Langmuir : the ACS journal of surfaces and colloids
Langmuir. 2012 Dec 10;: 23215537
Study of adhesion and friction properties on a nanoparticle gradient surface: Transition from JKR to DMT contact mechanics.
We have previously investigated the dependence of adhesion on nanometer-scale surface roughness by employing a roughness gradient. In this study, we correlate the obtained adhesion forces on nanometer-scale rough surfaces to their frictional properties. A roughness gradient with varying silica particle (diameter = 12 nm) density was prepared, and adhesion and friction forces were measured across the gradient surface in perfluorodecalin by means of atomic force microscopy with a polyethylene colloidal probe. Similarly to the pull-off measurements, the frictional forces initially showed a reduction with decreasing particle density and later an abrupt increase as the colloidal sphere began to touch the flat substrate beneath, at very low particle densities. The friction-load relation is found to depend on real contact area (Areal) between the colloid probe and the underlying particles. At high particle density, the colloidal sphere undergoes large deformations over several nanoparticles and the contact adhesion (JKR type) dominates the frictional response. However, at low particle density (before the colloidal probe is in contact with underlying surface), the colloidal sphere is suspended by a few particles only, resulting in local deformations of the colloid sphere, the frictional response to applied load being dominated by long-range, non-contact (DMT type) interactions with the substrate beneath.
Langmuir. 2012 Dec 6;: 23215530
Synthesis and surface properties of low surface free energy silane-functional polybenzoxazine films.
A series of low surface free energy silane-functional polybenzoxazine films were prepared from alkyl-phenol, paraformaldehyde, 3-aminopropyltrimethoxysilane (3-APTMOS) using a facile one-step thermal curing method. The chemical structures of these benzoxazine monomers were confirmed by FTIR, and 1H NMR. The surface properties of the series of polybenzoxazine films were proved through contact angle measurement and the lowest surface energy of these films was 14.91mJ/m2. Moreover, the hydrogen bond network of the polybenzoxazine systems was studied by using the FTIR spectra, and the result showed that the intramolecular hydrogen bonding transformed into intermolecular hydrogen bonding with increasing the curing time. Based on these findings, the transformation mechanism between the intermolecular and intramolecular hydrogen bonding during the progress of curing was proposed. The thermogravimetric analysis (TGA) results indicated that the silane-functional polybenzoxazine, with a high char yield of 57.01%, possessed excellent thermal stability.
Langmuir. 2012 Dec 7;: 23215450
Using atomic level simulation we aimed to investigate various intermediate phases of long chain alkyl sulfonate/water system. Overall, about 800ns parallel molecular dynamics simulation study was conducted for a surfactant/water system consisting of 128 sodium pentadecyl sulfonate and 2251 water molecules. The GROMACS software code with united atom force field was applied. Despite some differences, the analysis of main structural parameters is in agreement with X-ray experimental findings. The mechanism of self-assembly of SPDS molecules was also examined. At T=323K we obtained both tilted fully interdigitated and liquid crystalline-like disordered hydrocarbon chains, hence presence of either gel phase that coexists with a lamellar phase or metastable gel phase with fraction of gauche configuration can be assumed. Further increase of temperature revealed that the system underwent a transition to a lamellar phase, which was clearly identified by the presence of a fully disordered hydrocarbon chains. The transition from gel-to-fluid phase was implemented by simulated annealing treatment and the phase transition point at T=335K was identified. The surfactant force field in its presented set is surely enabled to fully demonstrate the mechanism of self-assembly and the behavior of phase transition making it possible to get important information around the phase transition point.
Langmuir. 2012 Dec 8;: 23215353
Fabrication of Microspheres via Solvent Volatization Induced Aggregation of Self-Assembled Nanomicellar structures and Their Use as a pH-Dependent Drug Release System.
