Surface Dipole Potential Research Articles

Controlling oxide surface dipole and reactivity with intrinsic nonstoichiometric epitaxial reconstructions

The composition and reconstruction of oxide surfaces can be deterministically controlled via ambient conditions. We demonstrate that such intrinsic alterations can have a crucial effect on the surface dipole and reactivity, even for surfaces with the same crystallographic plane. The surface dipole potential drops of ${\mathrm{BaTiO}}_{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{SrTiO}}_{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{LaFeO}}_{3}$, and ${\mathrm{TiO}}_{2}$ surfaces with various reconstructions and compositions are shown to vary by as much as 5 V, leading to significant variation of the band edge positions at these surfaces. These variations are shown to correlate with the calculated oxygen binding energy, demonstrating how oxide surface reactivity can be substantially manipulated using environmental changes.

The effect of the hydrophilic spacer length on the functionality of a mercury-supported tethered bilayer lipid membrane

A biomimetic membrane consisting of a thiolipid monolayer tethered to a mercury electrode, with a dioleoylphosphatidylcholine (DOPC) monolayer on top of it, was fabricated. The thiolipid, referred to as DPOL, consisted of an octaethyleneoxy (OEO) chain terminated at one end with a lipoic acid residue and covalently linked at the other end to two phytanyl chains. The functionality of this biomimetic membrane, referred to as a tethered bilayer lipid membrane (tBLM), was tested by incorporating gramicidin and alamethicin and verifying their ion channel activity. Advantages and drawbacks with respect to a tBLM using a thiolipid, referred to as DPTL, with a tetraethyleneoxy (TEO) chain were examined by using electrochemical impedance spectroscopy, potential-step chronocoulometry and cyclic voltammetry. The maximum charge surface density of potassium ions stored in the OEO spacer amounts to 70μCcm−2, as compared to a charge surface density of 45μCcm−2 in the TEO spacer. The lipid bilayer moiety of the DPOL/DOPC tBLM is somewhat leakier than that of the DPTL/DOPC tBLM at potentials negative of about −0.65V vs. the saturated calomel electrode. The estimated value of the surface dipole potential of the OEO spacer amounts to −0.180V and is, therefore, smaller than that, −0.230V, of the TEO spacer.

The physical properties of lipid monolayers and bilayers containing calixarenes sensitive to cytochrome c

We studied physical properties of the monolayers and bilayer lipid membranes (BLM) formed by calix[6]arene carboxylic acid derivative (CX) and its mixtures with diphytanoylphosphatidylcholine (DPhPC) by means of measurement surface pressure, surface dipole potential and electrostriction. CX forms stable monolayers at an air-water interface and complexes in mixed monolayers contained DPhPC. Calixarenes increase the elastic moduli of lipid monolayers as well as BLM. Cytochrome c (cyt c) specifically binds to CX by incorporation of the amino groups of lysine residues at the protein surface. This binding affected the physical properties of CX monolayers depending on their initial surface pressure. Addition of cyt c into the water subphase induced increase of surface pressure of CX monolayers at relatively low initial pressure (15 mN/m) when monolayer was in liquid expanded state (LE). This may be due interaction of positively charged cyt c with negatively charged carboxylic groups of CX and also by its penetration into the air-water interface. However, much subtle changes were observed for higher initial surface pressure (20 and 35 mN/m) when monolayer is in liquid condensed (LC) and solid (S) state, respectively. Lysine induced substantially lower changes in surface pressure in comparison with that of cyt c.

Impact of Film Structure on Ionization Energy of Titanyl-Phthalocyanine in Thin Films

ABSTRACTStructure-related ionization energy (IE) of vacuum-deposited titanyl-phthalocyanine (OTiPc) thin films was investigated by using in situ ultraviolet photoelectron spectroscopy (UPS) and X-ray diffractometry. Distinct molecular orientations (i.e. lying-flat and standing-up orientation) in different polymorphous (i.e. monoclinic β-phase and triclinic α-phase) were observed on a surface of polycrystalline (poly-) Au and octadecyltrichlorosilane-self assembled monolayer (OTS-SAM). For the two structures IE of the highest occupied molecular orbital (HOMO) of OTiPc thin films altered significantly by 0.55 eV. The different IE was attributed to surface dipole potential and strong intermolecular interaction.

