Multicomponent membranes on solid substrates: Interfaces for protein binding

Even though lipid membranes deposited on a solid support are now used for more than 20 years as a model system for cellular membranes, their potential has, as yet, not been fully exploited. Only in recent years, the composition of solid supported membranes became more and more complex, which is a prerequisite for the elucidation of biologically relevant protein adsorption processes. Multicomponent bilayers resemble the heterogeneity of the biological membrane, which is composed of hundreds of different lipids varying in their headgroup and acyl chain composition. The development of a multitude of elaborated surface sensitive techniques allows to study the phase behavior of these membranes and their interaction with proteins, some of which will be highlighted in this review.     

Formation of cellulose acetate membranes using a supercritical fluid assisted process

Microporous cellulose acetate membranes have been prepared from polymer–acetone solutions using a supercritical fluid phase inversion process in which CO₂ acts as the non-solvent. Series of experiments were performed at various polymer concentrations, temperatures and pressures. The structure of the resulting membranes was analysed using scanning electron microscopy. We operated with polymer concentrations ranging between 5 and 40% (w/w) in acetone obtaining different pore dimensions and membrane structures. Increasing the percentage of polymer in the solution, the structure of the membranes changed from beads-like structure to cellular structure. Polymer concentration also influenced the mean diameter of the pores that ranged from 2 to 50 μm for polymer concentrations from 40 to 5% (w/w). We also tested membrane formation pressures between 100 and 200 bar and at temperature between 45 and 65 °C. Pressure influences the change in membrane structure from cellular to beads-like, whereas temperature has a minor influence on pore size: both the effects can be partially related to CO₂ density. Cellulose acetate membrane formation mechanisms have also been discussed.     

Use of atomic force microscopy and fractal geometry to characterize the roughness of nano-, micro-, and ultrafiltration membranes

Membrane surface roughness alters the surface area accessible to foulants and may influence macroscopic properties, such as zeta potential. It is usually quantified by atomic force microscopy (AFM) at a single scan size. This would be appropriate if roughness is independent of scale. This study shows that the root-mean-square roughness, R_RMS, is scale (or scan size, L × L) dependent through the power law R_RMS = AL^3−D. The coefficient, A, is the roughness at a scan size of 1² μm². D is the fractal dimension that relates the increase in roughness to the increase in scan size. Values for A and D were determined for a range of micro- and ultrafiltration membranes using an AFM scan series covering at least three orders of magnitude in L. They were also determined for nanofiltration membranes by re-analysis of data in the literature. The results suggest that using the power law expression allows potentially greater discrimination among membrane types and provides a way to quantify membrane roughness over a range of scales. It was further observed that the coefficients A and D of PVDF membranes showed positive and negative correlations, respectively, with the molecular weight cut-off. Additionally, zeta potentials of PVDF membranes measured by the tangential streaming potential method became more negative with increasing A and more positive with increasing D, suggesting possible significant influence of roughness on hydrodynamic transport of ions.     

DOPC,DOPE和神经酰胺对鞘磷脂/胆固醇双层膜结构的影响

用LB技术和原子力显微镜(AFM)研究了1,2-二油酸甘油-3-磷脂酰胆碱(DOPC)、1,2-二油酸甘油-3-磷脂酰乙醇胺(DOPE)和神经酰胺(Ceramide)对鞘磷脂(SM)/胆固醇(Chol)结构的影响. 实验结果表明, 在表面压力较低时, 每种混合脂双层膜都呈现均匀分布的脂双层结构. 随着表面压力的增加, 形态发生了明显的变化: (1) SM/Chol二元组分双层膜形成均一的液态有序相微区结构, 衬底覆盖率达到80%; (2) DOPC的加入促使SM/Chol双层膜出现相分离现象, SM/Chol形成的液态有序相 “岛状” 微区结构漂浮在液态无序相的DOPC上部, 约占总面积的30%; (3) DOPE与SM/Chol形成的双层膜明显不同于DOPC/SM/Chol, 呈现出液态无序相、液态有序相及凝胶相3相共存的结构; (4) Ceramide诱导了SM/Chol双层膜结构发生重排, 两层脂分子间发生翻转形成囊泡结构, 部分神经酰胺从液态有序相中分离形成小颗粒结构. 在较高膜压下, 各系统都呈现出具有特定形态的双层膜结构. 分子官能团的成键能力决定了双层膜形态结构.     

