Characterization of 11-mercaptoundecanoic and 3-mercaptopropionic acids adsorbed on silver by surface-enhanced Raman scattering

Functionalized n-alkanethiols such as 11-mercaptoundecanoic (MUA) and 3-mercaptopropionic (MPA) acids are likely to adsorb in silver nanoparticles (AgNPs) solely through the thiol group (-SH) or also involving the carboxylate group (−COO⁻) in their structures. The relative tendency is closely related to pH conditions, solvent or the surface potential of the metallic nanoparticles. The SERS effect (Surface Enhancement Raman Scattering) was used for improving the understanding of MUA and MPA group interaction as well as the orientation of these organic compounds adsorbed on AgNPs and the influence of Cu(II) in solution. When analyzing the MPA SERS spectrum, it was verified that the thiol moiety was preferred to adsorb on the AgNPs surface in the thiolate form, presenting both anti and gauche conformations in both acidic and basic media. MUA SERS spectrum however, indicated that solely an anti conformation for the thiol moiety adsorbed on the AgNPs surface in both acidic and basic media. Adding Cu²⁺ ion resulted in coordination to the carboxyl or carboxylate moieties was confirmed by the downshift of the band assigned to OCO stretching. The presence of Cu(II) increased the tendency of gauche conformation for MPA; the coordination of MUA to Cu(II) resulted in a more upright conformation of the carboxylic/carboxylate moieties in both acidic and basic media, respectively.     

Raman scatterings of colloidal silver and gold prepared in the presence of a nonionic surfactant,Surfynol 465

The FT-Raman and resonance Raman scatterings of colloidal silver or gold formed in the presence of a nonionic surfactant, Surfynol 465, were studied. The intensity of Raman scattering of colloidal solution was strongly affected by the power of laser for excitation. At the low power, the intensity of scattering and the spectrum of colloidal solution were normal. However, at the high power, the intensity of Raman scattering anomalously increased over the whole frequency region, suggesting the surface enhanced Raman scattering (SERS) on colloidal silver or gold. And in the Raman spectra new bands were found in addition to bands of starting materials. Through the assignment of new bands, the working mechanism of Surfynol 465 for the formation of colloidal silver or gold was discussed.     

Labelling and binding of poly-(L-lysine) to functionalised gold surfaces. Combined FT-IRRAS and XPS characterisation

We compare herein the interfacial reactivity of self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA), 1-undecanethiol (UDT) and 11-mercaptoundecanol (MUD) on gold surfaces towards aqueous solutions of poly-(L-lysine) (PL). Liquid-phase labelling of PL with the alkyne dicobalt hexacarbonyl cluster 1 combined with analysis of the substrates by Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS) and X-ray photoelectron spectroscopy (XPS) revealed that irreversible binding of PL occurred in all cases. However, the mechanism of binding involved differed markedly from one monolayer to the other. The main mode of interaction of PL to MUA SAM was of electrostatic nature between the terminal carboxylate of MUA and the ammonium groups of PL. For a similar number of bound thiolate molecules, the UDT adsorbed layer was found less continuous than the MUA one, allowing a higher fraction of PL to directly bind to the gold surface. As for MUD, very little thiolate molecules were adsorbed, leaving bare gold surface areas for non specific adsorption of PL.     

Labelling and binding of poly-( -lysine) to functionalised gold surfaces. Combined FT-IRRAS and XPS characterisation

We compare herein the interfacial reactivity of self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA), 1-undecanethiol (UDT) and 11-mercaptoundecanol (MUD) on gold surfaces towards aqueous solutions of poly-( -lysine) (PL). Liquid-phase labelling of PL with the alkyne dicobalt hexacarbonyl cluster 1 combined with analysis of the substrates by Fourier transform infrared reflection–absorption spectroscopy (FT-IRRAS) and X-ray photoelectron spectroscopy (XPS) revealed that irreversible binding of PL occurred in all cases. However, the mechanism of binding involved differed markedly from one monolayer to the other. The main mode of interaction of PL to MUA SAM was of electrostatic nature between the terminal carboxylate of MUA and the ammonium groups of PL. For a similar number of bound thiolate molecules, the UDT adsorbed layer was found less continuous than the MUA one, allowing a higher fraction of PL to directly bind to the gold surface. As for MUD, very little thiolate molecules were adsorbed, leaving bare gold surface areas for non specific adsorption of PL.     

