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Investigation of the chemical enhancement contribution to SERS using a Kretschmann arrangement
Abstract
Using a Kretschmann configuration-based setup, surface-enhanced Raman scattering (SERS) experiments have been performed on a monolayer of Nile blue. The experiment has been optimized to achieve reproducible conditions. Laser excitation resulted in a fast decay of the Raman line intensities ending at a stable signal level. We have analyzed this intensity variation mode specifically. The Raman deactivation rate was found to be different for different vibrational modes where high-wavenumber vibrations showed slower decay than the group of low-wavenumber modes. SERS spectra were obtained excluding the contribution of this deactivation process for different angles of incidence of the exciting laser beam. The variation of the surface plasmon excitation in the thin silver film coated onto the prism surface of the Kretschmann configuration resulted in drastically different relative enhancements of the different Raman modes pointing to a major contribution of the chemical enhancement mechanism in the single-layer SERS experiment. The enhancement was found to be mode-specific. High-wavenumber modes showed a stronger enhancement than the low-wavenumber group. Copyright
... Details of this setup have already been explained earlier. 44 The experimental setup is designed such that well-defined excitation and signal collection conditions are achieved. Figure 2 shows the situation for excitation at the resonance angle (right) and preresonance excitation (left). ...
... In order to avoid the influence of photobleaching effects, the experiments have been done after irradiating the samples long enough in order to avoid time-dependent variation of the Raman intensity as we have reported in previous experiments. 44 We have demonstrated a strong dependence of the CE on the vibrational modes, which is not expected for a uniform field enhancement caused by EME. Figure 5 shows the KC SERS spectra of a dried droplet of NB in air on silver (45 nm film) measured under an angle of the incident laser beam fulfilling the SPP resonance condition and for an angle detuned from this angle by +3°. The same experiments have been repeated for NB separated from the silver layer by an ODT monolayer deposited on top of the silver film, again for resonance and nonresonance conditions. ...
... We find a considerable vibrational band dependence of the enhancement when we have no intermediate layer, i.e., different bands gain different enhancements when moving from preresonance to resonance conditions, as was already mentioned above and was observed by us earlier. 44 However, the enhancement seems to be nearly uniform for all Raman modes when we introduce the intermediate layer, which is in accordance to the assumption of a pure-field enhancement. In other words, in the case of pure EME, we would not expect that different bands gain considerably different enhancements by just going from resonance to preresonance conditions. ...
... Ultrathin carbon material has been applied in many fields because of its outstanding physical and chemical properties such as electrical and thermal conductivity and mechanical stability [18]. Its derivative graphene oxide has various surface radicals including epoxy, hydroxyl, and carboxyl, which offer good chemical affinity with target molecules whose electron structure and chemical properties can be tuned by altering the content and type of surface radicals [19]. Thus, graphene has attracted much attention since its discovery in 2004. ...
A new surface-enhanced Raman spectroscopy (SERS) biosensor of Graphene@Ag-MLF composite structure has been fabricated by loading AgNPs on graphene films. The response of the biosensor is based on plasmonic sensing. The results showed that the enhancement factor of three different spores reached 10⁷ based on the Graphene@Ag-MLF substrate. In addition, the SERS performance was stable, with good reproducibility (RSD<3%). Multivariate statistical analysis and chemometrics were used to distinguish different spores. The accumulated variance contribution rate was up to 96.35% for the top three PCs, while HCA results revealed that the spectra were differentiated completely. Based on optimal principal components, chemometrics of KNN and LS-SVM were applied to construct a model for rapid qualitative identification of different spores, of which the prediction set and training set of LS-SVM achieved 100%. Finally, based on the Graphene@Ag-MLF substrate, the LOD of three different spores was lower than 10² CFU/mL. Hence, this novel Graphene@Ag-MLF SERS substrate sensor was rapid, sensitive, and stable in detecting spores, providing strong technical support for the application of SERS technology in food safety.
Graphical abstract
... SPs can be generally divided into two categories: localized SP polaritons (LSPs) based on metal NPs and propagating SP polaritons (SPPs) based on metal nanostructures. Conventionally, there are two mechanisms to implement SERS: electromagnetic enhancement [8][9][10] (EM) originated from SPs and chemical enhancement [11][12][13] (CM) stemmed from charge transfer between metal structure and molecules. Due to the fact that the effect of EM is several orders of magnitude greater than CM [14], EM mechanism has a priority when designing more efficient SERS substrate. ...
... Here, beam falloff is due to limitations in the collection angle of the oil immersion lens. Still, the electromagnetic enhancement component introduced by SPPs on planar surfaces, while not particularly large, is highly uniform, reproducible and simple to control, therefore, suitable for the investigation of chemical enhancement contributions in particular 112 . ...
Over more than 40 years since its discovery, surface‐enhanced Raman scattering/spectroscopy (SERS) has attracted substantial attention as a tool for fundamental research in surface science and for potential analytical sensing applications. This review aims to concisely summarize the essential principles of both the physical and chemical processes underlying Raman signal enhancement, as well as elements of the broader field of plasmonics that are most relevant to SERS. In order to make sense of the overwhelming variety of SERS platforms that are reported, the fabrication, properties, and operational principles of enhancing substrates are given particular attention here in the context of specific application requirements. While the high spatial precision of lithographic techniques has helped to improve the understanding of plasmonic platforms, their prohibitive cost has encouraged the exploration of more facile fabrication methods for more widespread adoption of SERS. Selected examples are introduced to illustrate common themes that recur in fundamental studies, analytical applications, and efforts to produce stable, affordable, and robust SERS sensing platforms.
