Scanning force microscopic study of surface structure and properties of (alkylsilane/ fluoroalkylsilane) mixed monolayers

(Alkylsilane/fluoroalkylsilane) mixed monolayers were immobilized covalently on a silicon wafer surface with stable surface structure. Atomic force microscopic observation of the n-octadecyltrichlorosilane (OTS)/[2-(perfluorooctyl)ethyl]trichlorosilane (FOETS) mixed monolayer revealed that the crystalline OTS circular domains of ca. 1–2μm in diameter were surrounded by a sealike amorphous FOETS matrix, even though the molar fraction of OTS was above 75%. Also, the phaseseparated monolayer can be prepared from FOETS, and a non-polymerizable and crystallizable amphiphile such as lignoceric acid (LA). The phase separation of the (alkylsilane/fluoroalkylsilane) mixed monolayer might be attributed to both faster spreading of FOETS molecules on the water surface and the crystallizable characteristics of alkylsilane molecules. The mixed monolayer of crystalline alkylsilane (OTS) and amorphous alkylsilane (n-dodecyltrichlorosilane, DDTS) formed a phase-separated structure on the water surface because of the crystallizable characteristics of OTS. Lateral force microscopic (LFM) observation revealed that the order of the magnitude of lateral force generated against the silicon nitride tip was: n-triacontyltrichlorosilane (TATS) domain with longer alkyl chain > amorphous FOETS matrix > crystalline OTS domain. On the other hand, scanning viscoelasticity microscopic observation revealed that the order of the magnitude of modulus was: Si substrate > crystalline OTS domain > amorphous FOETS matrix.     

Molecular positional order in a dipalmitoylphosphatidic acid Langmuir–Blodgett monolayer by atomic force microscopy

Langmuir–Blodgett monolayers of dipalmitoylphosphatidic acid were studied by using atomic force microscopy on the large- and nano-scale. A molecularly resolved image was achieved at high surface pressure. The monolayer shows a dramatic long-range orientational and positional ordering of molecular organization of aliphatic tails. The ordered molecular arrangement of aliphatic tails may result from the strong intermolecular hydrogen bonding interactions between adjacent phosphate groups in the polar region.     

Amphiphilic Fullerene‐Cholesterol Derivatives: Synthesis and Preparation of Langmuir and Langmuir‐Blodgett Films

Amphiphilic fullerene bis‐adducts 11 and 14 containing two and four cholesterol moieties, respectively, were prepared starting from the corresponding bis‐malonate derivatives. In a systematic study, their spreading behavior at the air‐water interface was compared to that of bis‐adduct 6 with no polar head‐group. Compared to 6 , for which some three‐dimensional aggregation occurs, the polar head‐group in 11 and 14 is responsible for an attractive interaction with the aqueous subphase, forcing the molecules towards the water surface into a two‐dimensional arrangement. Even if homogeneous Langmuir films were obtained with both 11 and 14 , only the films of 14 show a reversible compression/expansion behavior. This suggests that, by increasing the number of cholesterol subunits, the encapsulation of the C‐sphere in its addend is more efficient, thus preventing fullerene‐fullerene interactions and aggregation phenomena. The Langmuir films of 11 and 14 were also efficiently transferred onto hydrophilic quartz slides, yielding Langmuir‐Blodgett films.     

Deposition and characterization of Langmuir‐Blodgett films of cadmium arachidate/SWCNTs composites

The incorporation of single‐wall carbon nanotubes (SWCNTs) in cadmium arachidate film by means of the Langmuir‐Blodgett (LB) technique was investigated as a function of arachidic acid/SWCNT mass ratio at the air/water interface and in Langmuir‐Blodgett films. The behaviour at the air/water interface shows that SWCNTs act as an independent phase with respect to the cadmium arachidate. Deposition conditions are optimized when the weight ratio between the arachidic acid (AA) and SWCNTs is in the range 0.018:1 to 1:1. The general order of the LB multilayered structure was destroyed by the progressive density increase in SWCNT quantity as evidenced by X‐ray reflectivity (XRR) analysis. Scanning electron microscopy images indicated that when a multilayered structure was formed its layers consisted of SWCNT bundles stacked one over the other. Copyright © 2006 John Wiley & Sons, Ltd.     

