Comparative study of monolayers self-assembled from alkylisocyanides and alkanethiols on polycrystalline Pt substrates

This paper compares the properties of self-assembled monolayers (SAMs) derived from octadecylisocyanide (ODI) and octadecanethiol (ODT) on polycrystalline Pt substrates. Both monolayers formed at a similar rate using 1.0 mM solutions in ethanol and achieved a thickness of 22-23 A after 24 h as determined by ellipsometry measurements. The advancing contact angles of ODI and ODT monolayers were found to be 113 and 117 degrees, respectively, suggesting a slight difference in structure between them. X-ray photoelectron spectroscopy revealed that SAMs of ODT were more stable than those of ODI, which was supported by experiments that probed desorption of these layers in prewarmed hexadecane. Cyclic voltammetry measurements indicated that both monolayer systems could diminish electron-transfer rates substantially, although ODT monolayers were more effective and robust than their ODI counterparts. The resistance of the SAMs to ion penetration differed in a similar way, and a microcontact-printed monolayer of ODT could protect the underlying Pt better in an HCl/Cl2-based etch process than the one formed from ODI.     

Kinetics of the formation of butanethiol monolayers on gold substrates, as studied by infrared spectroscopy

We have studied the formation of butanethiol self-assembled monolayers (C₄SAMs) and sub-monolayers chemisorbed on Au(111) surfaces using infrared reflection-absorption spectroscopy. Our IR study follows the evolution of the adsorbates from a disordered phase of sub-monolayer C₄ at short deposition times, to the more ordered arrangement of butanethiol films following extended exposure to the thiol solution. From the variation of the absorbances of CH₃ vibrations with the deposition time, we find that the initial deposition of the imperfect monolayer from solution is rapid at high concentrations, and can last many hours in very dilute solutions; this rapid absorption is followed by a much slower process of additional absorption and self-organisation. From the evolution of the frequency and intensity of the CH₂ related absorption bands, we observed that the angle of butanethiol molecular axis with respect to the surface normal is significantly greater in the initial state than for the final well-formed film; the intensities of CH₂ modes for fully equilibrated monolayers and sub-monolayers are extremely weak due to the nearly orthogonal orientation of the transition dipole moment with respect to the surface normal and to the polarisation of the incident radiation. Our data suggests that the initial deposition sites for individual butanethiol adsorbates are randomly distributed on the gold substrate.     

On the investigation of the bilayer functionalities of 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC) large unilamellar vesicles using cationic hemicyanines as optical probes: a wavelength-selective fluorescence approach

The behavior of the cationic hemicyanines trans-4-[4-(dimethylamino)-styryl]-1-methylpyridinium iodide (HC) and 4,(4-(dihexadecylamino)styryl-N-methyl-pyridinium iodide (DIA) were studied in large unilamellar vesicles (LUV) of 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC) using absorption, emission, depolarization and time resolved spectroscopies. Also, thorough spectroscopic studies were performed in homogeneous media to investigate the different interactions that the dyes can experience with its microenvironment. These results help us to comprehend the dye performance under different media and, consequently find interesting features of the DOPC membrane properties. The studies in homogeneous media analyzed by the Kamlet and Taft's solvatochromic comparison method demonstrate, for the first time, that the cationic hemycianines undergo specific interactions with the medium through the solvents ability to donate an electron pair as measured by the beta parameter. Thus, the absorption bands shifts bathochromically with beta while, the emission band shifts hypsochromically. In addition, for the relaxed hemicyanines the 00 energy, nu00, is invariant with the solvent properties. The results in LUV of DOPC show that, DIA undergoes a strong association with the vesicle bilayer while HC partitions between the water and the bilayer pseudophases. To monitor directly the microenvironment and dynamics around HC and DIA inside the DOPC bilayer, we use the wavelength-selective fluorescence approach, which is based on the red edge effect in fluorescence spectroscopy, in addition with the nu00 energy of the hemicyanines. The results show that the fluid state of the DOPC bilayer resembles the microenvironment of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) reverse micelles at W=[H2O]/[AOT] below 10 where there is no free water forming the water pool. Moreover, it is demonstrated for the first time, that the region of the bilayer close to the polar head of DOPC is a powerful electron donor environment.     

