In situ spectroelectrochemical study of acetate formation by CO2 reduction using Bi catalyst in amine-based capture solution.

Carbon capture and utilization (CCU) are technologies sought to reduce the level of CO2 in the atmosphere. Industrial carbon capture is associated with energetic penalty, thus there is an opportunity to research alternatives. In this work, spectroelectrochemistry was used to analyze the electrochemical CO2 reduction (eCO2R) in CO2 saturated monoethanolamine (MEA)-based capture solutions, in a novel CCU process. The in situ Fourier transform infrared (FTIR) spectroscopy experiments show that at the Bi catalyst, the active species involved in the eCO2R is the dissolved CO2 in solution, and not carbamate. In addition, the products of eCO2R were evaluated under flow, using commercial Bi2O3 NP as catalyst. Formate and acetate were detected, with normalized FE for acetate up to 14.5%, a remarkable result, considering the catalyst used. Acetate is formed either in the presence of cetrimonium bromide (CTAB) as surfactant or at higher current density (> -100 mA cm-2) and the results enabled the proposition of a pathway for its production. This work sheds light on the complex reaction environment of a capture medium electrolyte and is thus relevant for an improved understanding of the conversion of CO2 into value-added products and to evaluate the feasibility of a combined CCU approach.

Electrochemical and Infrared Studies of a Model Bilayer of the Outer Membrane of Gram-Negative Bacteria and its Interaction with polymyxin─the Last-Resort Antibiotic.

A model bilayer of the outer membrane (OM) of Gram-negative bacteria, composed of lipid A and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE), was assembled on the ß-Tg modified gold (111) single crystal surface using a combination of Langmuir-Blodgett and Langmuir-Schaefer transfer. Electrochemical and spectroscopic methods were employed to study the properties of the model bilayer and its interaction with polymyxin. The model bilayer is stable on the gold surface in the transmembrane potential region between 0.0 and -0.7 V. The presence of Mg2+ coordinates with the phosphate and carboxylate groups in the leaflet of lipid A and stabilizes the structure of the model bilayer. Polymyxin causes the model bilayer leakage and damage in the transmembrane potential region between 0.2 and -0.4 V. At transmembrane potentials lower than -0.5 V, polymyxin does not affect the membrane integrity. Polymyxin binds to the phosphate and carboxylate groups in lipid A molecules and causes the increase of the tilt angle of acyl chains and the decrease of the tilt of the C═O bond. The results in this paper indicate that the antimicrobial activity of polymyxin depends on the transmembrane potential at the model bilayer and provides useful information for the development of new antibiotics.

Effect of Lipid Composition on the Inhibition Mechanism of Amiloride on Alamethicin Ion Channels in Supported Phospholipid Bilayers.

The inhibition effect of amiloride on alamethicin ion channels was studied in a model zwitterionic floating bilayer lipid membrane (fBLM). The EIS studies indicated that amiloride prevents the transport of ions through the alamethicin channels leading to an overall increase in membrane resistance. The PM-IRRAS data demonstrated that amiloride has no influence on the secondary structure of alamethicin but restricts the insertion of the peptides into the bilayer and blocks ion transport through preformed alamethicin channels. The effect of amiloride on ion channel formation in the floating bilayer formed by a zwitterionic lipid was compared to those of previous studies involving negatively charged fBLMs and tethered zwitterionic lipid bilayers. The findings from these studies show that the effects of amiloride on ion channel formation strongly depend on the mobility and charge of the membrane lipids.

Ion-Pairing Mechanism for the Valinomycin-Mediated Transport of Potassium Ions across Phospholipid Bilayers.