In 1974 Fleischmann et al. observed a strong enhancement of Raman scattering when molecules were in contact or proximity to the surface of nanostructured coin metal surfaces. The “Surface-Enhanced Raman Scattering” (SERS) has attracted significant attention in many scientific research fields as a powerful spectroscopic tool and has been the subject of extensive research since then. This chapter will give an overview about applications of SERS but will also focus on the investigation of the contributions to the SE effect. Different techniques will be introduced including special arrangements like the so-called Kretschmann setup. In the discussion of the fundamentals of SERS, two main mechanisms play an important role, electromagnetic and chemical enhancement. These mechanisms have been studied by many researchers where most concentrated on the electromagnetic mechanism. The main goal was obtaining a bigger enhancement factor and several groups have reported high enhancement ratios, reaching up to 1012. The chemical or electronic SE mechanism on the other hand still is not completely understood. However, its contribution to SERS must not be neglected since it is based on a direct coupling between the electronic systems of metal and chemisorbed molecules, which results in changes of the Raman spectral features. The chapter will discuss results obtained from investigations of this mechanism. The results summarized in this chapter will help to use SERS as a reliable and robust tool in modern science.
In this paper, a multiplex signal amplification strategy was developed for the determination of miR-214 and miR-221 on a surface-enhanced Raman scattering (SERS)-enabled lab-on-a-chip (LoC) system to realize the early-stage diagnosis of Parkinson's disease (PD). The gold nanobipyramids (GNBPs) with great monodispersity were functionalized with Raman reporter molecules and hairpin DNA 1, serving as the SERS nanotags. The presence of targets can initial the strand displacement amplification (SDA) reaction and the numerous short-stranded trigger DNA (tDNA) can be released under the action of polymerase and nicking enzyme. Then, the tDNA can trigger the catalytic hairpin assembly (CHA) event between the SERS nanotags and the capture nanoprobes (Magnetic beads (MBs) modified with hairpin DNA 2), resulting in the aggregation of GNBPs on the MBs surface. The multiplex signal amplification contributed by the SDA-CHA strategy and the magnet-induced aggregation effect can ultimately lead to the significant improvement of the detection sensitivity and the limit of detection (LOD) was low to aM level with reproducibility and specificity meanwhile. Furthermore, a MPTP-induced PD mice model was established to verify the practicability and the expression level of miR-214 and miR-221 at different stages analyzed with the LoC system was confirmed by qRT-PCR.
A relatively simple experimental procedure is proposed for the simultaneous detection and quantitative assessment of moxifloxacin (Moxi) as an example for an antibiotic using surface‐enhanced Raman spectroscopy (SERS) performed in a Kretschmann configuration (KC). The example of Moxi shows the advantage of this approach, such as high sensitivity and relatively simple experimental procedure. The Moxi was reliably detected at levels of 100 nM using excitation laser powers as low as a few milliwatts. We also demonstrate that in the KC, the direct coupling between the electronic systems of analyte molecules and metal substrate contributing to the chemical enhancement mechanism in SERS plays a major role. For this, we have performed simulations based on density functional theory (DFT). The line profile of the SERS spectra can be explained by the direct coupling of molecular sites to the metal. This adds to the molecular specificity of SERS when using the KC. Detection of moxifloxacin down to 100 nM in aqueous solution by surface‐enhanced Raman spectroscopy (SERS) using a highly reproducible Kretschmann configuration. Experimental and DFT investigations are presented pointing to a major contribution of the chemical SERS mechanism when using a well‐defined flat silver layer on the prism.
This article reports the theoretical and experimental study of nonresonant mechanisms (CHEM) of surface‐enhanced Raman spectroscopy (SERS) of pyridine (Py). An improvement in the Raman dispersion intensity of Py is determined after the chemical absorption on CuO nanoparticles synthesized in NaCl. The density functional theory predicts the formation of new electronic states as result of the chemical bond (CB) established between the Py molecule and the (CuO)n clusters. This study is focused on the study of the most intense vibrational modes of Py before and after chemical adsorption. The CHEM by CB mechanisms is found as responsible for the SERS effect in Py. CuO nanoparticles were obtained in NaCl matrix. Overall enhancement of the Raman linesof Py have been found, consequence of chemical bonding stablished with CuO Nps. The DFTcalculation of Py chemical adsorbed in cluster of (CuO)n, reveal the formation of new electronicstates conducing to spatial redistribution of HOMO‐LUMO orbitals and the increase of Ramanlines intensities until 103.The main contribution to the SERS effect of Py is attributed tochemical enhancement.
Interleukin 8 is known as an important chemokine and plays an important role in tumor growth and angiogenesis. Over expression of IL-8 has been detected in a variety of human tumors, including gastric cancer and breast cancer. Therefore, the development of rapid and sensitive profiling methods is crucial for evaluating the level of IL-8 expression in human serum which is related to normal and tumor states. In our study, surface-enhanced Raman scattering (SERS) based on the double antibody sandwich format is reported for the determination of IL-8. Highly-branched gold nanoparticles (HGNPs) substrates were fabricated and conjugated with anti-IL-8 (coating) as capturing substrates, and gold nanocages (GNCs) adsorbed with 4-mercaptobenzonic acid are modified with anti-IL-8 (labeling) as SERS tags. After substrates capturing IL-8, the SERS tags were bonded to the captured IL-8. The interaction of SERS capturing substrates (HGNPs) and SERS tags (GNCs) show high sensitivity and a low detection limit for IL-8. The linear dynamic range of SERS for IL-8 was from 10 pg/mL to 1 g/mL which has a linear relationship between IL-8 concentration and SERS intensity. The detection limit was 6.04 pg/mL in phosphate buffered saline and 6.88 pg/mL in human serum. The analysis results of serum samples obtained from healthy subjects and two kinds of cancer patients (breast cancer and gastric cancer) by using the SERS method were acceptably consistent with those obtained from the enzyme-linked immunosorbent assay. The proposed immunoassay exhibits a great potential for application in clinical diagnosis.
