Surface enhanced Raman scattering of pyrazine adsorbed on silver colloidal particles

The surface enhaced Raman scattering of pyrazine in a silver sol was investigated. Strong enhancement of the g modes and appearance of Raman forbidden vibrations were observed, in agreement with existing results obtained on electrode surface. Chemical reactions of pyrazine on Ag were monitored by Raman and mass spectroscopy.     

Concentration-dependent surface-enhanced Raman scattering of 2-benzoylpyridine adsorbed on colloidal silver particles

Surface-enhanced Raman scattering (SERS) of 2-benzoylpyridine (2-BP) adsorbed on silver hydrosols has been investigated. It has been observed that with a small change in the adsorbate concentration, the SER spectra of 2-BP show significant change in their features, indicating different orientational changes of the different part of the flexible molecule on the colloidal silver surface with adsorbate concentration. The time dependence of the SER spectra of the molecule has been explained in terms of aggregation of colloidal silver particles and co-adsorption and replacement kinetics of the adsorbed solute and solvent molecules on the silver surface. The broad long-wavelength band in the absorption spectra of the silver sol due to solute-induced coagulation of colloidal silver particles is found to be red-shifted with the increase in adsorbate concentration. The surface-enhanced Raman excitation profiles indicate that the resonance of the Raman excitation radiation with the new aggregation band contributes more to the SERS intensity than that with the original sol band.     

Sers excitation profile investigation of a cyanine dye adsorbed on silver colloidal particles

An excitation profile investigation of 3,3′-diethyl-2,2′-benzthiatrimethinecyanine adsorbed on silver sol shows, for the pure surface enhancement, a relative minimum near the dye absorption (about 550 nm) and a strong increase for yellow-red wavelengths, which is explained qualitatively in terms of an “electromagnetic” model.     

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.     

Concentration-dependent surface-enhanced resonance Raman scattering of a porphyrin derivative adsorbed on colloidal silver particles

Surface-enhanced Raman spectra (SERS) of 5,10,15,20-tetrakis(1-decylpyridium-4-yl)-21H,23H-porphintetrabromide or Por 10 (H(2)Tdpyp) adsorbed on silver hydrosols are compared with the FTIR and resonance Raman spectrum (RRS) in the bulk and in solution. Comparative analysis of the RR and the FTIR spectra indicate that the molecule, in its free state, has D(2h) symmetry rather than C(2v). The SERS spectra, obtained on adsorption of this molecule on borohydride-reduced silver sol, indicate the formation of silver porphyrin. With the change in the adsorbate concentration, the SERS shows that the molecule changes its orientation on the colloidal silver surface. The appearance of longer wavelength band in the electronic absorption spectra of the sol has been attributed to the coagulation of colloidal silver particles in the sol. The long wavelength band is found to be red-shifted with the decrease in adsorbate concentration. The excitation profile study indicates that the resonance of the Raman excitation radiation with the original sol band is more important than that with the new aggregation band for the SERS activity. This indicates a large contribution of electromagnetic effect to surface enhancement.     

Surface-enhanced raman spectra and molecular orientation of phthalazine adsorbed on a silver electrode

SERS from phthalazine adsorbed on an Ag electrode was investigated under several conditions of applied voltage and solution concentration. Spectral assignments of the Raman bands were successfully performed and two differently oriented adsorbates, i.e. flat and end-on species, were identified. The contribution of the image field to the SERS intensity was considerable.     

Surface enhanced Raman spectroscopic studies on 1H-1,2,4-triazole adsorbed on silver colloidal nanoparticles

1H-1,2,4-triazole is a very effective corrosion inhibitor for copper. The adsorption of this compound on silver colloidal nanoparticles has been studied by means of surface enhanced Raman scattering (SERS). SERS data are interpreted with the help of DFT calculations of models of the surface complex formed by 1H-1,2,4-triazole on the silver colloidal nanoparticles surface. It was found that this compound is adsorbed on metal surface in its anionic form and that it interacts with silver through the N₁ and N₂ atoms. The molecular plane assumes a tilted orientation with respect to the silver surface.     