Organic field-effect transistor for label-free dopamine sensing

We describe a potentiometric sensor based on Electrolyte-Gated Organic Field-Effect Transistor (EGOFET) for “in vitro” detection of dopamine. The sensing element of this device resides at the Au gate–aqueous solution interface by means of a self-assembled monolayer (SAM) composed by cysteamine and 4-formylphenyl boronic acid. The covalent and selective adsorption of dopamine induces a surface dipole potential which shifts the electrode work function and modulates the double layer capacitance. As a result, our device is capable to detect dopamine up to pico-molar concentration showing higher sensitivity with respect to other approaches. For this reason the interface engineering of our EGOFET gate is a promising route for diagnostic applications.

Tert-Pentanol Adsorption on Ag(100) and Ag(110) from an Aqueous Solution and its Comparison with that on Ag(111), Hg, Ga and Bi(001)

Abstract The adsorption behavior of tert-pentanol (TPT) on Ag(100) and Ag(110) single-crystal faces from aqueous 0.05 M KClO4 was investigated on the basis of chronocoulometric and differential capacity measurements. The adsorption data on Ag(100) were analyzed thermodynamically and the standard Gibbs energy of adsorption, ΔG 0 ads , was determined as a function of the electrode potential and of the charge density, σ M , on the metal. The adsorption parameters for TPT adsorption on Ag(100) were compared with those on Ag(111), Hg, liquid Ga and Bi(001), available in the literature. The surface dipole potential of the TPT film at maximum coverage on the above metal surfaces was estimated by a novel approach that requires no a priori assumptions as to the predominance of either water desorption or TPT adsorption on the value of the shift, E N , of the potential of zero charge from zero to maximum coverage. Instead, the E N values for the different metals were estimated a posteriori, under the assumption that the water molecules that are still present at maximum TPT coverage maintain the same orientation as on the bare metal surface at the same charge density σ M . The lack of a correlation between the ΔG 0 ads values on the different metal surfaces and their hydrophilicity indicates that TBT adsorption depends primarily on its particular conformation and on the metal surface structure.

Partial charge transfer and surface dipole potential of the Hg–S bond layer at thiol monolayers tethered to mercury

The surface dipole potential χSAM due to the molecules of a monolayer self-assembled on a mercury drop can be estimated by tilting them via gradual expansion of the drop surface. Moreover, the potential difference due to distortional polarization of the self-assembled monolayer (SAM) can be estimated by measuring the charge experienced by the diffuse layer ions on the basis of the Gouy–Chapman theory. In the case of a physisorbed SAM, the sum Δϕ′ of these two potential differences differs from the extrathermodynamic potential difference Δϕ across the whole electrified interface by the surface dipole potential, χHgSAM, due to electron spillover; in the case of a chemisorbed thiol, this sum, herein denoted by Δϕ″, differs from Δϕ by χHgSAM plus the surface dipole potential χHg–SSAM due to partial charge transfer along the Hg–S bond. Comparing the two “partial” potential differences Δϕ′ and Δϕ″ at the same applied potential allows an estimate of χHg–SSAM. The magnitude of χHg–SSAM depends heavily upon the orientation of the dipole of the adsorbed molecules. If the χSAM value for a given thiol SAM estimated on Hg can be assumed not to vary in passing to Au or Ag, the above procedure can also be extended to the latter noble metals.