In this paper, we reported a simple one-step synthesis of highly-branched gold nanostructures (HGNs) in high yields. The reduction of HAuCl4 was accomplished by dopamine hydrochloride in the reaction system. By varying the amount of dopamine hydrochloride, HAuCl4 and the reaction temperature, we managed to tune the size of the HGNs from 200 to 600 nm. Systematic analysis revealed that the optical properties and surface-enhanced Raman scattering (SERS) activities of the HGNs were highly dependent on their morphology and size. In terms of their SERS activities, it was found that the HGNs synthesized at 60 °C with 2.0 mL dopamine hydrochloride (53 mM), 0.4 mL HAuCl4 (50 mM) exhibited the largest SERS enhancement. When the HGNs were assembled onto the silicon wafers, outstanding SERS efficiency was obtained with a detection limit of 5×10-10 M of 4-mercaptobenzoic acid (4-MBA) and the analytical enhancement factor (AEF) was calculated to be 7×107. Besides, the 3-aminopropyltriethoxysilane (APTES)-functionalized substrates with the HGNs displayed remarkable signal reproducibility with relative standard deviation (RSD) of 3.57%. All these results demonstrated that the SERS-active substrates held great promise to be applied in trace-level molecule detection in the future.
Compare with the two-dimensional (2D) structure of graphene, introducing three-dimensionality to graphene can provide more molecular adsorption sites by fully using the three-dimensional volume of laser. Here, we describe the fabrication of 3D wrinkled Ag NPs-graphene hybrid film which rarely contains two classes of wrinkles, micro wrinkles and nano wrinkles. We obtain a new 3D wrinkled Ag NPs-graphene hybrid structure by the buckling and crumpling of graphene to produce nano wrinkles with the formation of the micro wrinkles of the polystyrene (PS) polymer. We also have carefully examined evolution of crumpling and optimized the size and spacing of deposited Ag NPs on graphene for higher SERS intensity. The 3D Ag NPs-graphene hybrid film shows at least 1 order of magnitude lower detection limit than that of the flat one by SERS detection of triphenyl phosphine (PPh3). We believe that this wrinkled Ag NPs-graphene hybrid structure could be applied as a great potential platform for highly sensitive SERS substrate.
This review, published annually, provides an overview of advances in the field of Raman spectroscopy as found in papers published in the preceding calendar year in the Journal of Raman Spectroscopy (JRS), as well as in trends over the past decade across journals that have published papers important to the field of Raman spectroscopy. This information is obtained from statistical data on article counts obtained from Clarivate Analytics' Web of Science Core Collection by year and by subfield of Raman spectroscopy. Additional information is gleaned from presentations at the IX International Conference on Advanced Vibrational Spectroscopy (ICAVS-9) in Victoria, British Columbia, Canada, in June 2017 and those featuring Raman scattering at SCIX 2017 organized by the Federation of Analytical Chemistry and Spectroscopy Societies (FACSS) in Reno, Nevada, USA, in October 2017. Coverage is also provided for topics from the European Conference on Non-linear Optical Spectroscopy (ECONOS 2017) held in April 2017 in Jena, Germany, and the Ninth Congress on the Application of Raman in Art and Archaeology (RAA 2017) in October 2017 in Evora, Portugal. Finally, papers published in JRS in 2016 are highlighted and arranged by topics at the frontier of Raman spectroscopy. Based on this survey information, it is clear that Raman spectroscopy maintains a rapidly expanding influence across a wide range of novel disciplines and applications that provide sensitive photonic information of matter at the molecular level.
Nanomedicines intergrating both therapy and diagnosis functions provide promising strategy for anticancer treatment. As novel 2D materials, black phosphorus nanosheets (BPs) possess unique properties for biomedical applications, partically for photothermal therapy (PTT) of cancer, but its lack of air- and water-stability may hinder its application. Herein, a covalent functionalization strategy based on Nile Blue (NB) dye via diazonium chemistry is established to modify BPs, not only enhancing the stability of BPs, but also rendering BPs near-infrared (NIR) fluorescence, forming a novel multifunctional nanomedicine with both PTT and NIR-imaging capabilities. In vitro tests demonstrate that the dye-modified BPs (named NB@BPs) have good biocompatibility, and exhibit strong PTT and NIR-imaging efficiency. In vivo experiments show that the NB@BPs can mark the tumor site with red fluorescence, and lead efficient tumor ablation under NIR-irradiation. These results reveal a potential BPs-based nanomedicine with multiple functionalities boding well for anticancer applications.
We report a green and reusable surface-enhanced Raman scattering (SERS) film based on PMMA/Ag NPs/graphene. By using this Raman substrate, the SERS signals of R6G were significantly enhanced reaching a minimum detectable concentration of 5 × 10⁻⁸ M, due to having lots of hot spots adhered backside to the exposed graphene. The SERS film can be used for in-situ monitoring of trace thiram in apple juice with a detection limit of 1 × 10⁻⁶ M (0.24 ppm), which is below the maximal residue limit (MRL) of 7 ppm in fruit prescribed by the U.S. Environmental Protection Agency (EPA). Furthermore, reusability studies show that the SERS film can be used repeatedly. In addition, the graphene-enhanced SERS technique shows great potential applications for the in-situ detection and identification of pesticide residues in environmental water, fruits and vegetables.
Surface enhanced Raman scattering (SERS) effect is currently exploited as the basis of a new
type of optical labels for in vivo investigation of the tissues, especially for early medical diagnostic. Silver
colloidal nanoparticles decorated with chemisorbed cresyl violet molecular species could act as hybrid
SERS labels. Their Raman scattering properties have been characterized here using different SERS techniques,
and probing different excitation wavelengths, even in the near infrared. Obtaining FT-SERS signal
of cresyl violet is of particular importance for applying FT-Raman spectroscopy to the tissues or cells, in
providing sensitive information in the close vicinity of the SERS label incubated into the biological sample.
