LB Monolayers Research Articles

Probing the effect of nitro-substituents in the modulation of LUMO energies for directional electron transport through 4d6 ruthenium(II)-based metallosurfactants.

Electron-withdrawing nitro-substituents were installed onto terpyridine- and phenanthroline-based metallosurfactants with 4d6 ruthenium(II), which were deposited as Langmuir-Blodgett monolayers aiming to study the feasibility of charge transport in Au|LB|Au junctions. The nitro groups are intended to modulate the energy of the frontier molecular orbitals to near to, or match that of Fermi levels in the gold electrodes. A series of heteroleptic metallosurfactants [RuII(C18OPh-terpy)(X-terpy)](PF6)2 and [RuII(C18OPh-terpy)(X-phen)Cl]PF6 were synthesized, where C18OPh-terpy is the 4'-[4-(octadecyloxy)phenyl]-2,2':6',2''-terpyridine amphiphile common to all species, X-terpy is a terpyridine with-H (1) or-phenyl-NO2 (2) and X-phen is a phenanthroline with-H (3) or-NO2 (4) groups. These metallosurfactants were characterized by experimental and computational methods, and the presence of nitro groups affect more affordable reductions at less negative potentials, as well as slightly more positive oxidations, these changes are less pronounced in species 2 than in 4. Species 1 and 2 showed limited Pockels-Langmuir and Langmuir-Blodgett film formation with lower collapse pressure of 27 mN m-1. In contrast, metallosurfactants 3 and 4 showed enhanced hydrophilicity indicated by higher collapse pressures of ca. 36 mN m-1. The LB monolayers of 3 and 4 were deposited on gold electrodes to form Au|LB|Au junctions and electron transport was measured as I/V curves. The NO2-bearing species 4 showed asymmetric curves associated with directional electron transport with amplitudes up to -2.0 nA and rectification ratios from 5 to 26 between -1 to +1 V and from 3 to 14 between -3 to +3 V.

Observation of nanodomains and nanostripes in the Langmuir-Blodgett monolayers of Fe3O4 magnetic nanoparticles

Magnetic nanoparticles are of significant importance due to their potential applications in the fields of science, engineering and nanomedicine. Using the Langmuir-Blodgett technique, 2D assemblies of Fe3O4 nanoparticles were deposited on solid substrates at various surface pressures with and without an external magnetic field. Chain like and nanodomain structures were observed in LB monolayers. Shape deformation and aggregation of chains were investigated under lateral pressure and external magnetic field. Field Emission Scanning Electron Microscopy micrographs showed the formation of magnetic nanodomains and deformation of chains under mechanical compression. For the first time, we report the formation of magnetic nanodomains in LB films and their successful transformation to a glass substrate. This research provides fundamental insights into the formation of magnetic films under both dipole–dipole interactions and mechanical compression, which is very important for experimental and applied research. Furthermore, vast LB films of closely packed nanoparticles were prepared. The present study demonstrated that the presence of an external magnetic field improves the quality of LB films of closely packed nanoparticles. The effect of an external magnetic field at smaller surface pressures was to convert nanodomains and chains into striped structures.

Observation of current rectification by a new asymmetric iron(iii) surfactant in a eutectic GaIn|LB monolayer|Au sandwich.

In this paper we expand on the search for molecular rectifiers of electrical current and report on a hexacoordinate metallosurfactant [FeIII(LN3O)(OMe)2], where (LN3O)- is the deprotonated form of the new asymmetric ligand 2-((E)-((4,5-bis(2-methoxyethoxy)-2-(((E)-pyridin-2-ylmethylene)amino)phenyl)imino)methyl)-4,6-di-tert-butyl-phenol. This species rectifies current when deposited as a Langmuir-Blodget film in a "EGaIn/Ga2O3|LB|Au" sandwich with rectification ratios ranging from 25 to 300 at 1 Volt.

Nanoscale Chemical Imaging of Interfacial Monolayers by Tip‐Enhanced Raman Spectroscopy

AbstractWe report an investigation of interfacial fluorinated hydrocarbon (carboxylic‐fantrip) monolayers by nanoscale imaging using tip‐enhanced Raman spectroscopy (TERS) and density functional theory (DFT) calculations. By comparing TERS images of a sub‐monolayer prepared by spin‐coating and a π–π‐stacked monolayer on Au(111) in which the molecular orientation is confined, specific Raman peaks shift and line widths narrow in the transferred LB monolayer. Based on DFT calculations that take into account dispersion corrections and surface selection rules, these specific effects are proposed to originate from π–π stacking and molecular orientation restriction. TERS shows the possibility to distinguish between a random and locked orientation with a spatial resolution of less than 10 nm. This work combines experimental TERS imaging with theoretical DFT calculations and opens up the possibility of studying molecular orientations and intermolecular interaction at the nanoscale and molecular level.

