Covalent gluing and postgluing of Langmuir-Blodgett monolayers at hydrocarbon surfaces

Langmuir-Blodgett (LB) monolayers of 5,11,17,23,29,35-hexaformyl-37,38,39,40,41,42-hexakis(1-n-octyloxy)calix[6]arene (2), deposited onto silylated silicon wafers, were cross-linked (i.e., "covalently glued") via Schiff base formation with poly(allylamine). Direct evidence for imine formation was obtained from X-ray photoelectron spectroscopy and from attenuated total reflection IR spectroscopy. These modified surfaces could be removed from the aqueous subphase into air with retention of the assembly and its orientation relative to the surface, as evidenced by atomic force microscopy, water contact angle measurements, and film thickness determinations by ellipsometry. Similar assemblies were also synthesized via a postgluing procedure, in which the substrate containing the LB monolayer was removed from the subphase and rapidly immersed into an aqueous solution containing poly(allylamine). The potential of combining postgluing methods with continuous LB film deposition as a surface modification technique is briefly discussed.     

Transition from homogeneous Langmuir-Blodgett monolayers to striped bilayers driven by a wetting instability in octadecylsiloxane monolayers

We show that two dips of an oxidized silicon substrate through a prepolymerized n-octadecylsiloxane monolayer at an air-water interface in a rapid succession produces periodic, linear striped patterns in film morphology extending over macroscopic area of the substrate surface. Langmuir monolayers of n-octadecyltrimethoxysilane were prepared at the surface of an acidic subphase (pH 2) maintained at room temperature (22 +/- 2 degrees C) under relative humidities of 50-70%. The substrate was first withdrawn at a high dipping rate from the quiescent aqueous subphase (upstroke) maintained at several surface pressures corresponding to a condensed monolayer state and lowered soon after at the same rate into the monolayer covered subphase (downstroke). The film structure and morphology were characterized using a combination of optical microscopy, imaging ellipsometry, and Fourier transform infrared spectroscopy. An extended striped pattern, perpendicular to the pushing direction of the second stroke, resulted for all surface pressures when the dipping rate exceeded a threshold value of 40 mm min(-1). Below this threshold value, uniform deposition characterizing formation of a bimolecular film was obtained. Under conditions that favored striped deposition during the downstroke through the monolayer-covered interface, we observed a periodic auto-oscillatory behavior of the meniscus. The stripes appear to be formed by a highly correlated reorganization and/or exchange of the first monolayer, mediated by the Langmuir monolayer at the air-water interface. This mechanism appears distinctly different from nanometer scale stripes observed recently in single transfers of phospholipid monolayers maintained near a phase boundary. The stripes further exhibit wettability patterns useful for spatially selective functionalization, as demonstrated by directed adsorptions of an organic dye (fluorescein) and an oil (hexadecane).     

Polyoxometalate monolayers in Langmuir-Blodgett films

Langmuir and Langmuir-Blodgett (LB) films of a variety of polyoxometalates of different shapes, sizes, and charges were prepared by taking advantage of the adsorption properties of these polyanions on a positively charged monolayer of an organic surfactant spread on water. Three different aspects were investigated. 1) The electrochemical and electrochromic properties of LB films containing the easily reducible polyoxoanion [P2Mo18O62]6-. Absorbance changes of these LB films deposited onto an ITO substrate have been induced by repeated switching of the applied potential. These changes are due to the formation of the colored reduced forms of the polyanion. Coloration and bleaching of the LB film occur very quickly and are reversible. 2) The preparation of LB films based on magnetic polyoxometalates, such as the Keggin anions, [CoW12O40]6- and [SiMn(OH2)W11O39]6-, or containing magnetic clusters of increasing nuclearities such as [Co4(H2O)2(PW9O34)2]10- and [Co4(H2O)2(P2W15O62)2]16- based on a Co4O16 ferromagnetic cluster, and the polyoxometalates [Co9(OH)3(H2O)6(HPO4)2(PW9O34)3]16- and [Ni9(OH)3-(H2O)6(HPO4)2(PW9O34)3]16- based on a nonanuclear M9O36 cluster. 3) The preparation of LB films of the giant heteropolyoxomolybdate, [Na3(NH4)12][Mo57Fe6(NO)6O174(OH)3-(H2O)24]76 H2O.     

