Quantitative SERS sensors for environmental analysis of naphthalene

In the investigation of chemical pollutants, such as PAHs (Polycyclic Aromatic Hydrocarbons) at low concentration in aqueous medium, Surface-Enhanced Raman Scattering (SERS) stands for an alternative to the inherent low cross-section of normal Raman scattering. Indeed, SERS is a very sensitive spectroscopic technique due to the excitation of the surface plasmon modes of the nanostructured metallic film. The surface of quartz substrates was coated with a hydrophobic film obtained by silanization and subsequently reacted with polystyrene (PS) beads coated with gold nanoparticles. The hydrophobic surface of the SERS substrates pre-concentrates non-polar molecules such as naphthalene. Under laser excitation, the SERS-active substrates allow the detection and the identification of the target molecules localized close to the gold nanoparticles. The morphology of the SERS substrates based on polystyrene beads surrounded by gold nanoparticles was characterized by scanning electron microscopy (SEM). Furthermore, the Raman fingerprint of the polystyrene stands for an internal spectral reference. To this extent, an innovative method to detect and to quantify organic molecules, as naphthalene in the range of 1 to 20 ppm, in aqueous media was carried out. Such SERS-active substrates tend towards an application as quantitative SERS sensors for the environmental analysis of naphthalene.     

Mechanistic study of silver nanoparticle formation on conducting polymer surfaces

Conducting polymer (polyaniline) sheets are shown to be active substrates to promote the growth of nanostructured silver thin films with highly tunable morphologies. Using the spontaneous electroless deposition of silver, we show that a range of nanostructured metallic features can be controllably and reproducibly formed over large surface areas. The structural morphology of the resulting metal-polymer nanocomposite is demonstrated to be sensitive to experimental parameters such as ion concentration, temperature, and polymer processing and can range from densely packed oblate nanosheets to bulk crystalline metals. The deposition mechanisms are explained using a diffusion-limited aggregation (DLA) model to describe the semi-fractal-like growth of the metal nanostructures. We find these composite films to exhibit strong surface-enhanced Raman (SERS) activity, and the nanostructured features are optimized with respect to SERS activity using a self-assembled monolayer of mercapto-benzoic acid as a model Raman reporter. SERS enhancements are estimated to be on the order of 10(7). Through micro-Raman SERS mapping, these materials are shown to exhibit uniform SERS responses over macroscopic areas. These metal-polymer nanocomposites benefit from the underlying polymer's processability to yield SERS-active materials of almost limitless shape and size and show significant promise for future SERS-based sensing and detection schemes.     

Porous GaN as a template to produce surface-enhanced Raman scattering-active surfaces

Surface-enhanced Raman spectroscopy (SERS) substrates have been prepared by depositing Au or Ag on porous GaN (PGaN). The PGaN used as the template for the metal deposition in these studies was generated by a Pt-assisted electroless etching technique. PGaN was chosen as a potential SERS template due to its nanostructured surface and high surface area, two characteristics that are important for SERS substrates. Metal films were deposited either by solution-based electroless deposition or by thermal vacuum evaporation. SERS spectra were recorded at lambda = 752.5 nm for Au films and at lambda = 514.5 nm for Ag films deposited on PGaN. The SERS signal strength across the metal coated PGaN substrates was uniform and was not plagued by "hot" or "cold" spots on the surface, a common problem with other SERS surfaces. The Ag film deposited by electroless deposition had the highest overall SERS response, with an enhancement factor (EF) relative to normal Raman spectroscopy of 10(8). A portion of the increase in EF relative to typical SERS-active substrates can be assigned to the large surface area characteristic of the PGaN-Ag structures, but some of the enhancement is intrinsic and is likely related to the specific morphology of the metal-nanopore composite structure.     

