Langmuir-Schaefer Films Research Articles

Biomimetic collagen I and IV double layer Langmuir–Schaefer films as microenvironment for human pluripotent stem cell derived retinal pigment epithelial cells

The environmental cues received by the cells from synthetic substrates in vitro are very different from those they receive in vivo. In this study, we applied the Langmuir–Schaefer (LS) deposition, a variant of Langmuir–Blodgett technique, to fabricate a biomimetic microenvironment mimicking the structure and organization of native Bruch's membrane for the production of the functional human embryonic stem cell derived retinal pigment epithelial (hESC-RPE) cells. Surface pressure-area isotherms were measured simultaneously with Brewster angle microscopy to investigate the self-assembly of human collagens type I and IV on air-subphase interface. Furthermore, the structure of the prepared collagen LS films was characterized with scanning electron microscopy, atomic force microscopy, surface plasmon resonance measurements and immunofluorescent staining. The integrity of hESC-RPE on double layer LS films was investigated by measuring transepithelial resistance and permeability of small molecular weight substance. Maturation and functionality of hESC-RPE cells on double layer collagen LS films was further assessed by RPE-specific gene and protein expression, growth factor secretion, and phagocytic activity. Here, we demonstrated that the prepared collagen LS films have layered structure with oriented fibers corresponding to architecture of the uppermost layers of Bruch's membrane and result in increased barrier properties and functionality of hESC-RPE cells as compared to the commonly used dip-coated controls.

Water-binding phospholipid nanodomains and phase-separated diacylglycerol nanodomains regulate enzyme reactions in lipid monolayers.

Phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) nanodomains covered with bound water as well as diacylglycerol 1-palmitoyl-2-oleoyl-sn-glycerol (POG) nanodomains separated from a lipid membrane were studied, using monolayer surfaces of POPC hydrolyzed by phospholipase C (PLC). The investigation was based on the analysis of compression isotherms and on atomic force microscope (AFM) observations of Langmuir-Blodgett (LB) films and Langmuir-Schaefer (LS) films. The results included reaction rate constants obtained by kinetic analysis of phosphocholine at surface pressures from 0.1 to 31 mN/m and determined by a luminol-enhanced chemiluminescence method. Monolayer elastic modulus values and fluorescence microscopic images confirmed that hydrolysis by PLC progressed in the intermediate monolayer between a liquid-expanded (L1) film and a liquid-condensed (L2) film at 2-17 mN/m. Furthermore, the intermediate film was confirmed to consist of L1 film and the POPC nanodomains in the L2 state are covered with bound water, conclusions based on the following AFM results: (1) nanodomains in POPC LS films were catalyzed by PLC, (2) POG nanodomains extended out from LB films of mixed POPC/POG 9/1 (mol/mol) monolayers, and (3) POPC LS films were covered with bound water, as indicated by cross-sectional analysis. At the optimal surface pressure of 10 mN/m, when POPC nanodomains (L2), with internal diameters of ∼75 nm, were hydrolyzed by PLC, they shrank down into pockets of the same size as those that appeared with POG. The resulting pocket sizes on LS films were in agreement with POG nanodomain sizes on LB films. This study demonstrated that PLC reacted with POPC nanodomains (L2) dispersed in L1/L2 mixed phase monolayers selectively and that POG nanodomains were phase-separated from the monolayer as hydrolysis proceeded.

Surface plasmon coupled metal enhanced spectral and charge transport properties of poly(3,3'''-dialkylquarterthiophene) Langmuir Schaefer films.

