Air-subphase Interface Research Articles

Proline Enantiomers Discrimination by (L)-Prolinated Porphyrin Derivative Langmuir–Schaefer Films: Proof of Concept for Chiral Sensing Applications

A porphyrin derivative functionalized with the L-enantiomer of proline amino acid was characterized at the air–pure water interface of the Langmuir trough. The porphyrin derivative was dissolved in dichloromethane solution, spread at the air–subphase interface and investigated by acquiring the surface pressure vs. area per molecule Langmuir curves. It is worth observing that the behavior of the molecules of the porphyrin derivative floating film was substantially influenced by the presence of L-proline amino acid dissolved in the subphase (10−5 M); on the contrary, the physical chemical features of the floating molecules were only slightly influenced by the D-proline dissolved in the subphase. Such an interesting chirality-driven selection was preserved when the floating film was transferred onto solid supports by means of the Langmuir–Schaefer method, but it did not emerge when a spin-coating technique was used for the layering of the tetrapyrrolic derivatives. The obtained results represent proof of concept for the realization of active molecular layers for chiral discrimination: porphyrin derivatives, due to their intriguing spectroscopic and supramolecular properties, can be functionalized with the chiral molecule that should be detected. Moreover, the results emphasize the crucial role of the deposition technique on the features of the sensing layers.

Interfacial behavior of Lactate Oxidase at Air-Subphase interface

This article investigates the main aspects of the surface chemistry properties of the lactate oxidase (LacOx) enzyme monolayer at the air-subphase interface. Surface chemistry study determined the important properties like the surface packing and stability of the formed layer, whereas the spectroscopic experiments provided information regarding its secondary structure conformation of the enzyme. We have demonstrated that the LacOx in the monolayer form remained active for extended time period. In accordance to the data obtained from the isotherm it was also found that LacOx forms a stable monolayer that does not aggregate at the air-subphase interface. The stability of the monolayer at the air-subphase interface was studied by using compression-decompression cycles which revealed the stability with no significant evidence of aggregates or irreversible domains. This was further confirmed by UV–vis absorption and fluorescence measurements. Spectra from circular dichroism (CD) showed that the LB film retains the characteristic of an α-helix conformation.

Close-Packed Langmuir Monolayers of Saccharide-Based Carbon Dots at the Air-Subphase Interface.

Carbon dots (CDs) are zero-dimensional carbon-based spherical nanoparticles with diameters less than 10 nm. Here, we report for the first time CDs forming stable Langmuir monolayers at the air-subphase interface. Langmuir monolayers are of great interest both fundamentally to study the interactions at the interfaces and for many applications such as the development of sensors. However, CDs usually do not form Langmuir monolayers because of their highly hydrophilic nature. In this study, amphiphilic CDs were prepared through hydrothermal carbonization using saccharides as the precursors. The surface chemistry behavior and optical properties of CDs at the air-subphase interface were studied. CDs derived from saccharides consistently formed stable Langmuir monolayers which show all essential phases, namely, gas, liquid-expanded, liquid-condensed, and solid phases. The compression-decompression cycle method showed minimum hysteresis (4.3%), confirming theretaining capacity of the CDs as a monolayer. Limiting CD areas from surface pressure-area isotherm at the air-subphase interface were used to calculate the average diameter of the CDs at the air-subphase interface. UV/vis absorption spectra of CDs dispersed in water and in Langmuir monolayers had the same bands in the UV region. The intensity of the UV/vis absorption increases with increasing surface pressure at the air-subphase interface. Interestingly, photoluminescence (PL) of the Langmuir monolayer of CDs was excitation-independent, whereas the same CDs had excitation-dependent PL when dispersed in water.

Surface Chemistry and Spectroscopic Study of a Cholera Toxin B Langmuir Monolayer.

