Organization Of Monolayers Research Articles

Tracking the effect of binder length on colloidal stability and bioconjugation of gold nanoparticles

Understanding the organization of self-assembled monolayers (SAMs) on gold nanoparticles (AuNPs) as protective coatings is a key role of biological applications of nanomaterials. Here, we report the influence on the stability of the surface coverage of three mercaptocarboxylic lingands onto AuNPs, mercaptopropanoic acid (MPA), mercaptoundecanoic acid (MUA) and mercaptopropionic acid (MHA) under different conditions. In addition, we optimized a bioconjugation route using bovine serum protein (BSA) as a protein model. AuNPs and successful binding of ligands and BSA on the AuNPs were analyzed by UV–Vis, TEM, FTIR, RAMAN, DLS and zeta potential. The size of as-synthesized AuNPs was 18 ± 1,2 nm with surface plasmon resonance (SPR) peak at 522 nm. The magnitude of the bathochromic shift of AuNPs with MPA, MUA and MHA was determined by UV–Vis and the SPR band position of AuNP shifts to 1.5, 3 and 5 nm longer. Moreover, the data show the influence of chain length on colloidal stability and covalent and non-covalent coupling steps with nanomaterials. We demonstrate a method for quantitative determination of the coatings on gold nanoparticles and open new perspectives in understanding the influence of monolayer thickness on the generation of nanobioconjugates for biological applications.

Vibrational studies of saccharide-induced lipid film reorganization at aqueous/air interfaces

Vibrational sum frequency generation (VSFG) and surface tension experiments were used to examine the effects of aqueous phase soluble saccharides on the structure and organization of insoluble lipid monolayers adsorbed to aqueous-air interfaces. Changes in dipalmitoylphosphocholine (DPPC) chain structure as a function of aqueous phase saccharide concentration and pH are reported. Complementary differential scanning calorimetry (DSC) measurements performed on solutions containing soluble saccharides and DPPC vesicles measured the effects of the saccharides on the lipid membrane phase behavior. Data show that the saccharides glucosamine and glucuronic acid induce a higher degree of organization in compressed DPPC monolayers regardless of the saccharide’s charge.

Synthesis and Biophysical Characterization of the Chlorosulfolipids of Ochramonas danica

The freshwater algae Ochramonas danica produces a range of polychlorinated single-chain amphiphiles. Danicalipin A, a hexachlorosulfolipid makes up 90% of the polar lipid content of the flagellar membrane of O. danica. Its exotic structure presents both synthetic challenges and raises questions about its function within the membrane. To address these questions, we have combined total synthesis and membrane biophysics to investigate the effects of structural elements of chlorosulfolipids on their behavior in monolayers and bilayers. The discovery of a titanium-based catalytic, enantioselective dichlorination of allylic alcohols enabled the eight-step synthesis of (+)-Danicalipin A as a single stereoisomer in sufficient quantities for in vitro analysis. Nanoscale secondary ion mass spectrometry (NanoSIMS) confirmed that Danicalipin A is localized within plasma membrane of O. danica cells. Preliminary biophysical characterization of Danicalipin A has revealed that it alters the phase behavior and lateral organization in monolayers and bilayers of other lipids present in the membrane of O. danica. Danicalipin A incorporates into monolayers of phospho- and glycolipids at the air-water interface and increases the surface pressure at which the liquid-expanded to liquid-compact phase transition occurs, as well as increasing monolayer compressibility. Similarly, in giant unilamellar vesicles, Danicalipin A lowers the transition temperature of saturated phospho- and glycerolipids and causes phase separation. Natural and unnatural analogs to Danicalipin A are being synthesized to examine the effects of the stereochemistry, chlorination pattern, and sulfation on these biophysical properties. These results may reveal how the membrane of O. danica accommodates high concentrations of chlorosulfolipids, which are toxic to other organisms.

Two-Step Nanoscale Approach for Well-Defined Complex Alkanethiol Films on Au Surfaces.

