Self-organized hexagonal-nanopore SERS array

A robust SERS calibration using a pseudo-internal intensity reference

Surface-enhanced Raman scattering (SERS) with high molecular sensitivity and specificity is a powerful nondestructive analytical tool. Since its discovery, SERS measurements have suffered from the vulnerability of calibration curve, which makes quantification analysis a great challenge. In this work, we report a robust calibration method by introducing a referenced measurement as the intensity standard. This intensity reference not only has the advantages of the internal standard method such as reflecting the SERS substrate enhancement, but also avoids the introduction of competing adsorption between target molecules and the internal standard. Based on the normalized calibration curve, the magnitude of the R6G concentration can be well evaluated from 10^-7 M to 10^-12 M. Furthermore, we demonstrate that this pseudo-internal standard method can also work well using a different type of molecule as the reference. This SERS calibration method would be beneficial for the development of quantitative SERS analysis.

Trends in Application of SERS Substrates beyond Ag and Au, and Their Role in Bioanalysis

This article compares the applications of traditional gold and silver-based SERS substrates and less conventional (Pd/Pt, Cu, Al, Si-based) SERS substrates, focusing on sensing, biosensing, and clinical analysis. In recent decades plethora of new biosensing and clinical SERS applications have fueled the search for more cost-effective, scalable, and stable substrates since traditional gold and silver-based substrates are quite expensive, prone to corrosion, contamination and non-specific binding, particularly by S-containing compounds. Following that, we briefly described our experimental experience with Si and Al-based SERS substrates and systematically analyzed the literature on SERS on substrate materials such as Pd/Pt, Cu, Al, and Si. We tabulated and discussed figures of merit such as enhancement factor (EF) and limit of detection (LOD) from analytical applications of these substrates. The results of the comparison showed that Pd/Pt substrates are not practical due to their high cost; Cu-based substrates are less stable and produce lower signal enhancement. Si and Al-based substrates showed promising results, particularly in combination with gold and silver nanostructures since they could produce comparable EFs and LODs as conventional substrates. In addition, their stability and relatively low cost make them viable alternatives for gold and silver-based substrates. Finally, this review highlighted and compared the clinical performance of non-traditional SERS substrates and traditional gold and silver SERS substrates. We discovered that if we take the average sensitivity, specificity, and accuracy of clinical SERS assays reported in the literature, those parameters, particularly accuracy (93-94%), are similar for SERS bioassays on AgNP@Al, Si-based, Au-based, and Ag-based substrates. We hope that this review will encourage research into SERS biosensing on aluminum, silicon, and some other substrates. These Al and Si based substrates may respond efficiently to the major challenges to the SERS practical application. For instance, they may be not only less expensive, e.g., Al foil, but also in some cases more selective and sometimes more reproducible, when compared to gold-only or silver-only based SERS substrates. Overall, it may result in a greater diversity of applicable SERS substrates, allowing for better optimization and selection of the SERS substrate for a specific sensing/biosensing or clinical application.

Machine Learning-Driven 3D Plasmonic Cavity-in-Cavity Surface-Enhanced Raman Scattering Platform with Triple Synergistic Enhancement Toward Label-Free Detection of Antibiotics in Milk

The surface-enhanced Raman scattering (SERS) technique with ultrahigh sensitivity has gained attention to meet the increasing demands for food safety analysis. The integration of machine learning and SERS facilitates the practical applicability of sensing devices. In this study, a machine learning-driven 3D plasmonic cavity-in-cavity (CIC) SERS platform is proposed for sensitive and quantitative detection of antibiotics. The platform is prepared by transferring truncated concave nanocubes (NCs) to an obconical-shaped template surface. Owing to the triple synergistic enhancement effect, the highly ordered 3D CIC arrays improve the simulated electromagnetic field intensity and experimental SERS activity, demonstrating a 33.1-fold enhancement compared to a typical system consisting of Au NCs deposited on a flat substrate. The integration of machine learning and Raman spectroscopy eliminates subjective judgments on the concentration of detectors using a single feature peak and achieves accurate identification. The machine learning-driven CIC SERS platform is capable of detecting ampicillin traces in milk with a detection limit of 0.1 ppm, facilitating quantitative analysis of different concentrations of ampicillin. Therefore, the proposed platform has potential applications in food safety monitoring, health care, and environmental sampling.

Review on combining surface-enhanced Raman spectroscopy and electrochemistry for analytical applications

The discovery of surface enhanced Raman scattering (SERS) from an electrochemical (EC)-SERS experiment is known as a historic breakthrough. Five decades have passed and Raman spectroelectrochemistry (SEC) has developed into a common characterization tool that provides information about the electrode-electrolyte interface. Recently, this technique has been successfully explored for analytical purposes. EC was found to highly improve the performances of SERS sensors, providing, among others, controlled adsorption of analytes and increased reproducibility. In this review, we highlight the potential of EC-SERS sensors to be implemented for point-of-need (PON) analyses as miniaturized devices, and their ability to revolutionize fields like quality control, diagnosis or environmental and food safety. Important developments have been achieved in Raman spectroelectrochemistry, which now represents a promising alternative to conventional analytical methods and interests more and more researchers. The studies included in this review open endless possibilities for real-life EC-SERS analytical applications.