The role of the anion on the ionophore properties of valinomycin was studied in a model floating bilayer lipid membrane (fBLM) using supporting electrolytes containing K+ with four different counter anion species (ClO4-, H2PO4-, Cl-, and F-). The electrochemical impedance spectra indicate that the membrane resistance of the bilayer decreases with the decrease of Gibbs free energy of anion solvation. The IR spectra demonstrate that valinomycin does not readily bind to K+ in the KH2PO4, KCl, and KF electrolyte solutions, but in the presence of KClO4, valinomycin readily binds to K+, forming a valinomycin-K+ complex. The results in the present paper reveal the role of the counter anion on the transport of cations by valinomycin across the lipid bilayer. The valinomycin-cation complex creates an ion pair with the anion, and this ion pair can enter the hydrophobic region of the bilayer transporting the cation across the membrane. Anions with low solvation energies facilitate the formation of the ion pair improving the ion conductivity of valinomycin-incorporated bilayers. This paper sheds new light on the transport mechanism of valinomycin ionophores and provides new information about the bioactivity of this molecule.

Molecular recognition between guanine and cytosine-functionalized nucleolipid hybrid bilayers supported on gold (111) electrodes.

A hybrid bilayer lipid membrane (hBLM), constructed with a 1-hexadecanethiol self-assembled interior leaflet and a 1,2-dipalmitoyl-sn-glycero-3-cytidine nucleolipid exterior leaflet, was deposited at the surface of a gold (111) electrode. This system was used to investigate the molecular recognition reaction between the cytosine moieties of the lipid head group with guanine molecules in the bulk electrolyte solution. Electrochemical measurements and photon polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) were employed to characterize the system and determine the extent of the molecular recognition reaction. The capacitance of the hBLM-covered gold electrode was very low (~1 µF cm-2), therefore the charge density at the gold surface was small. Changing the electrode potential had a minimal effect on the complexation between the cytosine moieties and guanine molecules due to small changes in the static electric field across the membrane. This behavior favored the formation of the guanine-cytosine complex.

Alzheimer's disease-related amyloid ß peptide causes structural disordering of lipids and changes the electric properties of a floating bilayer lipid membrane.

Neurodegeneration in Alzheimer's disease is associated with disruption of the neuronal cell membrane by the amyloid ß (Aß) peptide. However, the disruption mechanism and the resulting changes in membrane properties remain to be elucidated. To address this issue, herein the interaction of amyloid ß monomers (AßMs) and amyloid ß oligomers (AßOs) with a floating bilayer lipid membrane (fBLM) was studied using electrochemical and IR spectroscopy techniques. IR measurements showed that both Aß forms interacted similarly with the hydrophobic membrane core (lipid acyl chains), causing conformational and orientational changes of the lipid acyl chains, thus decreasing acyl chain mobility and altering the lipid packing unit cell. In the presence of AßOs, these changes were more significant than those in the presence of AßMs. However, respective interactions of AßMs and AßOs with the membrane hydrophilic exterior (lipid heads) were quite different. AßMs dehydrated lipid heads without affecting their orientation while AßOs changed the orientation of lipid heads keeping their hydration level intact. Electrochemical measurements showed that only AßOs porated the fBLM, thus significantly changing the fBLM electrical properties. The present results provide new molecular-level insight into the mechanism of membrane destruction by AßOs and changes in the membrane properties.

Water Structure in the Submembrane Region of a Floating Lipid Bilayer: The Effect of an Ion Channel Formation and the Channel Blocker.

The structure of water in the submembrane region of the bilayer of DPhPC floating (fBLM) on a monolayer of 1-thio-ß-d-glucose (ß-Tg)-modified gold nanoparticle film was studied by the surface-enhanced infrared absorption spectroscopy (SEIRAS). SEIRAS employs surface enhancement of the mean square electric field of the photon, which is acting on a few molecular layers above the film of gold nanoparticles. Therefore, it is uniquely suited to probe water molecules in the submembrane region and provides unique information concerning the structure of the hydrogen bond network of water surrounding the lipid bilayer. The IR spectra indicated that water with a strong hydrogen network is separating the membrane from the gold surface. This water is more ordered than the water in the bulk. When alamethicin, a peptide forming ion channels, is inserted into the membrane, the network is only slightly loosened. The addition of amiloride, an ion channel blocker, results in a significant decrease in the amount of water in the submembrane region. The remaining water has a significantly distorted hydrogen bond network. This study provides unique information about the effect of the ion channel on water transport across the bilayer. The electrode potential has a relatively small effect on water structure in the submembrane region. However, the IR studies demonstrated that water is less ordered at positive transmembrane potentials. The present results provide significant insight into the nature of hydration of a floating lipid bilayer on the gold electrode surface.