Microphase separation at the surface of block copolymers, as studied with atomic force microscopy

Atomic force microscopy (AFM) is used to study the phase separation process occurring in block copolymers in the solid state. The simultaneous measurement of the amplitude and the phase of the oscillating cantilever in the tapping mode operation provides the surface topography along with the cartography of the microdomains of different mechanical properties. This technique thus allows to characterize the size and shape of those microdomains and their organization at the surface (e.g. cubic lattice spheres, hexagonal lattice of cylinders, or lamellae). In this study, a series of symmetric triblock copolymers made of a inner elastomeric sequence (poly(butadiene) or poly(alkylacrylate)) and two outer thermoplastic sequences (poly(methylmethacrylate)) is analyzed by AFM in the tapping mode. The microphase separation and their morphology are essential factors for the potential of these materials as a new class of thermoplastic elastomers. Special attention is paid to the control of the surface morphology, as observed by AFM, by the molecular structure of the copolymers (volume ratio of the sequences, molecular weight, length of the alkyl side group) and the experimental conditions used for the sample preparation. The molecular structure of the chains is completely controlled by the synthesis, which relies on the sequential living anionic polymerization of the comonomers. The copolymers are analyzed as solvent-cast films, whose characteristics depend on the solvent used and the annealing conditions. The surface arrangement of the phase-separated elastomeric and thermoplastic microdomains observed on the AFM phase images is discussed on the basis of quantitative information provided by the statistical analysis by Fourier transform and grain size distribution calculations.     

Micro-contact printing of poly(3-hexylthiophene) on silicon oxide: Effect of stamp stretching

Micro-contact printing (μCP) has been applied to deposit poly(3-hexylthiophene) (P3HT) layer onto silicon oxide substrate from a polydimethylsiloxane (PDMS) stamp. The effect of the stamp stretching onto the corresponding printed P3HT layer has been analysed both from morphological (AFM) and from structural (XRD) points of view. Results show an orienting effect of the stretching towards the P3HT ultimate morphological units (i.e. fibrils) along the stretching direction. The influence of other variables involved in the deposition (substrate polarity/apolarity, mechanical pressure onto the stamp) have been studied and reported.     

The effect of electrochemically treated glassy carbon on the activity of supported Pt catalyst in methanol oxidation

Effect of electrochemical oxidation of glassy carbon on deposition of platinum particles and electrocatalytic activity of platinum supported on oxidized glassy carbon (Pt/GC_OX) were studied for methanol oxidation in H₂SO₄ solution. Platinum was potentiostatically deposited from H₂SO₄ + H₂PtCl₆ solution. Glassy carbon was anodically polarised in 0.5 M H₂SO₄ at 2.25 V vs. saturated calomel electrode (SCE) during 35 s. Electrochemical treatment of GC support, affecting not significantly the real Pt surface area, leads to a better distribution of platinum on the substrate and has remarkable effect on the activity. The activity of the Pt/GC_OX electrode for methanol oxidation is larger than polycrystalline Pt and for more than one order of magnitude larger than Pt/GC electrode. This increase in activity indicates the pronounced role of organic residues of GC support on the properties of Pt particles deposited on glassy carbon.     

Atomic force microscope studies of membranes: Surface pore structures of Cyclopore and Anopore membranes

Non-contact atomic force microscopy has been used to investigate the surface pore structure of Cyclopore and Anopore microfiltration membranes in air. Three Cyclopore membranes and three Anopore membranes of different pore sizes were studied. Excellent high resolution images were obtained. Analysis of the images gave quantitative information on the surface pore structure, in particular the pore size distribution. Non-contact AFM is an excellent means of obtaining such information for microfiltration membranes.     

聚苯乙烯/聚二甲基硅氧烷嵌段共聚物的表面形态研究

采用原子力显微镜(AFM)和透射电镜(TEM)研究了聚苯乙烯/聚二甲基硅氧烷嵌段共聚物(PS-b-PDMS)薄膜的相形态.结果表明,当采用甲苯作为溶剂,旋转涂膜的薄膜样品呈现网络状的形态分布在表面,而样品所对应的透射电镜照片中,PDMS相作为球状分布在PS的连续相中.退火温度对共聚物表面形态有一定的影响,当退火温度高于PDMS的玻璃化温度,表面中PDMS相增多.PS-b-PDMS嵌段共聚物的表面形态随着所用溶剂的变化而有所不同,当采用甲苯作为溶剂时,样品的PS相形成凹坑分布在PDMS的相区之中,而采用环己烷作为溶剂时,PS相作为突起分布在PDMS相区之中.另外,基底对共聚物薄膜表面形态的有较大的影响,当采用硅晶片作为基底时,样品中的PDMS相和PS相呈现近似平行于表面的层状结构.     

