Three-dimensional Pt-coated Au nanoparticle arrays: applications for electrocatalysis and surface-enhanced Raman scattering

This study demonstrates a novel approach to synthesis methods for core-shell nanoparticle assembly using nanoparticle trapping at an interface and subsequent transfer onto a substrate for electrochemical ultrathin layer coating. The transferred nanoparticle array can have a tunable surface area depending on the number of transferred layers. Subsequently coating the surface with Pt-group metals that behave as an ultrathin film provides electrocatalytic activities with respect to a variety of chemical reactions, depending on the properties of the selected coating materials. The transferred 3D Au nanoparticle arrays act as a high-surface-area platform for the diversity of overlayer materials. The resulting 3D core-shell nanoparticle films could be utilized as a highly active electrocatalysis and Raman scattering substrate. The approach provides a versatile and convenient synthesis route to new nanoporous material with tailorable pore structure and material properties through bottom-up assembly.     

高度有序ZIF-8/Au纳米复合结构阵列的构筑及其表面增强拉曼光谱的应用

采用气-液界面自组装方式制备得到有序单层聚苯乙烯(PS)微球阵列,以此为模板,采用磁控溅射沉积方法结合热处理技术获得单层六方非密排Au纳米颗粒阵列。随后采用水热法成功制得高度有序ZIF-8/Au纳米复合结构有序阵列。探究了生长机理以及反应温度、反应时间对微观形貌和光学特性的影响,进一步探究了该复合结构阵列作为表面增强拉曼散射(SERS)活性基底的灵敏度和均一性。结果表明：当水热反应温度从25 ℃增加至100 ℃时,ZIF-8纳米颗粒的数量逐步增大,同时尺寸随之增大,表面等离激元共振(SPR)峰和衍射峰均发生了红移。当水热反应时间从10 min增至60 min时,ZIF-8纳米颗粒从围绕Au纳米颗粒选择性生长到蔓延到整个材料表面。在样品表面沉积特定厚度的Ag膜后,测得4-氨基苯硫酚(4-ATP)和罗丹明6G(R6G)两种探针分子的检测极限均为10^-11 mol·L^-1,4-ATP和R6G的SERS强度与分子溶液浓度呈线性关系,相关系数R²分别为0.980 1和0.984 4。随机选取10个不同位置测试4-ATP的SERS谱图,得到相对标准偏差(RSD)为8.86%,表明ZIF-8/Au纳米复合结构有序阵列作为SERS基底具有良好的均匀性和稳定性。     

Controlled fabrication of nanopillar arrays as active substrates for surface-enhanced Raman spectroscopy

Highly ordered gold nanopillar arrays were fabricated using anodized aluminum oxide (AAO) templates. Nanopillars with a dimension of 110 +/- 15 nm in vertical height and 75 +/- 10 nm in base diameter were formed with a density of 150 microm(-2). The ordered nanopillar arrays give reproducible surface-enhanced Raman scattering (SERS) at a detection limit of 10(-8) M using thionine as probing molecules. The enhancement by the Au nanopillar arrays was comparable with or better than that of dispersed gold nanoparticle SERS substrates. This work demonstrates a new technique for producing highly ordered and reproducible SERS substrates potentially applicable for chemical and biological assay.     

Silver nanovoid arrays for surface-enhanced Raman scattering

Highly ordered silver nanovoid arrays are fabricated on porous anodic alumina membranes to produce robust and cost-efficient surface-enhanced Raman scattering (SERS) substrates. Plasmonic tunability can be accomplished by adjusting the topography with different anode voltages. Evenly distributed plasmonic fields, high average enhancement factor, and excellent ambient stability due to the natural protective layer are some of the unique advantages, and the silver nanovoid arrays are applicable to sensing devices.     

