Organization Of Monolayers Research Articles

Effect of pH on acetylcholinesterase Langmuir and Langmuir–Blodgett films studied by surface potential and atomic force microscopy

Surface potential measurements have been used to determine the dipole interactions of the acetylcholinesterase (AChE) monolayer at the air/aqueous interface. The results show that it is the ionogenic groups of the enzyme that cause significant changes in the surface potential of the monolayer as the pH of the sub-phase is changed. The atomic force microscope (AFM) images of AChE Langmuir–Blodgett (LB) films indicate that at low pH, an unfolding of the enzyme occurs which leads to formation of large domains at the air/aqueous interface rather than an organized AChE monolayer. Furthermore, the AFM images of the AChE LB film prepared at basic pH show that the enzyme forms aggregates.

Monolayer properties of amphiphilic tetrakis(phenylethynyl)benzene compounds

The monolayer behaviour of new sheet-like tetrakis(arylethynyl)benzene derivatives bearing two hydrophilic head groups fixed to the para positions of the central benzene nucleus has been investigated at the air–water interface. The surface pressure-area isotherms indicate an edge-on orientation of the flat aromatic molecular part of each tetrayne within the compressed monolayers. Brewster angle microscopy shows spontaneous aggregation already in the expanded state. The surface pressure starts to rise only when the solid condensed islands formed immediately after spreading, make contact. Further information regarding the monolayer organization characteristics of the novel non-classical tetrayne amphiphiles was obtained by surface potential-area isotherms.

Organization of polymer-capped platinum colloidal particles at the air–water interface

This communication discusses the formation of thin films of platinum colloidal particles by organization at the air–water interface. The platinum clusters were prepared by sodium borohydride reduction of aqueous chloroplatinic acid with the nanoparticles being capped with a polymer, poly(vinylpyrrolidone) (PVP). After separation from the aqueous medium, the capped clusters were dispersed on the surface of water, using the hydrophobic component of the capping agent to organize the clusters at the air–water interface. Pressure-area measurements showed a region of rather low compressibility, characteristic of a reasonably compact, ordered phase of the colloidal particles. The Langmuir monolayer of the platinum clusters was transferred to suitable substrates and analyzed using transmission electron microscopy, quartz crystal microgravimetry and X-ray photoemission spectroscopy. An organized monolayer of platinum clusters was observed indicating that the air–water interface may be used as a medium for obtaining nanoparticle films for device applications.

Langmuir-blodgett films of amphiphilic cyanovinyl ferrocene derivatives and their electrochemical behaviour

Monolayer and Langmuir-Blodgett film forming properties of three amphiphilic cyanovinyl ferrocene derivatives containing different hydrophobic groups are reported. These materials showed a dependence of monolayer organisation (change in the surface pressure vs. molecular area isotherm) on the time that the floating film was allowed to remain in its uncompressed state. The molecular area changes were accompanied by changes in the cyclic voltammetric response for Langmuir-Blodgett monolayers transferred onto indium-tin-oxide electrodes.

Characterization of 6-mercaptopurine monolayers on Hg surfaces

Abstract The formation and organization of self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) has been studied on mercury surfaces. The organization process has been examined by cyclic voltammetry and chronoamperometry. The study confirms the occurrence of a two-dimensional (2D) condensation at more positive potentials than those of the cathodic/anodic adsorption peaks at about −0.2 V. The condensed phase is destroyed at more negative potentials. Charging current measurements as well as electron transfer experiments indicate the formation of a thin film monolayer. This film has no effect on the electron transfer rate constant of an outer-sphere redox couple but it shows a strong ability to inhibit electroreduction of dioxygen as it is characteristic of condensed organic monolayers. Cu 2+ is able to permeate the 6MP film, giving a current only three-fold lower than that at a bare hanging mercury drop electrode (HMDE) and with no-linear diffusion characteristics.