Study of poly[bis(p-toluene sulfonate) diacetylene] films prepared by a modification of the Langmuir–Blodgett technique

Coherent this films of poly[bis(p-toluene sulfonate) diacetylene] were successfully formed by modified Langmuir–Blodgett techniques using two methods: (i) Photopolymerization of the monomer film at the gas/liquid interface and then transfer to a solid substrate, and (ii) transfer of the monomer film to the solid substrate and subsequent photopolymerization on the substrate itself. The films thus obtained were characterized by traditional force–area isotherms while on pure water subphases. Segments were transferred at either 1 or 10 dyn/cm surface pressure, in different stages of photopolymerization, to glass or germanium substrates. The films on the substrate were characterized by the methods of multiple attenuated-internal-reflection infrared spectroscopy, ellipsometry, contact-potential measurement, and laser Raman spectroscopy. Our results show that the films are multimolecular and about 100 Å thick. Of special interest were the observation of significant anisotropy of oriented dipoles and the ability to obtain excellent spectral data for these very thin oriented films. Raman spectroscopic features are similar to those observed for the bulk polymer, even in the low-frequency region. Polarized Raman spectroscopy confirmed the presence of local anisotropy in these films.     

Detection of paraoxon by immobilized organophosphorus hydrolase in a Langmuir–Blodgett film

Langmuir–Blodgett (LB) film deposition technique was employed for the immobilization of organophosphorus hydrolase (OPH). OPH enzyme was covalently bonded to a fluorescent probe, fluorescein isothiocyanate (FITC), and used as a biological recognition element. Under optimal experimental conditions, OPH monolayers were deposited onto the surface of silanized quartz slides as LB film and utilized as a bioassay for the detection of paraoxon. Two different methods were employed for detection of paraoxon: the fluorescence quenching of the fluorescence probe (FITC) covalently bonded to OPH and the UV–vis absorption spectrum of the paraoxon hydrolysis product. The UV–vis absorption measurement demonstrated a linear relationship between the absorbance at 400 nm and the concentration of paraoxon solutions over the range of 1.0 × 10⁻⁷–1.0 × 10⁻⁵ M (0.27–27 ppm). By observing the FITC fluorescence quenching, the concentration of paraoxon can be detected as low as 10⁻⁹ M (S/N = 3). The research described herein showed that the LB film bioassay had high sensitivity, rapid response time and good reproducibility.     

On the formation and structure of self-assembling monolayers. I. A comparative atr-wettability study of Langmuir—Blodgett and adsorbed films on flat substrates and glass microbeads

Organized oleophobic monolayers of several long chain compounds and steroid derivatives produced on flat solid substrates by spontaneous adsorption from organic solutions are compared with Langmuir—Blodgett (LB) monolayers transferred on identical substrates from the water-air interface. Quantitative infrared ATR and polarized ATR spectroscopy, and wettability measurements are used to correlate the various films and to determine their molecular density and orientation, mode of film-to-surface binding, and other structural characteristics. Formation of oleophobic adsorbed monolayers on a model powder substrate—smooth glass microbeads—is also investigated. It is concluded that, irrespective of the mode of film-to-surface binding (ionic, covalent, or hydrogen bonding), and the nature of the substrate (Ge, Si, ZnSe, glass slides, glass microbeads), saturation of the adsorption leads in all studied systems to the formation of tightly packed and highly oriented monolayers, structurally equivalent to LB monolayers of same or similar compounds deposited on the bare surfaces of the respective substrates. These findings are interpreted in terms of a cooperative surface process leading to aggregation of molecules into a characteristic “monolayer phase.” Significant structural differences may develop in LB built-up films thicker than one monolayer. A mechanism for the formation of covalently bonded silane monolayers is proposed.     

