The SERS activity of a supported Ag nanocube strongly depends on its orientation relative to laser polarization

Surface-enhanced Raman spectroscopy studies of organophosphorous model molecules and pesticides

This work reports the analytical application of surface-enhanced Raman spectroscopy (SERS) in the trace analysis of organophosphorous pesticides (trichlorfon and glyphosate) and model organophosphorous compounds (dimethyl methylphosphonate and o-ethyl methylphosphonothioate) bearing different functional groups. SERS measurements were carried out using Ag nanocubes with an edge square dimension of ca. 100 nm as substrates. Density functional theory (DFT) with the B3LYP functional was used for the optimization of ground state geometries and simulation of Raman spectra of the organophosphorous compounds and their silver complexes. Adsorption geometries and marker bands were identified for each of the investigated compound. Results indicate the usefulness of SERS methodology for the sensitive analyses of organophosphorous compounds through the use of vibrational spectroscopy.

Direct observation of one-dimensional plasmon coupling in metallic nanofibers prepared by evaporation-induced self-assembly with DNA

Here we report a simple method for the preparation of highly aligned metallic nanofibers with anisotropic aggregates of silver nanoparticles (AgNPs) as well as a direct observation of localized plasmon field and its coupling in the prepared metallic nanofibers. Metallic nanofibers of several tens of nanometers wide and millimeters long were prepared. The preparation method, which is based on the process of evaporation-induced self-assembly with DNA and drying front movement, eliminates the need for lithography and an external field, and it is fast, cheap and easy. Dark-field scattering spectroscopy was used to study the strong plasmon coupling of AgNPs in the metallic nanofibers. We observed strong near-field coupling between neighboring nanoparticles, which results in red-shifted multipolar plasmon modes that are highly polarized along the fiber axis. The polarization dependence of plasmon coupling in the metallic nanofibers observed in this study was satisfactorily explained by the morphology of the metallic nanofibers, which was characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Furthermore, Raman spectra imaging of the metallic nanofibers revealed the existence of intense hot spots localized along their axes, which played a significant role in the intensity of surface enhanced Raman scattering (SERS) signals from DNA bases in the metallic nanofiber. Our results demonstrate the use of evaporation-induced self-assembly with DNA as a straightforward method to produce one-dimensional coupling of localized plasmons with a longer scale.

Gold nanoparticle thin films fabricated by electrophoretic deposition method for highly sensitive SERS application

We report an electrophoretic deposition method for the fabrication of gold nanoparticle (GNP) thin films as sensitive surface-enhanced Raman scattering (SERS) substrates. In this method, GNP sol, synthesized by a seed-mediated growth approach, and indium tin oxide (ITO) glass substrates were utilized as an electrophoretic solution and electrodes, respectively. From the scanning electron microscopy analysis, we found that the density of GNPs deposited on ITO glass substrates increases with prolonged electrophoresis time. The films possess high mechanical adhesion strength and exhibit strong localized surface plasmon resonance (LSPR) effect by showing high SERS sensitivity to detect 1 × 10-7 M rhodamine 6 G in methanol solution. Finally, the relationship between Raman signal amplification capability and GNP deposition density has been further investigated. The results of our experiment indicate that the high-density GNP film shows relatively higher signal amplification capability due to the strong LSPR effect in narrow gap regions between the neighboring particles on the film.

Plasmonic near-electric field enhancement effects in ultrafast photoelectron emission: correlated spatial and laser polarization microscopy studies of individual Ag nanocubes

Electron emission from single, supported Ag nanocubes excited with ultrafast laser pulses (λ = 800 nm) is studied via spatial and polarization correlated (i) dark field scattering microscopy (DFM), (ii) scanning photoionization microscopy (SPIM), and (iii) high-resolution transmission electron microscopy (HRTEM). Laser-induced electron emission is found to peak for laser polarization aligned with cube diagonals, suggesting the critical influence of plasmonic near-field enhancement of the incident electric field on the overall electron yield. For laser pulses with photon energy below the metal work function, coherent multiphoton photoelectron emission (MPPE) is identified as the most probable mechanism responsible for electron emission from Ag nanocubes and likely metal nanoparticles/surfaces in general.

