Water Subphase Research Articles

Unraveling Tryptophan Modulated 2D DPPC Lattices: An Approach toward Stimuli Responsiveness of the Pulmonary Surfactant

A molecular understanding on the preferential and selective interactions of L-tryptophan, a major component of surfactant proteins, with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) is important in the metabolic cycle of the pulmonary surfactant. In view of this, interfacial signals of interest in real time were tapped with aligned DPPC monolayers over a physiological tryptophan subphase using extremely surface sensitive 2D vibrational spectroscopy. Polarization-modulated and angle dependent Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) of DPPC monolayers on water and L-tryptophan subphases depicted fine structure/conformation differences in the interaction modes, evidenced from changes in the vibrational band intensities and frequencies under conditions of controlled 2D surface pressure. The computed 1:1 adducts of DPPC/H(2)O and DPPC/tryptophan in support of FT-IRRAS fine structure characteristics demonstrated binding in interfacial DPPC-tryptophan adducts to be driven by cation-π interactions alongside hydrogen bonding of carbonyl and phosphate groups of the lipid with NH(3)(+) of the zwitterionic tryptophan. In situ spectroscopy enabled assignment of relative orientations of the equivalent -CH(2) functional groups from the polarized XY plane transition moments with component intensities of the split orthorhombic CH(2) mode. A larger molecular tilt of 37° for the DPPC monolayer over tryptophan subphase in comparison with that over water (26°) substantiated the DPPC headgroup interaction with tryptophan, complemented through δ (N(+)(CH(3))(3)), ν(as) (PO(2)(-)), ν(s) (PO(2)(-)), ν(as) (C-N(+)-C), and ν (C═O) vibrational features. The IRRAS spectral features of the DPPC 2D condensed phase showed distinct tryptophan-induced temperature dependent lattice phase transitions: hexagonal → orthorhombic → triclinic → hexagonal packing of the hydrocarbon chains was noted over a subphase temperature range from 20 to 43 °C. The temperature dependent 2D DPPC lattice characteristics cited in this work will aid in understanding the impact of a temperature pulse toward the membrane functionality.

Synthesis of pH-responsive polysilane with polyelectrolyte side chains through γ-ray-induced graft polymerization

Polysilanes with polyelectrolyte side chains are synthesized by two methods utilizing γ-ray-induced grafting and the pH responsiveness for one of those polymers is revealed mainly by investigating interfacial behavior of its monolayer at the air/water interface. In the first synthetic method, poly(methyl acrylate) is grafted onto poly(methyl- n-propylsilane) (PMPrS) through γ-ray-induced grafting, and then the PMA chains are hydrolyzed to poly(acrylic acid) resulting in the yield of ca. 97%. Thus PMPrS with polyelectrolyte side chains is successfully synthesized by the graft chain hydrolysis. The other method is the direct grafting of electrolyte monomers. Poly(methacrylic acid)-grafted PMPrS (PMPrS- g-PMAA) can be obtained through γ-ray-induced grafting of methacrylic acid monomers onto PMPrS chains, which shows the effectiveness of radiation grafting for the synthesis of polyelectrolyte graft copolymers. PMPrS- g-PMAA exhibits pH responsive behavior. In addition to the pH-dependence of water solubility, interfacial behavior also depends on the pH. Langmuir monolayers of PMPrS- g-PMAA exhibit different surface pressure–area isotherms according to the grafting yield and the pH of the subphase water. This result suggests that radiation modification is useful for fabricating polysilane-based ordered materials responsive to outer stimuli.

Photofunctional multilayer films by assembling naked silver nanoparticles and a tailored nitric oxide photodispenser at water/air interface

This contribution reports the design, preparation, and characterization of nanostructured hybrid films of silver nanoparticles (AgNPs) and a tailored nitric oxide (NO) photodonor. They were achieved by exploiting effective interfacial interactions between an amino-terminated NO photodonor spread onto water surface and naked AgNPs dissolved in the water subphase. The morphology, the spectroscopic features, and the interaction between the two components in the floating films at the air/water interface were inspected by Brewster Angle Microscopy, UV–Vis reflection, and polarization–modulation infrared reflection–absorption spectroscopy. AgNPs and the NO photodonor were successfully transferred onto hydrophobized quartz substrates by horizontal lifting deposition and the resulting multilayer films were characterized by UV–Vis absorption spectroscopy and transmission electron microscopy, respectively. The results obtained showed the presence of both isolated AgNPs and assemblies of AgNPs having nanodimensional character in the films. The photochemical properties of the NO photodonor were well preserved in the hybrid multilayers. In fact, they were able to release NO under visible light excitation, as unambiguously demonstrated by the direct and in real-time monitoring of this transient species using an ultrasensitive electrode, and the transfer of the released NO to a protein such as myoglobin.