Furthermore, upon chemisorption on the silver nanoparticles, cresyl violet molecular orientation provided
both amino functional groups free of interaction with the silver, resulting double aminofunctionalized
Ag nanoparticles suitable for DNA tagging. SERS tests of melanoma induced in mouse using
CV-Ag SERS label suggested the possibility to setup the tissue labeling procedure for skin cancer
monitoring.
A comprehensive development of the charge‐transfer theory of surface enhanced Raman scattering (SERS) is presented. We incorporate the Herzberg–Teller mixing of zero‐order Born–Oppenheimer electronic states by means of vibronic interaction terms in the Hamiltonian. This is similar to the theory of Tang and Albrecht12 except that we include metal states as part of a molecule–metal system. When this is done we may no longer discard a term involving mixing of ground‐state vibrations. The theory is comprehensive in that both molecule‐to‐metal and metal‐to‐molecule transfer is considered. Furthermore, both Franck–Condon and Herzberg–Teller contributions to the intensity are obtained. The former, however, contribute only to the intensity of totally symmetric vibrations, while the latter contribute to nontotally symmetric vibrations as well. Since overtones are observed in SERS only weakly if at all, the Herzberg–Teller terms are most consistent with experimental findings. The resulting formulas may be interpreted as a type of resonance Raman effect in which intensity for the charge transfer transitions is borrowed from an allowed molecular transition. We may also carry out the sum over metal states. This procedure predicts a logarithmic resonance at the Fermi level of the metal. We thus predict intensity vs voltage profiles such as I ∝ ‖ln(ωFI−ω+iΓ)‖2 which fits the experimental curves quite well.
The use of surface-enhanced Raman scattering (SERS) as a detection method in gas chromatography (GC) was investigated by two different approaches; GC eluates were trapped either in liquid silver sol or on solid thin-layer chromatography (TLC) plates, previously coated with silver colloidal solution. Subsequently, the trapped analytes were monitored by their SERS spectrum. Pyridine was successfully used as probe molecule for both interfaces. The extension of this static system for application in on-line detection of GC eluates is discussed.
We present a unified expression for surface-enhanced Raman spectroscopy (SERS). The expression contains a product of three resonance denominators, representing the surface plasmon resonance, the metal−molecule charge-transfer resonance at the Fermi energy, and an allowed molecular resonance. This latter resonance is that from which intensity is borrowed for charge transfer, and when the molecular resonance is active it is responsible for surface-enhanced resonance Raman spectroscopy. We examine this expression in various limits, to explore the relative contribution or each resonance. First, we look at the situation in which only the surface plasmon resonance is active and examine the various contributions to the Raman signal, including the surface selection rules. Then we examine additional contributions from charge-transfer or molecular resonances. We show that the three resonances are not totally independent, since they are linked by a product of four matrix elements in the numerator. These linked matrix elements provide comprehensive selection rules for SERS. One involves a harmonic oscillator in the observed normal mode. This is the same mode which appears in the vibronic coupling operator linking one of the states of the allowed molecular resonance to the charge-transfer state. The charge-transfer transition moment is linked to the surface plasmon resonance by the requirement that the transition dipole moment be polarized along the direction of maximum amplitude of the field produced by the plasmon (i.e., perpendicular to the metal surface). We show that these selection rules govern the observed SERS spectral intensities and apply these to the observed spectra of several molecules. We also suggest a quantitative measure of the degree to which charge transfer contributes to the overall SERS enhancement.
We demonstrate the detection of molecular vibrations in single hemoglobin (Hb) protein molecules attached to isolated and immobilized silver nanoparticles by surface enhanced Raman scattering (SERS). A comparison between calculation and experiment indicates that electromagnetic field effects dominate the surface enhancement, and that single molecule Hb SERS is possible only for molecules situated between Ag particles. The vibrational spectra exhibit temporal fluctuations of unknown origin which appear to be characteristic of the single molecule detection limit.
Raman spectra are reported for liquids and Langmuir films enhanced by the electromagnetic fields of plasmon surface polaritons at flat silver surfaces. The Kretschmann prism coupling configuration is used to excite the incident surface polariton and to couple out the Raman shifted surface wave. The spectra are interpreted with the help of a simple quantum theory and compared with previously reported grating spectra. Enhancement factors, polarization characteristics, and the feasibility of using surface polaritons to enhance the Raman spectra of adsorbates on flat electrodes in contact with aqueous electrolyte are discussed.
A new method of exciting nonradiative surface plasma waves (SPW) on smooth surfaces, causing also a new phenomena in total reflexion, is described. Since the phase velocity of the SPW at a metal-vacuum surface is smaller than the velocity of light in vacuum, these waves cannot be excited by light striking the surface, provided that this is perfectly smooth.However, if a prism is brought near to the metal vacuum-interface, the SPW can be excited optically by the evanescent wave present in total reflection. The excitation is seen as a strong decrease in reflection for the transverse magnetic light and for a special angle of incidence. The method allows of an accurate evaluation of the dispersion of these waves. The experimental results on a silver-vacuum surface are compared with the theory of metal optics and are found to agree within the errors of the optical constants.
Surface-enhanced Raman scattering (SERS)-based signal amplification and detection methods using plasmonic nanostructures have been widely investigated for imaging and sensing applications. However, SERS-based molecule detection strategies have not been practically useful because there is no straightforward method to synthesize and characterize highly sensitive SERS-active nanostructures with sufficiently high yield and efficiency, which results in an extremely low cross-section area in Raman sensing. Here, we report a high-yield synthetic method for SERS-active gold-silver core-shell nanodumbbells, where the gap between two nanoparticles and the Raman-dye position and environment can be engineered on the nanoscale. Atomic-force-microscope-correlated nano-Raman measurements of individual dumbbell structures demonstrate that Raman signals can be repeatedly detected from single-DNA-tethered nanodumbbells. These programmed nanostructure fabrication and single-DNA detection strategies open avenues for the high-yield synthesis of optically active smart nanoparticles and structurally reproducible nanostructure-based single-molecule detection and bioassays.