Conceptual Progress for Explaining and Predicting Self-Organization on Anodized Aluminum Surfaces

Over the past few years, researchers have made numerous breakthroughs in the field of aluminum anodizing and faced the problem of the lack of adequate theoretical models for the interpretation of some new experimental findings. For instance, spontaneously formed anodic alumina nanofibers and petal-like patterns, flower-like structures observed under AC anodizing conditions, and hierarchical pores whose diameters range from several nanometers to sub-millimeters could be explained neither by the classical field-assisted dissolution theory nor by the plastic flow model. In addition, difficulties arose in explaining the basic indicators of porous film growth, such as the nonlinear current–voltage characteristics of electrochemical cells or the evolution of hexagonal pore patterns at the early stages of anodizing experiments. Such a conceptual crisis resulted in new multidisciplinary investigations and the development of novel theoretical models, whose evolution is discussed at length in this review work. The particular focus of this paper is on the recently developed electroconvection-based theories that allowed making truly remarkable advances in understanding the porous anodic alumina formation process in the last 15 years. Some explanation of the synergy between electrode reactions and transport processes leading to self-organization is provided. Finally, future prospects for the synthesis of novel anodic architectures are discussed.

Three-dimensional porous SERS powder for sensitive liquid and gas detections fabricated by engineering dense "hot spots" on silica aerogel

A three-dimensional porous SERS powder material, Ag nanoparticles-engineered-silica aerogel, was developed. Utilizing an in situ chemical reduction strategy, Ag nanoparticles were densely assembled on porous aerogel structures, thus forming three-dimensional "hot spots" distribution with intrinsic large specific surface area and high porosity. These features can effectively enrich the analytes on the metal surface and provide huge near field enhancement. Highly sensitive and homogeneous SERS detections were achieved not only on the conventional liquid analytes but also on gas with the enhancement factor up to ∼10⁸ and relative standard deviation as small as ∼13%. Robust calibration curves were obtained from the SERS data, which demonstrates the potential for the quantification analysis. Moreover, the powder shows extraordinary SERS stability than the conventional Ag nanostructures, which makes long term storage and convenient usage feasible. With all of these advantages, the porous SERS powder material can be extended to on-site SERS "nose" applications such as liquid and gas detections for chemical analysis, environmental monitoring, and anti-terrorism.

Convex-Meniscus-Assisted Self-Assembly at the Air/Water Interface to Prepare a Wafer-Scale Colloidal Monolayer Without Overlap

Self-assembly at the air/water interface (AWI) has proven to be an efficient strategy for fabricating two-dimensional (2D) colloidal monolayers, which was widely used as the template for nanosphere lithography in nanophononics, optofluidics, and solar cell studies. However, the monolayers fabricated at the AWI usually suffer from a small domain area and quasi-double layer structure caused by submerged particles. To overcome this, we proposed an improved protocol to prepare 2D colloidal monolayers free of overlapping nanospheres at the AWI. Utilizing the stable suspension infusion to the water surface, a convex meniscus, whose height is related to viscous force, was formed adjoining the three-phase boundary. As a result of the resistance of the convex meniscus, the polystyrene nanospheres in the initial suspension directly self-assembled into a preliminary monolayer, which proved effective in preventing nanospheres' sinking and increasing the colloidal crystal domain size. An optimal parameter for transferring the monolayer was also developed based on the numerical simulation results. Finally, a wafer-scale monolayer, covered with less than one nanosphere per 100 μm × 100 μm area, was achieved on the desired substrate with an average domain size attaining centimeter scale. The high-quality 2D colloidal crystal may further promote the application of nanosphere lithography, especially in the fields that require a defect-free template.

Controlling the Multiscale Topography of Anodized Aluminum Oxide Nanowire Structures for Surface-Enhanced Raman Scattering and Perfect Absorbers

In this study, a plasmonically active substrate is developed with the aim of controlling the perfect absorption and manipulating its optical properties for application in SERS (in NIR regime) and colorimetry. Based on modified fabrication method of anodized aluminum oxide (AAO), the cost-effective self-aggregation technique is presented to fabricate unique topography of bone-fire-like funnel-shaped collapsed and vertically aligned nanowire structures. The length of the nanowire and the modification of surface topography induced by capillary force inside the nanowire are set to structural parameters, and the effect of their changes is closely studied. After deposition of 40 nm gold (Au) film on numerous AAO nanowire structures with different wire lengths and unique topography, the localized surface plasmon resonance excitation is generated, and also its application on reflection and SERS spectra have been shown quantitatively. The length of the wire and surface topography modification are identified as suitable parameters to tune the reflection/absorption (from <40 to >90%) as well as colorimetric effect. Finally, an optimized wire length of Au-coated AAO substrate in SERS sensing application with 3.92 × 10⁵ order of enhancement of rhodamine 6G (R6G) Raman signal is demonstrated.