Ganglioside GD3 and GD3-lactone mediated regulation of the intermolecular organization in mixed monolayers with dipalmitoylphosphatidylcholine

The interactions of dpPC with ganglioside GD3 and two lactones, GD3LacI or GD3LacII, in lipid monolayers occur with reduced, unaltered, or increased molecular area and surface potential/molecule, respectively. dpPC is fully miscible with GD3 and GD3LacI but films with GD3LacII show immiscibility above 75 mol% lactone. At low proportions of GD3 in mixtures with dpPC, GD3 undergoes condensation and depolarization; dpPC is depolarized and its molecular area is reduced above 50 mol% GD3. GD3LacI forms ideally mixed films with dpPC. Mixtures of dpPC with GD3LacII at mole fractions below 0.3 show increased mean molecular area and surface potential/molecule mostly due to lactone alterations. Between mole fractions of 0.3 and 0.75 the surface parameters of dpPC are altered, and above these proportions both lipids are immiscible. Defined variations of molecular properties induced by ganglioside lactonization are selectively transduced to changes of the intermolecular organization and surface electrostatics in mixed interfaces with dpPC. Thus, changes in the relative proportions of a ganglioside and its lactone forms may act as sensitive biotransducers for membrane-mediated cellular functions, without the need for metabolically altering the concentration of gangliosides.

Electrochemical Modulation of Molecular Conversion in an Azobenzene-Terminated Self-Assembled Monolayer Film: An in Situ UV−Visible and Infrared Study

UV−visible absorption spectroscopy and Fourier transform infrared reflection absorption spectroscopy were utilized as in situ probes to investigate the electrochemical redox process in an azobenzene-terminated self-assembled monolayer (SAM) film in an aqueous electrolyte. The electrochemical reduction process, which is exceedingly slow due to disruption of the extremely densely packed SAM, produced the hydrazobenzene species. Upon reoxidation, the resulting hydrazobenzene is quickly converted back to trans-azobenzene. It appears that the electrochemical redox process is accompanied by a reversible orientational change in terms of the azobenzene terminal group. Therefore, by simply modulating the applied potential, a reproducible electrochemical redox process together with a reversible orientational variation can be achieved in an organized monolayer film.

Slow fusion of liposomes composed of membrane-spanning lipids

The fusion characteristics of large unilamellar liposomes composed of bipolar tetraether lipids extracted from the thermophilic archaeon Sulfolobus acidocaldarius, was investigated. These lipids span the entire membrane and form single monolayer liposomes in aqueous media [Elferink, M.G.L., de Wit, J.G., Demel, R., Driessen, A.J.M. and Konings, W.N., (1992) J. Biol. Chem. 267, 1375–1381]. In the presence of calcium-phosphate, slow mixing of the aqueous liposome contents and membrane lipids occurred, demonstrating that these liposomes are fusion-competent. The fusion process was essentially nonleaky. The rate of fusion increased with the pH and the concentration of calcium and phosphate. Fusion resulted in an increase of the size of the liposomes. These data demonstrate that a monolayer organization of lipids in a membrane does not per se interfere with membrane fusion competence.

Asymmetrical distribution of cardiolipin in yeast inner mitochondrial membrane triggered by carbon catabolite repression

Transmembrane asymmetry of cardiolipin in yeast was monitored during the switch from fermentative to gluconeogenic growth and the reverse. As soon as cells used ethanol as an electron donor to produce ATP by oxidative phosphorylation, rapid and abundant cardiolipin synthesis was observed on the matrix side of the inner mitochondrial membrane followed by a transverse rearrangement between the two leaflets. The cardiolipin distribution changed from about 20:80 (in/out) to 70:30 (in/out), and after translocation towards the outer leaflet it finally became 37:63 (in/out). At the same time, cytochrome c oxidase activity remained stable, then increased as a possible result of the topographical rearrangement. During the reverse process from gluconeogenic to fermentative growth, the amount of cardiolipin rapidly decreased by half, its bilayer distribution apparently changing to a monolayer organization before the 20:80 (in/out) asymmetry of repressed cells was re-established. Experimental impairment of cardiolipin topography by antibiotic inhibition of gene expression or in situ dissipation of mitochondrial membrane potential produced data that prove that the amount and transmembrane distribution of the phospholipid are two specific parameters of the mitochondrial inner membrane organization in both fermentative (2.2 fmol/cell and 20:80, in/out) and gluconeogenic (4.2 fmol/cell and 37:63, in/out) growing yeast cells. Finally, the inner mitochondrial membrane topography of cardiolipin appeared to be closely associated with the transmembrane redox potential.

3D organized self‐assembled monolayer electrodes: A novel biosensor configuration

A leak‐free, reagentless biosensor for glucose is demonstrated as one application of the bulk modified electrode with three‐dimensional self‐assembled monolayers reported here. The preparation of the electrode and the electrochemical investigations of its behavior are described. It is shown that model enzymes can be incorporated in these materials so that the biomolecules retain their activity and the SAMs their dense structure and charge‐mediation properties. A range of electrochemical applications are foreseen for such electrodes.