Nanoscale Chemical Imaging of Interfacial Monolayers by Tip-Enhanced Raman Spectroscopy.

We report an investigation of interfacial fluorinated hydrocarbon (carboxylic-fantrip) monolayers by nanoscale imaging using tip-enhanced Raman spectroscopy (TERS) and density functional theory (DFT) calculations. By comparing TERS images of a sub-monolayer prepared by spin-coating and a π-π-stacked monolayer on Au(111) in which the molecular orientation is confined, specific Raman peaks shift and line widths narrow in the transferred LB monolayer. Based on DFT calculations that take into account dispersion corrections and surface selection rules, these specific effects are proposed to originate from π-π stacking and molecular orientation restriction. TERS shows the possibility to distinguish between a random and locked orientation with a spatial resolution of less than 10 nm. This work combines experimental TERS imaging with theoretical DFT calculations and opens up the possibility of studying molecular orientations and intermolecular interaction at the nanoscale and molecular level.

Hemicyanine LB film—Silver nanoparticle composite: contrasting fluorescence responses sensitive to the ultrathin film assembly sequence

Fluorescence emission of molecules is strongly influenced by the plasmonic field of metal nanoparticles, with significant enhancement induced under optimal conditions. Nanocomposite ultrathin films fabricated with citrate-stabilized Ag nanoparticles and LB film of a cationic hemicyanine amphiphile, are shown to produce opposing fluorescence emission trends upon subtle variation in the assembly sequence. Monolayer LB films of the pure amphiphile show aggregation-induced quenching with increasing deposition pressure. Composite films formed by adsorption of Ag nanoparticles on the Langmuir film (self-assembly together with steered assembly) followed by LB transfer, show further quenching. However, adsorption of Ag nanoparticles on the pre-formed amphiphile LB film (self-assembly following steered assembly), causes the fluorescence to increase with the extent of adsorption. Spectroscopy and microscopy provide insight into the contrasting, tunable emission. Formation of Ag nanoparticle chains on the Langmuir film and their direct contact with the monolayer cause the fluorescence quenching; adsorption of isolated Ag nanoparticles on the LB film along with multilayer formation leads to the enhancement. The study illustrates the versatility of LB film—metal nanoparticle composites in producing distinct materials responses through subtle changes in the mode of assembly.

Molecular conformation of DPPC phospholipid Langmuir and Langmuir-Blodgett monolayers studied by heterodyne-detected vibrational sum frequency generation spectroscopy.

Heterodyne-detected (phase-sensitive) vibrational sum frequency generation spectroscopy was used to investigate molecular structures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers on water (Langmuir monolayer) and monolayers on a fused silica substrate (Langmuir-Blodgett [LB] monolayer). The spectral features in the CH stretching region depended on the phase of the Langmuir monolayer, which was controlled by the molecular area on water. From the spectral changes, the molecular structure of the monolayer in each phase was deduced at the molecular level. We discovered that when we compared Langmuir and LB monolayers, both of which correspond to a similar surface pressure, the LB monolayer tended to have fewer gauche defects and have less tilted terminal methyls in the n-pentadecyl groups than the corresponding Langmuir monolayer. In addition, weak vibrational bands, which have been hardly seen by the conventional (homodyne-detected) VSFG spectroscopy, were clearly observed with their phases, or arguments, for the first time.

MPP1 interacts with DOPC/SM/Cholesterol in an artificial membrane system using Langmuir-Blodgett monolayer.

The interaction between membrane palmitoylated protein -1 (MPP1) with lipid bi- and mono-layers composed of a DOPC/SM/Chol mixture was investigated. MPP1 co-migrates with liposomes to the top of the liposome flotation gradient, indicating binding of MPP1 with liposomes. The injection of MPP1 into the subphase of an LB monolayer of the above lipid composition induced an increase in surface pressure, indicating that MPP1 molecules were incorporated into the lipid monolayer. The compressibility modulus isotherms of MPP1, lipids and lipid-MPP1 films have essentially different shapes from one another. Pure MPP1 isotherms were characterized by a peak in surface pressure of 25-35 mNm-1. This transition disappears in isotherms obtained with lipid monolayers in the presence of MPP1, which suggests an interaction between the protein and the lipid monolayers. In addition, this interaction is sensitive to the presence of cholesterol in the lipid monolayer, as adding of MPP1 into the subphase of lipid monolayers containing cholesterol resulted in a much larger increase in surface area than when MPP1 is injected into the subphase of a lipid monolayer devoid of cholesterol. In conclusion, the data demonstrates that MPP1 interacts with lipid mixtures in two different model membrane systems.

