Lipid Nanotube Tailored Fabrication of Uniquely Shaped Polydopamine Nanofibers as Photothermal Converters

Helically coiled and linear polydopamine (PDA) nanofibers were selectively fabricated with two different types of lipid nanotubes (LNTs) that acted as templates. The obtained coiled PDA‐LNT hybrid showed morphological advantages such as higher light absorbance and photothermal conversion effect compared to a linear counterpart. Laser irradiation of the coiled PDA‐LNT hybrid induced a morphological change and subsequent release of the encapsulated guest molecule. In cellular experiments, the coiled PDA‐LNT efficiently eliminated HeLa cells because of its strong affinity with the tumor cells. This work illustrates the first approach to construct characteristic morphologies of PDA nanofibers using LNTs as simple templates, and the coiled PDA‐LNT hybrid exhibits attractive photothermal features derived from its unique coiled shape.     

Local environment and property of water inside the hollow cylinder of a lipid nanotube

We investigated the local environment of water confined inside the hollow cylinder of lipid nanotubes (LNTs) by time-resolved fluorescent measurements and attenuated-total-reflectance infrared (ATR-IR) spectroscopy. The LNT was obtained by self-assembly of cardanyl glucosides in water at room temperature and had an open-ended cylindrical nanospace with a diameter of 10-15 nm, a length of 10-100 microm, and hydrophilic inner and outer surfaces. We introduced a fluorescent probe of 8-anilinonaphthalene-1-sulfonate into the confined water and observed an extremely slow dynamic Stokes shift with a correlation time of 1.26 ns, which was 2-3 orders of magnitude longer than that of bulk-phase water. From the peak shift of the fluorescent spectrum, the local solvent polarity (ET(30)) of the confined water was estimated as 50 kcal/mol, which is 20% lower than that in bulk water. ATR-IR measurements showed that the hydrogen-bond network of water inside the LNT was more developed than that in bulk water at room temperature, which is in contrast to the water in other self-assembled confined geometries, such as Aerosol-OT (AOT) reversed micelles.     

FT-IR study of the interlamellar water confined in glycolipid nanotube walls

The local hydrogen-bonding environment of water confined in glycolipid nanotubes (LNTs) was investigated by Fourier transform infrared (FT-IR) spectroscopy. Using X-ray diffraction (XRD), we estimated the thickness of an interlamellar water layer, which was confined between the bilayer membranes constructing the walls of the LNTs, to be 1.3 +/- 0.3 nm. FT-IR spectroscopic measurement of the confined water showed an obvious reduction in IR absorption in both the low-energy (around 3000 cm(-1)) and high-energy regions (around 3600 cm(-1)) of the OH stretching band as compared to bulk water. The reduction around 3000 cm(-1) indicated a decrease in the relative proportion of the water molecules with a long-range network structure due to a geometrical restriction. This agrees with the results obtained for other multilamellar systems. On the other hand, the remarkable reduction around 3600 cm(-1), which was not observed in the other systems, indicated the absence of weakly hydrogen-bonded water aggregates due to the effect of sugar headgroups.     

Protein folding at emulsion oil/water interfaces

It has long been known that proteins change their conformation upon adsorption to emulsion oil/water interfaces. However, it is only recently that details of the specifics of these structural changes have emerged. The development of synchrotron radiation circular dichroism (SRCD), combined with advances in FTIR spectroscopy, has allowed the secondary and tertiary structure of proteins adsorbed at emulsion oil/water interfaces to be studied. SRCD in particular has provided quantitative information and has enabled new insights into the mechanisms and forces driving protein structure re-arrangement to be achieved.The extent of conformational re-arrangement of proteins at emulsion interfaces is influenced by several factors including; the inherit flexibility of the protein, the distribution of hydrophobic/hydrophilic domains within the protein sequence and the hydrophobicity of the oil phase. In general, proteins lose much of their tertiary structure upon adsorption to the oil/water interface and have considerable amounts of non-native secondary structure. Two key conformations have been identified in the structure of proteins at interfaces, intermolecular β-sheet and α-helix. The preferred conformation appears to be the α-helix which is the most compact amphipathic conformation at the oil/water interface. The polarity of the oil phase can have a considerable influence on the degree of protein conformational re-arrangement because it acts as a solvent for hydrophobic amino acids. The new conformation of proteins at interfaces also means that proteins undergo less heat induced re-arrangement at interfaces than in solution. Different conformations of proteins at interfaces impact on emulsification capability, emulsion stability and protein/emulsion digestion. Hence advances in the understanding of protein conformation at interfaces can help to identify suitable proteins and conditions for the preparation of emulsion based food products.     

