The theory of surface-enhanced Raman scattering

By considering the molecule and metal to form a conjoined system, we derive an expression for the observed Raman spectrum in surface-enhanced Raman scattering. The metal levels are considered to consist of a continuum with levels filled up to the Fermi level, and empty above, while the molecule has discrete levels filled up to the highest occupied orbital, and empty above that. It is presumed that the Fermi level of the metal lies between the highest filled and the lowest unfilled level of the molecule. The molecule levels are then coupled to the metal continuum both in the filled and unfilled levels, and using the solutions to this problem provided by Fano, we derive an expression for the transition amplitude between the ground stationary state and some excited stationary state of the molecule-metal system. It is shown that three resonances contribute to the overall enhancement; namely, the surface plasmon resonance, the molecular resonances, as well as charge-transfer resonances between the molecule and metal. Furthermore, these resonances are linked by terms in the numerator, which result in SERS selection rules. These linked resonances cannot be separated, accounting for many of the observed SERS phenomena. The molecule-metal coupling is interpreted in terms of a deformation potential which is compared to the Herzberg-Teller vibronic coupling constant. We show that one term in the sum involves coupling between the surface plasmon transition dipole and the molecular transition dipole. They are coupled through the deformation potential connecting to charge-transfer states. Another term is shown to involve coupling between the charge-transfer transition and the molecular transition dipoles. These are coupled by the deformation potential connecting to plasmon resonance states. By applying the selection rules to the cases of dimer and trimer nanoparticles we show that the SERS spectrum can vary considerably with excitation wavelength, depending on which plasmon and/or charge-transfer resonance is excited.     

Silver Nano Islands Enhanced Raman Scattering on Large Area Grating Substrates Fabricated by Two Beam Laser Interference

The authors preparedlarge area surface-enhanced Raman scattering(SERS) active substrates with tunable enhancement. First the large area gratings were fabricated by scanning a photoresist with two-beam laser interference and subsequently they were coated with silver nano islands via vacuum evaporation. SERS active metal island grating substrates with four different periods(300, 400, 515 and 600 nm) and Ag nano islands uniformly coated on an area of 2.5 cm×0.5 cm were obtained. The measured SERS spectra reveal the tuning effect of the period on the Raman signals period. The highest enhancement(ca. 10⁵) for Rhodamine 6G(R6G) as probing molecule is associated with a period of 515 nm due to the perfect matching of surface plasmons and Raman excitation line. A good reproducibility of SERS signals with almost the same SERS intensity at different spots was observed on all the larger area Ag island grating substrates.     

Activation of Oxygen on Gold and Silver Nanoparticles Assisted by Surface Plasmon Resonances

Surface plasmon resonances (SPRs) have been found to promote chemical reactions. In most oxidative chemical reactions oxygen molecules participate and understanding of the activation mechanism of oxygen molecules is highly important. For this purpose, we applied surface‐enhanced Raman spectroscopy (SERS) to find out the mechanism of SPR‐assisted activation of oxygen, by using p‐aminothiophenol (PATP), which undergoes a SPR‐assisted selective oxidation, as a probe molecule. In this way, SPR has the dual function of activating the chemical reaction and enhancing the Raman signal of surface species. Both experiments and DFT calculations reveal that oxygen molecules were activated by accepting an electron from a metal nanoparticle under the excitation of SPR to form a strongly adsorbed oxygen molecule anion. The anion was then transformed to Au or Ag oxides or hydroxides on the surface to oxidize the surface species, which was also supported by the heating effect of the SPR. This work points to a promising new era of SPR‐assisted catalytic reactions.     

Coupled subwavelength gratings for surface-enhanced Raman spectroscopy

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

Multilayer Substrate‐Mediated Tuning Resonance of Plasmon and SERS EF of Nanostructured Silver

A thin‐film of dielectric on a reflecting surface constituting a multilayer substrate modulates light intensity due to the interference effect. A nanostructure consisting of randomly oriented silver particles of different shapes, sizes, and interparticle spacings supports multiple plasmon resonances and is observed to have a broad extinction spectrum that spans the entire visible region. Combining the two systems by fabricating the nanostructure on the thin‐dielectric film of the multilayer substrate yields a new composite structure which is observed to modulate both the extinction spectrum and the SERS EF (surface enhanced Raman scattering enhancement factor) of the nanostructure as the thickness of the thin‐film dielectric is varied. The frequency and intensity of the visible extinction spectrum vary dramatically with the dielectric thickness and in the intermediate thickness range the spectrum has no visible band. The SERS EF determined for the composite structure as a function of the thin‐film dielectric thickness varies by several orders of magnitude. Strong correlation between the magnitude of the SERS EF and the extinction intensity is observed over the entire dielectric thickness range indicating that the extinction spectrum corresponds to the excitation of the plasmon resonances of the nanostructure. A significant finding which has potential applications is that the composite structure has synergic effect to boost SERS EF of the nanostructure by an order of magnitude or more compared to the same nanostructure on an unlayered substrate.     