Exploring the Effect of Ligand Structural Isomerism in Langmuir–Blodgett Films of Chiral Luminescent EuIII Self‐Assemblies

Here we have investigated the influence of the antenna group position on both the formation of chiral amphiphilic Eu^III‐based self‐assemblies in CH₃CN solution and, on the ability to form monolayers on the surface of quartz substrates using the Langmuir–Blodgett technique, by changing from the 1‐naphthyl ( 2(R) , 2(S) ) to the 2‐naphthyl ( 1(R) , 1(S) ) position. The evaluation of binding constants of the self‐ assemblies in CH₃CN solution was achieved using conventional techniques such as UV/Visible and luminescence spectroscopies along with more specific circular dichroism (CD) spectroscopy. The binding constants obtained for EuL , EuL₂ and EuL₃ species in the case of 2‐naphthyl derivatives were comparable to those obtained for 1‐naphthyl derivatives. The analysis of the changes in the CD spectra of 1(R) and 1(S) upon addition of Eu^III not only allowed us to evaluate the values of the binding constants but the resulting recalculated spectra may also be used as fingerprints for assignment of the chiral self‐assembly species formed in solution. The obtained monolayers were predominantly formed from EuL₃ (≈85 %) with the minor species present in ≈15 % EuL₂ .     

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.     

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.     

Assessment by XPS and electrochemical techniques of two molecular organosilane films prepared on stainless‐steel surfaces

The ability of silane groups to chemosorb on mechanically polished stainless‐steel surfaces has been investigated. Accordingly, tridecafluoro‐1,1,2,2‐tetrahydrooctyltrichlorosilane and the tridecafluoro‐1,1,2,2‐tetrahydrooctyltriethoxysilane, two organosilanes of similar chain length terminated with hydrophobic non‐reactive functions, have been tested as probe molecules. X‐ray photoelectron spectroscopy characterization, optical microscopy imaging, electrochemistry and contact angle measurements have been performed to characterize bare and modified samples. Results reported in this work show the influence of time of immersion and coupling agent reactive group on self‐assembled monolayer formation. It emerges that silanization by triethoxysilanes in millimolar solutions is not suitable for producing chemically bonded organic films with high density. Copyright © 2004 John Wiley & Sons, Ltd.     

Correlation of Molecular Assembly and Interactions in Crystals and Langmuir–Blodgett Films of N‐(2,4‐dinitrophenyl)‐n‐octadecylamine

Correlation of molecular organization in crystals and in ultrathin films is of fundamental interest in the design of molecular materials based on thin films. We have chosen as a test case, N‐(2,4‐dinitrophenyl)‐n‐octadecylamine (DNPOA), a potential candidate for the fabrication of Langmuir–Blodgett (LB) films for quadratic nonlinear optical applications. Like several other 4‐nitroaniline derivatives, DNPOA does not form stable monolayers at the air–water interface. This has precluded investigations of their organization in LB films. We have stabilized composite Langmuir films of DNPOA with the phospholipid molecule DSPC and fabricated their LB films. Successful growth of single crystals of DNPOA allowed structure determination and detailed analysis of molecular associations in the solid state. Electronic absorption spectra of DNPOA in solution, in the solid state and in the LB film are investigated. Modeling of the various spectral signatures by semiempirical computations on molecular clusters extracted from the crystal lattice provides insight into the correlation between the molecular organization in crystals and in LB films.     

Scanning force microscopy study of the morphology of spin‐cast molecular‐imprinted nylon thin films

The morphology of thin, selectively imprinted films of Nylon‐6 was investigated by scanning force microscopy. Four amino acids were used as template molecules in the spin‐cast films. Film thickness ranged from 2 µm to 500 nm, depending on the nylon and template concentration in the casting solution. The thin‐film properties, including the presence of nanometer‐ to micrometer‐sized pores, are clearly associated with the imprinting process. The larger features observed by scanning force microscopy are attributed to amino acid clustering during the casting process. Copyright © 2004 John Wiley & Sons, Ltd.     