Photoswitchable biomaterials as grounds for optobioelectronic devices

Optobioelectronic systems provide a means for the electronic transduction of recorded optical signals. The methodologies to assemble optobioelectronic devices are reviewed. One method involves the chemical modification of redox enzymes with photoisomerizable units and their integration with electrode surfaces. In one photoisomer state the protein structure is perturbed and its bioelectrocatalytic activities are blocked. In the second photoisomer state, the tertiary structure of the protein is retained and the enzyme exhibits bioelectrocatalytic functions. This method is exemplified by the chemical modification of glucose oxidase (GOx) with photoisomerizable nitrospiropyran units and with the reconstitution of apo-GOx with the photoisomerizable nitrospiropyran-FAD (flavin adenine dinucleotide phosphate) cofactor. The two systems enable the cyclic amplified amperometric transduction of optical signals recorded by the photoactive proteins. The second approach to assemble optobioelectronic systems involves the organization of photoisomerizable monolayers on electrode surfaces acting as ‘optical command surfaces’ for the control of the electrical contact between redox proteins (or redox enzymes) and the electrode. This method is exemplified by the control of the electrical contact of cytochrome c (Cyt. c) with a functionalized electrode consisting of a mixed monolayer of pyridine/nitrospiropyran assembled onto an Au-electrode. The ON-OFF photostimulated electrical contact of Cyt. c with the electrode is coupled to mediated electron transfer cascades, i.e. bioelectrocatalyzed reduction of oxygen by cytochrome oxidase (COx). The systems allow the amplified amperometric transduction of optical signals recorded by the photoisomerizable monolayer-electrode. The photochemically-triggered activation or deactivation of the electrical communication between Cyt. c and the electrode, originate from the association or repulsion of the protein from the photoisomerizable monolayer-electrode interface. This enables the microgravimetric transduction of the optical signals recorded by the monolayer using a quartz crystal microbalance (QCM). The photostimulated electrical contact of Cyt. c and the electrode, and the subsequent activation of an electron transfer cascade via the electrobiocatalyzed reduction of oxygen by COx, allow the amplified amperometric transduction of optical signals recorded by the monolayer-functionalized-electrode. Reversible immunosensors based on photoisomerizable antigen monolayers assembled onto electrodes represent another configuration of an optobioelectronic device. Assembly of an antigen monolayer on electrodes yields an active interface for the amperometric transduction of the association of the complementary antibody (Ab) to the monolayer. Binding of the Ab to the monolayer blocks the electrical contact of a redox enzyme, i.e. ferrocene-modified glucose oxidase (Fc-GOx), with the electrode, and inhibits the electrobiocatalyzed oxidation of glucose. A photoisomerizable antigen assembled as a monolayer on an electrode allows the tailoring of reversible immunosensing interfaces. In one photoisomer state, the monolayer acts as an active interface for amperometric detection of the antibody. The complementary photoisomer state lacks affinity for the Ab and allows the washing-off of the antibody and the regeneration of the active antigen monolayer by a second illumination cycle. This approach is exemplified by the application of a dinitrospiropyran monolayer assembled onto an Au-electrode as a reversible sensing interface for the dinitrophenyl antibody (DNP-Ab). The dinitrospiropyran monolayer, SP-state, acts as an active interface for the association of the DNP-Ab. Photoisomerization of the monolayer to the dinitromerocyanine configuration, MRH +-state, allows the washing-off of the DNP-Ab and regeneration of the active SP-monolayer electrode by a secondary photoinduced isomerization. The reversible photostimulated binding and dissociation of DNP-Ab to and from the electrode is transduced by the application of Fc-GOx as redox probe and is further supported by microgravimetric QCM analyses.

