Towards quantitatively reproducible substrates for SERS

There is a need for a method to facilitate the development of novel, reproducible colloidal surface-enhanced Raman scattering (SERS) substrates to encourage the use of SERS in applied studies. In this study we show for the first time that by using suitably designed SERS experiments in conjunction with multivariate analysis of variance (MANOVA), an objective assessment of colloidal SERS reproducibility can be made. This is demonstrated with the analyte cresyl violet, but could be extended to any analyte of interest for which reproducible SERS data are needed.     

Cantilever tip near-field surface-enhanced Raman imaging of tris(bipyridine)ruthenium(II) on silver nanoparticles-coated substrates

The near-field surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) images of tris(bipyridine)ruthenium(II) adsorbed on a silver nanoparticles-coated substrate were obtained with a scanning near-field optical microscope (SNOM, or near-field scanning optical microscopy, NSOM) using a cantilever tip. In comparison with the most widely used fiber tip for SNOM, the cantilever tip has higher optical throughput and better thermal stability, making it more suitable for detecting the extremely low Raman signal in the near-field spectroscopic investigations. Our preliminary results show that the near-field SERS with the higher spatial resolution can provide richer fingerprint information than the far-field SERS. A comparison of the two types of images shows that there are more SERS than SEF hot spots, and the two types of hot spots do not overlap. More surprisingly, the near-field SERS spectra differ from the far-field SERS spectra obtained on the same sample in the band frequency and relative intensities of some major Raman bands, and some IR-active bands were observed with the near-field mode. These results are explained mainly by the electric field gradient effect and heterogeneous polarization character that operate only in the near-field SERS.     

Influence of electric field on SERS: frequency effects, intensity changes, and susceptible bonds

The fundamental mechanism proposed to explain surface-enhanced Raman scattering (SERS) relies on electromagnetic field enhancement at optical frequencies. In this work, we demonstrate the use of microfabricated, silver nanotextured electrode pairs to study, in situ, the influence of low frequency (5 mHz to 1 kHz) oscillating electric fields on the SERS spectra of thiophenol. This applied electric field is shown to affect SERS peak intensities and influence specific vibrational modes of the analyte. The applied electric field perturbs the polar analyte, thereby altering the scattering cross section. Peaks related to the sulfurous bond which binds the molecule to the silver nanotexture exhibit strong and distinguishable responses to the applied field, due to varying bending and stretching mechanics. Density functional theory simulations are used to qualitatively verify the experimental observations. Our experimental and simulation results demonstrate that the SERS spectral changes relate to electric field induced molecular reorientation, with dependence on applied field strength and frequency. This demonstration creates new opportunities for external dynamic tuning and multivariate control of SERS measurements.     

Correlation of surface-enhanced Raman spectroscopy and laser desorption-ionization mass spectrometry acquired from silver nanoparticle substrates

Applying complementary experiments, like laser desorption-ionization mass spectrometry (LDI-MS) and confocal surface-enhanced Raman microscopy, to the same physical sample location has the potential to elucidate the behavior of complex chemical and biochemical systems in ways that are not available to either method applied in isolation. In these experiments surface-enhanced Raman scattering (SERS) and LDI-MS are applied to the same sample spot using a common structure, deposited Ag colloids, both as ionization matrix and simultaneously as enhancing media for surface-enhanced Raman scattering of small organic molecules, dyes and lipids, and the behavior is compared. Three compounds-p-aminothiophenol (ATP), rhodamine 6G and cholesterol-which exhibit different strengths of interaction with Ag are examined in detail by correlated SERS and LDI-MS. The related mechanisms of nanoparticle-assisted desorption-ionization and Raman enhancement are explored by correlating mass and Raman spectra. The correlated spectra highlight the manner in which the different test compounds interact with plasmonic metal nanostructures. These coupled studies yield new insight into the transition of analyte from the metal-solution interface to gaseous ions, including, in the case of organothiols, a rich set of mixed clusters that provide chemical insight into the ion formation process.     

