Synthesis of novel hybrid films of a layered silicate and alkylammonium cations on rough polymeric surfaces by Langmuir–Blodgett method

Hybrid films of a layered silicate and an amphiphilic alkylammonium (hexadecyltrimethylammonium) cation have been prepared by Langmuir–Blodgett (LB) method and transferred onto a polyamide surface by dip coating. This is the first time that stable LB hybrid monolayer and multilayer films have been formed on rough polymeric surfaces. The films were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and water contact angle measurements. XRD and FTIR showed that the hybrid multilayer was well-organized and the thickness of one layer was calculated to be 1.6 nm. Furthermore, the layered silicate was determined to be on the substrate side and the amphiphilic molecule layer was exposed to the air side. This provides a novel methodology for the surface modification of polymers.     

Photoreactions and lateral patterning in Langmuir and Langmuir-Blodgett films

Reversible morphological changes occur with photoisomerization of azobenzene in Langmuir-Blodgett (LB) films complexed with polycations, which contradicts an implicit assumption of the concept of free volume that two-dimensional film structures are preserved during the photoisomerization. J-aggregates of chromophores are formed by two processes. The first process is "light-induced J-aggregation" in which photoisomerized molecules form J-aggregates. The other process is "triggered J-aggregation," in which photoisomerization of one of the components triggers J-aggregation of another chemical species in the mixed films. Both processes of J-aggregation are in many cases accompanied by large morphological changes of the films. However, LB films fabricated using processes under isobaric conditions do not change their morphology during light-induced J-aggregation and are patterned with J-aggregates using ultraviolet illumination through a photomask. Phase separation in mixed LB films gives rise to two-dimensional patterns, which are used to fabricate templates by using an amphiphilic silane-coupling agent as one of the components in the mixed LB films. Nanopatterns are also fabricated.     

Langmuir monolayers and Langmuir–Blodgett films of 1-acyl-1,2,4-triazoles

New 1-acyl-1,2,4-triazoles (ATs) with different n-alkyl chain lengths from C2 to C18 were synthesized. ATs with long n-alkyl chains (C12 and larger) are non-charged amphiphilic molecules with a polar triazole head group. The Langmuir isotherms of ATs at the air–water interface with 12 (DoT-C12), 14 (MyT-C14), 16 (PaT-C16) and 18 (StT-C18) carbon atoms in their n-alkyl chains were studied using surface pressure-mean molecular area (π-mmA) measurements. Characteristic for the Langmuir isotherms of PaT-C16 and StT-C-18 was the first sharp increase of the surface pressure at a mmA value of 20 Å² marking a transition of the ATs from the gaseous state with n-alkyl chains already oriented perpendicular to the water surface to a condensed state. The collapse of the monolayers occurred between 20 and 36 mN m⁻¹ with a typical ‘spike’ in the isotherms of MyT-C14, PaT-C16 and StT-C18 which can be assigned to the buckling and subsequent folding of the monolayer. After the collapse point a pseudo-plateau region of slightly increasing surface pressure appeared for all ATs from C12 to C18 indicating a trilayer formation by the roll-over mechanism for MyT-C14, PaT-C16 and StT-C18. Upon further compression the final collapse occurred at π values between 59 and 67 mN m⁻¹. From reversibility studies it was found that the Langmuir isotherms of the ATs were irreversible. The morphology of Langmuir–Blodgett films of ATs transferred onto silicon wafers was studied by atomic force microscopy.     

Investigation of the origin of the glucose response in a glucose oxidase|polyaniline system

The origin of the signal seen in response to glucose in a polyaniline|glucose oxidase system is explored by immittance spectroscopy, by comparing data from an equivalent circuit model and the parameters obtained from a solution of the faradaic branch of the frequency dispersion for a coupled chemical—electrochemical reaction mechanism. It was shown that an RC subcircuit in the equivalent circuit model was sensitive to peroxide concentration, and the interaction of peroxide with polyaniline at potentials where it either oxidised or reduced the polyaniline was discussed. This information was used to compare the data obtained in a bulk and entrapped glucose oxidaselglucose system, and it was seen that the origin of the response could not be fully attributed to peroxide interaction in the latter case. Under anaerobic conditions with entrapped enzyme, it was proposed that a complex between the gluconolactone product of the enzyme reaction and the polymer leads to a more conducting polymer, with inherent charge compensation, and this results in the observed enhanced current signal.     