Surface-enhanced resonance Raman scattering and density functional calculations of hemicyanine adsorbed on colloidal silver surface

Resonance Raman (RR) and surface-enhanced resonance Raman scattering (SERRS) of 4'-(N,N'-dimethylaminostyryl)-4-propylpyridinium bromide (hemicyanine, HC dye) in acetonitrile solution and on a colloidal silver surface have been investigated. The structure of the dye in the ground (S0) and excited (S1) electronic states was optimized using density functional calculations along with the B3LYP and the configuration interaction with the singlet excitation (CIS) methods, respectively, using the 6-31G basis set. The vibrational frequencies of the molecule were computed at the optimized geometry and compared with the observed Raman bands. A complete normal-mode analysis has been carried out because it is essential for the accurate assignment of the vibrational spectra. From the observed enhancement along various in-plane and out-of-plane vibrations in the SERRS spectrum and from theoretical calculations, it has been inferred that the interaction with the silver surface occurs via the nitrogen lone pair of the pyridyl or the dimethylamino group of the molecule with a tilted orientation. The observed red-shifts in the SERRS spectrum along various vibrations indicate strong interaction (chemisorption) of the HC dye with the silver surface. This is also supported by the presence of a Ag-N stretching vibration at 241 cm(-1). The effect of the dye concentration on the orientation of the molecule is also discussed.     

Molecular structure and orientation of chemisorbed aromatic carboxylic acids: Surface enhanced Raman spectrum of benzoic acid adsorbed on silver sol

The surface enhanced Raman scattering (SERS) of benzoic acid adsorbed on silver sol is presented. A complete assignment of the SER spectrum is given and the nature, the structure and the orientation of the chemisorbed species are discussed. A comparison is made with related results available in literature.     

Molecular orientation of thiourea chemisorbed on copper and silver surfaces

Surface-enhanced Raman spectroscopy has been used to study chemisorbed thiourea complexes on Cu and Ag electrodes. It was found that the thiourea molecule was bonded via its S atom to the Cu and Ag ad-atoms on the substrates.     

pH dependent SERS and solvation studies of tyrosine adsorbed on silver colloidal nano particles combined with DFT calculations

The surface enhanced Raman spectra of tyrosine in colloidal Ag solution have been recorded over a range of pH. A line shape analysis of the bands at 1359, 1505 and 1577 cm⁻¹ was performed between pH 3.5 and 8.5. The variation of spectral linewidth (FWHM) of the band at 1359 cm⁻¹ with pH is explained in terms of two mechanisms in solution: (i) the fluctuation of the pH of a microscopic volume in a solution with an overall uniform pH and/or (ii) the role of changing viscosity and solvation at different pH values due to the intermolecular ionic interactions between different charged states of the tyrosine molecule. The blue shift in three bands with increasing pH has been explained in terms of charge transfer between the different charged states of tyrosine and metal ions upon chemisorption. The experimental spectra are compared with ab initio/DFT calculations of vibrational wavenumbers, bond geometries, binding energy and charge distributions obtained by means of Hartree–Fock (HF) analysis, the nonlocal density functional method (BLYP) and the hybrid functional method (B3LYP). Two basis sets, CEP-31G and lanl2DZ, were used for all calculations.     

Experimental and theoretical surface enhanced Raman scattering study of 2-amino-4-methylbenzothiazole adsorbed on colloidal silver particles

The adsorption of 2-amino-4-methylbenzothiazole (2-AMBT) on colloidal silver particles has been investigated by a surface enhanced Raman scattering (SERS) study. The SERS spectra of the 2-AMBT molecule at varied adsorbate concentrations recorded in different time domains are compared with its Fourier transform infrared (FTIR) spectrum and normal Raman spectrum (NRS) in the bulk and in solution. The experimentally observed SERS spectra are compared with the theoretically modeled surface complexes using ab initio restricted Hatree-Fock (RHF) and density functional theory (DFT) calculations. The most favorable adsorptive sites of the 2-AMBT molecule have been estimated by natural population analysis (NPA) using the above-mentioned high level of theories. The enhancement of the in-plane modes together with the appearance of Ag-N stretching frequency at 215 cm(-1) indicates that the 2-AMBT molecule is adsorbed on the silver surface through the lone pair electrons of both nitrogen atoms with the molecular plane nearly vertical to the surface.     

Determination of the orientation of azathioprine adsorbed on a silver electrode by SERS and ab initio calculations

We report the SERS spectrum of azthioprine (AZA) on a silver electrode surface and the results of normal mode calculations using empirical and ab initio calculations of the 6-mercaptopurione (6-MP) component of AZA. The empirical calculations were done with a Urey-Bradley force field (UBFF) and the ab initio calculations with the STO-3G basis set using the UHF, MP2 and BLYP methods. From the difference between the SERS and solid spectra, we determined that AZA attaches edge-on to the surface through the N3 site on the 6-MP component of the molecule. The UBFF calculation on an Ag adatom-molecule model reproduced most of the main observed frequency shifts in the SERS spectrum. With a similar model, the ab initio calculations yielded frequency shifts in the same direction as the one observed for the in-plane normal modes, but they yielded opposite shifts for the out-of-plane normal modes. This phenomenon may be attributed to a face-on interaction of the 6-MP component with a neighboring adatom made possible by an inclination of the molecule on the surface.     