Single molecule analysis by surfaced-enhanced Raman scattering

Our main objective in this tutorial review is to provide insight into some of the questions surrounding single molecule detection (SMD) using surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS). Discovered thirty years ago, SERS is now a powerful analytical tool, strongly tied to plasmonics, a field that encompasses and profits from the optical enhancement found in nanostructures that support localized plasmon excitations. The spectrum of the single molecule carries the quantum fingerprints of the system modulated by the molecule-nanostructure interactions and the electronic resonances that may result under laser excitation. This information is embedded in vibrational band parameters. The dynamics and the molecular environment will affect the bandwidth of the observed Raman bands. In addition, the localized surface plasmon resonances (LSPR) empower the nanostructure with a number of optical properties that will also leave their mark on the observed inelastic scattering process. Therefore, controlling size, shape and the formation of the aggregation state (or fractality) of certain metallic nanostructures becomes a main task for experimental SERS/SERRS. This molecule-nanostructure coupling may, inevitably, lead to spectral fluctuations, increase photobleaching or photochemistry. An attempt is made here to guide the interpretation of this wealth of information when approaching the single molecule regime.     

Surface-enhanced Raman scattering on nanoshells with tunable surface plasmon resonance

Fabrication, characterization, and optical enhancement applications of bimetallic AgAu nanoparticles and nanoshells are reported. Nanoparticles with tunable surface plasmon resonances are synthesized at room temperature and characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and photon correlation spectroscopy. The collective electron oscillation of the nanoparticles shows a controllable tunability in the 400-990 nm spectral range, in agreement with plasmon absorption calculated using Mie theory, providing an optimum substrate for surface plasmon-assisted enhanced spectroscopy. Surface-enhanced Raman scattering experiments show that the average enhancement factor obtained with nanoshells could be higher than those obtained with silver sols.     

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.     

Gold particle interaction in regular arrays probed by surface enhanced Raman scattering

Lithographically designed two-dimensional arrays consisting of gold nanoparticles deposited on a smooth gold film are used as substrate to examine the SERS effect of the trans-1,2-bis (4-pyridyl) ethylene molecule. These arrays display two plasmon bands instead of the single one observed for the same arrays of particles but deposited on indium tin oxide coated glass. Laser excitation within the short wavelength band does not bring about any SERS spectrum, while excitation within the long wavelength band yields SERS spectra with a gain per molecule rising up to 10(8). The simultaneous investigation of extinction and Raman spectra of arrays exhibiting various topography parameters enables us to suggest an interpretation for both the occurrence of the two plasmon resonances and for the high Raman enhancement. We suggest to assign the short wavelength band to a plasmon wave propagating at the gold glass interface and the long wavelength one to an air/gold surface plasmon mode modified by particle-particle interaction.     

Determination of the Degree of Charge‐Transfer Contributions to Surface‐Enhanced Raman Spectroscopy

We explore the application of a previously suggested formula for determining the degree of charge transfer in surface‐enhanced Raman scattering (SERS). SERS is often described as a phenomenon which obtains its enhancement from three major sources, namely the surface plasmon resonance, charge‐transfer resonances as well as possible molecular resonances. At any chosen excitation wavelength, it is possible to obtain contributions from several sources and this has led to considerable confusion. The formula for the degree of charge transfer enables one to separate these effects, but it requires that spectra be obtained either at two or more different excitation wavelengths or as a function of applied potential. We apply this formula to several examples, which display rather large charge‐transfer contributions to the spectrum. These are p‐aminothiophenol (PATP), tetracyano‐ ethylene (TCNE) and piperidine. In PATP we can show that several lines of the same symmetry give the same degree of charge transfer. In TCNE we are able to identify the charge‐transfer transition, which contributes to the effect, and are able to independently determine the degree of charge transfer by wavenumber shifts. This enables a comparison of the two techniques of measurement. In piperidine, we present an example of molecule to metal charge transfer and show that our definition of charge transfer is independent of direction.     

Aspect ratio dependence on surface enhanced Raman scattering using silver and gold nanorod substrates

Silver and gold nanorods with aspect ratios from 1 to 16 have been used as substrates for surface enhanced Raman spectroscopy (SERS) in colloidal solution. The nanorod aspect ratio is varied to give different degrees of overlap between the nanorod longitudinal plasmon band and excitation source in order to determine its effect on overall surface enhancement. Results suggest that enhancement factors are a factor of 10-10(2) greater for substrates that have plasmon band overlap with the excitation source than for substrates whose plasmon bands do not.     

