Showing papers in "Langmuir in 1994"

Patterning Self-Assembled Monolayers: Applications in Materials Science

Abstract: Self-assembled monolayers (SAMs) form when organic molecules spontaneously chemisorb on the surfaces of solids (e.g. organic thiols and disulfides on gold, silver, and copper or carboxylic acids on the surface of alumina).1'2 The most robust and best characterized SAMs are those comprising alkanethiolates on gold.l By variation of the length of the alkane chain and the identityof the functional group at its terminus, the thickness of the organic layer and the chemical properties of the exposed interface can be controlled with great precision. We and others have used these SAMs for studies in tribology,t adhesion,a wetting,s and other fields.6 In this paper, we describe a technique for patterning the formation of SAMs, using an elastomeric stamp, that can routinely produce patterns with dimensions from 1 to 100 lrm;7 features as small as 0.2 mm (200 nm) have been generated using this procedure, although these very small features are not always easily reproduced. These patterned surfaces have geometrically well-defined regions with different chemical and physical properties. We demonstrate a number of uses for them.

Sensing Discrete Streptavidin-Biotin Interactions with Atomic Force Microscopy

Abstract: Molecular recognition forms the basis for assembly and regulation in living organisms. We have used the atomic force microscope (AFM) to study the interaction of a model receptor streptavidin with its ligand biotin under physiological conditions. Surfaces functionalized with biotin and streptavidin exhibited adhesive forces 3-8 times greater than the nonspecific interactions observed between blocked streptavidin and biotinylated surfaces. The magnitude and distribution of the observed adhesive forces suggest they result from individual streptavidin-biotin interactions. This technique provides a means to directly study molecular recognition interactions at the molecular level. Nature has developed the unique ability for molecular recognition through the use of multiple noncovalent bonds (Le., van der Waals, hydrogen, ionic, and hydrophobic interactions) which possess a high degree of spatial and orientational specificity. Molecular recognition plays a central role in cellular behavior1 and the immunological response,2 and has also become the basis for a wide range of bioanalytical technique^.^ Although the structure and binding properties of molecular recognition systems can be measured, the forces involved in intermolecular interactions remain largely unknown. Molecular recognition interactions have been characterized through observations of the behavior of cells on which ligands and receptors naturally occur or have been atta~hed.~?~ Although these studies have largely been qualitative, the application of micropipet techniques to the study of cellular interactions5 has provided the meansfor micromanipulation with a force sensitivity of N. The surface force apparatus has also recently been used to characterize quantitatively the surface forces between model molecular recognition systems.6 The control of surface properties and the very small forces and distances involved in intermolecular interactions continue to limit the characterization of discrete molecular recognition interactions. The AFM has several properties that make it an ideal tool for measuring intermolecular forces: theoretical force sensitivity on the order of N, displacement sensitivity of 0.01 nm, contact areas as small as 10 nm2, and ability to operate under physiological condition^.^ In fact, the measurement of interactions as small as asingle hydrogen bond has recently been reported.8 The bond energies for specific molecular interactions fall between those typical * To whom correspondence should be addressed.

Direct Visualization of Surfactant Hemimicelles by Force Microscopy of the Electrical Double-Layer

Abstract: The morphology of ionic surfactant molecules adsorbed from aqueous solution onto hydrophobic substrates has been determined by atomic force microscopy. Near the critical micelle concentration (cmc), noncontact imaging using double-layer repulsion between the tip and sample shows parallel, epitaxially oriented stripes spaced apart by about twice the surfactant length. This represents the first direct imaging of «hemimicelles», liquid-crystalline aggregates of amphiphilic molecules (analogous to bulk micelles) which form at interfaces. The striped pattern is indicative of hemicylindrical hemimicelles, which is further corroborated by images of the monolayer adsorbate (in contact mode) below the cmc

Capillary forces between colloidal particles

Abstract: This work is devoted to a special kind of capillary interaction, which differs from the common lateral capillary forces between floating particles. It appears between particles protruding from a liquid film and ita physical origin is the capillary rise of the liquid along the surface of each particle. Special attention is paid to the case when the position of the contact line is fixed. The resulting capillary force is compared with that at fixed contact angle. It is demonstrated that the two alternative approaches to the calculation of capillary interactions, the force and the energetical one, are equivalent. When the liquid film is thin, the disjoining pressure affects the capillary interactions between particles attached to the film surfaces. The appearance of this effect is studied quantitatively for two specified systems modeling globular proteins in aqueous film on mercury substrate and membrane proteins incorporated in a lipid bilayer. For both systems the capillary forces appear to be strong enough to engender two-dimensional particle aggregation and ordering. This is a possible explanation of a number of experimental observations of such effects.

