An optofluidic device for surface enhanced Raman spectroscopy

We have developed an optofluidic device that improves the sensitivity of surface enhanced Raman spectroscopy (SERS) when compared to other SERS approaches. This device has a pinched and step microchannel-nanochannel junction that can trap and assemble nanoparticles/target molecules into optically enhanced SERS active clusters by using capillary force. These SERS active clusters provide an electromagnetic enhancement factor of approximately 10(8). In addition, due to the continuous capillary flow that can transport nanoparticles/target molecules into the junction sites, the numbers of nanoparticles/target molecules and SERS active sites are increased. As a result, the detection limit of SERS for adenine molecules was better than 10 pM.     

Whispering-gallery mode resonators: Surface enhanced Raman scattering without plasmons

Calculations based on the Mie theory are performed to determine the locally enhanced electric fields due to whispering-gallery mode resonances for dielectric microspheres, with emphasis on electromagnetic "hot spots" that are located along the wavevector direction on the surface of the sphere. The local electric field enhancement associated with these hot spots is used to determine the surface enhanced Raman scattering enhancement factors for a molecule, here treated as a classical dipole, located near the surface of the sphere. Both incident and Raman emission enhancements are calculated accurately using an extension of the Mie theory that includes interaction of the Raman dipole field with the sphere. The enhancement factors are calculated for dielectric spheres in vacuum with a refractive index of 1.9 and radii of 5, 10, and 20 microm and for wavelengths that span the visible spectrum. Maximum Raman scattering enhancement factors on the order of 10(3)-10(4) are found at locations slightly off the propagation axis when the incident excitation but not the Stokes-shifted radiation is coincident with a whispering-gallery mode resonance. The enhancement factors are found to vary inversely with the resonance width, and this determines the influence of the mode number and order on the results. Additional calculations are performed for the case where the Stokes-shifted radiation is also on-resonance and Raman enhancement factors as large as 10(8) are found. These enhancement factors are typically a factor of 10(2) smaller than would be obtained from /E/4 enhancement estimates, as enhancement of the Raman dipole emission is significantly reduced compared to the local field enhancement for micron size particles or larger. Conditions under which single-molecule or few-molecule measurements are feasible are identified.     

TDDFT studies of absorption and SERS spectra of pyridine interacting with Au20

We present time dependent density functional theory (TDDFT) calculations for a tetrahedral Au20 complex interacting with pyridine for the purpose of modeling absorption and surface enhanced Raman scattering, with emphasis on chemical and electrodynamic enhancement effects. These calculations are done using the ADF code with the BP86 functional, the zeroth-order regular approximation and with the resonant electronic response modeled using a short time approximation expression for the perturbed density matrix, with a damping factor that is empirically chosen. The absorption spectrum of bare Au20 shows strong intraband (sp-sp) and interband (sp-d) coupling with a low-energy peak at 2.89 eV that is mostly intraband and other peaks at 3.94 and 4.70 eV that have mixed intra- and interband character. SERS spectra are calculated for pyridine/Au20 for both vertex (V) and surface (S) configurations at their respective lowest energy absorption maxima (near 2.89 eV), and we find that the V configuration has higher intensities that correspond to SERS enhancements of 10(3)-10(4), whereas S has an enhancement of 10(2)-10(3). These enhancement values are significantly lower than the analogous results for pyridine/Ag20 primarily because of reduced oscillator strength associated with the intraband transition in Au20. Decomposition of the pyridine/Au20 enhancement factor into chemical and electromagnetic contributions (through an analysis of the static SERS intensities) shows enhanced chemical enhancements compared to Ag20 but reduced electromagnetic enhancements.     

Characterization of hotspots in a highly enhancing SERS substrate

Vapor deposition of silver and gold onto a porous anodized aluminum oxide template is shown to produce a SERS substrate with an average surface enhancement factor of 10(7)-10(8). The high level of enhancement is explored using a combination of dark-field Rayleigh scattering and Raman spectroscopy and imaging. The scattering spectrum of the surface indicates a Plasmon resonance at 633 nm and dark-field imaging shows a relatively uniform scattering intensity at this wavelength. These measurements are consistent with the uniform enhanced Raman intensity observed in Raman maps of the substrate. Scanning electron microscopy shows the surface exhibits heterogeneous nanostructures with diameters of approximately 100 nm, the size of the pores in the template. Our measurements indicate that interactions between adjacent structures forming junctions and crevices likely give rise to a high density of hotspots, which provide the extraordinary SERS enhancement. The advantage of substrates prepared in this way is the reproducibly dense distribution of hotspots across the surface, increasing the likelihood that an analyte will experience the largest enhancement.     