How Valinomycin Ionophores Enter and Transport K+ across Model Lipid Bilayer Membranes.

Valinomycin, a cyclic peptide, was incorporated into a biomimetic lipid membrane tethered to the surface of a gold (111) electrode. Electrochemical impedance spectroscopy was used to study the ionophore properties of the peptide, and polarization modulation infrared reflection absorption spectroscopy was employed to determine the conformation and orientation of valinomycin in the membrane. The combination of these two techniques provided unique information about the ionophore mechanism where valinomycin transports ions across the membrane by creating a complex with potassium ions and forming an ion pair with a counter anion. The ion pair resides within the hydrophobic fragment of the membrane and adopts a small angle of ∼22° with respect to the surface normal. This novel study provides new insights explaining the valinomycin ion transport mechanism in model biological membranes.

Electric-Field-Driven Molecular Recognition Reactions of Guanine with 1,2-Dipalmitoyl-sn-glycero-3-cytidine Monolayers Deposited on Gold Electrodes.

Monolayers of 1,2-dipalmitoyl-sn-glycero-3-cytidine were incubated with guanine in a 0.1 M NaF electrolyte at the surface of a Langmuir trough and transferred to gold (111) electrodes using the Langmuir-Schaefer technique. Chronocoulometry and photon polarization modulation infrared reflection absorption spectroscopy were employed to investigate the influence of the static electric field on the orientation and conformation of the cytidine nucleolipid molecules on the metal surface in the presence of guanine and to monitor the molecular recognition of guanine with the cytosine moiety. When the monolayer is exposed to guanine solutions, the cytosine moiety binds to the guanine residue in either a Watson-Crick complex at positively charged electrode surfaces or a noncomplexed state at negative surface charges. The positive electrostatic field causes the cytosine moiety and the cytosine-guanine complex to adopt a nearly parallel orientation with respect to the plane of the monolayer with a measured tilt angle of ∼10°. The parallel orientation is stabilized by the interactions between the permanent dipole of the cytosine moiety or the Watson-Crick complex and the static electric field. At negative charge densities, the tilt of the cytosine moiety increases by ∼15-20°, destabilizing the complex. Our results demonstrate that the static electric field has an influence on the molecular recognition reactions between nucleoside base pairs at the metal-solution interface and can be controlled by altering the surface charge at the metal.

In Situ Electrochemical and PM-IRRAS Studies of Colicin E1 Ion Channels in the Floating Bilayer Lipid Membrane.

Colicin E1 is a channel-forming bacteriocin produced by certain Escherichia coli cells in an effort to reduce competition from other bacterial strains. The colicin E1 channel domain was incorporated into a 1,2-diphytanoyl- sn-glycero-3-phosphocholine floating bilayer situated on a 1-thio-?-d-glucose-modified gold (111) surface. The electrochemical properties of the colicin E1 channel in the floating bilayer were measured by electrochemical impedance spectroscopy; the configuration and orientation of colicin E1 in the bilayer were determined by polarization-modulation-infrared-reflection absorption spectroscopy. The EIS and IR results indicate that colicin E1 adopts a closed-channel state at the positive transmembrane potential, leading to high membrane resistance and a large tilt angle of ?-helices. When the transmembrane potential becomes negative, colicin E1 begins to insert into the lipid bilayer, corresponding to low membrane resistance and a low tilt angle of ?-helices. The insertion of colicin E1 into the lipid bilayer is driven by the negative transmembrane potential, and the ion-channel open and closed states are potential reversible. The data in this report provide new insights into the voltage-gated mechanism of colicin E1 ion channels in phospholipid bilayers and illustrate that the floating bilayer lipid membrane at the metal electrode surface is a robust platform to study membrane-active proteins and peptides in a quasi-natural environment.

