An ensemble and single-molecule fluorescence microscopy investigation of phase-separated monolayer films stained with Nile Red

Phase-separated Langmuir-Blodgett monolayer films prepared from mixtures of arachidic acid (C19H39COOH) and perfluorotetradecanoic acid (C13F27COOH) were stained via spin-casting with the polarity sensitive phenoxazine dye Nile Red, and characterized using a combination of ensemble and single-molecule fluorescence microscopy measurements. Ensemble fluorescence microscopy and spectromicroscopy showed that Nile Red preferentially associated with the hydrogenated domains of the phase-separated films, and was strongly fluorescent in these areas of the film. These measurements, in conjunction with single-molecule fluorescence imaging experiments, also indicated that a small sub-population of dye molecules localizes on the perfluorinated regions of the sample, but that this sub-population is spectroscopically indistinguishable from that associated with the hydrogenated domains. The relative importance of selective dye adsorption and local polarity sensitivity of Nile Red for staining applications in phase-separated LB films as well as in cellular environments is discussed in context of the experimental results.     

Self-assembled arrays of silica particles on the patterns reflecting the phase-separated structures of mixed Langmuir–Blodgett films

In this study, we constructed self-assembled arrays of silica particles on the micro- and nanopatterns of functionalized templates fabricated from phase-separated mixed Langmuir–Blodgett (LB) films. Electrostatic interaction between the carboxylic groups of silica particles and amino groups on the functionalized templates results in the formation of the self-assembled arrays of silica particles. The patterns of the silica particle arrays can be designed by controlling the phase-separated structures of the original mixed LB films.     

Rippled domain formation in phase-separated mixed Langmuir-Blodgett films

The morphology and composition of phase-separated Langmuir and Langmuir-Blodgett films of stearic acid (C17H35COOH) (SA) mixed with perfluorotetradecanoic acid (C13F27COOH) (PA) have been investigated using a combination of atomic force microscopy (AFM) measurements and surface pressure-area isotherms. At elevated surface pressures, the mixed film phase-separated to form a distinct series of lines (ripples), as opposed to the hexagons that have previously been observed with mixed films with longer alkyl chain fatty acids. At low surface pressures, phase separation is still observed, though a range of different domain structures was formed. The chemical composition of the phase-separated domains has been investigated by AFM-based compositional mapping, which has allowed unambiguous identification of the chemical composition of the domains. A simple mechanistic model describing how domain formation takes place in this system is presented.     

Effect of subphase temperature on the phase-separated structures of mixed Langmuir and Langmuir-Blodgett films of fatty acids and hybrid carboxylic acids

We examined the phase-separated structures of the mixed Langmuir-Blodgett (LB) films of C(k)H(2k+1)COOH (HkA: k=17, 21) and C(m)F(2m+1)C(10)H(20)COOH (FmH10A: m=6, 8) fabricated isothermally or after isobaric thermal treatments. Under isothermal fabrication conditions, disks and wire-type domains formed in the H17A/F6H10A LB films at high and low fabrication temperatures, respectively, because the line tension and dipole-dipole interaction were comparable with each other. The thermodynamically stable phases of H21A/F6H10A LB films at high and low fabrication temperatures were disks and polygonal domains, respectively. Isobaric thermal treatments of the Langmuir films affected the domain size and not the domain shape when the transfer temperature was the same. Isobaric thermal treatments were effective in controlling the domain size. The thermodynamically stable phases of both H17A/F8H10A LB films and H21A/F8H10A LB films were nanowires in the range of the fabrication temperatures studied. Isobaric thermal treatments of the Langmuir films did not affect the domain shape significantly and affected the domain size in both the LB films studied. The change in the value m of FmH10A was more effective in controlling the phase-separated structures of the mixed LB films than the value of k of HkA.     

Micro- and nanopatterned copper structures using directed self-assembly on templates fabricated from phase-separated mixed Langmuir-Blodgett films

We report a versatile method to confine metal thin films in micro- and nanopatterns using directed self-assembly on the templates fabricated from phase-separated mixed Langmuir-Blodgett (LB) films. The pattern of the mixed LB films can be tuned by adjusting intermolecular interaction between the film-forming molecules in the LB films and by varying the fabrication conditions of the films such as the mixing ratio, subphase temperature, and surface pressure. We use the patterned LB films for templates to confine metal in patterned regions, taking advantage of the difference between the surface free energy of the patterned regions and that of the self-assembled monolayer of the silane coupling agent. Au nanoparticles are confined onto the patterned films as a catalyst for the succeeding Cu electroless deposition. The atomic force microscopic images, Auger electron spectra, and scanning Auger electron maps of a Cu-deposited film show that Cu is selectively deposited on the patterns of phase separation of the original mixed LB films.     

