Mixed Langmuir-Blodgett Films Research Articles

Interfacial interactions in urease/stearic acid and urease/DOPC binary mixed systems at the air-water and the air-solid interfaces

In the present study, the protein-lipid interactions between water-soluble metalloenzyme urease (Urs) and insoluble stearic acid (SA) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) were investigated at the air–water interface using the Langmuir-Blodgett (LB) technique. Surface pressure (π) - molecular area (A) isotherms were measured for the pure SA and pure DOPC monolayers in the presence of urease in the water subphase. From the (π – A) isotherms of the binary system, it was found that urease strongly interacts with SA and DOPC. The compressibility of the SA monolayer was reduced with the incorporation of urease; a similar trend was also found with DOPC. Moreover, an association of urease with lipid monolayers was found to be surface pressure (π) dependent. The presence of a little hump in the mixed isotherms of Urs-SA at low surface pressure supports this observation. However, the miscibility studies and the calculations of excess Gibb’s free energy indicate an attractive interaction between urease and SA molecules. The considerable morphological variations observed by atomic force microscopy (AFM) in urease-SA deposited films onto hydrophilic and hydrophobic Si(100) surfaces suggest that urease was strongly bound with the surrounding lipid SA on the hydrophilic surfaces but the urease molecules were rigorously denatured on the hydrophobic surfaces. The AFM analysis of Urs-DOPC mixed LB films at room temperature (RT) and elevated temperature suggests substantial denaturation of urease in a hydrophobic environment leading us to speculate the exposure of tryptophan moieties of the urease molecules to the hydrophobic air phase.

A voltammetry biosensor based on self-assembled layers of a heteroleptic tris(phthalocyaninato) europium triple-decker complex and tyrosinase for catechol detection

A highly efficient enzyme-based sensor was made from a heteroleptic tris(phthalocyaninato) europium triple-decker complex Eu2(Pc)[Pc(OPh)8]2, for the first time. The ITO working electrode was modified by a mixture of hybrid multi-layers consisting of Eu2(Pc)[Pc(OPh)8]2, stearic acid (SA) and tyrosinase (Tyr) (Eu2(Pc)[Pc(OPh)8]2/SA/Tyr-ITO) using the Langmuir-Blodgett (LB) technique. The microstructure and morphology of the resulting LB films can be characterized by their π–A isotherms, UV–vis absorption spectra, X-ray diffraction and atomic force microscopy (AFM) analysis. The experimental results revealed that the triple-decker molecules of Eu2(Pc)[Pc(OPh)8]2 take a H-type molecular stacking mode in both pure and mixed LB film, and the microstructures of films were effectively improved by mixing SA within the triple-decker Eu2(Pc)[Pc(OPh)8]2 molecules. The excellent electrocatalytic effect of the Eu2(Pc)[Pc(OPh)8]2/SA/Tyr LB films, leads to a good linear increase from 5.26 × 10−7 to 2.1 × 10−4 M for catechol. It also leads to an excellent sensitivity of 2.19 μA/μM, and a detection limits of 6.29 × 10−8 M (S/N = 3) of catehol at the oxidation peak, achieving best catechol sensing performance among the phthalocyanine-based biosensing mediators. The reduction peak also showed a good linear increase from 5.26 × 10−7 to 1.60 × 10−4 M for catechol with a sensitivity of 0.615 μA/μM, and a detection limit of 1.69 × 10−7 M (S/N = 3). Moreover, the Eu2(Pc)[Pc(OPh)8]2/SA/Tyr-ITO electrode are easy to reproduce, stable and resistant to interference when it comes to detection for catehol, and this indicates great potential of industrial application of tris(phthalocyaninato) rare earth complexes in ultrasensitive and specific biosensors.

