Exploring Three-Dimensional Nanosystems with Raman Spectroscopy: Methylene Blue Adsorbed on Thiol and Sulfur Monolayers on Gold (original) (raw)
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Surface-Enhanced Raman Spectroscopy of Aromatic Thiols and Disulfides on Gold Electrodes
Langmuir, 1998
Surface-enhanced Raman spectroscopy (SERS) has been used to characterize monolayers formed from benzenethiol (BT), benzenemethanethiol (BMT), p-cyanobenzenemethanethiol (CBMT), diphenyl disulfide (DPDS), and dibenzyl disulfide (DBDS) on roughened gold electrodes. All five species adsorb dissociatively as the corresponding thiolates. The charge transfer and electrostatic interactions between the adsorbates and the surface depend strongly on the applied potential. The aromatic ring in BT is tilted relative to the mean surface plane at all applied potentials. The rings in BMT and CBMT also are tilted, but stepping the potential to positive or negative extremes causes a reduction in surface coverage and permits the BMT and CBMT molecules to lie more flat on the surface. Full coverage can be restored by reimmersing the electrode in thiol solution. Monolayers formed from symmetric disulfides are exactly like those formed from the corresponding thiols. If the gold electrode is immersed for prolonged periods in disulfide solution and not rinsed with fresh ethanol, multilayers form, which can be electrochemically reduced (S-S bond cleavage). The monolayers formed from all five compounds are robust over the potential window for the gold electrode; complete desorption occurs between +800 and +1000 mV vs SCE, coincident with oxidation of the surface, and between-1000 and-1200 mV due to hydrogen generation (reduction of water) at the interface.
1993
Surface enhanced Raman scattering (SERS) and molecular fluorescence are used to probe the structure of a methylene blue monolayer adsorbed onto a sulfur-modified polycrystalline gold electrode. Changes in the wavelength of fluorescence for methylene blue on the gold electrode as compared to dilute solution indicate that the dye molecules' state of aggregation and electronic structure have been modified upon adsorption to the surface. An increase in the molecular fluorescence is observed on the sulfur-modified gold electrodes, and is attributed to the intervention of the sulfur between the methylene blue and the metal surface. To avoid interference from the methylene blue fluorescence, er: situ SERS spectra were obtained with a Fourier transform (FT) spectrometer that utilized an excitation wavelength of 1064 nm. The FT-SERS spectra from methylene blue on sulfur-modified and bare gold surfaces exhibit differences that can be ascribed to the interactions of the dye molecules with the sulfur monolayer.
Langmuir, 2004
A comparative study of charge-transfer processes from/to methyl-terminated and carboxylate-terminated thiolate-covered Au(111) surfaces to/from immobilized methylene blue (MB) molecules is presented. Scanning tunneling microscopy images with molecular resolution reveal the presence of molecular-sized defects, missing rows, and crystalline domains with different tilts that turn the thickness of the alkanethiolate SAM (the spacer) uncertain. The degree of surface heterogeneity at the SAMs increases as the number of C units (n) in the hydrocarbon chain decreases from n) 6. Defective regions act as preferred paths for MB incorporation into the methyl-terminated SAMs, driven by hydrophobic forces. The presence of negativecharged terminal groups at the SAMs reduces the number of molecules that can be incorporated, immobilizing them at the outer plane of the monolayer. Only MB molecules incorporated into the SAMs close to the Au(111) surface (at a distance < 0.5 nm) are electrochemically active. MB molecules trapped in different defects explain the broad shape and humps observed in the voltammogram of the redox couple. The heterogeneous charge-transfer rate constants for MB immobilized into methyl-terminated thiolate SAMs are higher than those estimated for carboxylate-terminated SAMs, suggesting a different orientation of the immobilized molecule in the thiolate environment.
Surface-Enhanced Raman Spectroscopy of Halogenated Aromatic Thiols on Gold Electrodes
Langmuir, 1998
Surface-enhanced Raman spectroscopy (SERS) has been used to characterize monolayers of p-substituted benzenethiols (XBTs) and p-substituted benzenemethanethiols (XBMTs) (substituent) X) F, Cl, Br) on gold electrodes. Detailed vibrational assignments have been made for the Raman and SER spectra of all six compounds. All of these molecules exist on the surface as thiolates, with the aromatic ring tilted relative to the surface normal. Monolayers of the XBTs and XBMTs remain intact on the surface throughout the potential range between the oxidation of the gold surface at ∼ +800 mV vs SCE and the reduction of water at ca.-1000 mV at neutral pH. Monolayers of ClBT and BrBT can be partially reduced electrochemically to form mixed monolayers of the halogenated BT and benzenethiol itself. The reductive elimination of the halide occurs at potentials more positive than are required for reduction of the same molecules in solution. FBT, FBMT, and ClBMT cannot be reduced at the surface, and the BrBMT monolayer is only slightly reduced. The electrochemical reactivities of the XBT and XBMT monolayers are explained in terms of facilitated electron transfer from the metal to the adsorbed thiolate, the properties of the leaving group (halogen), and the electronic consequences of having a methylene spacer group between the sulfur and the aromatic ring. This work shows the feasibility of modifying aromatic self-assembled monolayers in situ to form mixed monolayers. It also provides a framework for designing and fabricating monolayers with prescribed stabilities and electroactivities.