Self-assembled silver nanochains for surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) integrates high levels of sensitivity with spectroscopic precision and has tremendous potential for chemical and biomolecular sensing. The key to the wider application of Raman spectroscopy using roughened metallic surfaces is the development of highly enhancing substrates for analytical purposes. Here, we demonstrate a simple strategy for self-assembling silver nanochains on glass substrates for sensitive SERS substrates. The chain length of short Ag nanochains can be controlled by adjusting the concentration of cetyltrimethylammonium bromide (CTAB) and 11-mercaptoundecanoic acid (MUA). CTAB with appropriate concentration serves as the "glue" that can link the {100} facets of two neighboring Ag nanoparticles. MUA is found to be effective in "freezing up" the aggregation of Ag short chains and preventing them from further aggregating into a long chainlike network structure. The surface plasmon bands can be tuned over an extended wavelength range by controlling the length of the nanochains. The Ag monolayer, mainly composed of four-particle nanochains, exhibited the maximum SERS enhancement factor of around 2.6 x 108, indicating that a stronger SERS enhancement can be obtained in these interstitial sites of chainlike aggregated Ag nanoparticles.     

Vibrational spectra of isomeric pyridinic carboxylic acids in solid state and in solution and their SERS studies in silver sol

Raman spectroscopic studies of three isomeric pyridinic carboxylic acids, viz. picolinic, nicotinic and iso-nicotinic acid in solid state, in aqueous solution and in silver hydrosol, in the frequency range 900–1750 cm⁻¹, have been made. Assignments of the observed bands have been proposed in relation to the molecular forms present in solid state and in solution. Different degrees of intensity enhancements of the Raman bands in surface-enhanced Raman scattering (SERS) have been observed in all the three isomeric molecules. In iso-nicotinic acid, the intensity enhancement has been found to be minimum. Comparisons of Raman spectra in aqueous solution with those due to SERS in silver sol indicate that picolinic and nicotinic acid adsorb perpendicularly to the sol surface whereas in iso-nicotinic acid it occurs via donation of a π-electron of the aromatic ring, i.e. the plane of the ring lies parallel to the surface of the sol.     

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.     

Extending the metal interface generality of surface-enhanced Raman spectroscopy: Underpotential deposited layers of mercury,thallium, and lead on gold electrodes

Surface-enhanced Raman (SER) spectra are reported for several adsorbates at underpotential deposited (upd) layers of mercury, thallium, and lead on an electrochemically roughened gold electrode. For upd mercury monolayers, SER bands were obtained for the surface-halide stretching mode, ν_M-X, of adsorbed chloride and bromide that are of comparable intensities to those observed on the unmodified gold substrate. The ν_M-X peak frequencies are downshifted by 15–24 cm⁻¹ on the former relative to the latter surface, consistent with a smaller extent of halide bond covalency on mercury. These spectral changes induced by upd formation could be reversed by anodic stripping of the mercury monolayer. Comparable results were also obtained by prior formation of the upd monolayer in a separate solution followed by electrode transfer rather than by deposition in the solution of interest. Satisfactory SER spectra at upd mercury are reported additionally for thiocyanate and pyridine, as are similar experiments for upd thallium and lead layers on gold. Of the above adsorbates, only pyridine yielded easily measurable SER spectra for these layers, having 2–3 fold smaller signal intensities than on unmodified gold. The SERS intensity decreases upon thallium, and lead upd formation exhibited both irreversible and reversible components. Differential capacitance-potential plots for upd mercury indicate some similarities to liquid mercury interfaces. The results indicate that overlayers on gold provide a means of extending SERS to metals that in themselves do not exhibit suitable Raman scattering enhancements.     

