Neutron reflectivity studies of field driven transformations in a monolayer of 4-pentadecyl pyridine at Au electrode surfaces

What happens to the thiolates created by reductively desorbing SAMs? An in situ study using fluorescence microscopy and electrochemistry

In situ examination of the reductive desorption process for Au microelectrodes modified with a thiol self-assembled monolayer (SAM) using fluorescence microscopy enabled the study of the fate of the desorbed thiolate species. The Bodipy labeled alkyl-thiol SAM, when adsorbed, is not fluorescent due to quenching by the Au surface. Once reductively desorbed, the thiolate molecules fluoresce and their direction and speed are monitored. At moderately negative reduction potentials, the thiolate species hemispherically diffuse away from the microelectrode. Also observed is the influence of a closely positioned counter electrode on the direction of the desorbed thiolate movement. As the potential becomes more negative, the molecules move in an upward direction, with a speed that depends on the amount of dissolved H(2) produced by water reduction. Shown is that this motion is controlled, in large part, by the change in the electrolyte density near the electrode due to dissolved H(2). These results should help in explaining the extent of readsorption at oxidative potentials observed in cyclic voltammetry (CV) reductive desorption measurements, as well as improving the general understanding of the SAM removal process by reductive desorption. The electrogenerated H(2) was also shown to be able to reductively remove the thiol SAM from the Pt/Ir particles that decorate the microelectrode glass sheath.

Modulation of electrochemical hydrogen evolution rate by araliphatic thiol monolayers on gold

Electroreductive desorption of a highly ordered self-assembled monolayer (SAM) formed by the araliphatic thiol (4-(4-(4-pyridyl)phenyl)phenyl)methanethiol leads to a concurrent rapid hydrogen evolution reaction (HER). The desorption process and resulting interfacial structure were investigated by voltammetric techniques, in situ spectroscopic ellipsometry, and in situ vibrational sum-frequency-generation (SFG) spectroscopy. Voltammetric experiments on SAM-modified electrodes exhibit extraordinarily high peak currents, which di er between Au(111) and polycrystalline Au substrates. Association of reductive desorption with HER is shown to be the origin of the observed excess cathodic charges. The studied SAM preserves its two-dimensional order near Au surface throughout a fast voltammetric scan even when the vertex potential is set several hundred millivolt beyond the desorption potential. A model is developed for the explanation of the observed rapid HER involving ordering and pre-orientation of water present in the nanometer-sized reaction volume between desorbed SAM and the Au electrode, by the structurally extremely stable monolayer, leading to the observed catalysis of the HER.

Electrochemical and microscopic characteristics of thiolipid layers as simple models of cell membranes

The thiolipid 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE) has been proposed as a component of a simple model of cell membranes, which can be used for the studies of the interactions with drugs and other biologically important species. Depending on the deposition technique, Langmuir-Blodgett method or self-assembly, the obtained model membranes exhibit differences in the organization and properties, as shown by electrochemical techniques. The surface concentration and area per molecule of DPPTE model membranes were determined using chronocoulometry, which gives more reliable results than the widely used reductive desorption method. The mean surface concentration of self-assembled DPPTE monolayer deposited on gold electrode is equal to 4.52 × 10(-10) mol·cm(-2), which corresponds to the area per molecule of 36.7 Å(2). Moreover, model membranes prepared by means of LB method tend to be less compact than self-assembled DPPTE monolayers. Adsorption/desorption behavior of the DPPTE molecules on Au(111) was also visualized by EC-STM method. At the beginning of the process at negative potentials, the physisorbed molecules formed a flat-lying adlayer. Changing the potential in the positive direction resulted in the formation of Au-S bonds, and as a consequence the upstanding phase with higher packing density was observed. The thickness of such a layer determined by atomic force microscopy method is equal to 2.08 nm and corresponds to that of a monomolecular film.

Interfacial behavior of a hairpin DNA probe immobilized on gold surfaces

Neutron reflection has been used in combination with electrochemical and quartz microbalance techniques to characterize a mixed monolayer of thiolated hairpin ss-DNA and 4-mercaptobutan-1-ol monolayer self-assembled on gold before and after hybridization with a cDNA target. Neutron reflection has revealed the opening of the stem-loop configuration of the probe associated with helix formation. This change in conformation correlates to the modification of the electron-transfer resistance associated with the [Fe(CN)6](3-/4-) redox marker present in solution.