A quantum mechanical theory for single molecule-single nanoparticle surface enhanced Raman scattering

Single molecule-single nanoparticle surface enhanced Raman scattering event is analyzed using a quantum mechanical approach, resulting in an analytical expression for the electromagnetic enhancement factor that succinctly elucidates the fundamental aspects of SERS. The nanoparticle is treated as a dielectric spherical cavity, and the resulting increase in the spontaneous emission rate of a molecule adsorbed onto the surface of the nanoparticle is examined. The overall enhancement in Raman scattering is due to both the increased local electromagnetic field and the Purcell effect. The predictions of the present model are in agreement with the simulation results of the classical model.     

Exploring the chemical enhancement for surface-enhanced Raman scattering with Au bowtie nanoantennas

Single metallic bowtie nanoantennas provide a controllable environment for surface-enhanced Raman scattering (SERS) of adsorbed molecules. Bowties have experimentally measured electromagnetic enhancements, enabling estimation of chemical enhancement for both the bulk and the few-molecule regime. Strong fluctuations of selected Raman lines imply that a small number of p-mercaptoaniline molecules on a single bowtie show chemical enhancement >10(7), much larger than previously believed, likely due to charge transfer between the Au surface and the molecule. This chemical sensitivity of SERS has significant implications for ultra-sensitive detection of single molecules.     

用表面增强拉曼散射研究电解法制备的纳米银膜

用一种廉价的电解方法制备了纳米银膜,并详细研究了在这种银膜上的表面增强拉曼散射效果．结晶紫为本实验的检测性分子．通过实验发现,这种银膜用便携式拉曼光谱仪测试并计算出的表面增强拉曼散射的增强因子为603,并对结晶紫的最小检出限为0．1nmol／L．     

Dealloying Ag–Al Alloy to Prepare Nanoporous Silver as a Substrate for Surface‐Enhanced Raman Scattering: Effects of Structural Evolution and Surface Modification

Sensitive detection of molecules by using the surface‐enhanced Raman scattering (SERS) technique depends on the nanostructured metallic substrate and many efforts have been devoted to the preparation of SERS substrates with high sensitivity, stability, and reproducibility. Herein, we report on the fabrication of stable monolithic nanoporous silver (NPS) by chemical dealloying of Ag–Al precursor alloys with an emphasis on the effect of structural evolution on SERS signals. It was found that the dealloying conditions had great influence on the morphology (the ligament/pore size) and the crystallization status, which determined the SERS signal of rhodamine 6G on the NPS. NPS with small pores, low residual Al, and perfect crystallization gave high SERS signals. A high enhancement factor of 7.5×10⁵ was observed on bare NPS obtained by dealloying Ag₃₀Al₇₀ in 2.5 wt % HCl at room temperature followed by 15 min aging at around 85 °C. After coating Ag nanoparticles on the NPS surface, the enhancement factor increased to 1.6×10⁸ owing to strong near‐field coupling between the ligaments and nanoparticles.     

Surface enhanced Raman spectroscopic study of α-bithiophene and α-quaterthiophene molecules adsorbed on silver sols

Surface enhanced Raman scattering (SERS) has been observed from α-bithiophene (2T) and α-quaterthiophene (4T) adsorbed on Ag sols. Experimental results suggest that the adsorbed oligothiophene molecules are anti but twisted and the twist angle is smaller in the surface adsorbed state than in solution. It has been observed that the enhancement factor decreases with the increase of the chain length and maximum enhancement is obtained at a higher concentration for the longer oligomer. The Raman excitation frequency dependence of the enhancement suggests a significant classical electromagnetic contribution to the SERS of oligothiophenes in Ag sols     

Local and average electromagnetic enhancement in surface-enhanced Raman scattering from self-affine fractal metal substrates with nanoscale irregularities

We study the electromagnetic mechanism in surface-enhanced Raman scattering (SERS) from random self-affine fractal metal substrates with nanoscale irregularities by means of the rigorous Green’s theorem integral equation formulation. The SERS enhancement factor is calculated from the numerical results for the surface electric field intensity at the pump frequency for ensembles of self-affine fractal realizations a few microns wide with decreasing lower scale cutoff. We observe a dramatic increase of the local and average SERS enhancement factors throughout the visible and near infrared when the size of surface irregularities is diminished to a few nanometers.     

