A trough with radial compression for studies of monolayers and fabrication of Langmuir-Blodgett films

A new type of Langmuir trough with radial compression is reported for studies of monolayer properties and fabrication of Langmuir-Blodgett (LB) films. The trough included twenty curved diaphragms, which form a circular shape on water and compress monolayers. Typical monolayers of arachidic acid, DL--phosphatidylcholine dipalmitoyl, cellulose tridecanoate, and poly (butyl methacrylate) showed reproducible surface pressure-area isotherms with radial compression. Flow profiles were studied of monolayers on cellulose tridecanoate and poly(butyl methacrylate) and suggested that the monolayers are radially compressed with no particular film disturbance. A Wilhelmy glass plate preserved no deflection in the subphase surface from low to high surface pressures during the film compression and decompression; no pressure excess causing the deflection, which is frequently observed in the case of the film compression with a sliding barrier, was found on both sides of the Wilhelmy plate. Application of the radial compression enabled to do a symmetrical deposition for the fabrication of LB films.     

Photoinduced energy and electron transfer of covalently linked zinc(II)-pyropheophytin-anthraquinone molecules in monolayers of Langmuir-Blodgett films

Electron donor-acceptor compounds of optically pure covalently linked Zn(II)-pyropheophytin-anthraquinone (ZnPQ) have been synthesized. These compounds allow one to prepare highly ordered solid samples by means of the Langmuir-Blodgett technique. Monolayer films of the P4(S)-ZnPQ diastereoisomer were prepared in a dioleoyl phosphatidylcholine matrix with the concentration varied from 0.5 to 10 mol.%. The time-resolved fluorescence properties of the films were interpreted in terms of the presence of two types of P4(S)-ZnPQ conformers, the folded one and the opened one. The folded conformer predominated and underwent fast intramolecular electron transfer (rate constant > 10¹¹s⁻¹) from the Zn-pyropheophytin part to the anthraquinone moiety and was, therefore, not observed directly in the fluorescence study. The open conformer had a relatively long-lived singlet excited state and was quenched by an energy transfer process to the folded conformer at P4(S)-ZnPQ concentrations higher than 2 mol.% with an estimated quenching radius of 5±0.2 nm.     

PEDOT-PSS导电聚合物LB薄膜的制备和性能

研究了十八胺(ODA)及其与硬脂酸(SA)混合单分子膜在导电聚合物聚3,4-乙烯二氧噻吩/聚苯乙烯磺酸(PEDOT-PSS)胶体亚相上的成膜行为和复合LB膜在室温下的电学性能.结果表明:ODA-SA/PEDOT-PSS复合LB膜具有更好的成膜性能,表面粗糙度小且稳定可控,薄膜具有较好的有序结构;ODA-SA/PEDOT-PSS膜电导率高于ODA/PEDOT-PSS复合LB膜,其电导率呈各向异性,水平电导率(σ‖)与垂直电导率(σ⊥)之间相差3～4个数量级,Ⅰ-Ⅴ曲线呈指数关系,为典型的电子隧穿类型.     

Langmuir-Blodgett films from polyaniline/ruthenium complexes as modified electrodes for detection of dopamine

Langmuir-Blodgett (LB) films from mixtures of polyaniline (PANI) and the ruthenium complex mer-[RuCl₃(dppb)(py)] (dppb=PPh₂(CH₂)₄PPh₂; py=pyridine) (Rupy) were used to modify electrodes and to detect dopamine (DA). The electrochemical response has been used as the principle of detection in order to exploit the molecular-level interaction between PANI and Rupy in the LB films. Cyclic voltammograms were performed in a potential range where the Rupy complex is not electroactive and the response is dominated by the interconversion between the oxidation states leucoemeraldine and emeraldine of PANI. In the presence of DA a further redox pair appears at approximately 230 and 0 mV, associated with oxidation/reduction of DA to dopaminequinone. The current increased linearly in the range between 4.0×10⁻⁵ and 1.2×10⁻³ mol/l. The detection limit for DA concentrations for the 21-layer LB film from PANI/Rupy was 4.0×10⁻⁵ mol/l, which is sufficient to detect DA in a pharmaceutical product. The modified electrode based on PANI/Rupy LB films is selective to DA, particularly because the oxidation potential for DA is lowered, in comparison to a bare indium-tin-oxide electrode. The presence of ascorbic acid, which is an important interferent for DA, could be detected when its concentration was three times that of DA.     