Comprehensive reflectivity mapping of the angular dispersion of nanostructured arrays comprising of inverted pyramidal pits is demonstrated. By comparing equivalently structured dielectric and metallic arrays, diffraction and plasmonic features are readily distinguished. While the diffraction features match expected theory, localised plasmons are also observed with severely flattened energy dispersions. Using pit arrays with identical pitch, but graded pit dimensions, energy scaling of the localized plasmon is observed. These localised plasmons are found to match a simple model which confines surface plasmons onto the pit sidewalls thus allowing an intuitive picture of the plasmons to be developed. This model agrees well with a 2D finite-difference time-domain simulation which shows the same dependence on pit dimensions. We believe these tuneable plasmons are responsible for the surface-enhancement of the Raman scattering (SERS) of an attached layer of benzenethiol molecules. Such SERS substrates have a wide range of applications both in security, chemical identification, environmental monitoring and healthcare.
Optical detection and spectroscopy of single molecules and single nanoparticles have been achieved at room temperature with
the use of surface-enhanced Raman scattering. Individual silver colloidal nanoparticles were screened from a large heterogeneous
population for special size-dependent properties and were then used to amplify the spectroscopic signatures of adsorbed molecules.
For single rhodamine 6G molecules adsorbed on the selected nanoparticles, the intrinsic Raman enhancement factors were on
the order of 1014 to 1015, much larger than the ensemble-averaged values derived from conventional measurements. This enormous enhancement leads to
vibrational Raman signals that are more intense and more stable than single-molecule fluorescence.
Noble metal particles have long fascinated scientists because of their intense color, which led to their application in stained glass windows as early as the Middle Ages. The recent resurrection of colloidal and cluster chemistry has brought about the strive for new materials that allow a bottoms-up approach of building improved and new devices with nanoparticles or artificial atoms. In this review, we discuss some of the properties of individual and some assembled metallic nanoparticles with a focus on their interaction with cw and pulsed laser light of different energies. The potential application of the plasmon resonance as sensors is discussed.
The applicability of an etched and silver or gold coated SERS fiber probe in combination with a commercially available laboratory micro-Raman setup or a home built mobile micro-Raman setup to perform on-site field measurements was evaluated and successfully tested on different biological samples. The SERS fiber probe allows one to perform measurements with high spatial resolution. Simultaneously, the laser power used for Raman spectroscopy on biological samples as compared with conventional Raman experiments can be reduced by more than two orders of magnitude. This experimental arrangement was tested to investigate sensitive biological samples like mint plants (Bergamot mint, spear mint) and citrus fruits (kumquat). Furthermore, traces of fungicides on wine leaves were detected by means of such a SERS fiber probe setup.
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Considered one of the major fields of photonics of the beginning 21st century, plasmonics offers the potential to confine and guide light below the diffraction limit and promises a new generation of highly miniaturized photonic devices. Offering both a comprehensive introduction to the field and an extensive overview of the current state of the art, "Plasmonics: Fundamentals and Applications" should be of great value to the newcomer and to the experienced researcher.
The first part of the book describes the fundamentals of this research area, starting with a review of Maxwell’s equations in a form suited to the description of metals. Subsequent chapters introduce the two major ingredients of plasmonics, surface plasmon polaritons at metallic interfaces and localized plasmons in nanostructures. The mathematics of their description, excitation and imaging of the modes are discussed. This part closes with a presentation of electromagnetic surface waves at lower frequencies in the THz and microwave regime, comprising both spoof or designer plasmons and surface phonon polaritons.
Building on the fundamentals, the second part discusses some of the most prominent applications of plasmons: Plasmon waveguides, extraordinary transmission through aperture arrays, sensing and surface enhanced Raman scattering, spectroscopy as well as metamaterials. Exemplary studies in each of these fields taken from the original literature are presented.
Surface-Enhanced Raman Scattering (SERS) was discovered in the 1970s and has since grown enormously in breadth, depth, and understanding. One of the major characteristics of SERS is its interdisciplinary nature: it lies at the boundary between physics, chemistry, colloid science, plasmonics, nanotechnology, and biology. By their very nature, it is impossible to find a textbook that will summarize the principles needed for SERS of these rather dissimilar and disconnected topics. Although a basic understanding of these topics is necessary for research projects in SERS with all its many aspects and applications, they are seldom touched upon as a coherent unit during most undergraduate studies in physics or chemistry. This book intends to fill this existing gap in the literature. It provides an overview of the underlying principles of SERS, from the fundamental understanding of the effect to its potential applications. It is aimed primarily at newcomers to the field, graduate student, researcher or scientist, attracted by the many applications of SERS and plasmonics or its basic science. The emphasis is on concepts and background material for SERS, such as Raman spectroscopy, the physics of plasmons, or colloid science, all of them introduced within the context of SERS, and from where the more specialised literature can be followed. * Represents one of very few books fully dedicated to the topic of surface-enhanced Raman spectroscopy (SERS) * Gives a comprehensive summary of the underlying physical concepts around SERS * Provides a detailed analysis of plasmons and plasmonics. "Besides an overview of current promising research topics, this book is a self-contained introduction to Raman spectroscopy and fluorescence that summarises the main concepts and ideas needed for SERS. It is also a self-contained introduction to the physics of plasmon resonances within the broader scope of plasmonics. A detailed presentation of the SERS electromagnetic model and its extension to surface-enhanced fluorescence is included." "Aimed primarily at newcomers to the field, graduate students, and other researchers or scientists attracted by the many possible applications of SERS and plasmonics, or their basic science."--BOOK JACKET.