One-dimensional arrays of nanoshell dimers for single molecule spectroscopy via surface-enhanced Raman scattering

The optical properties of one-dimensional arrays of metal nanoshell dimers are studied systematically using the T-matrix method based on Mie theory, within the context of surface enhanced Raman scattering (SERS). It is shown that the local electromagnetic enhancement can be as high as approximately 4.5 x 10(13) for nanoshell dimer arrays with optimal geometry, and sensitive tunability in the resonant frequency can be gained by varying the geometrical parameters, making such structures appealing templates for SERS measurements with single molecule sensitivity. The extraordinarily high enhancement is attributed to a collective photonic effect constructively superposed onto the intrinsic enhancement associated with an isolated nanoshell dimer.     

Quantitative surface-enhanced Raman spectroscopy

The purpose of this tutorial review is to show how surface-enhanced Raman (SERS) and resonance Raman (SERRS) spectroscopy have evolved to the stage where they can be used as a quantitative analytical technique. SER(R)S has enormous potential for a range of applications where high sensitivity needs to be combined with good discrimination between molecular targets, particularly since low cost, compact spectrometers can read the high signal levels that SER(R)S typically provides. These advantages over conventional Raman measurements come at the cost of increased complexity and this review discusses the factors that need to be controlled to generate stable and reproducible SER(R)S calibrations.     

Reaction pathway change on plasmonic Au nanoparticles studied by surface-enhanced Raman spectroscopy

Gold nanoparticles (Au NPs) are nanoscale sources of light and electrons, which are highly relevant for their extensive applications in the field of photocatalysis. Although a number of research works have been carried out on chemical reactions accelerated by the energetic hot electrons/holes, the possibility of reaction pathway change on the plasmonic Au surfaces has not been reported so far. In this proof-of-concept study, we find that Au NPs change the reaction pathway in photooxidation of alkyne under visible light irradiation. This reaction produces benzil (COCO) without the presence of Au NPs. In contrast, as indicated by surface-enhanced Raman spectroscopic (SERS) results, the CC triple bonds (CC) adsorbed on Au NPs are converted into carboxyl (COOH) and acyl chloride (COCl) groups. The plasmonic Au NPs not only provide energetic charge carriers but also activate the reactant molecules as conventional heterogeneous catalysts. This study discloses the second role of plasmonic NPs in photocatalysis and bridges the gap between plasmon-driven and conventional heterogeneous catalysis.     

Ordered gold nanoparticle arrays as surface-enhanced Raman spectroscopy substrates for label-free detection of nitroexplosives

Nitroexplosives, such as 2,4,6-trinitrotoluene (TNT) which is a leading example of nitroaromatic explosives, are causing wide concern. Motivated by the urgent demand for trace analysis of explosives, novel surface-enhanced Raman spectroscopy substrates based upon highly ordered Au nanoparticles have been fabricated by a simple droplet evaporation method. It is noteworthy that an ethylhexadecyldimethyl ammonium bromide bilayer surrounding each individual nanoparticle not only is responsible for these periodic gap structures, but also tends to promote the adsorption of TNT on the composite NPs, thus resulting in a considerable increase of Raman signal. These desirable features endow the resulting SERS substrates with excellent enhancement ability and allow for a label-free detection of common plastic explosive materials even with a concentration as low as 10⁻⁹ M.     

Coupled subwavelength gratings for surface-enhanced Raman spectroscopy

Dual subwavelength Ag gratings with a small gap of about 15 nm are demonstrated to provide a huge additional SERS enhancement, more than 10(3) fold in scattering efficiency over normal SERS on an Ag film due to the strong plasmon coupling, which is simulated by theoretical calculation. The simulation also shows the advantages of the coupled two-layer gratings over the one-layer grating for SERS measurement. Our study provides a promising and feasible way of structure design for extremely sensitive substrates of SERS.     

