (surface‐enhanced Raman scattering) probes rely on random conjugation of Raman dyes onto metal nanostructures, but most of the Raman dyes are not located at Raman‐intense electromagnetic hotspots thus not contributing to SERS enhancement substantially. Herein, a competitive reaction between transverse gold overgrowth and dye conjugation is described to achieve site selective conjugation of Raman dyes to the hotspots (ends) on gold nanorods (GNRs). The preferential overgrowth on the nanorod side surface creates a barrier to prevent the Raman dyes from binding to the side surface except the ends of the GNRs, where the highest SERS enhancement factors are expected. The SERS enhancement observed from this special structure is dozens of times larger than that from conjugates synthesized by conventional methods. This simple and powerful strategy to prepare SERS probes can be extended to different anisotropic metal nanostructures with electromagnetic hotspots and has immense potential in in‐depth SERS‐based biological imaging and single‐molecule detection.     

Plasmonic Graphene Transparent Conductors

Plasmonic graphene is fabricated using thermally assisted self‐assembly of silver nanoparticles on graphene. The localized surface‐plasmonic effect is demonstrated with the resonance frequency shifting from 446 to 495 nm when the lateral dimension of the Ag nanoparticles increases from about 50 to 150 nm. Finite‐difference time‐domain simulations are employed to confirm the experimentally observed light‐scattering enhancement in the solar spectrum in plasmonic graphene and the decrease of both the plasmonic resonance frequency and amplitude with increasing graphene thickness. In addition, plasmonic graphene shows much‐improved electrical conductance by a factor of 2–4 as compared to the original graphene, making the plasmonic graphene a promising advanced transparent conductor with enhanced light scattering for thin‐film optoelectronic devices.     

Self‐Aligned Anisotropic Plasmonic Nanostructures

Great opportunities emerge not only in the generation of anisotropic plasmonic nanostructures but also in controlling their orientation relative to incident light. Herein, a stepwise seeded growth method is reported for the synthesis of rod‐shaped plasmon nanostructures which are vertically self‐aligned with respect to the surface of colloidal substrates. Anisotropic growth of metal nanostructure is achieved by depositing metal seeds onto the surface of colloidal substrates and then selectively passivating the seed surface to induce symmetry breaking in the subsequent seed‐mediated growth process. The versatility of this method is demonstrated by producing nanoparticle dimers and linear trimers of Au, Au–Ag, Au–Pd, and Au–Cu₂O. Further, this unique method enables the automatic vertical alignment of the resulting plasmonic nanostructures to the surface of the colloidal substrate, thereby making it possible to design magnetic/plasmonic nanocomposites that allow the dynamic tuning of the plasmon excitation by controlling their orientation using an external magnetic field. The controlled anisotropic growth of colloidal plasmonic nanostructures and their dynamic modulation of plasmon excitation further allow them to be conveniently fixed in a thin polymer film with a well‐controlled orientation to display polarization‐dependent patterns that may find important applications in information encryption.     

Highly controlled surface-enhanced Raman scattering chips using nanoengineered gold blocks

Well defined gold nanostructures of various sizes are fabricated on glass substrates using high-resolution electron-beam lithography/lift-off techniques and detailed surface-enhanced Raman scattering (SERS) properties of crystal violet molecules are studied in order to elucidate electromagnetic (EM) field enhancement effects on the fabricated structures. SERS measurements are performed with high reproducibility using in situ Raman microspectroscopy in aqueous solution. An analysis based on EM theory is performed using field-enhancement factors obtained from finite-difference time-domain (FDTD) simulations and the analysis reproduces experimental results very well. It is noteworthy, furthermore, that the proposed analytic method of EM effects on SERS allows the estimate of the ideal local temperature of gold nanostructures by canceling out the difference in EM field factors at Stokes and anti-Stokes Raman scattering wavelengths. Thus, these experimental results demonstrate that quantitative analysis based on EM theory can be obtained using highly controlled gold nanostructures for SERS measurements with high reproducibility, a result that is promising for the construction of a SERS analysis chip. Although no SERS chip reported so far has been usable for quantitative analysis, this study opens the door for construction of a quantitative SERS chip.     

Plasmonic Heterodimers with Binding Site‐Dependent Hot Spot for Surface‐Enhanced Raman Scattering

A novel plasmonic heterodimer nanostructure with a controllable self‐assembled hot spot is fabricated by the conjugation of individual Au@Ag core–shell nanocubes (Au@Ag NCs) and varisized gold nanospheres (GNSs) via the biotin–streptavidin interaction from the ensemble to the single‐assembly level. Due to their featured configurations, three types of heterogeneous nanostructures referred to as Vertice, Vicinity, and Middle are proposed and a single hot spot forms between the nanocube and nanosphere, which exhibits distinct diversity in surface plasmon resonance effect. Herein, the calculated surface‐enhanced Raman scattering enhancement factors of the three types of heterodimers show a narrow distribution and can be tuned in orders of magnitude by controlling the size of GNSs onto individual Au@Ag NCs. Particularly, the Vertice heterodimer with unique configuration can provide extraordinary enhancement of the electric field for the single hot spot region due to the collaborative interaction of lightning rod effect and interparticle plasmon coupling effect. This established relationship between the architecture and the corresponding optical properties of the heterodimers provides the basis for creating controllable platforms which can be exploited in the applications of plasmonic devices, electronics, and biodetection.     

M‐shaped Grating by Nanoimprinting: A Replicable,Large‐Area,Highly Active Plasmonic Surface‐Enhanced Raman Scattering Substrate with Nanogaps

Plasmonic nanostructures separated by nanogaps enable strong electromagnetic‐field confinement on the nanoscale for enhancing light‐matter interactions, which are in great demand in many applications such as surface‐enhanced Raman scattering (SERS). A simple M‐shaped nanograting with narrow V‐shaped grooves is proposed. Both theoretical and experimental studies reveal that the electromagnetic field on the surface of the M grating can be pronouncedly enhanced over that of a grating without such grooves, due to field localization in the nanogaps formed by the narrow V grooves. A technique based on room‐temperature nanoimprinting lithography and anisotropic reactive‐ion etching is developed to fabricate this device, which is cost‐effective, reliable, and suitable for fabricating large‐area nanostructures. As a demonstration of the potential application of this device, the M grating is used as a SERS substrate for probing Rhodamine 6G molecules. Experimentally, an average SERS enhancement factor as high as 5×10⁸ has been achieved, which verifies the greatly enhanced light–matter interaction on the surface of the M grating over that of traditional SERS surfaces.     

Resistless Nanoimprinting in Metal for Plasmonic Nanostructures

Metal nanostructures are the main building blocks of metamaterials and plasmonics which show many extraordinary properties not existing in nature. A simple and widely applicable method that can directly pattern metals with silicon molds without the need of resists, using pressures of <4 MPa and temperatures of 25–150 °C is reported. Three‐dimensional structures with smooth and vertical sidewalls, down to sub‐10 nm resolution, are generated in silver and gold films in a single patterning step. Using this nanopatterning scheme, large‐scale vivid images through extraordinary optical transmission and strong surface‐enhanced Raman scattering substrates are realized. Resistless nanoimprinting in metal (RNIM) is a new class of metal patterning that allows plasmonic nanostructures to be fabricated quickly, repeatedly, and at a low‐cost.