Effects of Amiloride, an Ion Channel Blocker, on Alamethicin Pore Formation in Negatively Charged, Gold-Supported, Phospholipid Bilayers: A Molecular View.

The effects of amiloride on the structure and conductivity of alamethicin ion pore formation within negatively charged, gold-supported, 1,2-dimyristoyl- sn-glycero-3-phosphocholine/Egg-PG membranes were investigated with the help of electrochemical impedance spectroscopy (EIS), photon polarization modulation-infrared reflection spectroscopy (PM-IRRAS), and atomic force microscopy (AFM). The EIS results indicate that ion conductivity across negatively charged phospholipid bilayers containing alamethicin decreases by an order of magnitude when amiloride is introduced to the system. Despite the reduction in ion conductivity, the PM-IRRAS data shows that amiloride does not inhibit ion channel formation by alamethicin peptides. High-resolution AFM images revealed that amiloride enlarges and distorts the shape of alamethicin ion pores when introduced to the system, indicating that it is inserting itself into the mouth of the alamethicin pores. This effect is driven by electrostatic interactions between positively charged amiloride molecules and the negative charge on the membrane.

Spectroelectrochemical Characterization of 1,2-Dipalmitoyl- sn-glycero-3-cytidine Diphosphate Nucleolipid Monolayer Supported on Gold (111) Electrode.

The effect of the electrode potential on the orientation and conformation of the 1,2-dipalmitoyl- sn-glycero-3-cytidine monolayer deposited on a gold (111) electrode surface was described. The potential of zero free charge ( Epzc) for the monolayer-covered electrode was determined to be -0.2 V vs SCE. The differential capacitance and charge density data indicated that the monolayer is stable at the electrode surface when ( E - Epzc) > 0.0 V. At negative rational potentials, a progressive detachment (electrodewetting) of the monolayer occurs. The monolayer is fully detached from the electrode surface at ( E - Epzc) < -0.6 V. The conformation and orientation of the acyl chains and the orientation of the cytosine moiety were determined with the help of photon polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The IR measurements demonstrate that the acyl chains are predominantly in the gel phase in the adsorbed state and tilted at an angle of ∼30° with respect to the electrode surface normal. The tilt angle of the acyl chains increases when the film is detached from the gold surface, indicating that the monolayer becomes more disordered. At ( E - Epzc) > 0.0 V, the plane of the cytosine moiety assumes a small angle of ∼20° with respect to the surface. At negative potentials, the tilt angle of the cytosine fragment increases and rotates. With the help of DFT calculations, these changes were explained by the repulsion of the positive pole of the cytosine permanent dipole moment by the positively charged gold surface and its attraction to the metal surface at negative electrode potentials. This work provides unique information for the future development of sensors based on the molecular recognition of nucleoside targets.

Pore Forming Properties of Alamethicin in Negatively Charged Floating Bilayer Lipid Membranes Supported on Gold Electrodes.

Electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM), and photon polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) were employed to investigate the formation of alamethicin pores in negatively charged bilayers composed of a mixture of 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) and egg-PG floating at gold (111) electrode surfaces modified by self-assembled monolayers of 1-thio-ß-d-glucose (ß-Tg). The EIS data showed that the presence of alamethicin decreases the membrane resistivity by about 1 order of magnitude. PM-IRRAS measurements provided information about the tilt angles of peptide helical axis with respect to the bilayer normal. The small tilt angles obtained for the peptide helical axis prove that the alamethicin molecules were inserted into the DMPC/egg-PG membranes. The tilt angles decreased when negative potentials were applied, which correlates with the observed decrease in membrane resistivity, indicating that ion pore formation is assisted by the transmembrane potential. Molecular resolution AFM images provided visual evidence that alamethicin molecules aggregate forming hexagonal porous 2D lattices with periodicities of 2.0 ± 0.2 nm. The pore formation by alamethicin in the negatively charged membrane was compared with the interaction of this peptide with a bilayer formed by zwitterionic lipids. The comparison of these results showed that alamethicin preferentially forms ion translocating pores in negatively charged phospholipid membranes.