Spectral-selective X-ray methods for structure diagnostics of ordered bioorganic nanosystems on a liquid surface

The X-ray standing wave method has been applied to study the formation of protein-lipid film on the water subphase. The influence of different toxic reagents (heavy metals and urea) on protein-lipid interaction in an ordered film of glucose oxidase and behenic acid has been investigated. Analysis of angular dependence of fluorescence yield provided information on molecular arrangement in bioorganic nanosystems on the water surface, where protein and phospholipid molecules were free to move over the water subphase.

Gold Nanoparticles Grown on Star-Shaped Block Copolymer Monolayers

We report on the growth of gold nanoparticles in polystyrene/poly(2-vinyl pyridine) (PS/P2VP) star-shaped block copolymer monolayers. These amphiphilic PS(n)P2VP(n) heteroarm star copolymers differ in molecular weight (149,000 and 529,000 Da) and the number of arms (9 and 28). Langmuir-Blodgett (LB) deposition was utilized to control the spatial arrangement of P2VP arms and their ability to reduce gold nanoparticles. The PS(n)P2VP(n) monolayer acted as a template for gold nanoparticle growth because of the monolayer's high micellar stability at the liquid-solid interface, uniform domain morphology, and ability to adsorb Au ions from the water subphase. UV-vis spectra and AFM and TEM images confirmed the formation of individual gold nanoparticles with an average size of 6 ± 1 nm in the P2VP-rich outer phase. This facile strategy is critical to the formation of ultrathin polymer-gold nanocomposite layers over large surface areas with confined, one-sided positioning of gold nanoparticles in an outer P2VP phase at polymer-silicon interfaces.

Control of photochemical properties of monolayers and Langmuir-Blodgett films of amphiphilic chromoionophores

Control of the structure of ultrathin films that determine the steric conditions of the passing of intermolecular interactions is one of the most promising methods of implementing the advantages of planar supramolecular systems as basic elements of nanosize information devices. This work studies the behavior of Langmuir monolayers of a new amphiphilic crown-substituted chromoionophore I on a water subphase. It is found that dithiaazacrown ether in molecule I selectively binds Hg2+ cations both in organic solvents and from aqueous subphase. The electronic absorption spectra of the monolayer showed that H-aggregation occurs actively in the course of two-dimensional compression on deionized water, which hinders the complexation process, while the presence of Na+ and Ba2+ cations in the subphase results in the effective inhibition of this aggregation. This conclusion is confirmed by spectral fluorimetric studies of monolayers of a dye with a similar structure that contains a chromophoric group with a much higher fluorescence quantum yield. Monolayer aggregation on deionized water at the surface pressure values of just 4–6 mN/m leads to the three- to fourfold fluorescence quenching, while in the case of subphases containing inert (noncomplementary to the dye ionophoric fragment) cations, the compression of the monolayer to pressures of 25–30 mN/m reduces the fluorescence intensity by no more than 25–35%. It was thus found that variations in the subphase composition allows one to monitor the degree of aggregation of the monolayer and the efficiency of cation recognition.

Physicochemical properties of solutions and ultrathin films of triple-decker gadolinium tetra-15-crown-5-phthalocyaninate

The ability of the triple-decker gadolinium complex with tetra-15-crown-5-phthalocyanine Gd2(R4Pc)3, (R = 15-crown-5) (1) to form monolayers and Langmuir-Blodgett films (LBFs) was studied for the first time. The charge characteristics of molecules of a triple-decker phthalocyaninate in monolayer, as well as their orientation and adhesion to the water subphase, were controlled by changing the surface pressure, pH, and subphase composition (aqueous solutions containing triethylenetetramine (TETA) and metal cations). It was shown that the presence of Na+ and protonated TETA aminogroups in resulted in an increasing limiting monolayer area and significant decreasing of the monolayer liquid state region. It is proven that the observed effects are caused by the conformational and charge transitions of peripheral crown ethers induced by their interaction with cations. A comparison of the differential reflectance spectra of the complex monolayer on the deionized water surface with UV-Vis absorption spectra of three-layer Gd2(R4Pc)3 LBF and complex solution in chloroform shows that partial complex oxidation and intensive stacking formation occurs already at the stage of monolayer formation on the subphase surface. Electrochemical studies of three-layer LBFs performed at the indium-tin-oxide electrodes (ITO-electrodes) using cyclic voltammetry (CV) showed three reversible redox waves in the potential range −200 to +1100 mV (vs. Ag+/AgCl). All registered peaks remain the same position and intensity upon multiple cycling. plasmon resonance (SPR) measurements allow to register three stable redox states of studied LBF upon applying the external potential. Such behavior shows the possibility to use multistep redox transformations of studied complex LBF for developing of stable and reproducible switchable optoelectronic systems.