Wavelength-scanned surface-enhanced Raman excitation spectroscopy

A detailed wavelength-scanned surface-enhanced Raman excitation spectroscopy (WS SERES) study of benzenethiol adsorbed on Ag nanoparticle arrays, fabricated by nanosphere lithography (NSL), is presented. These NSL-derived Ag nanoparticle array surfaces are both structurally well-characterized and extremely uniform in size. The WS SERES spectra are correlated, both spatially and spectrally, with the corresponding localized surface plasmon resonance (LSPR) spectra of the nanoparticle arrays. The surface-enhanced Raman scattering (SERS) spectra were measured in two excitation wavelength ranges: (1) 425-505 nm, and (2) 610-800 nm, as well as with the 532-nm line from a solid-state diode-pumped laser. The WS SERES spectra have line shapes similar to those of the LSPR spectra. The maximum SERS enhancement factor is shown to occur for excitation wavelengths that are blue-shifted with respect to the LSPR lambda(max) of adsorbate-covered nanoparticle arrays. Three vibrational modes of benzenethiol (1575, 1081, and 1009 cm(-1)) are studied simultaneously on one substrate, and it is demonstrated that the smaller Raman shifted peak shows a maximum enhancement closer to the LSPR lambda(max) than that of a larger Raman shifted peak. This is in agreement with the predictions of the electromagnetic (EM) enhancement mechanism of SERS. Enhancement factors of up to approximately 10(8) are achieved, which is also in good agreement with our previous SERES studies.     

Rationally designed nanostructures for surface-enhanced Raman spectroscopy

Research on surface-enhanced Raman spectroscopy (SERS) is an area of intense interest because the technique allows one to probe small collections of, and in certain cases, individual molecules using relatively straightforward spectroscopic techniques and nanostructured substrates. Researchers in this area have attempted to develop many new technological innovations including high sensitivity chemical and biological detection systems, labeling schemes for authentication and tracking purposes, and dual scanning-probe/spectroscopic techniques that simultaneously provide topographical and spectroscopic information about an underlying surface or nanostructure. However, progress has been hampered by the inability of researchers to fabricate substrates with the high sensitivity, tunability, robustness, and reproducibility necessary for truly practical and successful SERS-based systems. These limitations have been due in part to a relative lack of control over the nanoscale features of Raman substrates that are responsible for the enhancement. With the advent of nanotechnology, new approaches are being developed to overcome these issues and produce substrates with higher sensitivity, stability, and reproducibility. This tutorial review focuses on recent progress in the design and fabrication of substrates for surface-enhanced Raman spectroscopy, with an emphasis on the influence of nanotechnology.     

Swellable polymer films containing Au nanoparticles for point-of-care therapeutic drug monitoring using surface-enhanced Raman spectroscopy

Large (10 × 10 cm) sheets of surface-enhanced Raman spectroscopy (SERS) active polymer have been prepared by stabilising metal nanoparticle aggregates within dry hydroxyethylcellulose (HEC) films. In these films the aggregates are protected by the polymer matrix during storage but in use they are released when aqueous analyte droplets cause the films to swell to their gel form. The fact that these “Poly-SERS” films can be prepared in bulk but then cut to size and stored in air before use means that they provide a cost effective and convenient method for routine SERS analysis. Here we have tested both Ag and Au Poly-SERS films for use in point-of-care monitoring of therapeutic drugs, using phenytoin as the test compound. Phenytoin in water could readily be detected using Ag Poly-SERS films but dissolving the compound in phosphate buffered saline (PBS) to mimic body fluid samples caused loss of the drug signal due to competition for metal surface sites from Cl⁻ ions in the buffer solution. However, with Au Poly-SERS films there was no detectable interference from Cl⁻ and these materials allowed phenytoin to be detected at 1.8 mg L⁻¹, even in PBS. The target range of detection of phenytoin in therapeutic drug monitoring is 10–20 mg L⁻¹. With the Au Poly-SERS films, the absolute signal generated by a given concentration of phenytoin was lower for the films than for the parent colloid but the SERS signals were still high enough to be used for therapeutic monitoring, so the cost in sensitivity for moving from simple aqueous colloids to films is not so large that it outweighs the advantages which the films bring for practical applications, in particular their ease of use and long shelf life.     