Preparation of (ruthenium (II) complex)-bound cellulose derivative for photocurrent generation system

6-O-[Acetylacetonato-bis(2,2′-bipyridine)ruthenium(II)]-2,3-di-O-myristyl cellulose (3) was prepared from 2,3-di-O-myristyl cellulose (1) by two reaction steps in 37.7 % total yield. The LB monolayer film of compound 3 was successfully deposited onto an indium tin oxide electrode by a vertical dipping method and showed photocurrent generation performance in the range of 400–600 nm. Compound 3 was expected as a complementary material of porphyrin-bound cellulose derivative for biomaterial-based solar cells.

Self-Organization of Tetraazaporphyrin Derivatives in 2 and 3 Dimensions

Many applications of dyes, such as phthalocyanines (Pcs) and tetraazaporphyrins (TAPs), require specific supramolecular structures for the achievement of advanced properties. Most approaches towards nanostructured materials of Pcs and TAPs are based on their self-organizing properties that lead to many interesting properties but also highlighted some limitations. The work presented here describes new amphiphilic TAPs that form stable Langmuir and LB monolayers with a face-on orientation of the macrocycle and self-organize into columnar liquid crystaline structures. Both types of self-organized structures were successfully crosss-linked via cycloadditions of azide and acetylene groups with and without Cu catalyst. Surprisingly, the supramolecular structures of the self-organized structures were only marginally affected by the cross-linking processes.

Synthesis of (zinc(II) phthalocyanine)-containing cellulose derivative using phthalocyanine-ring formation reaction

2,3-Di-O-myristyl-6-O-(zinc(II) phthalocyaninyl) cellulose (5) was synthesized from cellulose (1) by five reaction steps via 6-O-(3′,4′-dicyanophenyl)-2,3-di-O-myristyl cellulose (4). The key reaction was phthalocyanine-ring formation on a cellulose backbone, that is, the reaction of compound 4 with o-phthalodinitrile in the presence of hexamethyldisilazane and zinc acetate in DMF afforded to compound 5 in 35.4 % yield. Consequently, the degree of substitution with phthalocyanine moieties of compound 5 was 0.38. The LB monolayer film of compound 5 on an indium tin oxide (ITO) electrode was found to show photocurrent generation performance at 680 nm.

Adsorption by Active and InactiveAspergillus Oryzae: Batch Studies

In this study, the optimal conditions for decolourizing Levafix brilliant blue EB (LB) were determined for both active (actively growing or active) and inactive (inactivated or inactive) Aspergillus oryzae. The adsorption capacity of active and inactive A. oryzae was compared using adsorption isotherm models. Our equilibrium data fitted well to Langmuir model. Under optimal conditions, the maximum monolayer LB adsorption capacities of active and inactive A. oryzae were 61 and 66 mg/g, respectively. The longevity of this decolourization activity under optimal conditions was investigated in repeated mode by adding 250 mg/l of dye in five dosages (50 mg/l each) in the growth medium of A. oryzae. LB decolourization efficiency of the fungus remained high after 5 days operation. Desorption of LB was achieved using 0.1 M NaOH. The experimental results demonstrated that the A. oryzae pellets could effectively be used as a sorbent for removing textile dyes.

Self-Assembly of Hydrophobin Protein Rodlets Studied with Atomic Force Spectroscopy in Dynamic Mode

We have investigated the self-assembling properties of the class I hydrophobin Vmh2 isolated from the fungus Pleurotus ostreatus. Five different hydrophobin self assembled samples including monolayers, bilayers, and rodlets have been prepared by Langmuir technique and studied at the nanoscale. Local wettability and visco-elasticity of the different hydrophobins samples were obtained from atomic force spectroscopy experiments in dynamic mode performed at different, controlled relative humidity (RH) values. It was found that hydrophobins assembled either in rodlets or in bilayer films, display similar hydropathicity and viscoelasticity in contrast to the case of monolayers, whose hydropathicity and viscoelasticity depend on the adopted preparation method (Langmuir-Blodgett or Langmuir-Schaeffer). The comparison with monolayers properties evidences a rearrangement of the bilayers adsorbed onto solid substrates. It is shown that this rearrangement leads to the formation of a stable hydrophobic film, and that the rodlets structure consists in fragments of restructured proteins bilayers. Our results support the hypothesis that the observed variations in the viscoelastic properties could be ascribed to the localization of the large flexible loop, typical of Class I hydrophobins which appears free at the air interface for LB monolayers but not for the other samples. These findings should now serve future developments and applications of hydrophobin films beyond the archetypal monolayer.

