Surface-enhanced Raman Research Articles

Surface-enhanced resonance raman scattering study of the reduction processes of meso-tetrakis(4- N-methylpyridyl) porphine adsorbed at silver electrode surfaces

The surface-enhanced resonance Raman scattering (SERS+RRS) spectra of meso-tetrakis(4- N-methylpyridyl)porphine (TMPyP) adsorbed at Ag electrode surfaces were measured at various electrode potentials. The potential dependence of the surface spectra were analysed by comparison with the ordinary resonance Raman spectra observed for TMPyP's (both in a diacid and free base forms) and reduction intermediates of TMPyP (i.e., a phlorin monocation, a neutral phlorin and a porphomethene). The SERS+RRS spectra of TMPyP in 1 M HCl indicate that an adsorbate at 0V (vs.Ag/AgCl) exists as a neutral phlorin, and on scanning the electrode potential to −0.2 V, the phlorin is reduced to a porphomethene. On the other hand, the SERS+RRS spectra of TMPyP in an aqueous solution at pH 2.0 show that the adsorbate on the Ag electrode at 0V exists as TMPyP in a free base form, which is reduced to the neutral phlorin at −0.2 V. Thus, the electrochemical processes at the Ag electrode surface are quite different from the processes in aqueous media which have already been studied by using polarography and cyclic voltammetry. The difference clearly indicates that an adsorption effect on the Ag electrode plays an essential role in determining the reduction pathways.

Use of surface-enhanced raman spectroscopy in the study of faradaic electrode processes

Use of surface-enhanced raman spectroscopy in the study of faradaic electrode processes

Normal, resonance enhanced, and surface enhanced raman spectra of ruthenium-pyridyl complexes

Normal Raman (NR) spectra of solid and solution phase 2,2′-bipyridine and solid phase Ru(bpy)(PVP)(ClO 4) 2 are shown. Resonance Raman (RR) spectra of solid and solution phase Ru(bpy)(pyr) 4 (ClO 4) 2 and Ru(pyr) 6 (BF 4) 2 are presented. Surface-enhanced Raman (SER) spectra of 2,2′-bipyridine, Ru(bpy)(pyr) 4(ClO 4) 2, (Ru(pyr) 6(BF 4) 2, and Ru(bpy)(PVP)(ClO 4) 2 at silver electrodes are presented. Above, pyr=pyridine, bpy-2,2′-bipyridine, and PVP=poly(4-vinylpyridine). The effect of elctrode potential on the SER spectra of these compounds is shown by spectra acquired at applied potentials from 0.0 to −1.0 V, and plots of SER intensity vs. electrode potential. Excitation profiles for selected bands of each compound are presented. Comparison of SER spectra of Ru(bpy)(pyr) 4(ClO 4) 2 and polymeric Ru(bpy)(PVP)(ClO 4) 2 suggests no potential effect intrinsic to the SER mechanism. Plots of SER intensity vs. electrode potential show the SER effect is dependent on the individual surface-active species. SER excitation spectra for these compounds show that colored ruthenium complexes exhibit a maximum with respect to excitation wavelength. The SER maxima are shifted toward longer wavelengths in comparison with the known UV-vis solution absorbance maxima.

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 spectroscopy of pentaammineosmium (III)/(II) and pentaammineruthenium (II) containing pyridine, pyrazine or 4,4′-bipyridine ligands at silver electrodes: vibrational assignments

Detailed vibrational assignments are described for surface-enhanced Raman (SER) spectra of pentaammineosmium (III), (II) and pentaammineruthenium (II) containing pyridine (py), pyrazine (pz) and 4,4′-bipyridine (bpy) ligands adsorbed at the silver—aqueous interface. The assignments are based on group symmetry analysis, deuteration effects and by comparison with corresponding normal Raman and i.r. spectra for the solid-phase complexes and surface and bulk Raman spectra for the corresponding free ligands. Most bands present in the normal Raman and/or i.r. spectra also appear in the SER spectra. This results from the high intensity of the SER spectra along with the relaxation of vibrational selection rules for adsorbed molecules. The appearance of intense SER spectra for Os (NH 3) 5py(III)/(II) and Ru II(NH 3) 5py is noteworthy since, unlike free pyridine, these lack an available nitrogen for surface coordination. The results illustrate the virtues of electrochemical SER spectroscopy for examining the detailed vibrational properties of coordination compounds, including those in oxidation states [such as Os(II) here] that are difficult to obtain pure in bulk media.