Characterization of self‐assembled phenyl and benzyl isothiocyanate thin films on Au surfaces

Structures of self‐assembled monolayers (SAMs) prepared from benzyl isothiocyanate (BZIT) and phenyl isothiocyanate (PHIT) were comparatively investigated by means of surface enhanced Raman scattering (SERS) on gold nanoparticle surfaces. Both the molecules are assumed to have upright geometries binding via the sulfur atom, from the SERS spectral features. The broad and split bands at ～2100 cm^?1 could be ascribed to the combination band ν_9a + ν₁₂ of the phenyl ring vibrations through a Fermi‐resonance interaction with the ν₃(NCS) band in the neat‐liquid state. Such broad bands became quite symmetric upon adsorption on Au. UV‐vis absorbance spectroscopy and cyclic voltammetry (CV) were used to compare the kinetics of the formation of SAMs on gold. Our results suggest that BZIT should have a relatively faster self‐assembly than PHIT, as indicated from signal decrease in the CV graph, which suggests a more vertical stance for BZIT in line with Raman measurements. Copyright © 2006 John Wiley & Sons, Ltd.     

Surface-enhanced Raman scattering on molecular self-assembly in nanoparticle-hydrogel composite

Surface-enhanced Raman scattering has been applied to study weak intermolecular interactions between small organic gelling molecules involved in the silver nanoparticle-hydrogel composite formation. Assembly and disassembly of the gelator molecules in close vicinity to embedded silver nanoparticles were followed by changes in Raman intensity of the amide II and carboxyl vibrational bands, whereas the strength of the bands related to benzene modes remained constant. This implied that the gelator molecules were strongly attached to the silver particles through the benzene units, while participating in gel structure organization by intermolecular hydrogen bonding between oxalyl amide and carboxyl groups.     

In situ infrared spectroscopic analysis of the adsorption of aromatic carboxylic acids to TiO2, ZrO2, Al2O3, and Ta2O5 from aqueous solutions

In situ infrared spectroscopy has been used to investigate the adsorption of a range of simple aromatic carboxylic acids from aqueous solution to metal oxides. Thin films of TiO2, ZrO2, Al2O3 and Ta2O5 were prepared by evaporation of aqueous sols on single reflection ZnSe prisms. Benzoic acid adsorbed very strongly to ZrO2, in a bridging bidentate fashion, but showed only weak adsorption to TiO2 and Ta2O5. Substituted aromatic carboxylic acids; salicylic, phthalic and thiosalicylic, were found to adsorb to each metal oxide. Salicylic and phthalic acids adsorbed to the metal oxides via bidentate interactions, involving coordination through both carboxylate and substituent groups. Thiosalicylic acid adsorbed to the metal oxides as a bridging bidentate carboxylate with no coordination through the thiol substituent group.     

The solvent trapping or co-adsorbing effect during the self-assembly monolayer studied by surface-enhanced Raman scattering

The possibility of solvent molecules being trapped within the monolayer interior during the self-assembly of 2,5-dimercapto-1,3,4-thiadiazole (DMTD) molecules from alcoholic solution, or co-adsorbing together with the solute molecules onto the silver surface was verified by means of surface-enhanced Raman scattering (SERS). Variations of the relative intensity of the solvent bands to the concentrations of the solution were investigated. To a certain range of the concentration, the smaller the concentration is, the larger the relative intensity of the solvent bands is. A new method for obtaining the SERS spectra of the organic compounds that are insoluble in water was developed in our laboratory, which also provides further evidence for the solvent trapping or co-adsorbing effect.     

Enhanced and normal Raman scattering from pyridine adsorbed on rough and smooth silver electrodes

A multiplex spectrograph has been used to record potential difference and modulation Raman spectra of pyridine adsorbed on silver electrodes in an electrochemical cell. Spectra have been obtained from rough silver surfaces which give SERS and from surfaces where SERS has been diminished by prolonged cathodic polarisation (DSERS). Raman scattering from pyridine at smooth silver surfaces in potassium perchlorate and fluoride solutions has been distinguished from solution scatter by a potential modulation technique. The results show that the enhanced scattering caused by silver atom or cluster sites is respresentative of the surface as a whole as similar Raman spectra are obtained on smooth surfaces at a count rate as low as ?1.4 photons s^?1 (incident laser power 500 mW).Correlation of simultaneous differential capacitance data and “snapshot” SER spectra indicate that pyridine molecules in aqueous chloride ion solutions adsorb on silver in a flat π-bonded configuration at potentials markedly positive to the point of zero charge and exhibit specific reorientation at ?0.3 V and ?0.45 V (vs. SCE) to become N-bonded, perpendicular to the surface. Results also show that the adsorption behaviour of pyridine in chloride and fluoride ion solutions is largely similar.     