A comparative study of the physicochemical properties of perfluorinated and hydrogenated amphiphiles

In this work we studied and compared the physicochemical properties of perfluorinated (sodium perfluoroheptanoate, C7FONa, and perfluorooctanoate, C8FONa) and hydrogenated (sodium octanoate, C8HONa, decanoate, C10HONa, and dodecanoate, C12HONa) amphiphiles. First, we determined their Krafft points to study the solubility and appropriate temperature range of micellization of these compounds. The critical micelle concentration (cmc) and ionization degree of micellization (beta) as a function of temperature (T) were estimated from conductivity data. Plots of cmc vs T appear to follow the typical U-shaped curve with a minimum T(min). The results show that the surfactants with CF2/CH2 ratio of 1.5 between alkyl chains (C12HONa-C8FONa and C10HONa-C7FONa) have nearly the same minimum value for cmc against temperature. The comparison between the cmc of hydrogenated amphiphiles and the corresponding perfluorinated amphiphiles must be done at this point. Thermodynamic functions of micellization were obtained by applying different theoretical models and choosing the one that best fit our experimental data. Although perfluorinated and hydrogenated amphiphiles present similar thermodynamic behavior, we have found a variation of 1.3 to 1.7 in the CF2/CH2 ratio, which did not remain constant with temperature. In the second part of this study the apparent molar volumes and adiabatic compressibilities were determined from density and ultrasound velocity measurements. Apparent molar volumes at infinite dilution presented the ratio 1.5 between alkyl chains again. However, apparent molar volumes upon micellization for sodium perfluoroheptanoate indicated a different aggregation pattern.     

A comparative Langmuir-Blodgett study on a set of covalently linked porphyrin-based amphiphiles: a detailed atomic force microscopic study

A set of covalently linked phenyl-amidophenyl-substituted porphyrin amphiphiles with n-C15H31 tails have been synthesized and completely characterized. These amphiphiles form good Langmuir-Blodgett (LB) films at the air/water interface. Mean molecular areas for the series were measured from the isotherms and found to increase as the number of aliphatic chains increased from one to four. No influence of the subphase pH was observed on the isotherms. LB films can be transferred successfully onto different solid surfaces. The LB films were characterized using tapping mode atomic force microscopy (AFM). Bis-, tris-, and tetra-substituted porphyrins were found to be fairly good film-forming amphiphiles, whereas irregular aggregates were seen in the case of the monosubstituted porphyrin amphiphile. Multilayers were also formed with tetra-substituted amphiphiles on mica. Detailed AFM studies of tetra-substituted amphiphiles have been carried out to investigate the effect of preparation procedure and solid substrates on film formation and transfer. The absorption and fluorescence spectra for the amphiphiles in solution and LB films deposited onto mica and glass were recorded, which demonstrated the successful transfer of LB films onto the substrates and provided more information about the arrangement of porphyrin molecules within the LB films. For comparison, self-assembled monolayers (SAMs) and the cast thin films of the amphiphiles were prepared and characterized.     

Characterization of domain growth kinetics in a mixed perfluorocarbon-hydrocarbon Langmuir-Blodgett monolayer

The rate of domain growth in phase-separated, mixed Langmuir-Blodgett (LB) monolayers of arachidic acid, C(19)H(39)COOH (AA) and perfluorotetradecanoic acid, C(13)F(27)COOH (PA) was tracked via atomic force microscope measurements. The growth rate of domains comprised of phase-separated AA was consistent with that predicted by the Lifshitz-Slyozov model for diffusion-limited Ostwald ripening. In addition to Ostwald ripening, some evidence for domain coalescence was also observed when LB films were deposited under conditions of low temperature and short incubation times, though this tendency disappeared at higher deposition temperatures.     

2-烷基-苯并咪唑在硝酸银亚相上的Langmuir膜及LB膜

对不同链长的2-烷基-苯并咪唑衍生物（BzCn,烷基链长从C5到C15）在硝酸银亚相上的成膜行为及形成的LB膜的结构进行了研究.表面压－面积曲线的结果表明，短链(C5～C9)的2-烷基-苯并咪唑可在银离子亚相上形成稳定的单分子膜，而长链(C13和C15)衍生物则形成多层膜.利用LB技术可将上述Langmuir膜转移到固体基板上形成LB膜，其吸收光谱的结果说明了苯并咪唑和银离子配位.利用AFM、XRD及FT-IR等技术研究了烷基链长对LB膜结构的影响.实验结果表明,除了BzC15，其余的衍生物都可形成规整的层状结构.短链衍生物的单层LB膜具有均一、平整的形貌；而对于BzC15，观察到多层结构.     