Study of polydiacetylenes and rhodamine-800 mixed film at air–water interface and onto solid support: Trace of fluorescence resonance energy transfer (FRET)

Here, we report the mixing behaviour of diacetylene monomer 10, 12-tricosadiynoic acid (TCDA) and rhodamine-800 (Rh8) using Langmuir–Blodgett (LB) technique. The presence of Rh8 affected the structures, phase behaviour as well as colorimetric properties of TCDA in the mixed films at air–water interface and onto solid support. The mixed LB films having TCDA molefraction ≥ 0.5 leads to multilayer formation, whereas multilayer formation was hardly observed for the films containing TCDA molefraction less than 0.5. It was observed that polymerization as well as phase change (blue to red) was possible only for multilayered films. Brewster angle microscopy, atomic force microscopy, and fluorescence inverted microscopy studies confirmed the formation of definite structures in the polymerized TCDA/Rh8 mixed films at air–water interface and onto solid support. Structure of TCDA polymer strands was largely influenced by the presence of Rh8 in the mixed films. Interestingly, energy transfer occurred from TCDA in red phase to Rh8 in the mixed films with TCDA molefraction greater than 0.4. Maximum energy transfer efficiency was found to be 47.3% for the mixed LB film with TCDA molefraction 0.9.

Incorporation of aptamer in mixed Langmuir-Blodgett films between lipid and graphene oxide sheets

Abstract Interaction between different molecules during formation of complex nanostructured films, which can be used as matrixes for fixing of macromolecules, plays important role in formation of sensitive films for biosensors. The Langmuir-Blodgett technique was used to obtain mixed films of dimethyldioctadecylammonium bromide (DODAB) and graphene oxide sheets (GO). Specific aptamers were subsequently attached. The obtained results after characterization of films on the air/water interface show that incorporation of GO sheets between lipid monolayers takes place. Transfer rates for mixed films indicates the formation of Z-type films after GO incorporation. Quartz Crystal Microbalance measurements of films density clearly demonstrated the presence of GO sheets in films. After drop casting method with the solution of aptamer marked with Cy5 fluorophores on the surface of the mixed films, measurements of fluorescence microscopy and contact angle were performed demonstrating the presence of these macromolecules even after cleaning procedures.

Stability of J-aggregated species in an indocarbocyanine dye in Langmuir–Blodgett Films

Here, the stability behaviour of J-aggregate of an indocarbocyanine dye 1,1′-Dioctadecyl-3,3,3′,3′-Tetramethylindocarbocynine perchlorate (DiI) in fatty acid mixed (XDiI=0.4) Langmuir–Blodgett (LB) films under different condition was reported. Surface pressure–area per molecule (π–A) isotherm measurement suggest that the molecular interaction between the constituent molecules changes with subphase temperature. UV–vis absorption and fluorescence spectroscopy and Atomic Force Microscopy (AFM) were used to study the stability behaviour. It has been observed that prominent J-aggregate of DiI occurred in the mixed films at subphase temperature 15°C. With the passage of time as well as post heat treatment DiI J-aggregate decayed to excimer in both mono- and multilayer LB films. Total transition from J-aggregate to excimer occurred in monolayer films, whereas, the transition was partial in multilayer films. The stability of DiI J-aggregate in monolayer films was increased by two ways – (i) depositing the DiI-SA mixed LB films at higher surface pressure, (ii) exposing the films at 50°C for 5min. It has been observed that after heat treatment the DiI J-aggregate remained almost stable even after 200 days. AFM investigations gave the compelling visual evidences of the same.

Conjugated polymers nanostructured as smart interfaces for controlling the catalytic properties of enzymes

The search for new molecular architectures to improve the efficiency of enzymes entrapped in ultrathin films is useful to enhance the effectiveness of biosensors. In this present work, conjugated polymers, based on thiophene and fluorine, were investigated to verify their suitability as matrices for the immobilization of urease. The copolymer poly[(9,9-dioctylfluorene)-co-thiophene], PDOF-co-Th was spread on the air-water interface forming stable Langmuir monolayers as determined by surface pressure-area isotherms, polarization-modulation reflection-absorption infrared spectroscopy (PM-IRRAS), and Brewster angle microscopy (BAM). Urease was incorporated in the floating monolayers being further transferred to solid supports as mixed Langmuir-Blodgett (LB) films. These films were then characterized with transfer ratio, fluorescence spectroscopy, PM-IRRAS and atomic force microscopy, confirming the co-transfer of the enzyme as well as its structuring in β-sheets. The catalytic activity was detected for urease, with a lower reaction rate than that encountered for the homogeneous environment. This was attributed to conformational constraints imposed to the biomacromolecule entrapped in the polymeric matrix.