Combined SPR and SERS microscopy in the Kretschmann configuration

A novel hybrid spectroscopic technique is proposed, combining surface plasmon resonance (SPR) with surface-enhanced Raman scattering (SERS) microscopy. A standard Raman microscope is modified to accommodate the excitation of surface plasmon-polaritons (SPPs) on flat metallic surfaces in the Kretschmann configuration, while retaining the capabilities of Raman microscopy. The excitation of SPPs is performed as in standard SPR-microscopy; namely, a beam with TM-polarization traverses off-axis a high numerical aperture oil immersion objective, illuminating at an angle the metallic film from the (glass) substrate side. The same objective is used to collect the full Kretschmann cone containing the SERS emission on the substrate side. The angular dispersion of the plasmon resonance is measured in reflectivity for different coupling conditions and, simultaneously, SERS spectra are recorded from Nile Blue (NB) molecules adsorbed onto the surface. A trade-off is identified between the conditions of optimum coupling to SPPs and the spot size (which is related to the spatial resolution). This technique opens new horizons for SERS microscopy with uniform enhancement on flat surfaces.     

One-dimensional arrays of nanoshell dimers for single molecule spectroscopy via surface-enhanced Raman scattering

The optical properties of one-dimensional arrays of metal nanoshell dimers are studied systematically using the T-matrix method based on Mie theory, within the context of surface enhanced Raman scattering (SERS). It is shown that the local electromagnetic enhancement can be as high as approximately 4.5 x 10(13) for nanoshell dimer arrays with optimal geometry, and sensitive tunability in the resonant frequency can be gained by varying the geometrical parameters, making such structures appealing templates for SERS measurements with single molecule sensitivity. The extraordinarily high enhancement is attributed to a collective photonic effect constructively superposed onto the intrinsic enhancement associated with an isolated nanoshell dimer.     

金纳米粒子组装体系粒子密度与SERS强度的关系

利用纳米粒子组装技术制备出金基底／巯基苯胺自组装膜偶联层／金纳米粒子的“三明治”结构。实验结果显示,该结构对偶联层分子的喇曼光谱显示出很好的增强效应,增强因子可达１０＾５;在表面粒子密度（粒子覆盖度）较低时,表面增强喇曼散射（ＳＥＲＳ）强度与表面粒子密度近似呈线性关系;随着表面粒子密度的增加,这种线性关系出现负偏差并在表面粒子密度较高区域出现一个平台;在６０￣１１０ｎｍ范围内大粒径金粒子对喇曼光谱     

Electromagnetic model and calculations of the surface-enhanced Raman-shifted emission from Langmuir-Blodgett films on metal nanostructures

We present a theoretical study of the electromagnetic contribution to surface-enhanced Raman scattering (SERS) from a Langmuir-Blodgett film close to a metal surface. This macroscopic dipolar model fully accounts for the Raman-shifted emission so that meaningful SERS (electromagnetic) enhancement factors that do not depend only on the local electromagnetic field enhancement at the pump frequency are defined. For a plane metal surface, analytical SERS enhancement factors that are consistent for all pump beam polarization and molecular orientation are obtained. In order to investigate SERS on complex nanostructured metal surfaces, we introduce this model into the formally exact, Green's theorem surface integral equation formulation of the scattered electromagnetic field. This formulation is thus employed to calculate numerically the near-field and far-field emissions at the Raman-shifted frequency for very rough, random nanostructured surfaces, with emphasis on the impact of collective processes for varying pump frequency and Raman shift. Our results reveal that the widely used |E|4 approximation tends to overestimate average SERS enhancement factors.     