Interfacial Behavior of OEG–Linear Dendron Monolayers: Aggregation, Nanostructuring, and Electropolymerizability

We report on the interesting interfacial behavior of oligoethylene glycol or OEGylated linear dendron monolayers at the air-water interface as a function of (a) carbazole dendron generation, (b) the length of the OEG units, and (c) the surface pressure applied upon compression. Surface pressure-area isotherms, hysteresis studies, and isobaric creep measurement revealed a structure-property relationship consistent with the hydrophilic-lipophilic balance of a linear dendron with the OEG group serving as the surface anchor to the water subphase. AFM studies revealed that all the OEGylated carbazole dendrons self-assemble into spherical morphology at low surface pressures but form ribbonlike structures as the surface pressure is increased. This nanostructuring is primarily imparted by the increase in van der Waals forces with increasing amount of carbazole units per dendron generation on a hydrophilic mica surface. Further, electrochemical cross-linking of the carbazole molecules by cyclic voltammetery (CV) on doped Si wafer has enabled the formation of an LB film monolayer with a secondary level of organization in the monolayer imparted by the inter- and intramolecular cross-linking among the carbazole units. This study should provide a basis for monolayer film materials based on combining the LB technique and electrochemical cross-linking for nanostructuring superstructures at the air-water interface.

The kinetics of Langmuir–Blodgett ZnO nano-film deposition for sensing electrodes

The kinetics of the monolayer formation of zinc oxide nanoparticles on water subphase was studied at different sample concentrations and barrier speeds. The recorded isotherms indicate that stearic acid can act as a suitable microenvironment for the deposition of zinc oxide when the spreading solution contains ~ 50% sample. The AFM image of the double layer shows that the film is nearly homogeneous with average thickness ~ 15 nm. The sensing ability of the nano-films was studied by cyclic voltammetry using ITO substrates, coated with zinc oxide nanoparticles as working electrode. It gave different reduction peaks for different pH electrolytes.

Nanopatterns of ABA triblock copolymer thin films formed on water surface

Nanopatterns of hydrophobic triblock copolymer SEBS thin films formed on a water surface by using drop spreading casting and dilute solution casting methods have been studied. It is found that the surface morphologies of thin films, as well as the spreading behavior of polymer solutions on water, depend strongly on the selectivity of the solvent and the functional group which interacts with the water subphase. The resulting nanopatterns were examined in terms of the relative interaction parameter (Δ χ) and the copolymer volume fraction ( ϕ) in the solution, based on the physics of solvent annealing and evaporation.

Study of relaxation process of dipalmitoyl phosphatidylcholine monolayers at air–water interface: Effect of electrostatic energy

The instability of organic monolayer composed of polar molecules at the air-water interface has been a spotlight in interface science for many decades. However, the effect of electrostatic energy contribution to the free energy in the system is still not understood. Herein, we investigate the mechanical and electrical properties by studying the isobaric relaxation process of a dipalmitoyl phosphatidylcholine monolayer on water subphase with various concentrations of divalent ions to reveal the effect of electrostatic energy on thermodynamics and kinetics of the collapse mechanism. Our results demonstrate that electrical energy among the dipolar molecules plays an important role in the stability of monolayer and enhances the formation of micelles into subphase under high pressure. In addition, to confirm the electrostatic energy contribution, the well-known thermal effect on the stability of the film is compared. Hence, the general description of the monolayer free energy with contribution of electrostatic energy is suggested to describe the phase transition.

Monolayer studies on poly(isobutylcyanoacrylate)-ampicillin association.

Surface pressure-area isotherms (pi-A) of poly(isobutylcyanoacrylate) monolayers with or without glucose and dextran as polymerization adjuvants used in the preparation of nanoparticles have been derived from measurements made at the air-water interface with the subphase pH at 2.7, 5.5 or 8.8. The isotherms were characteristic of the expanded type of polymer monolayer curves, yielding unusually low limiting area values compared with those of other known poly(acrylate) derivatives. This behaviour may be explained by the folding of polar moieties of the polymer groups in the water subphase. Ampicillin incorporated during preparation of nanoparticles had a negligible effect on the general behaviour of adjuvant-free monolayers, but a significant one on the adjuvant-loaded polymer monolayer system which showed an increase in surface area throughout the compression cycle, as compared with the surface area of the adjuvant-free polymer system although the collapse pressure was practically the same at 67 mN m-1.