Electrochemical oxidation of amine-containing compounds. A route to the surface modification of glassy carbon electrodes

Abstract: A method for the modification of glassy carbon electrodes (GCEs) with amine-containing compounds for electrocatalytic and biosensor purposes is investigated. The method utilizes the electrooxidation of amines to their analogous cation radicals to form a chemically stable covalent linkage between the nitrogen atom of the amine and edge plane sites at the GCE surface. By use of X-ray photoelectron spectroscopy (XPS) for coverage assessment, the capability of this route is demonstrated by the immobilization of a simple primary amine at the GCE surface. An investigation of the influence of substituents on the nitrogen atom (e.g., primary, secondary, tertiary amines) revealed that the surface coverage of primary amines was [approximately] 3 times higher than that of secondary amines, whereas tertiary amines were not immobilized at a detectable level. This behavior is attributed to a strong steric effect whereby bulky substituents on the nitrogen atom binder accessibility of the reactive amine cation radical to surface binding sites. Amine salts and amides also showed no detectable coverage by XPS. The utility of the method for creation of a GCE with electrocatalytic activity is demonstrated by the immobilization of dopamine (DA) at the GCE surface. 48 refs., 9 figs., 1 tab.

Surface Activity of Poly(ethylene oxide)-block-Poly(propylene oxide)-block-Poly(ethylene oxide) Copolymers

Abstract: The surface tension of aqueous solutions of seven poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) Pluronic copolymers, covering a wide range of molecular weights (3400-14600) and PPO/PEO ratios (0.19-1.79), was determined over the 10[sup [minus]5]-10% w/v concentration range, at two temperatures (25 and 35[degree]C). Two breaks (changes in slope) were observed in the surface tension vs log concentration curve for most of the copolymers. The low-concentration break, occurring at bulk copolymer concentrations of approximately 10[sup [minus]3]%, is believed to originate from rearrangement of the copolymer molecules on the surface at complete coverage of the air/water interface. The breaks at the high-concentration part of the surface tension curve occurred at concentrations that correspond to the critical micellization concentration values as determined by a dye solubilization technique. The surface area per copolymer molecule, A, increased as a function of the number of EO segments, N[sub EO], obeying a scaling law (A [approx] N[sub EO][sup 1/2]) similar to that of lower molecular weight C[sub i]E[sub j] nonionic surfactants. 56 refs., 6 figs., 2 tabs.

A new class of thiolipids for the attachment of lipid bilayers on gold surfaces

Abstract: A new class of lipid mols. is synthesized, based on two dipalmitoylphosphatidic mols., each extended at the lipid phosphate by a hydrophilic spacer chain of ethoxy groups of variable length, which are then coupled as a bilipid via a terminal disulfide group at the hydrophilic spacer. These anchor-bearing qthiolipidsq can attach to gold substrates by forming stable gold-sulfur bonds. In this way, the authors can couple lipid bilayers to gold surfaces, with the possibility of preserving a water layer between the support and the first monolayer. The thiolipid mols. are characterized on a Langmuir film balance using fluorescence microscopy. The mol. areas of the thiolipids on the water surface are 80-90 .ANG.2 at a fully compressed state. The thiolipid monolayers show a typical first-order phase transition on the water surface with regular, starlike domains. The formation of thiolipid-attached mono- and bilayers on gold surfaces was studied by surface plasmon resonance (SPR), impedance measurements, and cyclic voltammetry. Four different supported membrane systems are studied in detail: (1) pure thiolipid layers; (2) mixed lipid bilayers contg. a first pure thiolipid monolayer and a second one of conventional phospholipids; (3) bilayers, where the first gold-attached monolayer is composed of a mixt. of thio- and conventional phospholipids with another second phospholipid layer on top; (4) monolayers of pure 1-hexadecanethiol and layers with a second phospholipid film on top of the 1-hexadecanethiol. The electrochem. expts. reveal elec. blocking layers for all lipid systems investigated with specific resistances of 104-105 W cm2. The capacitance values for pure thiolipid bilayers are in the range of 0.5-0.7 mF/cm2 for the pure thiolipid bilayers and 0.7-0.8 mF/cm2 for the mixed thiolipid/phospholipid bilayers, which is comparable to the values found for unsupported, so-called black lipid membranes. SPR measurements confirm qual. the results of the electrochem. expts. [on SciFinder (R)]

Dispersions of Rhodamine-Labeled Silica Spheres: Synthesis, Characterization, and Fluorescence Confocal Scanning Laser Microscopy