Stimuli-responsive microgels for controlled deposition of gold nanoparticles on surfaces

A variety of gold nanoparticle (AuNP) core/poly(N-isopropylacrylamide) (pNIPAm) shell microgels (Au@pNIPAm) were generated using seed-mediated polymerization. The shell thickness and AuNP core diameter were easily tunable at the time of synthesis. The resultant Au@pNIPAm microgels were characterized via photon-correlation spectroscopy, transmission electron microscopy and ultraviolet-visible spectroscopy. AuNP arrays were generated by "painting" the microgels on a surface, using the shell thickness to define the distance between the AuNPs, followed by shell removal via plasma etching. We found that when the pNIPAm shell thickness decreased (via its tuning at the time of synthesis or deposition at elevated temperature at which the shell is collapsed) the AuNPs were closer to one another. We also showed that via sequential deposition Au@pNIPAm microgels with different AuNP core sizes could be deposited on a single surface. The presented "painting protocol" offers a facile way to coat large area surfaces quickly which is not easily achievable using other approaches. We envision that this approach is extremely versatile, allowing a number of different nanomaterials embedded in pNIPAm shells to be deposited/patterned on surfaces. With the control over the deposition on the surface that we show here, we hope that the Au@pNIPAm microgels will find use in lithography/surface patterning applications.

Fabrication of nano/microstructures for SERS substrates using an electrochemical method

Based on an electrochemical method, three-dimensional arrayed nanopore structures are machined onto a Mg surface. The structured Mg surface is coated with a thin gold (Au) film, which is used as a surface-enhanced Raman scattering (SERS) substrate. A rhodamine 6G (R6G) probe molecule is used as the detection agent for the SERS measurement. Different sizes of arrayed micro/nanostructures are fabricated by different treatment time using the electrochemical process. The topographies of these micro/nanostructures and the thickness of the Au film have an influence on the Raman intensity of the Mg substrate. Furthermore, when the thickness of Au film coating is held constant, the Raman intensity on the structured Mg substrates is about five times higher after a treatment time of 1 min when compared with other treatment times. The SERS enhancement factor ranges from 10⁶ to 1.75 × 10⁷ under these experimental conditions. Additionally, a 10^-6 mol·L^-1 solution of lysozyme was successfully detected using the Mg-Au nanopore substrates. Our low-cost method is reproducible, homogeneous, and suitable for the fabrication of SERS substrates.

A ten-minute, single step, label-free, sample-to-answer assay for qualitative detection of cytokines in serum at femtomolar levels

Label-free electronic affinity based immuno-sensing is an attractive candidate as a platform technology for analyzing biomarkers due to the ease of miniaturization and minimal use of reagents. Electronic based sensing approaches, however, have lagged behind their optical counterparts in terms of detection limit, selectivity, and reliability. Also, the matrix dependent nature of electronic sensing modalities makes difficult the analysis of biomarkers in high salt concentration samples such as serum due to charge screening. We present a novel sensing platform, the micro-well sensor, that works by functionalizing nanoscale volume wells with antibodies and monitoring the impedance change inside the wells due binding of target protein. This detection modality is advantageous to many label-free electronic sensors in that signal power scales with increase in salt concentration, thus improving the sensitivity of the platform. We demonstrate rapid label-free qualitative detection of cytokines within ten minutes at femtoMolar concentrations and a dynamic range of 3 orders of magnitude in serum samples. We describe the design, fabrication, and characterization of the micro-well sensor in serum samples using inflammatory protein biomarkers.

Fabrication of homogeneous waffle-like silver composite substrate for Raman determination of trace chloramphenicol

Waffle-like anodized aluminum oxide homogeneously immobilized with Ag nanoparticles (AAO/Ag) is rationally designed and fabricated as surface-enhanced Raman scattering (SERS) substrate. The as-prepared SERS substrate is characterized with transmission electron microscope (TEM), scanning electron microscopy (SEM), UV-Vis spectrophotometer, and Fourier transform infrared spectrometer (FT-IR). The AAO/Ag substrate shows good uniformity of the Raman signals (RSD = 7.02%) due to waffle-like AAO supporting the well-dispersed Ag nanoparticles. For real application, the AAO/Ag substrate is used for rapid determination of chloramphenicol (CAP) in honey with low detection limit (4.0 × 10^-9 mol L^-1) and good linearity from 1.0 × 10^-5 to 1.0 × 10^-8 mol L^-1 based on the SERS peak at 1348 cm^-1. The better accumulation in the short pore path of AAO improves the target molecule approaching into the vicinity of hot spots of Ag nanoparticles. The high selectivity for CAP is attributed to the strong interaction between -NO₂ group in CAP and the composite substrate. Schematic representation of the preparation of SERS substrate, AAO₁₅₀/Ag₁₀-5 composite nanoparticles, and antibiotic determination.