In Situ Study of Nanotemplate-Induced Growth of Lysozyme Microcrystals by Submicron Gisaxs

Nanoworld Institute, University of Genoa and Fondazione EL.B.A., Italy.AbstractA new in situ μGISAXS technique was used to study at ESRF the nucleation and growth mechanisms of lysozyme microcrystals with and without the thin Langmuir-Blodgett lysozyme film surface. Following recent studies on thaumatin crystal growth only in presence of LB monolayers (1.2) , in this work ultrasmall Lysozyme microcrystals are grown by classical hanging vapor diffusion and by its modification using homologous protein thin-film template displaying a long-range order. Intensity fluctuations in the μGISAXS pattern versus time for the LB-induced crystallization process appears associated to rapid seed formation and crystal growth, while the classical method continuous shift of intensity in the Yoneda region is compatible with slow crystal growth. The nucleation and growth mechanisms of lysozyme microcrystals are thereby studied at the thin lysozyme film surface by the new in situ μGISAXS technique developed at the microfocus beamline of the European Synchrotron Radiation in Grenoble. New insight on the nucleation and crystallization processes appear to emerge.Pechkova, E., Gebhardt, R., Riekel, C. & Nicolini, C. , In Situ GISAXS: I. Experimental Setup for Submicron Study of Protein Nucleation and Growth, Biophysical Journal(2010) doi:10.1016/j.bpj.2010.03.069 in pressGebhardt, R., Pechkova, E., Riekel, C. & Nicolini, C., In Situ mGISAXS: II. Thaumatin Crystal Growth Kinetic, Biophysical Journal(2010), doi:10.1016/j.bpj.2010.03, in press

First-Leaflet Phase Effect on Properties of Phospholipid Bilayer Formed Through Vesicle Adsorption on LB Monolayer

Phospholipid bilayers were formed on mica using the Langmuir-Blodgett technique and liposome fusion, as a model system for biomembranes. Nanometer-scale surface physical properties of the bilayers were quantitatively characterized upon the different phases of the first leaflets. Lower hydration/steric forces on the bilayers were observed at the liquid phase of the first leaflet than at the solid phase. The forces appear to be related to the low mechanical stability of the lipid bilayer, which was affected by the first leaflet phase. The first leaflet phase also influenced the long-range repulsive forces over the second leaflet. Surface forces, measured using a modified probe with an atomic force microscope, showed that lower long-range repulsive forces were also found at the liquid phase of the first leaflet. Force measurements were performed at 300mM sodium chloride solution so that the effect of the phase on the long-range repulsive forces could be investigated by reducing the effect of the repulsion between the second-leaflet lipid headgroups on the long-range repulsive forces. Forces were analyzed using the Derjaguin-Landau-Verwey-Overbeek theory so that the surface potential and surface charge density of the lipid bilayers were quantitatively acquired for each phase of the first leaflet.

Miscibility and Hydrolytic Behavior of Poly(trimethylene carbonate) and Poly(l-lactide) and Their Blends in Monolayers at the Air/Water Interface

In this study, two biodegradable polymers, poly(trimethylene carbonate) (PTMC) and poly(L-lactide) (PLLA) along with a series of PTMC/PLLA blends, were used as spreading materials to form LB monolayers at the air/water interface to study hydrolytic reaction kinetics of the monolayers with the Langmuir film balance technique. The pi-A isotherms of each homopolymer and their blends showed that blends of PTMC and PLLA were miscible on the neutral subphase (pH 7.4), whereas there was evidence of phase separation on the basic subphase (pH 10.7). The hydrolysis behavior of each homopolymer was investigated at these two different pH conditions. The PTMC monolayer showed faster hydrolysis on the neutral subphase (pH 7.4) than on the basic subphase (pH 10.7). However, in the case of the PLLA monolayer, the hydrolysis on the basic subphase is faster than that on the neutral subphase. On the basis of this result, hydrolysis mechanisms of PTMC and PLLA, considering a general hydrolysis mechanism and their stereo structures, are proposed. The hydrolysis rates of blends of PTMC and PLLA were much faster than that of each homopolymer on the basic subphase (pH 10.7). This result, which can be explained by a "dilution effect", was supported by the structure based mechanism proposed here.