Chiral tetrakis(4-carboxyphenyl)porphyrin aggregates induced by enantiomeric dibenzoyl tartaric acids at the air–water interface

Abstract Enantiomeric dibenzoyl tartaric acid (DBTarA)-induced formation of chiral tetrakis(4-carboxyphenyl)porphyrin (TCPP) aggregates have been investigated in solutions and at the air–water interface. Surface pressure–area isotherms of the monolayers, UV–vis, and infrared spectra, as well as X-ray photoelectron spectra of the transferred Langmuir–Blodgett (LB) films, indicated that, though the small DBTarA molecules had difficulty forming insoluble monolayers at the interfaces, they could insert themselves in the matrix of porphyrins to form TCPP-DBTarA mixed monolayers and LB films. The inorganic salt of CdCl2 in the subphase could stabilize the mixed monolayers due to formation of the coordination bond between Cd(II) ions and carboxylic acid substituents of TCPP and DBTarA. Mirror-imaged circular dichroism spectra were recorded with two signals centered at about 230–250 and 400–450 nm; the former indicated that chirality of the DBTarA remained in the LB films while the latter (corresponding to the Soret absorption band of porphyrins) indicated that the co-existing DBTarA could induce the formation of supramolecular chirality in the porphyrins. It was revealed that pure TCPP formed dot-like aggregates while its mixtures with DBTarA formed irregular nanowires on the CdCl2 subphase surface, a phenomenon which may be attributed to the fact that the coordination bond of Cd OOC connects TCPP and DBTarA molecules to form larger one-dimensional ternary molecular assemblies.

COMPARATIVE STUDY OF THE BEHAVIOR OF BATHOPHENANTHROLINE IN THE RESTRICTED GEOMETRY OF LANGMUIR–BLODGETT FILM WITH TWO DIFFERENT FATTY ACIDS

This communication reports the successful fabrication and the comparative study of Langmuir and Langmuir–Blodgett (LB) films of nonamphiphilic phenanthrene derivative, 1, 10 phenanthroline (bathophenanthroline) (BATH), when incorporated to a long chain fatty acid viz. heptadecanoic acid and stearic acid. Measurements of compressibility modulus from the surface pressure versus area per molecule isotherm indicate that the mixed film of BATH and SA is harder than that with HA. The plot of area per molecule versus mole fraction is the indication of some repulsive interactions between binary components of the mixed monolayer which facilitates the formation of aggregates. UV–Vis absorption spectroscopy and steady-state fluorescence spectroscopy confirms the formation of microcrystalline aggregates in the mixed LB films. Scanning electron microscopic study supports this observation. The dependence of various LB parameters in the mixed films such as mole fraction, number of layers and surface pressure of lifting have also been investigated in the light of electronic absorption spectroscopy.

Triggered J-aggregation in mixed Langmuir-Blodgett films of amphiphilic spiropyran having a methoxy group at the 5' position and an azobenzene derivative.