线状和树枝状银纳米结构、形成机理及表面增强拉曼散射性质(英文)

以聚丙烯酰胺(PAM)为模板,在液相中通过不同浓度的抗坏血酸还原硝酸银能够得到缠结的线状和树枝状银纳米结构.该方法合成条件温和(常温常压)、产率高、成本低、操作简单,并且得到了特殊形貌的缠结收光在谱一对起线的状线和状树银枝纳状米银结纳构.米通结过构透的射形电貌子和显性微质镜进(T行E了M)表,扫征描.研电究子表显明微,镜PA(SMEM对)线、拉形曼产光物谱的和形紫成外起?可了见决吸定性作用.在反应初期,大量新生成的银核被PAM链吸附,小颗粒逐渐长大,进而相连,导致生成了缠结的线状银纳米结构.另外,抗坏血酸的浓度越高,越不利于线状结构的生成.利用对巯基苯胺(PATP)为探针分子研究了银纳米结构的表面增强拉曼散射(SERS)活性,结果表明线状银纳米结构具有较强的表面增强拉曼散射效果.     

对巯基苯甲酸的表面增强拉曼光谱

应用表面增强拉曼光谱研究了吸附于粗糙银电极表面的对巯基苯甲酸。对巯基苯甲酸以去质子的形式通过巯基端进行吸附,表面Ag-S键的形成及羧基的结构改变直接影响苯环的电子结构。羧基的振动谱峰均对其质子化较为敏感,其峰强度随pH值的变化表明吸附态对巯基苯甲酸的pKa约为5．9。铜离子可与吸附对巯基苯甲酸形成表面络合物,配位反应与羧基的质子化反应密切相关。     

Interfacial study of benzenesulfinate chemisorbed on silver

The oxidative stability of self-assembled monolayer (SAM) of thiols on silver was examined with matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). A benzenesulfinate (BS) monolayer on silver was also prepared and investigated with MALDI-TOF-MS and surface-enhanced Raman scattering (SERS). The presence of sulfinate, sulfonate, and thiosulfonate fragment ions reveals that thiolate monolayers oxidize in the air. The relative abundance of C6H5SO2- and C6H5SO3- species in the mass spectra of the monolayer with different air-exposure times provides an estimation of the oxidation progress. In the vibrational spectrum, the large red shift of v(SOO-), combined with the lack of a shift of v(C-S) upon adsorption, indicates its bidentate O-coordination. The orientation of the confined molecules, based on the Raman surface selection rules, was derived from the preferential enhancement of the different functional groups. The benzene ring of BS was found almost normal to the metal surface as a result of the intermolecular forces; that geometry excludes the possibility of the pi-system of the benzene ring from participating as another binding site. The SER spectra of BS obtained via a selective etching process confirms these observations.     

Raman spectroscopic studies of terthiophenes for molecular electronics

The effect of thiol and selenol functionalization on the vibrational spectra and photochemical stability of terthiophene based molecular wires was investigated using surface-enhanced Raman scattering (SERS). The molecules were found to exhibit markedly different properties at the silver surface of the SERS substrate, despite having almost identical Raman spectra in solution and in the solid state. In contrast to terthiophene (3T), the bisthiolterthiophene (T3) and biselenol-terthiophene (Se3) molecules were stable against photoinduced structural changes when adsorbed to the metal surface at low concentrations. This indicates that the strong bonds to the silver surface, via S or Se terminal atoms, leads to a rapid decay of photoexcited states. Comparison with ab initio calculations shows that both T3 and Se3 bind with only one of the functional groups to the Ag surface.     

