Surface enhanced Raman spectroscopy study of the potential dependence of thymine on silver electrodes

The potential-induced changes in thymine coordination on polycrystalline silver electrodes are studied by surface enhanced Raman spectroscopy (SERS) for potentials positive to the potential of zero charge up to the end of the double layer range. Two distinct sets of spectra could be obtained in the range of potentials studied. Both states correspond to chemisorbed phases of thymine on silver, where a distinct heteroatom is deemed responsible for the bond with the surface. At less positive potentials, one of the ring oxygen atoms is responsible for the chemical bond and the molecule assumes a tilted position. At more positive potentials, one of the ring nitrogen atoms, possibly deprotonated, establishes a new bond with the surface, aligning the molecule's axis closer to the surface normal.This paper is dedicated to Prof. Wolf Vielstich on the occasion of his 80th birthday for his outstanding contributions to electrochemistry     

Atomistic Simulations of Dopamine Diffusion Dynamics on a Pristine Graphene Surface**

Carbon microelectrodes enable in vivo detection of neurotransmitters, and new electrodes aim to optimize the carbon surface. However, atomistic detail on the diffusion and orientation of neurotransmitters near these surfaces is lacking. Here, we employ molecular dynamics simulations to investigate the surface diffusion of dopamine (DA), its oxidation product dopamine-o-quinone (DOQ), and their protonated forms on the pristine basal plane of flat graphene. We find that all DA species rapidly adsorb to the surface and remain adsorbed, even without a holding potential or graphene surface defects. We also find that the diffusivities of the adsorbed and the fully solvated DA are similar and that the protonated species diffuse more slowly on the surface than their corresponding neutral forms, while the oxidized species diffuse more rapidly. Structurally, we find that the underlying graphene lattice has little influence over the molecular adsorbate's lateral position, and the vertical placement of the amine group on dopamine is highly dependent upon its charge. Finally, we find that solvation has a large effect on surface diffusivities. These first results from molecular dynamics simulations of dopamine at the aqueous-graphene interface show that dopamine diffuses rapidly on the surface, even without an applied potential, and provide a basis for future simulations of neurotransmitter structure and dynamics on advanced carbon materials electrodes.     

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.     

甲醇在粗糙铂电极上解离吸附的表面拉曼光谱

甲醇由于具有高能量密度及易于储存等优点，在燃料电池研究领域中备受关注．一般认为，甲醇在铂电极上的电氧化为双途径机理，即首先解离吸附在电极表面上，生成毒性中间体（CO）或活性中间体，然后再氧化生成CO。由于各研究小组使用的电极材料、预处理方法、溶液的组成以及检测手段不同，目前对甲醇的具体解离过程的细节尚未确定（如在反应条件下吸附物的特性及作用、表面结构和粗糙度的特殊影响等），还有待于建立或改进有关原位研究方法．在研究甲醇的电催化氧化的各种检测手段中，红外光谱应用得最为广泛，可用于原位红外技术中的电极…     

Au@SiO2膜/Ti电极吸附吡啶的表面增强拉曼光谱研究

基于壳层隔绝纳米粒子增强拉曼光谱技术,合成了Au@SiO₂纳米粒子,并对其进行了相关表征. 结果表明,包裹的二氧化硅层连续、致密,Au@SiO₂膜/Ti电极上可获得金属钛电极上吸附吡啶分子的高质量表面增强拉曼光谱（SERS）信号. 通过Pt、Ni电极的测试,证实该信号源于吸附在基底表面的吡啶分子. 此外,Au@SiO₂膜/Ti电极上吸附吡啶分子的现场SERS光谱研究表明,在-0.1 V ~ -0.6 V电位区间,吡啶分子平躺吸附,从-0.6 V起吸附的吡啶分子由平躺逐转变为垂直,而当电位为-1.2 V时,电极表面析氢,吡啶脱附.     

