Reproducible Surface-enhanced Raman Scattering Research Articles

Mechanically Flexible, Large-Area Fabrication of Three-Dimensional Dendritic Au Films for Reproducible Surface-Enhanced Raman Scattering Detection of Nanoplastics.

The escalating crisis of nanoplastic pollution in water and food products demands the development of novel methodologies for detection and recycling. Despite various techniques available, surface-enhanced Raman scattering (SERS) is emerging as a highly efficient technique for the trace detection of micro/nanoplastics. However, the development of highly reproducible and stable, flexible SERS substrates that can be used for sensitive detection in environmental medium remains a challenge. Here, we report a fabrication of large-area, three-dimensional (3D), and highly flexible SERS substrate based on porous dendritic Au films onto a flexible indium tin oxide (ITO) substrate via facile, thermal evaporation of Au over the vacuum-compatible deep eutectic solvent (DES)-coated glass substrate and subsequent direct transfer process. The as-fabricated 3D dendritic Au/ITO flexible substrates can be used for ultrasensitive SERS detection of crystal violet (CV) as probe analyte molecules with the limit of detection (LOD) as low as 6.4 × 10-15 M, with good signal reproducibility (RSD of 11.3%). In addition, the substrate showed excellent sensitivity in detecting nanoplastics such as poly(ethylene terephthalate) (200 nm) and polystyrene (100 nm) with LODs reaching up to 0.051 and 8.2 μg/mL, respectively. This work provides a facile approach for the preparation of highly flexible plasmonic substrates, showing great potential for the SERS detection of a variety of environmental pollutants.

Solvent Effect on the Synthesis of Oleylamine Modified Au Nanoparticles and Their Self-Assembled Film for SERS Substrate

The preparation and self-assembling of monodisperse gold nanoparticles (Au NPs) is of great significance for its SERS application. According to the oleylamine-reduced method, oleylamine (OAm) serves as the reducing agent and stabilizing agent, and the effects of different reaction parameters such as solvent and temperature on the size and dispersity of Au NPs have been evaluated. The Au NPs synthesized with toluene as the solvent have the best dispersity and narrowest particle size distribution with adjustable sizes. The particle size gradually increases with the increase in reaction temperature. The highly ordered self-assembly film of Au NPs was employed as surface-enhanced Raman scattering (SERS) substrate for the probing molecule of rhodamine 6G. The Au substrate exhibits excellent spatial uniformity and SERS reproducibility, which indicates its practicability as a substrate. This study provides a simple synthesis strategy of highly ordered monodispersed Au NPs, which can serve as a SERS substrate with excellent spatial uniformity and SERS re-producibility.

Gold Nanocone Array with Extensive Electromagnetic Fields for Highly Reproducible Surface-Enhanced Raman Scattering Measurements.

Surface-enhanced Raman scattering (SERS) is a technique used to distinguish the constitution of disease-related biomarkers in liquid biopsies, such as exosomes and circulating tumor cells, without any recognition elements. Previous studies using metal nanoparticle aggregates and angular nanostructures have achieved the detection of various biomarkers owing to strong hot spots and electromagnetic (EM) fields by localized surface plasmon resonance (LSPR). Although these SERS platforms enable significant enhancement of Raman signals, they still have some problems with the fabrication reproducibility of platforms in obtaining reproducible SERS signals. Therefore, highly reproducible fabrication of SERS platforms is required. Here, we propose the application of a polymer-based gold (Au) nanocone array (Au NCA), which extensively generates an enhanced EM field near the Au NCA surface by LSPR. This approach was experimentally demonstrated using a 785 nm laser, typically used for SERS measurements, and showed excellent substrate-to-substrate reproducibility (relative standard deviation (RSD) < 6%) using an extremely simple fabrication procedure and very low laser energy. These results proved that a Au NCA can be used as a highly reproducible SERS measurement to distinguish the constitution of biomarkers.

2D GaN for Highly Reproducible Surface Enhanced Raman Scattering.