密度泛函理论研究银上吸附对巯基吡啶的SERS化学增强效应

基于密度泛函理论计算和拉曼光谱理论分析,我们研究了对巯基吡啶(4MPY)分子的拉曼光谱和其在银上的表面增强拉曼光谱(SERS),并进一步探讨了SERS与界面吸附结构、异构化、质子化和氢键作用以及低能激发态的关系。首先,我们对两种分子异构体的相对稳定性和拉曼光谱进行了理论分析。在此基础上,进而研究了该分子与不同银簇作用时的拉曼光谱,结果表明,4MPY以巯基硫与银簇作用形成强的Ag―S键,导致拉曼光谱的线型不依赖于所选银簇的大小。接着我们考虑了吡啶氮端作用的两种情况。(1)当4MPY-银簇复合物同时以吡啶氮与水簇或水合质子簇形成氢键时,结果表明吡啶环的部分振动频率随氢键和质子化发生蓝移。(2)当考虑吡啶氮与银簇作用时,吡啶环三角畸变振动发生蓝移。上述情况不仅解释了实验观测的振动频率变化,而且表明了化学环境改变对相对拉曼强度的影响。最后,我们计算了当对巯基吡啶分子以单端或双端与银簇作用,在考虑激发光与低能激发态的能量匹配时,拉曼光谱强度与低能激发态的关系。计算结果表明,在双端吸附构型下,与吡啶氮成键的银簇受激发产生电荷转移态,不仅导致吡啶环v_(12)、v_1和v_(8a)振动的拉曼信号增强,而且选择性地增强吡啶环C―H面内对称弯曲振动v9a的拉曼信号。     

On the occurrence of the central line (1025 cm) in the SERS spectra of pyridine adsorbed on silver hydrosols

The occurrence of a central line at 1025 cm⁻¹ between the surface-enhanced Raman scattering (SERS) bands of pyridine at 1008 and 1036 cm⁻¹ has been first detected in silver hydrosols. This band, which has no counterpart in the Raman spectrum of the free ligand, is observed in acidic aqueous suspension as corresponding to that observed in an electrochemical cell and attributed to adsorption of pyridinium cation. When pyridine is adsorbed on an aged colloid in an alkaline medium two different species are detected. A central band at about the same wave number occurs, attributable to pyridine bound to silver ion cluster on the metal surface, oxidised by ambient air.     

Electrochemical and surface characterization of 4-aminothiophenol adsorption at polycrystalline platinum electrodes

The formation of a self-assembled monolayer (SAM) of 4-aminothiophenol (4-ATP) on polycrystalline platinum electrodes has been characterized by surface analysis and electrochemistry techniques. The 4-ATP monolayer was characterized by cyclic voltammetry (CV), linear sweep voltammetry, Raman spectroscopy, reflection-absorption infrared (RAIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). CV was used to study the dependence of the adsorption time and 4-ATP solution concentration on the relative degree of coverage of 4-ATP monolayers on polycrystalline Pt electrodes. The adsorption time range probed was 24-72 h. The optimal concentration of 4-ATP needed to obtain the highest surface at the lowest adsorption time was 10 mM. RAIR and Raman spectroscopy for 4-ATP-modified platinum electrodes showed the characteristic adsorption bands for 4-ATP, such as nuNH, nuCH(arom), and nuCS(arom), indicating the adsorption on the platinum surface. The XPS spectra for the modified Pt surface presented the binding energy peaks of sulfur and nitrogen. High energy resolution XPS studies, RAIR, and Raman spectrum for platinum electrodes modified with 4-ATP indicate that the molecules are sulfur-bonded to the platinum surface. The formation of a S-Pt bond suggests that ATP adsorption leads to an amino-terminated electrode surface. The thickness of the monolayer was evaluated via angle-resolved XPS (AR-XPS) analyses, giving a value of 8 A. As evidence of the terminal amino group on the electrode surface, the chemical derivatization of the 4-ATP SAM was done with 16-Br hexadecanoic acid. This surface reaction was followed by RAIR spectroscopy.