A procedure for estimating the surface dipole potential of monolayers adsorbed on electrodes

The extrathermodynamic potential difference Δϕ across an electrified interface enclosed between a bulk metal M and a bulk aqueous phase does not depend on the content of the interface, at constant applied potential E. By equating at constant E the expression of Δϕ for an electrode coated by a self-assembled monolayer (SAM) to that for the corresponding bare electrode immersed in the aqueous solution of a nonspecifically adsorbed electrolyte, it is possible to estimate the surface dipole potential χSAM of the SAM. If the molecules of the SAM form a bond M–X with the metal M, this procedure requires an independent knowledge of the surface dipole potential χM–XSAM due to such a bond. The other way round, if the χSAM value is known by independent means, the procedure allows an estimate of χM–XSAM. The self-consistency of this procedure was tested with SAMs of ten different thiolated peptides covalently bound to a mercury electrode, where χSAM can be determined independently by expanding a mercury drop. The procedure was then applied to the estimate of the χSAM value of a peptide SAM on a polycrystalline gold electrode.

Living systems and liquid crystals

The chemical, physical and electrical properties of naturally occurring liquid crystals are fundamentally linked to the functions of certain biological systems. This review focuses primarily on the liquid crystalline properties of cell membranes and the two-dimensional smectic A model used to describe the lipid bilayer structure that it forms. The composition of mammalian cells is considered along with the effects that disease has on the changes to the molecular composition, and the elastic and electrical properties of the cell membrane. Particular emphasis is given to the role that the flexoelectric effect in a two-dimensional lipid bilayer plays in mechanotransduction (the conversion of a mechanical stimulus to an electrical signal) in living systems. The intrinsic electrical properties arising from the membrane composition itself in terms of the transmembrane, surface and membrane dipole potential are considered in relation to the liquid crystalline structure of the cell membrane and the latest measurement techniques for the measurement of membrane electrical potentials are presented.

Estimate of the potential difference across metal/water interfaces and across the lipid bilayer moiety of biomimetic membranes: an approach

Mercury is a particularly advantageous support of lipid, thiol and disulfide self-assembled monolayers (SAMs), because it provides them with a perfectly smooth and fluid surface and allows their gradual expansion in aqueous solution with progressive tilt of the adsorbed molecules, without water incorporation. These unique advantageous features permit an estimate of the extrathermodynamic absolute potential difference, ϕ, across the Hg|water interface and of the surface dipole potential of mercury-supported SAMs. Two different models of metal-supported tethered bilayer lipid membranes (tBLMs) incorporating a cation-selective channel predict that the ϕ value at the inflection point of a plot of the in-phase component, Y′, of the electrochemical admittance against the applied potential E is almost coincident with the surface dipole potential, χs, of the hydrophilic spacer moiety of the tBLM. This prediction allows an estimate of the absolute potential difference, ϕ, across the interface between any metal capable of supporting tBLMs and the bulk aqueous phase, provided the χs value of the tBLM is known. Moreover, the potential difference across the lipid bilayer moiety of the tBLM (i.e., the transmembrane potential) is shown to be practically equal to χs at the inflection point of the corresponding Y′ vs.E plot. This approach is applied to polycrystalline Au and Ag(111).

Influence of gel-phase microdomains and lipid rafts in lipid monolayers on the electron transfer of a lipophilic redox probe: dioctadecylviologen

Gel-phase microdomains and lipid rafts form spontaneously in monolayers of lipid mixtures of dioleoylphosphatidylcholine (DOPC), palmitoylsphingomyelin (PSM) and cholesterol (Chol), self-assembled on mercury. The influence of microdomains on the electron transfer properties of 2 mol% dioctadecylviologen (DODV), incorporated in these lipid monolayers, was investigated by cyclic voltammetry. In pure DOPC, the DODV molecules tend to aggregate, giving rise to strong attractive lateral interactions. With an increase in the PSM mole fraction in DOPC/PSM binary mixtures, the edges of the resulting gel-phase microdomains act as docking sites for the DODV molecules, decreasing lateral interactions and modifying the DODV redox properties. A similar behavior is shown by lipid rafts formed by adding Chol to the above binary mixtures. By varying the DOPC/PSM molar ratio, the midpoint between the peak potentials of the DODV reduction and oxidation peaks shifts in parallel with the surface dipole potential of the lipid mixture. This behavior indicates that the formal (half-reduction) potential of a redox pair, as measured versus a given reference electrode, may include a surface dipole potential if one or both members of the redox pair are embedded in a medium different from the bulk phase containing the reference electrode.

