Single Langmuir-Blodgett Research Articles

Hyper-thin organic membranes with exceptional H2/CO2permeation selectivity: importance of ionic crosslinking and self-healing

Single Langmuir-Blodgett (LB) bilayers derived from calix[n]arene-based surfactants (n = 4, 5 and 6) and poly(acrylic acid) (PAA) exhibit exceptional H(2)/CO(2) permeation selectivities. Evidence for the importance of ionic crosslinking and self-healing processes is presented.

Dual-Emitting Langmuir−Blodgett Film-Based Organic Light-Emitting Diodes

Langmuir-Blodgett (LB) films containing alternating layers of the metallosurfactants bis(4,4'-tridecyl-2,2'-bipyridine)-(4,4'-dicarboxy-2,2'-bipyridine) ruthenium(II)-bis(chloride) (1) and bis[2-(2,4-difluorophenyl)pyridine](4,4'-dinonadecyl-2,2'-bipyridine)iridium(III) chloride (2) have been prepared. Langmuir monolayers at the air-water interface of 1 and 2 with different anions in the subphase have been characterized by pi-A compression isotherms and Brewster angle microscopy (BAM). The transferred LB films have been characterized by IR, UV-vis and emission spectroscopy, and atomic force microscopy (AFM). Electroluminescent devices formed by LB films containing alternating layers of these two molecules show dual emission by simple mixing of the two emitters in a single LB film, and by preparing two stacked configurations, in which a LB layer of the ruthenium complexes is deposited on top of a LB layer of the iridium complexes and the inverse situation. The color of the electroluminescence can be tuned by changing the thickness of each LB layer. Due to efficient hole blocking of a layer of the iridium complexes when deposited on top of the layer of ruthenium complexes, in that configuration the green emission of the iridium complexes is suppressed. In the opposite case, excitons are generated in both layers although most likely preferentially in the layer of the iridium complexes.

Insight into the Permeation Barrier of Glued Langmuir−Blodgett Bilayers

Permeation measurements have been made for He, CO2, and N2 across single Langmuir-Blodgett (LB) bilayers derived from 1,2,4,5-tetrakis[(N-(undecanoamidoethyl)-N,N-dimethyl ammonium) methyl]benzene tetrabromide (2) and 1,2,4,5-tetrakis[(N-(perfluoroundecanoamidoethyl)-N,N-dimethyl ammonium)methyl]benzene tetrabromide (3) in the absence and in the presence of entrapped poly(acrylic acid) (PAA). In the absence of PAA, single LB bilayers of 3 show a higher permeance for He and N2 but a lower permeation rate of CO2, as compared with analogous LB bilayers made from 2. The relatively low permeation rate of CO2 for the former has been attributed to reduced associative interactions with the fluorocarbon-rich bilayer. The same behavior has also been observed for LB bilayers containing PAA, formed under conditions that yield glued bilayers of 2 and 3 having similar diffusional pathways, as judged by He/N2 selectivities. These results, together with the fact that glued bilayers of 2 (having a thinner PAA layer as compared with those made from 3) exhibit lower He and N2 permeances, provide compelling evidence that the main barrier for gas transport is the combination of surfactant plus PAA and not simply a thin PAA layer that is encased within the surfactant bilayer.

Langmuir−Blodgett Films Formed by Continuously Varying Surface Pressure. Characterization by IR Spectroscopy and Epifluorescence Microscopy