A series of oleamide derivatives,(C(18)H(34)NO)(2)(CH(2))(n)[n = 2 (1a), 3 (1b), 4 (1c), or 6 (1d); C(18)H(34)NO = oleic amide fragment] and (C(18)H(34)NO)(CH(2))(6)NH(2)(2), have been synthesized and their self-assembly is investigated in ethanol/water media. Quantum chemical calculations and polymorph prediction show that compound 1a is best fitted with P2(1) space group, and all other compounds, 1b-1d and 2, are best fitted with P1. Self-assembly of 1a and 1b in ethanol/water (1/0.1 v/v) solution (5 mg mL(-1)) yields microspheres (MSs) with the average diameter ∼10 μm via a gradual temperature reduction and solvent volatilization processes. Under the same self-assembly conditions, microrods (average diameter ∼6 μm and several tens of micrometers in length), micro-necklace-like and shape-irregular microparticles are formed from the self-assembly of 1c, 1d and 2, respectively. The kinetics of evolution for their self-assemblies by dynamic light scattering technique and in situ observation by optical microscopy reveals that the microstructures formation is from a well-behaved aggregation of nanoscale micelles induced by solvent volatilization. The FT-IR and temperature-dependent 1H-NMR spectra demonstrate the hydrogen bonding force and π-π stacking drive the self-assembly of all oleamide derivatives in ethanol/water. Among the fabricated microstructures, the MSs from 1a exhibit the best dispersity, which thus have been used as a scaffold for the in vitro release of doxorubicin. The results demonstrate a pH-sensitive release process, enhanced release specifically at low pH 5.2.
Langmuir. 2012 Dec 5;: 23215347
Improved Analysis for Determining Diffusion Coefficients from Short Single-Molecule Trajectories with Photoblinking.
Bo Shuang, Chad P Byers, Lydia Kisley, Lin-Yung Wang, Julia Zhao, Hiroyuki Morimura, Stephan Link, Christy F Landes
Two Maximum Likelihood Estimation (MLE) methods were developed for optimizing the analysis of single-molecule trajectories that include phenomena such as experimental noise, photoblinking, photobleaching, and translation or rotation out of the collection plane. In particular, short, single-molecule trajectories with photoblinking were studied, and our method was compared with existing analytical techniques applied to simulated data. The optimal method for various experimental cases was established, and the optimized MLE method was applied to a real experimental system: single-molecule diffusion of fluorescent molecular machines known as nanocars.
Langmuir. 2012 Dec 7;: 23215250
Effect of the protonation degree of a self-assembled monolayer on the immobilization dynamics of a [NiFe] hydrogenase.
Tillmann Utesch, Diego Millo, Maria Ana Castro, Peter Hildebrandt, Ingo Zebger, Maria Andrea Mroginski
Understanding the interaction and immobilization of [NiFe] hydrogenases on functionalized surfaces is important in the field of biotechnology and in particular for the development of biofuel cells. In this study, we investigated the adsorption behavior of the standard [NiFe] hydrogenase of D. gigas on amino-terminated alkanethiol self-assembled monolayers (SAMs) with different levels of protonation. Classical all-atom MD simulations revealed a strong correlation between the adsorption behavior and the level of ionization of the chemically-modified electrode surface. While the hydrogenase undergoes a weak, but stable initial adsorption process on SAMs with a low degree of protonation, a stronger immobilization is observable on highly ionized SAMs, affecting protein reorientation and conformation. These results were validated by complementary SEIRA measurements on the comparable [NiFe] standard hydrogenases from D. vulgaris Miyazaki F, and allowed in this way for a detailed insight into the adsorption mechanism at the atomic level.
Langmuir. 2012 Dec 5;: 23215244
Simulating the Formation of Surfactant-Templated Mesoporous Silica Materials: A Model with Both Surfactant Self-Assembly and Silica Polymerization.