The coupling of organic molecule excitons with metal nano-structure surface plasmons can improve the performance of optoelectronic devices. This paper presents the effect of localized silver metal surface plasmons on spectral as well as charge transport properties of ordered molecular Langmuir Schaefer (LS) films of a fluorescent conducting multifunctional organic polymer: poly (3,3'''-dialkylquarterthiophene) [PQT-12]. The stability and thickness of the PQT-12 LS film were studied by the pressure vs. area isotherm curve. Atomic force microscopy images indicate the formation of a smooth ordered polymer thin LS film of PQT-12 over silver nanostructure island films [SNIF] (∼40 to 50 nm in size). Raman, electronic absorption and fluorescence spectral measurements of the PQT-12 LS film, near SNIF i.e. the near field, show a plasmon coupled enhancement of ∼13 fold in the intensity of Raman bands along with a two-fold enhancement in the absorption band (531 nm) and a six-fold enhancement in the fluorescence band (665 nm) coupled with a decrease in fluorescence decay time with improved photostability as compared to an identical control sample containing no SNIF i.e. the far field condition. These results indicate the formation of a plasmon coupled unified fluorophore system due to adsorption of the PQT-12 LS film over SNIF. The effect of plasmonic coupling is also studied by applying an electric field in sandwiched structures of Al/PQT-12 LS/SNIF/ITO with respect to Al/PQT-12 LS/ITO. Nearly three orders of magnitude enhancement in the current density (J-V plot) of the PQT-12 LS film is observed in the presence of SNIF, which further increases, on illuminating the film by green laser light [532 nm], while the fluorescence intensity and decay time decrease. X-ray photoelectron spectroscopic measurements of SNIF also show a red shift in 3d3/2 and 3d5/2 transitions of silver in the PQT-12 coated LS film, which indicates partial charge transfer from the PQT-12 polymer backbone to SNIF and causes an enhancement in conductivity. This again supports the formation of a field controlled radiating plasmon coupled fluorophore unified system. These findings show greater potential in developing a voltage controlled high photon flux electroluminescent material for multifarious applications.

Self-oriented β-crystalline phase in the polyvinylidene fluoride ferroelectric and piezo-sensitive ultrathin Langmuir-Schaefer film.

We report on the direct observation of ferroelectric switching and piezoelectric behaviour in ultrathin polyvinylidene fluoride (PVDF) films prepared by horizontal Langmuir-Schaefer (LS) technique. We have prepared pure β-phase by just increasing the number of LS layers without using additional non-ferroelectric assisting agents. Edge-on oriented CH2-CF2 units of PVDF at the air-water interface enable self-orientation of ferroelectric dipoles by means of the hydrogen bonding network. Such restricted conformation of PVDF at the air-water interface results in an increased net dipole moment with the number of LS layers. The film's ferroelectric switching and piezoelectric sensitivity are demonstrated by hysteretic polarization switching loops and butterfly-loops, respectively. Successful circular domain writing on ultrathin LS film, down to 5 monolayers of PVDF, is demonstrated. The achievement of pure β-phase of PVDF at room temperature without using any assisting agents may be promising for non-volatile memory and piezoelectric-based, ultrathin smart sensor devices.

Supramolecular arrangements of an organometallic forming nanostructured films

Organometallic materials have become subject of intensive research on distinct technological applications. The Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) techniques have proven to be suitable to address challenges inherent to organic devices, in which the film properties can be tuned at molecular level. Here, we report on the supramolecular arrangement of zinc(II)-protoporphyrin(IX) dimethyl ester (ZnPPIX-DME) using the Langmuir, LB and LS techniques, leading to nanostructured films. The π-A isotherms showed that π-π stacking interaction among ZnPPIX-DME molecules takes place at the air/water interface, favoring the formation of Langmuir films closely packed. The controlled growth of the LB and LS films was monitored via UV-Vis absorption spectroscopy, with the thickness per monolayer within 1.3 and 1.7 nm. The homogeneous topography found at microscale is no longer preserved at nanoscale, which is found rougher according to AFM data. The FTIR indicated that the ZnPPIX-DME is isotropically arranged on both LB and LS films.

Characterization of Langmuir monolayer, Langmuir–Blodgett and Langmuir–Schaefer films formed by POSS compounds

The properties of Langmuir monolayers and Langmuir–Blodgett as well as Langmuir–Schaefer films formed by two open-cage polyhedral oligomeric silsesquioxanes (POSS): 1,3,5,7,9,11,14-hepta-isooctyltricyclo [7.3.3.15,11] heptasiloxane-endo-3,7,14-triol (TSiO-POSS) and 1,3,5,7,9,11-octaisobutyltetracyclo [7.3.3.15,11] octasiloxane-endo-3,7-diol (DSiB-POSS) were studied. Both compounds were shown to form stable monolayers at the air/water interface. The morphology of the monolayers was visualized with a Brewster Angle Microscopy (BAM). The results obtained indicated that TSiO-POSS molecules formed dimers whereas in the case of DSiB-POSS was found the network structure, which was seen on the water/air interface as well as on L-S and L-B thin films. The difference in the interfacial behavior of POSS considered results from alkyl chains length and number of hydroxyl groups in the POSS molecule. The formation of network structure and presence of POSS dimers after transfer of the films on the solid substrates were reported.