Air–water interfacial behavior of linear-dendritic block copolymers containing PEG and azobenzene chromophores

Fabrication of Langmuir films at the air–water interface of four linear-dendritic block copolymers (LDBCs) is described. The LDBCs are composed of a linear hydrophilic chain of poly(ethylene glycol) (PEG) and the first four generations of hydrophobic aliphatic polyester dendrons functionalized at the periphery with cyanoazobenzene chromophores. Langmuir films of the LDBCs, coded as PEG-AZOn ( n indicates the number of cyanoazobenzene units at the periphery of the dendritic block), have been characterized by a combination of surface pressure versus area per molecule isotherms, UV–vis reflection spectroscopy and Brewster angle microscopy. The observed PEG-AZOn Langmuir film behavior depends strongly on the hydrophilic/hydrophobic ratio. A typical transition, related to PEG chains desorption from the air–water interface into the water subphase is observed for all the LDBCs, except for PEG-AZO16. In addition, PEG-AZO2 and PEG-AZO4 show a second transition whose nature has been studied in detail. Azobenzene chromophore interactions have been shown to be relevant in the organization of PEG-AZOn ( n = 4, 8 and 16) Langmuir films. Moreover, for PEG-AZO16 the orientation of the azobenzene units has been determined, revealing the formation of a well organized structure of azobenzene moieties at the air–water interface.

Nonequilibrium 2-Hydroxyoctadecanoic Acid Monolayers: Effect of Electrolytes

2-Hydroxyacids display complex monolayer phase behavior due to the additional hydrogen bonding afforded by the presence of the second hydroxy group. The placement of this group at the position α to the carboxylic acid functionality also introduces the possibility of chelation, a utility important in crystallization including biomineralization. Biomineralization, like many biological processes, is inherently a nonequilibrium process. The nonequilibrium monolayer phase behavior of 2-hydroxyoctadecanoic acid was investigated on each of pure water, calcium chloride, sodium bicarbonate and calcium carbonate crystallizing subphases as a precursor study to a model calcium carbonate biomineralizing system, each at a pH of ∼6. The role of the bicarbonate co-ion in manipulating the monolayer structure was determined by comparison with monolayer phase behavior on a sodium chloride subphase. Monolayer phase behavior was probed using surface pressure/area isotherms, surface potential, Brewster angle microscopy, and synchrotron-based grazing incidence X-ray diffraction and X-ray reflectivity. Complex phase behavior was observed for all but the sodium chloride subphase with hydrogen bonding, electrostatic and steric effects defining the symmetry of the monolayer. On a pure water subphase hydrogen bonding dominates with three phases coexisting at low pressures. Introduction of calcium ions into the aqueous subphase ensures strong cation binding to the surfactant head groups through chelation. The monolayer becomes very unstable in the presence of bicarbonate ions within the subphase due to short-range hydrogen bonding interactions between the monolayer and bicarbonate ions facilitated by the sodium cation enhancing surfactant solubility. The combined effects of electrostatics and hydrogen bonding are observed on the calcium carbonate crystallizing subphase.

Poly(ester)s containing germanium and silicon in the main chain, part 2: Static and dynamics behavior of monolayers at the air-water interface

The static and dynamic behavior of the Langmuir monolayers of poly(ester)s containing germanium and silicon in the main chain (Scheme 1) is reported. True monolayers are found for all the poly(ester)s studied. The isotherms of surface pressure versus surface area for poly(ester)s exhibit a behavior of the expanded-type monolayers and that of condensed-type monolayers. It was observed that the addition of an hydrophilic polymer such as poly(monomethyl itaconate) to the water subphase produces significant changes in the isotherm properties. The interfacial rheology of the poly(ester)s monolayers has also been studied by using Oscillatory Barrier Experiments in a low-frequency range. It was found that the elasticity and the dilational viscosity increase from diluted monolayers until a surface concentration about 20 × 10−5 mg cm−2. The static and dynamic elasticities were found to be very similar for the poly(ester)s over the dilute range. It was possible to conclude that these poly(ester)s form extremely viscoelastic monolayers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Langmuir-Blodgett Ultrathin Films Composed of ZnO Nanoparticles: Fabrication and Characterization