Correlating Dye Adsorption Behavior with the Open-Circuit Voltage of Triphenylamine-Based Dye-Sensitized Solar Cells

To study the relationship between dye adsorption behavior and open-circuit voltage (VOC) of dye-sensitized solar cells (DSCs), four triphenylamine-based organic sensitizers with closely related molecular structures but with two different adsorption orientations were engineered and compared. The origin of VOC was investigated in terms of band-edge movement of the TiO2 conduction band (CB) and interfacial charge recombination, with the latter found to be the governing factor. The two dyes with cyanoacetic acid as an anchoring group (TC dyes) adopt a standing adsorption mode and exert a larger surface dipole potential on TiO2 than their counterparts bearing rhodanine-3-acetic acid (TR dyes), which lie along the surface. TR dyes exhibit a greater extent of charge recombination than TC dyes because of the low surface-blocking efficiency of the dye layer and the intimacy between the I3−-bound dyes and the TiO2. The differences in both CB movement and charge recombination between TR and TC dyes amplify with the ...

Effect of a Strong Interfacial Electric Field on the Orientation of the Dipole Moment of Thiolated Aib-Oligopeptides Tethered to Mercury on Either the N- or C-Terminus

Four oligopeptides consisting of a sequence of alpha-aminoisobutyric acid (Aib) residues, thiolated at either the N- or C-terminus by means of a -(CH(2))(2)-SH anchor, were self-assembled on mercury, which is a substrate known to impart a high fluidity to self-assembled monolayers (SAMs). The surface dipole potential of these peptide SAMs was estimated in 0.1 M KCl aqueous solution at a negatively charged electrode, where the interfacial electric field is directed toward the metal. To the best of our knowledge, this is the first estimate of the surface dipole potential of peptide SAMs in aqueous solution. The procedure adopted consisted in measuring the charge involved in the gradual expansion of a peptide-coated mercury drop and then combining the resulting information with an estimate of the charge density experienced by diffuse layer ions. The dipole moment of the tethered thiolated peptides was found to be directed toward the metal, independent of whether they were thiolated at the C- or N-terminus. This result was confirmed by the effect of these SAMs on the kinetics and thermodynamics of the Eu(III)/Eu(II) redox couple. The combined outcome of these studies indicates that a strong interfacial electric field orients the dipole moment of peptide SAMs tethered to mercury, even against their "natural" dipole moment.

A New Method to Evaluate the Surface Dipole Potential of Thiol and Disulfide Self-Assembled Monolayers and Its Application to a Disulfidated Tetraoxyethylene Glycol

A procedure to evaluate the surface dipole potential chi of thiol and disulfide self-assembled monolayers (SAMs) is described. The procedure consists of self-assembling the monolayers on a hanging mercury drop electrode and of measuring the charge involved in a progressive expansion of the mercury drop. This measurement is then combined with an estimate of the charge density q experienced by diffuse layer ions, obtained by measuring the diffuse layer capacitance of the SAM at different electrolyte concentrations by electrochemical impedance spectroscopy. These chi measurements, combined with chronocoulometric measurements of the total charge density sigma(M) against potential, indicate that SAMs of tetraoxyethylene glycol-D,L-alpha-lipoic acid ester (TEGL), 2,3-di-O-phytanyl-sn-glycerol-1-tetraoxyethylene glycol-D,L-alpha-lipoic ester (DPTL), and trioxyethyleneoxythiol (EO3) on mercury may undergo a reversal in the surface dipole potential of their polyoxyethylene chain with a change in the interfacial electric field. Moreover, TEGL and EO3 form stable SAMs without electron transfer to the metal, while no such conclusion can be drawn for DPTL.