Here, we describe the formation of J-aggregates triggered by isomerization of an azobenzene derivative, N-[p-[(p-dodecylphenylazo)phenyloxy]dodecylpyridinium bromide (AzP), in mixed Langmuir-Blodgett (LB) films that contain an amphiphilic spiropyran with a methoxy group at the 5' position, MeO-SP1822. Pure LB films of MeO-SP1822 consist of multilayer domains embedded in a monolayer. UV irradiation of the films causes the isomerization of MeO-SP1822 to its merocyanine form, MeO-MC1822. Pure LB films of AzP comprise finger-like domains and granular domains. Irradiating mixed films of MeO-SP1822 and AzP with alternating UV and visible light causes J-aggregation of MeO-MC1822, with the amount of J-aggregates reaching a maximum at a 1:1 molar ratio. J-aggregation occurs in flat finger-like structures originating in the AzP-rich granular domains that are located on top of the MeO-MC1822-rich multilayer domains. J-aggregates are also present under the AzP-rich granular domains, though these domains do not serve as nucleation sites for the finger-like structures. We propose that granular domains serving as nucleation sites are partially buried in the multilayer domains, whereas those triggering the J-aggregation of MeO-MC1822 under the granular domains are situated on top of the multilayer domains.

Behavior of Coronene in the Mixed Langmuir-Blodgett (LB) Films of Two Different Matrixes on Water and Solid Surfaces

Excellent Langmuir-Blodgett films (LB) of non-amphiphilic coronene (COR) have been prepared by mixing with a long chain fatty acid, viz., arachidic acid (AA) and an inert polymer such as poly methyl methacrylate (PMMA) separately. Miscibility of the guest and host molecules in the mixed monolayer of COR and AA/PMMA have been studied with excess area criterion. Both the attractive and repulsive interactions are present in the COR/AA mixed monolayer, whereas only repulsive interaction is active between the component molecules of COR/PMMA mixed monolayer. Spectroscopic properties of COR in solution and mixed LB films have been compared by UV-Vis absorption spectroscopy. Surface morphology of mixed LB films has been examined by atomic force microscopy (AFM). Absorption spectroscopic study complemented with AFM image suggests the formation of some kind of aggregates in the mixed LB films.

Anomalous conformational transitions in cytochrome C adsorbing to Langmuir–Blodgett films

Helix to beta conformational transitions in proteins has attracted much attention due to their relevance to fibril formation which is implicated in many neurological diseases. This study reports on unusual conformational transition of cytochrome C adsorbing to hydrophilic surface containing pure cationic lipid and mixed Langmuir–Blodgett films (LB films) of cationic and neutral lipids. Evidence for conformational changes of the protein from its native helical state to beta sheet comes from Circular dichroic spectroscopy (CD spectroscopy). Analysis of these samples using High resolution TEM (HRTEM) shows a typical fibrillar pattern with each strand spacing of about 0.41nm across which can be attributed to the repeat distance of interdigitated neighboring hydrogen-bonded ribbons in a beta sheet. Changes in contact angles of protein adsorbing to the LB films together with the increased mass uptake of water using quartz crystal microbalance (QCM) confirm the role of positive charges in the conformational transition. Dehydration of the protein resulting from the excess water entrainment in the polar planes of the cationic lipid in hydrophilic surface seems to trigger the refolding of the protein to beta sheet while it retains its native conformation in hydrophobic films. The results suggest that drastic conformational changes in CytC adsorbing to cationic lipids may be of significance in its role as a peripheral membrane protein.

Patterning of Solution-Processable Materials on Templates Fabricated from Mixed Langmuir-Blodgett Films

We report a useful lithographic technique for the fabrication of organic or metal films on wettability-patterned surfaces fabricated solely through self-assembly. The phase-separated structures in mixed Langmuir-Blodgett (LB) films can be tuned by adjusting intermolecular interactions in film-forming molecules such as fatty acids, hybrid carboxylic acids, and silane-coupling agents. Templates are fabricated from phase-separated mixed LB films and organic phosphor and metal films can be formed on the templates using casting, dip-coating, and solvent evaporation techniques. This methodology enables low-cost and low-energy fabrication of patterned films comprising solution-processable materials in small facilities.