Imaging of reconstituted purple membranes by atomic force microscopy

The organization of bacteriorhodopsin (bR) within reconstituted purple membranes (RPM) was examined using atomic force microscopy (AFM). Five reconstituted species were examined: RPM 3 (bR/native polar lipids/dimyristoylphosphatidylcholine (DMPC) in a 1:9:14 molar ratio), RPM 4 (bR/native polar lipids in a 1:7 molar ratio), RPM 5 (bR/native polar lipids/1,2-di-O-phytanyl-sn-glycerol in a 1:3.5:6.1 molar ratio), RPM 6 (bR/native polar lipids/1,2-di-O-phytanyl-sn-glycero-3-phosphocholine in a 1:3.5:4.9 molar ratio), and RPM 7 (bR/native polar lipids/1,2-diphytanoyl-sn-glycero-3-[phospho-l-serine] in a 1:3.5:4.6 molar ratio). RPM 3 patches adsorbed onto mica exhibit domains of crystallized bR trimers arranged in a hexagonal packing structure, similar to those found in native purple membrane (NPM). These domains are enclosed by DMPC-rich regions. RPM 4 patches were observed to have larger domains of crystallized bR, with trimer orientation 30° different from that found in NPM. The bR-rich domains are enclosed by a large, protein-free, lipid-rich region. The topography of RPM 5 was difficult to resolve as the surface had no discernable patterns or structure. The topographies of RPM 6 and 7 were similar to that found in RPM 3 in that higher domains were formed within the patch adsorbed onto mica. They may contain protein-rich regions, but clear images of protein arrangement could not be obtained using AFM. This may be a result of imaging limitations or of the lack of organization of bR within these domains.     

血卟啉在胶束介质中的聚集状态研究

用光谱法研究了光敏剂血卟啉在水溶液中的聚集状态，并以胶束作为生物膜模型，探讨了血卟啉在胶束介质中的聚集状态状态变化，用原子力显微镜研究了血卟啉聚集状态的微观结构及其在胶束介质中的聚集状态。     

溶剂对相分离法制备聚偏氟乙烯微孔膜性质的影响

研究了用相转换法制备聚偏氟乙烯（PVDF）微孔膜时溶剂对成膜性质的影响.用浊点法测定了二甲基亚砜、N,N-二甲基甲酰胺、N,N-二甲基乙酰胺、N-甲基吡咯烷酮、磷酸三甲酯等五种溶剂配制的质量分数为wPVDF=0.12的铸膜液在30℃时的相分离点,显微镜拍照法测定了这些铸膜液与水接触时相分离前沿推进速率,泡点法测定了膜孔径,并测定了气体通量.结果表明,二甲基亚砜、磷酸三甲酯、N,N-二甲基乙酰胺是适于制作聚偏氟乙烯微孔膜的溶剂.     

Plasticized polymer electrolyte membranes based on PEO/PVdF-HFP for use as an effective electrolyte in lithium-ion batteries

Plasticized polymer electrolytes were prepared using poly(ethylene oxide)(PEO)/poly(vinylidene fluoridehexafluoro propylene)(PVd F-HFP) with lithium perchlorate(Li Cl O4) and different plasticizers. XRD and FTIR spectroscopic techniques were used to characterize the structure and the complexation of plasticizer with the host polymer matrix. The role of interaction between polymer hosts and plasticizer on conductivity is discussed using the results of alternating current(a.c.) impedance studies. TG-DTA and SEM were used for thermal and physical characterizations. Maximum ionic conductivity(3.26 × 10~(-4) S·cm~(-1)) has been observed for ethylene carbonate(EC)-based polymer electrolytes. Electrochemical performance of the plasticized polymer electrolyte is evaluated in LiFePO_4/plasticized polymer electrolytes(PPEs)/Li coin cell. Good performance with low capacity fading on charge discharge cycling is demonstrated.     

About the formation and characterisation of a new phase of polycationic monolayers at the air/water interface

The interfacial behaviour of highly charged cationic polyelectrolytes (PEs) (by alkylation of poly-4-vinylpyridine with C₈-, C₁₂- or C₁₆-alkyl halogenides) was investigated at the air/water interface. Great care must be taken with respect to the experimental technique for recording the isotherms. Only a stepwise procedure, applying small pressure increments and checking and recording potential area relaxations, reveals the intrinsic monolayer characteristics. After a true equilibrium range in the isotherm, starting from the onset of first film pressure to the almost closed packed layer, it could be demonstrated that at certain states of compression, a spontaneous kinetically controlled area relaxation is induced, which coincides with a mono- to triple-layer state transition. This region is visible as large shifts in area if the film is kept at constant pressure. The presence and coexistence of well-defined triple-layer states could be proved by Langmuir–Blodgett (LB) transfer and ellipsometric thickness measurements as well as by atomic force microscopy (AFM). Typical features of all area relaxations are induction periods and sigmoidal courses of the plots. According to these relaxations and the characterisation by AFM, the new phase formation is characterised by progressive nucleation and one-dimensional growth. Based on relaxation measurements, a saturation pressure can be derived, constituting the reference for defining supersaturation for nucleation and growth. All experimental findings together can be understood, if one assumes a stretched, stiff and rod-like structure as the most plausible and simple model for the PE in the presence of hard counterions. For the case of soft and polarisable counterions, for electrostatic reasons, the polymer behaviour changes to a worm-like structure. The rate of nucleation of the well-defined phase formation became highly irreproducible in this case.     

