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Knowledge of the structure of the electrical double layer in ionic liquids (IL) is crucial for their applications in electrochemical technologies. We report the synthesis and applicability of an imidazolium-based amphiphilic ionic liquid with a perdeuterated alkyl chain for studies of electric potential-dependent rearrangements, and changes in the microenvironment in a monolayer on a Au(111) surface. Electrochemical measurements show two states of the organization of ions on the electrode surface. In situ IR spectroscopy shows that the alkyl chains in imidazolium cations change their orientation depending on the adsorption state. The methylene-d2 stretching modes in the perdeuterated IL display a reversible, potential-dependent appearance of a new band. The presence of this mode also depends on the anion in the IL. Supported by quantum chemical calculations, this new mode is assigned to a second νas (CD2 ) band in alkyl-chain fragments embedded in a polar environment of the anions/solvent present in the vicinity of the imidazolium cation and electrode. It is a measure of the potential-dependent segregation between polar and nonpolar environments in the layers of an IL closest to the electrode.

A self-assembled monolayer (SAM) on gold was prepared from a diaminoterephthalate (DAT) derivative as functional molecule and 1-decanthiol as a backfiller. The DAT derivative is N-protected by a te...

In this paper we analyze the changes in molecular orientation triggered by electrochemical reduction of an iron-containing surfactant in Langmuir-Blodgett films deposited onto gold electrodes. The metallosurfactant [Feiii(LN2O3)] (1) is an established molecular rectifier capable of unidirectional electron transfer between two electrodes. A gradual decrease in the activity is observed in sequential current vs. potential curves upon repeated cycles. Here we evaluate the redox response associated with the reduction of the Feiii/Feii couple in a single monolayer, as well as in a 5-layer LB film of 1. We use polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) to follow structural and orientation changes associated with such applied potential scans. We observe that the reduction of the Fe center becomes increasingly irreversible because an Fe-Ophenolate bond is cleaved. This transformation is accompanied by an almost vertical change in the orientation of metallosurfactant molecules in LB films.

Determination of potential-dependent changes in the composition and structure of redox-active molecules involved in electrochemical reactions is one of the most important research objects in electrochemistry. Reduction and oxidation reactions take place on the electrode surface indicating that a redox-active substrate, at least during the redox reaction, is in direct contact with the electrode surface. In this chapter the applicability of PM IRRAS for studies of changes in the composition, structure and orientation of redox-active species in sub-monolayer, monolayer, multilayer films deposited on the metallic electrode surface is described. Use of non-metallic electrodes, such as glassy carbon electrodes, is of large practical importance in modern electrochemistry. Indeed, PM IRRAS with electrochemical control was successfully applied to study structural changes in complex redox reactions in thick polymer and composite material films on the glassy carbon electrode.

International audience; Polarization-modulation infrared reflection-absorption spectroscopy, surface pressure measurements and thermodynamic analysis were used to study enzymatic hydrolysis of lipid monolayers at the air/water interface. The Ca2+-requiring pork pancreatic phospholipase A2 was used as a catalyst. The substrates were pure 1,2-dilauroyl-sn-glycero-3-phosphocholine or mixed 1,2-dilauroyl-sn-glycero-3-phosphocholine - monosialotetrahexosylganglioside Langmuir films. The physicochemical properties of the monolayers were established with the aim of a correlation with enzyme activity. The infrared spectra were acquired upon the advancement of the catalysis; the latter was studied at a controlled surface pressure and area of the film. Changes of the intensity and frequency of different infrared signals characteristic for the two lipids were correlated with modification of the properties of the monolayer due to hydrolysis. The amide I signal characteristic for peptides permitted detecting the enzyme adsorbed at the interface. The thermodynamic and infrared results indicate that monosialotetrahexosylganglioside increases H-bonding of the lipid polar heads in the films. This effect, which may be responsible for the low activity of phospholipase A2 in the mixed films, could be used for developing enzyme-resistant lipid systems.

Control of the cell adhesion and growth on chemically patterned surfaces is important in an increasing number of applications in biotechnology and medicine, for example implants, in-vitro cellular assays, and biochips. This review covers patterning techniques for organic thin films suitable for site-directed guidance of cell adhesion to surfaces. Available surface patterning techniques are critically evaluated, with special emphasis on surface chemistry that can be switched in time and space during cultivation of cells. Examples from the authors' laboratory include the use of cell-repellent self-assembled monolayers (SAM) terminated by oligoethylene glycol (OEG) units and the lifting of the cell repellent properties by use of electrogenerated Br2/HOBr which can be performed with positionable microelectrodes. Structural changes of the SAM were analyzed by polarization-modulated infrared reflection absorption spectroscopy (PM IRRAS). Use of a soft array system of individually addressable microelectrodes enables formation of flexible and complex patterns in a short time and has the potential for further acceleration of probe-induced local manipulation of cell adhesion.