Superhydrophobic-Superhydrophilic Hybrid Surface with Highly Ordered Tip-Capped Nanopore Arrays for Surface-Enhanced Raman Scattering Spectroscopy

The designed superhydrophobic-superhydrophilic hybrid surface (SSHS) with highly ordered tip-capped nanopore arrays can be used as an intelligent and fast platform to realize different analyte solutions with different concentrations to be detected at the same time by surface-enhanced Raman spectroscopy. This surface is fabricated in a large area by a facile and low-cost method of programmed multistep anodization of aluminum and pore widening process followed by selective chemical modification. The highly ordered tip-capped nanopore arrays can induce the highly sensitive and reproducible Raman signal, whose enhanced factor for rhodamine 6G (R6G) at 1358 cm^-1 is 4.46 × 10⁶. The superhydrophobic-superhydrophilic hybrid property can realize the homogeneous distribution of the concentrated analyte in a droplet at the fixed place, which can avoid the diffusion-limit problem and further enhance the Raman signal. Surface-enhanced Raman spectroscopy of dried droplets with different concentrations of R6G or thiram is tested on SSHS, which show good reproducibility. The detection limits of R6G and thiram on SSHS are 10^-10 and 10^-7 M in 50 μL droplets, respectively. Due to the industrial compatibility of the fabrication technique, this smart surface has the potential to evolve into a general platform to develop various advanced chemical and biological sensors.

Label-free SERS diagnostics of radiation-induced injury via detecting the biomarker Raman signal in the serum and urine bio-samples based on Au-NPs array substrates

A sensitive approach based on surface enhanced Raman spectroscopy (SERS) has been developed to evaluate the radiation caused biological injury. To achieve the effective SERS substrate, canonical anodic aluminum oxide (AAO) templates with regular array of nanotips were fabricated, and by plasma sputtering the gold nanoparticles (Au-NPs) were distributed on the nanotips to form the Au-NPs array with plenty of hotspots. The SERS substrates were utilized to examine the serum samples taken from the mice with the treatment of total body irradiation (TBI) of X-ray. The impact of TBI on the mice was analyzed and it was found that the SERS peak intensity at 532 cm^-1 increased as a function of duration or dose of TBI. We confirmed that this Raman signature belongs to the myoglobin as a biomarker for the muscle damage due to the radiation caused injury. Furthermore, we also tested several blood and urine specimen of cancer patients who received radiotherapy. The results showed that our approach to some extent could distinguish the bio-samples from normal, X-ray treated and untreated individuals. Therefore, the proposed methodology may have the potential for prompt prognosis of radiation injury at early stage.

Cost-Effective and High-Throughput Plasmonic Interference Coupled Nanostructures by Using Quasi-Uniform Anodic Aluminum Oxide

Large-area and uniform plasmonic nanostructures have often been fabricated by simply evaporating noble metals such as gold and silver on a variety of nanotemplates such as nanopores, nanotubes, and nanorods. However, some highly uniform nanotemplates are limited to be utilized by long, complex, and expensive fabrication. Here, we introduce a cost-effective and high-throughput fabrication method for plasmonic interference coupled nanostructures based on quasi-uniform anodic aluminum oxide (QU-AAO) nanotemplates. Industrial aluminum, with a purity of 99.5%, and copper were used as a base template and a plasmonic material, respectively. The combination of these modifications saves more than 18 h of fabrication time and reduces the cost of fabrication 30-fold. From optical reflectance data, we found that QU-AAO based plasmonic nanostructures exhibit similar optical behaviors to highly ordered (HO) AAO-based nanostructures. By adjusting the thickness of the AAO layer and its pore size, we could easily control the optical properties of the nanostructures. Thus, we expect that QU-AAO might be effectively utilized for commercial plasmonic applications.

Recognition of plastic nanoparticles using a single gold nanopore fabricated at the tip of a glass nanopipette

A single gold nanopore with high surface enhanced Raman spectroscopy (SERS) activity is fabricated on the tip of a glass nanopipette.

Controlling surface morphology and sensitivity of granular and porous silver films for surface-enhanced Raman scattering, SERS

The design of efficient substrates for surface-enhanced Raman spectroscopy (SERS) for large-scale fabrication at low cost is an important issue in further enhancing the use of SERS for routine chemical analysis. Here, we systematically investigate the effect of different radio frequency (rf) plasmas (argon, hydrogen, nitrogen, air and oxygen plasma) as well as combinations of these plasmas on the surface morphology of thin silver films. It was found that different surface structures and different degrees of surface roughness could be obtained by a systematic variation of the plasma type and condition as well as plasma power and treatment time. The differently roughened silver surfaces act as efficient SERS substrates showing greater enhancement factors compared to as prepared, sputtered, but untreated silver films when using rhodamine B as Raman probe molecule. The obtained roughened silver films were fully characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron (XPS and Auger) and ultraviolet-visible spectroscopy (UV-vis) as well as contact angle measurements. It was found that different morphologies of the roughened Ag films could be obtained under controlled conditions. These silver films show a broad range of tunable SERS enhancement factors ranging from 1.93 × 102 to 2.35 × 105 using rhodamine B as probe molecule. The main factors that control the enhancement are the plasma gas used and the plasma conditions, i.e., pressure, power and treatment time. Altogether this work shows for the first time the effectiveness of a plasma treatment for surface roughening of silver thin films and its profound influence on the interface-controlled SERS enhancement effect. The method can be used for low-cost, large-scale production of SERS substrates.