Controlling the molecular organization of organic self-assembled monolayers (SAM) is of utmost importance in nanotechnology, molecular electronics, and surface science. Here we propose two well-differentiated approaches, double printing based on microcontact printing (μ-cp) and molecular backfilling adsorption, to produce complex alkanethiol films. The resulting films on model Au surfaces were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. Double printing alkanethiols results in clear coexisting regions where no molecular displacement is observed, highlighting the slow diffusion rates of long alkanethiols and large attractive interaction between long alkyl chains. Exposing a single-print μ-cp Au substrate to an additional alkanethiol solution yields the formation of differently ordered domain boundaries with different thickness and micrometer lateral size. The high order is a result of enhanced molecular mobility and restructuring during solution backfilling. The formed molecular assemblies constitute an excellent testing ground for nanoscale phenomena that strongly depend on the nanoscale geometrical and chemical features of the surface such as designed functionality or corrosion initiation and inhibition.

Nanofibrous carrier for transplantation of retinal pigment epithelial cells

SummaryThe most common retinal degenerative diseases such as age‐related macular degeneration (AMD) and retinitis pigmentosa belong to serious eye diseases that cause dominant number of cases of impaired vision or even legal blindness. Because there is no treatment method to stop progression of these diseases or even treat them, it is supposable that cell therapy based on replacement of retinal pigment epithelial (RPE) cells can serve as an effective treatment for AMD and other degenerative diseases. The study was aimed at development of a new delivery technique for transplantation of RPE cells using two complementary delivery devices: a membraneous cell carrier and an injector as trans‐scleral implantation instrument. The design of the suggested nanofibrous carrier with a peripheral frame was optimized to provide a minimal diffusion barrier for the flow of nutrients in the subretinal space and at the same time to resist surgical handling. An implantation instrument together with the cell carrier ensured a safe transfer of an organized RPE cell monolayer into the subretinal space. Different designs of the carrier seeded with cultured RPE cells and the implantation instrument was assessed in both ex vivo and in vivo animal experiments.

Influence of Fluorene and Spirobifluorene Regioisomerism on the Structure, Organization, and Permeation Properties of Monolayers

9,9′-Spirobifluorene (SBF) and fluorene derivatives are extensively used as active layer in many organic electronic devices nowadays; thus, careful examination of their structure and organization in thin films and monolayers is of primary importance. In the present work, immobilization of SBF and fluorene isomers was performed via Click Chemistry coupling reaction onto well-ordered ethynylbenzene monolayers on carbon substrates. Characterization of the as-prepared monolayers was performed by classical electrochemical techniques (CV), scanning electrochemical microscopy, and atomic force microscopy, highlighting the effect of SBF and fluorene onto the structure and organization of the obtained monolayers. The significant impact of their positional isomerism on the monolayers structuration was clearly evidenced. Indeed, 2-substituted SBF and fluorene monolayers appear to be more compact and ordered than their 4-substituted analogues and are orientated parallel to the surface. These findings should have an i...

Dynamics of Electron Transfer in Azulene-Based Self-Assembled Monolayers

Azulene is an attractive building block for molecular electronics design owing to the intrinsic charge separation within its sp2-carbon scaffold. In this context, the structure and molecular organization in self-assembled monolayers (SAMs) of unsubstituted and nitrile-functionalized azulenethiolates on Au(111) substrates were studied by a variety of complementary spectroscopic techniques. The molecule of 2-mercapto-6-cyanoazulene was specifically designed to be addressable within the core hole clock approach in the general framework of resonant Auger electron spectroscopy. The azulenic SAMs described herein were documented to be well-defined and densely packed, with their individual molecular constituents oriented upright with respect to the Au surface, but with considerable tilt and twist. The electron transfer (ET) properties of these azulenic SAMs were found to be comparable to those of analogous naphthalene-based monolayers, which suggests that the charge separation and the related dipole moment have ...

Side-Group-Induced Polymorphism in Self-Assembled Monolayers: 3,5-Bis(trifluoromethyl)benzenethiolate Films on Au(111).

