Cyclodextrin-based self-assembled nanotubes at the water/air interface

Native alpha-cyclodextrin (alpha-CD) is found to spontaneously form films at aqueous solution/air interfaces. Shape-response measurements to volume perturbations on drops hanging from a capillary indicate that temperature and sodium dodecyl sulfate (SDS) concentration strongly modify the viscoelastic properties of such films. By using isothermal titration calorimetry (ITC), Brewster angle microscopy (BAM), atomic force microscopy (AFM), and molecular dynamics (MD) simulations, it is shown that the films consist of self-assembled nanotubes whose building blocks are cyclodextrin dimers (alpha-CD2) and alpha-CD2-SDS1 complexes.     

Direct observation of chemical oscillation at a water/nitrobenzene interface with a sodium-alkyl-sulfate system.

The oscillation of the interfacial tension and electrical potential at a water/nitrobenzene interface was observed with homologous anionic surfactant molecules, sodium-alkyl-sulfates. Concerning small molecules with a short hydrophobic carbon chain, the oscillation period and amplitude decreased with a decrease of the length of the alkyl chain. On the other hand, when surfactant molecules with a long hydrophobic carbon chain were used, no remarkable periodic oscillation occurred after the first oscillation. In all systems, an interfacial flow by Marangoni convection was observed when the oscillation took place. By monitoring the movement of carbon powder scattered on the liquid/liquid interface with a CCD camera, we could observe that the liquid/liquid interface expanded outward from the area on which the surfactant molecules adsorbed when the oscillation occurred. When the small molecule was used, the speed of expansion of the interface (flow speed) was small and shrinkage followed by expansion of the interface repeatedly occurred. However, when the large molecule was used, the flow speed was large and expansion occurred only one time. These results show that hydrodynamic factors and surface activities are important in chemical oscillation systems.     

Electrochemical determination of electroinactive guests of b-cyclodextrin at a self-assembled monolayer interface

A novel determination method of electroinactive molecules by means of electrochemical technique is presented. A new self-assembled monolayer containing cyclodextrin(CD) is prepared with mono(6-o-p-tolylsulfonyl)-b-cyclodextrin. Although this derivatization process leads to a b-CD coverage of 10% of a full monolayer, this layer shows an effective host-guest response to ferrocene. The interfacial ferrocene complexation gives a response similar to that expected for a Langmuir adsorption isotherm yielding a stability constant of 4.2×104 mol-1@L and a maximum ferrocene coverage of 8.6′10-12 mol/cm2. The redox peak currents of the surface-confined ferrocene de-crease upon addition of competing b-CD guest species to the solution, such as m-toluic acid(mTA) and sodium dodecyl sulfonate(SDS). This principle has been used for the determination of the electroinactive molecules, mTA and SDS in the concentration ranges of 0.8-2.7 mmol/L and 5-100 nmol/L, respectively.     

Delocalized electron resonance at the alkanethiolate self-assembled monolayer/Au(111) interface

We probe the electronic structure of alkanethiolate self-assembled monolayers (SAMs) on Au(111) using two-photon photoemission spectroscopy. We observe a dispersive unoccupied resonance close to the vacuum level with a lifetime shorter than 30 fs. The short lifetime and the insensitivity of the energy level and dispersion to molecular length (and thus layer thickness) suggest that the probability density of the electron wave function is concentrated inside the molecular layer close to the SAM/Au interface. Such an interfacial resonance results from the image like potential at the SAM/Au interface.     