Gold films deposited over regular arrays of polystyrene nanospheres as highly effective SERS substrates from visible to NIR

Gold nanostructured films of various thicknesses (15, 30, and 60 nm) are deposited over regular arrays of polystyrene nanospheres in an attempt to evaluate their potential as SERS-active substrates. Atomic force microscopy is used to topographically characterize the substrates as well as to ensure the thickness of the deposited gold films. The optical response of the prepared substrates recommends their use in SERS experiments with multiple laser lines from visible and NIR spectral domains. The assessment of the substrates' SERS activity is performed by using the 532, 633, and 830 nm excitation lines and different average enhancement factor (EF) values are obtained depending on the film thickness and employed laser line. The 60 nm gold nanostructured film generates the greatest local electromagnetic field confinement under NIR excitation and consequently gives rise to maximum SERS enhancement. The large tunability of surface plasmon excitation combined with the advantage of relatively high exhibited average EF values obtained under NIR excitation recommends these substrates as outstanding candidates for upcoming investigations of biological relevant molecules.     

Surface-enhanced Raman scattering on silver nanostructured films prepared by spray-deposition

Silver nanostructured films were directly prepared by spray deposition of preformed polyol-based Ag-PVP nanoparticles. These homogeneous films of high optical quality were tested as SERS-active substrates. Laser excitation at 514.5 nm within the red part of the plasmon band leads to intense and reproducible SERS spectra of acridine, used as the probe molecule. From SERS measurements at different pH values, it was possible to determine the apparent pK(a) of acridine and to obtain specific surface properties of the film. Finally, these SERS titrations along with enhancement factor estimates allowed us to further depict the nature of the films.     

Surface-enhanced Raman scattering: realization of localized surface plasmon resonance using unique substrates and methods

Surface-enhanced Raman scattering (SERS) enhancement and the reproducibility of the SERS signal strongly reflect the quality and nature of the SERS substrates because of diverse localized surface plasmon resonance (LSPR) excitations excited at interstitials or sharp edges. LSPR excitations are the most important ingredients for achieving huge enhancements in the SERS process. In this report, we introduce several gold and silver nanoparticle-based SERS-active substrates developed solely by us and use these substrates to investigate the influence of LSPR excitations on SERS. SERS-active gold substrates were fabricated by immobilizing colloidal gold nanoparticles on glass slides without using any surfactants or electrolytes, whereas most of the SERS-active substrates that use colloidal gold/silver nanoparticles are not free of surfactant. Isolated aggregates, chain-like elongated aggregates and two-dimensional (2D) nanostructures were found to consist mostly of monolayers rather than agglomerations. With reference to correlated LSPR and SERS, combined experiments were carried out on a single platform at the same spatial position. The isolated aggregates mostly show a broadened and shifted SPR peak, whereas a weak blue-shifted peak is observed near 430 nm in addition to broadened peaks centered at 635 and 720 nm in the red spectral region in the chain-like elongated aggregates. In the case of 2D nanostructures, several SPR peaks are observed in diverse frequency regions. The characteristics of LSPR and SERS for the same gold nanoaggregates lead to a good correlation between SPR and SERS images. The elongated gold nanostructures show a higher enhancement of the Raman signal than the the isolated and 2D samples. In the case of SERS-active silver substrates for protein detection, a new approach has been adopted, in contrast to the conventional fabrication method. Colloidal silver nanoparticles are immobilized on the protein functionalized glass slides, and further SERS measurements are carried out based on LSPR excitations. A new strategy for the detection of biomolecules, particularly glutathione, under aqueous conditions is proposed. Finally, supramolecular J-aggregates of ionic dyes incorporated with silver colloidal aggregates are characterized by SERS measurements and correlated to finite-difference time-domain analysis with reference to LSPR excitations. Figure SPR and SERS images for isolated, elongated and two-dimensional gold nanostructures     

Fabrication and characterization of homogeneous surface-enhanced Raman scattering substrates by single pulse UV-laser treatment of gold and silver films

The fabrication of SERS-active substrates, which offer high enhancement factors as well as spatially homogeneous distribution of the enhancement, plays an important role in the expansion of surface-enhanced Raman scattering (SERS) spectroscopy to a powerful, quantitative, and noninvasive measurement technique for analytical applications. In this paper, a novel method for the fabrication of SERS-active substrates by laser treatment of 20, 40, and 60 nm thick gold and of 40 nm thick silver films supported on quartz glass is presented. Single 308 nm UV-laser pulses were applied to melt the thin gold and silver films. During the cooling process of the noble metal, particles were formed. The particle size and density were imaged by atomic force microscopy. By varying the fluence, the size of the particles can be controlled. The enhancement factors of the nanostructures were determined by recording self-assembled monolayers of benzenethiol. The intensity of the SERS signal from benzenethiol is correlated to the mean particle size and thus to the fluence. Enhancement factors up to 10(6) with a high reproducibility were reached. Finally we have analyzed the temperature dependence of the SERS effect by recording the intensity of benzenethiol vibrations from 300 to 120 K. The temperature dependence of the SERS effect is discussed with regard to the metal properties.     