Photo-induced structural changes of azobenzene Langmuir-Blodgett films

Structural changes of the Langmuir-Blodgett (LB) films of azobenzene accompanied by photoisomerization are described. First, photoisomerization is explained in terms of 'free volume'. In the polyion complex monolayers of amphiphiles having two azobenzene units at the air-water interface, the area per molecule depends on the polycation species. The fraction of cis-azobenzene in the LB films at the photostationary state under the illumination with UV light increased with increasing area per molecule, which is consistent with the concept of free volume. Second, a counter example of the concept of free volume is presented. Three-dimensional cone-shaped structures developed with trans-to-cis photoisomerization in the polyion complex LB film of a water-soluble amphiphilic azobenzene. These structures appeared and disappeared reversibly by alternate illumination with UV and visible light. The results indicate that the two-dimensional LB film structure exerts significant modification by photoisomerization. This is against the concept of free volume because this concept does not consider the possibility that the two-dimensional LB film structures may change into three-dimensional ones. Finally, photo-induced J-aggregate formation of non-photochromic and photochromic dyes is described. Two cyanine dyes were each mixed with an amphiphilic azobenzene in the LB films. These cyanine dyes are known to form J-aggregates in single-component LB films. In the mixed LB films, the J-aggregate formation was suppressed to some extent. The alternate illumination of the films with UV and visible light caused the photoisomerization of azobenzene in the mixed LB films, which triggered the J-aggregate formation of the cyanine dyes. The J-aggregate formation was accompanied by the development of three-dimensional cone-shaped structures from the film surface. When an amphiphilic merocyanine was mixed with the azobenzene in the LB films, J-aggregate formation was also induced by the alternate illumination with UV and visible light. This J-aggregate formation was also accompanied by a large morphological change: circular domains changed into fractal-like ones. The J-aggregate formation of the dyes and the concomitant morphological change were irreversible. In these cases, the photoisomerization of azobenzene served as a trigger to induce self-organization of the dye molecules.     

有机HTDIOO分子LB膜结构的AFM研究

利用原子力显微镜（AFM）对有机分子HTDIOO单层和多层LB膜结构进行了观察·实验结果表明，针尖与LB膜表面分子间的相互作用力会对成像的膜结构有影响．当悬臂针尖与LB膜表面分子的相互作用力较大时，针尖会扰动HTDIOO分子在单层LB膜中的有序排列．HTDIOO单层LB膜具有有序结构；而在多层LB膜中，HTDIOO分子则聚集在一起形成了一定的畴结构．     

有机HTDIOO分子LB膜结构的AFM研究

利用原子力显微镜（ＡＦＭ）对有机分子ＨＴＤＩＯＯ单层和多层ＬＢ膜结构进行了观察。实验结果表明，针尖与ＬＢ膜表面分子间的相互作用力会对成像的膜结构有影响。当悬臂针尖与ＬＢ膜表面分子的相互作用力较大时，针尖会扰动ＨＴＤＩＯＯ分子在单层ＬＢ膜中的有序排列。ＨＴＤＩＯＯ单层ＬＢ膜具有有序结构；而在多层ＬＢ膜中，ＨＴＤＩＯＯ分子则聚集在一起形成了一定的畴结构。     

Langmuir and Langmuir-Blodgett films of N-(4-octadecyloxy-2-hydroxybenzylidene) derivatives of amino acids

Langmuir and Langmuir-Blodgett monolayers of N-(4-octadecyloxy-2-hydroxybenzylidene) derivatives of glycine, tyrosine, and phenylalanine were studied using pi-A isotherms and photoelastic modulated FTIR (PEM-FTIR). Based on compression modulus and interaction parameters, mixed monolayers of these compounds with stearylamine (SAM) showed well-organized monolayers compared to mixed systems with stearic acid (SA) and stearyl alcohol (SAL). The pure amphiphiles exhibited fairly well-ordered packing in the films, and in the mixtures, the ordering increased and showed a triclinic packing arrangement. For the phenylalanine amphiphile the packing showed slight disorder compared to the other two compounds. Surface properties of the LB films of these compounds on solid substrates were analyzed using static and dynamic contact angles of a series of liquids. The surface tension of coated substrates reflected clearly the highly acidic character. Fluidlike monolayers having a molecularly rough surface indicated high wettability for n-alkanes. In contrast, the monolayer containing well-ordered, well-packed alkyl chains indicated low wettability and small hysteresis.     