Electrochemical determination of electroinactive guests of β-cyclodextrin at a self-assembled monolayer interface

A novel determination method of electroinactive molecules by means of electrochemical technique is presented. A new self-assembled monolayer containing cyclodextrin (CD) is prepared with mono(6-o-p-tolylsulfonyl)-β-cyclodextrin. Although this derivatization process leads to a β-CD coverage of 10% of a full monolayer, this layer shows an effective host-guest response to ferrocene. The interfacial ferrocene complexation gives a response similar to that expected for a Langmuir adsorption isotherm yielding a stability constant of 4.2 ×104 mol-1· L and a maximum ferrocene coverage of 8.6×10-12 mol/cm2. The redox peak currents of the surface-confined ferrocene decrease upon addition of competing β-CD guest species to the solution, such as m-toluic acid (mTA) and sodium dodecyl sulfonate (SDS). This principle has been used for the determination of the electroinactive molecules, mTA and SDS in the concentration ranges of 0.8-2.7 μmol/L and 5-100 nmol/L, respectively.     

Ion-transfer voltammetry at a polarized room-temperature molten salt/water interface.

Tetraoctylammonium cation forms a room-temperature molten salt (RTMS) with 2,4,6-trinitrophenolate anion. The RTMS is immiscible with water (W) and forms a stable RTMS/W interface. It has been shown that the RTMS/W interface can be electrochemically polarized. A well-defined voltammetric wave due to the transfer of thiocyanate ion across the RTMS/W interface was observed within the potential window. This is the first example of a polarized RTMS/W interface.     

A molecular simulation study of an organosilane self-assembled monolayer/SiO2 substrate interface

The bonding network of an alkylsilane self-assembled monolayer (SAM)SiO(2) substrate interface is investigated by means of canonical Monte Carlo (MC) simulations. SAMSiO(2) systems with different interfacial bonding topologies are sampled by the Metropolis MC method, and the AMBER potential with a newly developed organosilicon parameters are used to obtain an optimized structure with a given bonding topology. The underlying substrates are modeled as hydroxy-terminated (100) or (111) cristobalites. The SAMSiO(2) interface is characterized by a polysiloxane bonding network which comprises anchoring bonds and cross-linking bonds, namely, molecule-substrate and molecule-molecule Si-O-Si bonds, respectively. We show that at thermal equilibrium, the ratio of the number of anchoring bonds to cross-linking bonds decreases as a total Si-O-Si bond density increases, and that nevertheless, number of anchoring bonds always dominate over that of cross-linking bonds. Moreover we show that the total Si-O-Si bond density strongly affects the lateral ordering of the alkylsilane molecules, and that increase in the Si-O-Si bond density disorders the molecular packing. Our results imply that a lab-to-lab variation in the experimentally prepared SAMs can be attributed to different Si-O-Si bond densities at the SAMSiO(2) interface.     

Direct observation of a diblock copolymer-induced microemulsion at a polymer/polymer interface

We have studied the segregation of a block copolymer of poly(d₈-styrene-b-2-vinylpyridine) (dPS-PVP) at the interface between polystyrene and a random copolymer of poly(styreneran-4-hydroxystyrene) (PS-r-PPHS). Forward recoil spectrometry (FRES) was used to measure the equilibrium excess (z^*) of the dPS-PVP chains at the interface as a function of its volume fraction in the bulk PS phase (?_∞). It was found that there is a sharp increase in z^* at a critical value of ?_∞. This upturn indicates the formation of a microemulsion of PS and the random copolymer PS-r-PPHS due to a vanishing of the interfacial tension caused by the strong adsorption of the block copolymer. Cross-sectional transmission electron microscopy (TEM) of the interface shows that this microemulsion starts to form at the interface by forming a deeply corrugated structure where the “wavelength” of the corrugations is of the order of 50 nm. © 1995 John Wiley & Sons, Inc.     

Direct observation of single-molecule generation at a solid-liquid interface

Direct observation of single-molecule generation from a chemical reaction was achieved at a solid-liquid interface. The reaction between fluorescamine and immobilized N'-(3-trimethoxysilylpropyl)diethylenetriamine (DETA) was studied at the single-molecule level. Time-lapse fluorescence images of single-molecule products, excited by the evanescent field generated at a quartz-liquid interface, were recorded to follow the chemical reaction to its completion. The reactions were restricted to the approximately 1 nm thick layer nearest to the interface. Analysis of the photoelectron intensity of the fluorescent product of the reaction and its distribution shows that the reaction kinetics goes through a transition from zeroth-order to first-order as the reaction proceeds. This approach offered a novel means to study single-molecule reactions at the solid-liquid interface. It also enabled the investigation of reaction kinetics and chemical mapping of surface heterogeneity at the single-molecule level.     