In this article, we explored the surface chemistry properties of a cholera toxin B (CTB) monolayer at the air-subphase interface and investigated the change in interfacial properties through in situ spectroscopy. The study showed that the impact of the blue shift was negligible, suggesting that the CTB molecules were minimally affected by the subphase molecules. The stability of the CTB monolayer was studied by maintaining the constant surface pressure for a long time and also by using the compression-decompression cycle experiments. The high stability of the Langmuir monolayer of CTB clearly showed that the driving force of CTB going to the amphiphilic membrane was its amphiphilic nature. In addition, no major change was detected in the various in situ spectroscopy results (such as UV-vis, fluorescence, and IR ER) of the CTB Langmuir monolayer with the increase in surface pressure. This indicates that no aggregation occurs in the Langmuir monolayer of CTB.

β-Galactosidase Langmuir Monolayer at Air/X-gal Subphase Interface.

This article investigates the surface chemistry properties of the β-galactosidase monolayer at the air-subphase interface at the vicinity of its substrate, X-gal. We have demonstrated that the β-galactosidase in the monolayer form remained active and performed hydrolysis of the X-gal in the subphase. We investigated the β-galactosidase Langmuir monolayer in absence and presence of X-gal in the subphase of varying concentration of X-gal with the sodium chloride solution. It was found that the limiting molecular area as well as the collapse surface pressure kept on decreasing with the increasing concentration of X-gal. In accordance to the data obtained from the isotherm it was also found that β-galactosidase forms a stable monolayer that does not aggregate at the air-subphase interface. The stability of the monolayer at the air-subphase interface was studied by using compression-decompression cycles with and without X-gal at varying concentration and different surface pressures. The infrared reflection-absorption spectroscopy (IRRAS) and Brewster angle microscopy (BAM) of β-galactosidase Langmuir monolayer was also investigated for pure and mixed β-galactosidase at the air-subphase.

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.

A comparative study of two amphiphilic merocyanines: from monomers to aggregates in Langmuir and Langmuir–Blodgett mixed films

The Langmuir–Blodgett (LB) films with J aggregates of the amphiphilic MC2 merocyanine have been deeply investigated for more than two decades, mainly because of their interest for applications in photonic devices. To extend the excitation/emission wavelength range, we have studied an homologous dye, MC1, based on a shorter merocyanine chromophore, aiming at checking the ability of this compound to arrange into J aggregates within stable LB films. In this comparative MC1/MC2 investigation, we have addressed the structural and spectroscopic properties of the monomers in solution, the thermodynamic and morphological properties of the Langmuir monolayers at the air–water interface and the spectroscopic and photophysical properties, and the structural features thence obtainable, of the aggregates in the LB films. 1H NMR experiments have shown that, in chloroform solutions, both dyes adopt planar conformations with the hydrophilic and hydrophobic groups pointing to opposite directions. Strongly attractive interactions are exhibited by both dyes towards arachidic acid in monolayers at the air–subphase interface, with a maximum stability for the 1 : 2 MC(1/2) : AA mixture. Relatively homogeneous LB multilayers have been obtained from 1 : 4 and 1 : 2 MC : AA mixtures with good transfer ratios. Absorption and emission of the LB films of MC1 are dominated by J aggregates, while those of MC2 show some H aggregate contributions too. The J aggregates of both compounds exhibit a tendency to align with the transition dipoles along the film dipping direction, only slightly tilted relative to the substrate surface; however, this tendency is more pronounced for the MC1 J aggregates. Overall, MC1 yields slightly more stable monolayers and more solid multilayers compared to MC2.