Structure of the Langmuir Monolayers with Fluorinated Ethyl Amide and Ethyl Ester Polar Heads Creating Dipole Potentials of Opposite Sign

This study experimentally checks our previous hypothesis (Petrov, J. G.; Polymeropoulos, E. E.; Moehwald, H. Langmuir 2007, 23, 2623) that different conformations of the fluorinated heads of RCONHCH(2)CF(3) and RCOOCH(2)CF(3) monolayers cause the opposite signs and the striking difference of 1.480 V between their surface potentials Delta V. In situ X-ray diffraction at grazing incidence (GIXD) shows that both monolayers form orthorhombic lattices with closely packed chains tilted to the next-nearest neighbors in the RCONHCH(2)CF(3) film and upright in the RCOOCH(2)CF(3) monolayer. The packing of the chains in the plane perpendicular to them, which excludes the effect of the tilt, shows the same distance between the next-nearest neighbors, but significantly closer nearest neighbors in the RCONHCH(2)CF(3) film. This difference implies a specific anisotropic attraction between the adjacent amide heads. IR reflection absorption spectroscopy (IRRAS) shows that the -CONHCH(2)CF(3) heads have trans conformation and participate in H-bonds forming a -NH...O=C- lateral network. We speculate that such structure hinders the energetically optimal orientation of the hydrophobic -CH(2)CF(3) terminals toward air, so that the (delta+)C-(F (delta-))(3) dipoles at the monolayer/water boundary yield a strong positive contribution to Delta V. In contrast, most of the unbounded by H-bonds -COOCH(2)CF(3) heads statistically orient their hydrophobic (delta+)C-(F (delta-))(3) dipoles toward air, yielding a negative average dipole moment at the monolayer/water boundary and negative surface dipole potential.

Molecular Organization, Structural Orientation, and Surface Topography of Monoacylated β-Cyclodextrins in Monolayers at the Air−Aqueous Interface

The surface behavior of monoacylated beta-cyclodextrins, with hydrocarbon chains of 16, 14, and 10 carbons, has been assessed by the measurement of the surface pressure, surface (dipole) potential, optical reflectivity, and surface topography in monolayers at the air-water interface. For all the derivatives studied, the intermolecular organization adopted along compression-decompression isotherms reveals a rich variety of packing states which imply profound reorganization of the hydrophobic and hydrophilic moieties of the beta-cyclodextrin derivatives in the film, depending on the lateral surface pressure. The intermolecular arrangements are consistent with the adoption of a different and defined orientation of the cyclic oligosaccharide unit, relative to the interfacial plane and the aqueous subphase. This is different from the behavior of the per-substituted derivatives, and none of the changes exhibited by the monosubstituted forms are consistent with the oligosaccharide ring remaining in a fixed orientation along the interface when the surface pressure is varied.

Characterization of Solid-State Dye-Sensitized Solar Cells Utilizing High Absorption Coefficient Metal-Free Organic Dyes

Solid-state dye-sensitized solar cells were fabricated using the organic hole-transporting medium (HTM) 2,2'7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD), and three organic indoline-based sensitizer dyes with high molar extinction coefficients. The cells were characterized by several techniques, including spectral response measurements, photovoltage decay transients, intensity modulated photovoltage spectroscopy (IMVS), and charge extraction. The differences in apparent electron lifetime observed for cells fabricated using the three dyes are attributed in part to changes in the surface dipole potential at the TiO2/spiro-MeOTAD interface, which shift the TiO2 conduction band energy relative to the Fermi level of the HTM. These energy shifts influence both the open circuit voltage (as a result of changes in free electron density) and the short circuit current (as a consequence of changes in the overlap between the dye LUMO level and the conduction band). A self-consistent approach was used to derive the positions of the conduction band relative to the spiro-MeOTAD redox Fermi level for cells fabricated using the three dyes. The analysis also provided estimates of the free electron lifetime in spiro-MeOTAD cells. In order to evaluate the possible contribution of the adsorbed dyes to the observed changes in surface dipole potential, their dipole moments were estimated using ab initio density functional theory (DFT) calculations. Comparison of the calculated dipole contributions with the experimentally measured shifts in conduction band energy revealed that other factors such as proton adsorption may be predominant in determining the surface dipole potential.