Highly reproducible surface-enhanced Raman scattering-active Au nanostructures prepared by simple electrodeposition: Origin of surface-enhanced Raman scattering activity and applications as electrochemical substrates

The fabrication of effective surface-enhanced Raman scattering (SERS) substrates has been the subject of intensive research because of their useful applications. In this paper, dendritic gold (Au) rod (DAR) structures prepared by simple one-step electrodeposition in a short time were examined as an effective SERS-active substrate. The SERS activity of the DAR surfaces was compared to that of other nanostructured Au surfaces with different morphologies, and its dependence on the structural variation of DAR structures was examined. These comparisonal investigations revealed that highly faceted sharp edge sites present on the DAR surfaces play a critical role in inducing a high SERS activity. The SERS enhancement factor was estimated to be greater than 10⁵, and the detection limit of rhodamine 6G at DAR surfaces was 10⁻⁸ M. The DAR surfaces exhibit excellent spot-to-spot and substrate-to-substrate SERS enhancement reproducibility, and their long-term stability is very good. It was also demonstrated that the DAR surfaces can be effectively utilized in electrochemical SERS systems, wherein a reversible SERS behavior was obtained during the cycling to cathodic potential regions. Considering the straightforward preparation of DAR substrates and the clean nature of SERS-active Au surfaces prepared in the absence of additives, we expect that DAR surfaces can be used as cost-effective SERS substrates in analytical and electrochemical applications.     

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.     

Designed fabrication of ordered porous au/ag nanostructured films for surface-enhanced Raman scattering substrates

This article reports the designed preparation of two different kinds of novel porous metal nanostructured films, namely, an ordered macroporous Au/Ag nanostructured film and an ordered hollow Au/Ag nanostructured film. Different from previous reports, the presently proposed method can be conveniently used to control film structures by simply varying the experimental conditions. The morphology of these films has been characterized by scanning electron microscopy (SEM), and their performance as surface-enhanced Raman scattering (SERS) substrates has been evaluated by using rhodamine 6G (R6G) as a probe molecule. We show that such porous nanostructured films consisting of larger interconnected aggregates are highly desirable as SERS substrates in terms of high Raman intensity enhancement, excellent stability, and reproducibility. The interconnected nanostructured aggregate, long-range ordering porosity, and nanoscale roughness are important factors responsible for this large SERS enhancement ability.     

金纳米粒子组装体系SERS化学增强的研究

Gold nanoparticles were assembled on gold substrates with the self-assembled monolayer(SAM) of p-minothiophenol(PATP). AFM measurements disclose that gold nanoparticles are scattered over the surface of the substrate with a submonolayer coverage. The Raman signal of the coupling layer, the SAM of PATP, can be well observed. Potential-dependent measurements were performed to study the chemical enhancement in SERS of such a system. Based on the supposition that the direction of charge transfer is from gold nanoparticles to PATP, it is deduced that Herzberg-Teller contribution has ruled in the SERS of such a system.     

Spatially focused deposition of capillary electrophoresis effluent onto surface-enhanced Raman-active substrates for off-column spectroscopy