Human Cardiac Troponin I: A Langmuir Monolayer Study

Human cardiac troponin I (cTnI) is the preferred biomarker in the assessment of myocardial infarction. It is known to interact with troponin C and T to form a trimeric complex. Whereas small amounts are found in the cytoplasm, most of cTnI is in the form of a complex with actin located in myofilaments. To understand these interactions of cTnI better, we first investigated the surface chemistry of cTnI as a Langmuir monolayer spread at the air-water interface. We investigated the optimal conditions for obtaining a stable Langmuir monolayer in terms of changing the ionic strength of the subphase using different concentrations of potassium chloride. Monolayer stability was investigated by compressing the cTnI monolayer to a specific surface pressure and keeping the surface pressure constant while measuring the decrease in the molecular area as a function of time. Aggregation and/or domain formation was investigated by using compression-decompression cycles, in situ UV-vis spectroscopy, Brewster angle microscopy (BAM), and epifluorescence microscopy. To ensure that the secondary structure is maintained, we used infrared reflection-absorption spectroscopy (IRRAS) directly at the air-subphase interface. It was found that cTnI forms a very stable monolayer (after more that 5000 s) that does not aggregate at the air-subphase interface. The cTnI molecules maintain their secondary structure and, on the basis of the low reflectivity observed using BAM measurements and the low reflection-absorption intensities measured with IRRAS spectroscopy, lie flat on the subphase with the alpha-helices parallel to the air-subphase interface.

Effect of the deposition type on the structure of terthiophene-vinylbenzoate Langmuir–Blodgett films

The effect of the deposition type on the resulting overall structure of multilayer Langmuir–Blodgett (LB) films of terthiophene-vinylbenzoate (TSe) embedded in an octadecyl amine (ODA) matrix was studied. In UV–Vis polarized absorption spectra a large hypsochromic shift is observed for the LB-films compared with the solution indicating that TSe exists predominantly as aggregates in the films. In second harmonic generation (SHG) measurements the signal from the Y-type film was very weak as would be expected for an essentially centrosymmetric system. For X- and Z-type films, similar SHG signals were expected since the orientation of the active molecules should be uniform although opposite in these films. However, the SHG signal from the Z-type films was spatially inhomogeneous and much lower than that from the X-type film. The reason for these differences lies in the details of the film deposition process. In the preparation of Z-type films, the long time exposure of the hydrophobic part of the film to the aqueous subphase induces turning over of the molecules. Thus, the monolayer structure achieved at the air-subphase interface breaks down resulting in only partly organized films with decreased SHG intensity. For X-type films no such exposure takes place during the deposition and all TSe molecules remain in the original orientation.

Hydrogenase–poly(viologen) complex monolayers and electrochemical properties in Langmuir–Blodgett films

Abstract Mixed monolayers of a hydrogenase (H 2 ase) purified from Thiocapsa roseopersicina and poly(viologen) or hexadecyltrimethylammonium bromide (HTAB) were formed at the air–subphase interface. A larger H 2 ase-occupied area was measured in the H 2 ase–poly(viologen) or in the H 2 ase–HTAB mixed monolayers than that in the pure H 2 ase monolayer. These monolayers were transferred onto solid supports by the Langmuir–Blodgett (LB) method. Spectroscopic measurement indicated that H 2 ase activity was retained in the LB films. Cyclic voltammograms (CVs) of the H 2 ase–HTAB LB film showed two waves at ∼−560 and ∼−530 mV corresponding to the reduction and oxidation potentials of the [4Fe–4S] 2+/1+ clusters of H 2 ase. For the H 2 ase–poly(viologen) LB films, the CV curves showed two broad waves centered at ∼−510 and ∼−480 mV, which correspond to the reversible one-electron process of poly(viologen) 2+/1+ with a small contribution of the [4Fe–4S] 2+/1+ clusters of H 2 ase. Charge transfer properties of the modified electrodes are discussed by use of chronocoulometry.

Deposition of Langmuir monolayers using conical trough

Deposition of Langmuir monolayers of amphiphiles formed at the air–subphase interface has been done by effecting gradual descent of the Langmuir layer followed by deposition on a substrate kept immersed in the subphase. The conical shape of the indigenously built trough used for this purpose offers additional advantage of gradual radial compression. The films of stearic acid (SA), arachidic acid (AA) and riboflavin esterified with stearic acid (Srbf) thus prepared display excellent homogeneity as judged by atomic force microscopy (AFM). The films made out of Srbf, which contains flavin as a fluorophore, exhibit uniform fluorescence over the entire region as measured using a spectrofluorometer.