Study of electrical polarization hysteresis in ferroelectric polyvinylidene fluoride films

Electric properties of polyvinylidene (PVDF) films fabricated using the Langmuir-Schaefer method have been studied. Films of different thickness were deposited on silicon substrates and analyzed using several techniques. X-ray diffraction (XRD) data showed that PVDF films crystallize at an annealing temperature above 130 °C. Polarization versus electric field (PE) ferroelectric measurements was done for samples prepared with electrodes. PE measurements show that the coercivity of the films increases as the maximum applied electric field increases. The coercivity dependence on the frequency of the applied electric field can be fitted as f^0.6. The results also show that the coercivity decreases with increasing the thickness of PVDF film due to the pinning effect.     

Central metal effect on the organization of porphyrin LB films

Monolayers and Langmuir–Blodgett (LB) films of 5,10,15,20-tetra-4-oxy (2-stearic acid) phenylporphyrin (TSPP) and its complexes TSPPCu(II) and TSPPMn(III)Cl have been studied. π–A isotherms show that the molecular areas of the three molecules are similar to one another, while the polarized UV-vis spectra of the transferred films show different tilt angles of macrocycles with respect to the substrates: TSPP, 31°; TSPPCu(II), 0°; TSPPMn(III)Cl, 52°. The UV-vis spectra show that the π–π interactions between porphyrin macrocycles in monolayers of these three porphyrins are different. The Brewster angle microscopy (BAM) observations suggest that the monolayer behavior is also dependent on the central metal ions. These results indicate that the central metal ions have great influence on the organization of the films.     

Heat-induced cross-linking of nylon-like weak polyelectrolyte multilayer films: UV-visible studies of methylene blue loading and rates of release

Heat-induced cross-linking of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) films was studied as a potential method for controlling the rate of release of an absorbed dye, methylene blue (MB), from the film. Alternating layers of PAA and PAH films were prepared, loaded with MB and cured at different temperatures/times. Our studies identified the conditions best controlling both the amount of MB loading and its rate of release from PAA/PAH films to be curing at 150 °C for 10 min. In general, curing temperatures both above and below 150 °C showed slower rates of release as well as lower amounts of MB loading.     

Effects of interfacial modification on the performance of an organic transistor based on TCNQ LB films

The influence of interfacial modifications of n-type organic-based thin-film transistors (OTFTs), using low work-function metal electrodes and self-assembled monolayers (SAMs), were analyzed by electrical and structural measurements. We employed ultra thin films of long-alkyl-chained tetracyanoquinodimethane (C₁₈-TCNQ) Langmuir-Blodgett (LB) films as the n-type semiconducting layer with nano-scale thickness. A significant increase in the drain current was observed for the OTFT with low work-function metal electrodes, and this current increase was consistent with the decrease in the injection barrier height for carrier electrons to transfer from the electrodes into the LB films. On the other hand, it was found that the density of interfacial trapping sites decreased with the SAMs treatment, even though no structural modification was induced by the SAM. This behavior is considered to be due to the elimination of active SiOH groups on the SiO₂ substrate by the SAMs treatments. It was also found that the device parameters strongly depend on the alkyl chain length of the SAMs. In order to explain this phenomenon clearly, a new interfacial model based on the interaction between carrier electrons and interfacial SiOH groups is proposed.     

Surface properties of LB monolayers composed of aminimide derivatives having two alkyl chains

Langmuir-Blodgett monolayers were prepared using a series of aminimide derivatives having two alkyl chains. One of the alkyl chains had 19 carbons, while the number of the other alkyl chain was systematically changed from 8 to 20. Molecular organization of the aminimides on the water surface reflected the length of alkyl chains. Inter-chain packing and conformational order of the alkyl chains in the monolayer were affected by the length of the two alkyl chains. Steric and dynamic contact angles of a series of aprotic liquid having low surface tension were measured to investigate surface properties of the films with varying molecular organization of alkyl tail groups. Fluid-like monolayers having molecularly rough surface indicated high wettabilities for both of n-alkanes and naphthalene derivatives. However, contact angle hyteresis on the monolayer was sensitive to the sort of probe liquids owing to the interaction between the uppermost portion of the alkyl chains aligned homeotropically on the fluid-like monolayers. In contrast, the monolayer containing well-ordered, well-packed alkyl chains indicated low wettabilities, small hysteresis and random orientation of liquid crystals.     