Large-area nanogap plasmon resonator arrays for plasmonics applications

Large-area (∼8000 mm(2)) Au nanogap plasmon resonator array substrates manufactured using maskless laser interference lithography (LIL) with high uniformity are presented. The periodically spaced subwavelength nanogap arrays are formed between adjacent nanopyramid (NPy) structures with precisely defined pitch and high length density (∼1 km cm(-2)), and are ideally suited as scattering sites for surface enhanced Raman scattering (SERS), as well as refractive index sensing. The two-dimensional grid arrangement of NPy structures renders the excitation of the plasmon resonators minimally dependent on the incident polarization. The SERS average enhancement factor (AEF) has been characterized using over 30 000 individual measurements of benzenethiol (BT) chemisorbed on the Au NPy surfaces. From the 1(a(1)), β(CCC) + ν(CS) ring mode (1074 cm(-1)) of BT on surfaces with pitch λ(g) = 200 nm, AEF = 0.8 × 10(6) and for surfaces with λ(g) = 500 nm, AEF = 0.3 × 10(7) from over 99% of the imaged spots. Maximum AEFs > 10(8) have been measured in both cases.

Synthesis of Ag nanobars in the presence of single-crystal seeds and a bromide compound, and their surface-enhanced Raman scattering (SERS) properties

This Article describes the synthesis of Ag nanobars with different aspect ratios using a seed-mediated method and evaluation of their use for surface-enhanced Raman scattering (SERS). The formation of Ag nanobars was found to critically depend on the introduction of a bromide compound into the reaction system, with ionic salts being more effective than covalent molecules. We examined single-crystal seeds with both spherical and cubic shapes and found that Ag nanobars grown from spherical seeds had much higher aspect ratios than those grown from cubic seeds. The typical product of a synthesis contained nanocrystals with three different morphologies: nanocubes, nanobars with a square cross section, and nanobars with a rectangular cross section. Their formation can be attributed to the difference in growth rates along the three orthogonal <100> directions. The SERS enhancement factor of the Ag nanobar was found to depend on its aspect ratio, its orientation relative to the laser polarization, and the wavelength of excitation.

Polymer nanofibers embedded with aligned gold nanorods: a new platform for plasmonic studies and optical sensing

This paper reports the fabrication and characterization of polymer nanofibers embedded with gold nanorods in uniaxial alignment for applications in optical waveguiding and sensing. Using a waveguiding approach, we demonstrated highly efficient excitation of localized surface plasmon resonance in the embedded gold nanorods with a photon-to-plasmon-conversion efficiency as high as 70% for a single nanorod at its longitudinal resonance wavelength. On the basis of waveguiding polymer nanofibers embedded with gold nanorods, we further demonstrated compact optical humidity sensors with a response time of 110 ms and an operation optical power as low as 500 pW.

Individual nanostructured materials: fabrication and surface-enhanced Raman scattering

The progress of surface-enhanced Raman scattering (SERS) microscopy and spectroscopy on individual nanostructured materials has been reviewed in this feature article. After a brief introduction on individual nanomaterial SERS, we provide a systematic overview on the fabrication and SERS studies of individual nanoparticulates, nano-junctions and hierarchical nano-aggregate. These SERS-active nanomaterials have great potential in designing novel highly sensitive SERS substrates for the development of SERS-based sensing devices with a broad range of applications.

Topotaxial fabrication of vertical Aux Ag1-x nanowire arrays: plasmon-active in the blue region and corrosion resistant

Topotaxial growth of Au(x) Ag(1-x) alloy nanowires (NWs) by postepitaxial deposition of Ag vapor on Au NWs and investigation of their plasmonic properties are reported. Ag vapor is supplied onto the epitaxially grown Au NWs, topotaxially turning them into Au(x) Ag(1-x) alloy NWs. The original geometries and alignments of the Au nanostructures are well preserved, while the composition of the alloy NWs is controlled by varying the Ag vapor supply time. The Au(0.5) Ag(0.5) NWs show high surface-enhanced Raman scattering (SERS) activity comparable to that of Ag NWs as well as highly increased oxidation resistance. The plasmon-active wavelength range of the Au(0.5) Ag(0.5) NW is significantly extended to the blue region compared to Au NWs. The Au(x) Ag(1-x) alloy NWs that have plasmonic activity in the blue region in addition to high corrosion resistance will make a superb material for practical plasmonic devices including SERS sensors and optical nanoantennas.