Role of Transmembrane Potential and Defects on the Permeabilization of Lipid Bilayers by Alamethicin, an Ion-Channel-Forming Peptide.

The insertion and ion-conducting channel properties of alamethicin reconstituted into a 1,2-di- O-phytanyl- sn-glycero-3-phosphocholine bilayer floating on the surface of a gold (111) electrode modified with a 1-thio-ß-d-glucose (ß-Tg) self-assembled monolayer were investigated using a combination of electrochemical impedance spectroscopy (EIS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The hydrophilic ß-Tg monolayer separated the bilayer from the gold substrate and created a water-rich spacer region, which better represents natural cell membranes. The EIS measurements acquired information about the membrane resistivity (a measure of membrane porosity), and the PM-IRRAS experiments provided insight into the conformation and orientation of the membrane constituents as a function of the transmembrane potential. The results showed that the presence of alamethicin had a small effect on the conformation and orientation of phospholipid molecules within the bilayer for all studied potentials. In contrast, the alamethicin peptides assumed a surface state, where the helical axes adopted a large tilt angle with respect to the surface normal, at small transmembrane potentials, and inserted into the bilayer at sufficiently negative transmembrane potentials forming pores, which behaved as barrel-stave ion channels for ionic transport across the membrane. The results indicated that insertion of alamethincin peptides into the bilayer was driven by the dipole-field interactions and that the transitions between the inserted and surface states were electrochemically reversible. Additionally, the EIS measurements performed on phospholipid bilayers without alamethicin also showed that the application of negative transmembrane potentials introduces defects into the bilayer. The membrane resistances measured in both the absence and presence of alamethicin show similar dependencies on the electrode potential, suggesting that the insertion of the peptide may also be assisted by the electroporation of the membrane. The findings in this study provide new insights into the mechanism of alamethicin insertion into phospholipid bilayers.

Physicochemical Studies on Orientation and Conformation of a New Bacteriocin BacSp222 in a Planar Phospholipid Bilayer.

The behavior, secondary structure, and orientation of a recently discovered bacteriocin-like peptide BacSp222 in a lipid model system supported at a gold electrode was investigated by chronocoulometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and attenuated total reflectance infrared (ATR-IR) spectroscopy. The IR spectra show that the secondary structure of BacSp222 is predominantly α-helical. Analysis of the spectra in the amide I region shows that the α-helical fragment of the peptide is inserted into bilayer at the potential range at which the bilayer is stable and attached to the Au(111) surface, i.e., from -0.5 to 0.3 V vs Ag/AgCl. Insertion of BacSp222 to the membrane significantly changes the conformation of the acyl chains of lipid molecules, from all-trans to partially melted; however, the chains become less tilted. Based on these results, we propose that BacSp222 interacts with the DMPC bilayer through the barrel-stave pore formation. In this model, α-helix of BacSp222 inserts into the membrane with an angle between the α-helix axis and membrane normal equal to ∼18°. The changes in orientation of the α-helical fragment of the peptide indicate that the orientation of BacSp222 with respect to the bilayer surface is potential-dependent. The peptide is inserted into the membrane driven by the electrostatic field generated by negative charge at the metal surface. It is not inserted at negative potentials where the membrane is detached from the metal and no longer exposed to the electrostatic field of the metal.

Quantitative Subtractively Normalized Interfacial Fourier Transform Infrared Reflection Spectroscopy Study of the Adsorption of Adenine on Au(111) Electrodes.

