Molecular-level investigation on electrochemical interfaces by Raman spectroscopy

The structure and dynamics of electrode/liquid interfaces play an increasingly important role in electrochemistry. Raman spectroscopy is capable of providing detailed structural information at molecular level and new insight into the interfacial structure, adsorption, reaction, electrocatalysis and corro-sion. In this account we will summarize some progresses of surface Raman spectroscopy in the study of electrochemical interfaces, mainly based on our group's work, laying emphasis on the detection sensitivity, spectral resolution, time resolution and spatial resolution as well as the hyphenated technique.     

Surface-enhanced Raman spectroscopy using silver nanoparticles on a precoated microscope slide

A chemical reduction method was applied to deposit nano silver particles on a frosted microscope slide, precoated with indium tin oxide. The substrate was used to collect the surface-enhanced Raman spectrum of N1'-ethyl indirubin monooxime (EIM), a potential chemical for pharmaceutical application. From the observed surface-enhanced Raman scattering (SERS) spectrum, EIM might interact with silver surface through the lone pair electrons of the oxime nitrogen atoms. Crystal violet (CV) and p-nitrobenzoic acid (PNBA) were also used to test the SERS probe capability of the substrate. The surface morphorlogy of the substrate has been characterized by using scanning electron microscope (SEM) and atomic force microscope (AFM). The elementary composition was identified with Edex and X-ray element scanning.     

电化学原位表面增强拉曼光谱研究Au@Pd纳米粒子薄膜电极上吸附 CO的斯塔克效应

电化学 Stark效应是指电极溶液界面的吸附物或金属-吸附物之间的化学键的振动频率随电极电势而发生变化的现象.研究该效应,可以更好地理解吸附物与基底的相互作用(如吸附构型、吸附取向和覆盖度等随电位的变化),也可反过来推断电极基底的电子构型及其随电势的变化规律,对理解电化学双电层的结构以及电催化反应的构效关系都很有帮助.多年以来,电极表面吸附 CO的电化学 Stark效应广受关注,是由于 CO为很多小分子氧化的中间产物,研究 CO的谱学行为,可加深对 CO以及其它能产生 CO中间物有机小分子的电催化氧化机理和动力学的理解;另一方面, CO与过渡金属之间普遍存在s给电子以及p反馈电子作用,因此 CO也可作为探针分子,通过考察 COad以及 M–COad的振动频率的变化,可推断相应条件下基底的电子与几何结构等信息.
　　本文使用电化学原位表面增强拉曼技术,在一个大的电势范围内考察了 Au@Pd纳米粒子薄膜电极上饱和吸附 CO的振动光谱行为,以期更好地理解 COad与基底的成键作用与电极电势之间的关系.由于纯 Pd电极表面的拉曼信号太弱,实验使用具有核壳结构的 Au@Pd纳米粒子薄膜作为模型电极,并利用 Au核的拉曼增强特性.宽广的电势范围约–1.5到0.55V vs. NHE,通过使用酸性、中性以及碱性电解质得以实现.实验考察的电势上限由 COad氧化起始电位决定,而下限由强烈氢析干扰测量所限制.结果表明,在检测的电势范围内, C–OM(M指在电极表面的桥式吸附CO和穴位吸附 CO所形成的谱带重叠)和 Pd–COM键的振动频率可以分为三段： dνC–OM/dE在–1.5~–1.2 V范围内是185~207 cm–1/V,在–1.2~–0.15 V是83~84 cm–1/V,在–0.2~0.55 V是43 cm–1/V;而 dνPd–COM/dE在–1.5~–1.2 V范围内是–10~–8 cm–1/V,在–1.2~–0.15 V是–31~–30 cm–1/V,在–0.2~0.55 V是–15 cm–1/V.与同时记录的极化曲线对比,认为在中性和碱性介质中所观察到 dνC–OM/dE在–1.2 V附近的急剧变化与电极表面发生了强烈的析氢反应有关.另外,结合密度泛函理论模型计算,认为共吸附的 H减少了 COad从桥式构型到穴位构型的转变,在酸性介质中这种变化不明显,可能是由于对应的电势较高,桥式吸附的 CO比例越大,桥式向穴位的转变本身相对较少.     