Self-assembled lipid nanotube hosts: The dimension control for encapsulation of nanometer-scale guest substances

Supramolecular nanotube hosts with precisely controlled inner or outer diameters have been synthesized by self-assembly of unsymmetrical bolaamphiphilic monomers or glucopyranosylamide lipids, respectively. Time-resolved fluorescent measurement using 8-anilinonaphthalene-1-sulfonate (ANS) as a probe revealed that the water confined in a cardanyl-β-D -glucopyranoside lipid nanotube has relatively lower solvent polarity corresponding to that of propanol than bulk water. Extensively developed hydrogen bond networks also characterize the confined water in comparison to the case in bulk water. Encapsulation ability of the glucopyranosylamide lipid nanotube has been examined by filling the lyophilized LNTs with gold or silver nanoparticles, ferritin, or magnetic crystals. Filling the unsymmetrical bolaamphiphile nanotube possessing positively charged inner surfaces with negatively charged polymer beads or ferritin proved to be successful without depending on capillary action. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5137–5152, 2006     

Development of a new experimental system for monitoring biomembrane reactions: combination of laser spectroscopic techniques and biomembrane models formed at an oil/water interface.

We present a new experimental system to observe reactions in biomembranes by combining laser spectroscopic techniques with phospholipid monolayers formed at oil/water interfaces. The system can monitor reactions through changes in interfacial tension at oil/water interfaces induced by the reactions under non-destructive and non-contact conditions. In addition, oil/water interfaces with defined areas can define the composition of different kinds of phospholipids. Furthermore, the system allows using, as an oil phase, alkanes whose number of carbon atoms is close to the number of the alkyl chains of phospholipids in biomembranes (C > or = 16). We demonstrated the hydrolysis reaction in DPPC (dipalmitoyl phosphatidylcholine)/DPPS (dipalmitoyl phosphatidylserine)-mixed monolayers by phospholipase A2 by using the system.     

Preparation of microscopic and planar oil-water interfaces that are decorated with prescribed densities of insoluble amphiphiles

Langmuir monolayers (monolayers of insoluble molecules formed at the surface of water), and associated Langmuir-Blodgett/Schaefer monolayers prepared by transfer of Langmuir films to the surfaces of solids, are widely used in studies aimed at understanding the physicochemical properties of biological and synthetic molecules at interfaces. In this article, we report a general and facile procedure that permits transfer of Langmuir monolayers from the surface of water onto microscopic and planar interfaces between oil and aqueous phases. In these experiments, a metallic grid supported on a hydrophobic solid is used to form oil films with thicknesses of 20 mum and interfacial areas of 280 mum x 280 mum. Passage of the supported oil films through a Langmuir monolayer is shown to lead to quantitative transfer of insoluble amphiphiles onto the oil-water interfaces. The amphiphile-decorated oil-water interfaces hosted within the metallic grids (i) are approximately planar, (ii) are sufficiently robust mechanically so as to permit further characterization of the interfaces outside of the Langmuir trough, (iii) can be prepared with prescribed and well-defined densities of amphiphiles, and (iv) require only approximately 200 nL of oil to prepare. The utility of this method is illustrated for the case of the liquid crystalline oil 4-pentyl-4'-cyanobiphenyl (5CB). Transfer of monolayers of either dilauroyl- or dipalmitoylphosphatidylcholine (DLPC and DPPC, respectively) to the nematic 5CB-aqueous interface is demonstrated by epifluorescence imaging of fluorescently labeled lipid and polarized light imaging of the orientational order within the thin film of nematic 5CB. Interfaces prepared in this manner are used to reveal key differences between the density-dependent phase properties of DLPC and DPPC monolayers formed at air-water as compared to that of nematic 5CB-aqueous interfaces. The methodology described in this article should be broadly useful in advancing studies of the interfacial behavior of synthetic and biological molecules at liquid-liquid interfaces.     