Quantitative subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) was used to determine the molecular orientation and identify the metal-molecular interactions responsible for the adsorption of adenine from the bulk electrolyte solution onto the surface of the Au(111) electrode. The recorded p-polarized IR spectra of the adsorbed species were subtracted from the collected s-polarized IR spectra to remove the IR contributions of the vibrational bands of the desorbed molecules that are located within the thin layer cavity of the spectroelectrochemical cell. The intense IR band around 1640 cm(-1), which is assigned to the pyrimidine ring stretching vibrations of the C5-C6 and C6-N10 bonds, and the IR band at 1380 cm(-1), which results from a combination of the ring stretching vibration of the C5-C7 bond and the in-plane CH bending vibration, were selected for the quantitative analysis measurements. The transition dipoles of these bands were evaluated by DFT calculations. Their orientations differed by 85 ± 5°. The tilt angles of adsorbed adenine molecules were calculated from the intensity of these two vibrations at different potentials. The results indicate that the molecular plane is tilted at an angle of 40° with respect to the surface normal of the electrode and rotates by 16° around its normal axis with increasing electrode potential. This orientation results from the chemical interaction between the N10 and gold atoms coupled with the π-π parallel stacking interactions between the adjacent adsorbed molecules. Furthermore, the changes in the molecular plane rotation with the electric field suggests that the N1 atom of adenine must also participate in the interaction between the molecule and metal.

PM-IRRAS Studies of DMPC Bilayers Supported on Au(111) Electrodes Modified with Hydrophilic Monolayers of Thioglucose.

A phospholipid bilayer composed of 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (d54-DMPC) was deposited onto the Au(111) electrode modified with a self-assembled monolayer of 1-thio-ß-d-glucose (ß-Tg) via the Langmuir-Blodgett and Langmuir-Schaefer (LB-LS) techniques. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to characterize structural and orientational changes in this model biological membrane on a hydrophilic surface modified gold electrode. The results of the spectroscopic measurements showed that the tilt angle of acyl chains obtained for deuterated DMPC bilayers supported on the ß-Tg-modified gold is significantly lower than that reported previously for DMPC bilayers deposited directly on Au(111) electrodes. Moreover, tilt angles of ∼18° were obtained for d54-DMPC bilayers on ß-Tg self-assembled monolayers (SAMs) at positive potentials, which are similar to the values calculated for h-DMPC deposited on bare gold in the desorbed state and to those observed for a stack of hydrated DMPC bilayers. This data confirms that the ß-thioglucose SAM promotes the formation of a water cushion that separates the phospholipid bilayer from the metal surface. As a result, the DMPC polar heads are not in direct contact with the electrode and can adopt a zigzag configuration, which strengthens the chain-chain interactions and allows for an overall decrease in the tilt of the acyl chains. These novel supported model membranes may be especially useful in studies pertaining to the incorporation of peptides and proteins into phospholipid bilayers.

Attempts to prepare tethered bilayer lipid membranes using synthetic thioglycolipid anchors: synthesis of 6″-thiotrisaccharide glycolipid analogues and applications.

The synthesis of the three 6″-deoxy-6″-thio glycolipid analogues ß-d-Gal-(1→6)-ß-d-Gal-(1→4)-ß-d-Glu-(1→OCH2)-[1,2,3]-triazole-1-dodecane, ß-d-Gal-(1→4)-ß-d-Glu-(1→4)-ß-d-Glu-(1→OCH2)-[1,2,3]-triazole-1-dodecane and ß-d-Gal-(1→4)-ß-d-Glu-(1→4)-ß-d-Glu-(1→OCH2)-[1,2,3]-triazole-1-octadecane is presented. Glycosylation at position O-4' of a propargyl cellobioside glycosyl acceptor and position O-6' of a propargyl lactoside glycosyl acceptor with a 6-deoxy-6-thio galactosyl donor gave rise to two unique trisaccharides that in turn underwent copper-catalyzed azide-alkyne cycloadditions with either 1-azidododecane or 1-azidooctadecane. The potential for each of these analogues to function as tethers of lipid bilayers to Au(111) surface was assessed by differential capacitance experiments. A monolayer of the previously described monosaccharide 1-octadecane-4-(6-thio-ß-d-galacto-pyranosyloxymethyl)-[1,2,3]-triazole either self-assembled or prepared by Langmuir-Blodgett (LB) transfer was found to support an outer leaflet monolayer (DMPC/cholesterol, 70:30) deposited by Langmuir-Schaefer (LS) touch. The bilayers obtained with this monosaccharide analogue had minimum differential capacitances of 1.0 and 0.9µF/cm(2) when the inner monolayer was prepared by self-assembly and LS touch, respectively. Attempts to produce bilayers using the trisaccharides synthesized here were unsuccessful; we are attributing these unsuccessful results mostly to the high water solubility of trisaccharides combined with the relatively short length of the hydrocarbon chains used in this study.