Highly sensitive surface-enhanced Raman spectroscopy (SERS) platforms based on silver nanostructures fabricated on polyaniline membrane surfaces

Here, we demonstrate a facile synthesis of homogeneous Ag nanostructures fully covering the polyaniline (PANI) membrane surface simply by introducing organic acid in the AgNO(3) reaction solution, as an improved technique to fabricate well-defined Ag nanostructures on PANI substrates through a direct chemical deposition method [Langmuir2010, 26, 8882]. It is found that the chemical nature of the acid is crucial to create a homogeneous nucleation environment for Ag growth, where, in this case, homogeneous Ag nanostructures that are assembled by Ag nanosheets are produced with the assistance of succinic acid and lactic acid, but only scattered Ag particles with camphorsulfonic acid. Improved surface wettability of PANI membranes after acid doping may also account for the higher surface coverage of Ag nanostructures. The Ag nanostructures fully covering the PANI surface are extremely sensitive in the detection of a target analyte, 4-mercaptobenzoic acid (4-MBA), using surface-enhanced Raman spectroscopy (SERS), with a detection limit of 10(-12) M. We believe the facilely fabricated SERS-active substrates based on conducting polymer-mediated growth of Ag nanostructures can be promising in the trace detection of chemical and biological molecules.

Controlled fabrication of silver nanoneedles array for SERS and their application in rapid detection of narcotics

Novel surface-enhanced Raman scattering (SERS) substrates with high SERS-activity are ideal for novel SERS sensors, detectors to detect illicitly sold narcotics and explosives. The key to the wider application of SERS technique is to develop plasmon resonant structure with novel geometries to enhance Raman signals and to control the periodic ordering of these structures over a large area to obtain reproducible Raman enhancement. In this work, a simple Ar(+)-ion sputtering route has been developed to fabricate silver nanoneedles arrays on silicon substrates for SERS-active substrates to detect trace-level illicitly sold narcotics. These silver nanoneedles possess a very sharp apex with an apex diameter of 15 nm and an apex angle of 20°. The SERS enhancement factor of greater than 10(10) was reproducibly achieved by the well-aligned nanoneedles arrays. Furthermore, ketamine hydrochloride molecules, one kind of illicitly sold narcotics, can be detected down to 27 ppb by using our SERS substrate within 3 s, indicating the sensitivity of our SERS substrates for trace amounts of narcotics and that SERS technology can become an important analytical technique in forensic laboratories because it can provide a rapid and nondestructive method for trace detection.

Fabrication of high performance surface enhanced Raman scattering substrates by a solid-state ionics method

Silver nanostructures were prepared by a solid-state ionics method using fast ionic conductor RbAg(4)I(5) films under a direct current electric field (DCEF). The surface morphology of the silver nanostructures grown under different constant current fields was characterized by scanning electron microscopy (SEM). Rhodamine 6G (R6G) aqueous solutions were used as probe molecules to detect the Raman enhancement performance of the silver nanostructure substrates. The effect of external electric field current intensity on the surface morphology of the silver nanostructures during the preparation was studied in detail. The enhancement effect of the silver nanostructure surface enhanced Raman scattering (SERS) substrates with different surface morphologies toward R6G was determined. We found that disordered silver nanowires (DSNW), ordered silver nanowires (OSNW), densely arranged silver nanobamboo arrays (SNBA) and compactly arranged silver nanobud clusters (SNBC) were respectively obtained when the constant current intensity was 3 μA, 5 μA, 8 μA and 12 μA under the same vacuum evaporation plating conditions. The limiting concentrations of R6G for these SERS substrates were found to be 10(-7) mol l(-1), 10(-13) mol l(-1), 10(-13) mol l(-1) and 10(-16) mol l(-1), respectively.

Ultralocal modification of surface plasmons properties in silver nanocubes

The plasmonic properties of individual subwavelength-sized silver nanocubes are mapped with nanometric spatial resolution by means of electron energy-loss spectroscopy in a scanning transmission electron microscope. Three main features with different energies and spatial behavior (two peaked at the corners, one on the edges) are identified and related to previous measurements on ensemble or individual nanoparticles. The highly subwavelength mapping of the energy position and intensity of the excitations shows that the surface plasmon modes, localized at specific areas of the particles, for example, the corners or the edges, are modified by their size, the presence of a substrate, and the very local environment. Helped by discrete dipole approximation numerical simulations, we discuss how local modifications of the environment affect the global modes of the particles. In particular, we show both experimentally and theoretically that absorption resonances at different corners of the same nanocube are largely independent of each other in energy and intensity. Our findings provide a better understanding of the spatial coherence of the surface plasmons in nanoparticles but also give useful insights about their roles in the nanoparticle sensing properties.