Novel strategy for fabrication of sensing layer on thiol-functionalized fiber-optic tapers and their application as SERS probes

This work presents a new strategy to fabricate optical fiber surface-enhanced Raman scattering (SERS) probes with high-performance remote sensing prepared by thiol functionalization of silica fiber taper, and further in situ nucleation and growth of silver nanoparticles (AgNPs). The prepared fiber probes can effectively identify the analyte 4-aminothiophenol (4-ATP) with a limit of detection (LOD) as low as 2.15 × 10^-11 M using a portable commercial Raman spectrometer. Simultaneously, such fiber probes have shown a good reproducibility with the relative standard deviation (RSD) value of 7.6%, and possessed high signal stability at room temperature over one month. Furthermore, this approach provides new insight into the fabrication of fiber SERS probe integrated the advantages in terms of sensitivity, reproducibility and stability, which shows great potential for practical SERS applications.

Enhanced Raman scattering in porous silicon grating

The enhancement of Raman signal on monocrystalline silicon gratings with varying groove depths and on porous silicon grating were studied for a highly sensitive surface enhanced Raman scattering (SERS) response. In the experiment conducted, porous silicon gratings were fabricated. Silver nanoparticles (Ag NPs) were then deposited on the porous silicon grating to enhance the Raman signal of the detective objects. Results show that the enhancement of Raman signal on silicon grating improved when groove depth increased. The enhanced performance of Raman signal on porous silicon grating was also further improved. The Rhodamine SERS response based on Ag NPs/ porous silicon grating substrates was enhanced relative to the SERS response on Ag NPs/ porous silicon substrates. Ag NPs / porous silicon grating SERS substrate system achieved a highly sensitive SERS response due to the coupling of various Raman enhancement factors.

Application of nanotechnology in biosensors for enhancing pathogen detection

Rapid detection and identification of pathogenic microorganisms is fundamental to minimizing the spread of infectious disease, and informing clinicians on patient treatment strategies. This need has led to the development of enhanced biosensors that utilize state of the art nanomaterials and nanotechnology, and represent the next generation of diagnostics. A primer on nanoscale biorecognition elements such as, nucleic acids, antibodies, and their synthetic analogs (molecular imprinted polymers), will be presented first. Next the application of various nanotechnologies for biosensor transduction will be discussed, along with the inherent nanoscale phenomenon that leads to their improved performance and capabilities in biosensor systems. A future outlook on characterization and quality assurance, nanotoxicity, and nanomaterial integration into lab-on-a-chip systems will provide the closing thoughts. This article is categorized under: Diagnostic Tools > Diagnostic Nanodevices Diagnostic Tools > Biosensing.

Large-scale fabrication of highly ordered sub-20 nm noble metal nanoparticles on silica substrates without metallic adhesion layers

Periodic noble metal nanoparticles offer a wide spectrum of applications including chemical and biological sensors, optical devices, and model catalysts due to their extraordinary properties. For sensing purposes and catalytic studies, substrates made of glass or fused-silica are normally required as supports, without the use of metallic adhesion layers. However, precise patterning of such uniform arrays of silica-supported noble metal nanoparticles, especially at sub-100 nm in diameter, is challenging without adhesion layers. In this paper, we report a robust method to large-scale fabricate highly ordered sub-20 nm noble metal nanoparticles, i.e., gold and platinum, supported on silica substrates without adhesion layers, combining displacement Talbot lithography (DTL) with dry-etching techniques. Periodic photoresist nanocolumns at diameters of ~110 nm are patterned on metal-coated oxidized silicon wafers using DTL, and subsequently transferred at a 1:1 ratio into anti-reflection layer coating (BARC) nanocolumns with the formation of nano-sharp tips, using nitrogen plasma etching. These BARC nanocolumns are then used as a mask for etching the deposited metal layer using inclined argon ion-beam etching. We find that increasing the etching time results in cone-shaped silica features with metal nanoparticles on the tips at diameters ranging from 100 nm to sub-30 nm, over large areas of 3×3 cm². Moreover, subsequent annealing these sub-30 nm metal nanoparticle arrays at high-temperature results in sub-20 nm metal nanoparticle arrays with ~10¹⁰ uniform particles.

High-performance surface-enhanced Raman scattering substrate prepared by self-assembling of silver nanoparticles into the nanogaps of silver nanoislands

We report an effective and simple method to further enhance the surface-enhanced Raman scattering (SERS) by silver (Ag) nanoparticles (AgNPs) self-assembling into the nanogaps of an Ag nanoisland (AgNIs). The AgNIs prepared by dewetting of Ag film created a nanorough surface, which induced the Ag nanoparticles to regularly deposit into the nanogaps. AgNPs and AgNIs samples were also prepared for comparative analysis. Their SERS activities were investigated theoretically and experimentally. Experimental enhancement factors (EFs) for AgNPs, AgNIs, and AgNPs decorated AgNIs substrate (AgNPs-AgNIs) were ∼10⁷, ∼10⁶, ∼10⁸, respectively, with relative standard deviation (RSD) of 66.1%, 12.9%, and 13.2%. Remarkable enhancement (EF≈10⁸) and excellent reproducibility (RSD=13.2%) indicated the AgNPs-AgNIs had a high potential in practical application. Electromagnetic simulation using COMSOL Multiphysics demonstrated that the additional enhancement of the SERS effect could be mainly attributed to the improvement of the local electromagnetic field. Moreover, the deposition process of Ag nanoparticles was analyzed in detail to understand the reproducibility of AgNPs-AgNIs.