Apparatus for the Continuous Monitoring of Surface Morphology via Fluorescence Microscopy during Monolayer Transfer to Substrates

Langmuir monolayers can be characterized at the air- subphase interface with fluorescence microscopy at the micron scale, but must be transferred to solid substrates for higher resolution imaging using atomic force microscopy. To utilize both techniques to characterize the same region of interest or to ensure that the deposition process is artifact-free, we have developed a transfer technique that allows for the continuous optical imaging of surface morphology as the monolayer is transferred from the air-subphase interface to the substrate. The advantages of this deposition technique are that it is simple and inexpensive and can be implemented in almost all Langmuir troughs and that artifacts associated with nonideal transfer can readily be identified. This new transfer technique is ideal for transferring rigid and viscous films that are difficult to deposit using the traditional Langmuir-Blodgett technique. This technique can also be used for transferring monolayers to germanium attenuated total reflection (ATR) crystals for Fourier transform infrared (FTIR)-ATR spectroscopic measurements of protein conformation and lipid orientation while maintaining the desired monolayer morphologies.

Stability and superstructure of squarylium dye TSQ Langmuir–Blodgett films

The stability of monolayer Squarylium Dye TSQ in air-subphase interface was studied. Langmuir–Blodgett films of this dye were deposited on mica and on hydrophobic silicon (100) wafers and a contact mode atomic force microscope (AFM) was used to characterize the structures of these films. It was found that the stability of a TSQ monolayer in the air-subphase interface was increased substantially when cadmium chloride was added to the subphase. Although TSQ dye does not have a long alkyl tail typically required to form a LB film, AFM images of these films and their 2D Fourier Transform analyses show that TSQ LB films possess a triclinic superstructure. Based on the analyses of pressure-area ( π-A) isotherms and AFM images of these films, models of TSQ molecules packing on the subphase were developed.

Scanning probe characterization of Langmuir-Blodgett and self-assembled films of pyrrole derivatives

Atomic force microscopy was used to investigate the space molecular organization of Langmuir-Blodgett (LB) and self-assembled (SA) films of pyrrole derivatives on different substrates. The LB films were obtained from monomers of 3-hexadecyl-pyrrole (3HP); the polymerization of the film was performed directly at the air-subphase interface, and the films were collected on hydrophilic glass. Very homogeneous polymeric and monomeric LB films were collected on glass with a transfer rate near unity, the typical average roughness range was 0.9 ± 0.4 nm, and the quality of films was independent of the dipping speed. A functionalized molecule, 11-(3-pyrrolyl)undecyl mercaptane (3UPSH), was used to modify Ti and TiN electrodes by immersing the substrates in a solution containing the redox active molecule. The modification of the metal surfaces was proved by means of time of flight secondary ion mass spectrometry (TOF SIMS) and AFM characterization.

Manipulation of Multicomponent Langmuir-Blodgett Films of Electrically Conductive Polymers: Polypyrrole and Poly(3-Hexylthiophene)

Abstract Multicomponent Langmuir-Blodgett (LB) films containing poly-pyrrole, 3-octadecanoyl pyrrole, and poly(3-hexylthiophene) were fabricated via a novel in-situ polymerization process. In this process, pyrrole monomer is polymerized at the air-subphase interface in the presence of poly(3-hexylthiophene) by using a subphase containing 1 wt% FeCl3. The resultant electrically conductive films could be readily deposited onto solid substrates as Z-type films by the vertical lifting method. By using visible absorption, x-ray diffraction, and FTIR measurements, it was found that all three components were present within the transferred films, with the polypyrrole chains assuming a preferential orientation parallel to the film surface. The conductivities of the mixed LB films were as high as 1.0 S/cm, and the stability of the conductivity was very good. Chemical doping of poly(3-hexylthiophene) within the film with strong oxidizing agents such as I2 further enhanced the conductivity of the film.