Ordering by collapse: formation of bilayer and trilayer crystals by folding Langmuir monolayers

Neutron and synchrotron X-ray studies of arachidic-acid monolayers compressed to the collapse region, beyond their densely packed molecular area, reveal that the resulting structures exhibit a surprising degree of reproducibility and of order. The structure of the collapsed monolayers differs for films that are spread on pure water or on CaCl2 solutions. On pure water, the collapsed monolayer forms a stable crystalline trilayer structure, with acyl-chain in-plane packing practically identical to the three-dimensional (3D) crystal structure of fatty acids. For monolayers spread on Ca2+ solutions, the collapsed film consists of a bi- and trilayer mixture with a ratio that changes by the collapse protocol. Our analysis suggests that the bilayer structure is inverted, i.e., with the hydrophobic tails in contact with the water surface and the calcium ions bridging the polar heads. The inverted bilayer structure possesses a well-ordered crystalline slab of calcium oxalate monohydrate intercalated between two acyl chains. We provide theoretical arguments rationalizing that the observed structures have lower free energies compared with other possible structures and contend that the collapsed structures may, under certain circumstances, form spontaneously.     

Reorganization of lipid nanocapsules at air-water interface 3. Action of hydrolytic enzymes HLL and pancreatic PLA2

The action of the hydrolytic enzymes humicola lanuginosa lipase (HLL) and pancreatic phospholipase A2 (PLA2) on monolayers formed from lipid nanocapsules (LNC) and model monolayers containing their components, Labrafac, Solutol and Lipoid, is studied by simultaneous measuring the changes in the film area and the surface potential in the "zero order" trough at constant surface pressure (pi). The kinetic models describing the hydrolysis by HLL of the Labrafac, Solutol and their mixtures have been proposed. By using the developed theoretical approach together with the experimental results the surface concentrations of the substrates, hydrolysis products and values of the global kinetic constants were obtained. The comparison between the global kinetic constants in the case of HLL hydrolysis of pure Labrafac, Solutol monolayers and those of the model mixed Labrafac/Solutol monolayers, shows that the rates of hydrolysis are of the same order of magnitude, i.e. an additively of the HLL enzyme action is observed. The composition of the mixed Labrafac/Solutol monolayer, formed after the interfacial LNC destabilization, was estimated.     

Memory effects of monolayers and vesicles formed by the non-ionic surfactant, 2C(18)E(12)

The behaviour of monolayers and bilayers formed by the dialkyl chain non-ionic surfactant, 1,2-di-O-octadecyl-rac-glycerol-3-omega-methoxydodecaethylene glycol (2C(18)E(12)) in water at 297 K has been investigated. Using a surface film balance (or Langmuir trough) the compression-expansion cycle of the 2C(18)E(12) monolayer was found to be reversible when compressed to surface pressures (pi) less than 42 mN m(-1). Compression of 2C(18)E(12) monolayer to pi greater than 42 mN m(-1) above this resulted in a considerable hysteresis upon expansion with the pi remaining high relative to that obtained upon compression, suggesting a time/pressure dependent re-arrangement of 2C(18)E(12) molecules in the film. Morphology of the 2C(18)E(12) monolayer, investigated using Brewster angle microscopy, was also found to depend upon monolayer history. Bright, randomly dispersed domains of 2C(18)E(12) of approximately 5 mum in size were observed during compression of the monolayer to pi less than 42 mN m(-1). At pi of 42 mN m(-1) and above, the surfactant film appeared to be almost completely 'solid-like.' Regardless of the extent of compression of the monolayer film, expansion of the film caused formation of chains or 'necklaces' of individual surfactant domains, with the extent of chain formation dependent upon pressure of compression of the monolayer and the length of time held at that pressure. Irreversible effects on 2C(18)E(12) vesicle size were also seen upon temperature cycling the vesicles through their liquid-crystalline phase transition temperature with vesicles shrinking in size and not returning to their original size upon standing at 298 K for periods of more than 24 h. No comparable hysteresis, time, pressure or temperature effects were observed with the monolayer or vesicles formed by the corresponding phospholipid, disteaorylphosphatidylcholine, under identical conditions. The effects observed with 2C(18)E(12) are attributed to the ability of the polyoxyethylene head group to dehydrate and intrude into the hydrophobic chain region of the mono- and bilayers. These studies have important implications for the use of the vesicles formed by 2C(18)E(12) as drug delivery vehicles.     

