Time-dependent picture of the charge-transfer contributions to surface enhanced Raman spectroscopy

We reexamine the Herzberg-Teller theory of charge-transfer contributions to the theory of surface enhanced Raman scattering (SERS). In previous work, the Kramers-Heisenberg-Dirac framework was utilized to explain many of the observed features in SERS. However, recent experimental and theoretical developments suggest that we revise the theory to take advantage of the time-dependent picture of Raman scattering. Results are obtained for molecular adsorption on nanoparticles in both the strong confinement limit and the weak confinement limit. We show that the Herzberg-Teller contributions to the charge-transfer effect in SERS display a resonance at the molecule-to-metal or metal-to-molecule transition while retaining the selection rules associated with normal Raman spectroscopy (i.e., harmonic oscillator, as opposed to Franck-Condon overlaps). The charge-transfer contribution to the enhancement factor scales as Gamma(-4), where Gamma is the homogeneous linewidth of the charge-transfer transition, and thus is extremely sensitive to the magnitude of this parameter. We show that the Herzberg-Teller coupling term may be associated with the polaron-coupling constant of the surface phonon-electron interaction. A time-dependent expression for the Raman amplitude is developed, and we discuss the implications of these results for both metal and semiconductor nanoparticle surfaces.     

Single-molecule surface enhanced resonance Raman spectroscopy of the enhanced green fluorescent protein

In the present contribution, we demonstrated that surface-enhanced resonance Raman scattering spectra from single green fluorescent proteins (GFPs) were obtained. The most important findings are the direct detection of the conversion between a deprotonated and a protonated form of the chromophore at the single-molecule level via the corresponding vibrational fingerprints, and the fact that the enhanced green fluorescent protein (EGFP) also shows a high surface enhanced resonance Raman scattering (SERRS) signal. Our findings show the potential of the technique to study structural dynamics of protein molecules at a single-molecule level.     

Rapid monitoring of antibiotics using Raman and surface enhanced Raman spectroscopy

Comparatively few studies have explored the ability of Raman spectroscopy for the quantitative analysis of microbial secondary metabolites in fermentation broths. In this study we investigated the ability of Raman spectroscopy to differentiate between different penicillins and to quantify the level of penicillin in fermentation broths. However, the Raman signal is rather weak, therefore the Raman signal was enhanced using surface enhanced Raman spectroscopy (SERS) employing silver colloids. It was difficult by eye to differentiate between the five different penicillin molecules studied using Raman and SERS spectra, therefore the spectra were analysed by multivariate cluster analysis. Principal components analysis (PCA) clearly showed that SERS rather than the Raman spectra produced reproducible enough spectra to allow for the recovery of each of the different penicillins into their respective five groups. To highlight this further the first five principal components were used to construct a dendrogram using agglomerative clustering, and this again clearly showed that SERS can be used to identify which penicillin molecule was being analysed, despite their molecular similarities. With respect to the quantification of penicillin G it was shown that Raman spectroscopy could be used to quantify the amount of penicillin present in solution when relatively high levels of penicillin were analysed (>50 mM). By contrast, the SERS spectra showed reduced fluorescence, and improved signal to noise ratios from considerably lower concentrations of the antibiotic. This could prove to be advantageous in industry for monitoring low levels of penicillin in the early stages of antibiotic production. In addition, SERS may have advantages for quantifying low levels of high value, low yield, secondary metabolites in microbial processes.     

Application of surface enhanced Raman spectroscopy to the study of SOFC electrode surfaces

SERS provided by sputtered silver was employed to detect trace amounts of chemical species on SOFC electrodes. Considerable enhancement of Raman signal and lowered detection threshold were shown for coked nickel surfaces, CeO(2) coatings, and cathode materials (LSM and LSCF), suggesting a viable approach to probing electrode degradation and surface catalytic mechanism.     

On-chip ultra-thin layer chromatography and surface enhanced Raman spectroscopy

We demonstrate that silver nanorod (AgNR) array substrates can be used for on-chip separation and detection of chemical mixtures by combining ultra-thin layer chromatography (UTLC) and surface enhanced Raman spectroscopy (SERS). The UTLC-SERS plate consists of an AgNR array fabricated by oblique angle deposition. The capability of the AgNR substrates to separate the different compounds in a mixture was explored using a mixture of four dyes and a mixture of melamine and Rhodamine 6G at varied concentrations with different mobile phase solvents. After UTLC separation, spatially-resolved SERS spectra were collected along the mobile phase development direction and the intensities of specific SERS peaks from each component were used to generate chromatograms. The AgNR substrates demonstrate the potential for separating the test dyes with plate heights as low as 9.6 μm. The limits of detection are between 10(-5)-10(-6) M. Furthermore, we show that the coupling of UTLC with SERS improves the SERS detection specificity, as small amounts of target analytes can be separated from the interfering background components.     