Langmuir and Langmuir-Blodgett monolayers of molecular tweezers and clips

Molecular clips and tweezers are able to selectively bind electron-deficient aromatic and aliphatic substrates. By means of pressure-area isotherms and Brewster angle microscopy (BAM), the self-association process and phase behavior of dimethylene-bridged molecular clips and tetramethylene-bridged molecular tweezers each substituted with two acetoxy groups as polar head groups were investigated. In a series of experiments, we observed that the molecular surface area of the clips and tweezers only depended on the skeletal structure and not on the polar groups. The measured areas agreed with the effective molecular diameters of the molecules if the aromatic side walls of the clips or tweezers were assumed to be aligned perpendicularly to the water surface. We compared the phase behavior of the pure molecular clips and tweezers with that of the host-guest complexes of these molecules, which were formed with 1,2,4,5-tetracyanobenzene (TCNB) as the guest molecule. For the clips with a central benzene (I) and naphthalene spacer unit (II), the complex formation with TCNB had no measurable influence on the phase diagrams of the films. We observed, however, a dramatic difference in the BAM images and pi-A isotherms between the pure molecular tweezers III and its complex with TCNB (TCNB@III). In addition to the pi-A isotherms, we used the surface potential (V)-area (A) isotherms to compare the pure tweezers III with the corresponding complex (TCNB@III). There was a strong difference in the maximum surface potential value for the pure tweezers (450 mV) and that for the complex (300 mV). In additional experiments, we prepared LB layers of such molecules, which were investigated by fluorescence spectroscopy. In comparison to the pure tweezers III, a luminescence emission of charge-transfer (CT) origin was observed for the host-guest complex (TCNB@III) fixed on the solid substrate. It turned out that the spectra were in good agreement with the results observed in chloroform solution.     

PEG-covered lipid surfaces: bilayers and monolayers

In this review paper we survey the ways in which various micropipet techniques have been used to study the mechanochemical and interactive features of lipid bilayer vesicles and monolayer-coated gas bubbles. Special emphasis will be made on characterizing the barrier properties of grafted PEG layers and how a hierarchical approach that uses a short barrier and extended ligand allows us to start to mimic nature's own solution to the problem of ubiquitous repulsion and specific attraction. The information gained from such studies not only characterizes the membrane and other lipid surfaces and their intersurface interactions from a fundamental materials science perspective, but also provides essential materials property data that are required for the successful design and deployment of lipid-based carriers and other capsules in applications involving this so-called ‘stealthy’ surface.     

Solid-supported monolayers and bilayers of amphiphilic beta-cyclodextrins

This paper describes the adsorption and spreading of beta-cyclodextrin (CD) vesicles on hydrophobic and hydrophilic substrates, which involves a transition from bilayer vesicles to planar molecular monolayers or bilayers. On substrates that are patterned with self-assembled monolayers by microcontact printing (muCP), the CD vesicles preferentially adsorb on hydrophobic areas instead of hydrophilic (nonionic) areas, and on cationic areas instead of hydrophilic (nonionic) areas. Supported monolayers of amphiphilic cyclodextrins CD1 and CD2 were obtained by adsorption of CD vesicles to hydrophobic substrates, and supported bilayers of amphiphilic cyclodextrins CD1 and CD2 were prepared by adsorption of CD vesicles on cationic substrates. Contact angle goniometry, atomic force microscopy and confocal fluorescence microscopy (CFM) were used to analyze the supported CD layers. The fluidity of the supported CD layers was verified using fluorescence recovery after photobleaching experiments. The supported layers function as a supramolecular platform that can bind suitable guest molecules through inclusion in the CD host cavities. Additionally, the CD host layers were patterned with fluorescent guest molecules by supramolecular muCP on the supported CD layers. The host-guest interactions were investigated with CFM and fluorescence resonance energy transfer experiments.     

Self-organization of a sulfonamido-porphyrin in Langmuir monolayers and Langmuir-Blodgett films

Langmuir monolayers (LM) and Langmuir-Blodgett (LB) films of pure lipophilic meso-tetra(4-dodecylaminosulfophenyl)porphyrin (PC12) and mixed with the anionic surfactant sodium hexadecylsulfate (SHS) were studied. The molecular packing and structure of PC12 and PC12-4SHS with variable surface pressure were investigated by surface pressure-area measurements, steady-state absorption, fluorescence emission and anisotropy, as well as by fluorescence lifetime imaging microscopy (FLIM). At low surface pressure, the porphyrin molecules are organized with the rings tilted on the water surface whereas at high surface pressure the porphyrin rings achieve a more perpendicular arrangement. Using the FLIM images a gradual change of aggregates into large "islands" is observed. Different patterns are observed in the pure PC12 multilayer films (n = 3 and 5) with ordered patches superimposed which are not observed in the PC12-4SHS multilayer LB films.     