We have used Monte Carlo simulations to study the formation of the MCM-41 mesoporous silica material, with a new lattice model featuring explicit representations of both silicic acid condensation and surfactant self-assembly. Inspired by experimental syntheses, we have adopted the following two-step ``synthesis'' during our simulations:(i) high pH and low temperature allowing the initial onset of mesostructures with long-range order;(ii) lower pH and higher temperature promoting irreversible silica condensation. During step (i), the precursor solution was found to spontaneously separate into a surfactant-silicate-rich phase in equilibrium with a solvent-rich phase. Lamellar and hexagonal ordering emerged for the surfactant-silicate-rich mesosphases under different synthesis conditions, consistent with experimental observations. Under conditions where silica polymerization can be neglected, our simulations were found to transform reversibly between hexagonal and lamellar phases by changing temperature. During step (ii), silica polymerization was simulated at lower pH using reaction ensemble Monte Carlo to treat the pH-dependence of silica deprotonation equilibria. Monte Carlo simulations produced silica-surfactant mesostructures with hexagonal arrays of pores and amorphous silica walls, exhibiting Q_n distributions in reasonable agreement with 29-Si NMR experiments on MCM-41. Compared with bulk amorphous silica, the wall domains of these simulated MCM-41 materials are characterized by even less order, larger fractions of three- and four-membered rings, and wider ring-size distributions.
Langmuir. 2012 Dec 7;: 23215163
Nanoscale Interfacial Friction and Adhesion on Supported versus Suspended Monolayer and Multilayer Graphene.
We determine the effect of subsurface van der Waals interactions on nanoscale friction and adhesion for suspended and silicon dioxide-supported monolayer and multilayer graphene surfaces, using atomic force microscopy (AFM) supported by semi-continuum numerical simulations. While adhesive force measurements reveal no layer number dependence for supported graphene, suspended graphene exhibits an increase in adhesive force with thickness. Further, at low applied loads friction increases with increasing number of layers for suspended graphene, in contrast to reported trends for supported graphene. We attribute these results to a competition between local forces which determine the deformation of the surface layer, the profile of the membrane as a whole, and van der Waals forces between the AFM tip and subsurface layers. We find that friction on supported monolayer graphene can be fit using generalized continuum mechanics models, from which the work of adhesion and interfacial shear strength may be extracted. In addition, we show that tip-sample adhesive forces depend on interactions with subsurface material and increase in the presence of a supporting substrate or additional graphene layers.
Langmuir. 2012 Dec 5;: 23215160
The assembly of ordered dicolloid monolayers is directed by an electric field. The dicolloid particles are polystyrene latex with a maximum equatorial diameter 3.45 μm and length 4.63 μm. The monolayer structure is characterized using small angle light scattering and bright field microscopy. With increasing field strength from 26.7 to 200 V(RMS)/cm, a transition from a disordered monolayer, to first orientationally-ordered, and then translationally ordered two-dimensional (2D) arrays occurs. A c2mm plane group symmetry dominates the ordered structure, but is present alongside structures with p2 symmetry, leading to a spread in the angular distribution of the light scattering peaks. The order-disorder transition dependence on field strength and frequency is similar to that observed for colloidal spheres; at higher frequencies, stronger fields are required to assemble particles. Optimal ordered structures reflect a balance between inducing sufficiently strong interparticle interactions while limiting the rate of formation to ensure the growth of large crystalline domains.
Langmuir. 2012 Dec 7;: 23215148
Molecular reorganization induced morphology alteration in asymmetric substrate-supported lipid bilayers (SLB) was directly visualized by means of atomic force microscopy (AFM) and total internal reflection fluorescence (TIRF) microscopy. SLB samples were fabricated on mica-on-glass and glass substrates by Langmuir-Blodgett (LB)/ Langmuir-Schaeffer (LS) using binary lipid mixtures, namely 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/ 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and ternary mixtures DOPC/DPPC/ 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS), labeled with 0.2 mol% Texas Red 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine triethylammonium salt (TR-DHPE) dye. Phase segregations were characterized by TIRF imaging, and DPPC enriched domain structures were also observed. Interestingly for ~40%(n=6) of the samples with binary mixtures in LB leaflet and single component in LS leaflet, i.e.(DOPC/DPPC)LB+DOPCLS, the contrast of the DPPC domains changed from the original dark (without dye) to bright (more TR dye partitioning) on TIRF images, returning to dark again. This contrast reverse was also correlated to AFM height images, where a DPPC-DPPC gel phase was spotted after the TIRF image contrast returned to dark. The rupture force mapping results measured on these binary mixture samples also confirmed unambiguously the formation of DPPC domains components during the contrast change. The samples were tracked over 48 hours to investigate the lipid molecule movements in both the DPPC domains and the DOPC fluid phase. The fluorescence contrast changes from bright to dark in SLB indicate that the movement of dye molecules was independent of the movement of lipid molecules. In addition, correlated multimodal imaging using AFM, force mapping and fluorescence provides a novel route to uncover the re-organization of lipid molecules at the solid-liquid interface, suggesting that the dynamics of dye molecules is highly structure dependent.