Protein-nanoparticle labelling probed by surface enhanced resonance Raman spectroscopy

A benzotriazole dye has been attached to a heme protein via a Michael addition and the unique potential of surface enhanced resonance Raman scattering (SERRS) to provide informative in situ recognition of more than one label on one protein demonstrated.     

Determination of carbendazim in tea using surface enhanced Raman spectroscopy

Surface-enhanced Raman scattering(SERS) is applied to detect the concentration of carbendzim(CBZ) in tea leaves. Au colloid is selected and used for active surfaces, and the extraction conditions are optimized in the experiment. The linearity range for the SERS intensity and the concentration of CBZ is found to be0.5 to 8 mg kgà1. The detection limit for CBZ is 0.1 mg kgà1and its recovery in tea samples is 72.3%. The detection results for CBZ using this method are compared with those of HPLC, and no obvious difference can be found. In addition, by dripping the condensed Au colloid on the tea leaves, the proposed SERS approach could be used to the in-situ determination of the half life period of CBZ on tea leaves.     

表面增强拉曼光谱在食品安全分析中的应用

拉曼光谱技术具有样品用量少、快速高效、无损分析等特点,表面增强拉曼光谱克服了常规拉曼光谱灵敏度低的缺点,可以获得更多物质结构信息,在现场快速筛查、检测和鉴别农兽残、限用或禁用添加剂分析检测中具有广阔的应用前景。本文综述了表面增强拉曼光谱在食品中农药残留、兽药残留和限/禁用添加剂检测中的研究进展,并展望了其发展前景。     

Efficient disc on pillar substrates for surface enhanced Raman spectroscopy

In this work, geometrical optimizations of Ag disc on pillar (DOP) hybrid plasmonic nanostructures were conducted and allowed us to achieve reproducible average enhancement factors of 1 × 10(9) and greater.     

A simple filter-based approach to surface enhanced Raman spectroscopy for trace chemical detection

We demonstrate an extremely simple and practical surface enhanced Raman spectroscopy (SERS) technique for trace chemical detection. Filter membranes first trap silver nanoparticles to form a SERS-active substrate and then concentrate analytes from a mL-scale sample into a μL-scale detection volume. We demonstrate a significant improvement in detection limit as compared to colloidal SERS for the pesticide malathion and the food contaminant melamine. The measured SERS intensity exhibits low variation relative to traditional SERS techniques, and the data can be closely fit with a Langmuir isotherm. Thus, due to the simple procedure, the low-cost of the substrates, the quantitative results, and the performance improvement due to analyte concentration, our technique enables SERS to be practical for a broad range of analytical applications, including field-based detection of toxins in large-volume samples.     

Spatially resolved surface enhanced Raman spectroscopy: Feasibility,intensity dependence on sampling area and attomole mass sensitivity

Spatially resolved surface enhanced Raman scattering (SR SERS) from μm sized sampling areas has been observed in the electrochemical environment for the first time. Analysis of the equation for SR SERS intensity reveals that the detected signal should be area independent. This is demonstrated experimentally for the aqueous pyridine/Ag model system using 200 kW cm⁻² peak laser irradiance. The mass detection limits for SR SERS are found to be on the order of 10⁵ molecules or 0.17 attomoles!     

Temperature dependent surface enhanced Raman spectroscopy of piperidine in AgBr sol

The temperature dependent surface enhanced Raman (SER) spectra of piperidine in AgBr sols are presented with emphasis on the study of the intensity variation as the temperature increases. Most of the SER intensities decrease as the temperature increases. This is interpreted as due to the increase of the adsorption distance from the sol particle surface. It is also inferred from the various slopes of the decrease in intensity of various modes that there is variation of the SER effect in the dimension of a single bond, i.e. in 1.5 Å. Moreover, from the temperature dependent SER spectra, it is concluded that due to thermal agitation, both axial and equatorial piperidine molecules are adsorbed on the sol particle surface as the temperature increases. The energy difference between these two forms are calculated to be around 20 kcal/mol which is believed to be larger than that in solution due to the adsorption effect. Besides, two peaks at 1390 and 1231 cm⁻¹ show an anomalous positive thermal effect which could be due to the very complicated SERS mechanism. Finally, it is discussed that the temperature dependent SERS study possesses potentiality in revealing the chemical structure near the sol particle and noble metal electrode surfaces.     