Oriented films of layered rare-earth hydroxide crystallites self-assembled at the hexane/water interface

Layered rare-earth hydroxide crystallites self-assembled at the hexane/water interface were transferred to various substrates to form a monolayer film, which exhibited photoluminescence properties and ion-exchange ability.     

Electron transport through rectifying self-assembled monolayer diodes on silicon: Fermi-level pinning at the molecule-metal interface

We report the synthesis and characterization of molecular rectifying diodes on silicon using sequential grafting of self-assembled monolayers of alkyl chains bearing a pi group at their outer end (Si/sigma-pi/metal junctions). We investigate the structure-performance relationships of these molecular devices, and we examine the extent to which the nature of the pi end group (change in the energy position of their molecular orbitals) drives the properties of these molecular diodes. Self-assembled monolayers of alkyl chains (different chain lengths from 6 to 15 methylene groups) functionalized by phenyl, anthracene, pyrene, ethylene dioxythiophene, ethylene dioxyphenyl, thiophene, terthiophene, and quaterthiophene were synthesized and characterized by contact angle measurements, ellipsometry, Fourier transform infrared spectroscopy, and atomic force microscopy. We demonstrate that reasonably well-packed monolayers are obtained in all cases. Their electrical properties were assessed by dc current-voltage characteristics and high-frequency (1-MHz) capacitance measurements. For all of the pi groups investigated here, we observed rectification behavior. These results extend our preliminary work using phenyl and thiophene groups (Lenfant et al., Nano Lett. 2003, 3, 741). The experimental current-voltage curves were analyzed with a simple analytical model, from which we extracted the energy position of the molecular orbital of the pi group in resonance with the Fermi energy of the electrodes. We report experimental studies of the band lineup in these silicon/alkyl pi-conjugated molecule/metal junctions. We conclude that Fermi-level pinning at the pi group/metal interface is mainly responsible for the observed absence of a dependence of the rectification effect on the nature of the pi groups, even though the groups examined were selected to have significant variations in their electronic molecular orbitals.     

Electrochemical study of insulin at the polarized liquid-liquid interface

This paper reports on the electrochemical behavior of bovine insulin at the interface between two immiscible electrolyte solutions (ITIES). The voltammetric ion-transfer response obtained in the presence of insulin was dependent on the aqueous phase pH conditions and on the nature of the organic phase electrolyte employed in experiments. Optimal detection was obtained at acidic pH below the isoelectric point of insulin where it was positively charged. A shift in transfer potentials to lower potential values was observed with decreasing hydrophobicity of the anion of the organic phase electrolyte. No ion-transfer response was observed at pH values of the aqueous phase above the isoelectric point, where insulin was negatively charged. These results suggest that the voltammetric response is due to ion-pairing interactions at the ITIES between positively charged insulin and the hydrophobic anion of the organic phase electrolyte, together with adsorption of the ion-pair at the interface. The voltammetric response was obtained for insulin at concentrations down to 1 muM. These results show that electrochemistry is useful in studying the behavior of this important protein molecule at the polarized water-1,2-DCE interface and provides an alternative detection mode for bioanalytical applications.     