Electrochemical and STM studies of 1-thio-ß-D-glucose self-assembled on a Au(111) electrode surface.

In this study, a Au(111) electrode is functionalized with a monolayer of 1-thio-ß-D-glucose (ß-Tg), producing a hydrophilic surface. A monolayer of ß-Tg was formed on a Au(111) surface by either (1) potential-assisted deposition with the thiol in a supporting electrolyte or (2) passive incubation of a gold substrate in a thiol-containing solution. For each method, the properties of the ß-Tg monolayer were investigated using cyclic voltammetry (CV), differential capacitance (DC), and chronocoulometry. In addition, electrochemical scanning tunneling microscopy (EC-STM) was used to obtain images of the self-assembled monolayer with molecular resolution. Potential-assisted assembly of ß-Tg onto a Au(111) electrode surface was found to be complicated by oxidation of ß-Tg molecules. The EC-STM images revealed formation of a passive layer containing honeycomb-like domains characteristic of a formation of S(8) rings, indicating the S-C bond may have been cleaved. In contrast, passive self-assembly of thioglucose from a methanol solution was found to produce a stable, disordered monolayer of ß-Tg. Since the passive assembly method was not complicated by the presence of a faradaic process, it is the method of choice for modifying the gold surface with a hydrophilic monolayer.

Quantitative SNIFTIRS studies of (bi)sulfate adsorption at the Pt(111) electrode surface.

Subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) was applied to study (bi)sulfate adsorption on a Pt(111) surface in solutions of variable pH while maintaining a constant total bisulfate/sulfate ((bi)sulfate) concentration without the addition of an inert supporting electrolyte. The spectra were recorded for both the p- and s-polarizations of the IR radiation in order to differentiate between the IR bands of the (bi)sulfate species adsorbed on the electrode surface from those species located in the thin layer of electrolyte. The spectra recorded with p-polarized light consist of the IR bands from both the species adsorbed at the electrode surface and those present in the thin layer of electrolyte between the electrode surface and ZnSe window whereas the s-polarized spectra contain only the IR bands from the species located in the thin layer of electrolyte. A new procedure was developed to calculate the angle of incidence and thickness of the electrolyte between the Pt(111) electrode surface and the ZnSe IR transparent window. By combining these values with the knowledge of the optical constants for Pt, H(2)O and ZnSe, the mean square electric field strength (MSEFS) at the Pt(111) electrode surface and for thin layer of solution were accurately calculated. The spectra recorded using s-polarization were multiplied by the ratio of the average MSEFS for p- and s-polarizations and subtracted from the spectra recorded using p-polarization in order to remove the IR bands that arise from the species present within the thin layer cavity. In this manner, the resulting IR spectra contain only the IR bands for the anions adsorbed on the Pt(111) electrode surface. The spectra of adsorbed anions show little change with respect to the pH ranging from 1 to 5.6. This behavior indicates that the same species is predominantly adsorbed on the metal surface for this broad range of pH values and the results suggest that sulfate is the most likely candidate for this adsorbate.