Probing supported model and native membranes using AFM

Atomic Force Microscopy (AFM) has gained lots of interest since its ability to get high resolution imaging in liquid environment. In the last years, this technique was particularly successful in probing the surface of membrane model systems of biological interest and spectacular results have been obtained with native specialized membranes. In this review, we aim at highlighting the recent developments that illustrate the unique powerfulness of AFM in determining the nanoscale organization of membranes and their local physical properties. An important part will focus on AFM high resolution imaging of transmembrane proteins in model and native membranes and on the study of few applications such as biosensors. An overview of main recent developments of AFM as well as new possibilities gained by combination with other techniques will also be addressed.     

Formation of microporous Teflon PFA membranes via thermally induced phase separation

Poly(tetrafluoroethylene-co-perfluoro-(propyl vinyl ether)) (Teflon^® PFA) membranes of a variety of structures have been produced through thermally induced phase separation of Teflon^® PFA-chlorotrifluoroethylene melt-blends of different compositions. A phase diagram of the two component system was constructed, and electron microscopy was used to characterize the structures of membranes produced. The morphological characteristics of the Teflon^® PFA membranes have been explained on the basis of equilibrium driving forces for liquid-liquid and solid-liquid phase separations.     

The influence of micro- and nanoparticles on model atomically flat surfaces on crystallization of polyethylene

Crystallization of high density polyethylene (PE) from the melt on model atomically flat solid surfaces decorated with micro- and nanoparticles of gold or NaCl of different size and densities is investigated. The morphology of the contact layer of PE after its detachment from the support is studied using atomic force microscopy (AFM). It is shown that the nucleating and ordering effect of the solid on PE crystallization depends to a large extend on the nanostructure of its surface, in particular on the size of the atomically flat domains and on the presence of nanoscopic obstacles. The minimum size of the flat domain which can significantly influence the PE crystallization is estimated to be of the order of 150 nm.     

Fabrication and evaluation of a polymer Li-ion battery with microporous gel electrolyte

This paper introduces an easy method for the fabrication of polymer Li-ion batteries with microporous gel electrolyte (MGE). The MGE is a multiphase electrolyte, which is composed of liquid electrolyte, gel electrolyte, and polymer matrix. The MGE not only has high ionic conductivity and good adhesion to the electrodes at low temperatures, but also retains good mechanical strength at elevated temperatures. Therefore, the MGE batteries are able to operate over a wide temperature range. During battery fabrication, the MGE is formed in situ by introducing liquid electrolyte into a swellable microporous polymer membrane and then heating or cycling the battery. In this work, the chemical compatibility of MGE with metal lithium during 60 °C storage and with LiMn₂O₄ cathode during cycling was studied. In addition, graphite/MGE/LiMn₂O₄ Li-ion batteries were made and evaluated.     

Label-free Microcantilever Immunosensor Based on a Competitive Immunoassay for the Determination of Clenbuterol

A label-free microcantilever immunosensor based on a competitive immunoassay is reported for the determination of clenbuterol. The immunosensor was fabricated by modifying clenbuterol–ovalbumin on the gold surface of the microcantilever with crossing linkage by L-cysteine and glutaraldehyde. Atomic force microscopy was utilized to characterize the construction of immunosensor and to measure the deflection of the microcantilever. The deflection response of the microcantilever was in negatively proportional to the concentration of clenbuterol from 1.0?×?10^?2 to 20?µg/L with a limit of detection of 1.0?×?10^?2?µg/L. The fabricated immunosensor was used to determine clenbuterol in pork samples with satisfactory results. In addition, the results were in accordance with those obtained by high-performance liquid chromatography. The reported immunosensor displayed high sensitivity and specificity together with excellent repeatability and reliability.     

有机HTDIOO分子LB膜结构的AFM研究

利用原子力显微镜（AFM）对有机分子HTDIOO单层和多层LB膜结构进行了观察·实验结果表明，针尖与LB膜表面分子间的相互作用力会对成像的膜结构有影响．当悬臂针尖与LB膜表面分子的相互作用力较大时，针尖会扰动HTDIOO分子在单层LB膜中的有序排列．HTDIOO单层LB膜具有有序结构；而在多层LB膜中，HTDIOO分子则聚集在一起形成了一定的畴结构．