The structure and molecular organization of self-assembled monolayers (SAMs) depend on a complex interplay of intermolecular and molecule-substrate interactions, so that even a small change in molecular composition can result in noticeable changes in the SAM structure. Herein we show that decoration of the most basic aromatic SAM constituent, benzenethiol, with two trifluoromethyl groups leads to distinct polymorphism in the respective SAMs, in which the appearance of a specific structural phase or a combination of several different phases is dependent on the parameters of the preparation procedure. High-quality films with a single crystallographic phase and significantly large domains could only be prepared after a short immersion time (5 min) and an additional re-immersion of the sample in pure ethanol at an elevated temperature. A standard 24 h immersion at room temperature led to poorly defined films with a large defect density and only a small portion of the surface covered by well-ordered molecular domains.

Structural and Functional Insight into Recombinant Lung Surfactant Protein B (rSP-B)

Surfactant associated-protein B (SP-B) is the most essential protein component of lung surfactant; its absence is fatal. Lung surfactant associated proteins, especially SP-B, assist in the organization and rearrangement of the lipid monolayers and multilayers that coat the air-water interface of the alveolus, and so help to reduce the work of breathing and prevent lung collapse. Due to SP-B's unique hydrophobic nature however, the overall three dimensional structure of the protein is not yet determined, therefore the molecular basis for its activity is not clearly understood. Recently, our lab has managed to produce the protein in bacteria using recombinant DNA technology. Recombinant SP-B (rSP-B) is shown to retain the expected alpha helical conformation in different membrane mimetic conditions, over a range of temperatures as observed by far UV circular dichroism (CD) spectroscopy. Dynamic Light Scattering (DLS) of rSP-B suspended in detergent micelles indicates two populations, one with a hydrodynamic size of ∼3 nm and the other at ∼100 nm. Nanoparticle Tracking Analysis (NTA) confirmed the ∼100 nm species to be prominent. Preliminary 1D solution NMR experiments have also been carried out.Moreover, rSP-B also shows promising results in in vitro measures of functionality when tested with a Langmuir-Blodgett trough. Lipid films in the presence of rSP-B are able to promote multilayer formation when compressed to a sufficiently high surface pressure, comparable to that of clinical lung surfactants. What is more, films in the presence of rSP-B demonstrate superior compressibility and film recovery when compared to films compressed without rSP-B.

BLT2 expression improves skin integrity and protects from alterations caused by hyperglycemia in type 2 diabetes.

ABSTRACTType 2 diabetes (T2D) can go undiagnosed for years, leading to a stage where chronic high blood sugar produces complications such as delayed wound healing. Reports have shown that BLT2 activation improves keratinocyte migration and wound healing, as well as protecting the epidermal barrier through the promotion of actin polymerization.The goal of this study was to elucidate the role of BLT2 expression in skin epithelial integrity in T2D.For this purpose, we used both wild type (WT) and BLT2 knockout mice in a model, in which a T2D-like phenotype was induced by keeping the animals on a high fat (HF) diet over 5 weeks. In a parallel in vitro approach, we cultured BLT2-transfected HaCaT cells at both low and high glucose concentrations for 48 h. Structure, transepithelial resistance (TEER), IL-1ß, IL-8 or CXCL2, MMP9, Filaggrin, Loricrin and Keratin 10 (K10) were evaluated ex vivo and in vitro. Additionally, wound healing (WH) was studied in vitro.The skin from T2D and BLT2 knockout mice showed a reduction in TEER and the expression of IL-1ß, and in increase in CXCL2, MMP9, Filaggrin, Loricrin and K10 expression. The structure suggested an atrophic epidermis; however, the skin was dramatically affected in the BLT2 knockout mice kept on a HF diet.HaCaT-BLT2 cells presented as an organized monolayer and showed higher TEER and wound healing compared with vector only-transfected HaCaT-Mock cells. Likewise, alterations in the expression of skin inflammatory, matrix degradation and differentiation markers under low and high glucose conditions were less severe than in HaCaT-Mock cells.Our results suggest that BLT2 improves epithelial integrity and function by regulating differentiation markers, cytokines and MMP9. Furthermore, BLT2 attenuates the damaging effects of high glucose levels, thereby accelerating wound healing.