Infrared studies of pyroelectric Langmuir–Blodgett films of arachidic acid and 1,2-bis(dodecyloxy)-4,5-diaminobenzene

Fourier Transform Infrared (FTIR), p-polarized grazing angle (GAIR) and Horizontal Attenuated Total Reflectance (HATR) spectra have been recorded of arachidic acid (AA)/1,2-bis(dodecyloxy)-4,5-diaminobenzene (DADB) Y-type alternate LB films deposited on an aluminium plate with 31 layers. It is well known that the frequencies of CH₂ stretching bands of a hydrocarbon chain are sensitive to the conformational ordering of the chain. Changes in frequency and intensity can be used to characterize film ordering and preferential molecular packing. The observed peak frequencies and intensities of these bands indicate that the alkyl chains are present in a mostly trans conformation and tilted from the normal direction with respect to the substrate in LB films. The FTIR–GAIR and HATR spectra of 31 layers alternate film show significant changes in the region 1700–1400 cm⁻¹ due to the partial proton transfer between acid and amine head groups. According to the HATR spectrum, the peak at 1731 cm⁻¹ is observed due to a proportion of the carboxylic acid groups forming sideways dimers indicating that if the carboxylic acid groups form sideways dimers, they are less likely to undergo proton transfer with the amino groups.     

Voltammetric Detection of Copper Ions on a Gold Electrode Modified with a N-methyl-2-naphthyl-cyclam film

A voltammetric sensor based on a methyl-naphthyl cyclam (1,4,8,11-tetraazacyclotetradecane) film deposited through dip coating on gold electrodes allowed the sensitive detection of copper (II). The obtained film was characterized in terms of the composition and morphology using Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy. In the presence of divalent metallic ions (copper and nickel) and trivalent metallic ions (iron), at pH 7 and 7.7, the current density of the ionic oxidation peak maximum was determined using square wave voltammetry. The relative variation of this current density varies linearly with the base-ten logarithm of the ion concentration. From this calibration curve, a detection limit of 7?×?10^?12?M was obtained for copper (II) at pH 7. At this pH value, the sensitivity of detection of copper (II) was 2.5 times higher than for nickel (II) and 5.8 times higher than for iron (III). The methyl-naphthyl cyclam film-modified gold sensor was validated for the detection of copper in spiked urine samples.     

Spin state transitions in Langmuir–Blodgett films of recombinant cytochrome P450scc and adrenodoxin

Langmuir–Blodgett (LB) films of recombinant cytochrome P450scc, of P450scc–adrenodoxin (Adx) complex and alternated layers of Adx and P450scc have been obtained. Spectral properties of these proteins in thin films were investigated by UV–Vis absorption spectroscopy. It has been found that cytochrome P450scc exists in LB films only in low-spin state while before the deposition it was in high-spin state. The data suggest that transferring the hemoprotein or its complex with redox partner results in the modification of the spin state by a conformational transition. In order to investigate further the P450scc and Adx interaction, the mass density of the films formed from these molecules has been studied by nanogravimetric measurements. Comparative study between nanogravimetric and spectral characterisation was performed. The results indicate that the protein–protein interaction is disrupted, when the complex is organised in thin film.     

Effect of water on the interfacial structures of room-temperature ionic liquids

The interfacial structures of cyano-based room-temperature ionic liquids play a very important role in reducing friction. However, the presence of water impairs their tribological performance. The interfacial structures and friction forces of 1-ethyl-3-methylimidazolium dicyanamide, [EMIM][DCN], and the effect of water on these structures and forces were investigated using atomic force microscopy. In addition, the interaction of water and [EMIM][DCN] was evaluated using Fourier-transform infrared (FT-IR) spectroscopy. Multiple repulsive layers were observed in the [EMIM][DCN] solution. This solution showed low friction force because these repulsive layers worked as protective layers against friction. On the other hand, the specific repulsive layer characteristics of [EMIM][DCN] could not be observed in a [EMIM][DCN] + 2 wt% H₂O solution. FT-IR results indicated that the layer structure of [EMIM][DCN] was disturbed by the addition of H₂O. Therefore, the solution containing water exhibited a high friction force.     