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.     

A spatial distribution study of analyte concentration in solid substrate surface-enhanced Raman spectroscopy

As surface-enhanced Raman spectroscopy becomes more widely used as a technique for routine analysis, attention must be given to factors which affect the reproducibility of the technique. One such factor which has not been considered is the effect of the spatial distribution of molecules in an analyte spot on a solid SERS substrate. In this study, we demonstrate that the spatial distribution of analyte can be quite non-uniform due to chromatographic interactions between the analyte molecules, solvent and support used. The SERS signal obtained is highly dependent on the position of the laser beam in the analyte spot. The SERS signal is also dependent on the relative sizes of the analyte spot and the laser beam at the sample. This latter dependence can not be explained with current theoretical treatments. However, we demonstrate that a simple modification of the current theory can qualitatively explain the results we observe.     

Polarized surface enhanced Raman and absorbance spectra of aligned silver nanorod arrays

Polarized surface-enhanced Raman scattering (SERS) and UV-vis absorbance spectra were measured for a nonplanar Ag nanorod array substrate prepared by oblique angle vapor deposition. The anisotropy of the SERS polarization was shown to differ from that of the polarized UV-vis absorbance. The maximum SERS intensity was observed in the polarization direction perpendicular to the long axis of the Ag nanorods, while the UV-vis absorbance was strongly polarized along the direction of the long axis of the nanorod array. Analysis of the polarization data showed that molecular orientation was not the cause of the anisotropic SERS scattering. Rather, the SERS anisotropy was primarily attributed to the lateral arrangement of the three-dimensional tilted nanorod lattice in which highly localized plasmon modes are created by strong electromagnetic coupling between adjacent metallic nanorods.     

Portable Microfluidic Chip Based Surface-Enhanced Raman Spectroscopy Sensor for Crystal Violet

This paper describes the development of a portable microfluidic chip based on a surface-enhanced Raman spectroscopy (SERS) sensor for crystal violet analysis. A Y-shape microfluidic chip with a staggered herringbone structure was designed to efficiently mix the analyte and SERS active silver colloid. The subsequent detection of the analyte was performed on the microfluidic chip by a portable Raman system. Compared with other methods, this sensor is easy to operate and is expected to have applications for rapid and sensitive on-site analysis. A good linear correlation over the concentration range of 10 to 750 nM of crystal violet with a correlation coefficient of 0.992 was obtained. The recovery was between 98.6% and 102.9% for crystal violet in river water with relative standard deviations between 2.43% and 4.26%.     

Surface-enhanced Raman scattering imaging of single living lymphocytes with multivariate evaluation

This paper is aimed to show the possibility to determine individual organic compounds introduced into single living cells with surface-enhanced Raman spectroscopy (SERS). Surface enhancement was achieved with gold colloids that were allowed to diffuse into lymphocytes. An introduced analyte, rhodamine 6G, could be imaged together with for example nucleotides and amino acids of the cell. Multivariate evaluation of surface-enhanced Raman images proved to be a powerful tool for the separation of spectral information of various intracellular components. The principal component analysis (PCA) enabled identification of spectra containing different chemical information and separation of the spectral contribution of rhodamine 6G from the complex cellular matrix.     

Polarized surface-enhanced Raman spectroscopy on coupled metallic nanowires

Regular-shaped metal nanocrystals and their ensembles can serve as ideal substrates for studying surface-enhanced Raman scattering (SERS). We synthesized well-defined silver nanowires for a systematic study of SERS signal with respect to polarization and structural ordering. The observed dependence on polarization direction confirms prior theoretical predictions that large electromagnetic (EM) fields are localized in the interstitials between adjacent nanowires. We show that these modes are largely dipolar in nature and rely on short-range EM coupling between nanowires.     