Monolayer films of phospholipids at the air-water interface have been transferred to solid substrates under conditions of continuously varying surface pressure, an approach termed COVASP. The molecular and supramolecular properties of the film constituents have been characterized with two complementary techniques. IR spectroscopy was used to monitor chain conformation as a function of transfer surface pressure. Results were compared to those from Langmuir films determined directly at the A/W interface by IR reflection-absorption spectroscopy (IRRAS). The methylene stretching frequencies for both proteated and acyl chain perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC and DPPC-d62) in the transferred molecules indicate that the phospholipids retain at least, in part, their surface pressure-dependent chain-conformational order characteristics. The line widths of these modes are somewhat reduced, suggestive of slower rates of reorientational motion in the Langmuir-Blodgett (LB) films. Epifluorescence microscopy reveals a progressive condensation gradient, including nucleation and growth of probe-excluding condensed domains along the transfer line. DPPC condensation, observed along a single LB film, was qualitatively comparable to compression-driven condensation as observed in situ or in conventional LB films transferred at constant pressures. However, condensation along the compression isotherm in COVASP-LB films was reduced by 15-20% as compared to films equilibrated at different constant pressures, probably the result of kinetic differences in equilibration processes. As a preliminary demonstration of the utility of this new approach, the monolayer --> multilayer transition known to occur (Eur. Biophys. J. 2005, 34, 243) in a four-component model for pulmonary surfactant has been examined. IR parameters from both the lipid and the protein constituents of the film all indicate that the transition persists during the transfer process. This new approach for the study of transferred films will permit the efficient characterization of lipid-protein interactions and structural transitions occurring in pulmonary surfactant films subjected to dynamic compression.

Sticky Monolayers and Defect-Free Langmuir−Blodgett Bilayers Using Poly(acrylamide) Glue

The introduction of poly(acrylamide) (PAM, Mw 10 000) to an aqueous subphase resulted in increased intermolecular association in the monolayer formed from 5,11,17,23,29,35-hexaamideoxime-37,38,39,40,41,42-hexakis(1-hexadecyloxy) calix[6]arene (2) at the air−water interface, as evidenced by surface viscosity measurements. Langmuir−Blodgett (LB) bilayers of 2, which were deposited onto silylated silicon wafers, showed the presence of PAM, as determined by X-ray photoelectron spectroscopy (XPS). On the basis of XPS analysis at two different takeoff angles (15 and 75°), both the amidoxime groups of the calix[6]arene and the amide groups of PAM, appear to be buried within the LB bilayer. Deposition of single LB bilayers of 2 containing PAM onto poly[1-(trimethylsilyl)-1-propyne] (PTMSP) supports and measurement of their their permeability with respect to He, N2, and CO2 showed a substantial improvement in their permeation selectivity properties, compared with analogous membranes that were devoid of PAM. Taken together, these results indicate that the use of nonionic, water-soluble polymers that are capable of hydrogen bonding with surfactant monolayers have considerable promise as “gluing” agents for enhancing the cohesiveness and quality of corresponding LB films.

Infrared Study of the Molecular Orientation in Ultrathin Films of Behenic Acid Methyl Ester: Comparison between Single Langmuir−Blodgett Monolayers and Spin-Coated Multilayers

Single Langmuir−Blodgett (LB) monolayers of behenic acid methyl ester (BAME) were studied by infrared spectroscopy. Anisotropic optical constants of this system were calculated from a perpendicular-polarized attenuated total reflectance spectrum and a parallel-polarized reflection−absorption spectrum and were compared to those obtained, in a previous article, for spin-coated multilayers (Pelletier, I.; Bourque, H.; Buffeteau, T.; Blaudez, D.; Desbat, B.; Pezolet, M. J. Phys. Chem. B 2002, 106, 1968). These optical constants were used to calculate the tilt angle of the alkyl chains in both types of films deposited on solid substrates and to simulate polarization modulation reflection−absorption spectra of the Langmuir film at the air/water interface. The results indicate that the molecular tilt angle is near 30° for spin-coated multilayers, while single LB monolayers have a tilt angle of 11 ± 2°. A similar orientation of the alkyl chains is obtained on single monolayers deposited onto solid substrates or s...