Hybrid platinum nanoparticle ensemble for the electrocatalytic oxidation of H2O2: Toward nanostructured biosensor design

This paper reports the electrocatalysis of H2O2 oxidation by Langmuir–Blodgett (LB) thin films based on polymer-grafted platinum nanoparticles (PtNPs). 4-mercaptoaniline-functionalized PtNPs have been chemically modified to obtain poly(methacrylic acid)-grafted platinum nanoparticles (PMAA-PtNP) via surface-ATRP. These elementary bricks are used to form mixed LB films consisting of controlled mixtures of PMAA-PtNP and a redox-inactive fatty acid molecule to tune the PtNP surface density. Nanostructures are formed in which the number of layers and the number of particles in each layer may be varied. The nanostructure morphology strongly depends on the amount of PtNP. High Pt content leads to quite large nanoparticle domains whereas low content gives small domains with a finely divided nanostructure. The H2O2 oxidation current increases when the PMAA-PtNP surface density decreases. This result is discussed in terms of electrocatalyst accessibility modification related to the nanostructure of the films.

Fluorescencing behavior of thin solid films based on polyelectrolyte‐surfactant complex and dye molecules

AbstractThis article is devoted to investigation of interaction between polycomplex and dye molecules assembled in fluorescent thin solid Langmuir‐Blodgett (LB) films. Preparation procedure of organosoluble stoichiometric polycomplex based on cationic polyelectrolyte and anionic surfactant is described. Formation of mixed monolayers consisting of polyelectrolyte‐surfactant complex (PSC) and dye molecules–Nile Red (NR) at water‐air interface is shown. Assembling conditions of LB films based on PSC and NR are defined and the spectral‐luminescent properties of obtained films are studied. It is shown that when nonamphiphilic NR and PSC are mixed the excellent monolayers formed, which can easily be transferred onto solid substrates by Langmuir‐Blodgett method with a high transfer coefficient. Absorption and fluorescence spectra of mixed LB films reveal the formation of dimers and existence of monomers in LB films. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Formation of nanoscale aggregates of a coumarin derivative in Langmuir–Blodgett film

In the present communication, we report the formation of organized nanoscale aggregates of a coumarin derivative 7 Hydroxy-N-Octadecyl Coumarin-3-Carboxamide (7HNO3C) at the air–water interface and in Langmuir–Blodgett (LB) films in the presence and absence of stearic acid (SA). A pressure-area isotherm reveals that the 7HNO3C form stable monolayer at the air–water interface. However, the stability can be improved by mixing it with a fatty acid stearic acid (SA). The miscibility study shows that the nature of interaction is strongly dependent on the mixing ratio and surface pressure. At a mole fraction of 0.4 of 7HNO3C in SA, the attractive and repulsive interaction between these two molecules balance each other forming a stable film with nanoscale aggregates. UV-Vis absorption spectroscopic studies reveal the nature of the aggregates in LB films. Scanning electron microscopy gives compelling visual evidence of formation of nanoscale aggregates in the mixed LB films.

Preparation and characterization of mixed monolayers and Langmuir−Blodgett films of merocyanine 540/octadecylamine mixture

Mixed monolayer of merocyanine 540 (MC540) dye and octadecylamine (ODA) at the air/water interface has been prepared using the co-spreading method. The pressure–area (π–A) isotherm studies revealed that the mixtures of MC540/ODA at a different ratio formed a stable monolayer at the air/water interface and these floating layers were easily transferred onto hydrophilic substrates as the Y-type Langmuir–Blodgett (LB) film. The specific area per molecule in the π–A isotherm of the mixed monolayer of MC540/ODA was larger than that of the pristine ODA. The area can also get increased up to 40mN/m surface pressure by the increase of the dye concentration in the mixtures. The LB films of MC540/ODA mixture were formed at 30mN/m surface pressure by transferring the mixed monolayer at the air/water interface on hydrophilic glass substrate via vertical dip-coating as mono- and multilayer films. The photophysical properties of MC540 in chloroform and LB films have been investigated using the absorption, steady-state and time-resolved fluorescence spectroscopy techniques. H-aggregate formation of MC540 both in chloroform and LB film were concluded from the spectroscopic results. The morphology of the one-layer mixed LB film of MC540/ODA on the glass substrate has been characterized by AFM (atomic force microscopy). The non-contact mode AFM image of the one-layer mixed LB film showed that the film surface consisted of MC540/ODA nanoclusters. In conclusion, our results contribute to understanding the strong interaction between MC540 and ODA at the air/water interface, and show the effects of some parameters on the mixed LB films of MC540.