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.

Applications of surface-enhanced raman scattering (SERS) spectroscopy in biochemistry

These results demonstrate that SERS spectroscopy creates a new and powerful method to study the vibrational behaviour of biomolecules at rather dilute aqueous solutions down to 10−7M.

Surface-enhanced raman scattering and nonlinear optics applied to electrochemistry

Abstract The most recently developed diagnostic technique in metal-electrolyte and metal-gas interfaces adapts spontaneous Raman scattering and nonlinear optical generation, techniques normally applied to bulk media, to surface science investigation. For certain metallic surfaces, an enormous increase exists in the Raman (as much as 106 to 108 times) and nonlinear optical signals resulting from submonolayer coverage of molecular adsorbates at the interface. Spontaneous Raman scattering and nonlinear optical scattering are well developed in both theory and practice for the analysis of molecular structure and concentration in bulk media. Instrumentation to generate and detect these inelastically scattered signals is readily available and is adequate for adaption to surface science. However, the mechanism (or mechanisms) giving rise to such a large enhancement at the interfaces is still being actively researched and remains controversial. Theoretical and experimental investigations related to the underlying ...

Can surface-enhanced raman scattering be caused by waveguide resonances?

The amplitude of the standing wave within each channel of a rough metallic surface consisting of a set of deep narrow grooves is shown to increase dramatically at certain (resonant) frequencies thereby leading to abnormal optical absorption and large field enhancement within the channels. Adsorbed molecules located therein might account for the SERS signals observed with quenched Ag films.

A surface-enhanced raman scattering study of CTAB adsorption on copper

A surface-enhanced raman scattering study of CTAB adsorption on copper

Surface-enhanced resonance raman spectra of meso-tetraphenylporphine and its metal complexes in a layered structure having an Ag/sample/CaF 2-roughened-substrate configuration

Surface-enhanced resonance Raman scattering spectra were measured for meso-tetraphenylporphine (TPP) and its Ag(II), Mg and Zn complexes in a layered structure, having an Ag/sample/CaF 2-roughened-substrate configuration. The structural data of SERS-active species give information about and Ag incorporation and/or a metal-exchange reaction of TPP and MgTPP during the Ag deposition process.

Surface-enhanced raman scattering of polymethine dyes on silver colloidal particles

SERS was observed from adsorbed cationic cyanine and neutral merocyanine dyes, whereas the tested oxonol dye did not show any adsorption on the silver colloids. A comparison between Raman scattering of the first three representatives of the homologous series of one dye and Raman experiments performed on the same dyes in solution confirms the existence of SERS.

Surface-enhanced raman spectroscopy of N-methylpyridinium cation and pyridine Identification of surface species

In this paper are described the results of a series of experiments undertaken to clarify the role of the anion in surface-enhanced Raman scattering (SERS) measurements involving the N-methylpyridinium cation (NMP+) and to identify the actual surface species observed in such experiments. We have studied the effects of varying the anion, pH and electrochemical potential, have performed competition experiments in which both pyridine and NMP+ were present in the electrolyte and have compared the ordinary Raman spectrum of the solid silver-pyridine complex, Ag(py)2NO3, with the various SER spectra obtained. By these experiments, we have demonstrated that different pyridine/NMP+ species are responsible for the observed spectra. In addition, it appears that surface-induced and/or photochemical reactions may produce species not originally present in the sample solution. Consequently, care must be exercised in the assignment of SER bands to sample components without consideration of possible decomposition to other species capable of producing SERS.