Langmuir. 2012 Dec 7;: 23215040
Structure, Stability and Electronic Interactions of Polyoxometalates on Functionalized Graphene Sheets.
Polyoxometalates (H3PMo12O40, H3PW12O40, H4PMo11VO40) supported on oxygen- and alkyl-functionalized graphene sheets were investigated. Discrete molecular species were directly observed by electron microscopy at loadings below 20 wt%. The interaction between the polyoxometalates and the graphene surface was found to significantly impact their vibrational spectra and a linear correlation between the frequency of the M-Oc-M vibration and the dispersion was evidenced by FTIR. While bulk-like electronic properties were observed for small aggregates (2-5 nm), UV-Vis spectroscopy and cyclic voltammetry revealed changes in the electronic structure of isolated molecular species as a result of their interaction with graphene. Because of the ability to disperse alkyl-functionalized graphene in a variety of polar and non-polar solvents, the materials synthesized in this work provide an opportunity to disperse polyoxometalates in media in which they would not dissolve if unsupported.
Langmuir. 2012 Dec 7;: 23215031
Modulation of the Photophysical Properties of 2,2'-Bipyridine-3,3'-diol inside Bile Salt Aggregates: A Fluorescence Based Study for the Molecular Recognition of Bile Salts.
Sarthak Mandal, Surajit Ghosh, Hari Hara Kumar Aggala, Chiranjib Banerjee, Vishal Govind Rao, Nilmoni Sarkar
2,2'-bipyridine-3,3'-diol (BP(OH)2) has been used as a sensitive ESIPT fluorophore to assess different bile salt aggregates as one of the potential biologically relevant host systems useful for carrying many sparingly water soluble drug molecules. The formation of inclusion complexes, complex induced fluorescence behaviour and their binding ability have been investigated from the modulated photophysics of BP(OH)2 by means of photophysical techniques. The constrained hydrophobic environment provided by the aggregates significantly reduces the water assisted nonradiative decay channels and lengthens the fluorescence lifetime of the proton transferred DK tautomer. Both the absorption and fluorescence properties of BP(OH)2 are found to be sensitive to the change in the structure, size and hydrophobicity of the aggregates. Fluorescence quenching experiments were performed to gain insight into the differential distribution of the probe molecules between bulk aqueous phase and nanocavities of various aggregates. The observation of longer fluorescence lifetime and rotational relaxation time in NaDC aggregates compared to that in NaCh and NaTC aggregates indicates that the binding structures of NaDC aggregates are more rigid due to its greater hydrophobicity and larger size and therefore provide better protection to the bound guest. It is noteworthy to mention that the hydrophobic microenvironments provided by bile salt aggregates are much stronger than that provided by micelles and cyclodextrins. The accessibility of water to the aggregate bound guest can significantly be enhanced with the addition of organic cosolvents. However the efficiency decreases in the order of dimethylformamide, acetonitrile and methanol.
Langmuir. 2012 Dec 10;: 23215028
Correction to Surface Modification of Silicone for Biomedical Applications Requiring Long-Term Antibacterial, Antifouling, and Hemocompatible Properties.