Self-assembly nano-structure of type I collagen adsorbed on Gemini surfactant LB monolayers

The self-assembly nano-structures of type I collagen adsorbed on anionic Gemini surfactant LB monolayer were observed by using atomic force microscopy (AFM) images. It was found that the adsorption behavior and self-assembly structure of collagen could be controlled by the concentration of the collagen solution, adsorption interval and the properties of substrates. With the increase of the adsorption interval and concentration of collagen, the strands size of collagen changed. The self-assembly structures of collagen were also influenced by the interaction between collagen molecules and Gemini surfactant monolayer substrates. Finally, the adsorption behaviors of collagen molecules on cationic Gemini monolayer were compared with those on anionic Gemini monolayer.

Thermo-responsiveness of poly( [formula omitted]-diethylacrylamide) polymers at the air–water interface: The effect of a hydrophobic block

The poly( N , N -diethylacrylamide) (h-PDEA) homopolymer and the poly( N-decylacrylamide)- b-PDEA (PDcA 11- b-PDEA 231) diblock copolymer were studied in the range of 10 to 40 °C, at the air–water interface. The π – A isotherms of h-PDEA appear nearly invariant with temperature while the π – A isotherms of PDcA 11- b-PDEA 231 deviate significantly to lower areas with the temperature increase evidencing the thermo-responsiveness of this diblock copolymer at the interface. For the copolymer, the limiting area per segment versus temperature shows a break point around 29 °C, slightly lower than the lower critical solution temperature (LCST) of h-PDEA in water (31–33 °C). AFM images of LB monolayers transferred at 40 °C revealed for both polymers the presence of hydrophobic aggregates due to the conformational changes (collapse) of chains that occur at the LCST. Differences in the morphology of these aggregates, flat irregular structures for h-PDEA and round-shaped domains for PDcA 11- b-PDEA 231, were related with the condensing effect of the hydrophobic block. The PDcA 11 block, anchoring the polymer to the interface, ensures a better stability and cohesion to the film and preserves the thermo-sensitivity of the h-PDEA at the interface.

Brewster angle microscopic study of mixed lipid–protein monolayer at the air–water interface and its application in biosensing

This study investigated lipid–protein LB film formation with Brewster angle microscopy. Our experimental results show that hemoglobin (Hb) molecules can enter the lipid layer and remain for an extended time. We investigated the KCl effect on the LB monolayer of lipid–protein. The lipid–Hb monolayer was transferred from the air–water interface to a QCM gold electrode. UV–vis spectra showed that Hb retained its natural structure in the lipid layer. Cyclic voltammetric (CV) and amperometric systems were applied in this study in order to confirm the remaining bioactivity and sensitivity of Hb to hydrogen peroxide (H 2O 2). Lipid–Hb-modified electrodes showed well-defined redox peaks, indicating that the direct electron transfer between Hb and the electrode was enhanced by Hb incorporated in lipid layer. Based on this phenomenon, a novel biosensor for H 2O 2 was designed. Experimental conditions influencing the biosensor performance such as pH, and potential were optimized and assessed. The levels of the R.S.D.'s (<5%) for the entire analyses reflected the highly reproducible sensor performance. Using optimized conditions the linear range for the detection of H 2O 2 was observed from 1 × 10 −6 to 1.00 × 10 −4 mol L −1 with a detection limit of 4.00 × 10 −7 mol L −1 (based on the S/N = 3).

Electron transfer studies on cholesterol LB films assembled on thiophenol and 2-naphthalenethiol self-assembled monolayers

We have formed the cholesterol monolayer and multilayer LB films on the self-assembled monolayers of 2-naphthalenethiol (2-NT) and thiophenol (TP) and studied the electrochemical barrier properties of these composite films using cyclic voltammetry and electrochemical impedance spectroscopy. We have also characterized the cholesterol monolayer film using grazing angle FTIR, scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Cholesterol has a long hydrophobic steroid chain, which makes it a suitable candidate to assemble on the hydrophobic surfaces. We find that the highly hydrophobic surface formed by the self-assembled monolayers (SAM) of 2-NT and TP act as effective platforms for the fabrication of cholesterol monolayer and multilayer films. The STM studies show that the cholesterol monolayer films on 2-NT form striped patterns with a separation of 1.0 nm between them. The area per cholesterol molecule is observed to be 0.64 nm2 with a tilt angle of about 28.96 degrees from the surface normal. The electrochemical studies show a large increase in charge transfer resistance and lowering of interfacial capacitance due to the formation of the LB film of cholesterol. We have compared the behavior of this system with that of cholesterol monolayer and multilayers formed on the self-assembled monolayer of thiophenol.