Surface enhanced Raman scattering of 2,2' biquinoline adsorbed on colloidal silver particles

Surface enhanced Raman scattering (SERS) in silver sol and normal Raman spectra in the bulk and in solution of 2,2' biquinoline (BQ) molecule have been investigated. The observed Raman bands along with their corresponding FTIR bands have been assigned based on the established assignments of the vibrational bands of the parent napthalene and quinoline molecules. Existence of both the cis and trans form of the BQ molecule in solution and in the bulk are inferred from the normal Raman and FTIR spectra, whereas SERS study reveal that in the surface adsorbed state the molecule exists in the cis form. Definite evidence of the charge transfer interaction to the overall contribution in the SER enhancement have been reported. The excitation profile also supports the CT interaction. Estimated enhancement factor of the principal SERS bands indicate that the molecule is adsorbed on the silver surface through both the nitrogen atoms with the molecular plane almost perpendicular to the surface. This preferred orientation of the molecule is in conformity with its existence in the cis form in the surface adsorbed state.     

Exploring three-dimensional nanosystems with Raman spectroscopy: methylene blue adsorbed on thiol and sulfur monolayers on gold

Resonant Raman and surface-enhanced Raman scattering (SERS) spectroscopies, complemented with scanning tunnel microscopy and electrochemical techniques, have been used to obtain information about the amount and spatial distribution of methylene blue (MB) molecules immobilized on sulfur and four ultrathin molecular alkanethiolate films self-assembled on Au(111) and rough Au electrodes. The intensity of the Raman signals allow one to estimate the amount of immobilized MB at different organic films, whereas the decrease in the SERS intensity as a function of distance for the rough Au electrodes is used to locate the average position of the MB species with respect to the Au substrate. We found that significant amounts of cationic MB species are able to diffuse into methyl-terminated thiols, but they are stopped at the outer plane of the self-assembled monolayer (SAM) by negatively charged carboxylate groups. The relative shift of C-N stretching Raman modes indicates that the binding of MB to S is different from that found for MB on thiols. Most of the molecules immobilized on methyl- and carboxylate-terminated thiols are electrochemically inactive, suggesting that strong coupling between the Au electrode and the MB molecules is needed for charge transfer. Our results are consistent with a small population of electrochemically active MB species very close to the Au surface that reach this position driven by their lipophilic (hydrophobic) character through defects at SAMs.     

Surface enhanced Raman scattering at Ag electrodes in non-aqueous pyridine solutions

The surface enhanced Raman scattering (SERS) of interfacial pyridine and Br⁻ species at Ag electrodes from neat pyridine solutions containing tetra-n-butylammonium bromide is presented. The potential dependence of the SERS peak frequencies, bandwidths, and relative intensities is reported and interpreted in terms of pyridine reorientation in the double layer. A band at 221 cm⁻¹ is observed at certain potentials and is assigned to the AgN stretch for Ag surface pyridine species.     

粗糙镍电极上吡啶和二巯基嘧啶的表面拉曼光谱

粗糙镍电极上吡啶和二巯基嘧啶的表面拉曼光谱①黄群健高劲松蔡雄伟毛秉伟郑明森田中群＊（厦门大学化学系，固体表面物理化学国家重点实验室，厦门３６１００５）自从人们发现和认知表面增强拉曼散射（ＳＥＲＳ）效应以来［１］，ＳＥＲＳ技术以其极高的表面灵敏度为电极...     

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.     

Surface enhanced Raman spectroscopy on copper hydrosols

Surface enhanced Raman spectroscopy (SERS) allows the detection of trace quantities of molecular species adsorbed onto a surface. The potential use of silver colloids as substrates for analytical SERS measurements is demonstrated. Detection limits and other analytical figures of merit for pyridine, p-aminobenzoic acid and p-nitrobenzoic acid are presented.     

