Surface-enhanced Raman scattering at the silver electrode/ionic liquid (BMIPF6) interface

This is the first report of in situ SER spectra of chemical species adsorbed on a Ag/room temperature ionic liquid (RTIL) interface. We have investigated the dependence of the SERS intensity of the RTIL derived from 1-n-butyl-3-methylimidazolium hexafluorophosfate (BMIPF6) adsorbed on a silver electrode. It has been shown that the BMI+ adsorbs on the silver electrode for potentials more negative than -0.4 V vs a Pt quasireference electrode (PQRE). In the -0.4 to -1.0 V potential range the SER spectra are similar to the Raman spectrum of the RTIL BMIPF6. At potentials more negative than -1.0 V some imidazolium ring vibrational modes and N-CH3 vibrations are enhanced, suggesting that the imidazolium ring is parallel to the surface and for potentials <-2.8 V the BMI+ is reduced to the BMI carbene. The potential dependence of the SERS intensities of Py adsorbed on a silver electrode in BMIPF6 has also been investigated. The results have shown that at potentials less negative than -0.8 V (vs PQRE) Py adsorbs at an end-on configuration forming an Ag-N bond. In the -0.9 to -1.4 V potential range Py molecules lie flat on the electrode surface and at potentials <-1.4 V Py is replaced by the BMI+. The electrochemical and SERS results have shown that Py has the effect of changing the oxidation of silver in that medium as well as the reduction of BMI+ to the BMI carbene. In the presence of Py the BMI+ reduction is observed at potentials near -2.4 V. The Ag electrode has presented SERS activity from 0.0 to -3.0 V.     

Raman spectral study of metal-cytosine complexes: a density functional theoretical (DFT) approach

The fluctuation of surface-enhanced Raman scattering (SERS) spectra has been an obstacle to the analysis of the adsorbate on the metal surface. In this paper, we aim at using the density functional theory (DFT) to study the fluctuant Raman spectra of the cytosine molecule which interacts with a coinage metal atom or cation via N1 and N3 sites. The results show that the adsorption site strongly influences the Raman spectral property of cytosine molecule, especially the relative intensity of some bands. In addition, the SERS spectra of cytosine which is adsorbed on the gold, silver, and copper electrodes are measured, and the possible orientation and adsorption site of the cytosine molecule adsorbed on metal electrodes surface are proposed with the help of DFT simulations.     

Surface enhanced Raman spectra of nucleic acid components adsorbed at a silver electrode

High-resolution vibrational spectra of nucleic acid components adsorbed on a silver electrode were obtained using a spectroelectrochemical method based on the large-intensity enhancement for Raman scattering at electrode surfaces.The laser surface Raman spectra of purine, adenine, adenosine, deoxyadenosine, adenine mononucleotides, adenylyl-3′, 5′-adenosine and polyriboadenylic acid were recorded in the range of 150–3500 cm^?1. The intensities of the vibrational bands were highly dependent upon the electrochemical preparation of the electrode, the applied potential and the nature of the adsorbate species. High-intensity spectra in rather dilute bulk solutions were obtained.The phosphate derivatives of adenosine exhibited strongly enhanced Raman scattering. Spectral band frequencies corresponded closely with normal Raman spectra of these molecules in solution. The adenine ring breathing mode at 740 cm^?1 and the adenine ring skeletal vibration at 1335 cm^?1 produced prominent Raman scattering. A strong band at about 240 cm^?1 for the adenine mononucleotides was attributed to silver/adsorbed phosphate group vibrations.     