Improving the selectivity of a free base tetraphenylporphyrin based gas sensor for NO2 and carboxylic acid vapors

Langmuir-Schaefer (LS) films of a free base porphyrin 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO) are used in conjunction with a PMMA and carboxylic acid substituted calix[8]arene barrier layers deposited on top of the porphyrin films to enhance the selectivity of this class of vapor sensor. Initially, the monolayer properties of EHO without a barrier layer are investigated by reporting Π-A isotherms and the effect of transfer pressure on the gas sensing mechanism is studied. Optimal surface pressures for transferring the EHO films onto glass substrates and the number of layers to be transferred for gas sensing applications are presented. The prospect of tuning these vapor sensors in order to differentiate between large and small analyte molecules is investigated by applying overlying barrier layers which consists of a PMMA:calix[8]arene mixtures. EHO LS films with different barrier layers are exposed to NO2, acetic acid, butyric acid and hexanoic acid vapors. EHO films without any barrier layer are found to be highly sensitive to all four analytes studied; but the respective response speed depends on the vapor being sensed. Upon addition of the PMMA:calix[8]arene barrier layers on top of the EHO films this selectivity is further enhanced. The PMMA:calix[8]arene barrier layer acts as a size selective layer and the degree of selectivity is determined by the relative quantity of calix[8]arene molecules in the selective layer. For all the sensors tested recovery is achieved by gently heating, and the reversibility of sensor material is shown to be very good.

Structural and electrochemical properties of lutetium bis-octachloro-phthalocyaninate nanostructured films. Application as voltammetric sensors.

Thin films of the bis[2,3,9,10,16,17,23,24-octachlorophthalocyaninate] lutetium(III) complex (LuPc2Cl32) have been prepared by the Langmuir-Blodgett and the Langmuir-Schaefer (LS) techniques. The influence of the chlorine substituents in the structure of the films and in their spectroscopic, electrochemical and sensing properties has been evaluated. The π-A isotherms exhibit a monolayer stability greater than the observed in the unsubstituted analogue (LuPc2), being easily transferred to solid substrates, also in contrast to LuPc2. The LB and LS films present a linear growth forming stratified layers, monitored by UV-VIS absorption spectroscopy. The latter also revealed the presence of LuPc2Cl32 in the form of monomers and aggregates in both films. The FTIR data showed that the LuPc2Cl32 molecules present a non-preferential arrangement in both films. Monolayers of LB and LS were deposited onto 6 nm Ag island films to record surface-enhanced resonance Raman scattering (SERRS), leading to enhancement factors close to 2 x 10(3). Finally, LB and LS films deposited onto ITO glass have been successfully used as voltammetric sensors for the detection of catechol. The improved electroactivity of the LB and LS films has been confirmed by the reduction of the overpotential of the oxidation of catechol. The enhancement of the electrocatalytic effect observed in LB and LS films is the result of the nanostructured arrangement of the surface which increases the number of active sites. The sensors show a limit of detection in the range of 10(-5) mol/L.

Discrimination of mercuric ions in piezoelectric sensors with a conjugated polymeric active layer.