Min Li, Koon Gee Neoh, Li Qun Xu, Rong Wang, En-Tang Kang, Titus Lau, Dariusz Piotr Olszyna, Edmund Chiong
Langmuir. 2012 Dec 7;: 23215012
Transparency sheets, which are normally associated with use on overhead projectors, offer lowered costs and high amenability for optical diagnostics in microplate instrumentation. An alternative microplate design in which circles are scribed on the surface of the transparency to create the boundaries to hold the drop in place is investigated here. The 3D profile of the scribed regions obtained optically showed strong likelihood of affecting three-phase contact line interactions. During dispensation, the contact angle (95(o)) was larger than the drop advancing state (80(o)) due to a period of non adhesion, where the contact angle later reduced to the drop advancing state followed by increase in the liquid area coverage on the substrate. It was established that 50μL was needed to fill the well fully and the maximum volume retainable before breaching was 190μL. While the tilt angle needed for displacement reduced significantly from 50μL to 95μL, this was markedly better than non-scribed surfaces, where tilt angles always had to be kept to within 30o. It was found that there was greater ability to fill the well with smaller volumes with dispensation at the center. This was attributed to the growing contact line not meeting the scribed edge in parallel if liquid was dispensed closer to it, wherein pinning reduction in some directions permitted liquid travel along the scribed edge to undergo contact angle hysteresis. Fluorescence measurements conducted showed no performance compromise when using scribed transparency microplates over standard microplates.
Langmuir. 2012 Dec 7;: 23214993
We have found for the first time the reentrant lamellar/onion (lamellar-onion-lamellar) transition with varying temperature under constant shear rate by using simultaneous measurements of shear stress and small-angle X-ray scattering (Rheo-SAXS) for a nonionic surfactant (C14E5)/water system which exhibits the lamellar phase in a wide temperature range from 15 - 75 °C. The onion state exists in a closed region in the temperature-concentration diagram at a constant shear rate. Temperature dependence of the lamellar repeat distance (d) at rest has also been measured at several concentrations. It is shown that the increase of d with increasing temperature is necessary for the existence of the lower transition. We have investigated the change in the lamellar orientation in the lamellar-to-onion and onion-to-lamellar transition processes near the upper and lower transition temperatures. For all the four kinds of transition processes, the following change in the lamellar orientation is observed; lamellar state (oriented to the velocity gradient direction) ↔ further enhancement of the orientation to the velocity gradient direction ↔ enhancement of the orientation to the neutral direction ↔ onion state.
Langmuir. 2012 Dec 10;: 23214978
Kinetic Process of Formation and Reconstruction of Small Unilamellar Vesicles Consisting of Long- and Short-Chain Lipids.
Neutron Science Laboratory, High Energy Accelerator Research Organization , Tokai, Ibaraki 319-1106, Japan.
Whereas the phase separation of normal phospholipids induces formation of microdomains on the surface of spherical vesicles, the separation of a long- and short-chain lipids can induce perforation of small unilamellar vesicles (SUVs) and transformation into bilayered micelles (bicelles) because the edges of the bilayers are stabilized by the short-chain lipid microdomain. In this study, the effect of the phase separation of lipids on the transformation behavior of SUVs consisting of a mixture of long- and short-chain lipids was investigated using small-angle neutron scattering. At the temperature jump from below to above the chain melting temperature of the long-chain lipid, T(c), bicelles fused together and transformed into SUVs when their size reached a critical radius. In contrast, a sequential transformation of small SUVs to small bicelles, small bicelles to large bicelles, and large bicelles to large SUVs occurred when the temperature jumped from a value far above T(c) to one slightly above T(c). To the best of my knowledge, this is the first report of such reconstruction of vesicles. By considering the bending energy of the membrane, the line tension of the rim, and the perforation due to the phase separation, the mechanism of the transformation processes was clarified.
Langmuir. 2012 Dec 7;: 23214974
A Strategy for Synthesizing Quantum Dot-Layered Double Hydroxide Nanocomposites and Their Enhanced Photoluminescence and Photostability.