Preparation of highly stable organic steps with a fullerene-based molecule

We report the formation of highly stable Langmuir-Blodgett (LB) organic steps made with a hexa-adduct fullerene-based amphiphile. This amphiphile forms films of excellent quality, with a very low roughness, that are structurally stable: X-ray reflectivity spectra recorded on fresh and 12-month-old samples are undiscernible. Such a behavior contrasts with that of more traditional amphiphiles, which are unfortunately well-known for their instability in time. The stability of the films stems, among others, from the spheroidal shape of the constitutive molecules. These experiments show that it is possible to circumvent the major drawback of LB films and to prepare materials more suited for applications. We show that the LB film prepared with this fullerene derivative can successfully be used as thickness gauges for atomic force microscopy or light microscopy studies.     

Microstructure of Mixed Cadmium Stearate and Behenate Langmuir-Blodgett Films

X-ray diffraction (XRD) was used to investigate the microstructure and phase separation of mixed multilayers of cadmium stearate (SA) and behenate (BA) deposited onto hydrophilic glass by the Langmuir-Blodgett technique. No unitary fatty acid diffraction peaks in the XRD spectra of the mixed LB films, which reveals that domains in these samples are small and uniform. The interplanar spacing of the mixed LB films changes with the ratio of BA to SA in a step-shaped curve, which suggests that with the changing of the ratio between BA and SA, only three kinds of ordered structure form in the mixed system, and each microstructure, in particular, the short chains against long chains meshed microstructure can be maintained in a certain ratio range . In addition, for the meshed microstructure, the alkyl chains of both SA and BA stand straight or may have exactly the same small tilted angle from the substrate normal, as is indicated by the symmetric and asymmetric stretching vibration of methylene (CH2) peaks which are at 2847.80 and 2914.37 cm-1respectively in the Fourier transform infrared (FT-IR) transmission spectra. The mixed system always goes through a longitudinal regularity decreasing process when BA/SA is out of the intermediate ratio range (BA/SA: 1/5-1/1), which is illustrated by the relative XRD intensity changing with the ratio of BA/ SA in a \W" shape. These results for the mixed LB films of BA/SA provide meaningful data for choosing the mixture ratio when fabricating composite films with special structure.     

Probing chitosan and phospholipid interactions using Langmuir and Langmuir-Blodgett films as cell membrane models

The interaction between chitosan and Langmuir and Langmuir-Blodgett (LB) films of dimyristoyl phosphatidic acid (DMPA) is investigated, with the films serving as simplified cell membrane models. At the air-water interface, chitosan modulates the structural properties of DMPA monolayers, causing expansion and decreasing the monolayer elasticity. As the surface pressure increased, some chitosan molecules remained at the interface, but others were expelled. Chitosan could be transferred onto solid supports alongside DMPA using the LB technique, as confirmed by infrared spectroscopy and quartz crystal microbalance measurements. The analysis of sum-frequency vibration spectroscopy data for the LB films combined with surface potential measurements for the monolayers pointed to chitosan inducing the ordering of the DMPA alkyl chains. Furthermore, the morphology of DMPA LB films, studied with atomic force microscopy, was affected significantly by the incorporation of chitosan, with the mixed chitosan-DMPA films displaying considerably higher thickness and roughness, in addition to chitosan aggregates. Because chitosan affected DMPA films even at pressures characteristic of cell membranes, we believe this study may help elucidate the role of chitosan in biological systems.     

Proton sponge and fatty acid interactions at the air-water interface. Thermodynamic, spectroscopic, and microscopic study

Mixed Langmuir and Langmuir-Blodgett (LB) films of a proton sponge, namely, diphenyl bis(octadecylamino)phosphonium bromide, and a fatty acid, docosanoic acid, with different molar ratios have been fabricated. Surface pressure versus area per molecule isotherms were registered, and the excess areas and excess Gibbs energy of mixing were calculated. Strong interactions between the proton sponge and the fatty acid take place at the air-water interface. The existence of a stoichiometric 1:1 acid-base reaction between the two components forming a complex on the water surface at high surface pressures has been demonstrated. Furthermore, the reaction had an efficiency close to 100% at the air-water interface; meanwhile, it hardly takes place in organic solvents such as chloroform or even a mixture of chloroform and dimethyl sulfoxide. The floating films were transferred to solid supports and characterized by means of several techniques including IR spectroscopy, X-ray diffraction, and atomic force microscopy, revealing the presence of highly ordered alkyl chains and a constant architecture along the different layers as well as the presence of different domains in the LB films, except those having a 1:1 proton sponge-fatty acid ratio that are homogeneous. Such domains have been interpreted as the presence of two different phases, the 1:1 complex plus the excess component in the mixture.     