Patterned growth of vertically aligned silicon nanowire arrays for label-free DNA detection using surface-enhanced Raman spectroscopy

Patterning is of paramount importance in many areas of modern science and technology. As a good candidate for novel nanoscale optoelectronics and miniaturized molecule sensors, vertically aligned silicon nanowire (SiNW) with controllable location and orientation is highly desirable. In this study, we developed an effective procedure for the fabrication of vertically aligned SiNW arrays with micro-sized features by using single-step photolithography and silver nanoparticle-induced chemical etching at room temperature. We demonstrated that the vertically aligned SiNW arrays can be used as a platform for label-free DNA detection using surface-enhanced Raman spectroscopy (SERS), where the inherent “fingerprint” SERS spectra allows for the differentiation of closely related biospecies. Since the SiNW array patterns could be modified by simply varying the mask used in the photolithographic processing, it is expected that the methodology can be used to fabricate label-free DNA microarrays and may be applicable to tissue engineering, which aims to create living tissue substitutes from cells seeded onto 3D scaffolds.

Electrochemical surface-enhanced Raman spectroscopy of nanostructures

This tutorial review first describes the early history of SERS as the first SERS spectra were obtained from an electrochemical cell, which led to the discovery of the SERS effect in mid-1970s. Up to date, over 500 papers have been published on various aspects of SERS from electrochemical systems. We then highlight important features of electrochemical SERS (EC-SERS). There are two distinctively different properties of electric fields, the electromagnetic field and static electrochemical field, co-existing in electrochemical systems with various nanostructures. Both chemical and physical enhancements can be influenced to some extent by applying an electrode potential, which makes EC-SERS one of the most complicated systems in SERS. Great efforts have been made to comprehensively understand SERS and analyze EC-SERS spectra on the basis of the chemical and physical enhancement mechanisms in order to provide meaningful information for revealing the mechanisms of electrochemical adsorption and reaction. The EC-SERS experiments and applications are then discussed from preparation of nanostructured electrodes to investigation of SERS mechanisms and from characterization of adsorption configuration to elucidation of electrochemical reaction mechanisms. Finally, prospective developments of EC-SERS in substrates, methods and theory are discussed.     

Surface-enhanced Raman scattering from helical silver nanorod arrays

Helical silver nanorod arrays with different arm numbers are designed by oblique angle deposition and their surface-enhanced Raman scattering properties are characterized. Assuming that the hot spots are located at the bends between the arms, and considering the optical absorbance of different layers, the SERS behavior can be predicted qualitatively.     

Preparation of gold-tellurium hybrid nanomaterials for surface-enhanced Raman spectroscopy

We report a simple method for preparing three different SERS-active substrates. Concentrated hydrazine solution as the reducing agent and tellurium dioxide as the precursor were used to prepare Te nanowires (NWs). The as-prepared Te NWs have an average length of 547.7 +/- 111.6 nm and an average width of 15.1 +/- 2.7 nm. Through the reaction of Te NWs with sodium tetrachloroaurate in the presence of hexadecyltrimethylammonium bromide (CTAB) over reaction times of 10, 20, and 60 min, gold-tellurium nanodumbbells, gold-tellurium nanopeapods, and gold pearl-necklace nanomaterials (Au PNNs) were obtained, respectively. By controlling the reaction time, the distance between adjacent gold nanoparticles (Au NPs) in each Te nanowire was tunable, allowing us to investigate its effect on the SERS signals. Having shorter distances among Au NPs (greater electromagnetic fields), the Au PNNs provided a reproducible enhancement factor of 5.6 x 10(9).     