Si/SiO2-templated formation of ultraflat metal surfaces on glass, polymer, and solder supports: their use as substrates for self-assembled monolayers

This paper describes the use of several methods of template stripping (TS) to produce ultraflat films of silver, gold, palladium, and platinum on both rigid and polymeric mechanical supports: a composite of glass and ultraviolet (UV)-curable adhesive (optical adhesive, OA), solder, a composite of poly(dimethyl siloxane) (PDMS) and OA, and bare OA. Silicon supporting its native oxide layer (Si/SiO2) serves as a template for both mechanical template stripping (mTS), in which the metal film is mechanically cleaved from the template, and chemical template stripping (cTS), in which the film-template composite is immersed in a solution of thiols, and the formation of the SAM on the metal film causes the film to separate from the template. Films formed on all supports have lower root-mean-square (rms) roughness (as measured by atomic force microscopy, AFM) than films used as-deposited (AS-DEP) by electron-beam evaporation. Monolayers of n-dodecanethiolate formed by the mTS and cTS methods are effectively indistinguishable by scanning tunneling microscopy (STM); molecularly resolved images could be obtained using both types of surfaces. The metal surfaces, before being cleaved, are completely protected from contact with the atmosphere. This protection allows metal surfaces intended to support SAMs to be prepared in large batch lots, stored, and then used as needed. Template stripping thus eliminates the requirement for evaporation of the film immediately before use and is a significant extension and simplification of the technology of SAMs and other areas of materials science requiring clean metal surfaces.     

Densely stacked multilamellar and oligovesicular vesicles, bilayer cylinders, and tubes joining with vesicles of a salt-free catanionic extractant and surfactant system

In the phase diagram of an excellent extractant of rare earth metal ions, di(2-ethylhexyl) phosphate (HDEHP, commercial name P204), mixing with a cationic trimethyltetradecylammonium hydroxide (TTAOH) in water, a birefringent Lalpha phase was found, which consists of densely stacked multilamellar vesicles. The densely stacked multilamellar vesicles are remarkably deformed, as observed by means of cryotransmission electron microscopy (cryo-TEM). Further, self-assembled structures-oligovesicular vesicles, bilayer cylinders, and tubes joining with vesicles-were also observed. The self-assembled phase is transparent, anisotropic, and highly viscous, possessing elastic properties determined by rheological measurements. This is the first time that birefringent Lalpha phase with remarkably deformed amphiphilic bilayer membranes has been constructed through combining a hydrophobic organic extractant having double chains with a water-soluble surfactant having a single chain, which may direct primarily toward acquiring an understanding of the mechanism of salt-free catanionic vesicles and secondarily to determine if vesicle-extraction technology utilizing extractants is possible.     

Orientationally disordered solid monolayers of tetramethylsilane on graphite and a bilayer nature of the 2-D fluid

A calorimetric investigation of the monolayers (MLs) of tetramethylsilane (TMS) adsorbed on graphite demonstrates two remarkable features: (i) occurrence of an orientational order–disorder phase transition in the 2-D solid (at 107 K at the coverage θ<1 and at 138 K at 1<θ<2) accompanied by a large entropy change (ranging from Rln8 to Rln3) depending on the coverage, and (ii) possible formation of a fluid bilayer around 220 K, above the 2-D critical temperature. A high-resolution incoherent neutron scattering experiment reveals a considerable motional disorder in the disordered 2-D solid.     

Attachment and phospholipase A2-induced lysis of phospholipid bilayer vesicles to plasma-polymerized maleic anhydride/SiO2 multilayers

This article describes a method by which intact vesicles can be chemically attached to hydrolyzed maleic anhydride films covalently bound to plasma-polymerized SiO2 on Au substrates. Surface plasmon field-enhanced fluorescence spectroscopy (SPFS) combined with surface plasmon resonance spectroscopy (SPR) was used to monitor the activation of plasma-deposited maleic anhydride (pp-MA) film with EDC/NHS and the subsequent coupling of lipid vesicles. The vesicles were formed from a mixture of phosphatidylcholine and phosphatidylethanolamine lipids, with a water-soluble fluorophore encapsulated within. Vesicle attachment was measured in real time on plasma films formed under different pulse conditions (plasma duty cycle). Optimum vesicle attachment was observed on the pp-MA films containing the highest density of maleic anhydride groups. Phospholipase A2 was used to lyse the surface-bound vesicles and to release the encapsulated fluorophore.     

Thermodynamics of partitioning of phenothiazine drugs between phosphatidylcholine bilayer vesicles and water studied by second-derivative spectrophotometry