Super-resolution imaging reveals a difference between SERS and luminescence centroids

Super-resolution optical imaging of Rhodamine 6G surface-enhanced Raman scattering (SERS) and silver luminescence from colloidal silver aggregates are measured with sub-5 nm resolution and found to originate from distinct spatial locations on the nanoparticle surface. Using correlated scanning electron microscopy, the spatial origins of the two signals are mapped onto the nanoparticle structure, revealing that, while both types of emission are plasmon-mediated, SERS is a highly local effect, probing only a single junction in a nanoparticle aggregate, whereas luminescence probes all collective plasmon modes within the nanostructure. Calculations using the discrete-dipole approximation to calculate the weighted centroid position of both the |E|(2)/|E(inc)|(2) and |E|(4)/|E(inc)|(4) electromagnetic fields were compared to the super-resolution centroid positions of the SERS and luminescence data and found to agree with the proposed plasmon dependence of the two emission signals. These results are significant to the field of SERS because they allow us to assign the exact nanoparticle junction responsible for single-molecule SERS emission in higher order aggregates and also provide insight into how SERS is coupled into the plasmon modes of the underlying nanostructure, which is important for developing new theoretical models to describe SERS emission.

Quantitative analysis of the role played by poly(vinylpyrrolidone) in seed-mediated growth of Ag nanocrystals

This article presents a quantitative analysis of the role played by poly(vinylpyrrolidone) (PVP) in seed-mediated growth of Ag nanocrystals. Starting from Ag nanocubes encased by {100} facets as the seeds, the resultant nanocrystals could take different shapes depending on the concentration of PVP in the solution. If the concentration was above a critical value, the seeds simply grew into larger cubes still enclosed by {100} facets. When the concentration fell below a critical value, the seeds would evolve into cuboctahedrons enclosed by a mix of {100} and {111} facets and eventually octahedrons completely covered by {111} facets. We derived the coverage density of PVP on Ag(100) surface by combining the results from two measurements: (i) cubic seeds were followed to grow at a fixed initial concentration of PVP to find out when {111} facets started to appear on the surface, and (ii) cubic seeds were allowed to grow at reduced initial concentrations of PVP to see at which concentration {111} facets started to appear from the very beginning. We could calculate the coverage density of PVP from the differences in PVP concentration and the total surface area of Ag nanocubes between these two samples. The coverage density was found to be 140 and 30 repeating units per nm(2) for PVP of 55,000 and 10,000 g/mol in molecular weight, respectively, for cubic seeds of 40 nm in edge length. These values dropped slightly to 100 and 20 repeating units per nm(2), respectively, when 100 nm Ag cubes were used as the seeds.

Monomer adsorption of indocyanine green to gold nanoparticles

NIR-dye encoded gold nanoparticles (GNP) are rapidly emerging as contrast agents in many bio-imaging/sensing applications. The coding process is usually carried out without control or a clear understanding of the metal-liquid interface properties which, in contrast, are critical in determining the type and extension of dye-metal interaction. In this paper, we investigated the effect of gold surface composition on the adsorption of indocyanine green (ICG) on GNP, simulating the surface conditions of gold nanorods on citrate-capped gold nanospheres. These substrates allowed a careful control of the metal-liquid interface composition and, thus, detailed absorption and fluorescence concentration studies of the effects of each individual chemical in the colloidal solution (i.e. bromide anions, cetyl trimethylammonium ions and Ag(+) ions) on the ICG-gold interaction. This study reveals the drastic effect that these experimental parameters can have on the ICG adsorption on GNP.

Surface-enhanced Raman scattering-active nanostructures and strategies for bioassays

Surface-enhanced Raman scattering (SERS) techniques offer a number of advantages in molecular detection and analysis, particularly in terms of the multiplex detection of biomolecules. So far, many new SERS-based substrates and analytical techniques have been reported. For easy understanding, various SERS techniques are classified into the following four categories: adsorption-mediated direct detection; antibody- or ligand-mediated direct detection; binding catalyzed indirect detection; and tag-based indirect detection. Among these, recent successes of SERS tagging/encoding (nano/micro) materials and detection methods are highlighted, including our recent works. Some novel SERS-based strategies for the detection of several biological molecules are also introduced.

