SERS Intensity Research Articles

Gold nanoparticle superlattices as functional solids for concomitant conductivity and SERS tuning

Mercaptosuccinic acid protected gold nanoparticles (Au@MSA) self assemble to form superlattice (SL) crystals at the air-water interface. These have been used for gas adsorption. The current-voltage (I-V) characteristics of the SL film with embedded SL crystals, obtained by four probe measurements, show Ohmic conduction. The conductance observed was proportional to the polarizability of the adsorbed gases. The current through the SL decreases on adsorption of the gas along with decrease in the SERS intensity of a probe molecule from the crystals. We rationalise our observation of the linear dependence of the conductance on the polarizability of the adsorbed gas using a simple model calculation. Variation of the conductance may be useful in designing electrical switches operating at the nanometre length scales.

Label-free Rapid Semiquantitative Detection of Proteins Down to Sub-monolayer Coverage by Using Surface-enhanced Raman Scattering of Nitrate Ion

Abstract A novel heat-induced SERS method was used to selectively enhance a band of NO3− at 1049 cm−1 for semi-quantitative detection of lysozyme and insulin down to 10−9 and 10−8 M, respectively, and bell shape variations of SERS intensities were observed for the concentration dependency of the proteins.

Development of highly reproducible nanogap SERS substrates: Comparative performance analysis and its application for glucose sensing

We report a new class of a SERS substrate with ordered nanostructures fabricated on silicon wafer using a deep UV (DUV) lithography technique followed by surface coating of silver and/or gold film. These substrates possess sharp edged nanogaps, which are responsible for the SERS enhancement. SERS performance of these substrates was analyzed by studying its reproducibility, repeatability and signal enhancement measured from 2-naphthalene thiol (NT) molecule covalently anchored on to the substrate. SERS performance of this substrate was also compared with a commercial substrate and metal film over nanosphere (MFON) substrate, which is one of the most promising reported substrates. It was found that MFON substrate showed a slightly higher SERS intensity among all three chosen substrates, but the relative standard deviation (RSD) of the intensity for the two prominent peaks of NT was about 7–14% while for our nanogap DUV substrate the RSD was less than 3% with comparable SERS signal intensities to MFON. For the commercial substrate, the relative standard deviation was about 7–9% but with a much lower SERS signal intensity. To our knowledge, this observed reproducibility along with good SERS enhancement with nanogap substrate is the best among the reported SERS substrates. These observed results with the nanogap substrate show great potential for the development of a sensitive SERS biosensing platform. Efficacy of the nanogap DUV substrate for biosensing was demonstrated for in vitro glucose sensing under physiologically relevant conditions.

Super-resolution Optical Imaging of Single-Molecule SERS Hot Spots

We present the first super-resolution optical images of single-molecule surface-enhanced Raman scattering (SM-SERS) hot spots, using super-resolution imaging as a powerful new tool for understanding the interaction between single molecules and nanoparticle hot spots. Using point spread function fitting, we map the centroid position of SM-SERS with +/-10 nm resolution, revealing a spatial relationship between the SM-SERS centroid position and the highest SERS intensity. We are also able to measure the unique position of the SM-SERS centroid relative to the centroid associated with nanoparticle photoluminescence, which allows us to speculate on the presence of multiple hot spots within a single diffraction-limited spot. These measurements allow us to follow dynamic movement of the SM-SERS centroid position over time as it samples different locations in space and explores regions larger than the expected size of a SM-SERS hot spot. We have proposed that the movement of the SERS centroid is due to diffusion of a single molecule on the surface of the nanoparticle, which leads to changes in coupling between the scattering dipole and the optical near field of the nanoparticle.