Quantitative analysis of thymine with surface-enhanced Raman spectroscopy and partial least squares (PLS) regression

Silver sol surface-enhanced Raman spectroscopy (SERS) was considered as a technique in the quantitative analysis of low-concentration thymine. Because of the poor stability and reproducibility of SERS signal, a polymer of polyacrylic acid sodium was selected as a stable medium to add into silver sol in order to obtain a surface-enhanced Raman spectroscopy signal. Assignments of Raman shift for solid thymine, SERS of thymine, and SERS of thymine containing stable medium were given. The comparison of Raman peaks between them showed that the addition of stable medium had a little influence on the SERS of thymine and is suitable for the quantitative analysis of low-concentration thymine.     

Comparative study of the morphology, aggregation, adherence to glass, and surface-enhanced Raman scattering activity of silver nanoparticles prepared by chemical reduction of Ag+ using citrate and hydroxylamine

Two different silver colloids were prepared by chemical reduction of silver nitrate with trisodium citrate and hydroxylamine hydrochloride to compare their characteristics in relation to their possible use in surface-enhanced Raman scattering (SERS) spectroscopy. The morphology and plasmon resonance of the single nanoparticles and aggregates integrating these colloids were characterized by means of UV-vis absortion spectroscopy and scanning electron microscopy, revealing important differences between each type of nanoparticle as concerns their physical properties. These metallic systems also manifested differences in the aggregation and the adherence to glass surfaces, revealing significant differences in the chemical surface properties of these nanoparticles. SERS and surface-enhanced IR also indicated the presence of interference bands which can overlap the spectra of the analyte, mainly in the case of the citrate colloid. All these differences have an important influence on the applicability of these nanostructured systems in SERS. In fact, the enhancement factor and spectral pattern of the SERS obtained by using alizarin as a molecule probe are different.     

Role of nanoparticle surface charge in surface-enhanced Raman scattering

In this work, the role of nanoparticle surface charge in surface-enhanced Raman scattering (SERS) is examined for the common case of measurements made in colloidal solutions of Ag and Au. Average SERS intensities obtained for several analytes (salicylic acid, pyridine, and 2-naphthalenethiol) on Ag and Au colloids are correlated with the pH and zeta potential (zeta) values of the nanoparticle solutions from which they were recorded. The consequence of the electrostatic interaction between the analyte and the metallic nanoparticle is stressed. The zeta potentials of three commonly used colloidal solutions are reported as a function of pH, and a discussion is given on how these influence SERS intensity. Also examined is the importance of nanoparticle aggregation (and colloidal solution collapse) in determining SERS intensities, and how this varies with the pH of the solution. The results show that SERS enhancement is highest at zeta potential values where the colloidal nanoparticle solutions are most stable and where the electrostatic repulsion between the particles and the analyte molecules is minimized. These results suggest some important criteria for consideration in all SERS measurements and also provide important insights into the problem of predicting SERS activities for different molecular systems.     

Fluorescence dye as novel label molecule for quantitative SERS investigations of an antibiotic

Within this contribution, the proof-of-principle for a new concept for indirect surface-enhanced Raman spectroscopy (SERS) detection is presented. The fluorescence dye FR-530 is applied as a label molecule for the antibiotic erythromycin. The antibiotic binds directly to the label molecule. Changes within the SERS spectrum of the fluorescence dye appearing with the presence of the antibiotic are utilized for the detection and quantitative investigations of erythromycin. With the new concept of binding the label molecule directly to the analyte molecule, the application of linkage compounds like antibodies or any other recognition molecules becomes dispensable.     

表面增强拉曼散射增强机理的部分研究进展

概述了我们在表面增强拉曼散射(SERS)化学增强机理方面的一些研究工作, 介绍了分子-金属的成键作用和光诱导电荷转移对Raman谱峰强度与电位关系的影响, 基于光电场下纳米粒子光学性质的物理增强机理, 并针对SERS机理研究中尚存在的基本问题提出了建立SERS统一理论的展望.     