The partition coefficients (Kps) of phenothiazine drugs (trifluoperazine, triflupromazine, chlorpromazine and promazine) between phosphatidylcholine (PC) small unilamellar vesicles (SUV) and water were determined over the temperature range of 10-40 degrees C by a second-derivative spectrophotometric method. The second derivative spectra of each drug solution containing various amounts of SUV showed distinct derivative isosbestic points confirming the entire elimination of the residual background signal effects of the SUV. The Kp values were calculated from the derivative intensity change of the drugs induced by the addition of SUV to the drug buffer solutions (pH 7.4) and obtained with the R.S.D. below 10% (n=3). The van't Hoff analysis of the temperature dependence of Kp values revealed negative deltaH(w-->l) and positive deltaS(w-->l), suggesting an enthalpy/entropy driven mechanism for the phenothiazine partitioning. The negative deltaH(w-->l) implies that the electrostatic interaction, positively charged alkyl amino groups of phenothiazine drugs with negatively charged phosphate groups on the surface of PC SUV, partly contributes to the partitioning. The existence of halogen atom(s) on the phenothiazine ring at position C-2 enhanced the Kp value (Hl) value (Hl) increase is considered to be enhancement of disorder in the hydrophobic acyl chain regions of PC SUV membranes derived from the phenothiazine ring insertion and thus depends on the bulkiness of the substituent. The enthalpy-entropy correlation analysis yielding a good linear relationship also suggests that the phenothiazine drugs studied have identically an enthalpy-entropy compensation mechanism for the partitioning.     

Contact area lithography and pattern transfer of self-assembled organic monolayers on SiO2/Si substrates

We describe a procedure for one-step patterning and transfer of self-assembled organic monolayers (SAMs) on SiO(2)/Si substrates. This procedure was inspired from an idea of pattern formation at contact area, which realizes high patterning fidelity, and enables a universal approach for the micro/nanometre scale patterning of SAMs.     

Raman enhancement of azobenzene monolayers on substrates prepared by Langmuir-Blodgett deposition and electron-beam lithography techniques

Nanostructured metallic platforms for Raman enhancement were fabricated using Langmuir-Blodgett and electron beam (e-beam) lithography techniques. The gold platforms were inscribed on thin glass slides with the purpose of using them in a transmission geometry experimental setup under a confocal microscope. The plasmon frequency of the gold nanostructures was determined in the visible-near-infrared range for various pattern sizes prepared by Langmuir-Blodgett transfer and e-beam lithography. The surface Raman enhancement factors were determined for a monolayer of azobenzene molecules adsorbed on gold through thiol bonding and compared for both LB transfer and e-beam samples for nanostructures of comparable geometries.     

Comparative study of ZnO and CuSCN semiconducting nanowire electrodeposition on different substrates

In this study, we are reporting on the electrochemical deposition of two kinds of semiconducting nanowires (ZnO and CuSCN) on different substrates. ZnO and CuSCN are n- and p-type transparent semiconductors whose electrochemical preparation has some similarity, and it is a combination of two steps: an electrochemical reduction with consecutive chemical precipitation. Here, we show that despite the different physicochemical nature of the studied materials, when they are deposited electrochemically, their dimensions depend mainly on the surface state of the used substrate. Thus, depending on the substrate morphology, nanowires with diameters between 50 and 380 nm from both semiconductors could be grown. It is also shown that ZnO and CuSCN nanowires could be successfully grown on glass and plastic substrates as well as on a metallic one. The possibility of growing these transparent semiconductors on flexible substrates opens new perspectives for their use in “invisible” electronic devices.     

Comparative study of the adhesion, friction, and mechanical properties of CF3- and CH3-terminated alkanethiol monolayers

We report the results of a direct comparison of the adhesion, friction, and mechanical properties between alkanethiol self-assembled monolayer films terminated by either CH(3) or CF(3) end groups using both interfacial force (IFM) and atomic force (AFM) microscopies. The purpose of this work is to gain insight into the detailed origins of the differing frictional behavior previously observed with AFM. The IFM results reveal an increased adhesive interaction for the CF(3)-terminated film due to the highly polar nature of the end groups. In agreement with earlier studies, the AFM results show two linear regions with differing frictional slopes for the CH(3)-terminated film but only a single slope for the CF(3)-terminated film. We contrast the differences between these techniques, approximately 100 times smaller tips for the AFM, and discuss the role of the mechanical properties, the increased adhesive interaction, and the amount of disorder present in the film in creating differences in frictional behavior between the two systems. We conclude that increased adhesion for the CF(3)-terminated film plays an important role in the observed differences in frictional behavior, while the differences between the two techniques can be traced to the different tip sizes and the consequent responses to the presence of disorder in the films.     