Stability of dimethyldioctadecylammonium bromide Langmuir-Blodgett films on mica in aqueous salt solutions—implications for surface force measurements

Langmuir-Blodgett (LB) monolayer films of dimethyldioctadecylammonium bromide (DDOA) on muscovite mica have been studied using Wilhelmy plate type wetting measurements, surface force measurements, atomic force microscopy (AFM), and Brewster angle microscopy (BAM) on insoluble monolayers of DDOA before deposition. In particular, the effect of exposure to aqueous KBr salt solutions was investigated. BAM shows a heterogeneous monolayer with small condensed domains of dendritic shape under conditions normally used for deposition. A stick-jump behaviour of the meniscus is seen during deposition, leading to a large-scale heterogeneity measurable in wetting studies. These also show breakdown and hydrophilization of the LB film at the three-phase contact line (meniscus) and when exposed to salt solutions of approximately 10⁻² M concentration. The advancing contact angle against water is approximately 105°. Surface force measurements show long-range attraction in water, but also a surface charge which depends on salt concentration, and breakdown when surfaces are brought into contact in high salt concentrations. AFM images of untreated films show small holes, and breakdown when exposed to salt solution, especially at the three-phase line. The LB film is judged to be less suitable as a model hydrophobic surface owing to its heterogeneity and instability in salt solution.     

Thin films of liquid crystal fumarates and a related acrylate

Five fumarate esters and one acrylate ester have been synthesized and their properties examined. Four of them can be deposited as multilayers by evaporation in vacuo, and three of this group behave as thermotropic smectic liquid crystals. We have attempted to stabilize the homeotropic smectic phase by polymerizing the film using UV light. In two cases we have been successful and have produced films which are resistant to prolonged heating in THF. We infer that these materials are both polymerized and cross linked. They retain their regular layer structure after polymerization, a behaviour which we have demonstrated by X-ray diffraction. We have also attempted to form Langmuir-Blodgett multilayers with these compounds and in four cases have been successful.     

Comparative study of porphyrin derivatives in monolayers at the air-water interface and in Langmuir-Blodgett films

The orientation and aggregation of various porphyrin derivatives at the air-water interface and in Langmuir-Blodgett films were investigated. Monolayer properties of these molecules, where long alkyl chain(s) were covalently bound through different functionality of varying hydrophilicity were studied by measuring surface pressure area isotherms. Such derivatives, where ether functionality (functionalities) was (were) used for linking long alkyl chain(s), did not form uniform monolayer; instead they were found to form multilayer clusters or aggregates on the water surface. On the other hand, porphyrin derivative functionalized at the four peripheral phenyl rings with eight hexadecyl ether chains formed stable spherical vesicles when deposited on mica. Tetra N-alkyl pyridinium porphyrins with long alkyl chain were found to form various phases on the water surface. Evidence of transition from horizontal orientation to vertical orientation of porphyrin rings of porphyrin molecules having C₁₄ chains was observed. This type of transition was lost with the porphyrin molecule with a relatively smaller chain (C₈).     

Non-linear optical effects in layer-by-layer alternate films of polycations and an azobenzene-containing polyanion

The technique of polycation/ polyanion layer-by-layer adsorption was applied to fabrication of non-linear optical films. Optical second-harmonic generation (SHG) was clearly observed from the films prepared by alternate adsorption of polycations and an azobenzene-containing polyanion (PAZO) on quartz substrate modified with polyion precursor. Regular film growth during adsorption cycles was proved by means of UV spectroscopy and quartz crystal microbalance technique. The SHG signal increased with the number of PAZO layers up to four layers, while further assembly reduced the signal. The SHG decreased considerably by heating the film up to 120 °C.     