Formation and characterization of mixed polyelectrolyte–surfactant Langmuir layer templates for silver nanoparticle growth

The hydrophilic characteristic of the polyelectrolyte, poly(4-styrenesulfonic acid) (PSS), was modified by associating with the surfactant, dodecyltrimethylammonium bromide (DTMAB), to form polyelectrolyte–surfactant (PSS–DTMA) Langmuir layers at air/liquid interfaces. The interfacial behavior of the PSS–DTMA complexes was investigated with the Langmuir trough technique. The mixed PSS–DTMA Langmuir layers were then used as the two-dimensional templates to incorporate with silver precursors from the subphase, and were transferred onto mica substrates with the Langmuir–Blodgett (LB) deposition technique. The silver nanoparticles were fabricated in the resulting LB films with UV irradiation, and the morphology of the silver nanoparticle structures was analyzed by atomic force microscopy (AFM). The results indicated that increasing the DTMA⁺ content in the mixed PSS–DTMA system would enhance the hydrophobic characteristic of the complexes and then form stable PSS–DTMA Langmuir layers at interfaces. In addition, by varying the DTMA⁺ content, one could adjust the charge density in the Langmuir layer templates and thus control the association behavior between the two-dimensional templates and the silver precursors in the subphases. The AFM images demonstrated that the formation of the silver nanoparticle structures in the UV-treated LB films could be regulated with the DTMA⁺ content in the Langmuir layer templates. It is inferred that the polyelectrolyte–surfactant template offers a potential of designing structures of polyelectrolyte–nanoparticle materials with a template-synthesis procedure.     

Deposition and photopolymerization of phase-separated perfluorotetradecanoic acid-10,12-pentacosadiynoic acid Langmuir-Blodgett monolayer films

Mixed monolayer surfactant films of perfluorotetradecanoic acid and the photopolymerizable diacetylene molecule 10,12-pentacosadiynoic acid were prepared at the air-water interface and transferred onto solid supports via Langmuir-Blodgett (LB) deposition. The addition of the perfluoroacid to the diacetylene surfactant results in enhanced stabilization of the monolayer in comparison with the pure diacetylene alone, allowing film transfer onto a solid substrate without resorting to addition of cations in the subphase or photopolymerization prior to deposition. The resulting LB films consisted of well-defined phase-separated domains of the two film components, and the films were characterized by a combination of atomic force microscope (AFM) imaging and fluorescence emission microscopy both before and after photopolymerization into the highly emissive "red form" of the polydiacetylene. Photopolymerization of the monolayer films resulted in the formation of diacetylene bilayers, which were highly fluorescent, with the apparent rate of photopolymerization and the fluorescence emission of the films being largely unaffected by the presence of the perfluoroacid.     

Molecular dipoles at substrate/film interfaces influencing surface energy of Langmuir-Blodgett films

Langmuir and Langmuir-Blodgett (LB) films of novel rigid Schiff base amphiphiles with different polar groups on aromatic rings have been studied at air/water interfaces and on solid substrates. The local surface potentials for the different substituents on the aromatic groups of the amphiphiles are correlated to the surface energy of LB films on solid substrates. Their contributions are probed on high-surface-energy (hsfe) quartz and hydrophobically modified low-surface-energy (lsfe) quartz. Using theoretical estimates, the long-range effects of the substituents of the polar groups on the alkyl chains have been studied from charge on the carbon and hydrogen atoms of the alkyl tails. The calculations indicate that a substituted carbon atom has a charge that is directly dependent on the nature of the atoms to which it is attached rather than on the charges of any other atoms of the substituents. This work shows that the polar substituents on the head group clearly influence the distal methylene groups of the tail. Thus, any treatment, either theoretical or experimental, of these films in contact with solid substrates should include the local relative permitivities of the polar moieties, which seem to play an important role in determining macroscopic properties such as surface energy.