Preparation and characterization of 5,10,15,20‐tetraphenylporphyrin Langmuir films for gas chemsensor applications

In this work is reported the preparation and characterization of 5,10,15,20‐tetraphenylporphyrin (H₂TPP) films at the water‐air interfaces. The surface pressure‐area isotherms (π‐A) and UV‐Vis spectroscopy were used to investigate the effect of the spreading methods and parameters on the porphyrin monolayer formation. Also, Langmuir‐Blodgett (LB) and Langmuir‐Schaefer (LS) films were deposited onto glass substrates in order to study the conformation changes in porphyrin molecular packing. Quartz crystal microbalance (QCM) was utilized as the active solid substrate for the development of the NO₂ gas sensor based on the H₂TPP molecular films. The results of π‐A curves have clearly shown the significant contribution of the preparation methods and processing parameters on the conformation of porphyrin molecular films. The UV‐Vis spectroscopy results using polarized absorption dichroism have indicated different molecular packing for porphyrin films deposited by LB and LS methods, with relative tilted angles of 50° ± 5° and 35° ± 5°, respectively. Moreover, the QCM response has given strong evidence that H₂TPP porphyrin molecular films have performed as NO₂ chemsensor. Copyright © 2011 John Wiley & Sons, Ltd.     

Statistical thermodynamics approach for estimation of the interaction parameters in the mixed Langmuir films at the water/air interface

The treatment of two-dimensional system of three components of molecules of different molecular sizes, adapted from statistical thermodynamics, is used to derive parameters which could be derive parameters which could be related to the interaction between molecules in the mixed Langmuir films formed at the water/air interface.     

Chemical and structural changes at the ABS polymer-copper metal interface

Creating oxygen containing moieties (hydroxyl or carbonyl) on polymer substrate surfaces is known to increase the adhesion strength of polymers to metals. However, we noticed adhesion increase with time even though no pre- or post-treatment of the polymer substrate was done. In the case of sputtered and galvanically strengthened copper coatings on acrylonitrile-butadiene-styrene polymer (ABS) substrate, the adhesion strength increased from approximately 6 J/m² to 53 J/m² during a 1008-h period. During this period structural and chemical changes of the polymer near the interface take place. Carbonyl functionalities developed on the ABS surface are most likely responsible for the large increase in the adhesion strength. Chemical changes of the polymer are probably a consequence of the galvanic deposition and a close contact of ABS with copper which is known to facilitate the oxidation of ABS.     

About the formation and characterisation of a new phase of polycationic monolayers at the air/water interface

The interfacial behaviour of highly charged cationic polyelectrolytes (PEs) (by alkylation of poly-4-vinylpyridine with C₈-, C₁₂- or C₁₆-alkyl halogenides) was investigated at the air/water interface. Great care must be taken with respect to the experimental technique for recording the isotherms. Only a stepwise procedure, applying small pressure increments and checking and recording potential area relaxations, reveals the intrinsic monolayer characteristics. After a true equilibrium range in the isotherm, starting from the onset of first film pressure to the almost closed packed layer, it could be demonstrated that at certain states of compression, a spontaneous kinetically controlled area relaxation is induced, which coincides with a mono- to triple-layer state transition. This region is visible as large shifts in area if the film is kept at constant pressure. The presence and coexistence of well-defined triple-layer states could be proved by Langmuir–Blodgett (LB) transfer and ellipsometric thickness measurements as well as by atomic force microscopy (AFM). Typical features of all area relaxations are induction periods and sigmoidal courses of the plots. According to these relaxations and the characterisation by AFM, the new phase formation is characterised by progressive nucleation and one-dimensional growth. Based on relaxation measurements, a saturation pressure can be derived, constituting the reference for defining supersaturation for nucleation and growth. All experimental findings together can be understood, if one assumes a stretched, stiff and rod-like structure as the most plausible and simple model for the PE in the presence of hard counterions. For the case of soft and polarisable counterions, for electrostatic reasons, the polymer behaviour changes to a worm-like structure. The rate of nucleation of the well-defined phase formation became highly irreproducible in this case.     