Surface Enhanced Vibrational Spectroscopy (SEVS) has reached maturity as an analytical technique, but until now there has been no single work that describes the theory and experiments of SEVS. This book combines the two important techniques of surface-enhanced Raman scattering (SERS) and surface-enhanced infrared (SEIR) into one text that serves as the definitive resource on SEVS. Discusses both the theory and the applications of SEVS and provides an up-to-date study of the state of the art. Offers interpretations of SEVS spectra for practicing analysts. Discusses interpretation of SEVS spectra, which can often be very different to the non-enhanced spectrum - aids the practicing analyst.
In 1978 it was discovered, largely through the work of Fleischmann, Van Duyne, Creighton, and their coworkers that molecules adsorbed on specially prepared silver surfaces produce a Raman spectrum that is at times a millionfold more intense than expected. This effect was dubbed surface-enhanced Raman scattering (SERS). Since then the effect has been demonstrated with many molecules and with a number of metals, including Cu, Ag, Au, Li, Na, K, In, Pt, and Rh. In addition, related phenomena such as surface-enhanced second-harmonic generation, four-wave mixing, absorption, and fluorescence have been observed. Although not all fine points of the enhancement mechanism have been clarified, the majority view is that the largest contributor to the intensity amplification results from the electric field enhancement that occurs in the vicinity of small, interacting metal particles that are illuminated with light resonant or near resonant with the localized surface-plasmon frequency of the metal structure. Small in this context is gauged in relation to the wavelength of light. The special preparations required to produce the effect, which include among other techniques electrochemical oxidation-reduction cycling, deposition of metal on very cold substrates, and the generation of metal-island films and colloids, is now understood to be necessary as a means of producing surfaces with appropriate electromagnetic resonances that may couple to electromagnetic fields either by generating rough films (as in the case of the former two examples) or by placing small metal particles in close proximity to one another (as in the case of the latter two). For molecules chemisorbed on SERS-active surface there exists a "chemical enhancement" in addition to the electromagnetic effect. Although difficult to measure accurately, the magnitude of this effect rarely exceeds a factor of 10 and is best thought to arise from the modification of the Raman polarizability tensor of the adsorbate resulting from the formation of a complex between the adsorbate and the metal. Rather than an enhancement mechanism, the chemical effect is more logically to be regarded as a change in the nature and identity of the adsorbate.
We present an introduction to surface-enhanced Raman scattering (SERS) which reviews the basic experimental facts and the essential features of the mechanisms which have been proposed to account for the observations. We then review very recent fundamental developments which include: SERS from single particles and single molecules; SERS from fractal clusters and surfaces; and new insights into the chemical enhancement mechanism of SERS.
Surface enhanced resonance Raman scattering (SERRS) has considerable potential as an ultra-sensitive analytical technique; for the first time the synthesis of specific dye ligands designed to overcome major problems in obtaining reliable SERRS is described.
We have measured the enhanced Raman scattering by a liquid (methyl alcohol) in contact with an evaporated film of silver (∼400 Å) deposited on the flat surface of a hemispherical prism. The exciting laser light (5145 Å) was incident on the film through the prism and the scattered light from the liquid was collected also through the prism. The Raman-scattering intensity is a sharp function of both the incident and the scattered angles. The observed angular dependence of the Raman intensity can be fitted remarkably well by assuming that the enhanced scattering process is mediated by surface-plasmon polaritons at the silver-liquid interface. The observed scattering intensity is consistent with the predicted enhancement factor of 4 × 104.
Systematic variation of the internal geometry of a dielectric core-metal shell nanoparticle allows the local electromagnetic field at the nanoparticle surface to be precisely controlled. The strength of the field as a function of core and shell dimension is measured by monitoring the surface enhanced Raman scattering (SERS) response of nonresonant molecular adsorbates (para-mercaptoaniline) bound to the nanoparticle surface. The SERS enhancement appears to be directly and exclusively due to nanoparticle geometry. Effective SERS enhancements of 106 are observable in aqueous solution, which correspond to absolute enhancements of 1012 when reabsorption of Raman emission by nearby nanoparticles is taken into account. © 2003 American Institute of Physics.
The essential oils of Thymus vulgaris and Origanum vulgaris are studied by means of micro-Raman spectroscopy. The containing monoterpenes can be identified by their Raman spectra. Further the essential oils are investigated in their natural environment, the so-called oil cells of these Lamiaceae plants, with surface enhanced Raman spectroscopy (SERS). This method has the advantage to enhance Raman signals and furthermore the SERS effect leads to fluorescence quenching.
A range of pseudo-random silver structures, where there is a choice of clustered spheres or pillars or tori, have been fabricated on silicon using the method of island lithography combined with electroless plating. Pyridine has been adsorbed on these structures and the surface-enhanced Raman scattering spectrum (SERS) measured using 633 nm laser radiation. Measurements of SERS spectra as a function of pyridine solution concentration have enabled an adsorption isotherm to be obtained and the standard free energy of adsorption to be determined (24 kJ mol-1), in good agreement with the literature. The substrates are found to give a uniform signal, as sampled over the prepared area, for both saturation coverage (±12%) and for a fraction of a monolayer, ca. 0.08, (±31%). It is concluded that, in the system studied, the effective area of the SERS “site” must be large compared with the area occupied by the adsorbed pyridine, so that the SERS signal is proportional to the surface coverage, averaged over the adsorbent. The formalism due to Tian and co-workers has been adopted to determine G, the SERS enhancement factor. G is calculated as follows: G = (scattering intensity per adsorbed molecule)/(scattering intensity per solution molecule). G from 1.9 × 106 (pillars) to 2.5 × 107 (tori) have been estimated, and larger values are expected. These silver substrates, which are robust and reproducible, would seem to be good candidates for various analytical applications.