Surface-enhanced Raman scattering (SERS) is employed to obtain distinctive spectra of compounds that are efficiently separated by capillary electrophoresis (CE) and deposited onto planar SERS-active substrates. A simple method is described that explains how to prepare SERS-active substrates by depositing a silver-colloid solution onto frosted-glass microscope slides, using a high-efficiency nebulizer. Scanning electron micrographs reveal a layered coating of fairly uniform-sized, 100-nm silver nanoparticles with interstitial spaces ranging from a few to tens of nanometers. The on-column separation is monitored by laser-induced fluorescence, while electrofilament depositing the CE effluent onto a moving SERS-substrate. Subsequently, the SERS spectra and off-column electropherograms are obtained with a simple confocal Raman spectrometer. The test compounds used to demonstrate this technique include compounds of biological significance: benzyloxyresorufin, riboflavin, and resorufin. CE and Raman conditions are evaluated to determine their affects on the SERS signals. An average off-column efficiency of 100,000 plates/m and a signal reproducibility of 11% relative standard deviation were achieved. Characteristic spectra with major Raman bands exhibiting signal-to-noise ratios of greater than 3 were obtained for a 3.2-nL injection of 10(-6) M (706 fg) resorufin. Forming a self-assembled monolayer (SAM) on the substrate increases the sensitivity of the SERS technique and decreases the on-substrate broadening. Calibration plots for both plain- and SAM-SERS substrates are demonstrated.     

Adsorption Behaviors of 4-Mercaptobenzoic Acid on Silver and Gold Films

The adsorption behaviors of 4-mercaptobenzoic acid on silver and gold nanoparticles were studied by surface-enhanced Raman scattering (SERS) and density functional theory. The silver and gold films by electrodeposition have the same excellent characteristics as SERS-active substrates. At the same, the SERS spectra indicate that 4-mercaptobenzoic acid molecules are adsorbed on the surfaces of gold nanoparticles through the S atom, and that the carboxyl group is far away from surface of gold nanoparticles, and that there is a certain angle between the plane of benzene ring and gold film. However, 4-mercaptobenzoic acid molecules are adsorbed on the surfaces of silver nanoparticles through the carboxyl group, and the S atom is far away from surface of silver nanoparticles, and there is also a certain angle between the plane of benzene ring and the surface of silver nanoparticles. Here it is demonstrated the calculated Raman frequencies are in good agreement with experimental values, and the calculated Raman frequencies are also helpful to infer the adsorption behaviors of 4-mercaptobenzoic acid molecules.     

Lab-on-a-bubble: synthesis, characterization, and evaluation of buoyant gold nanoparticle-coated silica spheres

This paper describes the development and preparation of a new class of materials for surface-enhanced Raman scattering (SERS) consisting of gold nanoparticles coated onto hollow, buoyant silica microspheres. These materials allow for a new type of molecular assay designated as a lab-on-a-bubble (LoB). LoB materials serve as a convenient platform for the detection of analytes in solution and offer several advantages over traditional colloidal gold and planar SERS substrates, such as the ability to localize and concentrate analytes for detection. An example assay is presented using the LoB method and cyanide detection. Cyanide binds to SERS-active, gold-coated LoBs and is detected directly from the corresponding SERS signal. The abilities of LoBs and a gold colloid to detect cyanide are compared, and in both cases, a detection limit of ~170 ppt was determined. Differences in measurement error using LoBs versus gold colloid are also described, as well as an assay for 5,5'-dithiobis(2-nitrobenzoic acid) that shows the benefit of using LoBs over SERS analyses in colloids, which are often plagued by particle aggregation.     

Temperature-assisted ionic liquid dispersive liquid-liquid microextraction combined with high performance liquid chromatography for the determination of PCBs and PBDEs in water and urine samples

We report on silver–gold core-shell nanostructures that contain Methylene Blue (MB) at the gold–silver interface. They can be used as reporter molecules in surface-enhanced Raman scattering (SERS) labels. The labels are stable and have strong SERS activity. TEM imaging revealed that these nanoparticles display bright and dark stripe structures. In addition, these labels can act as probes that can be detected and imaged through the specific Raman signatures of the reporters. We show that such SERS probes can identify cellular structures due to enhanced Raman spectra of intrinsic cellular molecules measured in the local optical fields of the core-shell nanostructures. They also provide structural information on the cellular environment as demonstrated for these nanoparticles as new SERS-active and biocompatible substrates for imaging of live cells.