Surface‐enhanced Raman scattering from polystyrene on gold clusters

AbstractSurface‐enhanced Raman scattering (SERS) constitutes a spectroscopic method of rapidly growing importance, and polystyrene is a widely used compound of great industrial importance. In this work, SERS data were obtained from polystyrene samples prepared by vapor deposition of gold and plasma‐induced polymerization of styrene gas. A thorough examination of this data is presented. The relationships between sample preparation parameters, gold‐cluster morphology, and SERS intensity were elucidated. Using Wilson's notation, vibrations were assigned to all bands between 250 and 1750 cm−1 in the ordinary Raman and SERS spectra of polystyrene. The correct assignment of these bands would be a significant achievement because they have been controversial in the literature for ∼30 years. Our assignments were made by reviewing the literature and comparing the assignments found there to spectral data acquired during this study; they were confirmed using density functional theory (DFT) calculations performed on the styrene monomer. The orientation of polystyrene's phenyl ring, relative to the gold surface, was determined. It has been suggested that reactions involving silver catalyze polystyrene degradation during SERS, but we found that silver is not necessary for the degradation to occur. Copyright © 2009 John Wiley & Sons, Ltd.

Strong location dependent surface enhanced Raman scattering on individual gold semishell and nanobowl particles

We demonstrate strong spatial localization of SERS on single symmetry-reduced gold semishell and nanobowl particles. A ∼30 nm carbon nanoparticle acts as a Raman reporter and is placed on different locations on a single semishell or nanobowl by e-beam induced deposition method, and remarkably different SERS intensities are observed.

Charge-Transfer-Induced Surface-Enhanced Raman Scattering on Ag−TiO2 Nanocomposites

In this paper, a series of silver-deposited TiO2 (Ag−TiO2) nanoparticles (NPs) with a varying content of Ag were prepared by a photoreduction method and were attempted to serve as SERS-active substrates for the first time. SERS signals of 4-MBA molecules adsorbed on Ag−TiO2 NPs were further enhanced considerably relative to those enhancements on pure TiO2 NPs. The surface-deposited Ag on TiO2 can inject additional electrons into molecules adsorbed on the TiO2 surface through the conduction band of TiO2 NPs because of plasmon resonance absorption of Ag under incident visible laser, besides the intrinsic TiO2-to-molecule charge-transfer (CT) contribution. The two contributions mentioned are responsible for the whole SERS intensity of the molecules adsorbed on Ag−TiO2 NPs. This work is valuable in developing nanosized TiO2 used as a promising, nontoxic and biologically compatible SERS-active substrate as well as in studying the CT mechanism between Ag and TiO2 for potential photoelectrochemical applications.

Aggregation of Gold Nanoframes Reduces, Rather Than Enhances, SERS Efficiency Due to the Trade-Off of the Inter- and Intraparticle Plasmonic Fields

It is usually observed and understood that aggregation of silver and gold solid nanoparticles gives rise to enhanced SERS spectra due to the increased plasmon field between the particles. In the present work, we observed that the increase of aggregation of Langmuir-Blodgett assembled 80 nm gold nanoframe particles reduces the efficiency of the surface-enhanced Raman spectra of adsorbed thiophenol molecules. Using discrete dipole approximation simulation of the plasmonic fields of a pair of nanoframes as a function of their interparticle separation, it is found that at large separation the fields inside the cavities are stronger than those outside. As the interpair separation decreases, the gain in the interparticle field does not make up for the loss in the field within the cavities, supporting the observation of the decrease in the SERS intensity with aggregation.

Wavelength-Dependent Surface-Enhanced Resonance Raman Scattering by Excitation of a Transverse Localized Surface Plasmon

We measured the polarization dependences of surface-enhanced (resonance) Raman scattering (SER(R)S) and localized surface plasmon (LSP) resonance Rayleigh scattering for the same single Ag nanoaggregate to explore whether SER(R)S is dominated by the resonance of incident light with LSPs. We excited the LSPs polarized parallel to the short axis of the Ag nanoaggregate (transverse LSPs) using laser lines from an Ar ion laser (λ = 458, 488, and 514 nm). The polarization dependence of SERRS intensities is opposite to that of the transverse LSPs whose maximum wavelengths are below 500 nm. On the contrary, the polarization dependence of SERS intensities becomes similar to that of the transverse LSPs with the maximum wavelengths above 550 nm. The wavelength-dependent correlation between the transverse LSP and SER(R)S due to the absence and presence of just one junction on the short axis of the Ag aggregate will lead to an accurate understanding of the correlation between LSP and SER(R)S.