This paper presents a theoretical and preliminary experimental evaluation of a Kretschmann prism coupler method used in Raman scattering experiments with molecular adsorbates. The theoretical enhancement of Raman scattering intensity, which can be as high as a factor of 1000, is calculated by using an exact method of 2 X 2 transfer matrices. Preliminary experiments, designed to investigate the feasibility of this technique under ultra-high-vacuum conditions, show that monolayer sensitivity can be practically achieved. The authors discuss the optimum experimental configuration, as well as the factors that can seriously limit achievable enhancement levels.
The surface plasmon response of metal nanoparticles is studied for different shapes and physical environments. For polyhedral nanoparticles, the surface plasmon resonances are studied as a function of the number of faces and vertices. The modification of these surface plasmons by different surrounding media and the presence of a substrate or other nanoparticles is also discussed. We found that polyhedral nanoparticles composed with less faces show more surface plasmon resonances, and as the nanoparticle becomes more symmetric, the main surface plasmon resonance is blue-shifted. It is also found that the corners induce more surface plasmons in a wider energy range. In the presence of a substrate, multipolar plasmon resonances are induced, and as the nanoparticle approaches the substrate, such resonances are red-shifted. The interaction among nanoparticles also induces multipolar resonances, but they can be red or blue-shifted depending on the polarization of the external field.
The model of surface-enhanced Raman scattering (SERS) by time-dependent evolution in the intermediate anionic state of the adsorbate is analogous to intramolecular Franck–Condon resonance Raman scattering. For adsorbates with a π* state, the residence time of some femtoseconds (10−15 s) in the anionic state leads to a separation of electron (e) and hole (h), which quenches SERS at a smooth surface. At so-called SERS-active sites, the residence time of the hole is enhanced and therefore there is no final e–h pair and the excitation of only a molecular vibration leads to SERS. In contrast, for molecules with only high-energy σ* states, the residence time in the anionic state is <1 fs (analogous to the impulse mechanism in electron scattering), and the creation of e–h pairs is less likely. This leads to first-layer electronic Raman scattering, especially by CH stretch vibrations with an average enhancement of about 30–40-fold. Copyright © 2005 John Wiley & Sons, Ltd.
We present the measured dependence of the Raman intensity on the thickness of the metal film in a Kretschmann geometry when surface plasmons are excited. To perform our work, we deposited a Ag film of varying thickness onto a prism, and then we coated it with a thin layer of copper phthalocyanine. We show that the maximum of the Raman signal is achieved for an overall optimization of the coupling at both pump and Stokes frequencies, demonstrating the simultaneous excitation of surface plasmons at these two frequencies.
Raman spectroscopy has been employed for the first time to study the role of adsorption at electrodes. It has been possible to distinguish two types of pyridine adsorption at a silver electrode. The variation in intensity and frequency of some of the bands with potential in the region of the point of zero charge has given further evidence as to the structure of the electrical double layer; it is shown that the interaction of adsorbed pyridine and water must be taken into account.
By exploiting the extremely large effective cross sections ( 10-17-10-16 cm2/molecule) available from surface-enhanced Raman scattering (SERS), we achieved the first observation of single molecule Raman scattering. Measured spectra of a single crystal violet molecule in aqueous colloidal silver solution using one second collection time and about 2×105 W/cm2 nonresonant near-infrared excitation show a clear ``fingerprint'' of its Raman features between 700 and 1700 cm-1. Spectra observed in a time sequence for an average of 0.6 dye molecule in the probed volume exhibited the expected Poisson distribution for actually measuring 0, 1, 2, or 3 molecules.
Single molecule photo-bleaching of Rhodamine 6G (RH6G) dye molecules is observed by surface enhanced resonant Raman scattering in gold (Au) colloids under double resonant excitation conditions. The effect is revealed through a breakdown in the normal exponential decay of the dye population at extremely low concentrations. Step-like structures in the decay of the Raman signals show that individual molecules are being destroyed as a function of time. This arrangement allows a study of photochemical degradation at single molecular level, and creates the possibility for the determination of absolute enhancement factors in the Raman cross-sections.
The excitation of surface plasmons at silver electrodes by attenuated total reflection allows one to intensify the Raman signal from adsorbed molecules. The enhancement of the Raman intensity as a result of the amplification of the surface electromagnetic field at the metal-electrolyte interface is demonstrated for pyridine on silver.
Nanosphere lithography (NSL) is an inexpensive, simple to implement, inherently parallel, high throughput, materials general nanofabrication technique capable of producing an unexpectedly large variety of nanoparticle structures and well-ordered 2D nanoparticle arrays. This article describes our recent efforts to broaden the scope of NSL to include strategies for the fabrication of several new nanoparticle structural motifs and their characterization by atomic force microscopy. NSL has also been demonstrated to be well-suited to the synthesis of size-tunable noble metal nanoparticles in the 201000 nm range. This characteristic of NSL has been especially valuable for investigating the fascinating richness of behavior manifested in size-dependent nanoparticle optics. The use of localized surface plasmon resonance (LSPR) spectroscopy to probe the size-tunable optical properties of Ag nanoparticles and their sensitivity to the local, external dielectric environment (viz., the nanoenvironment) is discussed
Caught in a trap: Colloids of gold nanoparticles coated with a thermally responsive poly‐(N‐isopropylacrylamide) (pNIPAM) microgel can trap molecules in different ways as a function of temperature (see scheme). The porous pNIPAM shells prevent electromagnetic coupling between metal particles, thus providing highly reproducible surface‐enhanced Raman scattering (SERS) signals and intensity.