Applications of Biomaterials in Corneal Endothelial Tissue Engineering.

When corneal endothelial cells (CECs) are diseased or injured, corneal endothelium can be surgically removed and tissue from a deceased donor can replace the original endothelium. Recent major innovations in corneal endothelial transplantation include replacement of diseased corneal endothelium with a thin lamellar posterior donor comprising a tissue-engineered endothelium carried or cultured on a thin substratum with an organized monolayer of cells. Repairing CECs is challenging because they have restricted proliferative ability in vivo. CECs can be cultivated in vitro and seeded successfully onto natural tissue materials or synthetic polymeric materials as grafts for transplantation. The optimal biomaterials for substrata of CEC growth are being investigated. Establishing a CEC culture system by tissue engineering might require multiple biomaterials to create a new scaffold that overcomes the disadvantages of single biomaterials. Chitosan and polycaprolactone are biodegradable biomaterials approved by the Food and Drug Administration that have superior biological, degradable, and mechanical properties for culturing substratum. We successfully hybridized chitosan and polycaprolactone into blended membranes, and demonstrated that CECs proliferated, developed normal morphology, and maintained their physiological phenotypes. The interaction between cells and biomaterials is important in tissue engineering of CECs. We are still optimizing culture methods for the maintenance and differentiation of CECs on biomaterials.

Patchy and Janus Nanoparticles by Self-Organization of Mixtures of Fluorinated and Hydrogenated Alkanethiolates on the Surface of a Gold Core.

The spontaneous self-organization of dissimilar ligands on the surface of metal nanoparticles is a very appealing approach to obtain anisotropic "spherical" systems. In addition to differences in ligand length and end groups, a further thermodynamic driving force to control the self-assembled monolayer organization may become available if the ligands are inherently immiscible, as is the case of hydrogenated (H-) and fluorinated (F-) species. Here, we validate the viability of this approach by combining 19F NMR experiments and multiscale molecular simulations on large sets of mixed-monolayer-protected gold nanoparticles (NPs). The phase segregation of blends of F- and H-thiolates grafted on the surface of gold NPs allows a straightforward approach to patterned mixed monolayers, with the shapes of the monolayer domains being encoded in the structure of the F/H-thiolate ligands. The results obtained from this comprehensive study offer molecular design rules to achieve a precise control of inorganic nanoparticles protected by specifically patterned monolayers.

Formation of 2-D Crystalline Intermixed Domains at the Molecular Level in Binary Self-Assembled Monolayers from a Lyotropic Mixture

The organization and intermolecular interactions of mixed self-assembled monolayers (SAMs) of 1-decanethiol (DT) and 11-mercaptoundecanoic acid (MUA) formed on Au(111) from a lyotropic medium have been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and infrared reflection–absorption spectroscopy (IRRAS) as a function of their surface composition. The presence of a single reductive desorption peak in the voltammograms and thiol surface coverages close to 7.6 × 10–10 mol/cm2 reveal the formation of homogeneous mixtures of both molecules in an upright configuration in the mixed SAMs. Despite this fact, two different distributions of the individual components have been found in these MUA-enriched SAMs according to their different electron transfer blocking behavior and conformational order shown by EIS and IRRAS results. At short modification times, MUA–MUA and DT–DT homomolecular interactions prevail during the assembly process, and nanodomains of like molecules are probabl...