Surface-enhanced Raman scattering (SERS) based on 2D semiconductors has been rapidly developed due to their chemical stability and molecule-specific SERS activity. High signal reproducibility is urgently required towards practical SERS applications. 2D gallium nitride (GaN) with highly polar Ga-N bonds enables strong dipole-dipole interactions with the probe molecules, and abundant DOS (density of states) near its Fermi level increases the intermolecular charge transfer probability, making it a suitable SERS substrate. Herein, 2D micrometer-sized GaN crystals are demonstrated to be sensitive SERS platforms with excellent signal reproducibility and stability. Strong dipole-dipole interaction between the dye molecule and 2D GaN enhances the molecular polarizability. Furthermore, 2D GaN benefits its SERS enhancement by the combination of increased DOS and more efficient charge transfer resonances when compared with its bulk counterpart.

Highly Sensitive and Reproducible SERS Substrates Based on Ordered Micropyramid Array and Silver Nanoparticles.

The construction of a highly sensitive and reproducible surface-enhanced Raman scattering (SERS) substrate is the key factor that restricts its practical application. In this paper, a three-dimensional (3D) SERS substrate based on ordered micropyramid array and silver nanoparticles (MPA/AgNPs 3D-SERS) was constructed using the roll-to-plate embossing technology and a hydrothermal method, which provided an efficient and low-cost preparation process for the SERS substrate. Using rhodamine 6G (R6G) as a probe molecule, the performance of an MPA/AgNP 3D-SERS substrate was studied in detail, whose minimum detection limit was 10-12 M and the enhancement factor was calculated as 8.8 × 109, indicating its high sensitivity. In addition, the minimum relative standard deviation (RSD) for the MPA/AgNP 3D-SERS substrate was calculated as 4.99%, and SERS performance basically had no loss after 12 days of placement, which indicated that the prepared SERS substrate had excellent stability and repeatability. At last, the thiram detection application of the MPA/AgNP 3D-SERS substrate was also investigated. The results showed that the minimum detection limit was 1 × 10-7 M, and quantitative analysis of pesticide residues could be realized. This research could provide useful guidance for the efficient and low-cost fabrication of highly sensitive and reproducible SERS substrates.

Bimetallic Au-Ag on a Patterned Substrate Derived from Discarded Blu-ray Discs: Simple, Inexpensive, Stable, and Reproducible Surface-Enhanced Raman Scattering Substrates.

A simple and reproducible surface enhanced Raman scattering (SERS) substrate of bimetallic gold-silver (Au-Ag) based on discarded a Blu-ray disc read only memory (BD-ROM) was developed by simply incorporating electrochemical (EC) treatment and chemical reaction. The resurfaced AgBD-ROM substrate (r-AgBD-ROM) was fabricated by EC treatment on a Ag film layer in BD-ROM (AgBD-ROM) to generate silver nanoparticles (AgNPs) on the indented pattern surface. Then, galvanic displacement reaction of Au and Ag was carried out to prepare the bimetallic Au-Ag structure (Au-r-AgBD-ROM). The suitable size and density as well as location of NPs on the surface can be tuned via EC treatment conditions to obtain highly active SERS performance. The SERS enhancement phenomenon on our developed substrate was studied by observing the location of the SERS hot spot obtained by Raman mapping. The developed SERS substrate offers excellent stability (90 days), good uniformity [6.14% relative standard deviation (RSD)], and reproducibility (3.79% of RSD). Moreover, this substrate can be used as a promising sensor for detecting acetaminophen, ibuprofen, and mefenamic acid. This finding suggests a simple and low-priced process, which potentially facilitates fabrication of highly sensitive SERS substrates for practical applications.

A sensitive and reproducible SERS sensor based on natural lotus leaf for paraquat detection

A facile and green approach of using natural lotus leaf as a superhydrophobic platform was proposed for sensitive and reproducible surface enhanced Raman scattering (SERS) detection. The surface of a lotus leave provides hydrophobic concentrating effect. After the mixture of an Ag nanoparticle (NP) dispersion and the target solution is dropped on the lotus leave chip, Ag NPs assemble into closely packed Ag arrays after solvent evaporation, producing numerous hot spots for SERS enhancement. The hydrophobic concentrating effect of the lotus leave surface greatly enhances the assembling. The final assembled Ag spot can shrink a small disc with a diameter of only about 0.5 mm. This leads to easy fabrication of a sensitive and reproducible SERS sensor based on natural lotus leaves. It was found that the state of the natural lotus leaves (growth stage, size and parts, water loss degree, storing condition) did not influence markedly the detection performance of the lotus leave-based SERS sensor. When the sensor was used to detect paraquat, the SERS signal intensity of paraquat was linearly correlated with its concentration in a very wide range from 5 μg L−1 and 50 mg L−1, with a detection limit of as low as 1.2 μg L−1. The multiple detection experiment demonstrated that this sensor method had high reproducibility with relative standard deviation (RSD) of 11.6%.

