Izabella Zawisza

Electrochemical and photon polarization modulation infrared reflection absorption spectroscopy study of the electric field driven transformations of a phospholipid bilayer supported at a gold electrode surface.

Electrochemistry and polarization modulation Fourier transform infrared reflection absorption spectroscopy (PM-FTIRRAS) was employed to investigate fusion of small unilamellar vesicles of 1,2dioyl-sn-glycero-3-phosphatidyl choline (DOPC) onto the Au(111) electrode. Electrochemical studies demonstrated that the DOPC vesicles fuse and spread onto the gold electrode surface at small charge densities -8 microC cm(-2)<sigmaM<0 microC cm(-2)(if the static electric field is <2 x 10(8) V/m) to form a bilayer. At sigmaM<-8 microC cm(-2), the film is detached from the electrode surface; however, the film remains in close proximity to the surface. The PM-FTIRRAS experiments demonstrated that the field-driven transformation of the film involves changes in hydration, orientation, and conformation in the polar headgroup region and that changes in the packing and tilt of the acyl chains are consequences of the headgroup rearrangements.

Microelectrochemical Modulation of Micropatterned Cellular Environments

Patterned cell cultures obtained by microcontact printing have been modified in situ by a microelectrochemical technique. It relies on lifting cell-repellent properties of oligo(ethylene glycol)-terminated self-assembled monolayers (SAMs) by Br2, which is produced locally by an ultramicroelectrode of a scanning electrochemical microscope (SECM). After Br2 treatment the SAM shows increased permeability and terminal hydrophobicity as characterized by SECM approach curves and contact angle measurements, respectively. Polarization-modulation Fourier transform infrared reflection-absorption spectroscopic (PM FTIRRAS) studies on macroscopic samples show that the Br2 treatment removes the oligo(ethelyene glycol) part of the monolayer within a second time scale while the alkyl part of the SAM degrades with a much slower rate. The lateral extension of the modification can be limited because heterogeneous electron transfer from the gold support destroys part of the electrogenerated Br2 once the monolayer is locally damaged in a SECM feedback configuration. This effect has been reproduced and analyzed by exposing SAM-modified samples to Br2 in the galvanic cell Au|SAM|5 microM Br2 + 0.1 M Na2SO4||10 microM KBr + 0.1 M Na2SO4|Au followed by an PM FTIRRAS characterization of the changes in the monolayer system.

Modified electrode surfaces for catalytic reduction of carbon dioxide

Ni(II) complexes with substituted tetraazacyclotetradecanes were incorporated into Nafion films or assembled in monolayers on electrodes in order to obtain electrocatalytic systems for the reduction of carbon dioxide. Reproducible and stable loading of the Nafion films with the complexes has been observed. Systematic increase of N-methyl substitution of the catalytic [Ni(cyclam)]2+ complex leads to a shift of the Ni(II)L/Ni(I)L formal potential to more positive values, however, at the same time to a decrease of the catalytic current of CO2 reduction. Monolayer techniques — the Langmuir–Blodgett and self-assembly methods — were found advantageous for the preparation of electrode surfaces active in the catalytic reduction of CO2 compared to Nafion coatings containing the catalyst. Glassy carbon electrode was suitable for the transfer of Langmuir–Blodgett monolayers. Catalysis was observed only when the electrode was touching the monolayer at the air–water interface or was covered in the dipping mode, hence when the catalyst molecules were oriented with their alkyl chains towards the substrate and macrocyclic head-group towards the solution. Most efficient catalysis was found using electrodes coated by the self-assembly procedure with monolayers of the cyclam complex modified with a pyridine side group. The role of the pyridine moieties was to anchor the catalytic monolayer to the electrode surface.

Polarization modulation infrared reflection absorption spectroscopy investigations of thin silica films deposited on gold. 2. Structural analysis of a 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer.