Photophysical investigations of Langmuir–Blodgett mono- and multilayer films of pheophorbide-a

Absorption, fluorescence and time-resolved fluorescence spectroscopy has been applied to quasi two-dimensional Langmuir–Blodgett monolayers and three-dimensional arrangements of Langmuir–Blodgett films with different numbers of layers. Pure dye films of pheophorbide-a and films in which pheophorbide-a molecules were embedded in a behenic acid matrix have been investigated. Optical spectroscopy is used in order to obtain evidence for intra- and interlayer interaction in the membrane-like environment and distinguish between these different types of interactions. Depending on how the dye molecule is embedded a broad range of π-system interactions occur. Optical band shifts as well as band shape changes and the formation of new bands are indicators for these molecular interactions [M.B. Grieve, A.J. Hudson, T. Richardson, Thin Solid Films, 1994, 243, 581.]. Among such interactions, exciton coupling and excimer formation are considered to be possible reasons for the observed major differences between spectra and kinetics of mono- and multilayer Langmuir–Blodgett films. Exciton coupling is indicated by red shifts of the Q-band of about 600 cm⁻¹ in the absorption spectra of monolayer films. The observed broad red shifted fluorescence in the near-infrared region and their short lifetimes (≤100 ps) are assigned to excimer formation. Time-resolved fluorescence spectra of bilayer films as well as polarization dependencies of the Langmuir–Blodgett film absorption are presented.     

The anisotropic gas response behavior of LB films of an amphiphilic asymmetrically substituted phthalocyanine

1,4-Di(2-hydroxyethoxy)-9,10,16,17,23,24-hexapentyloxy phthalocyanine (asy-Pc) has been deposited as well-ordered Langmuir–Blodgett (LB) films on a glass substrate and glass-bearing interdigital electrodes. The molecular orientation of the asy-Pc LB films has been characterized by polarized UV-vis absorption spectra and in-plane dichroism has been observed. The LB films show good room temperature response to NO₂ gas and the recovery is almost complete. It has been found that the magnitude of the response increases with increasing gas concentration and is also dependent upon the LB dipping direction relative to the arms of the electrodes, consistent with conduction being favored along the axes of the stacks of asy-Pc molecules.     

Photo- and electro-luminescent properties of 5,10,15,20-tetra-p-tolyl-21H,23H-porphine doped poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] films

In this work we studied both photoluminescence (PL) and electroluminescence (EL) properties of 5, 10, 15, 20-tetra-p-tolyl-21H, 23H-porphine (TTP) doped poly[2-methoxy-5-(2′-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) with the weight percentages of 0, 0.5, 1, 3, 5, 8 and 12, respectively. In the process of PL the significant energy transfer occurs from MEH-PPV to TTP, even though there is a small spectral overlap between the absorption of TTP and the emission of MEH-PPV. For investigation of the process of EL a series of organic light-emitting diodes were fabricated with the device structure of ITO/PEDOT:PSS/TTP-doped polymer layer/Al (ITO = Indium Tin Oxides; PEDOT:PSS = poly (3,4-oxyethyleneoxythiophene): poly-(styrene sulfonate)). In devices in which the TTP was present at 5% the emission of EL was dominated by TTP; at lower doping levels MEH-PPV emission dominated. Moreover, multi-color emission was observed at the doping level below 5%. On the other hand, the mechanism for the EL process was reported.     

一种新型可溶喹吖啶酮衍生物LB膜的制备及其光谱特性

采用表面压-分子面积(π一A)等温曲线和紫外-可见吸收谱的方法研究了一种新型喹吖啶酮衍生物材料LB膜的制备及其光谱特性．实验表明,这种新型喹吖啶酮衍生物能够在水面上形成稳定的单分子膜,它与花生酸(AA)混合后不仅可以形成很好的单分子膜,而且可以较好地转移到固体基片上制备成LB膜多层膜．这种新型喹吖啶酮衍生物LB膜的紫外-可见吸收谱的吸收峰位较稀溶液发生了红移,这是由极性溶剂分子与其相互作用的结果．它在溶液和LB膜中都是以单体的形式存在．     

Atomic force microscopy studies of lateral phase separation in mixed monolayers of dipalmitoylphosphatidylcholine and dilauroylphosphatidylcholine