Supramolecular self-assembly study of a flexible perylenetetracarboxylic diimide dimer in Langmuir and Langmuir–Blodgett films

A novel perylenetetracarboxylic diimide molecule (2PDI-TAZ), which contains two perylenetetracarboxylic diimide (PDI) attached to a melamine headgroup, was designed and synthesized. Supramolecular self-assemblies were studied in Langmuir and Langmuir–Blodgett films. Surface pressure–area isotherm measurements and the spectroscopic studies indicate that the 2PDI-TAZ molecules adopted a face-to-face configuration and edge-on orientation in Langmuir or the multilayer LB films. The presence of the barbituric acid in subphase change the hydrophilicity of 2PDI-TAZ due to the hydrogen bonding between melamine and barbituric acid, which has been revealed by the π–A isotherms and the FT-IR spectra. Transmission electron microscopy images of the LB films deposited from the barbituric acid solution revealed uniform nanowire morphology while the X-ray diffraction studies indicate that the molecules in the solid film packed with high order. The strong excimer emission of 2PDI-TAZ in LB films suggests enforced face-to-face configuration for the PDI unites in LB films in relative to that in solution.     

Catalysis of the electrochemical oxidation of glucose by glucose oxidase and a single electron cosubstrate: kinetics in viscous solutions

Catalysis of the electrochemical oxidation of glucose by glucose oxidase with a single electron mediator (cosubstrate) may be used to transform mixtures of concentrated industrial sugars. How the high viscosity of such media may affect the enzymatic reaction and the transport of the mediator can be mimicked by addition of large concentrations of sucrose to glucose solutions. Cyclic voltammetry then provides a simple means of investigating the effect of an increased viscosity on the kinetics of the enzymatic reaction and the diffusion of the mediator. The diffusion coefficient of the mediator is decreased 10 times by addition of 1.6 M sucrose. At pH 8, in the presence of the same concentration of sucrose, the catalytic activity of the enzyme towards its substrate is only slightly affected. A 35% decrease of the glucose Michaelis constant is observed. The reaction of the reduced enzyme with the cosubstrate is six times slower and the mediator Michaelis constant undergoes a three-fold increase. It follows that glucose oxidase remains an efficient catalyst in such viscous media.     

Symmetric esters derived from 1,3,4-oxadiazole: synthesis,mesomorphic properties and structural study by semi-empirical calculations

Three homologous series of symmetric esters derived from 2-alkylthio-5-(p -hydroxy)phenyl-1,3,4-oxadiazole (series 3, 4 and 5) were synthesized and their liquid crystalline properties investigated by optical microscopy and differential scanning calorimetry. Depending on the chain length, nematic and smectic C phases were observed in the series 3 ; smectic A and smectic C phases in series 4. None of the homologues of series 5 shows mesomorphic properties. These three series are compared with other previously reported oxadiazoles. A structural study of AM1 semi-empirical calculations is also described.     

Application of Polyfolates in the Development of Electrochemical Glucose Biosensors

Folic acid was polymerised electrochemically at a glassy carbon electrode surface from 0.1 mol L^?1 phosphate buffer saline solution, pH 5.0, containing 0.1 mmol L^?1 monomer. The obtained thin film was porous with a pore size of 50–60 nm. Since its electrochemical stability was rather short, the polyfolate film was covered with a graphene‐chitosan composite layer which increased its stability significantly. The best strategy to immobilise the enzyme was crosslinking with glutaraldehyde. The lifetime of this glucose biosensor in use was at least 12 days, on‐shelf life time was at least 30 days. The linear range was up to 1 mmol L^?1 and the LOD was 0.6 µmol L^?1. The first polyfolate‐based biosensor was applied to analysis of natural samples.