The synthetic conjugated poly(1,4-arylene-2,5-thienylene) containing benzo[c][2,1,3]thiadiazole monomeric units (Bz-PAT) is proposed as active layer for the selective detection of mercuric ions. The Bz-PAT polymer chemical structure induces the formation of a disordered film with numerous vacancies and the size of these defects could be exploited for a reversible trapping of mercuric ions. For these reasons the Langmuir-Schaefer (LS) deposition method has been employed for transferring Bz-PAT layers with the desired accurate bi-dimensional organization control of the layer and with a high control of the deposition parameters. In this contribution, the frequency variation of quartz crystal microbalances functionalized with 10, 20, 30 and 40 LS runs of Bz-PAT have been investigated in response to the injection of aqueous solutions of HgCl2, Pb(NO3)2, NiCl2, CdCl2 and ZnSO4 at different concentrations (0.5 mM, 1 mM, 5 mM). An almost linear dependence on the number of the LS layers and hence on the film thickness, measured by means of ellipsometric spectroscopy, has been found in terms of sensor response to concentration of Hg2+ ions fluxed. By means of UV-Vis spectroscopy, the variations in the π-π* absorption band of the polymer, attributed to the thiophene segment, induced by HgCl2 injection has been analyzed and explained as a consequence of the electron transfer from the mercuric ion to the polymer solid film. These results, together with the linear relation found between the number of deposited layers and LS film thickness, suggest that the sensing mechanism can be explained both by an electron interaction between active layer and analyte and a diffusion mechanism of Hg2+ into the solid film that reaches an asymptotic value at 30 runs (about 80 nm), then a higher number of layers does not influence the sensor sensibility.

Langmuir and Langmuir–Schaefer Films of Poly(3-hexylthiophene) with Gold Nanoparticles and Gold Nanoparticles Capped with 1-Octadecanethiol

This paper reports on the production of Langmuir films of regioregular poly(3-hexylthiophene) (P3HT) and their characterization by surface pressure isotherms and UV–vis in situ spectroscopy aiming at the best spreading conditions. Hybrid films containing the polymer and gold nanoparticles were obtained through the use of the optimized parameters for neat P3HT. Two types of hybrid films with P3HT and gold nanoparticles were studied. In the first type, gold nanoparticles in an aqueous solution were used in a subphase of a Langmuir trough and the polymer spread on the air/water interface. In the second type, 1-octadecanethiol capped gold nanoparticles (AuNpOctathiol) were used in a mixture with P3HT in chloroform and spread on the air–water interface. The Langmuir films were transferred to several solid substrates by the Langmuir–Schaefer (LS) method. The surface morphology was characterized by Scanning Electron Microscopy with Field Emission Guns (SEM-FEG), and the presence of gold nanoparticles in the films was confirmed. The growth of the layers in the films was monitored by transmission UV–vis spectroscopy, and the existence of optical anisotropy was investigated by polarized UV–vis spectroscopy. The results imply that the polymer backbone adopts a preferential orientation during the compression, which has a strong component parallel to the trough barrier, and is influenced by the presence of the nanoparticles in the films. The photostability of the films was studied by their exposition to a white light, and the hybrid films formed with P3HT and AuNpOctathiol showed an increase in the photostability, in comparison to the neat P3HT. Such an increase indicates that somehow the AuNpOctathiol molecules provide some protection against the photodegradation of P3HT.

Formation of a stable polymer blue phase under UV irradiation of Langmuir-Schaefer films of diin N-arylcarbamate derivative

Polymerization under UV irradiation is studied in monolayer Langmuir-Schaefer films of diin N-arylcarbamate derivative. The formation of a stable polymer blue phase of diacetylene derivative stable against excess UV light and heating to a temperature of 120 °C was detected. The formation of the structurally ordered Langmuir monolayer of monomers is studied using Brewster angle microscopy.

Spectroscopic Investigation of the Selective Interaction of Mercuric and Cupric Ions with a Porphyrin Active Layer

The mechanisms of interaction between a tetrapyridyl-substituted porphyrin and Hg(II) and Cu(II) ions have been investigated by means of different spectroscopic techniques. Reflection spectroscopy investigations of the porphyrin Langmuir floating film and by polarization modulation infrared (IR) reflection adsorption spectroscopy at the air–water interface provided evidence of the active role played by the pyridyl substituents of the porphyrin in the interaction with the analyte. Such behavior seems to be very selective toward Cu2+ and Hg2+ ions, as demonstrated by the IR measurements in difference mode. UV–vis and IR characterizations suggest a deeply different interaction between the active molecules and the two analytes. In fact, the interaction of Hg2+ ions with the tetrapyrrolic derivative molecules involves both the pyridyl substituents and the central bite of the ring. On the other hand, in the case of Cu2+ ions, spectroscopic evidence suggests that the cupric ions interact with only the porphyrin peripheral substituents. A relevant fluorescence quenching of the Langmuir–Schaefer (LS) film is observed when even a 0.5 nM HgCl2 or 0.5 nM CuCl2 aqueous solution is fluxed on the LS film.