Surface-enhanced Raman spectroscopic detection of Bacillus subtilis spores using gold nanoparticle based substrates

The detection of bacterial spores requires the capability of highly sensitive and biocompatible probes. This report describes the findings of an investigation of surface-enhanced Raman spectroscopic (SERS) detection of Bacillus subtilis spores using gold-nanoparticle (Au NP) based substrates as the spectroscopic probe. The SERS substrates are shown to be highly sensitive for the detection of B. subtilis spores, which release calcium dipicolinate (CaDPA) as a biomarker. The SERS bands of CaDPA released from the spores by extraction using nitric acid provide the diagnostic signal for the detection, exhibiting a limit of detection (LOD) of 1.5×10(9) spores L(-1) (or 2.5×10(-14) M). The LOD for the Au NP based substrates is quite comparable with that reported for Ag nanoparticle based substrates for the detection of spores, though the surface adsorption equilibrium constant is found to be smaller by a factor of 1-2 orders of magnitude than the Ag nanoparticle based substrates. The results have also revealed the viability of SERS detection of CaDPA released from the spores under ambient conditions without extraction using any reagents, showing a significant reduction of the diagnostic peak width for the detection. These findings have demonstrated the viability of Au NP based SERS substrates for direct use with high resolution and sensitivity as a biocompatible probe for the detection of bacterial spores.     

Surface-enhanced raman spectroscopy of electrochemically characterized interfaces: Relations between Raman scattering intensity and surface coverage for simple anionic adsorbates

Surface-enhanced raman scattering (SERS) obtained as a function of electrode potential for chloride, bromide, iodide, thiocyanate, azide, and cyanide adsorbed at roughened silver electrodes is compared with corresponding surface concentration-potential data extracted from differential capacitance measurements in order to examine the relation between SERS and surface coverage for these structurally simple adsorbates. After generating SERS by means of an oxidation-reduction cycle, it was found that altering the potential to a more negative value, where the adsorbate coverage fell below a monolayer, corresponded closely in most cases to the onset of a potential-dependent decay in the SERS intensity. Monitoring the potential dependence of the Raman intensity with an optical multichannel analyzer as well as with a conventional scanning spectrometer allowed a rapid “reversible” component of the potential dependence to be separated from an additional “irreversible” signal decay associated with the loss of Raman-active sites. Examination of potential-dependent SERS sufficiently rapidly so that reversible conditions prevail has the crucial advantage of holding constant the concentration of Ramanactive sites. For adsorbed chloride and bromide, an approximate correlation was found between the fractional coverage and the variation in the corresponding reversible Raman intensity brought about by altering the electrode potential. The present results suggest that the adsorption energetics of the Raman-active surface sites do not differ substantially from those for the sites occupied by the majority of the adsorbate.     

Surface-enhanced Raman scattering spectroscopy of explosive 2,4-dinitroanisole using modified silver nanoparticles

2,4-Dinitroanisole (DNAN) is being used as a replacement for 2,4,6-trinitrotoluene (TNT) as a less-sensitive melt-cast medium explosive than TNT. In this paper, we studied the surface-enhanced Raman spectroscopy (SERS) analysis of DNAN using Ag nanoparticles (AgNPs) modified by L-cysteine methyl ester hydrochloride. Due to the formation of a Meisenheimer complex between DNAN and the modifier, the modified AgNPs can detect 20 μg/L (0.2 ng) and 0.1 mg/L (1 ng) DNAN in deionized water and aged tap water, respectively. Three other chemicals (L-cysteine, N-acetyl-L-cysteine, and L-cysteine ethyl ester hydrochloride) were used as AgNPs modifiers to study the mechanism of the SERS of DNAN. It was confirmed that the amino group of L-cysteine methyl ester hydrochloride was the active group and that the methyl ester group significantly contributed to the high SERS sensitivity of DNAN. In order to further test the mechanism of Meisenheimer complex formation, the effect of anions and cations present in natural water on the SERS of DNAN was studied. It was found that CO(3)(2-), Cl(-), and K(+) at 100 mg/L did not negatively affect the SERS of 10 mg/L DNAN, while SO(4)(2-), Na(+), Mg(2+), and Ca(2+) at 100 mg/L significantly quenched the SERS of 10 mg/L DNAN. The negative effect of the bivalent cations could be offset by SO(4)(2-).     

Surface-enhanced Raman spectroscopy of blood plasma and serum using Ag and Au nanoparticles: a systematic study

Surface-enhanced Raman spectroscopy (SERS) is a good candidate for the development of fast and easy-to-use diagnostic tools, possibly used on biofluids in point-of-care or screening tests. In particular, label-free SERS spectra of blood serum and plasma, two biofluids widely used in diagnostics, could be used as a metabolic fingerprinting approach for biomarker discovery. This study aims at a systematic evaluation of SERS spectra of blood serum and plasma, using various Ag and Au aqueous colloids, as SERS substrates, in combination with three excitation lasers of different wavelengths, ranging from the visible to the near-infrared. The analysis of the SERS spectra collected from 20 healthy subjects under a variety of experimental conditions revealed that intense and repeatable spectra are quickly obtained only if proteins are filtered out from samples, and an excitation in the near-infrared is used in combination with Ag colloids. Moreover, common plasma anticoagulants such as EDTA and citrate are found to interfere with SERS spectra; accordingly, filtered serum or heparin plasma are the samples of choice, having identical SERS spectra. Most bands observed in SERS spectra of these biofluids are assigned to uric acid, a metabolite whose blood concentration depends on factors such as sex, age, therapeutic treatments, and various pathological conditions, suggesting that, even when the right experimental conditions are chosen, great care must be taken in designing studies with the purpose of developing diagnostic tests.     