Study of SERS Chemical Enhancement Factors Using Buffer Layer Assisted Growth of Metal Nanoparticles on Self-Assembled Monolayers

Highly controlled morphology Au nanoparticle films can be formed on the surfaces of self-assembled monolayers (SAMs) by vapor deposition at cryogenic temperatures (approximately 10 K) with intervening condensed Xe layers on the SAMs serving as a buffer to reduce the kinetic energy of the Au atoms impinging on the surface (buffer layer assisted growth or BLAG). Under these conditions pristine Au nanoparticles (AuNp) of a uniform shape and size were deposited onto two SAMs differing only by their terminal groups, 4-benzenedithiol (BDT) and 4-methylbenzenethiol (MBT), to form -S/Au and -CH(3)/Au interfaces with essentially identical AuNp overlayer morphologies. A surface enhanced Raman (SERS) enhancement factor ratio EF(BDT)/EF(MBT) of approximately 130 was observed uniformly across the surfaces (approximately <10% variation). Since equal electromagnetic contributions to the SERS enhancements are expected from the two identically structured Au overlayer films, the observed SERS intensity ratio accordingly reflects a pure chemical enhancement (CE) contribution arising from the -S/Au relative to the -CH(3)/Au interface and thereby provides the first quantitative experimental data for the magnitude of the SERS CE for well-defined Au-molecule contacts.

Subdiffraction Limited, Remote Excitation of Surface Enhanced Raman Scattering

We demonstrate that focused laser excitation at the end of silver nanowires of 50-150 nm diameter excites SERS hot-spots at points of nanoparticle adsorption many micrometers along the wire due to the plasmon waveguide effect. The total SERS intensity detected at the hot-spots following wire-end excitation correlates with the known wavelength, polarization, and distance dependences of surface plasmon polariton (SPP) propagation in nanowires. The SERS spectra obtained at the hot-spots following wire-end excitation show very little background compared to when excitation occurs directly at the hot-spot, suggesting that a much smaller SERS excitation volume is achieved by remote, waveguide excitation. The ability to transfer SERS excitation over several micrometers, through a structure with a subdiffraction limit diameter, is discussed with respect to potential high-resolution SERS imaging applications.

Direct evidence of chemical effect of surface-enhanced Raman scattering observed on electrochemically prepared rough gold substrates

In this study, polypyrrole (PPy) films were electrochemically deposited on gold substrates roughened by an electrochemical triangular-wave oxidation–reduction cycles (ORC) in an aqueous solution containing 0.1N KCl. Then the substrates were heated from 25 to 50 °C and the corresponding SERS performances of PPy were observed in situ. The results indicate that the SERS enhancement capabilities of substrates are gradually raised from 25 °C to a maximum at 40 °C and monotonically decreased from 40 to 50 °C. These SERS enhancement capabilities ascribed to the charge transfers from PPy to Au, which are responsible for the chemical effects of SERS mechanisms, are successfully observed via SERS and high resolution X-ray photoelectron spectroscopy (HRXPS) analyses. The variation in content of the oxidized PPy peak of the double peaks in the range of 1000–1150 cm −1 in SERS spectrum obtained on an Au substrate at different temperatures is consistent with its corresponding variation in the SERS intensity of PPy. The variation in content of the oxidized nitrogen of PPy deposited on an Au substrate at different temperatures revealed from an HRXPS analysis also confirms this consistence.