Langmuir–Schaefer Films for Aligned Carbon Nanotubes Functionalized with a Conjugate Polymer and Photoelectrochemical Response Enhancement

Single-walled carbon nanotubes (SWCNTs) were suspended in 1,2-dichloroethane by noncovalent functionalization with a low-band-gap conjugated polymer 1 alternating dialkoxyphenylene-bisthiophene units with benzo[c][2,1,3]thiadiazole monomeric units. The suspended 1/SWCNT blend was transferred onto different solid substrates by the Langmuir-Schaefer deposition method, resulting in films with a high percentage of aligned nanotubes. Photoelectrochemical characterization of 1/SWCNT thin films on indium-tin oxide showed the benefits of SWCNT alignment for photoconversion efficiency.

Molecular orientation of Ru(II) complexes introduced in hybrid Langmuir–Schaefer films of clay nanosheets and alkylammonium cations

Ternary component hybrid films of amphiphilic alkylammonium cations, clay nanosheets, and ruthenium(II) complex cations were prepared by the modified Langmuir–Schaefer method. When a solution of the amphiphilic ammonium salt (octadecylammonium chloride, ODAH+Cl−) was spread at an air–clay dispersion interface, the clay nanosheets in the dispersion were adsorbed on the floating molecular film of ODAH+ by the electrostatic interaction. Then the floating film of ODAH+ and the clay was deposited on a glass plate and immersed in an aqueous solution of [RuL3]Cl2 (L=2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 5,6-dimethyl-1,10-phenanthroline (dmphen), and 4,7-diphenyl-1,10-phenanthroline (dpp)) to form a ternary component hybrid monolayer. By repeating these procedures, a hybrid multilayer was fabricated. Characterization of the hybrid multilayers was carried out by electronic and IR spectral measurements and X-ray diffraction (XRD) method. Areas per [RuL3]2+ estimated from the analysis of the electronic spectra suggested that the Ru(II) complex cations interacted with each other through hydrophobic interaction. IR and XRD data indicated disordered alkylchains of ODAH+ which would occupy spaces between the [RuL3]2+ cations in the film. The tilt angles of 3-fold axes of [RuL3]2+ were determined to be 32–35° by analyzing electronic spectra measured with p- and s-polarized beams. An orientation model of the [RuL3]2+ cations in the film was proposed, in which substitution groups at 4- or 5-positions of 1,10-phenanthroline ligand were in contact with the clay nanosheet layers. It was likely that the sizes of the ligands and the interaction between the ligands affected the structure of the hybrid film.

Registration of the immunological reaction in bionanocomposite Langmuir-Schaefer films

Bionanocomposite Langmuir-Schaefer films containing single-wall carbon nanotubes noncovalently bound to immunoglobulin G are prepared. The possibility of using such films for the registration of antigen-antibody reactions is demonstrated. The interaction is recorded by means of surface plasmon resonance (SPR) and an Agilent 86140B spectrum analyzer. The concentration of the specific antigen studied, bovine serum albumin, ranges within 10−8–10−12 M. Application of several layers of stearic acid between the gold layer and the bionanocomposite layer increases of the sensitivity of the system (SPR method) to the antigen by a factor of 2 to 7.3.

Selective Detection of Calcium Cations on Macromolecular Langmuir Monolayer and Langmuir-Schaefer Films

In this study, we report the interfacial Langmuir monolayer behavior and surface morphological features of a dendronic terthiophene linked via a tetraethylene glycol moiety (3T5O3T) and its complexation with calcium cations. Surface pressure-mean molecular area (pi-A) isotherms show distinct behavior due to the secondary interaction of cations with the ionophore units. Furthermore, two Langmuir-Schaefer (LS) films with and without calcium cations exhibited significantly different surface morphological features from atomic force microscopy (AFM) imaging. Quartz crystal microbalance (QCM) measurements reveal changes in frequency and resistance enabling selective sensing of calcium cations on the LS films. Thus, well-organized 3T5O3T LS films on the solid substrates are potential candidates for highly selective chemosensing.