Behavior of polymer chains grafted from latex particles at soft interfaces

We have studied the behaviors of a poly(methyl methacrylate) (PMMA) chains anchored to polystyrene particles at air/water and oil/water interfaces to recognize the roles of oil molecules in the PMMA property at the interfaces. Through the comparison of π-A isotherms we found two aspects of unique structural and rheological characteristics observed in PMMA-grafted polystyrene latex (PSL-PMMA) monolayer system in common. (1) The π-A isotherms showed surface pressure increase at larger occupied area compared to the PSL-PMMA size in solution at three different types of interfaces in most cases. (2) Compressional modulus, C _s ^?1, obtained by π-A isotherm analysis for PSL-PMMA at interfaces, showed the tendency to decrease with molecular weight of PMMA. This is opposite to that of PMMA homopolymer at interfaces. The effect of oil molecules on PSL-PMMA system at interfaces are found both in the difference of occupied area and C _s ^?1. The occupied areas were larger for the isotherms at the oil/water interfaces than those at the air/water interface in most cases, which suggested the reduced attractive interactions between anchored polymers by oil molecules. On the other hand, C _s ^?1 of PMMA monolayers is strongly dependent on the constituents of the interface and the order of C _s ^?1 is air/water?>?decane/water?>?dibutyl ether/water interfaces. The difference between oil species was not explained only by PMMA/oil interaction in bulk, but we suggested that interfacial tension of oil/water interface affects the miscibility of oil molecules with PMMA to cause higher miscibility between PMMA and dibutyl ether at the oil/water interfaces.     

Hydroxide and hydronium ion adsorption — A survey

The propensity of hydroxide and hydronium ions to accumulate at interfaces is the subject of ongoing scientific debate. Electrokinetic and surface force measurements suggest elevated interfacial concentrations of hydroxide ions across a wide range of pHs. Contrary to this, however, surface-sensitive spectroscopic techniques and molecular dynamic (MD) simulations indicate that hydronium ions have strong surface affinity under similar conditions. Here we review results obtained for gas/water, oil/water and solid/water interfaces. Emphasis is placed on ion adsorption phenomena occurring on polymer films of different hydrophobicity and structure. The results clearly show that asymmetric water ion adsorption is independent of the hydrophobicity of the solid surface. Recently obtained data reveal significant effects of the hydroxide and hydronium ions even on the charging of hydrophobic polymers in the presence of multivalent electrolytes and on the charging of zwitterionic lipid membranes.     

Three distinct water structures at a zwitterionic lipid/water interface revealed by heterodyne-detected vibrational sum frequency generation