Control of Pd catalyst selectivity with mixed thiolate monolayers

Metal catalysts coated with self-assembled monolayers have been found to exhibit improved performance in a number of reactions, but previous work has focused on deposition of uniform layers from a single precursor. In this study, we investigate how the use of variable-composition monolayers formed from mixtures of 1-adamantanethiol (AT) and 1-octadecanethiol (C18) can be used to control catalyst performance and provide insights into both monolayer ordering and surface reaction mechanisms. Benzyl alcohol hydrodeoxygenation to toluene was used as a probe reaction for these studies. The mixed monolayer composition was controlled by varying the proportion of AT to C18 in an ethanol solution used for deposition. For AT-modified catalysts, toluene selectivity was higher than that for the uncoated catalyst. Increases in the C18 surface fraction (yC18) in the monolayers resulted in further increases in toluene selectivity, with selectivity approaching 98% near complete conversion at full C18 coverage. Infrared spectra collected after dosing CO or benzyl alcohol also indicated that increasing yC18 lowered the ratio of bridging and three-fold adsorption on high coordination sites compared to linear adsorption on low coordination sites. These results suggested that the mechanism for selectivity enhancement was at least partly due to selective poisoning of terrace sites and reduction of contiguous active sites. Whereas selectivity was observed to follow a continuous increasing trend with increasing thiolate coverage, trends in reaction rates were more complex. A sharp 60% decrease in the rate of the undesired decarbonylation (DC) reaction was observed when yC18 was increased from zero to 0.15. This sharp decrease at low yC18 was attributed to filling in defects in AT monolayers by C18. Control of mixed monolayer composition allowed for tuning of the catalyst performance such that the rate of toluene production could match that over the uncoated catalyst, while the decarbonylation rate was decreased by an order of magnitude, providing an active, highly selective catalyst. Beyond catalyst design, these investigations demonstrate that catalyst evaluation can serve as a valuable tool for understanding monolayer organization.

Cold denaturation induces inversion of dipole and spin transfer in chiral peptide monolayers.

Chirality-induced spin selectivity is a recently-discovered effect, which results in spin selectivity for electrons transmitted through chiral peptide monolayers. Here, we use this spin selectivity to probe the organization of self-assembled α-helix peptide monolayers and examine the relation between structural and spin transfer phenomena. We show that the α-helix structure of oligopeptides based on alanine and aminoisobutyric acid is transformed to a more linear one upon cooling. This process is similar to the known cold denaturation in peptides, but here the self-assembled monolayer plays the role of the solvent. The structural change results in a flip in the direction of the electrical dipole moment of the adsorbed molecules. The dipole flip is accompanied by a concomitant change in the spin that is preferred in electron transfer through the molecules, observed via a new solid-state hybrid organic–inorganic device that is based on the Hall effect, but operates with no external magnetic field or magnetic material.

Dielectric relaxation properties of carboxylic acid-terminated n-alkyl monolayers tethered to Si(1 1 1): dynamics of dipoles and gauche defects

Molecular-level insights into the organization and dynamics of n-alkyl monolayers covalently bonded to Si(1 1 1) were gained from admittance measurements of dipolar relaxation in rectifying Hg HOOC-C10H25–n Si junctions performed as a function of applied voltage and temperature. A collective behavior of dipole dynamics is inferred from the non-Debye asymmetric relaxation peak shape and strong coupling of the dipole relaxation path with some bending vibrations of the n-alkyl OML (multi-excitation entropy model). A variety of relaxation mechanisms is observed in the frequency range (0.1 Hz–10 MHz) with different dependence of relaxation frequency and dipolar strength on measurement temperature and applied voltage. Their microscopic origin is discussed by comparing the activation energy of relaxation frequency with previous molecular mechanics calculations of saddle point energy barriers for structural defects such as gauche conformations or chain kinks in n-alkanes assemblies. Gauche conformations organized in pairs (kinks) have vanishing relaxation strength below an order–disorder transition temperature TD = 175 K and their probability strongly increases with applied reverse voltage, above TD. The presence of hydrogen bonds between terminal carboxylic acid functionalities is inferred from a comparison with a similar junction bearing a low density of carboxylic acid end groups. This temperature-dependent hydrogen-bond network provides some additional stiffness against external electrostatic stress, as deduced from the rather weak sensitivity of relaxation frequencies to applied bias voltage.

Molecular Packing in Langmuir Monolayers Composed of a Phosphatidylcholine and a Pyrene Lipid.