Directed assembly of gold nanorods using aligned electrospun polymer nanofibers for highly efficient SERS substrates

Nonspherical metal nanoparticles are very attractive plasmonic nanostructures owing to the facile tunability of the plasmonic properties and the presence of sharp corners and edges, which act as electromagnetic hot spots for surface enhanced Raman scattering (SERS). However, such anisotropic nanostructures exhibit strong polarization dependence in their plasmonic properties, exhibiting significantly higher SERS intensity in certain orientations. In this paper, we demonstrate a facile strategy to achieve directed assembly of aligned gold nanorods using highly aligned electrospun nanofibers. We believe that the interstices between the nanofibers act as micro-and nanochannels, resulting in hydrodynamic drag forces on the gold nanorods, thus inducing massive alignment of the same on the nanofibers. Apart from exhibiting nearly 50 times higher SERS intensity compared to a planar SERS substrate with randomly oriented nanorods, our results highlight the importance of the orientation of anisotropic nanostructures. Finite difference time domain (FDTD) simulations employed to understand the electromagnetic field distribution around an aligned nanorod array showed excellent agreement with the experimental observations.

Electrochemically fabricated self-aligned 2-D silver/alumina arrays as reliable SERS sensors

A novel SERS sensor for adenine molecules is fabricated electrochemically using an ordered two-dimensional array of self-aligned silver nanoparticles encapsulated by alumina. Silver is electro-deposited on the interior surfaces at the bottom of nano-channels in a porous anodic aluminum oxide (AAO) film. After etching aluminum, the back-end alumina serves as a SERS substrate. SERS enhancement factor greater than 10(6) is measured by 532 nm illumination. It exhibits robust chemical stability and emits reproducible Raman signals from repetitive uses for eight weeks. The inexpensive mass production process makes this reliable, durable and sensitive plasmon based optical device promising for many applications.

Generation of hot spots with silver nanocubes for single-molecule detection by surface-enhanced Raman scattering

This paper presents a simple strategy for the formation of surface-enhanced Raman scattering (SERS) hot spots, or regions with extraordinary large electric-field enhancements, by depositing a silver nanocube on a metal substrate. Our experimental and theoretical results show that hot spots form at the corners of a nanocube in contact with the substrate and the hot spots derived from a single silver nanocube are capable of detecting SERS from a single molecule. By varying the electrical property of the substrate, and the distance between the nanoparticle and the substrate, we show that the substrate can dramatically affect the SERS from a supported nanoparticle. In addition, by comparing the SERS for nanocubes and nanospheres of similar sizes, we show that this effect is also sensitive to the shape of the supported nanoparticle, and enhancement factors of 9.7×106 and 2.1×108 were obtained for a nanosphere and a nanocube on a metal substrate, respectively. This new approach requires minimum fabrication efforts and offers great simplicity for the formation of robust and fully accessible hot spots, providing an effective SERS platform for single-molecule detection.

Uniform gold nanoarray formed by controlled IP6 micelles for chemical mapping

A uniform Au nanoarray is successfully formed at an indium tin oxide (ITO) glass surface modified with well-distributed inositol hexakisphosphoric (IP(6)) micelle layers by controlling the pH of the medium at 10. When Rhodamine 6G (R6G) and 2-mercaptopyridine (2-MPy) are used as the Raman probes, the uniform Au nanoarray presents a sound surface enhanced Raman scattering (SERS) efficiency and a reproducible Raman signal in two dimensions. The relative standard deviation (RSD) of Raman intensities of R6G or 2-MPy on the uniform Au nanoarray recorded by point to point is less than 12%, which is beneficial to its application for chemical mapping or imaging. A case of Raman point-mapping for onion epidermis is demonstrated in the present work. A uniform IP(6)-Au nanoarray might be mass-produced by this protocol.