Photoinduced electron transfer in Langmuir-Blodgett monolayers of porphyrin-fullerene dyads

A series of electron donor-acceptor (DA) dyads, composed of a porphyrin donor and a fullerene acceptor covalently linked with two molecular chains, were used to fabricate solid molecular films with the Langmuir-Blodgett (LB) technique. By means of the LB technique, the DA molecules can be oriented perpendicular to the plane of the substrate. In DHD6ee and its zinc derivative hydrophilic groups are attached to the phenyl moieties in the porphyrin end of the molecule; while in the other three dyads, TBD6a, TBD6hp, and TBD4hp, the hydrophilic groups are in the fullerene end of the molecule. This makes it possible to alternate the orientation of the molecules in two opposite directions with respect to the air-water interface and to fabricate molecular assemblies in which the direction of the primary photoinduced vectorial electron transfer can be controlled both by the deposition direction of the LB monolayer and by the selection of the used DA molecule. This was proved by the time-resolved Maxwell displacement charge measurements. The spectroscopic properties of the DA films were studied with the steady-state absorption and fluorescence methods. In addition, the time correlated single photon counting technique was used to determine the fluorescence properties of the dyad films.     

Structural investigation of Langmuir and Langmuir-Blodgett monolayers of semifluorinated alkanes

The behavior of a semi-fluorinated alkane (C(10)F(21)C(19)H(39)) has been studied at the air-water interface by using surface pressure and surface potential-area isotherms as well as infrared spectroscopy for the Langmuir-Blodgett films. In addition, based on the quantum chemical PM3 semiempirical approach, the dimer structure was investigated, and the double helix was found to be the most stable conformation of the dimer. The obtained results allow us to imply that the phase transition observed in the course of the surface pressure/area isotherm is due to a conformational change originating from the double helix to a vertical, single helix configuration.     

Interaction of curcumin with lipid monolayers and liposomal bilayers

Curcumin shows huge potential as an anticancer and anti-inflammatory agent. However, to achieve a satisfactory bioavailability and stability of this compound, its liposomal form is preferable. Our detailed studies on the curcumin interaction with lipid membranes are aimed to obtain better understanding of the mechanism and eventually to improve the efficiency of curcumin delivery to cells. Egg yolk phosphatidylcholine (EYPC) one-component monolayers and bilayers, as well as mixed systems containing additionally dihexadecyl phosphate (DHP) and cholesterol, were studied. Curcumin binding constant to EYPC liposomes was determined based on two different methods: UV/Vis absorption and fluorescence measurements to be 4.26 × 10⁴ M⁻¹ and 3.79 × 10⁴ M⁻¹, respectively. The fluorescence quenching experiment revealed that curcumin locates in the hydrophobic region of EYPC liposomal bilayer. It was shown that curcumin impacts the size and stability of the liposomal carriers significantly. Loaded into the EYPC/DPH/cholesterol liposomal bilayer curcumin stabilizes the system proportionally to its content, while the EYPC/DPH system is destabilized upon drug loading. The three-component lipid composition of the liposome seems to be the most promising system for curcumin delivery. An interaction of free and liposomal curcumin with EYPC and mixed monolayers was also studied using Langmuir balance measurements. Monolayer systems were treated as a simple model of cell membrane. Condensing effect of curcumin on EYPC and EYPC/DHP monolayers and loosening influence on EYPC/DHP/chol ones were observed. It was also demonstrated that curcumin-loaded EYPC liposomes are more stable upon interaction with the model lipid membrane than the unloaded ones.     

Specific adsorption of cytochrome C on cardiolipin-glycerophospholipid monolayers and bilayers

In this study, we examined the adsorption of cytochrome c (cyt c) on monolayers and liposomes formed from (i) pure 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), or cardiolipin (CL) and on (ii) the more thermodynamically stable binary mixtures of POPE/CL (0.8:0.2 mol/mol) and POPC/CL (0.6:0.4 mol/mol). Constant surface pressure experiments showed that the maximum and minimum interactions occurred in the pure CL (anionic phospholipid) and the pure POPE (zwitterion) monolayers, respectively. Observation by atomic force microscopy (AFM) of the images of Langmuir-Blodgett (LB) films extracted at 30 mN m-1 suggests that the different interactions of cyt c with POPE/CL and the POPC/CL monolayers could be due to lateral phase separation occurring in the POPE/CL mixture. The competition between 8-anilino-1-naphthalene sulfonate (ANS) and cyt c for the same binding sites in liposomes that have identical nominal compositions with respect to those of the monolayers was used to obtain binding parameters. In agreement with the monolayer experiments, the most binding was observed in POPE/CL liposomes. All of our observations strongly support the existence of selective adsorption of cyt c on CL, which is modulated differently by different neutral phospholipids (POPE and POPC).     