Disposable, stable media for reproducible surface-enhanced Raman spectroscopy

Large numbers of identical and stable SE(R)RS [surface-enhanced (resonance) Raman]-active media, which are convenient to handle and manipulate but sufficiently inexpensive that they can be used once and then discarded, have been prepared by isolating nanoparticles from Ag and Au sols in hydrophilic polymer gels. The preparation simply involves mixing a suitable polymer with the sol to give a viscous suspension that can be coated onto a substrate and dried to form a hard translucent film. The films remain inactive until they are treated with aqueous analyte solution, which causes the film to swell and brings the analyte into contact with the active metal particles. The swollen films give strong SERS spectra which are effectively identical to those obtained from simple sols. The advantage of this method is that the dried polymers can be stored indefinitely before use and that they give a high degree of spectral reproducibility.     

Towards single-microorganism detection using surface-enhanced Raman spectroscopy

The identification and discrimination of microorganisms is important not only for clinical reasons but also for pharmaceutical clean room production and food-processing technology. Vibrational spectroscopy such as IR, Raman, and surface-enhanced Raman scattering (SERS) can provide a rapid ‘fingerprint’ on the chemical structure of molecules and is used to obtain a ‘fingerprint’ from microorganisms as well. Because of the requirement that a single bacterium cell and noble metal nanoparticles must be in close contact and the lack of a significant physical support to hold nanoparticles around the single bacterium cell, the acquisition of SERS spectra for a single bacterium using colloidal nanoparticles could be a challenging task. The feasibility of SERS for identification down to a single bacterium is investigated. A Gram-negative bacterium, Escherichia coli, is chosen as a model for the investigation. Because the adsorption of silver nanoparticles onto the bacterial cell is an exclusive way for locating nanoparticles close to the bacterium cell, the absorption characteristics of silver nanoparticles with different surface charges are investigated. It is demonstrated that the citrate-reduced colloidal silver solution generates more reproducible SERS spectra. It is found that E. coli cells aggregate upon mixing with silver colloidal solution, and this may provide an additional benefit in locating the bacterial cell under a light microscope. It is also found that a laser wavelength in the UV region could be a better choice for the study due to the shallow penetration depth. It is finally shown that it is possible to obtain SERS spectra from a single cell down to a few bacterial cells, depending on the aggregation properties of bacterial cells for identification and discrimination.     

Detection of bacteria by surface-enhanced Raman spectroscopy

The detection and identification of dilute bacterial samples by surface-enhanced Raman spectroscopy has been explored by mixing aqueous suspensions of bacteria with a suspension of nanocolloidal silver particles. An estimate of the detection limit of E. coli was obtained by varying the concentration of bacteria. By correcting the Raman spectra for the broad librational OH band of water, reproducible spectra were obtained for E. coli concentrations as low as approximately 10³ cfu/mL. To aid in the assignment of Raman bands, spectra for E. coli in D₂O are also reported. Figure Light scattering apparatus used to detect bacteria     

Investigation of the effects of the local environment on the surface-enhanced Raman spectra of striped gold/silver nanorod arrays

The effects of the local environment on surface-enhanced Raman scattering (SERS) spectra utilizing gold, silver, and gold/silver striped nanorod array substrates was investigated. The arrays were fabricated using an electrochemical metal deposition into an anodic aluminum oxide template. The analyte chosen for this study was p-nitroso-N,N-dimethylaniline (p-NDMA), which has an electronic structure that is highly sensitive to its surrounding environment. Changes in the peak positions and peak ratios were used to probe the influence of water and the striping pattern on the SERS signal of p-NDMA. We present the results of the fabrication and characterization of the nanorod array substrates, as well as SERS spectra of p-NDMA in both polar and nonpolar environments and SERS spectra on a variety of striped nanorod arrays. The Raman data suggests that the p-NDMA molecule exists in a more polarized state when bound to the gold as compared to the silver rods. We have attempted to use these differences to determine whether the SERS signal predominantly arises from the tips of the rods or from the interior of the array.