A novel reversed reporting agent method for surface-enhanced Raman scattering; highly sensitive detection of glutathione in aqueous solutions

In this study, a new application method for SERS named "reversed reporting agent" method is proposed for selective detection of biomolecules with a thiol group. In this method reporting agents such as rhodamine 6G (R6G) are capped on surfaces of silver colloidal nanoparticles. Analytes having a thiol group will replace the positions of reporting agents due to the strong interactions between silver and the thiol group, and then the SERS intensity of the reporting agents will be reduced. By monitoring the difference in the SERS intensity of reporting agents before and after the replacement of analytes, the concentration of the analytes can be estimated. To demonstrate the feasibility of this proposed method, glutathione was used as the analyte and R6G, crystal violet (CV), and thiacyanine (TC) were employed as the reporting agents, and factors that affect the capability of the determination of glutathione were investigated. The results indicate that R6G is the most suitable reporting agent, and the concentration of the reporting agents affects the sensitivity and selectivity of glutathione determination. Under the optimal conditions, the detection of glutathione can be finished within 20 minutes and the detection limit of ca. 1 microM can be achieved. Furthermore, the proposed method holds specific selectivity towards glutathione, which means it has possible practical applications.

Multiplex Immunoassay Using Fluorescent-Surface Enhanced Raman Spectroscopic Dots for the Detection of Bronchioalveolar Stem Cells in Murine Lung

Immunoassays using nanomaterials have been rapidly developed for the analysis of multiple biomolecules. Highly sensitive and biocompatible surface enhanced Raman spectroscopy-active nanomaterials have been used for biomolecule analysis by many research groups in order to overcome intrinsic problems of conventional immunoassays. We used fluorescent surface-enhanced Raman spectroscopic dots (F-SERS dots) to detect biomolecules in this study. The F-SERS dots are composed of silver nanoparticle-embedded silica nanospheres, organic Raman tagging materials, and fluorescent dyes. The F-SERS dots demonstrated highly sensitive, selective, and multifunctional characteristics for multiplex targeting, tracking, and imaging of cellular and molecular events in the living organism. We successfully applied F-SERS dots for the detection of three cellular proteins, including CD34, Sca-1, and SP-C. These proteins are simultaneously expressed in bronchioalveolar stem cells (BASCs) in the murine lung. We analyzed the relative expression ratios of each protein in BASCs since external standards were used to evaluate SERS intensity in tissue. Quantitative comparisons of multiple protein expression in tissue were first attempted using SERS-encoded nanoprobes. Our results suggested that immunoassays using F-SERS dots offered significant increases in sensitivity and selectivity. Such immunoassays may serve as the primary next-generation labeling technologies for the simultaneous analysis of multiple biomolecules.

Electrochemical Approach to and the Physical Consequences of Preparing Nanostructures from Gold Nanorods with Smooth Ends

We have developed a method to smooth the end sections of nanowires and nanograps generated via the On-Wire Lithography process and studied these rods with optical spectroscopies and theoretical modeling (Discrete Dipole Approximation). The first step of the smoothing process is a reductive one aimed at controlling the diffusion and migration of metal ions to the growing nanorod surface by adjusting the applied potential and concentration of the metal ions in the growth solution. A second oxidative smoothing step, based in part on the energetic differences between topologically rough and smooth surfaces, is used to further smooth the nanorod. The RMS roughness can be reduced over five fold to approximately 5 nm. The properties of these smoothed rods were investigated by empirical and theoretical methods, where it was found the smoothed rods have sharper plasmon resonances and decreased SERS intensity.

Sculpturing Effect of Chloride Ions in Shape Transformation from Triangular to Discal Silver Nanoplates

The sculpturing effect of chloride ions on the shape transformation of silver nanoparticles is presented. UV−vis spectroscopy and transmission electron microscopy (TEM) were used to monitor the evolution of silver nanoplates. Cl− can etch the corners and side faces of the silver nanoprism, and the resulting nanoparticles are disk-like in shape. The dissolved silver atoms would aggregate to form small silver clusters, which were stabilized by the Cl− and citrate ions. The facet-selective etching effect of Cl− is mainly attributed to the surface energy difference of each face of the nanoplate. The thickness of the nanodisk increased during the etching process because of the redeposition of sliver clusters on the {111} planes. The prepared nanodisk also gave rise to high SERS intensity of the probing molecule.