Abstract: Monodisperse colloidal silica spheres, with a total radius of 200 nm, labeled with the fluorescent dye rhodamine isothiocyanate (RITC) in a core of 100 nm radius have been synthesized. The particles were characterized by transmission electron microscopy and by static and dynamic light scattering. The fluorescence properties and dye concentration in the particles were measured with fluorescence and absorption spectroscopy. Stable dispersions of these hydrophilic, charge-stabilized silica spheres in polar solvents and of organophilic, sterically stabilized, 1-octadecanol-coated silica spheres in apolar solvents were studied with fluorescence confocal scanning laser microscopy (CSLM). The bleachability of these particles was determined with CSLM and compared with particles labeled with the dye fluorescein isothiocyanate (FITC). The possibilities and limitations of confocal microscopy to study individual fluorescent particles in the bulk of concentrated model dispersions are discussed and demonstrated with CSLM graphs of a time series showing colloidal crystallization (crystal growth rate approximately 4 pm/s), an equilibrium between a colloidal liquid and colloidal crystal interface, and a binary mixture of RITCand FITC-labeled spheres. Further, the feasibility of using the RITC-labeled spheres with other techniques relying on fluorescence is discussed as well.

Stabilization of Asphaltenes in Aliphatic Solvents Using Alkylbenzene-Derived Amphiphiles. 1. Effect of the Chemical Structure of Amphiphiles on Asphaltene Stabilization

Abstract: Stabilization of crude oil asphaltenes in apolar alkane solvents was investigated using a series of alkylbenzene-derived amphiphiles as the asphaltene stabilizers. In this paper (i.e., part I), we present the study on the influences of the chemical structure of these amphiphiles on the effectiveness of asphaltene solubilization and on the strength of asphaltene-amphiphile interaction using both UV/vis and FTIR spectroscopies. The results showed that the amphiphile's effectiveness of asphaltene stabilization was primarily controlled by the polarity of the amphiphile's head group and the length of the amphiphile's alkyl tail. Increasing the acidity of the amphiphile's head group could promote the amphiphile's ability to stabilize asphaltenes by increasing the acid-base attraction between asphaltenes and amphiphiles. On the other hand, although decreasing the amphiphile's tail length increased the asphaltene-amphiphile attraction slightly, it still required a minimum tail length (six carbons for p-alkylphenol amphiphiles) for amphiphiles to form stable steric layers around asphaltenes. We also found additional acidic side groups of amphiphiles could further improve the amphiphile's ability to stabilize asphaltenes. The effect of the molecular weight of alkane solvents on the amphiphile's ability to stabilize asphaltenes was also studied. 18 refs., 12 figs., 3 tabs.

Real-Space Observation of Nanoscale Molecular Domains in Self-Assembled Monolayers

Abstract: Scanning tunneling microscopy with very high gap impedance reveals several stable molecular conformations of organic monolayers self-assembled from alkanethiols on gold. Thermal annealing of the preassembled films favored well-packed large molecular domains that were studied in real space with unprecedented detail and accuracy. At room temperature the system has several stable molecular conformations, which implies that chemisorption energies balance the energy of interaction between molecular chains. A similar probability of different conformations indicates almost equal total energies, which makes it an interesting-and challenging-object for molecular dynamics simulations. Our results contribute to a long-standing scientific debate on the structure of self-assembled monolayers and provide the basis for an improved understanding of organic interfaces.

Force Microscopy Study of Friction and Elastic Compliance of Phase-Separated Organic Thin Films

Abstract: A correlation between friction and elasticity is drawn from measurements on organic thin films with a modified scanning force microscope (SFM). Local elastic compliance has been measured simultaneously with both topography and friction on the submicrometer scale on thin films of phase-separated mixtures of fluorocarbons and hydrocarbons. On the fluorocarbon domains, higher friction and lower Young`s modulus than on the hydrocarbon domains have been found. Variations in pH during sample preparation lead to differences in film formation, detected with the SFM as changes in topography, elasticity, and friction. On increasing pH, both the Young`s modulus and friction force are found to decrease. This unexpected result is discussed in terms of film formation and cohesive energy mechanisms.

Preparation and Catalysis of Novel Colloidal Dispersions of Copper/Noble Metal Bimetallic Clusters

Abstract: Colloidal dispersions of polymer-protected Cu/Pt alloy clusters have been prepared for the first time by reduction of the corresponding metal hydroxide with a cold alloying process. The Cu/Pt (mole ratio=1/1) colloid with an average diameter of 19 A and Cu/Pt (3/1) with an average diameter of 23 A have the alloy structure of CuPt and Cu 3 Pt, respectively. Colloidal dispersions of polymer-protected Cu/Pd bimetallic alloy clusters have also been prepared by the same method at various Cu/Pd ratios. These bimetallic clusters exhibit excellent properties as the active and selective catalyst for the hydration of acrylonitrile to acrylamide as well as the hydrogenation of 1,3-cyclooctadiene to cyclooctene