Atomic force microscopy imaging of dipalmitoylphosphatidylcholine (DPPC)/dilauroylphosphatidylcholine (DLPC) monolayers deposited onto alkanethiol modified-gold surfaces by the Langmuir–Schaefer technique was used to investigate domain formation in a binary system where phase separation arises from a difference in the alkyl chain lengths of the lipids. We have established how the condensed domain structure (shape and size) in DPPC/DLPC monolayers depends on the surface pressure and lipid composition. The mixed monolayers exhibit a positive deviation from an ideal mixing behavior at surface pressures of 32 mN/m. Lateral compression to pressures greater than the liquid-expanded-to-liquid-condensed (LE-to-LC) phase transition pressure of the mixed monolayer (8–16 mN/m) induces extensive separation into condensed DPPC-rich domains and a fluid DLPC matrix. The condensed structures observed at a few milliNeutons per meter above the LE-to-LC transition pressure resemble those reported for pure DPPC monolayers in the LE/LC co-existence region. At a bilayer equivalence pressure of 32 mN/m and 20 °C, condensed domains exist between x_DPPC 0.25 and 0.80, analogous to aqueous DPPC/DLPC dispersions. Compression from 32 to 40 mN/m results in either a striking distortion of the DPPC domain shape or a break-up of the microscopic DPPC domains into a network of nanoscopic islands (at higher DPPC mol fractions), possibly reflecting a critical mixing behavior. The results of this study provide a fundamental framework for understanding and controlling the formation of lateral domain structures in mixed phospholipid monolayers.     

Morphology and electrochemical properties of amphiphilic viologen functionalized multiwalled carbon nanotube hybrids in Langmuir-Blodgett films

Amphiphilic viologens were electrostatically adsorbed on the surface of multiwalled carbon nanotubes (MWCNT) to form viologen-MWCNT hybrids, in which the content of viologens was about 5-10% in weight. Although both viologens and MWCNT hardly dispersed in the water-insoluble organic solvents, the as-prepared viologen-MWCNT hybrids were well dispersed in them with a strong long-term stability, the features of which provided a possibility to prepare their insoluble monolayers at the air-water interface. The surface pressure-area isotherms of these hybrids revealed that they could form stable monolayers, which were transferred on the substrate surfaces by the Langmuir-Blodgett (LB) method. Morphologies of the LB films were characterized by using scanning electron microscopy and atomic force microscopy, the images of which revealed the formation of network two- or three-dimensional films of the functionalized MWCNT. Cyclic voltammograms of the LB films revealed one or two couples of one electron transfer process corresponding to the viologen-MWCNT hybrids with the cathodic and anodic potentials closely related to the alkyl chains of the viologens.     

Supramolecular architectures in layer-by-layer films of single-walled carbon nanotubes,chitosan and cobalt (II) phthalocyanine

The building of supramolecular structures in nanostructured films has been exploited for a number of applications, with the film properties being controlled at the molecular level. In this study, we report on the layer-by-layer (LbL) films combining cobalt (II) tetrasulfonated phthalocyanine (CoTsPc), chitosan (Chit) and single-walled carbon nanotubes (SWCNTs) in two architectures, {Chit/CoTsPc}_n and {Chit-SWCNTs/CoTsPc}_n (n = 1–10). The physicochemical properties of the films were evaluated and the multilayer formation was monitored with microgravimetry measurements using a quartz microbalance crystal and an electrochemical technique. According to atomic force microscopy (AFM) results, the incorporation of SWCNTs caused the films to be thicker, with a thickness ca. 3 fold that of a 2-bilayer LbL film with no SWCNTs. Cyclic voltammetry revealed a quasi-reversible, one electron process with E_1/2 at −0.65 V (vs SCE) and an irreversible oxidation process at 0.80 V in a physiological medium for both systems, which can be attributed to [CoTsPc(I)]⁵⁻/[CoTsPc(II)]⁴⁻ and CoTsPc(II) to CoTsPc(III), respectively. The {Chit-SWCNTs/CoTsPc}₅ multilayer film exhibited an increased faradaic current, probably associated with the supramolecular charge transfer interaction between cobalt phthalocyanine and SWCNTs. The results demonstrate that an intimate contact at the supramolecular level between functional SWCNTs immobilized into biocompatible chitosan polymer and CoTsPc improves the electron flow from CoTsPc redox sites to the electrode surface.