Computer simulation studies of surfactant monolayer mixtures at the water/oil interface: charge distribution effects

Charge distribution effects on polar head groups for a mixture of amphiphilic molecules at the water/oil interface were studied. For this purpose a model which allowed us to investigate the charge effects exclusively was created. As a molecular model we used the structure of sodium dodecyl sulfate. Then we prepared molecules with the same molecular structure but with different charge distributions in order to have one cationic and one nonionic molecule. So, in this way, we were able to focus only in the charge effects. The monolayer mixtures were composed of anionic/nonionic and cationic/nonionic surfactants. Simulations of these systems show that the location of the different surfactants at the interface is determined by the interaction and the charge distribution of the molecules. Due to the difference in the charge distribution of the surfactant monolayers, the water molecules present distinct orientations in the mixture. Finally, it was found that the electrostatic potential difference across the interface depended on the interactions (charge distribution) of the anionic, cationic, and nonionic molecules in the mixture.     

Structure of the complex monolayer of gemini surfactant and DNA at the air/water interface

The properties of the complex monolayers composed of cationic gemini surfactants, [C(18)H(37)(CH(3))(2)N(+)-(CH(2))(s)-N(+)(CH(3))(2)C(18)H(37)],2Br(-) (18-s-18 with s = 3, 4, 6, 8, 10 and 12), and ds-DNA or ss-DNA at the air/water interface were in situ studied by the surface pressure-area per molecule (π-A) isotherm measurement and the infrared reflection absorption spectroscopy (IRRAS). The corresponding Langmuir-Blodgett (LB) films were also investigated by the atomic force microscopy (AFM), the Fourier transform infrared spectroscopy (FT-IR), and the circular dichroism spectroscopy (CD). The π-A isotherms and AFM images reveal that the spacer of gemini surfactant has a significant effect on the surface properties of the complex monolayers. As s ≤ 6, the gemini/ds-DNA complex monolayers can both laterally and normally aggregate to form fibril structures with heights of 2.0-7.0 nm and widths of from several tens to ~300 nm. As s > 6, they can laterally condense to form the platform structure with about 1.4 nm height. Nevertheless, FT-IR, IRRAS, and CD spectra, as well as AFM images, suggest that DNA retains its double-stranded character when complexed. This is very important and meaningful for gene therapy because it is crucial to maintain the extracellular genes undamaged to obtain a high transfection efficiency. In addition, when s ≤ 6, the gemini/ds-DNA complex monolayers can experience a transition of DNA molecule from the double-stranded helical structure to a typical ψ-phase with a supramolecular chiral order.     

The interaction between G-quadruplex-forming oligonucleotide and cationic surfactant monolayer at the air/water interface

We report on the interactions between a 21-mer quadruplex-forming oligonucleotide bearing human telomere sequence of dG(3)(T(2)AG(3))(3) (G4 DNA) and a positively charged dioctadecyldimethylammonium bromide (DODAB) monolayer at the air-aqueous interface, studied by surface film balance measurements. In the presence of G4 DNA, the π-A isotherm of the cationic Langmuir film shifted to lower molecular areas when compared with the reference isotherm recorded on the subphase containing only 50 mM triethylamine-acetate (TEAA) buffer. The presence of quadruplex-stabilizing metal cations (K(+) or Na(+)) further affected profiles of π-A isotherms. Further insight into processes related to the G4 DNA-monolayer interactions was provided by recording time profiles of the surface pressure of monolayer at a constant mean molecular area. In these experiments G4 DNA and/or metal ions were sequentially injected under the monolayer surface. Results indicated that multistranded assemblies of G4 DNA were formed at the monolayer interface even in the absence of metal ions, which suggested that the charged cationic surface of Langmuir monolayer induced aggregation of guanine-rich DNA strands. The presence of sodium and potassium ions inhibited formation of multi-stranded assemblies through the competitive G-quadruplex formation but to different extent that might be related to the differences in stability and topology of both quadruplexes.     