乙腈溶液中银电极的表面增强拉曼光谱研究

利用共焦显微拉曼系统研究了非水体系0.1mol·L^-1LiClO4/CH3CN溶液中,乙腈分子在粗糙银和金电极表面的吸附和解离行为。结果表明非水体系中乙腈可在银、金表面发生还原反应,产物CN^-离子与电极表面作用形成的表面配合物可以较宽的电位区间吸附于电极表面。溶液中的微量水、激光照射以及电极电位均对该反应有较大的影响。通过拉曼谱图的比较,得出乙腈分子解离出的CN^-在金电极表面比在银电极表面有更强的吸附作用。     

Surface enhanced raman study of benzidine on a silver electrode: an interpretation for different adsorption configurations at various applied voltages

The surface enhanced Raman spectra of benzidine in a silver sol and on a silver electrode at different applied voltages are reported. These spectra are compared with those in the crystalline form and in dimethyl sulfoxide (DMSO) solution, with group theoretical analysis. It is interpreted that in the silver sol and on the silver electrode with voltages larger than −0.2 V (with respect to the standard calomel electrode) the benzidine molecule is adsorbed perpendicularly with one of its two amino groups attached on the silver surface. When the applied voltage on the silver surface is less than −0.4 V both of its amino groups are adsorbed on the silver surface with the flat configuration. The adsorption mechanism for these two configurations is also proposed in the report.     

Enhanced Raman spectra from species formed by the coadsorption of halide ions and water molecules on silver electrodes

The enhanced Raman spectra of water molecules and chloride ions coadsorbed at an electrochemically roughened silver electrode are reported. The spectra are discussed in terms of specifically adsorbed chloride ions.     

Surface-enhanced Raman spectroscopy of 12CN− and 13CN− adsorbed at silver electrodes

Measurements are reported of the surface-enhanced Raman of ¹²CN and ¹³CN (and of isotopically labelled mixtures) adsorbed at silver electrodes. The spectra are shown to arise from a complex species whose coordination number does not change with electrode potential. This species is probably a [Ag(CN₂)]^? entity having C_2v symmetry; at very negative potentials a reduced form of this complex [Ag(CN)₂]^2? coexists with the formally Ag¹ species at the surface. The shifts in band position are interpreted in terms of changes in the bond character of the adsorbed CN^? species. The spectrum of water coadsorbed with CN^? is also markedly dependent on the charge density of the adsorbed CN^? groups.     

Enhanced and normal Raman scattering from pyridine adsorbed on rough and smooth silver electrodes

A multiplex spectrograph has been used to record potential difference and modulation Raman spectra of pyridine adsorbed on silver electrodes in an electrochemical cell. Spectra have been obtained from rough silver surfaces which give SERS and from surfaces where SERS has been diminished by prolonged cathodic polarisation (DSERS). Raman scattering from pyridine at smooth silver surfaces in potassium perchlorate and fluoride solutions has been distinguished from solution scatter by a potential modulation technique. The results show that the enhanced scattering caused by silver atom or cluster sites is respresentative of the surface as a whole as similar Raman spectra are obtained on smooth surfaces at a count rate as low as ?1.4 photons s^?1 (incident laser power 500 mW).Correlation of simultaneous differential capacitance data and “snapshot” SER spectra indicate that pyridine molecules in aqueous chloride ion solutions adsorb on silver in a flat π-bonded configuration at potentials markedly positive to the point of zero charge and exhibit specific reorientation at ?0.3 V and ?0.45 V (vs. SCE) to become N-bonded, perpendicular to the surface. Results also show that the adsorption behaviour of pyridine in chloride and fluoride ion solutions is largely similar.     

时间分辨表面增强拉曼光谱(TRSERS)研究硫脲衍生物与ClO4^-的共吸附行为

运用时间分辨表面增强拉曼光谱(TRSERS)结合电位双阶跃的方法, 研究了硫脲衍生物甲基硫脲(MTU)和烯丙基硫脲(ATU)在银电极表面与ClO4^-离子的共吸附行为, SERS强度-时间曲线表明它们各自相应的SERS谱峰强度随电位阶跃的响应速率不同, 离电极表面较近的基团的特征谱峰强度的响应速率较快; MTU以S端且垂直吸附在电极表面,ATU也以S端和表面发生化学吸附, 但整个分子斜躺吸附在电极表面上。     