A self-assembled monolayer (SAM) on gold was formed with specifically perdeuterated hexaethylene glycol-terminated alkanethiol HS(CD(2))(12)(O-CH(2)-CH(2))(6)OCH(3) (D-OEG). The structure of the d-alkane and the oligoethylene glycol (OEG) parts of the molecule in a SAM was studied by means of polarization modulation infrared reflection absorption spectroscopy. The D-OEG monolayers are highly ordered and exist in a crystalline phase. The d-alkane chain adopts an all-trans conformation. Both, the d-alkane chain and long axis of the OEG part make an angle of 26.0 degrees +/- 1.5 degrees with respect to the surface normal, a value characteristic for the tilt of solid n-alkane thiols in the SAMs on Au. The positions of nu(as)(COC) and CH(2) wagging and rocking modes indicate a helical arrangement of the OEG chains. The D-OEG SAMs were exposed to 25 muM Br(2) in two ways: (i) by immersion into the Br(2) solution and (ii) in the galvanic cell Au|D-OEG SAM|25 muM Br(2) + 0.1 M Na(2)SO(4)|| 50 muM KBr + 0.1 M Na(2)SO(4)|Au. In the galvanic cell, the oxidant (Br(2)) is scavenged by a heterogeneous electron transfer reaction, slowing the reaction of D-OEG with Br(2). The slow progress of the reaction with Br(2) allowed us to draw conclusions about molecular rearrangements taking place during this reaction. The reaction with Br(2) starts on boundaries and/or defects present in the SAM. First, at the defect place, the alpha-C atom of the OEG chain reacts with Br(2) and the OEG part of the molecule is removed from the monolayer. In consequence an increase in disorder in the OEG part of the SAM is observed. The same mechanism of the reaction with Br(2) is applied for the d-dodecane alkanethiol part of the molecule. This reaction is slow, thus the order and the tilt of the hydrocarbon chain changes only a little during the reaction time.

The GM1 ganglioside in the monolayer at the air/water interface shows a liquid expanded-liquid condensed phase transition. Due to a combination of the PM IRRAS results with quantum chemical calculations the structure and orientation of the GM1 molecule in the monolayer on the sub-molecular level is provided. The PM IRRAS studies of GM1 monolayers demonstrate that the phase transition is accompanied by a reorientation of sugar residues with simultaneous changes in the network of hydrogen bonds. The calculation of the IR spectrum of the GM1 molecule allowed us to describe individual nu(as)(COC) modes in sugar residues of the GM1 pentasaccharide. The visualization of the dipole moment vector of each analyzed nu(as)(COC) band allowed us to discuss the orientation of the polar head group of the GM1 in the monolayer. In the liquid expanded state the planes of two sugar residues: beta-Gal-(1-3)-betaGlc-(1-1)-Cer of the GM1 molecule are tilted by ca. 55 degrees with respect to the surface normal. The plane of the beta1,4-GalNAc sugar ring is inclined by 70 degrees towards the gold surface. The phase transition to the liquid condensed state causes simultaneous reduction (by approximately 10 degrees) of the tilt of planes of sugar residues. The plane of the alpha2,3-Nue5Ac residue (sialic acid) has a nearly perpendicular orientation to the gold surface. Upon the transition to the liquid condensed state the strength of hydrogen bonds formed by the carboxylate group in the alpha2,3-Nue5Ac residue decreases. In parallel, the strength of hydrogen bonds formed by N atoms of amide groups of the GM1 molecule increases. These events may be explained by loosening and/or breaking of hydrogen bonds between the carboxylate and hydroxyl groups in the alpha2,3-Nue5Ac residue in a densely packed monolayer, due to steric hindrances. The ceramide group coupling the polar head group with the hydrophobic hydrocarbon chains, forms strong hydrogen bonds. The C=O bond of the amide group is almost perpendicular to the gold surface orientation. The hydrocarbon chains of the GM1 exist in a liquid disordered state and their physical state and orientation are not affected by the phase transition.