Hydrolysis and Condensation of Self-Assembled Monolayers of (3-Mercaptopropyl)trimethoxysilane on Ag and Au Surfaces

Self-assembled monolayer films of (3-mercaptopropyl)trimethoxysilane (3MPT) and their hydrolysis products on Ag and Au surfaces are characterized using Raman spectroscopy, FTIR spectroscopy, ellipsometry, X-ray photoelectron spectroscopy (XPS), and electrochemistry. 3MPT monolayers are formed through metal−thiolate bonding through the S atom on both metals, similar to other alkanethiol chemisorption chemistries. The orientation of 3MPT molecules in these monolayers is similar on both metals. Prior to hydrolysis, the molecules form an organized monolayer with the methoxy headgroups oriented largely parallel to the surface and the propyl chain in a largely trans conformation. When the methoxy groups are hydrolyzed, the 3MPT molecules cross-link through the formation of siloxane bonds. Although the surface vibrational spectroscopy indicates the presence of a small number of unreacted silanol groups on the siloxane surface, cyclic voltammetry of underpotential deposition of Pb suggests that the Si−O−Si networ...

Surface-Enhanced Raman Scattering Spectroscopy and AFM Studies of Amphiphilic Nitroaniline Derivatives

Surface-enhanced Raman scattering (SERS) Spectroscopy and atomic force microscopy (AFM) were employed to characterize the Langmuir-Blodgett (LB) monolayers of an amphiphilic nitroaniline derivatives, n-docosyl-2-methyl-4-nitroaniline (DCMNA) on both Ag surface and mica. DCMNA formed a stable, organized and homogeneous monolayer at the air-water interface only under the specific conditions. Structural changes at the interface induced by surface pressure, temperature and pH of the subphase, and the mole ratio of mixed molecules have been studied. The stable monolayers of DCMNA were obtained on a neutral subphase maintained lower than 10°C. The compressibility of the monolayer was improved by addition of arachidic acid or spreading the monolayer on an alkaline subphase. Micro-Raman and AFM studies indicated that pure DCMNA on a neutral subphase spontaneously formed a well ordered crystalline phase as well as an organized monolayer even under a lower surface pressure. AFM and lateral force microscopy (LFM) confirmed the arachidic acid minimizes self-aggregation of DCMNA and induces formation of monodispersed nano sized domain in the mixed monolayer.

Tissue Culture of Retinal Pigment Epithelium Following Isolation with a Gelatin Matrix Technique

Prior to transplantation of the retinal pigment epithelium, it is necessary to develop techniques to harvest viable retinal pigment epithelium as an organized monolayer. Unfortunately, current techniques result in contraction of the harvested monolayer and coiling of the cell sheets, which hinders successful transplantation. The purpose of this study was to develop a method for harvesting retinal pigment epithelium from eye cups and from tissue culture prior to transplantation. Passage 1 porcine retinal pigment epithelium and native retinal pigment epithelium from fresh porcine eye cups were pretreated with 0.25% edetic acid for 12 minutes and coated with a 100 micron layer of 12£ gelatin. Patches of the retinal pigment epithelial cell monolayer were harvested and transferred to culture plates, and cell viability and the ability of the transferred cells to proliferate in culture was determined. Our results demonstrated that retinal pigment epithelium can be harvested from tissue culture and eye cups as an organized monolayer with high efficiency (94.7±3.5% and 99.7±0.3% harvesting rates, respectively) and high cell viability (91.3±2.9% and 89.4±4.3%, respectively). Cells harvested from tissue culture plates divided and became confluent within 10 to 14 days. Cells harvested from eye cups maintained a differentiated phenotype and migrated outward from the margin of the exoplant. There was no contraction of the retinal pigment epithelial monolayer isolated from either substrate. Thus, we were able to harvest retinal pigment epithelium as an organized monolayer from tissue culture plates and freshly enucleated eyes with edetic acid and gelatin. The harvested cells were viable and proliferated without contraction of the monolayerin vitro.

Alpha-Spectrin is required for ovarian follicle monolayer integrity in Drosophila melanogaster.

To understand the role of the spectrin-based membrane skeleton in generating epithelial polarity, we characterized the distribution of membrane skeletal components in Drosophila ovarian follicle cells and in somatic clones of mutant cells that lack alpha-spectrin. Immunolocalization data reveal that wild-type follicle cells contain two populations of spectrin heterodimers: a network of alphabeta heterodimers concentrated on the lateral plasma membrane and an alphabetaH population targeted to the apical surface. Induction of somatic clones lacking alpha-spectrin leads to follicle cell hyperplasia. Surprisingly, elimination of alpha-spectrin from follicle cells does not appear to prevent the assembly of conventional beta-spectrin and ankyrin at the lateral domain of the follicle cell plasma membrane. However, the alpha-subunit is essential for the correct localization of betaH-spectrin to the apical surface. As a consequence of disrupting the apical membrane skeleton a distinct sub population of follicle cells undergoes unregulated proliferation which leads to the loss of monolayer organization and disruption of the anterior-posterior axis of the oocyte. These results suggest that the spectrin-based membrane skeleton is required in a developmental pathway that controls follicle cell monolayer integrity and proliferation.