Pyrene lipids are useful tools to investigate membrane organization and intracellular lipid trafficking. The molecular interactions controlling the organization of lipid monolayers composed of a cationic amphiphile tagged with a pyrene residue and a saturated or unsaturated phospholipid, namely, 1,2-dimyristoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphocholine, were investigated by Langmuir trough isotherms to understand how the molecular structure of the components and their relative amount affect the physicochemical properties of lipid monolayers. The obtained results show that the cationic headgroups and unsaturation of hydrophobic chains strongly affect the organization of the lipid monolayer as a function of the amount of components. On the other hand, the presence of the pyrene moiety does not seem to have a marked influence on the interaction within lipid assembly.

Parameters affecting monolayer organisation of substituted polysaccharides on solid substrates upon Langmuir–Schaefer deposition

Cellulose esters with varying ester and residual hydroxyl group contents represent a cheap and pliable bio-based polysaccharide to examine. In this work, the molecular organization of cellulose triacetate, cellulose acetate propionate and cellulose acetate butyrate as a response to a variety of external parameters are investigated. The morphology of ultrathin films of the cellulose esters, transferred by means of Langmuir–Schaefer (LS) deposition from the air/water interface to the surface of solid substrates, are shown to be induced by the total surface free energy of each substrate. Additionally, the spreading capability of the esters is investigated — the spread of each polymer is directly linked to its chemical composition, namely the proportion of unsubstituted hydroxyl groups remaining on the glucose backbone. Monomer arrangement at the air/water interface has a distinct dependency on the rate at which their monolayers are compressed. The study provides a systematic approach to the investigations of LS deposition of cellulose esters.

Investigation of Mixed Surfactant Films at Water Surface Using Molecular Dynamics Simulations.

Multicomponent Langmuir monolayers are important models of organic coatings of naturally occurring water-vapor interfaces such as the surfaces of oceans or aerosol particles. We investigated mixed monolayers comprised of palmitic acid, C15H31COOH (PA) and 1-bromoalkanes of different chain length (C5, C10, and C16) at the air-water interface employing classical molecular dynamics simulations. Different composition ratios and lateral compression of the monolayers were considered. The structural parameters, such as density profiles, and deuterium order parameter, evaluated as functions of composition and the lateral film packing, provide microscopic information about organization and dynamics of the mixed monolayers. Simulations demonstrate that stable and well mixed monolayers are formed by the mixtures of PA and BrC16H33 (BrCl6), whereas the two considered shorter bromoalkanes, BrC5H11 (BrC5) and BrC10H21 (BrC10), do not form stable films. This is in accord with earlier experimental studies. Under high lateral pressures, in PA/BrC10 mixed systems molecules of the bromoalkane readily flip in the monolayer and subsequently leave the film, while the molecules of the longer BrC16 are expelled from the PA film but no flipping occurs. These results suggest that the film collapse under pressure is preceded by squeezing-out of bromoalkanes from the PA monolayer.

Disruption of lipid domain organization in monolayers of complex yeast lipid extracts induced by the lysophosphatidylcholine analogue edelfosine in vivo

The lysophosphatidylcholine analogue edelfosine is a potent antitumor and antiparasitic drug that targets cell membranes. Previous studies have shown that edelfosine alters membrane domain organization inducing internalization of sterols and endocytosis of plasma membrane transporters. These early events affect signaling pathways that result in cell death. It has been shown that edelfosine preferentially partitions into more rigid lipid domains in mammalian as well as in yeast cells. In this work we aimed at investigating the effect of edelfosine on membrane domain organization using monolayers prepared from whole cell lipid extracts of cells treated with edelfosine compared to control conditions. In Langmuir monolayers we were able to detect important differences to the lipid packing of the membrane monofilm. Domain formation visualized by means of Brewster angle microscopy also showed major morphological changes between edelfosine treated versus control samples. Importantly, edelfosine resistant cells defective in drug uptake did not display the same differences. In addition, co-spread samples of control lipid extracts with edelfosine added post extraction did not fully mimic the results obtained with lipid extracts from treated cells. Altogether these results indicate that edelfosine induces changes in membrane domain organization and that these changes depend on drug uptake. Our work also validates the use of monolayers derived from complex cell lipid extracts combined with Brewster angle microscopy, as a sensitive approach to distinguish between conditions associated with susceptibility or resistance to lysophosphatidylcholine analogues.