Molecular Recognition Capabilities of a Nucleolipid Amphiphile (3‘,5‘-Distearoyl)-2‘-Deoxythymidine to Adenosine at the Air/Water Interface and Langmuir−Blodgett Films Studied by Molecular Spectroscopy

Monolayer behavior of a nucleolipid amphiphile, (3‘,5‘-distearoyl)-2‘-deoxythymidine, on pure water, aqueous 1 mM and 5 mM adenosine solutions was investigated by means of surface pressure−molecular area (π−A) isotherms, which indicate that both hydrogen-bonding and stacking interactions are present between the (3‘,5‘-distearoyl)-2‘-deoxythymidine monolayer and adenosine in subphase, and the recognition of this amphiphile to adenosine is more complete in higher subphase concentration. The ultraviolet−visible (UV−vis), Fourier transform infrared (FTIR) transmission, FTIR-attenuated total reflection (ATR), and Fourier transform surface-enhanced Raman scattering (FT-SERS) spectroscopic results indicate that the adenosine molecules in the subphase can be transferred onto solid substrates by Langmuir−Blodgett (LB) technique as a result of the formation of Watson−Crick base-pairing at the air/water interface. The regular and closed packing of the constituent molecules facilitates the photodimerization of the thymine moieties in the headgroup under ultraviolet irradiation even at room temperature. FT-SERS technique, for the first time, was introduced into the research area of molecular recognition occurring at an interface system. High-quality FT-SERS spectra of single LB monolayers obtained in the 1750−500 cm-1 region, together with the FTIR-ATR results in the NH-stretching high-frequency region, provide direct evidences for the formation of multiple complementary hydrogen bonds between the base pairs.

A Single Langmuir−Blodgett Monolayer for Gas Separations

This paper reports the synthesis of novel composite membranes that use a single Langmuir-Blodgett (LB) monolayer as a separating layer. These membranes have been fabricated by using poly[1-(trimethylsilyl)-1-propyne] (PTMSP) as support materials, {alpha}-cyclodextrin/sodium phenoxide or {beta}-cyclodextrin/sodium phenoxide as a gutter layer, and a single monolayer of a calix[6]arene as the primary barrier for transport.

Study of the Conformational Structure and Cluster Formation in a Langmuir−Blodgett Film Using Second Harmonic Generation, Second Harmonic Microscopy, and FTIR Spectroscopy

Nonlinear second harmonic generation (SHG), second harmonic microscopy (SHM), and infrared spectroscopy are used to determine the structural and optical properties of the Langmuir−Blodgett (LB) monolayer assemblies of NLO-active, 4-eicosyloxo-(E)-stilbazolium iodide (4-EOSI) on a glass substrate. The packing characteristics of the pretransferred interfacial films are determined using π−A isotherm measurements. The molecular coverage of the transferred films is determined by ellipsometry. The films formed on both sides of the glass substrate show substantial second harmonic (SH) conversion from p-polarized 1064 nm fundamental excitation. The SHG and FTIR measurements imply that the single LB layer on glass is composed of oriented clusters of 4-EOSI molecules that are laterally discontinuous. Ordered clusters up to 40 μm in diameter are directly observed using SHM. Subsequent LB transfers using the same 4-EOSI molecule or an amphiphile of comparable chain-length (eicosanoic acid) fill in the unoccupied vacancies in the first layer. The magnitude of the dominant element of the nonlinear susceptibility and the average molecular orientation angle of the chromophore are determined by modeling the characteristic SHG Maker fringes.

Minority-carrier MIS tunnel diode hydrogen sensors

The electrical characteristics of Pd/LB film/n-Si tunnel diode hydrogen sensors containing one and three cadmium stearate Langmuir-Blodgett (LB) films are reported. The devices exhibit good rectification behaviour. However, the IV characteristics are complex and deviate considerably from ideal Schottky-barrier theory. The semiconductor surface appears to be inverted at zero bias. Saturation of the forward current is attributed to tunnel-limited transport in the case of single LB film devices and to insulator-limited conduction in the case of devices containing three LB films. Exposure to hydrogen causes pronounced degradation of the diode behaviour. In particular, the forward IV characteristics shows an IαV 2 behaviour that cannot be satisfactorily explained. A plausible explanation, however, is offered for much of the observed data based upon a minority-carrier metal-insulator-semiconductor (MIS) tunnel diode model. However, the full range of behaviour observed cannot be treated adequately by any one theoretical model.