Surface-enhanced Raman scattering of pyridine using different metals: differences and explanation based on the selective formation of alpha-pyridyl on metal surfaces

A simple method has been developed to produce SERS-active metal surfaces. Six metal surfaces (cadmium, nickel, gold, iron, copper, and silver) have been prepared on an aluminum foil underlayment by chemical reduction and strong surface-enhanced Raman signals have been observed for pyridine species on these surfaces. This permits the direct comparison of pyridine spectra on different metal surfaces prepared by the same chemically clean method. The differences among the SER spectra of the aqueous pyridine species using different metals generally follow the trend of silver, cadmium, nickel, iron, gold, and copper, which can be explained by the selective formation of alpha-pyridyl species and the equilibria between end-on adsorbed pyridines and edge-on adsorbed alpha-pyridyl species on the different metal surfaces.     

Raman Spectroscopic Studies on the Complexation and Adsorbed Behaviour of Thiourea

Raman spectroscopic studies on thiourea (TU) in highly acidic media (e. g. >3 mol/L HC1O4) show that TU as a solution species is protonated through its sulphur atom and forms complex with anions. However, as an adsorbed species, TU is protonated via the nitrogen atom and adsorbed at the silver electrode surface through the sulphur atom. The distinct effects of pH and anions of the solution on the surface enhanced Raman (SER) spectra of TU were investigated. In acidic and neutral solutions, TU is coadsorbed with anions through its NH3 and - NH2 groups respectively.     

Raman spectra of formate and acetate ions adsorbed on silver particles in aqueous solutions

Raman spectra from the surfaces of black silver particles formed by reduction of silver ions in aqueous solutions have been interpreted as arising from adsorbed formate and adsorbed acetate ions.     

Adsorption of 2-aminobenzothiazole on colloidal silver particles: an experimental and theoretical surface-enhanced Raman scattering study

Surface-enhanced Raman scattering (SERS) spectra of the biologically important 2-aminobenzothiazole (2-ABT) molecule adsorbed on silver hydrosols are compared with its FTIR spectrum and normal Raman spectroscopy (NRS) spectrum in the bulk and in solution. The optimized structural parameters and the computed vibrational wavenumbers of the compound have been estimated from ab initio (Hatree-Fock) and density functional calculations. Some vibrational modes of the molecule have been reassigned. Concentration-dependent SERS spectra of the molecule reveal the existence of two types of vertically adsorbed species on colloidal silver particles, whose relative population varies with the adsorbate concentrations. The adsorption geometry and structural parameters of one type of adsorbed species are related to the NRS spectrum of the chemically prepared and theoretically modeled 2-ABT-Ag(I) coordination compound.     

Raman spectroscopy of cobalt phthalocyanine adsorbed on a silver electrode

Raman spectra of cobalt tetrasulfonated phthalocyanine adsorbed on a silver electrode in aqueous electrolytes have been recorded in situ. It is shown that the entensity of the Raman bands is directly related to the amount of charge transfered during the electrochemical activation of the silver. The strong potential dependence of distinct Raman bands is discussed with respect to the resonance properties of the adsorbate, taking into account the orientation of the molecule on the surface.     

Surface enhanced Raman spectroscopy analysis of the adsorption of 2-thiouracil to Au,Ag and Cu electrodes: Surface potential dependence

Surface enhanced Raman spectroscopy has been used to study the potential controlled behavior of 2-thiouracil adsorbed on silver, gold and copper electrodes. Comparison of the normal Raman and SER spectra shows that the molecule adsorbs chemically to the metallic surfaces through its deprotonated sulfur atom. The position of the molecular aromatic ring in respect to the surface normal under potential modulation was analyzed using the electromagnetic theory's selection rules. Our data indicate that for all electrodes the surface potential induces a molecular reorientation leading to a more upright or tilted position.     