Cell transport via electromigration in polymer-based microfluidic devices

Electrokinetic transport of Escherichia coli and Saccharomyces cerevisiae (baker's yeast) cells was evaluated in microfluidic devices fabricated in pristine and UV-modified poly(methyl methacrylate)(PMMA) and polycarbonate (PC). Chip-to-chip reproducibility of the cell's apparent mobilities (micro(app)) varied slightly with a RSD of approximately 10%. The highest micro(app) for baker's yeast cells was observed in UV-modified PC with 0.5 mM PBS (pH = 7.4), and the lowest was measured in pristine PMMA with 20 mM PBS (pH = 7.4). Baker's yeast in all devices migrated toward the cathode because of their smaller electrophoretic mobility compared to the EOF. In 0.5 mM and 1 mM PBS, E. coli cells migrated toward the anode in all cases, opposite to the direction of the EOF due to their larger electrophoretic mobility. E. coli cells in 20 mM PBS migrated toward the cathode, which indicated that the electrophoretic mobility of E. coli cells decreased at higher ionic strengths. Observed differential migrations of E. coli and baker's yeast cells in appropriately prepared polymer microchips were used as the basis for selective introduction into microfluidic devices of only one type of cell. As a working model, experiments were performed with E. coli and RBCs (red blood cells). RBCs migrated toward the cathode in pristine PMMA with 1 mM and 20 mM PBS (pH = 7.4), opposite to the direction of the E. coli cells. By judicious choice of the buffer concentration in which the cell suspension was prepared and the polymer material, RBCs or E. coli cells were selectively introduced into the microdevice, which was monitored via laser backscatter signals.     

Interactions and reactions of monolayers and Langmuir-Blodgett multilayers with compounds in the bulk phase

Studies performed on the interactions and reactions of compounds in the bidimensional state, essentially in monolayers and Langmuir-Blodgett multilayers, with substances in the aqueous subphase are reported. More precisely, the following is illustrated: (i) interactions between acid amphiphiles and prevalently bivalent ions placed in the aqueous support and between compounds capable of functioning like ion carriers in monolayers and ions in the subphase, in order to build mimetic membranes capable of selective ion transport; and the complexation of amphiphiles in monolayer with ions in the bulk liquid phase, in order to build chemical sensors to ions; (ii) the reactions of photoinduced electron transfer between a partner in mono- or multimolecular films and a partner in the subphase, which may determine the fundamental parameters and the differences with the same reactions in the bulk phase; and (iii) the reactions of enzymatic hydrolysis between the monolayer of a glyceride, which constitutes the reaction support, and the enzyme in the liquid bulk phase, which constitutes the subphase. The mechanism of the reactions and its inhibition are clarified. To conclude, possible future developments connected with the areas studied are examined.     

Effect of dipolar moments in domain sizes of lipid bilayers and monolayers

Lipid domains are found in systems such as multicomponent bilayer membranes and single component monolayers at the air-water interface. It was shown by Keller et al. [J. Phys. Chem. 91, 6417 (1987)] that in monolayers, the size of the domains results from balancing the line tension, which favors the formation of a large single circular domain, against the electrostatic cost of assembling the dipolar moments of the lipids. In this paper, we present an exact analytical expression for the electric potential, ion distribution, and electrostatic free energy for different problems consisting of three different slabs with different dielectric constants and Debye lengths, with a circular homogeneous dipolar density in the middle slab. From these solutions, we extend the calculation of domain sizes for monolayers to include the effects of finite ionic strength, dielectric discontinuities (or image charges), and the polarizability of the dipoles and further generalize the calculations to account for domains in lipid bilayers. In monolayers, the size of the domains is dependent on the different dielectric constants but independent of ionic strength. In asymmetric bilayers, where the inner and outer leaflets have different dipolar densities, domains show a strong size dependence with ionic strength, with molecular-sized domains that grow to macroscopic phase separation with increasing ionic strength. We discuss the implications of the results for experiments and briefly consider their relation to other two dimensional systems such as Wigner crystals or heteroepitaxial growth.     