Comparative study of paclitaxel physically encapsulated in and chemically conjugated with PEG‐PLA

Paclitaxel‐loaded poly(ethylene glycol)‐b‐poly(l ‐lactide (LA)) (PEG‐PLA) micelles were prepared by two methods. One is physical encapsulation of paclitaxel in micelles composed of a PEG‐PLA block copolymer and the other is based on a PEG‐PLA–paclitaxel conjugate, abbreviated as “conjugate micelles”. Their physicochemical characteristics, e.g. critical micelle concentration (CMC), morphology, and micelle size distribution were then evaluated by means of fluorescence spectroscopy, scanning electron microscopy (SEM), and dynamic light scattering (DLS). The results show that the CMC of PEG‐PLA–paclitaxel and PEG‐PLA are 6.31 × 10^?4 and 1.78 × 10^?3 g L^?1, respectively. Both micelles assume a spherical shape with comparable diameters and have unimodal size distribution. Moreover, invitro drug delivery behavior was studied by high performance liquid chromatography (HPLC). The antitumor activity of the paclitaxel‐loaded micelles against human liver cancer H7402 cells was evaluated by 3‐(4,5‐Dimethylthiazol‐2‐yl)‐2,5‐diphenyl tetrazolium bromide (MTT) method. The conjugate micelles show a lower burst release during the initial stage and higher accumulative release amount of paclitaxel after a period of time while the encapsulated ones behave in the opposite way. Both the paclitaxel‐loaded micelles showed comparable anticancer efficacy with the free drug. Copyright © 2008 John Wiley & Sons, Ltd.     

Scanning tunneling microscopy-based replacement lithography on self-assembled monolayers: comparison of gold, palladium, and platinum substrates

Self-assembled monolayers of dodecanethiol on gold, palladium, and platinum can be locally replaced with a different thiol under the action of a scanning tunneling microscopy tip at elevated bias. This process is characterized and the bias dependence of the degree of replacement for each metal substrate is illustrated. Replacement on gold substrates occurs at lower applied potentials than on platinum substrates, provided care has been taken to remove surface oxides.     

A comparative study of gold nanocubes, octahedra, and rhombic dodecahedra as highly sensitive SERS substrates

Gold nanocubes, octahedra, and rhombic dodecahedra with roughly two sets of particle sizes have been successfully synthesized via a seed-mediated growth approach. All six samples were analyzed for comparative surface-enhanced Raman scattering (SERS) activity. All of these Au nanostructures were found to yield strong enhancement at a thiophenol concentration of 10(-7) M and are excellent SERS substrates. Rhombic dodecahedra with a rhombus edge length of 32 nm showed significantly better enhancement than the other samples and can reach a detection limit of 10(-8) M. Simulations of the binding energies of thiophenol on the different faces of gold and electric near-field intensities of these nanocrystals have been performed to evaluate the experimental results. Superior SERS activity of these nanocrystals can be expected toward the detection of many other molecules.     

基于结构设计合成新型喹啉类分泌型PLA2抑制剂

磷脂酶A2(PLA2)在很多人类疾病的病理研究中起重要作用,是药物化学研究所的热点之一。因此,发展新型的PLA2抑制剂对生物有机研究和临床应用均有重要意义。我们设计合成分泌型PLA2的非底物类似物以寻找新型PLA2抑制剂。本文基于结构设计并合成了喹啉－4－乙酰胺作为PLA2抑制剂,目标化合物结构经1^HNMR,IR,MS和元素分析确认,初步活性检测显示该类化合物具有较好体外活性及动物活性。     

Comparative study of Factor Xa fluorogenic substrates and their influence on the quantification of LMWHs

Low molecular weight heparins (LMWHs) are recognised as the preferred anticoagulants in the prevention and treatment of venous thromboembolism. Anti-Factor Xa (anti-FXa) levels are used to monitor the anticoagulant effect of LMWHs and such assays are routinely employed in hospital diagnostic laboratories. In this study, a fluorogenic anti-FXa assay was developed using a commercially available fluorogenic substrate with an attached 6-amino-1-naphthalene-sulfonamide (ANSN) fluorophore and was used for the determination of two LMWHs, enoxaparin and tinzaparin and the heparinoid, danaparoid. The assay was based on the complexation of heparinised plasma with 100 nM exogenous FXa and 25 μM of the fluorogenic substrate Mes-D-LGR-ANSN (C₂H₅)₂ (SN-7). The assay was tested with pooled plasma samples spiked with anticoagulant concentrations in the range 0–1.6 U mL⁻¹. The statistically sensitive assay range was 0–0.4 U mL⁻¹ for enoxaparin and tinzaparin and 0–0.2 U mL⁻¹ for danaparoid, with assay variation typically below 10.5%. This assay was then compared with a previously published fluorogenic anti-FXa assay developed with the peptide substrate, methylsulfonyl-d-cyclohexylalanyl-glycyl-arginine-7-amino-4-methylcoumarin acetate (Pefafluor FXa). Both assays were compared in terms of fluorescence intensity, lag times and sensitivity to anticoagulants.