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。     

Nanoscale Chemical Imaging Using Tip‐Enhanced Raman Spectroscopy: A Critical Review

Methods for chemical analysis at the nanometer scale are crucial for understanding and characterizing nanostructures of modern materials and biological systems. Tip‐enhanced Raman spectroscopy (TERS) combines the chemical information provided by Raman spectroscopy with the signal enhancement known from surface‐enhanced Raman scattering (SERS) and the high spatial resolution of atomic force microscopy (AFM) or scanning tunneling microscopy (STM). A metallic or metallized tip is illuminated by a focused laser beam and the resulting strongly enhanced electromagnetic field at the tip apex acts as a highly confined light source for Raman spectroscopic measurements. This Review focuses on the prerequisites for the efficient coupling of light to the tip as well as the shortcomings and pitfalls that have to be considered for TERS imaging, a fascinating but still challenging way to look at the nanoworld. Finally, examples from recent publications have been selected to demonstrate the potential of this technique for chemical imaging with a spatial resolution of approximately 10 nm and sensitivity down to the single‐molecule level for applications ranging from materials sciences to life sciences.     

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.     

SERS Enhancement on the Basis of Temperature‐Dependent Chemisorption: Microcalorimetric Evidence

The mechanism of surface‐enhanced Raman spectroscopy (SERS) is not very clear in view of the magnitude of the contribution of electromagnetic factor as well as the chemical mechanism. This report presents the extent of adsorption at different temperatures in terms of signal enhancements in SERS employing silver nanoparticles (AgNPs) of various shapes as substrate and dye molecules, crystal violet or Rhodamine 6G, as model Raman probes. Initially, the SERS signal increases with increasing temperature until a maximum intensity is reached, before it gradually decreases with increasing temperature. This trend is independent of the shape of the Raman substrates and probes. However, the temperature at which maximum intensity is obtained may depend upon the nature of the Raman probe. The energetics involved in the chemisorption process between dye molecules and AgNPs were determined through isothermal titration calorimetry and their implications for the observed SERS signals were assessed. The maximum heat change occurred at the temperature at which the maximum signal enhancement in SERS was obtained and the enhanced interaction at optimum temperature was confirmed by absorption spectroscopy.     

Applications of surface enhanced Raman scattering to the study of metal-adsorbate interactions

Surface enhanced Raman scattering (SERS) is a powerful technique for characterizing adsorbed species and processes at metallic surfaces. The giant signal enhancement (10⁴–10⁶ larger than normal Raman scattering) makes this technique sensitive to even sub-monolayer amounts of adsorbate on a surface. Consequently, the application of SERS to the in situ study of electrochemical processes provides useful mechanistic and structural information. In this review, advantages and limitations of electrochemical SERS techniques are presented along with experimental information about the nature of the metal-adsorbate interactions occurring in various aqueous and non-aqueous systems. Special emphasis is given to experimental results; however, the salient features of the enhancement theories are highlighted. Adsorbate orientation and SERS surface selection rules are discussed.     

Surface enhanced Raman scattering of p-aminothiophenol self-assembled monolayers in sandwich structure fabricated on glass

A sandwich structure consisting of Ag nanoparticles (NPs), p-aminothiophenol (p-ATP) self-assembled monolayers (SAMs), and Ag NPs was fabricated on glass and characterized by surface enhanced Raman scattering (SERS). The SERS spectrum of a p-ATP SAM in such sandwich structure shows that the electromagnetic enhancement is greater than that on Ag NPs assembled on glass. The obtained enhancement factors (EF) on solely one sandwich structure were as large as 6.0 +/- 0.62 x 10(4) and 1.2 +/- 0.62 x 10(7) for the 7a and 3b(b(2)) vibration modes, respectively. The large enhancement effect of p-ATP SAMs is likely a result of plasmon coupling between the two layers of Ag NP (localized surface plasmon) resonance, creating a large localized electromagnetic field at their interface, where p-ATP resides. Moreover, the fact that large EF values (approximately 1.9 +/- 0.7 x 10(4) and 9.4 +/- 0.7 x 10(6) for the 7a- and b(2)-type vibration modes, respectively) were also obtained on a single sandwich structure of Au NPsp-ATP SAMsAg NPs in the visible demonstrates that the electromagnetic coupling does not exist only between Ag NPs but also between Au and Ag NPs. The lower EF values on Au-to-Ag NPs compared to those on Ag-to-Ag NPs demonstrate that the Au-to-Ag coupling must be less effective than the Ag-to-Ag coupling for the induction of SERS in the visible.