Layered double hydroxide (LDH)-quantum dot (QD) composites are synthesized via a room temperature LDH formation reaction in the presence of QDs. InP/ZnS (core/shell) QD, a heavy metal free QD, is used as a model constituent. Interactions between QDs (with negative zeta potentials), decorated with dihydrolipoic acids, and inherently positively charged metal hydroxide layers of LDH during the LDH formations are induced to form the LDH-QD composites. The formation of the LDH-QD composites affords significantly enhanced photoluminescence quantum yields, thermal- and photo-stabilities compared to their QD counterparts. In addition, the fluorescence from the solid LDH-QD composite preserved the initial optical properties of the QD colloid solution without noticeable deteriorations such as red-shift or deep trap emission. Based on their advantageous optical properties, we also demonstrate the pseudo white light emitting diode, down-converted by the LDH-QD composites.
Langmuir. 2012 Dec 7;: 23214950
We characterize recently introduced novel nanobilayer technique (Nano Letters 2011, 11 (8) 3334-3340) and its practical aspects for incorporating the biological nanopore - α-haemolysin from Staphylococcus aureus and subsequent studies on translocation of biomolecules under various conditions. This technique provides advantages over classical bilayer methods, especially the quick formation, extended stability of a bilayer. We have also developed a methodology to prepare uniform quality of giant unilamellar vesicles (GUVs) in a reproducible way for producing nanobilayers. The process and the characteristics of the reconstitution of α-haemolysin in nanobilayers were examined by exploiting various important parameters including, pH, applied voltage, salt concentration and number of nanopores. Protonation of α-haemolysin residues in the low pH region affects the translocation durations, which, in turn changes the statistics of event types due to electrostatics and potentially the structural changes in DNA. By varying pH and applied voltage it was possible to investigate and partly control the capture rates and type of translocation events through α-haemolysin nanopores. This study could be helpful to utilize the nanobilayer technique for further explorations, particularly owing to its advantages and technical ease compared to existing bilayer methods.
Langmuir. 2012 Dec 7;: 23214946
Fabrication and Modification of Ordered Nanoporous Structures from Nanophase-Separated Block Copolymer/Metal Salt Hybrids.
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University , Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan.
We report facile preparation of nanoporous thin films by rinsing out a metal salt from nanophase-separated hybrid films composed of a block copolymer and a water-soluble metal salt. Nanophase-separated hybrids were prepared by mixing polystyrene-b-poly(4-vinylpyridine)(PS-P4VP) and iron(III) chloride in a solvent of pyridine, followed by solvent-casting and thermal-annealing. Film samples with a thickness of ca. 100 nm were fabricated from the nanophase-separated hybrids by using a microtoming technique. Metal salts in the films were removed by immersion into water to fabricate nanopores. Morphological observations were conducted by using transmission electron microscopy (TEM). Ordered cylindrical nanopores were clearly observed in the thin films prepared from the water-immersed hybrids which originally present cylindrical nanodomains. These nanoporous films were modified by loading another metal salt, samarium(III) nitrate, into the nanopores on the basis of the coordination ability of P4VP tethered to the pore walls. The samples after loading treatment were evaluated by TEM observation and elemental analysis with energy dispersive X-ray spectroscopy.
Langmuir. 2012 Dec 6;: 23214931
Corina Curschellas, Joachim Kohlbrecher, Thomas Michael Geue, Peter Fischer, Bertrand Schmitt, Martine Rouvet, Erich Windhab, Hans Joerg Limbach
The importance of surfactant self-assemblies in foam stabilization is well known. The aim of the current study was to investigate the self-assemblies of the non-ionic surfactant polyglycerol ester (PGE) in bulk solutions, at the interface and within foams, using a combined approach of small angle neutron scattering, neutron reflectivity and electron microscopy. PGE bulk solutions contain vesicles, as well as open lamellar structures. Upon heating of the solutions the lamellar spacing increases, with significant differences in the presence of NaCl or CaCl2 as compared to the standard solution. The adsorption of the multi-lamellar structures present in the bulk solutions lead to a multi-layered film at the air-water interface. The ordering within this film was increased as a result of a 20\% area compression mimicking a coalescence event. Finally, PGE foams were shown to be stabilized both by strong interfacial films, but also by agglomerated self-assemblies within the interstitial areas of the foams.