Lipid/water interfaces and associated interfacial water are vital for various biochemical reactions, but the molecular-level understanding of their property is very limited. We investigated the water structure at a zwitterionic lipid, phosphatidylcholine, monolayer/water interface using heterodyne-detected vibrational sum frequency generation spectroscopy. Isotopically diluted water was utilized in the experiments to minimize the effect of intra/intermolecular couplings. It was found that the OH stretch band in the Imχ((2)) spectrum of the phosphatidylcholine/water interface exhibits a characteristic double-peaked feature. To interpret this peculiar spectrum of the zwitterionic lipid/water interface, Imχ((2)) spectra of a zwitterionic surfactant/water interface and mixed lipid/water interfaces were measured. The Imχ((2)) spectrum of the zwitterionic surfactant/water interface clearly shows both positive and negative bands in the OH stretch region, revealing that multiple water structures exist at the interface. At the mixed lipid/water interfaces, while gradually varying the fraction of the anionic and cationic lipids, we observed a drastic change in the Imχ((2)) spectra in which spectral features similar to those of the anionic, zwitterionic, and cationic lipid/water interfaces appeared successively. These observations demonstrate that, when the positive and negative charges coexist at the interface, the H-down-oriented water structure and H-up-oriented water structure appear in the vicinity of the respective charged sites. In addition, it was found that a positive Imχ((2)) appears around 3600 cm(-1) for all the monolayer interfaces examined, indicating weakly interacting water species existing in the hydrophobic region of the monolayer at the interface. On the basis of these results, we concluded that the characteristic Imχ((2)) spectrum of the zwitterionic lipid/water interface arises from three different types of water existing at the interface: (1) the water associated with the negatively charged phosphate, which is strongly H-bonded and has a net H-up orientation, (2) the water around the positively charged choline, which forms weaker H-bonds and has a net H-down orientation, and (3) the water weakly interacting with the hydrophobic region of the lipid, which has a net H-up orientation.     

Oil/Alkanethiol Layers for the Study of Emulsified Protein Conformation by Surface Plasmon Resonance Using Monoclonal Antibodies

A method combining surface plasmon resonance and epitope mapping was developed to study the protein conformation at the oil/water interface of an emulsion. The conformation of beta-lactoglobulin stabilizing dodecane/water and miglyol/water interfaces was investigated using five anti-beta-lactoglobulin monoclonal antibodies. The developed method allows us to specifically recognize the emulsified beta-lactoglobulin at the surface of a sensor chip with good repeatability; i.e., standard deviations range between 0.7 and 3.6%. Considering that the monoclonal antibodies, recognizing conformational epitopes, still bind to beta-lactoglobulin at oil/water interfaces, it is concluded that the protein retains a globular conformation. It is shown that the inhibition-binding values of two pairs of Mabs are different for beta-lactoglobulin stabilizing dodecane/water and miglyol/water interfaces. This indicates that the conformations of emulsified beta-lactoglobulin are slightly different according to the nature of the oil phase. Copyright 2000 Academic Press.     

Interfacial rheology of natural silk fibroin at air/water and oil/water interfaces

The interfacial viscoelastic behavior of natural silk fibroin at both the air/water and oil/water interfaces is reported. This natural multiblock copolymer is found to be strongly amphiphilic and forms stable films at these interfaces. The result is an interfacial layer that is rheologically complex with strong surface elastic moduli that are only slightly frequency-dependent. The kinetics of surface viscoelastic evolution are reported as functions of time for various concentrations of the spread films. Films deposited by Langmuir-Blodgett deposition were studied by scanning electron microscopy (SEM) to reveal a fibrous structure at the interface. The production of stable O/W emulsions by silk fibroin further confirms the generation of the elastic films at the oil/water interfaces.     

Interaction of α-Synuclein with Phospholipids and the Associated Restructuring of Interfacial Lipid Water: An Interface-Selective Vibrational Spectroscopic Study

Interaction of α-Synuclein (αS) with biological lipids is crucial for the onset of its fibrillation at the cell membrane/water interface. Probed herein is the interaction of αS with membrane-mimicking lipid monolayer/water interfaces. The results depict that αS interacts negligibly with zwitterionic lipids, but strongly affects the pristine air/water and charged lipid/water interfaces by perturbing the structure and orientation of the interfacial water. The net negative αS (−9 in bulk water; pH 7.4) reorients the water as hydrogen-up (H-up) at the air/water interface, and electrostatically interacts with positively charged lipids, making the interface nearly net neutral. αS also interacts with negatively charged lipids: the net H-up orientation of the interfacial water decreases at the anionic lipid/water interface, revealing a domain-specific interaction of net negative αS with the negatively charged lipids at the membrane surface.     