Surface-enhanced raman scattering from H2O and D2O at silver electrodes Influence of temperature

Surface-enhanced Raman scattering (SERS) for water and deuterium oxide can be observed only under particular conditions. At first, a strong oxidation-reduction cycle (ORC) is necessary in order to create the type of rough surface required for high effiency in the excitation of SERS. Secondly, a large concentration of halide salt has to be used. Obviously, SERS for water is related to the presence of active sites stabilized by halide ions at the interface. The concept of active sites is confirmed by the fact that SERS for water vanishes irreversibly when these ions are desorbed temporarily from the electrode by applying a sufficiently cathodic potential pulse. A decrease of the temperature should reduce the metastability of these sites and, hence, extend the potential range accessible for SERS from water beyond the halide adsorption region. Indeed, at room temperature water can be studied only at potentials positive of −1.1 V; at low temperature (250 K) SERS for water is detectable up to −1.5 V. Spectra recorded at various electrode potentials reveal the interfacial composition and its change: the stretching vibrations exhibit distinct band shapes and decrease to lower frequencies. Both effects indicate a decrease of the OH oscillator strength caused by enlarged intermolecular interactions. This indicates an increase of the hydrogen bonding of adsorbed molecules with desorption of halide ions and reduction of ionic strength at the interface. In other words, SERS allows monitoring the replacement of solvating water by hydrogen bonded water.

The dependence of the metal—molecule vibrational frequency on the mass of the adsorbate and its relevance to the role of adatoms in surface-enhanced raman scattering

It has been suggested by Lombardi, Shields Knight and Birke that one can distinguish between an adsorbed molecule bonded “directly” to a metal surface as opposed to one bonded to a metal adatom which is itself bound to a metal surface by plotting the adsorbate—adsorbent vibrational frequency for a series of cognate molecules, alternatively against m ads −1/2 and (1/ m ads + 1/ m metal) 1/2. ( m ads and m metal are, respectively the mass of the adsorbate and a metal atom.) If a proportional relationship is obtained in the former case then the molecule is bonded to the bulk metal, if in the latter case then it is bonded to an adatom. We show using two model calculations that that proposal is incorrect. Both proportional and non-proportional dependences may be obtained for either mode of bonding.

Electrochemical processes of meso-tetrakis (4-sulfonatophenyl) porphine at a silver electrode studied by surface-enhanced resonance raman spectroscopy

The potential dependence of surface-enhanced resonance Roman spectra of meso-tetrakis (4-sulfonatophenyl) porphine (TSPP) in 0.05 M H 2SO 4 reveals two electrochemical processes at a silver electrode surface. One which begins around −0.3 V is interpreted as dissociation of aggregated TSPP to monomers. The other which occurs near −0.4 V is ascribed to a partial Ag incorporation of TSPP molecules.

Ethylene on silver: Development of surface enhanced Raman lines with exposure

Adsorption of ethylene on silver films ( T s =120 K) as a function of exposure has been studied by following the surface enhanced Raman (SER) signal of the symmetric scissors mode. Exposures as low as 10 −3 L were easily detected demonstrating the very high sensitivity of SERS for this system.

Surface-enhanced raman scattering from amphiphilic and polymer molecules on silver and gold sols

Surface-enchanced Raman spectra from cetylpyridinium and cetylquinolinium halides at sub-monolayer coverage of silver and gold colloids are reported. SERS is also observed from polyvinylpyrrolidone on these sols, but only when aggregation of the colloidal metal particles has occured.

Efficient gold and silver electrodes for surface enchanced raman spectral studies of electrochemical systems: The behaviour of pyridine and naphthalene adsorbed on roughened gold electrodes

A method is described whereby surface enhanced Raman (SER) active electrode surfaces of gold and silver may be made without recourse to the oxidation—reduction techniques which have been used hitherto. This method involves electroplating at low current density from dilute (< 10 −2 M) solutions of a suitable salt or complex in the absence of supporting electrolyte. Scanning electron microscopy shows the surfaces to consist of small spherical particles of fairly constant diameter packed together on the electrode. The sphere diameters are typically 70 nm (gold) and 180 nm (silver) for electrodes prepared in this manner. These electrodes exhibit intense SER scattering and have advantages over oxidation—reduction roughened electrodes. As examples of their utility some results are presented relating to the gold/pyridine and gold/naphthalene systems and these are discussed in relation to results obtained by other workers in similar systems using different methods, i.e., ellipsometry and differential capacitance variation.

Surface-enhanced raman spectrometry of ruthenium complexes at silver electrodes

Surface-enhanced raman spectrometry of ruthenium complexes at silver electrodes