Raman spectroscopy of adsorbates on thin film electrodes deposited on silver substrates

Is is shown that enhanced Raman spectra can be detected for CN⁻ absorbed on thin Ni, Co, Cu and Zn layers deposited from cyanide plating baths onto silver substrate electrodes roughened in a preceding controlled oxidation-reduction cycle; adsorption of CN⁻ on Ag stabilises the roughened substrate structure during the electroplating step. Mechanisms for the generation of enhanced spectra at such composite layered electrodes are discussed briefly.     

In-situ monitoring of redox processes of viologen at Au(hkl) single-crystal electrodes using electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy

In-situ Raman/SERS studies of molecular adsorption/reaction behaviors at well-defined electrochemical interfaces are important for understanding the fundamentals of electrochemical processes. However, it is still a great challenge to perform such studies on model single-crystal surfaces as the smooth surface cannot support surface plasmon resonance (SPR). In this work, shell-isolated nanoparticle-enhanced Raman spectroscopy was combined with an electrochemical method (EC-SHINERS) to study the adsorption and redox transformation of a resonant molecule viologen HS-8V8H at Au(hkl) single-crystal electrodes. Changes in the molecular structure with potential were identified on different single-crystal surfaces, which explained the transformation process of viologen from V^2 + state to V⁺ and then V⁰. Facet-dependent SERS enhancement was also observed, which results from the different imaginary part of the dielectric function on Au(111), Au(100) and Au(110), and is supported by the FEM simulations. Furthermore, a nonlinear resonant Raman process has been directly observed in our experiments, which is consistent with the simulation results. These findings increase our understanding of the electrochemical behavior of molecules in model systems.     

Surface-enhanced vibrational Raman spectroscopy of C60 and C70 on rough silver surfaces

The observation of the surface-enhanced vibrational Raman spectra of vapor-deposited C₆₀ and C₇₀ on rough silver films is reported. Both near-monolayer and multilayer films of pure C₆₀ and of C₆₀/C₇₀ mixtures are studied. The films are obtained by evaporating fullerene samples at temperatures of 683–875 K in ultra-high vacuum. Mixed fullerene samples were greatly enriched in C₇₀ by making use of the slightly different vapor pressures of the two major components at the low end of this temperature range. The spectra contain all the lines of the normal Raman spectra as well as several additional lines caused by a reduction in the stringency of the normal Raman selection rules. These results demonstrate the potential of this technique for detecting small quantities of fullerenes and obtaining their vibrational spectra.     

Local electrochemical impedance spectroscopy: A window into heterogeneous interfaces

The ability to probe surface reactivity on a local scale has led to a new insight into the comprehension of the electrochemical reactivity in relation with the microstructure of the surface. Among the different techniques developed in recent years, local electrochemical impedance spectroscopy has the advantage of using a transient approach to locally characterize a stationary electrochemical system without the need to add any redox mediator in solution, which is a great advantage for the study of different systems.In this review, particular attention is paid to the different ways of measuring the local impedance, and the technique implementing a local current measurement in solution is deeply discussed. This local electrochemical impedance spectroscopy journey also encompasses a discussion about technical and experimental limitations.     

Surface-enhanced Raman spectrometry for detection in liquid chromatography using a windowless flow cell

A windowless flow cell has been developed for surface-enhanced Raman (SER) detection in liquid chromatography. Using colloidal silver as an active substrate, SER spectrometry of banned drugs in sport is presented. The experimental factors of primary influence on the analytical performance of the cell have been studied. The results of SER detection with the proposed cell have been compared to UV detection.     

Fourth-order coherent Raman spectroscopy in a time domain: applications to buried interfaces

A Raman-based, nonlinear optical spectroscopy is a promising method for observing vibrational modes localized at buried interfaces. The principles of Raman excitation and interface-selective detection of coherent vibrations are described. Applications to air-liquid, liquid-liquid, air-solid, liquid-solid, and solid-solid interfaces are reviewed.     

Probing polymer surfaces and interfaces using sum frequency generation vibrational spectroscopy - a powerful nonlinear optical technique

Sum frequency generation (SFG) vibrational spectroscopy has been proved to be a powerful technique which substantially impacts on many research areas in surface and interfacial sciences. This paper reviews the recent progress of applying this nonlinear optical technique in the studies of polymer surfaces and interfaces. The theoretical background of SFG is introduced first. Current applications of SFG in polymer science are then described in more detail to demonstrate the significance of this technique. Finally, a short summary is presented on this relatively new but widely applicable spectroscopic technique.