Tuning plasmons on nano-structured substrates for NIR-SERS

Surface-Enhanced Raman Spectroscopy (SERS) is a very sensitive and selective technique for detecting surface species. Colloidal crystal-templated 'inverse opal' nanostructured gold films have been demonstrated to be excellent SERS substrates by various researchers around the globe. However, visible excitation laser sources commonly used in SERS experiments can cause photochemical reactions on the surface as well as fluorescence from the adsorbed molecules. A way to circumvent this possibility is the use of Near Infra-Red (NIR) laser sources. This demands appropriate design of substrates for NIR-SERS in order to obtain maximum enhancement of signals from analytes. In the current paper, we use systematic variation of sphere size and electrochemical control over film height to tune plasmons on such nanovoid substrates. We use plasmon maps as a tool for predicting NIR-SERS enhancements recorded with a 1064 nm laser source for benzenethiol as the probe molecule. Direct correlation is observed between Raman enhancements and plasmonic resonances with ingoing and outcoming radiation. Our study demonstrates the feasibility of plasmon engineering and the predictive power of their mapping on our substrates. It also demonstrates the ability to design reproducible NIR-SERS substrates and its empirical fruition.     

Surface-Enhanced Raman Scattering (SERS) on transition metal and semiconductor nanostructures

Surface-Enhanced Raman Scattering (SERS) spectroscopy has experienced a rapid growth over the past 30 years, and has become a valuable tool in various research areas. In conjunction with recent explosive development of nanoscience and nanotechnology, the SERS-active substrates have also expanded from traditional Group 11 metals (Au, Ag, Cu) to non-Group 11 nanostructures. This paper gives an overview of historical advances in the use of non-Group 11 nanostructures as substrates for SERS. Several possible mechanisms and important factors for SERS from non-Group 11 nanostructures are discussed in detail. The SERS from non-Group 11 nanostructures provides many significant applications in surface, interface analysis and biochemical detection. It is reasonable to believe that the advancement in the non-Group 11 nanostructures-based SERS-active substrates will lead to a more promising future for the SERS technology in surface science, spectroscopy and biomedicine.     

Characterization of the surface enhanced raman scattering (SERS) of bacteria

The surface enhanced Raman scattering (SERS) of a number of species and strains of bacteria obtained on novel gold nanoparticle (approximately 80 nm) covered SiO(2) substrates excited at 785 nm is reported. Raman cross-section enhancements of >10(4) per bacterium are found for both Gram-positive and Gram-negative bacteria on these SERS active substrates. The SERS spectra of bacteria are spectrally less congested and exhibit greater species differentiation than their corresponding non-SERS (bulk) Raman spectra at this excitation wavelength. Fluorescence observed in the bulk Raman emission of Bacillus species is not apparent in the corresponding SERS spectra. Despite the field enhancement effects arising from the nanostructured metal surface, this fluorescence component appears "quenched" due to an energy transfer process which does not diminish the Raman emission. The surface enhancement effect allows the observation of Raman spectra of single bacterial cells excited at low incident powers and short data acquisition times. SERS spectra of B. anthracis Sterne illustrate this single cell level capability. Comparison with previous SERS studies reveals how the SERS vibrational signatures are strongly dependent on the morphology and nature of the SERS active substrates. The potential of SERS for detection and identification of bacterial pathogens with species and strain specificity on these gold particle covered glassy substrates is demonstrated by these results.     

Preparation of gold colloid monolayer by immunological identification

The design and initial characterization of the self-assembled gold colloid monolayer by a sandwich structure via the immunological identification are reported. The 13 nm gold colloid nanoparticles and the silicon or quartz substrates have been modified with the mouse polyclonal antibody against hepatitis B virus surface antigen (PAb) and the mouse monoclonal antibody against hepatitis B virus surface antigen (MAb), respectively. They can be linked by a special reaction with their corresponding hepatitis B virus surface antigen (Antigen) as a sandwich structure. Thus, the density of gold nanoparticles self-assembled on the substrate can be readily controlled by the amount of the antigen added. The resulting substrates have been characterized by atomic force microscopy (AFM) and surface-enhanced Raman scattering (SERS) spectroscopy when the gold nanoparticles were modified with SERS-active probe molecules of 4-mercaptobenzoic acid (MBA) after silver enhancement. These data show that the gold nanoparticles are separately fixed onto the substrate and form a uniform monolayer, which possess a set of features that make them very attractive for both basic and applied uses, including roughness, high stability, and biocompatibility.