Evaporating metal nanocrystal arrays

Anodic aluminum oxide (AAO) substrates with a self-ordered triangular array of nanopores provide the means to fabricate multiple forms of nano materials, such as nanowires and nanoparticles. This study focuses on nanostructures that emerge in thin films of metals thermally evaporated onto the surface of AAO. Previous work showed that films of different evaporated metals assume dramatically different structures, e.g. an ordered triangular array of nearly monodisperse nanoparticles forms for lead (Pb) while a polycrystalline nanohoneycomb structure forms for silver (Ag). Here, we present investigations of the effects of substrate temperature and deposition angle that reveal the processes controlling the nano particle array formation. Our findings indicate that arrays form provided the grain nucleation density exceeds the pore density and the atomic mobility is high enough to promote grain coalescence. They introduce a method for producing films with anisotropic grain array structure. The results provide insight into the influence of substrate nano-morphology on thin film growth energetics and kinetics that can be harnessed for creating films with other novel nano-structures.

Ultrasensitive SERS performance in 3D "sunflower-like" nanoarrays decorated with Ag nanoparticles

Low-cost, stabilized and ultrasensitive three-dimensional (3D) hierarchical surface-enhanced Raman scattering substrates ("sunflower-like" nanoarrays decorated with Ag nanoparticles, denoted as SLNAs-Ag) have been obtained by fabricating binary colloidal crystals and then decorating with Ag nanoparticles. In order to provide a larger density of hot spots within the laser-illumination area, the silica sphere arrays were chosen as the island-type platform for the polystyrene (PS) nanosphere deposition, and the distances between the PS nanospheres were tuned by etching for different durations. Compared with conventional 2D planar systems, the as-fabricated 3D SLNAs-Ag exhibited extremely high SERS sensitivity ascribed to the larger SERS active regions. Quantitative detection of molecules with an extremely low incident laser power was achieved on the "sunflower-like" nanoarrays in which the PS nanospheres were etched for 5 minutes and decorated with Ag nanoparticles, and the corresponding analytical enhancement factor is calculated to be 2 × 10¹⁴ with the concentration of rhodamine 6G down to 10^-15 M. Based on the achieved SERS substrates, we have further demonstrated the highly sensitive detection of molecules such as melamine for food safety inspection.

Nanoporous Silver Film Fabricated by Oxygen Plasma: A Facile Approach for SERS Substrates

Nanoporous metal films are promising substrates for surfaced-enhanced Raman scattering (SERS) measurement, owing to their homogeneity, large surface area, and abundant hot-spots. Herein, a facile procedure was developed to fabricate nanoporous Ag film on various substrate surfaces. Thermally deposited Ag film was first treated with O2 plasma, resulting in porous Ag/AgxO film (AgxO-NF) with nanoscale feature. Sodium citrate was then used to reduce AgxO to Ag, forming nanoporous Ag film (AgNF) with similar morphology. The AgNF substrate demonstrates 30-fold higher Raman intensity than Ag film over polystyrene nanospheres (d = 600 nm) using 4-mercaptobenzoic acid (4-MBA) as the sensing molecule. Comparing with ordinary Raman measurement on 4-MBA solution, an enhancement factor of ∼6 × 10(6) was determined for AgNF. The AgNF substrate was evaluated for benzoic acid, 4-nitrophenol, and 2-mercaptoethanesulfonate, showing high SERS sensitivity for chemicals that bind weakly to Ag surface and molecules with relatively small Raman cross section at micromolar concentration. In addition to its simplicity, the procedure can be applied to various materials such as transparency film, filter paper, hard polystyrene film, and aluminum foil, revealing similar Raman sensitivity. By testing the durability of the substrate, we found that the AgxO films can be stored in ambient conditions for more than 90 days and still deliver the same SERS intensity if the films are treated with sodium citrate before use. These results demonstrate the advantage of the proposed approach for mass production of low-cost, sensitive, and durable SERS substrates. The transferable nature of these AgNF to different flexible surfaces also allows their easy integration with other sensing schemes.

Tailoring Size and Coverage Density of Silver Nanoparticles on Monodispersed Polymer Spheres as Highly Sensitive SERS Substrates

Silver nanoparticles (AgNPs) were deposited onto the monodispersed carboxylic polystyrene (CPS) spheres by an improved in situ reduction method. The size and coverage density of the AgNPs on the surface of CPS spheres could be easily tailored by tuning the concentrations of carboxylic functional groups and silver precursor. The morphologies and structures of the resulting CPS/Ag hybrid particles were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-Vis-NIR spectrometer and X-ray photoelectron spectroscopy (XPS), etc. The surface enhanced Raman scattering (SERS) performances of the resulting uniform CPS/Ag hybrid particles were investigated using 4-aminobenzenethiol (4-ABT) as the probe molecule. The optimized CPS/Ag hybrid particles show high enhancement factor (EF) of 2.71×10(7) , low limit of detection (LOD) of 10(-10)  m and good reproducibility with relative standard deviation (RSD) of 9.64 %. The good SERS improvement properties demonstrate these hybrid particles could be employed as simple and effective substrates in the SERS spectroscopy.