Surface-enhanced Raman spectra of langmuir-blodgett monolayers of (hexadecamethyltetralino) porphyrazines

Langmuir monolayers of metal-free tetra(1,1,4,4-tetramethyl-6,7-tetralino) porphyrazine (HDMTLPH2) and its copper complex (HDMTLPCu) were formed on a water subphase. The surface pressure-area isotherms were measured and the floating monolayers were transferred to various substrates. The spectra of Langmuir-Blodgett (LB) films deposited on metal surfaces were studied. Surface-enhanced Raman spectra of a single LB monolayer of HDMTLPH2 and HDMTLPCu deposited on rough surfaces (silver-coated tin spheres and island films of silver and gold) are reported. Reflection absorption Fourier transform IR spectra of LB films on smooth silver are also presented. The spectroscopic characterization includes the UV-visible spectra of multilayer films on glass slides, solutions and the bulk materials. Characteristic vibrational wavenumbers are listed and used to discuss molecular orientation in the LB assemblies.

Low temperature SERRS and fluorescence spectra of a Langmuir-Blodgett monolayer of N-hexyl, perylenetetracarboxylic monoimide

Surface-enhanced resonant Raman scattering (SERRS) and fluorescence of N-hexyl, perylenetetracarboxylic monoimide (HPTCO) were measured at room, liquid nitrogen and liquid helium temperatures. The sample was a single Langmuir-Blodgett (LB) of HPTCO and arachidic acid mixed in a 1:10 ratio. For comparison, similar measurements were carried out with a mixed LB monolecular layers of N-hexyl, N′-ethyl-3,4:9,10-perylenetetracarboxylic diimide (HPTCDE). The objective of the present study was to examine spectral features that could help in the structural assignment of molecular aggregates. SERRS of the mixed LB layers was dominated by the inelastic scattering of aggregates. There was experimental evidence that the excimer fluorescence decreases at low temperatures.

Hole-burning spectroscopy of a Langmuir-Blodgett monolayer of etioporphyrin and octaethylporphyrin

Langmuir-Blodgett (LB) mixed monolayers of octaethylporphyrin (OEP) and etioporphyrin I (EI) and arachidic acid were transferred to glass slides and glass slides coated with a silver island film. Persistent spectral holes in the region of the S 1←S 0 electronic transition of a single LB monolayer of OEP and EI were observed at 4.2 K. No significant differences between holewidths of a monolayer on glass and of a monolayer on a Ag island film were observed. From the measurements of the holewidths it was concluded that the homogeneous width of the S 1←S 0 electronic transition of both OEP and EI in LB films should be less than 0.5 cm −1. The bandwidth measured in the absorption spectra were 42 cm −1 and ca. 150 cm −1 for OEP and EI respectively. The large FWHM values were attributed to inhomogeneous broadening. The electronic spectra of OEP and EI monolayer (measured at 4.2 K) were not significantly changed by the presence of the Ag island film. Surface enhanced Raman scattering and fluorescence spectra were also measured.

Atomic force microscopy of Langmuir-Blodgett monolayers and bilayers

Phospholipid monolayers and bilayers prepared by the Langmuir-Blodgett technique are simplified model systems of biological membranes. The molecular organization of these monolayers and bilayers has been intensely studied as they are excellent tools for understanding biological processes such as adhesion, fusion, protein structure and organization, etc. Recent structural studies have included electron diffraction of monolayers deposited on substrates and X-ray diffraction of monolayers at the air-water interface. Both diffraction techniques suggest that in the gel phase, the lipid hydrocarbon chains possess long range orientational order, but relatively short range positional order. As a result, L-B films in the gel state closely resemble the “hexatic” phases better known in thermotropic liquid crystals. However, both shortrange and long-range order have not yet been studied in a single Langmuir-Blodgett film.