Silver films coated inverted cone-shaped nanopore array anodic aluminum oxide membranes for SERS analysis of trace molecular orientation

Abstract Reproducible surface enhanced Raman scattering (SERS) enhanced performance, affected by molecular orientations in plasmonic nanostructures, attracts more attention than SERS detection sensitivity for satisfying requirements of practical applications. In this work, by coating silver films on the inverted cone-shaped nanopore array anodic aluminum oxide membranes, we provide a way to study the effect of molecular orientations on SERS enhancement with mapping technology. SERS enhanced performance of the fabricated substrates is assessed with trace Rhodamine 6G detection, enhancement factor calculation and 3D finite-difference time-domain simulation respectively. It is found that inverted cone-shaped nanopore and silver films both contribute to the highly SERS enhancement. And special inverted cone-shaped nanopore plasmonic structures limit molecular vibrations so as to affect Raman scattering enhancement. These findings can help us to understand the electromagnetic mechanism of molecular orientations on SERS enhancement, so as to design highly reproducible SERS substrates for various applications.

Cellulose nanofibrils coated paper substrate to detect trace molecules using surface-enhanced Raman scattering

Acquisition of reproducible surface-enhanced Raman scattering (SERS) signals using paper-based SERS sensors is difficult because of the non-uniformity of the paper surface, which possesses a large number of pores and high surface roughness. To overcome the poor reproducibility of paper-based SERS sensors, they were coated with cellulose nanofibrils (CNF) obtained from pulp fibers, which fill the pores and decrease the local height differences on the paper’s surface. The combination of hydrophobic modification and the improved uniformity and smoothness of the surface after CNF coating increased the coverage of silver nanoparticles (AgNPs) in the SERS spots from 87 to 95% and gave a more uniform distribution of the AgNPs on the SERS substrate. When a high magnification lens was used for sensitive molecular detection, the hydrophobic CNF-coated paper-based SERS sensor reduced the relative standard deviation of the SERS intensity and improved the limit of detection for rhodamine 6G. The CNF coating on paper was found to afford a more effective and reproducible paper-based SERS sensor than conventional filter paper.

Investigation of Mass-Produced Substrates for Reproducible Surface-Enhanced Raman Scattering Measurements over Large Areas.

Surface-enhanced Raman scattering (SERS) is a versatile spectroscopic technique that suffers from reproducibility issues and usually requires complex substrate fabrication processes. In this article, we report the use of a simple mass production technology based on Blu-ray disc manufacturing technology to prepare large area SERS substrates (∼40 mm2) with a high degree of homogeneity (±7% variation in Raman signal) and enhancement factor of ∼6 × 106. An industrial high throughput injection molding process was used to generate periodic microstructured polymer substrates coated with a thin Ag film. A short chemical etching step produces a highly dense layer of Ag nanoparticles at the polymer surface, which leads to a large and reproducible Raman signal. Finite difference time domain simulations and cathodoluminescence mapping experiments suggest that the sample microstructure is responsible for the generation of SERS active nanostructures around the microwells. Comparison with commercial SERS substrates demonstrates the validity of our method to prepare cost-efficient, reliable, and sensitive SERS substrates.

Facile Fabrication of Large-Scale Silver Nanowire Bilayer Films and Its Application as Sensitive and Reproducible Surface-Enhanced Raman Scattering Substrates.

The homogeneous surface-enhanced Raman scattering (SERS) active hot spots on a SERS substrate is the most crucial factor in ensuring their application as reproducible and ultrasensitive sensing platforms. In this paper, we report on a simply shaking-assisted liquid–liquid (water-chloroform) interfacial assembly process for fabricating aligned Ag nanowire (AgNW) bilayer films on solid substrates. A scalable fabrication process can be easily realized by using a large size of container. These AgNW bilayer films can be used as ideal SERS active substrates for chemical and biomolecular detection with highly sensitivity and excellent reproducibility. Significantly, sensitive and quantitative detection of carbaryl with a detection limit of 0.1 ppm using these SERS substrates to demonstrate potential applications for environmental pollutant analysis.