Exploring point defects in hexagonal boron-nitrogen monolayers

A comprehensive theoretical study of selected point defects for a monolayer of hexagonal boron nitride (h-BN) is presented. Two-dimensional structures were simulated through large h-BN molecular clusters and used to examine various defects, like: atom vacancies, atom substitutions, or distortions of the hexagonal lattice. Since carbon contaminations are very common in the h-BN technology, a particular attention has been paid to carbon impurities. The calculations of IR spectra for the doped molecular clusters reveal the presence of additional frequencies, which in many cases correspond to defect-bound modes. In particular, when two carbon atoms are close to each other, a localized stretching C C mode of a high intensity has been found, with a frequency value of about 100-200 cm^–1 higher than for collective BN stretch frequencies. Absorption UV-Vis spectra obtained from time-dependent density functional theory show that the inclusion of impurities results in an emergence of several low-energy electronic excitations, from which some are localized on a defect, while other are delocalized. Energies of these excitations are strongly dependent on the defect type, and they range from about 0.7 to 6.1 eV for the lowest excitations. Based on UV-Vis spectra we propose several candidates which could be responsible for the experimental 4 eV color band. These defects are built from two or four adjacent carbon atoms and have the lowest excitation ranging from 3.9 to 4.8 eV, which is strongly localized on the defect.     

Effect of acidity on morphologies and photodimerization kinetics in Langmuir-Blodgett monolayers of styrylquinoline derivatives

The real-time second harmonic generation was employed to investigate the[2+2] photodimerization of styrylquinoline (SQ) derivatives in Langmuir-Blodgett (LB) monolayers deposited from aqueous subphases with different acidity. It was discovered that the photodimerization rate constant significantly decreased upon the addition of acid. The additional atomic force microscopy measurements revealed that surface morphologies were correlated to the photodimerization kinetics. These experimental results provide direct evidence for the topo-chemical mechanisms of[2+2] photodimerization in LB films. The current study demonstrates that the intermolecular interactions and aggregation structures play important roles in the photochemical properties at surfaces.     

Polymeric amphiphiles with hydrophilic main chain spacers: Studies in monolayers and Langmuir-Blodgett multilayers

A methacrylic lipid was polymerized to form an amphiphilic homopolymer and random copolymers with various levels of acrylic acid comonomers. The behavior of these polymeric lipids was investigated in monolayers at the air-water interface and in Langmuir-Blodgett multilayers on polymer foil supports. The acrylic acid acts as a hydrophilic spacer along the polymer backbone, and improves the ability of the polymer to self-organize into highly ordered monolayers and ultimately LB multilayers. It is shown that the quality and stability of the multilayers increase substantially with the increase in comonomer content.     

Rearrangement of Langmuir-Blodgett stearic acid films and band assignments in deuterated stearic acid monolayers

Band assignments in the C-D stretching region of straight chain hydrocarbon species are derived from the spectra of stearic acid monolayers on gold. The fatty acid molecules reorient with respect to the metal surface as the films age. Correlating the changes in the IR spectra of both the undeuterated and deuterated acids allows one to identify the vibrational modes of the latter based on the accepted assignments of the former. The CD₂ asymmetric and symmetric stretches are observed at 2194 and 2086 cm^–1, respectively. Bands at 2212 and 2221 cm^–1 are attributed to asymmetric in-plane and out-of-plane CD₃ stretches. Assignments of several other features in this region are given while one band remains unassigned.     

Helfrich's concept of intrinsic force and its molecular origin in bilayers and monolayers

Bilayers and monolayers are excellent models of biological membranes. The constituents of the biological membranes such as lipids, cholesterols and proteins are chiral. Chiral molecules are abundant in nature (protein, nucleic acid and lipid). It is obvious that relationship between chirality and morphology (as well as function) of biological membrane is of interest for its fundamental importance and has technological implication regarding various membrane functions. The recent years have witnessed that a number of experimental studies in biomimetic systems have shown fascinating morphologies where chirality of the constituent molecule has decisive influence. Significant progress is made towards the understanding of these systems from the theoretical and computational studies. Helfrich's concept of intrinsic force arising from chirality is a milestone in understanding the biomimetic system such as bilayer and the related concepts, further progresses in molecular understanding made in recent years and experimental studies revealing the influence of chirality on morphology are the focus of the present review. Helfrich's concept of intrinsic force arising due to chirality is useful in understanding two-dimensional bilayers and one-dimensional monolayers and related mimetic systems. Various experimental techniques are used, which can probe the molecular architecture of these mimetic systems at different length scales and both macroscopic (thermodynamic) as well as microscopic (molecular) theories are developed. These studies are aimed to understand the role of chirality in the molecular interaction when the corresponding molecule is present in an aggregate. When one looks into the variety of morphologies exhibited by three-dimensional bilayer and two-dimensional monolayer, the later types of systems are more exotic in the sense that they show more diversity and interesting chiral discrimination. Helfrich's concept of intrinsic force may be considered useful in both cases. The intrinsic force due to chirality is the decisive factor in determining morphology which is explained by molecular approaches. Finally, biological and technological implications of such morphological variations are briefly mentioned.