Three‐Component Langmuir–Blodgett Films Consisting of Surfactant,Clay Mineral,and Lysozyme: Construction and Characterization

The Langmuir–Blodgett (L–B) technique has been employed for the construction of hybrid films consisting of three components: surfactant, clay, and lysozyme (Lys). The surfactants are octadecylammonium chloride (ODAH) and octadecyl ester of rhodamine B (RhB18). The clays include saponite and laponite. Surface pressure versus area isotherms indicate that lysozyme is adsorbed by the surfactant–clay L–B film at the air–water interface without phase transition. The UV‐visible spectra of the hybrid film ODAH–saponite–Lys show that the amount of immobilized lysozyme in the hybrid film is (1.3±0.2) ng mm^?2. The average surface area (Ω) per molecule of lysozyme is approximately 18.2 nm² in the saponite layer. For the multilayer film (ODAH–saponite–Lys)_n, the average amount of lysozyme per layer is (1.0±0.1) ng mm^?2. The amount of lysozyme found in the hybrid films of ODAH–laponite–Lys is at the detection limit of about 0.4 ng mm^?2. Attenuated total reflectance (ATR) FTIR spectra give evidence for clay layers, ODAH, lysozyme, and water in the hybrid film. The octadecylammonium cations are partially oxidized to the corresponding carbamate. A weak 1620 cm^?1 band of lysozyme in the hybrid films is reminiscent of the presence of lysozyme aggregates. AFM reveals evidence of randomly oriented saponite layers of various sizes and shapes. Individual lysozyme molecules are not resolved, but aggregates of about 20 nm in diameter are clearly seen. Some aggregates are in contact with the clay mineral layers, others are not. These aggregates are aligned in films deposited at a surface pressure of 20 mN m^?1.     

Determination of Copper at a Glassy Carbon Electrode Modified with Langmuir–Blodgett Film of p‐tert‐Butylthiacalix[4]arene

A new type of voltammetric sensor, Langmuir–Blodgett film of p‐tert‐butylthiacalix[4]arene modified glassy carbon electrode, was advanced and used for determining copper at trace levels by differential pulse stripping voltammetry. Calibration plot was found to be linear in the range of 2×10^?8 M to 5×10^?6 M; the detection limit was 2×10^?9 M. Possible recognition mechanism was also discussed. From determination of Copper in real samples (river, lake and tap water) it can be concluded that the method is rapid, sensitive in determining of copper and can be used in the analysis of natural water samples.     

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.     

Design and Synthesis of Aviram–Ratner‐Type Dyads and Rectification Studies in Langmuir–Blodgett (LB) Films

The design and synthesis of Aviram–Ratner‐type molecular rectifiers, featuring an anilino‐substituted extended tetracyanoquinodimethane (exTCNQ) acceptor, covalently linked by the σ‐spacer bicyclo[2.2.2]octane (BCO) to a tetrathiafulvalene (TTF) donor moiety, are described. The rigid BCO spacer keeps the TTF donor and exTCNQ acceptor moieties apart, as demonstrated by X‐ray analysis. The photophysical properties of the TTF‐BCO‐exTCNQ dyads were investigated by UV/Vis and EPR spectroscopy, electrochemical studies, and theoretical calculations. Langmuir–Blodgett films were prepared and used in the fabrication and electrical studies of junction devices. One dyad showed the asymmetric current–voltage (I–V) curve characteristic for rectification, unlike control compounds containing the TTF unit but not the exTCNQ moiety or comprising the exTCNQ acceptor moiety but lacking the donor TTF part, which both gave symmetric I–V curves. The direction of the observed rectification indicated that the preferred electron current flows from the exTCNQ acceptor to the TTF donor.     

Electrochemical behavior of nitroxy radical monolayers prepared on tin oxide electrodes by the Langmuir—Blodgett method: An unusual effect of concentration of supporting electrolytes on cyclic voltammograms

The photoemission current (j) and the differential capacitance of the double layer (C) on 20-200 nm thick bismuth films have been measured. The current j as a function of potential (E) follows the 5/2 law for films of thickness exceeding 80 nm and for bulk bismuth. The current j as a function of E for thin films exhibits steps whose width increases as the thickness decreases. The absolute values of j and C also decrease for thinner films. These effects are manifestations of the quantum size effect in the energy spectrum of charge carriers in bismuth. Estimations of the degree of metallization of the bismuth surface are presented.