Plasmonic ladder–like structure and graphene assisted high surface enhanced Raman scattering detection

Surface enhanced Raman scattering (SERS) study is an interesting active area of research, where periodically patterned plasmonic substrates play a key role in SERS enhancement. Surface plasmon resonance excitation generates tremendous electromagnetic near-fields (E) in the form of localized or propagating near-fields contributing to the Raman signal (E4 process) to a great extent and overall enhancement is reported to be as high as 1010 or even more. Independently, graphene alone can enhance the Raman signal due to chemical enhancement. In the present study, we have attempted to achieve high SERS from the R6G Raman active probe using plasmonic patterned substrates in the presence and absence of graphene oxide. Plasmonic ladder-like patterned substrates are fabricated using laser interference lithography, which is cost effective, simple to operate, and has potential for large scale nanofabrication. By combining graphene oxide with R6G, we have found additional two time enhancement compared to that obtain from R6G alone on the plasmonic patterned substrate. Further, we have also attempted to understand the underlying mechanism to correlate the uniform and reproducible SERS through Raman mapping and Finite difference time domain computation. Our finding can potentially be applied for SERS investigation at a low molecular concentration.

Highly reproducible surface-enhanced Raman scattering substrate for detection of phenolic pollutants

The ordering degree of nanostructures is the key to determining the uniformity of surface-enhanced Raman scattering (SERS). However, fabrication of large-area ordered nanostructures remains a challenge, especially with the ultrahigh-density (>1010 cm−2). Here, we report a fabrication of large-area ultrahigh-density ordered Ag@Al2O3/Ag core–shell nanosphere (NS) arrays with tunable nanostructures. The ultrahigh-density (2.8 × 1010 cm−2) ordered NS arrays over a large-area capability (diameter >4.0 cm) enable the uniform SERS signals with the relative standard deviation of less than 5%. The as-fabricated highly reproducible SERS substrate can be applied to detect trace phenolic pollutants in water. This work does not only provide a new route for synthesizing the ultrahigh-density ordered nanostructures, but also create a new class of SERS substrates with high sensitivity and excellent reproducibility.

Fabrication of Superior Au–Ag Composites Surface-Enhanced Raman Scattering Active Substrates Based on One-Step Method of Chemical Etching

Bimetallic silver–gold composites are currently among the most studied substrates for detection based on surface-enhanced Raman scattering (SERS). Here, we developed Au–Ag composite films as high sensitivity, large-scale, chemically stable and reproducible SERS substrates. Au–Ag composite films were fabricated by chemical etching of Ag–Au alloy leaves with HAuCl4 aqueous solutions. Along with galvanic replacement reaction and dealloying, films with distinct microstructural features exhibit dramatic improvement in the SERS intensity. Especially, the Au–Ag composite films with legible nano-ridges fabricated by alloy leaves of 9 carat present the strongest SERS enhancement. The superior SERS enhancement is attributed to the confluence effect of enhanced local surface plasmon fields and electromagnetic coupling within nanogaps, sharp edges and corners. In addition, results showed that features of the films mightily depend on the initial proportions of Ag–Au alloys.

Facile Synthesis of Silver Nanoparticles with Application of Reproducible Surface Enhanced Raman Scattering Substrates

ABSTRACTSurface enhanced Raman scattering (SERS) allows the determination of analytes at low concentrations on silver substrates. A novel facile method is reported to reproducibly synthesize large area SERS substrates. A composite of silver nanoparticles was fabricated by drop casting silver nitrate followed by thermal reduction to immobilize the nanoparticles on the surface. The morphology of the nanoparticles was characterized by optical and scanning electron microscopy, and the composition was evaluated by X-ray photoelectron spectroscopy. Reproducible substrates were prepared using Rhodamine 6G at concentrations from 1 to 50 µM as the SERS probe with relative standard deviations of 22% for ten locations on one sample. An inverse correlation between probe concentration and precision was obtained. The influence of microstructure morphology on the reproducibility was characterized; the more uniform the morphology of the silver microstructure, the better the precision of the substrates. Some silver patterns provided better precision than others, showing the significance of this work for basic and applied SERS.