In this paper we report on the structural analysis of bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) using polarization modulation infrared reflection absorption spectroscopy (PM IRRAS). The lipid bilayers were formed on SiO2|Au and Au surfaces using the Langmuir-Blodgett and Langmuir-Schaeffer techniques. As we showed in part 1 (Zawisza, I.; Wittstock, G.; Boukherroub, R.; Szunertis, S. Langmuir 2007, 23, 9303-9309), SiO2 layers of 7 nm thickness, synthesized by plasma-enhanced chemical vapor deposition on 200 nm thick gold covered glass slides, allow PM IRRAS investigations. Only minor changes in the order and structure of the lipid bilayer are observed when deposited on SiO2|Au and Au surfaces. The choline moiety in the leaflet directed toward the SiO2 surface exists in trans conformation and shows a tilt of 28 degrees with the surface normal of the CN bond. On the silica surface in the second leaflet directed toward air and in two layers deposited on the Au surface, trans and gauche isomers of the choline moiety are present and the tilt of the CN bond increases to 55 degrees with respect to the surface normal. The order and molecular orientation in the DMPC bilayers on SiO2 and Au surfaces are not affected by time. The analysis of the phosphate stretching mode on the Au surface shows slight dehydration of this group and reorientation of the phosphate moiety.

Application of Thin Titanium/Titanium Oxide Layers Deposited on Gold for Infrared Reflection Absorption Spectroscopy : Structural Studies of Lipid Bilayers

Ultrathin titanium layers when deposited on the surface of gold can be successfully applied for infrared reflection absorption spectroscopy (IRRAS) investigations. It was shown that the reflectivity, the phase shift, and the mean square electric field of the p- and s-polarized IR radiation in up to 20 nm thick titanium layers covered with a 3-4 nm thick layer of native oxide are comparable to those of the air/gold interface. The surface selection rule is fulfilled. Thus, qualitative and quantitative analysis of 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) bilayers transferred in liquid expanded (LE) and liquid condensed (LC) states can be performed. Differences are found in the hydration state and molecular arrangement of the two investigated bilayers. In the DMPC bilayer in the LE state, the C-N bond in the positively charged choline moiety is inclined by approximately 70 degrees toward the surface of the negatively charged titanium substrate. In the phosphate moiety, the in-plane vector of the O-P-O group makes a small angle of approximately 15 degrees to the surface normal. This open structure of the lipid molecule corresponds to the B crystal structure of the DMPC molecule and provides space for strong hydration of the polar headgroup. In the DMPC bilayer in the LC state, the intermolecular distances are reduced; the C-N bond of the choline group makes a smaller angle to the surface normal, and the in-plane vector of the O-P-O group in the phosphate moiety displays a larger tilt. The degree of hydration is reduced. The arrangement of the polar headgroup region corresponds to the A crystal structure of the DMPC molecule.

Inkjet-printed thiol self-assembled monolayer structures on gold: quality control and microarray electrode fabrication.

Laterally structured, self-assembled monolayers (SAMs) of different thiols (HS-R-X, R = (CH 2) 3-16, X = -CH 3, -COOH, -NH 2) on gold have been prepared by inkjet printing. The printer is a modified, low-cost desktop printer (Epson Stylus Photo R200), the ink is a 1 mM solution of the thiol in ethanol/glycerol (6:1). The quality of inkjet-printed large area SAMs obtained in this study is between that of a layer self-assembled from a thiol solution and that obtained by soft lithography, according to cyclic voltammetry, electrochemical impedance spectroscopy, scanning electrochemical microscopy (SECM), and polarization-modulated Fourier transform infrared reflection-absorption spectroscopy (PM IRRAS). For the first time, simultaneous printing of two different thiols in a single print job as an alternative to sequential printing and backfilling is demonstrated. The smallest structures consisting of conductive disks of 40 microm diameter were analyzed as single spots by SECM and as random array electrodes with different average disk-disk distance. Conductive band electrodes with variable bandwidth (300 microm to 1 cm) are presented, as well as a pH switchable band structure. As compared to stamping, inkjet printing allows for simultaneous multiple thiol printing in a single print job with the resolution limited only by the droplet size and the precision of the translation stage.