Time dependent surface enhanced Raman spectroscopy of pyridine in AgBr sol

Time and concentration dependent surface enhanced Raman spectra of pyridine adsorbed on silver bromide colloids are studied. It is found that the time     

Non aggregated colloidal silver nanoparticles for surface enhanced resonance Raman spectroscopy

Silver nanoparticles with a tuneable λ max were produced as colloids by heterogeneous nucleation. The synthesis process is both fast and repeatable, producing stable PVA capped nanoparticles. The colloid's effectiveness in the SERRS system was investigated using Rhodamine 6G, R6G, Crystal Violet, CV, and Malachite Green, MG, as probe molecules. A clear sensing trend was observed, where the Raman signal emitted was significantly enhanced by the addition of silver nanoparticles. A build up of signal intensity is observed until an optimum ratio is achieved, followed by a decline in signal intensity as the concentration of nanoparticles is further increased. The sensing trend appeared to be dependant on the structure of these model molecules with similarly structured compounds exhibiting similar trends. Thus a maximum enhancement with the Ag: analyte molar ratio of ～ 5.56: 1, was seen for CV and MG whereas R6G had a maximum enhancement at the Ag: analyte molar ratio of ～ 2.25: 1.     

Recent progress in surface enhanced Raman spectroscopy for the detection of environmental pollutants

Surface enhanced Raman spectroscopy (SERS) has emerged as one of the most promising analytical tools in recent years. Due to advantageous features such as sensitivity, specificity, ease of operation and rapidity, SERS is particularly well suited for environmental analysis. We summarize here some considerations with respect to the detection of pollutants by SERS and provide an overview on recent achievements in the determination of organic pollutants, heavy metal ions, and pathogens. Following an introduction into the topic and considering aspects of sensitivity, selectivity, reproducibility and portability, we are summarizing applications of SERS in the detection of pollutants, with sections on organic pollutants (pesticides, PAHs and PCBs, explosives), on heavy metal ions, and on pathogens. In addition, we discuss current challenges and give an outlook on applications of SERS in environmental analysis. Contains 174 references.

Physics of single molecule fluctuations in surface enhanced Raman spectroscopy active liquids

Surface enhanced Raman spectroscopy (SERS) of dyes in solution allows the study of the differences between ensemble averaged spectroscopic signals and single molecular events. We address several outstanding issues in single molecule detection via SERS; in particular, evidence for single molecule vibrational pumping and/or single molecule laser heating, the statistics of hotspots in the liquid, and anti-Stokes/Stokes anomalies. We demonstrate that anti-Stokes/Stokes ratios are a very unreliable measure of temperature, because the two processes are affected differently by the underlying frequency-dependent plasmon resonances. Subtle hints of vibrational pumping and/or heating in single molecules can only obtained via careful cross correlations between the parameters (frequency position, width, and intensity) of the Stokes signals for different excitation lasers. We introduce the use of single-peak parameter cross correlations for the study of these phenomena.     

Magnetic separation and immunoassay of multi-antigen based on surface enhanced Raman spectroscopy

A novel and highly sensitive immunoassay method based on surface enhanced Raman spectroscopy (SERS) and magnetic particles has been developed. This method exhibits great potential application in bio-separation and immunoassay.     

金银合金纳米粒子表面处理及其表面增强拉曼光谱研究

采用水合肼还原的方法制备了金银比例为1∶1的金银合金纳米粒子, 紫外可见吸收光谱显示合成的溶胶只有一个介于金和银之间的吸收峰, 证明了合金结构的形成. 通过氨基耦联方法将合金纳米粒子组装到硅片表面, 利用氯金酸与合金中银的反应对基底上合金纳米粒子表面进行了改性处理. 以吡啶为探针分子, 研究了表面处理前后基底的SERS效应的差别, 结果表明随着浸泡时间延长, 信号强度先逐渐增强后降低至不变, 这与合金纳米粒子表面结构的变化有关, 氯金酸与表面银的反应经历了两个过程, 即粒子表面形成小的孔洞(去合金过程)和AgCl(s)在粒子表面的沉积, 前者有利于SERS效应的提高, 而后者导致SERS效应快速衰减.     

2D correlation analysis of the continuum in single molecule surface enhanced Raman spectroscopy

The role and the nature of the continuum in Surface Enhanced Raman Spectroscopy (SERS) are unclear. Here, two-dimensional (2D) covariance and correlation analysis is applied to single molecule SERS spectra on silver colloids with and without rhodamine 6G (native colloid). The resulting 2D covariance and correlation maps show that the sharp molecular Raman peaks from rhodamine 6G and the molecule responsible for the SERS peaks from the native colloid are correlated to different continua even though both continua are present in each data set. This suggests that two distinct active sites on the silver colloids produce the two different continua, and that each site has some molecular specificity.