Naturally occurring spore particles at planar fluid interfaces and in emulsions

We have investigated the potential of utilizing naturally occurring spore particles of Lycopodium clavatum as sole emulsifiers of oil and water mixtures. The preferred emulsions, prepared from either oil-borne or aqueous-borne dispersions of the monodispersed particles of diameter 30 microm, are oil-in-water. The particles act as efficient stabilizers for oils of different polarity. Droplets as large as several millimeters are stable to coalescence indefinitely, despite the low coverage of interfaces by particles observed microscopically. Consistent with the emulsion findings, we discover that particles spontaneously adsorb to bare oil-water interfaces of single drops from oil dispersions, whereas adsorption is less spontaneous and extensive from aqueous dispersions. Monolayers of the spore particles at both air-water and oil-water planar interfaces contain particles in an aggregated state forming clusters and chains. The influence of particle concentration, oil/water ratio, and additives in the aqueous phase is studied.     

Self-assembly of biocidal nanotubes from a single-chain diacetylene amine salt

We describe the facile two-step synthesis of nanotubes that form pure, well-defined, nanostructured materials. We have synthesized a secondary amine HBr salt as the headgroup of a single-chain diacetylenic lipid. This molecule can form a number of different self-assembled nanostructures in aqueous or organic solvents. In water, this lipid forms a monodisperse preparation of nanotubes at high yields. Partially dissolving a preparation of nanotubes dried from aqueous solution results in a remarkably organized structure that resembles a nanocarpet. Details of the nanotube structure were investigated by scanning electron microscopy, transmission electron microscopy, and small-angle X-ray spectroscopy. The aqueous nanotubes have a cross-sectional diameter of 89 nm. The walls of the tubes are an exquisitely uniform 27 nm thick and are shown to consist of five lipid bilayers with a repeat spacing of 57.8 A. The chemical structure of the material shows no chiral centers, but suspensions of the nanotubes in an aqueous medium show an unexpected circular dichroism signal. The versatility of this new material as a platform for nanostructure design and synthesis is enhanced by its biocidal activity. This antimicrobial activity along with the regularity the nanostructures will enhance the development of a range of applications from biosensors to artificial retinas.     

Molecular dynamics simulations of charged nanoparticle self-assembly at ionic liquid-water and ionic liquid-oil interfaces

Nanoparticle self-assembly at liquid-liquid interfaces can be significantly affected by the individual nanoparticle charges. This is particularly true at ionic liquid (IL) based interfaces, where Coulombic forces play a major role. Employing 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)]) as a model IL, we have studied the self-assembly of hydrophobic nanoparticles with different surface charges at the IL/water and IL/oil (hexane) interfaces using molecular dynamics simulations. In the IL/water system, the nanoparticles were initially dispersed in the water phase but quickly equilibrated at the interface, somewhat in favor of the IL phase. This preference was lessened with increased nanoparticle charge. In the IL/hexane system, all charged nanoparticles interacted with the IL to some extent, whereas the uncharged nanoparticles remained primarily in the hexane phase. Potential of mean force calculations supported the observations from the equilibrium studies and provided new insights into the interactions of the nanoparticles and ionic liquid based interfaces.     

Stability of water/crude oil emulsions based on interfacial dilatational rheology

The dilatational viscoelasticity behaviors of water/oil interfaces formed with a crude oil and its distilled fractions diluted in cyclohexane were investigated by means of an oscillating drop tensiometer. The rheological study of the w/o interfaces at different frequencies has shown that the stable w/o emulsions systematically correspond to interfaces which present the rheological characteristics of a 2D gel near its gelation point. The stability of emulsions was found to increase with both the gel strength and the glass transition temperature of the gel. As expected, the indigenous natural surfactants responsible for the formation of the interfacial critical gel have been identified as the heaviest amphiphilic components present in the crude oil; i.e., asphaltenes and resins. Nevertheless, we have shown that such a gel can also form in the absence of asphaltene in the oil phase.     