小尺寸金石墨纳米颗粒的合成与表征

在表面增强拉曼光谱(SERS)的研究领域中,基于局域表面等离子体共振效应的等离子体SERS基底的制备成为过去几十年的研究热点。然而,通常开发的等离子体金属基底具有较差的稳定性和重现性。对于SERS而言,石墨烯类材料具有拉曼化学增强效应,除此之外,还具有分子富集、强的稳定性与荧光猝灭能力等优点,因此基于石墨金属复合纳米材料的SERS基底受到了研究人员的重视。我们利用化学气相沉积(CVD)法制备了小尺寸的金石墨核壳纳米颗粒(Au@G),其粒径约为17 nm。我们通过在Au NP上包覆介孔二氧化硅来控制Au@G的尺寸,同时还研究了包覆二氧化硅过程中,正硅酸乙酯(TEOS)的浓度对于石墨壳层形成的影响。结果表明当TEOS在一定浓度范围内,其浓度的降低有利于得到石墨化程度高的Au@G。进一步利用Au@G对结晶紫分子进行拉曼检测,也表明了Au@G具有较好的拉曼增强效果。这种小尺寸的Au@G在分子检测与细胞成像分析领域中具有广泛的应用潜力。     

A Comparative Study on the Influence of Submonolayer Coverage Sn on SERS of Au and Pt Substrates

Since the discovery of the surface enhanced Raman scattering (SERS) in mid-1970's,great efforts have been devoted to understand the enhancement mechanism as well as to extend the SERS system and application. There has been a consensus that the electromagnetic enhancement (EM) and chemical enhancement are the two important SERS mechanisms but each of them can only explain some of experimental results^[1,2] The EM mechanism relies on the surface plasmon resonance under a proper incident laser excitation. Strong EM enhancement has been observed on metals such as Cu, Ag and Au but not on transition metals such as Pt. However, the surface electronic properties can be modulated through submonolayer quantity modification of foreign metal atoms, hi this paper, we report a comparative study on SERS of Au and Pt in the presence of underpotentially deposited (UPD) submonolayer Sn.     

The effects of Au aggregate morphology on surface-enhanced Raman scattering enhancement

We have identified empirically a relationship between the surface morphology of small individual aggregates (<100 Au nanoparticles) and surface-enhanced Raman scattering (SERS) enhancement. We have found that multilayer aggregates generated greater SERS enhancement than aggregates limited to two-dimensional (2D) or one-dimensional structures, independent of the number of particles. SERS intensity was measured using the 730 cm(-1) vibrational mode of the adsorbed adenine molecule on 75 nm Au particles, at an excitation wavelength of 632.8 nm. To gain insight into these relationships and its mechanism, we developed a qualitative model that considers the collections of interacting Au nanoparticles of an individual aggregate as a continuous single entity that retains its salient features. We found the dimensions of the modeled surface features to be comparable with those found in rough metal surfaces, known to sustain surface plasmon resonance and generate strong SERS enhancement. Among the aggregates that we have characterized, a three 75 nm nanoparticle system was the smallest to generate strong SERS enhancement. However, we also identified single individual Au nanoparticles as SERS active at the same wavelength, but with a diameter twice in size. For example, we observed a symmetric SERS-active particle of 180 nm in diameter. Such individual nanoparticles generated SERS enhancement on the same order of magnitude as the small monolayer Au aggregates, an intensity value significantly stronger than predicted in recent theoretical studies. We also found that an aspect of our model that relates the dimensions of its features to SERS enhancement is also applicable to single individual Au particles. We conclude that the size of the nanoparticle itself, or the size of a protrusion of an irregularly shaped single Au particle, will contribute to SERS enhancement provided that its dimensions satisfy the conditions for plasmon resonance. In addition, by considering the ratio of the generated intensities of typical 2D Au aggregates to the enhancement of individual SERS-active particles, a value of approximately 2 is determined. Its moderate value suggests that it is not the aggregation effect that is responsible for much of the observed SERS enhancement but the surface region associated with the SERS-active site.     

Surface-enhanced Raman scattering (SERS) based on surface plasmon resonance coupling techniques

Surface plasmon resonance (SPR) can provide a remarkably enhanced electromagetic field around metal surface. It is one of the enhancement models for explaining surface-enhanced Raman scattering (SERS) phonomenon. With the development of SERS theories and techniques, more and more studies referred to the configurations of the optical devices for coupling the excitation and radiation of SERS, including the prism-coupling, waveguide-coupling, and grating-coupling modes. In this review, we will summarize the recent experimental improvements on the surface plasmoncoupled SERS.     

