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.     

An investigation of the potential of near-IR Raman spectroscopy for recording non-resonance Raman spectra of azo dyes

The use of near-IR FT-Raman spectroscopy to obtain resonance-free spectra of a substituted azobenzene dye has been investigated. Although the incident laser wavelength was detuned by ∼500 nm from resonance with the dye absorption maximum, bands due to the dye chromophore still dominated the spectrum even though strongly-scattering groups (methacrylate, sulphone) were also present in the molecule. These results are similar to those obtained by other workers in studies on chromophoric proteins.     

Raman spectra of poly(ethylene glycols) in solution

Raman spectra have been obtained from poly(ethylene glycol) (PEG) in the solid and molten phases and in aqueous and chloroform solutions. Several new lines are observed or resolved in the Raman spectrum of the solid state as a result of using a high-power argon-ion laser as a source. The Raman spectra of the molten polymer and the chloroform solutions are indicative of a disordered structure, since additional Raman lines appear as a result of the additional rotational isomers. The Raman spectrum of the aqueous solution shows that considerably less structural change occurs upon dissolution in water.     

Raman and surface Raman spectroscopy with ultraviolet excitation

We record the accurate and reliable Raman spectra of benzoic acid (BA), p-nitrobenzoic acid (PNBA) and o-nitrobenzoic (ONBA) in aqueous solution with ultraviolet excitation. And we find that the ultraviolet (UV) Raman spectrum of aqueous BA solution has one-to-one correspondence to that of BA solid whereas the others are less resemble to the solid counterparts. We also report surface Raman spectroscopy of them in silver colloid without any enhancement in UV region and call it surface-unenhanced Raman spectroscopy (SUERS) while the surface-enhanced Raman scattering (SERS) effects are perfect in near infrared or visible regions. It demonstrates the SERS effects are strongly dependent on the excitation wavelength. On the basis of the experiments, we discuss the mechanism of SERS excited in different regions.     

Solute-solvent interactions in formamide and zinc chloride solutions: An investigation by Raman spectroscopy

Raman experiments of formamide and zinc chloride solutions in a wide concentration range (0.1-5.0 mol kg⁻¹) have been carried out. The spectral changes were interpreted on three different ways: (i) the rupture of the H-bonds of FA was evidenced by the trend observed in the ν_CO, δ_HNH and restricted (translation or libration) modes; (ii) the appearance of a new band in the ν_CN region (∼1338 cm⁻¹) was assigned to FA coordinated to Zn (II) through nitrogen atom; (iii) the electronic delocalization in the FA structure upon complexation provided the appearance of new features in the δ_CH and ν_CH regions. The quantitative treatment performed at the ν_CN region of FA allowed the determination of an average number of 3 FA molecules per Zn (II) in the first solvation shell. This value is supported by the appearance of features assigned to ZnCl⁺ and ZnCl₂ entities that also occupy vertices of the tetrahedron at higher salt concentrations. The present study may be useful for a better understanding on electrochemical processes employed in the production of dendritic zinc films as well as FA hydrolysis catalyzed by this metal.     

Raman spectroscopy of thiosulphates

The narrow line-width of Fourier transform Raman spectroscopy has been utilized to produce spectra of various simple thiosulphates. The accuracy of the technique has enabled us to show wide frequency shifts and splitting from counter ion to counter ion, and has thrown doubt on the use of vibrational spectra to indicate ion interactions.     

Structural investigation of zeolite-templated, ordered microporous carbon by scanning tunneling microscopy and Raman spectroscopy

Scanning tunneling microscopy (STM) and Raman spectroscopy have been employed for a detailed structural characterization of an ordered microporous carbon synthesized in the nanochannels of zeolite Y by a templating approach. The carbon exhibited an exceptionally high adsorption capacity together with a long-range structural organization on the nanometer scale. As revealed by STM, this material exhibited both terrace-like and periodic (approximately 1.4 nm) stripe-like nanostructures. The vertical separation between contiguous terraces was measured to be also about 1.4 nm and was thus coincident with the structural periodicity deduced by X-ray diffraction. The terraces of the carbon material were shown to consist of arrays of approximately 1 nm wide carbon clusters. The carbon clusters displayed only a limited degree of local order within the terraces but not long-range periodicity. Likewise, STM indicated that the micropore structure of this carbon originated from the large number of voids that separate adjacent clusters, being morphologically very different from that commonly found in activated carbons. The range of void sizes measured by STM (0.8-2.3 nm) was in complete agreement with the pore size distribution determined from nitrogen adsorption measurements. The origin of the nanostructural features observed for this microporous carbon was discussed on the basis of the surface structure of the zeolite Y template. Finally, Raman spectroscopy provided evidence that the carbon clusters were made up of nanographenes with a curved topology.     

Raman spectroscopy of diamondoids

A selection of diamondoid hydrocarbons, from adamantane to [121321] heptamantane, have been analysed by multi-wavelength laser Raman spectroscopy. Spectra were assigned using vibrational frequencies and Raman intensities were calculated by employing the B3LYP functional and the split valence basis set of Schafer, Horn and Ahlrichs with polarisation functions on carbon atoms. The variation of the spectra and associated vibrational modes with the structure and symmetry of the molecules are discussed. Each diamondoid was found to produce a unique Raman spectrum, allowing for easy differentiation between molecules. Using the peak assignments derived from the calculations we find that the low frequency region of the spectra, corresponding to CCC-bending/CC-stretching modes, is particularly characteristic of the geometric shape of the diamondoid molecules.     