Properties of Z and E isomers of azocrown ethers in monolayer assemblies at the air-water interface

Abstract Monomolecular films of amphiphilic derivatives of crown ethers bearing an azo group in the macrocycle were prepared on surfaces of pure water or aqueous solutions of alkali metal cations, and studied using the surface pressure and surface potential measurements. The organization of the azocrown molecules in monolayers at the air–water interface was found to depend on the geometry around the –N N– moiety and on the presence of alkali metal cations in the subphase. Transition from E to Z isomer occurred under influence of UV-irradiation, while switching back from Z to E isomer was induced by addition of appropriate alkali metal cation forming a complex with the azocrown ligand. The Z and E isomers can be recognized by the different values of typical monolayer parameters: compressibility modulus and the mean molecular area in the monolayer. fChanges in the monolayer parameters allow to follow isomerization processes in the monolayer. The ability to transform one isomer of the azocrown ether to the other under influence of UV-irradiation or metal ion addition may be useful for the applications of these ligands as molecular switches.

Voltammetric Recognition of Cis (Z) and Trans (E) Isomers of Azobenzene and Azocrown Ethers

Abstract An electroanalytical approach allowing the recognition of isomers of azo compounds and the determination of their ratio in the solution is introduced. Adsorptive preaccumulation of the title compounds at the electrode surface retains on the electrode the ratio of isomers present in the solution. Voltammetric reduction of the adsorbed species allows recognition of the cis (Z) and trans (E) forms of the azo-crowns because their reduction potentials in alkaline solutions are different. In the region below pH 10 fast isomerization follows the first electron transfer, hence both forms appear to be reduced at the same potential. At pH>10 interactions of the large crown radical anion with alkali metal cations slow down the isomerization reaction at the electrode surface and a separate reduction peak for each isomer is seen on the voltammogram. The adsorptive voltammetric method provides a simple and useful way of monitoring the progress of isomerization reactions in the solution. Comparison of azobenzen...

Electron Transport Through Composite Monolayers

Well-organized thiol monolayers on electrode surfaces are prepared using the Langmuir–Blodgett and self-assembly methods. Planned modification of the molecules building the monolayer allow the electron tunneling efficiency across the monolayer to be controlled. The barrier properties of the monolayers are probed by electrochemical methods. The extent of blocking for all systems under study indicates that contribution of the electroactive molecules that find direct access to the electrode surface can be neglected. These observations permit us to use the monolayers for the determination of the kinetic parameters of Fe(CN)3–6 and IrCl2–6 ion reduction. Such monolayers are employed for the studies of long-range electron transport. We show that insertion of amide bonds in appropriate positions of the alkyl chains of all molecules building the monolayer makes it possible to create a lateral hydrogen-bond network linking the internal amide groups in the monolayer and contributing to the electronic coupling between the redox probe and the electrode. The relation between the location of the amide moiety in the molecule and its importance for the electron tunneling efficiency through the intervening organic medium is discussed.

Electrochemistry of Azocrown Ethers in Langmuir-Blodgett Monolayers

Abstract Monomolecular films of amphiphilic derivatives of crown ethers bearing an azo group in the macrocycle were prepared on surfaces of pure water and transferred onto electrodes using the Langmuir-Blodgett technique. The azocompounds studied were separated into Z and E stereoisomers. Monolayers of both isomers of azocrowns were transferred onto the surfaces of hydrophilic (thin mercury film electrode, TMFE) and hydrophobic (indium-tin oxide, ITO) electrodes. The electrode processes showed more reversible cyclic voltammetry profiles when mercury was used instead of ITO as the electrode substrate. This difference was ascribed to the different orientation of the molecules on the electrode surface.