Investigation of the effects of the local environment on the surface-enhanced Raman spectra of striped gold/silver nanorod arrays

The effects of the local environment on surface-enhanced Raman scattering (SERS) spectra utilizing gold, silver, and gold/silver striped nanorod array substrates was investigated. The arrays were fabricated using an electrochemical metal deposition into an anodic aluminum oxide template. The analyte chosen for this study was p-nitroso-N,N-dimethylaniline (p-NDMA), which has an electronic structure that is highly sensitive to its surrounding environment. Changes in the peak positions and peak ratios were used to probe the influence of water and the striping pattern on the SERS signal of p-NDMA. We present the results of the fabrication and characterization of the nanorod array substrates, as well as SERS spectra of p-NDMA in both polar and nonpolar environments and SERS spectra on a variety of striped nanorod arrays. The Raman data suggests that the p-NDMA molecule exists in a more polarized state when bound to the gold as compared to the silver rods. We have attempted to use these differences to determine whether the SERS signal predominantly arises from the tips of the rods or from the interior of the array.     

In situ laser-induced photochemical silver substrate synthesis and sequential SERS detection in a flow cell

A new, simple, and effective approach for multianalyte sequential surface-enhanced Raman scattering (SERS) detection in a flow cell is reported. The silver substrate was prepared in situ by laser-induced photochemical synthesis. By focusing the laser on the 320 μm inner diameter glass capillary at 0.5 ml/min continuous flow of 1 mM silver nitrate and 10 mM sodium citrate mixture, a SERS active silver spot on the inner wall of the glass capillary was prepared in a few seconds. The test analytes, dacarbazine, 4-(2-pyridylazo)resorcinol (PAR) complex with Cu(II), and amoxicillin, were sequentially injected into the flow cell. Each analyte was adsorbed to the silver surface, enabling the recording of high intensity SERS spectra even at 2 s integration times, followed by desorption from the silver surface and being washed away from the capillary. Before and after each analyte passed the detection window, citrate background spectra were recorded, and thus, no “memory effects” perturbed the SERS detection. A good reproducibility of the SERS spectra obtained under flow conditions was observed. The laser-induced photochemically synthesized silver substrate enables high Raman enhancement, is characterized by fast preparation with a high success rate, and represents a valuable alternative for silver colloids as SERS substrate in flow approaches.     

SERS from molecules adsorbed on small Ag nanoparticles: A microscopic model

A microscopic approach to surface-enhanced Raman scattering (SERS) from molecules adsorbed on noble-metal nanoparticles is developed. For nanoparticle sizes smaller than 10 nm, the classical electromagnetic enhancement mechanism is modified by quantum-size effects. Using time-dependent local field approximation, we perform systematic analysis of SERS in nanometer-sized Ag nanoparticles. We find that, in small nanoparticles, Raman cross-section enhancement is governed by the interplay between Landau damping of the surface plasmon and interband screening in the nanoparticle surface layer.     

Combined linear response quantum mechanics and classical electrodynamics (QM/ED) method for the calculation of surface-enhanced Raman spectra

A multiscale method is presented that allows for evaluation of plasmon-enhanced optical properties of nanoparticle/molecule complexes with no additional cost compared to standard electrodynamics (ED) and linear response quantum mechanics (QM) calculations for the particle and molecule, respectively, but with polarization and orientation effects automatically described. The approach first calculates the total field of the nanoparticle by ED using the finite difference time domain (FDTD) method. The field intensity in the frequency domain as a function of distance from the nanoparticle is calculated via a Fourier transform. The molecular optical properties are then calculated with QM in the frequency domain in the presence of the total field of the nanoparticle. Back-coupling due to dipolar reradiation effects is included in the single-molecule plane wave approximation. The effects of polarization and partial orientation averaging are considered. The QM/ED method is evaluated for the well-characterized test case of surface-enhanced Raman scattering (SERS) of pyridine bound to silver, as well as for the resonant Raman chromophore rhodamine 6G. The electromagnetic contribution to the enhancement factor is 10(4) for pyridine and 10(2) for rhodamine 6G.