Supramolecular organization of meso-substituted derivatives of tetraphenylporphine in thin films

Films of meso-substituted derivatives of tetraphenylporphine substituted in the paraor ortho-positions of the phenyl rings with alkoxy groups (-OC4H9 or -OC16H33) and films of their metal complexes are prepared using the Langmuir-Schaefer (LS) method. The effect of the molecular structure on the supramolecular organization of thin films is determined. The films are obtained by transferring layers of the compounds from the surface of the water onto silicon substrates using the Langmuir-Schaefer technique. The structures of the one-component LS films of the investigated compounds are studied by the small-angle X-ray scattering method, and also the lattice periodicities are calculated. The supramolecular organization of the meso-substituted tetraphenylporphine derivatives is modeled and refined by means of the X-ray diffraction method. When Ni or Cu serve as chelating metal, it is found that the macrocycle-macrocycle interaction is so large that it leads to violation of the molecular linearity, i.e., the lateral substituents are folded toward the macrocycle, reducing the area occupied by the structural units.

Porphyrin–polyoxometalate hybrids connected via a Tris-alkoxo linker for the generation of photocurrent

Films on ITO of [N(C4H9)4]5H[P2V3W15O59{(OCH2)3CNHCO(ZnTPP)}] (ZnTPP=zinc(II)-tetraphenylporphyrin) were obtained by the Langmuir–Schaefer inverted transfer method.The films were characterized by UV–vis absorption spectroscopy, XPS, AFM, and electrochemistry. In addition, the photovoltaic performances of the Langmuir–Schaefer films were investigated by photocurrent transient measurements, which show significant photocurrent response.

Formation of two-dimensional (M) and three-dimensional (V) nanoaggregates of substituted cobalt porphyrin in the Langmuir layers and Langmuir-Schaefer films

Floating molecular layers of cobalt tetra(p-metoxyphenyl)porphyrin (CoTMPP) were studied experimentally and theoretically. The structure of the layers was analyzed within the framework of an original model and method for quantitative analysis of compression isotherms of a monolayer comprised of Major nanoaggregates (M-monolayer). The existence regions as well as the structural parameters and properties of monolayers in stable states were determined. These include the size of M-nanoaggregates and the number of molecules in them, the distance between aggregates, the content of bound water and water between aggregates, the compressibility, and the pressure and current surface concentration ranges corresponding to stable states. Quantitative correlations between the fabrication conditions, nanoaggregate size, and properties of M-monolayers were obtained. The state diagrams and M-isotherms were plotted and a passport of CoTMPP floating layers was compiled. A classification of porphyrin M-monolayers including the description of rarefied and condensed M-aggregates in the floating monolayers was proposed; a quantitative model for such monolayers was developed. Constants governing the structure of CoTMPP monolayers and the formation conditions of stable floating layers with 3D nanoaggregates (V-nanoaggregates) were determined. A specific feature of CoTMPP which can discriminate it from azaporphyrins is as follows: the size of M edge-aqua aggregates and the distance between them are independent on the initial surface concentration. CoTMPP thin films on solid substrates were prepared by various methods including the Langmuir-Schaefer technique. A comparison of UV-Vis spectra of the solution, Langmuir-Schaefer films, and spin-cast films suggests different degrees of CoTMPP aggregation in these systems.

Quantification of Ligand Binding to G-Protein Coupled Receptors on Cell Membranes by Ellipsometry

G-protein-coupled receptors (GPCRs) are prime drug targets and targeted by approximately 60% of current therapeutic drugs such as β-blockers, antipsychotics and analgesics. However, no biophysical methods are available to quantify their interactions with ligand binding in a native environment. Here, we use ellipsometry to quantify specific interactions of receptors within native cell membranes. As a model system, the GPCR-ligand CXCL12α and its receptor CXCR4 are used. Human-derived Ishikawa cells were deposited onto gold coated slides via Langmuir-Schaefer film deposition and interactions between the receptor CXCR4 on these cells and its ligand CXCL12α were detected via total internal reflection ellipsometry (TIRE). This interaction could be inhibited by application of the CXCR4-binding drug AMD3100. Advantages of this approach are that it allows measurement of interactions in a lipid environment without the need for labelling, protein purification or reconstitution of membrane proteins. This technique is potentially applicable to a wide variety of cell types and their membrane receptors, providing a novel method to determine ligand or drug interactions targeting GPCRs and other membrane proteins.