In Situ Raman Spectroscopy of Redox Species Confined in Self-Assembled Molecular Films
The Journal of Physical Chemistry C, 2008
In situ Raman spectroscopy and electrochemical experiments are used to study different methylene blue (MB) species immobilized in molecular films (sulfur, propanethiol, dodecanethiol, mercaptopropionic acid, and mercaptoundecanoic acid monolayers), self-assembled on SERS-active Au surfaces in aqueous electrolyte under potential control. The presence of three different MB species along reduction-oxidation cycles is inferred, irrespective of the nature of the molecular film. Residual Raman intensity in the spectroscopically silent leuco methylene blue (LMB) potential range demonstrate a dissimilar degree of connectivity for electron transfer between MB molecules and substrate depending on the spacer. In addition, the intensity of the Raman signal depends on the monolayer chemistry where hydrophobic and/or electrostatic interactions tend to stabilize the MB species against desorption from the molecular film driven by concentration gradients.
The Journal of Physical Chemistry B, 2000
Molecular resolution scanning tunneling microscopy (STM) imaging and high sensitivity infrared reflection spectroscopy (IRS) measurements have been combined to characterize the structures of mixed self-assembled monolayers (SAMs) of fully conjugated, linear thiolate-terminated molecules and short chain (8-12) n-alkanethiolates on Au{111}. Immersion of preformed, ordered alkanethiolate SAMs into dilute solutions of the conjugated molecules results in two-dimensional matrix isolation of conjugated adsorbates in the host SAM. The post-immersion host SAM matrix shows retained alkanethiolate ordering with the guest molecules inserted both singly into boundaries between SAM structural domains and in bundles at substrate step edges. Inserted molecules of lengths in the ∼15 Å range adopt surface orientations similar to those of alkanethiolate molecules at all compositions, including the pure conjugated SAMs. In contrast, the configuration of an inserted, long conjugated molecule (∼40 Å), varies monotonically with the final SAM composition. When inserted at decreasing fractions into an n-octanethiolate matrix, the average tilt of the long molecular axis decreases and approaches alignment with the host SAM. Combined quantum chemical calculations and IRS data for the short guest-host SAMs support a picture of a dense local environment of the host SAM around the guest molecules, despite their insertion at host defects. These results have important implications for designing electronic devices based on the addressing of individual, fully conjugated molecules self-assembled at gold electrodes.
Journal of the Brazilian Chemical Society, 2004
Tióis de cadeias carbônicas com tamanhos diferentes e contendo grupo terminal COOH foram usados para formar monocamadas auto-organizadas sobre a superfície de eletrodos de ouro. A transferência de elétrons do par Fe(CN) 6 3-/4para o eletrodo foi estudada em diferentes pH usandose as técnicas de voltametria cíclica (CV) e de espectroscopia de impedância eletroquímica (EIS). Mudanças no pH da solução resultaram em variações na carga do grupo terminal das monocamadas auto-organizadas e, conseqüentemente, em alterações na interação eletrostática da SAM com as espécies eletroativas em solução.
Mixed Methyl- and Propyl-Thiolate Monolayers on a Au(111) Surface
Langmuir, 2013
Mixed methyl-and propyl-thiolate self-assembled monolayers (SAMs) are prepared on a Au(111) surface by exposing the gold substrate to methyl-propyl-disulfide vapor at room temperature. Scanning tunneling microscopy imaging of such SAMs reveals a (3 × 4) phase consisting of CH 3-S-Au-S-CH 3 , CH 3-S-Au-S-(CH 2) 2 CH 3 , and CH 3-(CH 2) 2-S-Au-S-(CH 2) 2 CH 3. Partial desorption of methyl-thiolate occurs when samples are thermally annealed to 373 K, leading to the formation of a striped phase consisting of primarily CH 3-(CH 2) 2-S-Au-S-(CH 2) 2 CH 3 .
Langmuir
In situ scanning tunneling microscopy (STM) was employed to study the structure of adlayers of crystal violet (CV) and water-soluble porphyrin formed on a sulfur-modified Au(111) electrode in perchloric acid solution. It was confirmed that a well-defined sulfur adlayer with ( 3 × 3)R30°structure can be prepared on Au(111) under potential control. Highly ordered adlayers of the molecules were found to form on the sulfur-modified Au(111) surface. In situ STM revealed the characteristic shape, packing arrangement, and even internal structure of each molecule. It is demonstrated that the sulfur-modified Au(111) electrode can be used as a substrate suitable for the investigation of the adsorption of organic molecules in solution.
Electrochimica Acta, 2010
Electron transfer (ET) kinetics through n-dodecanethiol (C 12 SH) self-assembled monolayer on gold electrode was studied using cyclic voltammetry (CV), scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). An SECM model for compensating pinhole contribution, was used to measure the ET kinetics of solution-phase probes of ferrocyanide/ferricyanide (Fe(CN) 6 4−/3−) and ferrocenemethanol/ferrociniummethanol (FMC 0/+) through the C 12 SH monolayer yielding standard tunneling rate constant (k 0 ET) of (4 ± 1) × 10 −11 and (3 ± 1) × 10 −10 cm s −1 for Fe(CN) 6 4−/3− and FMC 0/+ respectively. Decay tunneling constants (ˇ) of 0.97 and 0.96Å −1 for saturated alkane thiol chains were obtained using Fe(CN) 6 4− and FMC respectively. Also, it was found that methylene blue (MB) molecules are effectively immobilized on the C 12 SH monolayer and can mediate the ET between the solution-phase probes and underlying gold substrate. SECM-mediated model was used to simultaneously measure the bimolecular ET between the solution-phase probes and the monolayer-immobilized MB molecules, as well as tunneling ET between the monolayer-immobilized MB molecules and the underlying gold electrode, allowing the measurement of k BI = (5 ± 1) × 10 6 and (4 ± 2) × 10 7 cm 3 mol −1 s −1 for the bimolecular ET and k 0 ET/MB = (1 ± 0.3) × 10 −3 and (7 ± 3) × 10 −2 s −1 for the standard tunneling rate constant of ET using Fe(CN) 6 4−/3− and FMC 0/+ probes respectively.