Site-selective localization of analytes on gold nanorod surface for investigating field enhancement distribution in surface-enhanced Raman scattering

Understanding detailed electric near-field distributions around noble metal nanostructures is crucial to the rational design of metallic substrates for maximizing surface-enhanced Raman scattering (SERS) efficiency. We obtain SERS signals from specific regions such as the ends, the sides and the entire surfaces of gold nanorod by chemisorbing analytes on the respective areas. Different SERS intensities from designated surfaces reflect their electric near-field intensities and thus the distributions. Our experimental results show that approximately 65% of the SERS enhancement emanated from the ends of gold nanorods which occupies only 28% of the total surface area, quantitatively exhibiting the strongly localized electric field around the ends. The reliability and generality of the investigation is confirmed by employing analytes with different chemical characteristics: positively and negatively charged, neutral, hydrophobic and hydrophilic ligands, which are selectively adsorbed on the different sites. Numerical simulations of the electric near-field distributions around the nanorod are in well agreement with our experimental results. In addition, we observed that the SERS intensities of colloidal gold nanospheres are independent of surface areas being functionalized by analytes, indicating a homogenous electric near-field distribution around gold nanospheres.

Nanoparticle SERS substrates with 3D Raman-active volumes

This Perspective reviews a new class of surface-enhanced Raman scattering (SERS) nanoparticle substrates defined by their three-dimensional (3D) confinement of localized electromagnetic fields. First, we describe the critical design parameters and recent advances in nanofabrication to create reproducible nanoparticle assemblies for SERS. Next, we highlight a promising platform-gold nanopyramids-for testing how the local arrangement of particles in an assembly affects the overall SERS response. The dimensions and optical properties of the nanopyramids can be tuned easily, and their unique anisotropic shape allows them to be organized into different particle configurations with 3D Raman-active volumes. Because of their large hot-spot volumes, this unique class of nanoparticle substrates offers an attractive alternative for ultra-sensitive sensors and trace chemical analysis.

Engineering the Properties of Metal Nanostructures via Galvanic Replacement Reactions

In this review, we will bring the reader up to date with recent advances in the use of galvanic replacement reactions to engineer highly tunable nanostructures for a variety of applications. We will begin by discussing the variety of templates that have been used for such reactions and how the structural details (e.g., shape, size, and defects, among others) have interesting effects on the ultimate product, beyond serving as a simple site for deposition. This will be followed by a discussion of how we can manipulate the processes of alloying and dealloying to produce novel structures and how the type of precursor affects the final properties. Finally, the interesting optical properties of these materials and some innovative applications in areas of biomedical engineering and catalysis will be discussed, completing our overview of the state of the art in galvanic replacement.

Dispersion in the SERS enhancement with silver nanocube dimers

The SERS phenomenon was studied using a large set of silver nanocube dimers programmed to self-assemble in preset locations of a patterned substrate. This SERS substrate made it possible to demonstrate the dependence of the SERS enhancement on the geometry of the silver nanocube dimers and to quantify the dispersion in the SERS enhancement obtained in an ensemble of dimers. In addition to the effects of the gap distance of the dimer and the orientation of the dimer axis relative to the laser polarization on SERS enhancement, the data reveal an interesting dependence of the site-to-site variations of the enhancement on the relative orientation of the nanocubes in the dimer. We observed the highest heterogeneity in the SERS signal intensity with face-to-face dimers and a more robust SERS enhancement with face-to-edge dimers. Numerical calculations indicate that the plasmon resonance frequencies of face-to-face dimers shift considerably with small changes in gap distance. The resonance frequency shifts make it less likely for many of the dimers to satisfy the matching condition between the photon frequencies and the plasmon resonance frequency, offering an explanation for the large site-to-site variations in SERS signal intensity. These results indicate that plasmonic nanostructure designs for SERS substrates for real-world applications should be selected not only to maximize the signal enhancement potential but also to minimize the heterogeneity of the substrate with respect to signal enhancement. The latter criterion poses new challenges to experimentalists and theorists alike.

Seed-mediated synthesis of Ag nanocubes with controllable edge lengths in the range of 30-200 nm and comparison of their optical properties

Silver nanocubes with edge lengths controllable in the range of 30-200 nm were synthesized using an approach based on seeded growth. The keys to the success of this synthesis are the use of single-crystal Ag seeds to direct the growth and the use of AgNO(3) as a precursor to elemental Ag, where the byproduct HNO(3) can block both the homogeneous nucleation and evolution of single-crystal seeds into twinned nanoparticles. Either spherical (in the shape of a cuboctahedron) or cubic seeds could be employed for this growth process. The edge length of the resultant Ag nanocubes can be readily controlled by varying the amount of Ag seeds used, the amount of AgNO(3) added, or both. For the first time, we could obtain Ag nanocubes with uniform edge lengths controllable in the range of 30-200 nm and then compare their localized surface plasmon resonance and surface-enhanced Raman scattering properties.