Over the past year, the BIAcore system (which is based on the surface plasmon resonance phenomenon) has become increasingly popular for the study of macromolecular interactions. This biomolecular interaction analysis system allows the detection of macromolecular interactions in real time and in a label-free mode. The real-time detection properties of this technique suggest its potential in the generation of data relating to the kinetics of interaction of biomolecules.
Since the advent of surface plasmon resonance (SPR)-based interaction analysis techniques in 1990 the field has grown rapidly. So far, more than 220 publications and hundreds of laboratories have reported useful applications for this label-free real-time binding approach. Milestones passed during the past year include the direct detection of low molecular mass (200 Da) binding events and applications in several new fields as disparate as chaperonins, cellular adhesion, molecular biology, transcription and small-molecule screening.
T-lymphocyte activation is initiated by the interaction of the alpha beta TCR with a complex consisting of a class I or class II MHC-encoded molecule and an antigenic peptide, displayed on the surface of an antigen-presenting cell. Real-time binding measurements using surface plasmon resonance have revealed kinetic and equilibrium parameters for the interactions between purified MHC molecules and peptides, between TCR and MHC-peptide complexes, and between TRC and superantigens. The MHC-peptide interaction is characterized by its high affinity and long half-life, the TCR-MHC/peptide interaction by its low affinity and short half-life, and the TCR-superantigen interaction by its low-to-moderate affinity, which is dependent on the particular superantigen involved. The consistent finding is that both MHC-peptide complexes and superantigens interact with TCR with a low affinity attributable to rapid dissociation. That an MHC-peptide complex that encounters a single TCR only briefly can still deliver the necessary activation signals offers a mechanistic conundrum for which several solutions have been proposed.
We have made in situ measurements of attenuated total reflection (ATR) and Raman scattering from a layered structure consisting of a glass prism, a thin silver film, an MgF2 spacer, and a liquid mixture whose refractive index is matched to that of MgF2. When the incident angle of the laser beam coincides with the ATR angle, the surface-plasmon polariton (SPP) of the silver film is excited resonantly and the Raman scattering intensity of the liquid shows a maximum. The same effect is observed at the frequency of the Stokes scattered light. By measuring the decrease of the Raman scattering intensity of the liquid with increase of the thickness of the MgF2 spacer layer, we have determined the decay length (ld) of the SPP field into the liquid. The measured value of ld=1539 Å agrees with the calculated value, 1534 Å.
Polarization and angle-of-incidence dependences of the surface-enhanced Raman scattering (SERS) signal of benzoic acid adsorbed on isolated oblate particles of silver have been measured in an attenuated-total-reflection geometry. Simultaneously, the absorption of photons in the particles has been determined by a photoacoustic method. At the critical angle and in the wavelength range of the present study, only one dipole resonance mode, the nonazimuthally symmetric mode, is excited by the component of the electric field parallel to the reflecting surface. This has allowed us to evaluate contributions of the resonance absorption and the nonresonant intrinsic absorption to the total absorption. We show that the absorption of p-polarized light at the critical angle is solely due to the intrinsic absorption. No Raman signal can be detected in the absence of the surface-plasmon mode or electromagnetic resonance. We show also that the dependence of SERS on the polarization and angle of incidence can be explained by considering only the excitation of localized plasmons.
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This work presents a detailed analysis of enhanced Raman scattering of the pyridine-Ag(20) model system using time-dependent density functional theory. A consistent treatment of both the chemical and electromagnetic enhancements (EM) is achieved by employing a recently developed approach based on a short-time approximation for the Raman cross section. A strong dependence of the absolute and relative intensities on the binding site and excitation wavelength is found. The analysis of the Raman scattering cross sections shows the importance of different contributions to the enhancements, including static chemical enhancements (factor of 10), charge-transfer enhancements (10(3)), and EM enhancements (10(5)). The largest enhancement found (10(5)-10(6)) is due to the EM mechanism, with a small contribution from the chemical interaction. This suggests that the enhanced Raman scattering due to atomic clusters is comparable to findings on single nanoparticles. A combination of information about the vibrational motion and the local chemical environment provides a simple picture of why certain normal modes are enhanced more than others.
Beads labelled using surface enhanced resonance Raman scattering (SERRS) are highly sensitive and specific tags, with potential applications in biological assays, including molecular diagnostics. The beads consist of a nucleus containing dye labelled silver-nanoparticle aggregates surrounded by a polymer core. The nuclei generate strong SERRS signals. To illustrate the coding advantage created by the sharp, molecularly specific SERRS signals, four specially designed SERRS dyes have been used as labels and three of these have been combined in a multiplex analysis. These dyes use specific groups such as benzotriazole and 8-hydroxyquinoline to improve binding to the surface of the silver particles. The aggregation state of the particles is held constant by the polymer core, this nucleus also contains many dye labels, yielding a very high Raman scattering intensity for each bead. To functionalise these beads for use in biological assays an outer polymer shell can be added, which allows the attachment of oligonucleotide probes. Oligonucleotide modified beads can then be used for detection of specific oligonucleotide targets. The specificity of SERRS will allow for the detection of multiple targets within a single assay.
This critical review highlights recent advances in using electronic structure methods to study surface-enhanced Raman scattering. Examples showing how electronic structure methods, in particular time-dependent density functional theory, can be used to gain microscopic insights into the enhancement mechanism are presented (150 references).
The unique ability to obtain molecular recognition of an analyte at very low concentrations in situ in aqueous environments using surface enhanced Raman scattering (SERS) and surface enhanced resonance Raman scattering (SERRS) detection makes these spectroscopies of considerable interest. Improved understanding of the effect coupled to improvements in practical techniques make the use of SERS/SERRS much simpler than has been the case in the past. This article is designed as a tutorial review targeted at aiding in the development of practical applications.
- M Fleischmann
- P J Hendra
- A J Mcquillan
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