Investigation the Corrosion Process of the Co Electrode by Surface Raman Spectroscopy

Due to its special magnetic property, Co has become a very important material in surface science. Furthermore, its unique high catalytic activity has made it an important catalyst for syngas in the catalytic industries. On the other hand, in electrochemistry, Co and its oxides are wide-used electrode materials in the Ni battery and alloy electrodes for hydrogen storage. Thus it is very important to perform the study on the surface process on the Co surface. Raman Spectroscopy is a powerful technique in characterizing the surface species, however Co has been considered as not a suitable material for creating enormous surface enhanced Raman process. Theoretically, it has been reported that Co with suitable particle size could be able to generate weak surface enhancement, but no experimental evidence is available. In this study,we developed a method to roughen the Co surface to create possible enhancement in the Raman signal. Based on that, the oxidation process of the Co surface (or the etching process of the Co surface in electrolytes) was studied. Benzotriazole(BTA), a very typical corrosion inhibitor, was added to the electrolyte to study its effect on the corrosion of the Co electrode.     

Adsorption and corrosion inhibition behavior of imidazole on cobalt electrodes studied by SERS and electrochemical methods

As one of the most widely used transition-metals, Co plays an important role in the study of metal mate-rials. The alloys of Co are often used as magnetic ma-terials because of their intense magnetism[1,2]. As an active metal and owing to high catalytic a…     

Raman spectroscopic investigation of silver electrodes

Raman spectroscopy has been applied to the study of the reduction of carbon dioxide and of formate and carbonate ions at a silver electrode. Raman spectra of adsorbed intermediate species, which are as yet only partially identified, have been detected and show marked variations with electrode potential. These spectral variations are clearly correlated with the voltammetric features for carbonate solutions and suggest that these reduction products complicate most measurements on silver electrodes in the cathodic region. The interpretation of the previously reported spectra due to adsorbed pyridine at silver electrodes has been reconsidered; interactions with surface carboxy species may be significant.     

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 scattering of benzotriazole: a molecular orientational study

Surface enhanced Raman scattering of benzotriazole in aqueous silver sol at pH approximately 9 has been investigated. The spectra of the molecule at various concentrations were recorded and the maximum enhancement was observed with a 10(-5) M solution. From the comparison of the surface enhanced Raman spectra with the conventional Raman spectra and applying "surface selection rule", it is inferred that the molecular ion interacts with the silver sol via the nitrogen atoms of the benzotriazole ring and the molecule assumes a near "end-on" orientation.     

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。     

几种粗糙铂电极上表面拉曼增强效应初探

Good-quality surface enhanced Raman spectra have been obtained from various roughened Pt electrodes using a confocal Raman measuring system. A new equation is presented to estimate the enhancement factor G for the electrodispersed and platinized Pt electrodes in a pyridine+NaClO4 solution. It is shown that the platinum electrodes undergoing the special roughening procedures exhibit a weak SERS effect with an enhancement factor of 10 to 1.2×102, depending on the surface pretreament.     

钴电极在碱性溶液中氧化还原反应的现场共聚焦显微喇曼光谱研究

采用现场共聚焦显微喇曼光谱研究钴电极在碱性溶液中的氧化还原行为和生成物的喇曼光谱特征.研究结果表明:电位正向扫描时,在-0.64V左右Co氧化生成Co(OH)₂和CoO,随着电位正移逐步生成Co₃O₄,在正电位区电极表面层主要是Co₃O₄、CoOOH和CoO₂等;电位负向扫描时,电极表面上的高价含氧化合物相继还原为Co₃O₄和Co(OH)₂,并最终还原为Co.由不同电位下的生成物的喇曼光谱可以看出:电极表面上的氧化还原反应是随电位变化而逐步进行的连续化反应过程,并主要形成复合含氧化合物.     

Combining SERS and Electrochemical Transient Technique to Study Structural Change of Coadsorbed Species at Au electrodes

Although surface-enhanced Raman spectroscopy (SERS) is probably one of the most sensitive surface spectroscopic techniques, most of the previously electrochemical SERS studies have been carried out in steady state. Recently, much attention has been attracted to combine time-dependent SERS with electrochemical transient techniques to investigate the electrode/solution interfacial dynamics at molecular level[1,2]. Our previous studies indicate that thiourea (TU) can coadsorb with ClO₄^- at Au electrodes in steady state. In the present work, time-dependent SERS has been used to study the structural change with the potential step for the coadsorption system.