Nanoscale colloidal particles: Monolayer organization and patterning

The monolayer deposition of nanoscale colloidal particles (Au citrate sols) was demonstrated by employing an aminofunctional silane [3-(2-Aminoethlyamino) propyltrimethoxysilane] as a coupling agent. The compatibility of this colloidal Au deposition method with conventional electron beam lithography techniques was examined, and the two-dimensional patterning of the Au colloidal monolayer was demonstrated. Using this fabrication method, a proposal for a single electron device structure based on nanoscale (2 nm diameter) gold colloidal particles was made.

Registered growth of mesoporous silica films on graphite

It has recently been demonstrated that hexagonal mesoporous silica films can be grown at mica/water interfaces. In this report it is established that the films can also be synthesized on cleaved pyrolytic graphite. Diffraction and microscopy studies of the growth, structure and form of the films indicate that the channels are aligned along the hexagonal symmetry axes of the graphite surface. The registry of the mesoporous silica film with the underlying substrate may be facilitated by an organized hemicylindrical surfactant monolayer assembly that is observed at the boundary between the graphite and water. Growth of the mesoporous silica film most likely begins within a liquid-crystal surfactant/silicate film that is aligned with the hemimicelle–graphite overstructure.

Organized self‐assembled lipoyl‐β‐cyclodextrin derivative monolayer on a gold electrode

AbstractAn organized self‐assembled monolayer of β‐cyclodextrin (β‐CD) derivatives on a gold surface has been prepared using two lipoyl‐β‐cyclodextrin derivatives with different chain lengths, which are more flexible than heptakis (dodecyl sulfide) β‐cyclodextrin. The surface coverage of the monolayers were found to be in agreement with the surface coverage of a highly organized monolayer of β‐CD. It has been revealed by cyclic voltammetry that the β‐CD derivative monolayers have the characteristics of: (a) selective barriers blocking the permeability for those molecules, such as ferricyanide, and p‐aminophenol, which are incapable of complexing with β‐CD; and (b) have selective transport for electroactive organic species, such as ferrocene carboxylic acid and hydroxyquinone, which can form host‐guest inclusion complexes with β‐CD receptors. The selective transport can be controlled by blocking the β‐CD cavities with some guest molecules such as cyclohexanol borneol and ursodeoxycholic acid, and the controlling mode can be expected for detection of organic compounds.

New disc-shaped triarylamino-1,3,5-triazines with heteroaromatic central cores

Abstract New disc-shaped triarylamino-1,3,5-triazines were prepared by reaction of cyanuric chloride with two-fold alkoxy substituted anilines. Each of the triarylmelamines exhibits an enanti-otropic mesophase. X-ray investigations reveal the formation of columnar liquid crystalline structures with either a hexagonal or a rectangular lattice. The lattice parameters, as well as the intracolumnar ordering, depend on the length of the flexible lateral side chains. The new melamines form Langmuir monolayers at the air/water interface due to their amphiphilic nature. The monolayer organization is characterized by a side-on arrangement of the molecules. The polar central molecular parts lie flat on the water, whereas the lateral alkyl chains are oriented perpendicular to the water surface.

Enhanced binding of porphyrin by a laterally organized monolayer

The effect of lateral organization of a mixed monolayer of dioctadecyl-dimethyl-ammonium bromide (DOMA) and methyl stearate (SME) at the air-water interface by interaction with a water-soluble porphyrin (PO) on binding of PO has been investigated with the aim of achieving molecular recognition. The organized monolayers were formed by co-spreading of the lipids DOMA and SME with PO. This technique should be more efficient in generating a laterally organized monolayer than reorganization of the lipid monolayer by molecules adsorbed from the aqueous subphase. By measuring surface pressure-area and surface potential-area isotherms as well as reflection spectra, an optimal molar ratio of PO:DOMA:SME was determined. The monolayers were transferred to hydrophobic glass plates by vertical dipping, and the PO was removed from the immersed plate by changing the pH. Readsorption of PO from dilute aqueous solution showed stronger binding by the organized monolayer as compared with a lipid monolayer of the same average composition.