PH modulation of transport properties of alamethicin oligomers inserted in zwitterionic-based artificial lipid membranes

Electric features of biological membranes are major determinants of the function and physiological manifestation of membrane-penetrating peptides, and such features are prone to be modulated by the properties of the surrounding aqueous medium. In this work, we demonstrate that pH plays crucial roles in modulating electric characteristics of zwitterionic-based artificial lipid membranes. The effect of pH on electrical properties of such membranes was probed by evaluating the transport properties of embedded alamethicin oligomers over a wide range of pH values (i.e., 0.65, 2.08, 2.94, 7 and 10.1). Our data strongly support the paradigm of a pH-dependent variation of the surface and membrane dipole potential which, in conjunction with possible lateral pressure effects within the lipid membrane, lead to a non-monotonic modulation of the electrical conductance of alamethicin oligomers. As expected, pH modulation of transport properties through the alamethicin oligomer is more visible for narrower pores (that is, the 1st conductive state) with slightly better cation selectivity as compared to larger oligomers.

New BODIPY lipid probes for fluorescence studies of membranes

Many fluorescent lipid probes tend to loop back to the membrane interface when attached to a lipid acyl chain rather than embedding deeply into the bilayer. To achieve maximum embedding of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) fluorophore into the bilayer apolar region, a series of sn-2 acyl-labeled phosphatidylcholines was synthesized bearing 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene-8-yl (Me(4)-BODIPY-8) at the end of C(3)-, C(5)-, C(7)-, or C(9)-acyl. A strategy was used of symmetrically dispersing the methyl groups at BODIPY ring positions 1, 3, 5, and 7 to decrease fluorophore polarity. Iodide quenching of the phosphatidylcholine probes in bilayer vesicles confirmed that the Me(4)-BODIPY-8 fluorophore was embedded in the bilayer. Parallax analysis of Me(4)-BODIPY-8 fluorescence quenching by phosphatidylcholines containing iodide at different positions along the sn-2 acyl chain indicated that the penetration depth of Me(4)-BODIPY-8 into the bilayer was determined by the length of the linking acyl chain. Evaluation using monolayers showed minimal perturbation of <10 mol% probe in fluid-phase and cholesterol-enriched phosphatidylcholine. Spectral characterization in monolayers and bilayers confirmed the retention of many features of other BODIPY derivatives (i.e., absorption and emission wavelength maxima near 498 nm and approximately 506-515 nm) but also showed the absence of the 620-630 nm peak associated with BODIPY dimer fluorescence and the presence of a 570 nm emission shoulder at high Me(4)-BODIPY-8 surface concentrations. We conclude that the new probes should have versatile utility in membrane studies, especially when precise location of the reporter group is needed.

Study of calix[4]resorcinarene–dopamine complexation in mixed phospholipid monolayers formed at the air–water interface

We have studied the physical properties of monolayers formed by calix[4]resorcinarene and in mixtures with dipalmitoyl phosphatidylcholine (DPPC) in various molar ratios formed at the air–water interface and at presence of dopamine in water subphase by means of measurements of surface pressure and dipole potential. We showed that both calix[4]resorcinarene as well as its mixture with DPPC form stable monolayers at the water subphase. The presence of dopamine resulted in an increase of the mean molecular area and in a decrease of the compressibility modulus of the monolayers. For mixed monolayers at higher content of calix[4]resorcinarene (> 0.2 molar fraction) a deviation from ideal miscibility took place especially for monolayers in a solid state. This can be connected with formation of aggregates of calix[4] resorcinarene. Lowest miscibility and weakest interaction of dopamine with a monolayer was observed for calix[4]resorcinarene molar fraction of 0.33 in the monolayer.