Quantitative electrochemical SERS of flavin at a structured silver surface

In situ electrochemical surface enhanced Raman spectra (SERS) for an immobilized monolayer of a flavin analogue (isoalloxazine) at nanostructured silver surfaces are reported. Unique in the present study, the flavin is not directly adsorbed at the Ag surface but is attached through a chemical reaction between cysteamine adsorbed on the Ag surface and methylformylisoalloxazine. Even though the flavin is held away from direct contact with the metal, strong surface enhancements are observed. The nanostructured silver surfaces are produced by electrodeposition through colloidal templates to produce thin (<1 microm) films containing close-packed hexagonal arrays of uniform 900 nm sphere segment voids. The sphere segment void (SSV) structured silver surfaces are shown to be ideally suited to in situ electrochemical SERS studies at 633 nm, giving stable, reproducible surface enhancements at a range of electrode potentials, and we show that the SER spectra are sensitive to subfemtomole quantities of immobilized flavin. Studies of the SER spectra as a function of the electrode potential show clear evidence for the formation of the flavin semiquinone at the electrode surface at cathodic potentials.     

The raman spectroscopy of the ferricyanide/ferrocyanide system at gold, β-palladium hydride and platinum electrodes

The surface enhanced Raman spectroscopy (SERS) of ferricyanide and ferrocyanide anions at gold electrodes shows that both species are adsorbed at the interface. The variation of the spectra with potential and the nature of the cations of the support electrolyte shows that anions and cations are co-adsorbed and that the structure of the anions is markedly perturbed compared to that of the solution phase species. It is also shown that SERS of these ions at palladium electrodes can be readily generated by converting the surface to the β-palladium hydride. In this case the spectra are more simple but both anions are again adsorbed. Adsorption is also shown to take place at smooth platinum electrodes, measurements in this case being carried out by potential modulated surface unenhanced Raman spectroscopy (SUERS) but with resonance Raman enhancement of the scattering.The spectroscopic results show that electron transfer in the ferrocyanide/ferricyanide system cannot be considered to be a purely outer sphere reaction. Results indicate that the smallest change in structure of the adsorbed species takes place in supporting electrolytes containing caesium ions; these solutions give the largest value of the standard rate constant for the reaction.     

pH值对2-(2-吡啶基)咪唑荧光光谱的影响研究

合成了2-(2-吡啶基)咪唑,并通过稳态荧光光谱研究了它在溶液中的光物理行为及溶液酸碱度的影响.发现2-(2-吡啶基)咪唑化合物存在分子内光诱导电子转移反应(PET),并且这一过程强烈地依赖于溶液的pH值,但是氮原子的质子化会影响PET过程.     

纯锌电极上的表面增强拉曼光谱研究

通过化学刻蚀、电化学沉积和电化学氧化还原等粗糙方法,寻找合适的条件对锌电极表面进行预处理,以期获得吡啶在纯锌电极上的表面增强拉曼光谱(SERS).实验证明,电化学氧化还原处理是最佳的选择.以0.5mol/LNaClO4中性溶液作为电解质溶液,分别进行电化学氧化还原循环和电位阶跃两种处理.结果表明,将还原电位和氧化电位分别控制在-1.6和-0.7V时,利用共焦显微拉曼系统成功地得到了粗糙锌电极表面吸附吡啶的SERS信号.     

Surface enhanced vibrational spectra of thymine

Surface-enhanced vibrational spectroscopy (SEVS) is discussed using 5-methyluracil (thymine) as a model compound. Surface-enhanced Raman scattering (SERS) and surface-enhanced infrared (SEIR) are reported and a characterization of thymine adsorbed onto silver island films is provided. The thymine SERS spectra obtained using silver colloids, silver roughened electrodes and silver island films are remarkably different due to several binding possibilities of thymine during chemical adsorption onto a silver surface. It is shown that laser induced photo dissociation may lead to further changes in the recorded spectra of the adsorbate. The surface enhanced-infrared (SEIR) spectra of thymine on silver island are reported here for the first time. The infrared spectra of thymine films were also been obtained to help the assignment of molecular vibrations in the surface enhanced spectra.