A new liquid surface neutron reflectometer and its application to the study of DPPC in a monolayer at the air/water interface

A constant wavelength neutron reflectometer is described. Using this reflectometer, the neutron reflectivities from phosphatidylcholine monolayers in the highly condensed LC phase on ultra pure H₂O and D₂O have been measured on a Wilhelmy film balance. The neutron reflectivities have been carefully compared with those obtained by the X-ray method applied to the same monolayer under similar conditions. A new approach to analyzing a combined set of data composed of X-ray and neutron reflectivities has been used. From the analysis it is concluded that despite their limited q_z range neutron reflectivities are as essential as X-ray reflectivities for the unique determination of the monolayer structure.     

Properties of a water layer on hydrophilic and hydrophobic self-assembled monolayer surfaces: A molecular dynamics study

The microscopic behaviors of a water layer on different hydrophilic and hydrophobic surfaces of well ordered self-assembled monolayers (SAMs) are studied by molecular dynamics simulations. The SAMs consist of 18-carbon alkyl chains bound to a silicon(111) substrate, and the characteristic of its surface is tuned from hydrophobic to hydrophilic by using different terminal functional groups ( CH 3 , COOH). In the simulation, the properties of water membranes adjacent to the surfaces of SAMs were reported by comparing pure water in mobility, structure, and orientational ordering of water molecules. The results suggest that the mobility of water molecules adjacent to hydrophilic surface becomes weaker and the molecules have a better ordering. The distribution of hydrogen bonds indicates that the number of water-water hydrogen bonds per water molecule tends to be lower. However, the mobility of water molecules and distribution of hydrogen bonds of a water membrane in hydropho- bic system are nearly the same as those in pure water system. In addition, hydrogen bonds are mainly formed between the hydroxyl of the COOH group and water molecules in a hydrophilic system, which is helpful in understanding the structure of interfacial water.     

Spontaneous non-linear surface tension oscillations in the presence of a spread surfactant monolayer at the air/water interface

The phenomena accompanying the dissolution of a surfactant droplet under the water/air interface covered by a spread monolayer are studied experimentally and theoretically. It is shown that the variation of the initial surface coverage changes the way of the system evolution. With respect to the character of changes of the interfacial tension with time one can distinguish between three different regimes which replace each other by increase of the initial surface coverage: (i) single oscillation followed by a long period of the monotonous decrease of the surface tension after which repeated non-linear oscillations develop spontaneously; (ii) repeated non-linear oscillations of the surface tension (without period of the monotonous decrease); (iii) monotonous decrease of the surface tension without any oscillation. The hydrodynamics of the observed regimes are discussed.     

Molecular details of ovalbumin-pectin complexes at the air/water interface: a spectroscopic study

To stabilize air-water interfaces, as in foams, the adsorption of surface-active components is a prerequisite. An approach to controlling the surface activity of proteins is noncovalent complex formation with a polyelectrolyte in the bulk phase. The molecular properties of egg white ovalbumin in a complex with pectin in the bulk solution and at air/water interfaces were studied using drop tensiometry (ADT) and time-resolved fluorescence anisotropy techniques. The complex formation of ovalbumin with pectin in the bulk resulted in the formation of a compact structure with a different spatial arrangement depending on the protein/pectin ratio. Complex formation did not provide an altered protein structure, whereas the conformational stability was slightly increased in the complex. In excess pectin, an overall condensed complex structure is formed, whereas at limited pectin concentrations the structure of the complex is more "segmental". The characteristics of these structures did not depend on pH in the 7.0 to 4.5 regime. Interaction with pectin in the bulk solution resulted in a significantly slower adsorption of the protein to the air/water interface. The limited mobility of the protein at the interface was found for both ovalbumin and ovalbumin-pectin complexes. From both the rotational dynamics and total fluorescence properties of the protein in the absence and presence of pectin, it was suggested that the complex does not dissociate at the interface. Ovalbumin in the complex retains its initial "aqueous" microenvironment at the interface, whereas in the absence of pectin the microenvironment of the protein changed to a more nonpolar one. This work illustrates a more general property of polyelectrolytes, namely, the ability to retain a protein in its microenvironment. Insight into this property provides a new tool for better control of the surface activity of complex biopolymer systems.