Aligned gold nanoneedle arrays for surface-enhanced Raman scattering

A simple Ar(+)-ion irradiation route has been developed to prepare gold nanoneedle arrays on glass substrates for surface-enhanced Raman scattering (SERS)-active substrates. The nanoneedles exhibited very sharp tips with an apex diameter of 20 nm. These arrays were evaluated as potential SERS substrates using malachite green molecules and exhibited a SERS enhancement factor of greater than 10(8), which is attributed to the localized electron field enhancement around the apex of the needle and the surface plasmon coupling originating from the periodic structure. This work demonstrates a new technique for producing controllable and reproducible SERS substrates potentially applicable for chemical and biological assays.

Facile fabrication of homogeneous 3D silver nanostructures on gold-supported polyaniline membranes as promising SERS substrates

We report a facile synthesis of large-area homogeneous three-dimensional (3D) Ag nanostructures on Au-supported polyaniline (PANI) membranes through a direct chemical reduction of metal ions by PANI. The citric acid absorbed on the Au nuclei that are prefabricated on PANI membranes directs Ag nanoaprticles (AgNPs) to self-assemble into 3D Ag nanosheet structures. The fabricated hybrid metal nanostructures display uniform surface-enhanced Raman scattering (SERS) responses throughout the whole surface area, with an average enhancement factor of 10(6)-10(7). The nanocavities formed by the stereotypical stacking of these Ag nanosheets and the junctions and gaps between two neighboring AgNPs are believed to be responsible for the strong SERS response upon plasmon absorption. These homogeneous metal nanostructure decorated PANI membranes can be used as highly efficient SERS substrates for sensitive detection of chemical and biological analytes.

Single-order, subwavelength resonant nanograting as a uniformly hot substrate for surface-enhanced Raman spectroscopy

The surface-enhanced Raman spectroscopy (SERS) activity and the optical reflectance of a subwavelength gold nanograting fabricated entirely using top down technologies on silicon wafers are presented. The grating consists of 120 nm gold cladding on top of parallel silica nanowires constituting the grating's lines, with gaps between nanowires <10 nm wide at their narrowest point. The grating produces inordinately intense SERS and shows very strong polarization dependence. Reflectance measurements for the optimized grating indicate that (when p-polarization is used and at least one of the incident electric field components lies across the grating lines) the reflectance drops to <1% at resonance, indicating that essentially all of the radiant energy falling on the surface is coupled into the grating. The SERS intensity and the reflectance at resonance anticorrelate predicatively, suggesting that reflectance measurements can provide a nondestructive, wafer-level test of SERS efficacy. The SERS performance of the gratings is very uniform and reproducible. Extensive measurements on samples cut from both the same wafer and from different wafers, produce a SERS intensity distribution function that is similar to that obtained for ordinary Raman measurements carried out at multiple locations on a polished (100) silicon wafer.

Understanding the SERS Effects of Single Silver Nanoparticles and Their Dimers, One at a Time

This perspective article highlights recent developments in a class of surface-enhanced Raman scattering (SERS) experiments that aim to correlate SERS enhancement factors with the physical parameters of metal nanostructures. In a typical study, the SERS substrate is fabricated by depositing colloidal nanoparticles on a silicon wafer to obtain individual particles isolated from each other, or small aggregates such as dimeric units. With the help of registration marks, the same nanoparticle, or dimer of nanoparticles, can be quickly located under a Raman microscope (for SERS spectra) and a scanning electron microscope (for structural characterization). The nanoscale characterization achieved by these studies has resulted in unparalleled investigations into the nature of polarization dependency for SERS, the hot spot nature of single nanoparticles and dimers, and the manipulation of hot spots through shape-controlled synthesis and self-assembly. We discuss the new insights these studies have offered, and the future progress they can deliver to the advancement of SERS.

Optical Properties of Nested Pyramidal Nanoshells

This paper describes the fabrication and characterization of nested Au pyramidal nanoshells. These particles exhibited two plasmon resonances at visible and near-infrared wavelengths that could be manipulated depending on the size of the gap between inner and outer pyramidal shells. We found that larger gaps (30 nm) exhibited much larger Raman scattering responses compared to smaller gaps (5 nm) in the nested pyramidal shells. The SERS-activity of these anisotropic particles can be optimized by adjusting the distances between the inner and outer Au shells.