SERS activity with tenfold detection limit optimization on a type of nanoporous AAO-based complex multilayer substrate

Most of SERS applications are constricted by heterogeneous hotspots and aggregates of nanostructure, which result in low sensitivity and poor reproducibility of characteristic signals. This work intends to introduce SERS properties of a type of SERS-active substrate, Au-CuCl2-AAO, which is innovatively developed on a porous anodic alumina oxide (AAO) template. Spectral measuring results of Rhodamine 6G (R6G) on this substrate optimized by controlling morphology and gold thickness showed that enhancement factor (2.30 × 10(7)) and detection limit (10(-10) M) were both improved and represented better performance than its template AAO. Homogenous hot spots across the region of interest were achieved by scanning SERS intensity distribution for the band at 1505 cm(-1) in 5 × 5 μm(2) area. Furthermore, the promising SERS activity of the flower-patterned substrate was theoretically explained through simulation of the electromagnetic field distribution. In addition, this SERS substrate is proposed for applications within the field of chemical and biochemical analyses.

Elevated gold ellipse nanoantenna dimers as sensitive and tunable surface enhanced Raman spectroscopy substrates

We demonstrate large area arrays of elevated gold ellipse dimers with precisely controlled gaps for use as sensitive and highly controllable surface enhanced Raman scattering (SERS) substrates. The enhanced Raman signal observed with SERS arises from both localized and long range plasmonic effects. By controlling the geometry of a SERS substrate, in this case the size and aspect ratio of individual ellipses, the plasmon resonance can be tuned in a broad wavelength range, providing a method for designing the response of SERS substrates at different excitation wavelengths. Plasmon effects exhibited by the elevated gold ellipse dimer substrates are also demonstrated and confirmed through finite difference time domain (FDTD) simulations. A plasmon resonance red shift with an increase of the ellipse aspect ratio is observed, allowing systematic control of the resulting SERS signal intensity. Optimized elevated ellipse dimer substrates with 10 ± 2 nm gaps exhibit uniform SERS enhancement factors on the order of 10(9) for adsorbed p-mercaptoaniline molecules.

Superhydrophobic SERS Substrates Based on Silver-Coated Reduced Graphene Oxide Gratings Prepared by Two-Beam Laser Interference

Reported here is the fabrication of reduced graphene oxide (RGO) grating structures by two-beam laser interference (TBLI) for the development of highly efficient SERS substrates via simple physical vapor deposition (PVD) coating of silver. TBLI has been utilized to make hierarchical RGO grating structures with microscale gratings and nanoscale folders through a laser treatment induced ablation and photoreduction process. The hierarchical structures contribute to the formation of plasmonic structures after silver coating, giving rise to the formation of plenty of SERS "hot spots", while the RGO substrate would provide chemical enhancement of Raman signal through interaction with analytes molecules. The significantly increased roughness with respect to the hierarchical structures in combination with the removal of hydrophilic oxygen-containing groups endow the resultant substrates with unique superhydrophobicity, which leads to the enrichment of analytes and further lowers the detection limit. The synergistic effects make the silver coated RGO gratings a highly efficient SERS substrate; in the detection of Rhodamine B, our SERS substrates showed high SERS enhancement and good reproducibility, a detection limit of 10(-10) M has been achieved.

Fabrication of silver nanoplate hierarchical turreted ordered array and its application in trace analyses

Silver nanoplate hierarchical turreted ordered arrays were fabricated through an electro-deposition method on ordered acuate silicon nanocone templates. Such arrays can be used as SERS substrates for trace analyses of streptomycin sulphate, and exhibit high activity and stability. This work is of importance in practical applications based on the SERS effect of noble metal micro/nano-structured arrays.

Mechanically Self-Assembled, Three-Dimensional Graphene-Gold Hybrid Nanostructures for Advanced Nanoplasmonic Sensors

Hybrid structures of graphene and metal nanoparticles (NPs) have been actively investigated as higher quality surface enhanced Raman spectroscopy (SERS) substrates. Compared with SERS substrates, which only contain metal NPs, the additional graphene layer provides structural, chemical, and optical advantages. However, the two-dimensional (2D) nature of graphene limits the fabrication of the hybrid structure of graphene and NPs to 2D. Introducing three-dimensionality to the hybrid structure would allow higher detection sensitivity of target analytes by utilizing the three-dimensional (3D) focal volume. Here, we report a mechanical self-assembly strategy to enable a new class of 3D crumpled graphene-gold (Au) NPs hybrid nanoplasmonic structures for SERS applications. We achieve a 3D crumpled graphene-Au NPs hybrid structure by the delamination and buckling of graphene on a thermally activated, shrinking polymer substrate. We also show the precise control and optimization of the size and spacing of integrated Au NPs on crumpled graphene and demonstrate the optimized NPs' size and spacing for higher SERS enhancement. The 3D crumpled graphene-Au NPs exhibits at least 1 order of magnitude higher SERS detection sensitivity than that of conventional, flat graphene-Au NPs. The hybrid structure is further adapted to arbitrary curvilinear structures for advanced, in situ, nonconventional, nanoplasmonic sensing applications. We believe that our approach shows a promising material platform for universally adaptable SERS substrate with high sensitivity.