Potential-Modulation Spectroscopy at Solid/Liquid and Liquid/Liquid Interfaces

Potential-modulation spectroelectrochemical methods at solid/liquid and liquid/liquid interfaces are reviewed. After a brief summary of the basic features and advantages of the methods, practical applications of potential-modulation spectroscopy are demonstrated using our recent studies of solid/liquid and liquid/liquid interfaces, including reflection measurements for a redox protein on a modified gold electrode and fluorescence measurements for various dyes at a polarized water/1,2-dichloroethane interface. For both interfaces, the use of linearly polarized incident light enabled an estimation of the molecular orientation. The use of a potential-modulated transmission-absorption measurement for an optically transparent electrode with immobilized metal nanoparticles is also described. The ability of potential-modulated fluorescence spectroscopy to clearly elucidate the charge transfer and adsorption mechanisms at liquid/liquid interfaces is highlighted.

ADSORPTION AND REACTION AT ELECTROCHEMICAL INTERFACES AS PROBED BY SURFACE-ENHANCED RAMAN SPECTROSCOPY

Over the past three decades, surface-enhanced Raman spectroscopy (SERS) has gone through a tortuous pathway to develop into a powerful surface diagnostic technique for in situ investigation of surface adsorption and reactions on electrodes. This review presents the recent progress achieved mainly in our laboratory on the improvement of detection sensitivities as well as spectral, temporal, and spatial resolutions. Various surface roughening procedures for electrodes of different metals coupled with maximum use of a high-sensitivity confocal Raman microscope enable us to obtain good-quality SER spectra on the electrode surfaces made from net Pt, Ni, Co, Fe, Pd, Rh, Ru, and their alloys that were traditionally considered to be non-SERS active. A novel technique called potential-averaged SERS (PASERS) has been developed for the quantitative study of electrochemical sorption. Applications are exemplified on extensively studied areas such as coadsorption, electrocatalysis, corrosion, and fuel cells, and several advantages of in situ electrochemical SERS are demonstrated. Finally, further developments in this field are briefly discussed with emphasis on the emerging methodology.

Layer by layer characterization of 1-octadecanol films on a Au(111) electrode surface. "In situ" polarization modulation infrared reflection absorption spectroscopy and electrochemical studies

Bilayers of 1-octadecanol were transferred from the gas-solution interface onto a Au(111) electrode surface using either the horizontal touch or the Langmuir-Blodgett (LB) technique. The horizontal touch method gives an X-type film in which the molecule in the bottom layer is oriented with the tail toward the metal and the head toward the tail of the molecule in the top layer (head to tail orientation). The LB technique allows one to build not only X but also a Y bilayer, in which the molecules in the two leaflets are oriented tail to tail with the polar heads facing the metal and the solution. The properties of these films were characterized by electrochemistry and polarization modulation infrared reflection absorption spectroscopy (PM IRRAS). The freshly prepared X bilayer is defect free and is characterized by a low capacity of 0.84 microF cm(-2). The bilayer may be desorbed from the electrode surface at E < -0.6 V and readsorbed again at more positive potentials. The potential-driven readsorption gives a Y film with a higher capacity of 2.2 microF cm(-2). The IR studies of a mixed bilayer composed of one hydrogenated leaflet and a second deuterated leaflet showed that 1-octadecanol molecules assume a more vertical orientation in the leaflet facing the solution than in the leaflet in contact with the metal. Upon desorption of the X film, the molecules in the bottom layer flip-flop and the readsorbed film assumes the Y structure. Smaller tilt angles are observed in the X film as opposed to the Y film.

Electric field-driven transformations of a supported model biological membrane--an electrochemical and neutron reflectivity study

A mixed bilayer of cholesterol and dimyristoylphosphatidylcholine has been formed on a gold-coated block of quartz by fusion of small unilamellar vesicles. The formation of this bilayer lipid membrane on a conductive surface allowed us to study the influence of the support's surface charge on the structure and hydration of the bilayer lipid membrane. We have employed electrochemical measurements and the specular reflection of neutrons to measure the thickness and water content in the bilayer lipid membrane as a function of the charge on the support's surface. When the surface charge density is close to zero, the lipid vesicles fuse directly on the surface to form a bilayer with a small number of defects and hence small water content. When the support's surface is negatively charged the film swells and incorporates water. When the charge density is more negative than -8 micro C cm(-2), the bilayer starts to detach from the metal surface. However, it remains in a close proximity to the metal electrode, being suspended on a thin cushion of the electrolyte. The field-driven transformations of the bilayer lead to significant changes in the film thicknesses. At charge densities more negative than -20 micro C cm(-2), the bilayer is approximately 37 A thick and this number is comparable to the thickness determined for hydrated multilayers of dimyristoylphosphatidylcholine from x-ray diffraction experiments. The thickness of the bilayer decreases at smaller charge densities to become equal to approximately 26 A at zero charge. This result indicates that the tilt of the acyl chains with respect to the bilayer normal changes from approximately 35 degrees to 59 degrees by moving from high negative charges (and potentials) to zero charge on the metal.