Polarization-Dependent Surface-Enhanced Raman Scattering from a Silver-Nanoparticle-Decorated Single Silver Nanowire

Single silver nanowires produced by DC electrodeposition in porous anodic alumina templates were surface-functionalized with the bifunctional molecule 4-aminobenzenthiol (ABT) then exposed to aqueous silver nanoparticles resulting in a silver nanoparticle-decorated silver nanowire. The polarization dependent surface-enhanced Raman (SERS) signal from this system showed significant intensity anisotropy when measured at a midsection of the nanowire, where the largest SERS intensities were observed when the incident light was polarized perpendicular to the nanowire's long axis but was almost isotropic near the tip of the nanowire. The observed effects were accounted for in terms of the electromagnetic fields concentrated in the collection of hot spots created through the ABT-linker-driven nanoparticle-nanowire self-assembly process.

Gold and magnetic oxide/gold core/shell nanoparticles as bio-functional nanoprobes

The ability to create bio-functional nanoprobes for the detection of biologicalreactivity is important for developing bioassay and diagnostic methods. Thispaper describes the findings of an investigation of the surface functionalization ofgold (Au) and magnetic nanoparticles coated with gold shells (M/Au) byproteins and spectroscopic labels for the creation of nanoprobes for use insurface enhanced Raman scattering (SERS) assays. Highly monodispersed Aunanoparticles and M/Au nanoparticles with two types of magnetic nanoparticle cores(Fe2O3 and MnZn ferrite) were studied as model systems for the bio-functionalization and Ramanlabeling. Comparison of the SERS intensities obtained with different particle sizes(30–100 nm) and samples in solution versus on solid substrates have revealed importantinformation about the manipulation of the SERS signals. In contrast to the salt-induceduncontrollable and irreversible aggregation of nanoparticles, the ability to use acentrifugation method to control the formation of stable small clustering sizes ofnanoparticles was shown to enhance SERS intensities for samples in solution as comparedwith samples on solid substrates. A simple method for labeling protein-capped Aunanoparticles with Raman-active molecules was also described. The functionalized Au andM/Au nanoparticles are shown to exhibit the desired functional properties for the detectionof SERS signals in the magnetically separated reaction products. These results arediscussed in terms of the interparticle distance dependence of ‘hot-spot’ SERS sites and thedelineation of the parameters for controlling the core–shell reactivity of the magneticfunctional nanocomposite materials in bio-separation and spectroscopic probing.

Optimization of the Preparation of Glass-Coated, Dye-Tagged Metal Nanoparticles as SERS Substrates

Dye-tagged metal nanoparticles are of significant interest in SERS-based sensitive detection applications. Coating these particles in glass results in an inert spectral tag that can be used in applications such as flow cytometry with significant multiplexing potential. Maximizing the SERS signal obtainable from these particles requires care in partitioning available nanoparticle surface area (binding sites) between the SERS dyes and the functionalized silanes necessary for anchoring the glass coating. In this article, we use the metal-mediated fluorescence quenching of SERS dyes to measure surface areas occupied by both dyes and silanes and thus examine SERS intensities as a function of both dye and silane loading. Notably, we find that increased surface occupation by silane increases the aggregative power of added dye but that decreasing the silane coverage allows a greater surface concentration of dye. Both effects increase the SERS intensity, but obtaining the optimum SERS intensity will require balancing aggregation against surface dye concentration.

Surface-Enhanced Raman Spectroscopy and Nanogeometry: The Plasmonic Origin of SERS

Using a series of highly regular nanostructures consisting of periodic Ag nanowires fabricated in porous aluminum oxide, we validate the overwhelmingly plasmonic origin of the most intense SERS signals such as those responsible for single-molecule SERS, demonstrating its sensitive dependence on the system's nanogeometry. By varying the interwire gap distance from 35 to 10 nm, the SERS intensity excited with 785 nm laser light, increased over 200-fold. These observations were shown to agree quantitatively with electromagnetic field calculations carried out using the free space green's tensor method.