By the Langmuir-Blodgett(LB) technique, the zinc ion containing multilayer was prepared by transferring the zinc acetate, spread on the surface of subphase of ultrapure water and stearic acid-chloroform mixtures, onto a hydrophilic silicon wafer or glass plate. Then the multilayer was converted into ZnO ultrathin films after pre-heating and annealing. The optimized parameters for monolayer formation, such as concentration of subphase, barrier speed and spreading volume, were determined by the measurement of the surface pressure-surface area (Π-A) isotherms. The expended areas after deposition with zinc ions inferred the interaction of stearic acid with zinc ion during the formation of monolayer at air-water interface. The optimized parameters for multilayer deposition, such as surface pressure and lifting speed were determined by the measurement of the transfer coefficient. The X-ray diffraction(XRD) was used to investigate the crystal structures of ZnO nanoparticles and ultrathin film. The surface morphologies of the LB multilayer and ultrathin film were observed by the atomic force microscopy (AFM). A uniform and flat surface of ZnO ultrathin film within nanometer ranges(<35nm) were obtained.

Structure of porphyrin-fullerene dyad monolayer on the water surface and solid substrate

Monolayers of porphyrin-fullerene dyad TBD6a were formed on the surface of a water subphase and then transported on a solid substrate by the Langmuir-Schaefer method. A simulation was performed for the structure of a single molecule and for a molecular monolayer, according to the area per molecule in the monolayer formed, which was calculated based on an analysis of the π-A isotherm. A unit cell was chosen for the proposed molecular packing (a = 1.54 nm, b = 1.50 nm, c = 1.75 nm, α = 80.0°, β = 90.0°, and γ = 90.0°), and the atomic coordinates were calculated. A comparison of the interplanar spacings and diffraction peak intensities in the experimental and calculated (for the unit cell proposed) diffraction patterns indicates that a platelike texture is formed in the monolayer and that the crystal structure of the domains corresponds to the model chosen.

Zinc modified polydiacetylene Langmuir films

Polydiacetylene (PDA) Langmuir films (LF) containing Zn2+ cations were prepared in different experimental procedures. The incorporation of zinc cations resulted in new morphological and spectral properties. We found that zinc cations stabilize the blue phase of PDA, and inhibit its transformation into the red phase, withstanding prolonged exposure to UV irradiation. The increased stability was accompanied with the loss of the linear strand morphology and appearance of a new, transient purple phase before the final red phase stage. It was found that the most significant differences in film properties occur when the zinc cations are already present in solution during the polymerization stage at sufficient concentration. In such case zinc ions penetrate into the headgroup interlayer in the trilayer organization, and are implicated in the film's chromatic properties. In cases where polymerization was carried out prior to introduction of the cations, the properties of PDA/Zn2+ films are similar to films produced on a pure water subphase.

Preparation, luminescence and photoelectric properties of Langmuir–Blodgett films of alkynylplatinum(II)–zinc(II) porphyrinate/heteropolyoxometalate hybrid composites

A new class of organometallic/inorganic hybrid Langmuir–Blodgett (LB) films, consisting of rigid-rod alkynylplatinum(II)–zinc(II) porphyrinate complex (OMA) as the π-conjugated donor–acceptor-type molecule and tungsto(molybdo)phosphoric heteropolyacids (HPA) (HPA = H 3PMo 12O 40 and H 3PW 12O 40, abbreviated as HPMo 12 and HPW 12, respectively) of the Keggin structure as the inorganic component, were prepared and characterized by π–A isotherms, UV–vis absorption and luminescence spectra, low-angle X-ray diffraction, scanning tunneling microscopy and surface photovoltage spectroscopy. Our experimental results indicate that stable, well-defined and well-organized Langmuir and LB films have been formed in pure water and heteropolyacid subphase. They typically have a highly organized lamellar structure in which a monolayer of HPA is most likely embedded inside the OMA molecular space formed by long chains of PBu 3. Luminescence spectra of these hybrid LB films show that HPMo 12 and HPW 12 can enhance the emission of OMA to some extent. These LB composites show good photovoltage responses and a photovoltage of 79 μV can be obtained for the OMA/HPMo 12 system when it is excited by light. The monolayer LB films on ITO wafer can also display interesting electrical conductivity.

Photoelectric Response from D–π–A Dye's Langmuir-Blodgett Monolayer Film Modified Indium-Tin Oxide Electrode

Langmui-Blodgett (LB) monolayer films of two new amphiphilic D–π–A dyes were fabricated on both pure and acidic water subphases and transferred onto the ITO electrode by the LB) technique. The ultraviolet-visible absorption spectra and photocurrent properties of the ordered LB films were investigated.