Synergetic Light-Harvesting and Near-Field Enhancement in Multiscale Patterned Gold Substrates

Sphere-segment void (SSV) cavities have emerged as promising substrates for reproducible Surface Enhanced Raman Scattering (SERS), offering strong and uniform electromagnetic enhancement associated with the excitation of cavity-like localized surface plasmon resonances tunable across the UV–vis-near IR range, with a facile large-scale fabrication process. High-resolution electron micrographs of these structures reveal a considerable departure from the idealized smooth spherical cavity shape; notably, the electrochemical deposition of gold yields an important surface roughness. We investigate this contribution to the SERS activity of SSV substrates with a series of experiments, varying the degree of surface roughness using thermal annealing and gradual electrochemical roughening. Notably, we observe that both roughness features and cavity-like modes operate in conjunction as a multiscale antenna to provide larger SERS efficiency than the two mechanisms considered separately. We conclude that the main role ...

High Aspect-Ratio Iridium-Coated Nanopillars for Highly Reproducible Surface-Enhanced Raman Scattering (SERS).

A variety of different gold and silver nanostructures have been proposed over the years as high sensitivity surface-enhanced Raman scattering (SERS) sensors. However, efficient use of SERS has been hindered by the difficulty of realizing SERS substrates that provide reproducible SERS response over the whole active area. Here, we show that atomic layer deposition (ALD) grown iridium can be used to produce highly reliable SERS substrates. The substrates are based on a periodic array of high aspect-ratio iridium coated nanopillars that feature efficient and symmetrically distributed hot spots within the interpillar gaps (gap width<10 nm). We show that the enhancement with the iridium based nanostructures is of significant magnitude and it equals the enhancement of silver based reference substrates. Most notably, we demonstrate that the ordered and well-defined plasmonic nanopillars offer a measurement-to-measurement variability of 5%, which paves the way for truly quantitative SERS measurements.

Highly reproducible SERS substrate based on polarization-free Ag nanoparticles decorated SiO2/Si core-shell nanowires array

SiO2/Si core-shell nanowires array coated with gap-rich silver nanoparticles were demonstrated as a highly reproducible surface-enhanced Raman scattering (SERS) substrate. SERS detection of a relative standard deviation of 8% for 10−4 M R6G with a spot size of ∼2 μm and 900 spots over an area of 150 × 150 μm2 was reported. The high reproducibility is ascribed to the polarization-independent electrical field distribution among three-dimensional nanowire structure with an optimized thickness of SiO2 shell layer.

Toward hyperuniform disordered plasmonic nanostructures for reproducible surface-enhanced Raman spectroscopy.

We report on the self-assembling of clusters of gold-nanoparticles (Au-NPs) directed by the phase separation of poly(styrene)-b-poly(methylmethacrylate) (PS-b-PMMA) block-copolymer (BCP) on indium tin oxide coated glass, which induces the onset of vertical lamellar domains. After thermal evaporation of gold on BCP, Au-NPs of 4 nm are selectively included into PS-nanodomains by thermal annealing, and then clustered with large density of hot spots (> 10(4) μm(2)) in a random two-dimensional pattern. The resulting nanostructure exhibits near-hyperuniform long-range correlations. The consequent large degree of homogeneity of this isotropic plasmonic pattern gives rise to a highly reproducible Surface-Enhanced Raman Scattering (SERS) enhancement factor over the centimeter scale (std. dev. ∼ 10% over 0.25 cm(2)). We also discuss the application of a static electric field for modulating the BCP host morphology. The electric field induces an alignment of Au-NP clusters into ordered linear chains, exhibiting a stronger SERS activity, but reduced SERS spatial reproducibility.

Side-to-side alignment of gold nanorods with polarization-free characteristic for highly reproducible surface enhanced Raman scattering

A large-batch method for fabricating large-area, low-cost, and flexible surface enhanced Raman scattering (SERS) arrays with Au nanorods (NRs) using an inkjet printing technique was presented. The Raman mapping results of 441 points reveal clearly that the Au NRs arrays are a well reproducible SERS substrate for the detection of various chemicals. The neighbouring Au NRs aligned side-to-side nanoclusters were formed on printer paper as a highly reproducible SERS platform due to its polarization-free properties which were further confirmed by theoretical calculations of the electric field enhancement change about only two. Therefore, the SERS signals can be more evenly enhanced.