Molecular orientation of EVA chains adsorbed on chemically controlled surfaces: influence of specific interactions

Molecular orientation of ethylene–vinyl acetate (EVA) copolymer nanofilms adsorbed on chemically controlled surfaces is studied. Four EVA copolymers with different contents of vinyl acetate (VA) were spin‐coated onto gold, COOH and NH₂ functionalized substrates in order to study chain behaviour when adsorbed in a quasi‐two‐dimensional system. Polarization‐modulation infrared reflection–absorption spectroscopy (PM‐IRRAS), a very suitable technique to study thin films, was the key to quantitative calculation of EVA chain orientational angles. Acid–base interactions between carbonyl groups of the chain ramification (vinyl acetate units) and the surface functionalities are evidenced on the basis of infrared spectra. Their incidence on the molecular orientation is also discussed. Our results show a quasi‐parallel orientation of EVA main chains with respect to the surface plane for all adsorption substrates. At the same time, orientation changes of the acetate groups are observed when the EVA copolymer is adsorbed onto functionalized substrates, suggesting that acid–base interactions could influence the orientation of these groups. However, these changes are limited and cannot reorient the main chain axis. Moreover, our results show that increasing VA content in the chain does not lead to more carbonyl functions involved in acid–base interactions with the adsorption surface. This fact also will be discussed. Copyright © 2003 John Wiley & Sons, Ltd.     

Carey Lea's colloidal silver

Summary Carey Leas silver sol is investigated with electron microscopy and analytical methods. Its colloidal behaviour is studied. Coagulation, repeptization and fractionation are described. TheCarey Lea sol is not a protected colloid. It consists of small (80 Å diameter) spherical, rather monodisperse, positively charged silver particles. Citrate ions are adsorbed at the particle surface in superequivalent adsorption.

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Zusammenfassung DasCarey Leasche Silbersol wurde untersucht mit den Methoden der Elektronenmikroskopie und der chemischen Analyse sowie nach seinen kolloidchemischen Eigenschaften. Flockung. Repeptisation und fraktionierte Koagulation werden beschrieben. Es stellt sich heraus, daß das Sol aus feinkörnigen (80 Å Durchmesser), kugelförmigen, ziemlich monodispersen, positiv geladenen Silberteilchen besteht, an denen Zitrationen, in superäquivalenter Adsorption angelagert sind.

     

Surface-enhanced Raman spectrometry of amiloride on colloidal silver

Surface-enhanced Raman spectrometry (SERS) of the diuretic drug amiloride is discussed. The SERS-active substrate used is colloidal silver, which is prepared at room temperature by simple tetrahydroborate reduction of an aqueous solution of silver nitrate. SERS detection of amiloride in human urine is illustrated. The reproducibility of quantitative SERS data is supported by the simultaneous measurement of the scattering signal and transmitted light. Relative standard deviations at the 5% level are compatible with many practical analytical situations.     

Effects of chromophore orientation and molecule conformation on surface-enhanced Raman scattering studied with alkanoic acids and colloidal silver nanoparticles

Experimental studies have been carried out to gain a better understanding of the effects of chromophore orientation and molecular conformation on surface-enhanced Raman scattering (SERS) based on metal nanostructures. A series of alkanoic acids that contain a phenyl ring separated by methylene groups from the carboxylic acid, including phenylacetic acid, 3-phenylpropionic acid, 4-phenylbutyric acid, 5-phenylvaleric acid, and 6-phenylhexanoic acid, was investigated as model molecules with colloidal silver nanoparticles as SERS substrates. As the number of methylene groups increases, the molecules display an interesting zigzag intensity pattern of the phenyl ring bending mode around 1000 cm(-1) as well as a trend of appearance and disappearance of either the degenerate ring breathing mode or C[Double Bond]O vibrational mode near 1585 and 1630 cm(-1), respectively. Molecules containing an odd number of methylene units display a higher ring bending intensity and degenerate ring breathing mode and are suggested to have a trans conformation on the particle surface. Molecules with an even number of methylene units show a C[Double Bond]O vibrational mode and weaker ring bending in their SERS spectra and are suggested to have a gauche conformation on the silver nanoparticle surface. The different conformation is attributed to the varying interactions of the carboxylic group or the phenyl ring pi electrons with the silver surface. The SERS intensity was found to change little as the length between the phenyl ring and the carboxylic group was increased by adding CH(2) spacers. This is possibly because the effective distance between the phenyl ring and the silver surface does not change much with increasing number of CH(2) spacers as a result of changes in molecular conformation and variations in the phenyl ring orientation with CH(2) addition. The insight gained from this study is important for understanding SERS of complex molecules for which chromophore orientation and molecular conformation must be taken into careful consideration.