Interfacial activity of particles at PI/PDMS and PI/PIB interfaces: analysis based on Girifalco–Good theory

Particles that are partially wetted by oil and water are known to adsorb at oil/water interfaces. By the same mechanism, particles that are partially wetted by two immiscible polymers should adsorb at the interface between those two polymers. However, since chemical differences between immiscible polymers are relatively modest, particle adsorption at polymer/polymer interfaces may be expected to be relatively uncommon. We have conducted experiments with several particle types added to two pairs of model polymers, polyisoprene/polydimethylsiloxane and polyisoprene/polyisobutylene. Contrary to our expectation, in every case, particles readily adsorbed at the polymer/polymer interfaces. We evaluated the Girifalco–Good theory as a means to predict the interfacial activity of the particles. The solid surface energy required by the Girifalco–Good theory was assumed to be equal to the critical surface tension, which was then found by float/sink tests. Our results suggest that this approach is not able to predict the observed interfacial activity of particles at polymer/polymer interfaces.     

Viscoelastic Properties of Crude Oil Components at Oil‐Water Interfaces. 2: Comparison of 30 Oils

The dilational properties of diluted (0.7 vol/vol in toluene) and undiluted crude oil‐water interfaces have been studied using the oscillation drop method with the objective of understanding the properties contributing to the overall stability of crude oil emulsions. The importance of working with undiluted crude oils instead of model systems when dilational properties of real oil‐water systems are going to be reproduced in the laboratory setting has been discussed. For such studies, molecular exchange mechanisms and the aggregation state of asphaltenes are too dependent on concentration to justify the use of model compounds, i.e. fractionated asphaltenes diluted in a solvent. As expected in the low frequency range (0.01–1 Hz), molecular exchange from the bulk oil phase strongly affected the measured dilational parameters. For the diluted crude oils, the frequency dependence of the dilational modulus increased with its magnitude. The systems that exhibited particularly low magnitude of the dilational modulus were of the heaviest crude oils in the sample set, whereas the systems with greatest dilational modulus were among the lightest crude oils. The overall characteristic time of relaxation of the crude oil‐water interfaces was in the range below 10 seconds. The undiluted crude oil‐water interfaces had similar interfacial properties as the diluted samples except for slightly reduced magnitude of the dilational modulus. The crude oil‐water interfaces appeared to be soluble, but some observations pointed to intrinsic rheological properties of the interfaces. Intrinsic elasticity and viscosity of the films should be studied outside the frequency range used here at low (ω～0 Hz) and high (ω→500 Hz), respectively.     

A many-body dissipative particle dynamics study of forced water-oil displacement in capillary

The forced water-oil displacement in capillary is a model that has important applications such as the groundwater remediation and the oil recovery. Whereas it is difficult for experimental studies to observe the displacement process in a capillary at nanoscale, the computational simulation is a unique approach in this regard. In the present work, the many-body dissipative particle dynamics (MDPD) method is employed to simulate the process of water-oil displacement in capillary with external force applied by a piston. As the property of all interfaces involved in this system can be manipulated independently, the dynamic displacement process is studied systematically under various conditions of distinct wettability of water in capillary and miscibility between water and oil as well as of different external forces. By analyzing the dependence of the starting force on the properties of water/capillary and water/oil interfaces, we find that there exist two different modes of the water-oil displacement. In the case of stronger water-oil interaction, the water particles cannot displace those oil particles sticking to the capillary wall, leaving a low oil recovery efficiency. To minimize the residual oil content in capillary, enhancing the wettability of water and reducing the external force will be beneficial. This simulation study provides microscopic insights into the water-oil displacement process in capillary and guiding information for relevant applications.