An Ion Sensitive Organic Field-Effect Transistor Incorporating the Ionophore Valinomycin

We report on the effect of depositing the ionophore valinomycin onto the polymethylmethacrylate (PMMA) gate insulator of an ion-sensitive organic field-effect transistor (ISOFET) based on poly(3-hexylthiophene). The ionophore was deposited onto the PMMA using the Langmuir-Blodgett (LB) technique; thin films based on pure valinomycin and those in which valinomycin was mixed with arachidic acid were investigated. The pH sensitivity of the reference ISOFET could be improved significantly when the devices were coated with an LB film of arachidic acid. However, the response to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm K} ^{+}$</tex></formula> was low. By adding a small amount (5% w/w) of the ionophore valinomycin to the fatty acid LB film, an improved response to potassium ions was achieved, but no selectivity over sodium. It was necessary to use an LB membrane of pure valinomycin in order to realise an ISOFET with some <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm K} ^{+}$</tex></formula> selectivity. We suggest that the presence of the ionophore in the fatty acid matrix disrupts the packing of the hydrocarbon chains in the mixed LB film and that the monovalent ion response originates from interactions with the carboxylic acid groups in the fatty acid. In contrast, for the case of the pure valinomycin coating, it is thought that <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\rm K} ^{+}$</tex></formula> response is controlled by complex formation with the ionophore.

Influence of magnetic field on delayed fluorescence of coumarin dye in Langmuir–Blodgett films

The triplet–triplet annihilation (TTA) was studied in mixed Langmuir–Blodgett films of 7-decyloxy-3 (4′-ethoxyphenyl)-coumarin molecules and palmitic acid. The thermal activation of the TTA process is observed due to the inhomogeneous broadening of triplet energy levels. Modulation of the TTA rate constant in the external magnetic field for the multilayer films shows the dependence that is characteristic of crystals. Only negative magnetic effect is observed for the monolayers. Time dependence of the magnetic effect in conditions of high rate constant of triplet excitons migration is connected with the dominating contribution of ordered clusters or randomly oriented molecule clusters into the TTA process.

Investigation of Miscibility and Aggregate Formation in the Mixed Langmuir–Blodgett Films of 2-aminoanthracene by Surface Pressure and Spectroscopic Methods

Nonamphiphilic 2-aminoanthracene has been incorporated within the Langmuir monolayer and Langmuir–Blodgett (LB) films when mixed with behenic acid (BA). Miscibility of 2-aminoanthracene and BA at the air—water (A–W) interface has been studied by using surface phase rule and excess area criterion. Our studies reveal that repulsive type interactions exist between the component molecules in mixed monolayer and this may facilitate the formation of aggregates. Spectroscopic as well as morphological studies of mixed LB films of 2-aminoanthracene and BA undoubtedly confirms the formation of aggregates.

Electrochemical behaviour of mixed LB films of ubiquinone – DPPC

The structure and the electrochemical behaviour of Langmuir and Langmuir–Blodgett (LB) films of the biological ubiquinone-10 (UQ) and a mixture of dipalmytoilphosphatidylcholine (DPPC) and UQ at the molar ratios DPPC:UQ 5:1 and 10:1 have been investigated. The surface pressure-area isotherms of the Langmuir films and the AFM images of the LB films show the formation of a monolayer in the DPPC:UQ mixture till a certain surface pressure is attained, and then at higher surface pressures the UQ is progressively expelled. The cyclic voltammograms of DPPC:UQ LB films formed on indium tin oxide, ITO, at different surface pressures show one reduction and one oxidation peak at low surface pressures, but two or even more reduction and oxidations peaks at medium and high surface pressures. The electrochemical behaviour is correlated with the film structure.