Surface-enhanced Raman spectroscopy investigation of the potential-dependent acid-base chemistry of silver-immobilized 2-mercaptobenzoic acid

The acid-base chemistry of 2-mercaptobenzoic acid (2-MBA) immobilized on a polycrystalline silver surface was investigated by surface-enhanced Raman spectroscopy under potential control. The COO(-) bending mode of the benzoate form and the C-COOH stretching mode of the benzoic acid form of 2-MBA were used to determine the relative deprotonated and protonated populations of the bound ligand, respectively. In addition, shifts in the symmetric carboxylate stretching mode of 2-MBA reveal interactions between the benzoate group and the silver surface, interactions which could be displaced by acetate and other buffer anions from solution. It was found that the applied potential has a significant effect on the proton dissociation equilibrium of immobilized 2-MBA. This effect arises from the surface potential governing the activity of protons at the interface, which changes the interfacial pH relative to bulk solution. The results are fit to a Poisson-Boltzmann model, corrected for potential distribution across the monolayer and interactions between adjacent immobilized ligands. The results show a significant increase in the intrinsic pK(a) of the immobilized ligand compared to 2-MBA in free solution, which is likely due to an increase in electron density on the benzoic acid group that occurs upon binding of the thiol group to the silver surface.     

Laser Raman spectra of mono-, oligo- and polysaccharides in solution

We examined the Raman spectra of thirteen sugars—seven monosaccharides, two disaccharides, one trisaccharide and three polysaccharides—in the wavelength range 200—1700 cm⁻¹ and (i) varied the phosphate buffered solution from pH 6.0 to 8.5 at constant ionic strength of 0.1 and (ii) varied HCl solutions from pH 0.8 to 5.0. As is to be expected with molecules containing COH groupings, all the molecular spectra are distinct. Of the thirteen sugars examined, only d-fructose 1,6-diphosphate (FDP) demonstrated spectral changes for the pH range 6.0—8.5 in phosphate buffer; but all exhibited band intensity enhancement in HCl at the lower pHs, but not band wavenumber changes.The results indicate that: (i) changes in the pH of the major intracellular buffer, phosphate, toward acidity, are able to hydrolyze the 1-phosphate group of FDP and the relative concentration of fructose 1-phosphate to fructose 6-phosphate is indicated by the intensity ratio of the 982 and 1080 cm⁻¹ bands; (ii) it appears that all phosphate groups of FDP are hydrolyzed at pH 0.8 in HCl; and (iii) although conditions of extreme acidity are able to hydrolyze other sugars examined, there is no major degradation.     

Quantitative surface-enhanced Raman spectroscopy

The purpose of this tutorial review is to show how surface-enhanced Raman (SERS) and resonance Raman (SERRS) spectroscopy have evolved to the stage where they can be used as a quantitative analytical technique. SER(R)S has enormous potential for a range of applications where high sensitivity needs to be combined with good discrimination between molecular targets, particularly since low cost, compact spectrometers can read the high signal levels that SER(R)S typically provides. These advantages over conventional Raman measurements come at the cost of increased complexity and this review discusses the factors that need to be controlled to generate stable and reproducible SER(R)S calibrations.     

Raman spectroscopy in astrobiology

Raman spectroscopy is proposed as a valuable analytical technique for planetary exploration because it is sensitive to organic and inorganic compounds and able to unambiguously identify key spectral markers in a mixture of biological and geological components; furthermore, sample manipulation is not required and any size of sample can be studied without chemical or mechanical pretreatment. NASA and ESA are considering the adoption of miniaturised Raman spectrometers for inclusion in suites of analytical instrumentation to be placed on robotic landers on Mars in the near future to search for extinct or extant life signals. In this paper we review the advantages and limitations of Raman spectroscopy for the analysis of complex specimens with relevance to the detection of bio- and geomarkers in extremophilic organisms which are considered to be terrestrial analogues of possible extraterrestial life that could have developed on planetary surfaces.     

Filament-driven impulsive Raman spectroscopy

Vibrational Raman spectroscopy is performed in the gas phase using a femtosecond laser pulse undergoing filamentation as an impulsive excitation source. The molecular coherence induced by the filamentary pulse is subsequently probed using a narrowband, sub-picosecond laser pulse to produce Raman spectra of gas phase species in a few tens of milliseconds (~10 laser shots). Pulse shortening with concomitant spectral broadening during filamentation results in a pulse that is both sufficiently short and of sufficient spectral power density to impulsively excite the highest energy ground state vibrations (up to 4158 cm(-1) corresponding to H(2)). Gas phase detection of chloroform, methylene chloride, cyclohexane, toluene, pentane, triethylamine, ammonia, nitromethane, and gasoline is performed.     

Raman spectroscopy on zeolites

The zeolite Raman literature is reviewed, with an emphasis on zeolite structure and synthesis, adsorption and metal complex formation in zeolites     

Non-linear coherent Raman spectroscopy

A general introduction to several coherent Raman methods, which are based on the third-order non-linear susceptibility is given. These methods are described basically under a common point of view, which is the excitation of coherent molecular vibrations in the field of two strong laser beams operating at different frequencies. Most of these methods can be applied successfully also under electronic resonance conditions. As a particular example, studies of coherent anti-Stokes continuum resonance Raman scattering in iodine vapour will be presented and discussed in detail.     

Ionization-detected stimulated Raman spectroscopy

We report a new spectroscopic technique which we call “ionization-detected stimulated Raman spectroscopy” that combines the high sensitivity of resonant laser ionization methods with the high spectral resolution of stimulated Raman spectroscopy. A Q-branch spectrum of NO at 10 mTorr pressure illustrates an improvement in sensitivity of over 1000-fold compared to previous stimulated Raman methods.