Highly Reproducible and Sensitive SERS Substrates with Ag Inter-Nanoparticle Gaps of 5 nm Fabricated by Ultrathin Aluminum Mask Technique

Applicable surface enhanced Raman scattering (SERS) active substrates require high enhancement factor (EF), excellent spatial reproducibility, and low-cost fabrication method on a large area. Although several SERS substrates with high EF and relative standard deviation (RSD) of signal less than 5% were reported, reliable fabrication for large area SERS substrates with both high sensitivity and high reproducibility via low-cost routes remains a challenge. Here, we report a facile and cost-effective fabrication process for large-scale SERS substrate with Ag inter-nanoparticle (NP) gaps of 5 nm based on ultrathin alumina mask (UTAM) surface pattern technique. Such closely packed Ag NP arrays with high density of electromagnetic field enhancement ("hot spots") on large area exhibit high SERS activity and excellent reproducibility, simultaneously. Rhodamine 6G molecules with concentration of 1 × 10(-7) M are used to determine the SERS performance, and an EF of ∼10(9) is obtained. It should be noted that we obtain RSDs about 2% from 10 random spots on an area of 1 cm(2), which implies the highly reproducible signals. Finite-difference time-domain simulations further suggest that the enhanced electric field originates from the narrow gap, which agrees well with the experimental results. The low value of RSD and the high EF of SERS signals indicate that the as-prepared substrate may be promising for highly sensitive and uniform SERS detection.

Fabrication and robotization of ultrasensitive plasmonic nanosensors for molecule detection with Raman scattering

In this work, we introduce the history and mechanisms of surface enhanced Raman scattering (SERS), discuss various techniques for fabrication of state-of-the-art SERS substrates, and review recent work on robotizing plasmonic nanoparticles, especially, the efforts we made on fabrication, characterization, and robotization of Raman nanosensors by design. Our nanosensors, consisting of tri-layer nanocapsule structures, are ultrasensitive, well reproducible, and can be robotized by either electric or magnetic tweezers. Three applications using such SERS nanosensors were demonstrated, including location predictable detection, single-cell bioanalysis, and tunable molecule release and monitoring. The integration of SERS and nanoelectromechanical system (NEMS) devices is innovative in both device concept and fabrication, and could potentially inspire a new device scheme for various bio-relevant applications.

Morphology genetic materials templated from natural species

The structural characteristics of natural species have been optimized by natural selection for millions of years. They offer specific functions much more effectively than artificial approaches. Morphology genetic materials utilize morphologies gleaned from natural selection into their hierarchical structures. The combination of natural morphologies and manually selected functional materials makes these novel materials suitable for many applications. This review focuses on the strategies by which the structures and functions of natural species can be utilized. Specific functions inherited from both the natural microstructures and coupled functional materials are highlighted with regard to various applications, including photonics, light-harvesting, surface-enhanced Raman scattering (SERS), and electrodes for supercapacitors and batteries, as well as environmentally friendly materials.

Optical properties of thin metal films with nanohole arrays on porous alumina–aluminum structures

Enhanced plasmonic attenuation of reflection is observed in a gold–alumina–aluminum multilayer system near the interferometric anti-reflection condition.

Porous metallic nanocone arrays for high-density SERS hot spots via solvent-assisted nanoimprint lithography of block copolymer

We present a facile method of fabricating SERS substrate by combining solvent-assisted nanoimprint lithography and selective etching of block copolymer.

Carbon nanotube/gold nanoparticle composite-coated membrane as a facile plasmon-enhanced interface for sensitive SERS sensing

1D/0D-structured carbon nanotube/gold nanoparticle composites were synthesized and used to fabricate a simple and sensitive SERS sensor by filtering through a commercial membrane for on-site applications.

Self-assembled plasmonic nanoparticles on vertically aligned carbon nanotube electrodes via thermal evaporation

This study details the development of a large-area, three-dimensional (3D), plasmonic integrated electrode (PIE) system. Vertically aligned multiwalled carbon nanotube (VA-MWNT) electrodes are grown and populated with self-assembling silver nanoparticles via thermal evaporation. Due to the geometric and surface characteristics of VA-MWNTs, evaporated silver atoms form nanoparticles approximately 15-20 nm in diameter. The nanoparticles are well distributed on VA-MWNTs, with a 5-10 nm gap between particles. The size and gap of the self-assembled plasmonic nanoparticles is dependent upon both the length of the MWNT and the thickness of the evaporated silver. The wetting properties of water of the VA-MWNT electrodes change from hydrophilic (∼70°) to hydrophobic (∼120°) as a result of the evaporated silver. This effect is particularly pronounced on the VA-MWNT electrodes with a length of 1 μm, where the contact angle is altered from an initial 8° to 124°. Based on UV-visible spectroscopic analysis, plasmonic resonance of the PIE systems occurs at a wavelength of approximately 400 nm. The optical behavior was found to vary as a function of MWNT length, with the exception of MWNT with a length of 1 μm. Using our PIE systems, we were able to obtain clear surface-enhanced Raman scattering (SERS) spectra with a detection limit of ∼10 nM and an enhancement factor of ∼10(6). This PIE system shows promise for use as a novel electrode system in next-generation optoelectronics such as photovoltaics, light-emitting diodes, and solar water splitting.