Electrochemical pathway for the quantification of SERS enhancement factor

This communication presents a new pathway for the more precise quantification of surface-enhanced Raman scattering (SERS) enhancement factor via deducing resonance Raman scattering (RRS) effect from surface-enhanced resonance Raman scattering (SERRS). To achieve this, a self-assembled monolayer of 1,8,15,22-tetraaminophthalocyanatocobalt(II) (4α-Co^IITAPc) is formed on plasmon inactive glassy carbon (GC) and plasmon active GC/AuNP surface. The surfaces are subsequently used as common probes for electrochemical and Raman (RRS and SERRS) studies. The most crucial parameters required for the quantification of SERS substrate enhancement factor (SSEF) such as real surface area of GC/AuNPs substarte and the number of 4α-Co^IITAPc molecules contributing to RRS (on GC) and SERRS (on GC/AuNPs) are precisely estimated by cyclic voltammetry experiments. The present approach of SSEF quantification can be applied to varieties of surfaces by choosing an appropriate laser line and probe molecule for each surface.     

Functionalized Ag nanoparticles with tunable optical properties for selective protein analysis

We present a preparation procedure for small sized biocompatibly coated Ag nanoparticles with tunable surface plasmon resonances. The conditions were optimised with respect to the resonance Raman signal enhancement of heme proteins and to the preservation of the native protein structure.     

An ionic surfactant-mediated Langmuir-Blodgett method to construct gold nanoparticle films for surface-enhanced Raman scattering

A gold nanoparticle film for surface-enhanced Raman scattering (SERS) was successfully constructed by an ionic surfactant-mediated Langmuir-Blodgett (LB) method. The gold film was formed by adding ethanol to a gold colloid/hexane mixture in the presence of dodecyltrimethylammonium bromide (DTAB). Consequently, gold nanoparticles (AuNPs) assembled at the water/hexane interface due to the decrease in surface charge density of AuNPs. Since DTAB binds the gold surface by a coulombic force, rather than a chemical bonding, it is easily replaced by target molecules for SERS purposes. The SERS enhancement factor of the 80 nm gold nanoparticle film was approximately 1.2 × 10(6) using crystal violet (CV) as a Raman dye. The SERS signal from the proposed DTAB-mediated film was approximately 10 times higher than that from the octanethiol-modified gold film, while the reproducibility and stability of this film compared to an octanethiol-modified film were similar. This method can also be applied to other metal nanostructures to fabricate metal films for use as a sensitive SERS substrate with a higher enhancement factor.     

Gold Nanowire Forests for SERS Detection

Simple wet chemistry has been applied to control the vertical growth of gold nanowires on a glass substrate. As a consequence, the longitudinal localized surface plasmon band position can be tuned from 656 to 1477 nm in a few minutes by simply controlling the growth rate and time. This allowed us to select the optimum conditions for maximum electromagnetic enhancement and performance in surface enhanced Raman scattering (SERS) detection. SERS measurements confirmed the uniform and reproducible distribution of the nanowires on the substrate, with the subsequent high reproducibility of hot spot formation. Detection of malachite green in water and of 1-naphthalenethiol from the gas phase are demonstrated as proof-of-concept applications of these three-dimensional SERS substrates.     

Design of Silica@Au Hybrid Nanostars for Enhanced SERS and Photothermal Effect

Core-shell nanostructures of silicon oxide@noble metal have drawn a lot of interest due to their distinctive characteristics and minimal toxicity with remarkable biocompatibility. Due to the unique property of localized surface plasmon resonance (LSPR), plasmonic nanoparticles are being used as surface-enhanced Raman scattering (SERS) based detection of pollutants and photothermal (PT) agents in cancer therapy. Herein, we demonstrate the synthesis of multifunctional silica core – Au nanostars shell (SiO₂@Au NSs) nanostructures using surfactant free aqueous phase method. The SERS performance of the as-synthesized anisotropic core-shell NSs was examined using Rhodamine B (RhB) dye as a Raman probe and resulted in strong enhancement factor of 1.37×10⁶. Furthermore, SiO₂@Au NSs were also employed for PT killing of breast cancer cells and they exhibited a concentration-dependent increase in the photothermal effect. The SiO₂@Au NSs show remarkable photothermal conversion efficiency of up to 72 % which is unprecedented. As an outcome, our synthesized NIR active SiO₂@Au NSs are of pivotal importance to have their dual applications in SERS enhancement and PT effect.