Probing small unilamellar EggPC vesicles on mica surface by atomic force microscopy

Sonicated small unilamellar egg yolk phosphatidylcholine (EggPC) vesicles were investigated using atomic force microscopy (AFM) imaging and force measurements. Three different topographies (convex, planar, and concave shape) of the EggPC vesicles on the mica surface were observed by tapping mode in fluid, respectively. It was found that the topography change of the vesicles could be attributed to the interaction force between the AFM tip and vesicles. Force curves between an AFM tip and an unruptured vesicle were obtained in contact mode. During approach, two breaks corresponding to the abrupt penetration of upper and lower bilayer of vesicle were exhibited in the force curve. Both breaks spanned a distance of around 4 nm close to the EggPC bilayer thickness. Based on Hertz analysis of AFM approach force curves, the Young's modulus (E) and the bending modulus (kc) for pure EggPC vesicles were measured to be (1.97 +/- 0.75) x 10(6)Pa and (0.21 +/- 0.08) x 10(-19)J, respectively. The results show that the AFM can be used to obtain good images of intact and deformed vesicles by tapping mode, as well as to probe the integrity and bilayer structure of the vesicles. AFM force curve compare favorably with other methods to measure mechanical properties of soft samples with higher spatial resolution.     

Interaction between dendrons directly studied by single-molecule force spectroscopy

In this article, we have investigated the interaction between two poly(benzyl ether) dendrons directly by single-molecule force spectroscopy. For this purpose, one dendron was immobilized on an AFM tip through a poly(ethylene glycol) (PEG) spacer, and the other dendron was anchored on a gold substrate as a self-assembled monolayer. Two dendrons approached and then interacted with each other when the AFM tip and the substrate moved close together. The rupture force between dendrons was measured while the AFM tip and the substrate separated. PEG as a flexible spacer can function as a length window for recognizing the force signals and avoiding the disturbance of the interaction between the AFM tip and the substrate. The interaction between two first-generation dendrons is measured to be about 224 pN at a force loading rate of 40 nN/s. The interaction between second- and first-generation dendrons rises to 315 pN at the same loading rate. Such interactions depend on the force loading rate in the range of several to hundreds of nanonewtons per second, indicating that the rupture between dendrons is a dynamic process. The study of the interaction between surface-bound dendrons of different generations provides a model system for understanding the surface adhesion of molecules with multiple branches. In addition, this multiple-branch molecule may be used to mimic the sticky feet of geckos as a man-made adhesive.     

Formation and finite element analysis of tethered bilayer lipid structures

Rapid solvent exchange of an ethanolic solution of diphytanoyl phosphatidylcholine (DPhyPC) in the presence of a mixed self-assembled monolayer (SAM) [thiolipid/β-mercaptoethanol (βME) (3/7 mol/mol) on Au] shows a transition from densely packed tethered bilayer lipid membranes [(dp)tBLMs], to loosely packed tethered bilayer lipid membranes [(lp)tBLMs], and tethered bilayer liposome nanoparticles (tBLNs) with decreasing DPhyPC concentration. The tethered lipidic constructs in the aqueous medium were analyzed by atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS). Finite element analysis (FEA) was applied to interpret spectral EIS features without referring to equivalent circuit modeling. Using structural data obtained earlier from neutron reflectometry and dielectric constants of lipid bilayers, we reproduced experimentally observed features of the electrochemical impedance (EI) spectra of complex surface constructs involving small pinhole defects, large membrane-free patches, and bound liposomes. We demonstrated by FEA that highly insulating (dp)tBLMs with low-defect density exhibit EI spectra in the shape of a perfect semicircle with or without low-frequency upward "tails" in the Cole-Cole representation. Such EI spectra were observed at DPhyPC concentrations of >5 × 10(-3) mol L(-1). While AFM was not able to visualize very small lateral defects in such films, EI spectra unambiguously signaled their presence by increased low frequency "tails". Using FEA we demonstrate that films with large diameter visible defects (>25 nm by AFM) produce EI spectral features consisting of two semicircles of comparable size. Such films were typically obtained at DPhyPC concentrations of <5 × 10(-3) mol L(-1). At DPhyPC concentrations of <1.0 × 10(-3) mol L(-1) the planar bilayer structures were replaced by ellipsoidal liposomes with diameters ranging from 50 to 500 nm as observed in AFM images. Despite the distinct surface morphology change, the EI curves exhibited two semicircle spectral features typical for the large size defects in planar tBLMs. FEA revealed that, to account for these EI features for bound liposome systems (50-500 nm diameter), one needs to assume much lower tBLM conductivities of the submembrane space, which separates the electrode surface and the phospholipid bilayer. Alternatively, FEA indicates that such features may also be observed on composite surfaces containing both bound liposomes and patches of planar bilayers. Triple semicircular features, observed in some of the experimental EI curves, were attributed to an increased complexity of the real tBLMs. The modeling demonstrated that such features are typical for heterogeneous tBLM surfaces containing large patches of different defectiveness levels. By integrating AFM, EIS, and FEA data, our work provides diagnostic criteria allowing the precise characterization of the properties and the morphology of surface supported bilayer systems.     

Atomic force microscopy of soil and stream fulvic acids

Atomic force microscopy (AFM) was used to image fulvic acid (FA) deposited from aqueous solution on to the basal-plane surfaces of freshly cleaved muscovite, and allowed to air dry. Two fulvic acid samples were used: a soil fulvic acid (SFA) prepared by NaOH extraction from a muck soil underlying a freshwater fen in the New Jersey Pinelands and the IHSS standard Suwannee River fulvic acid (SRFA). The use of tapping-mode AFM (TMAFM), a relatively new technique which reduces the lateral frictional forces generally associated with contact-mode AFM, allowed excellent images of delicate FA structures to be obtained with minimal sample disturbance. Four main structures were observed on SFA. At low concentrations, sponge-like structures consisting of rings ( 15 nm in diameter) appeared, along with small spheres (10–50 nm). At higher concentrations, aggregates of spheres formed branches and chain-like assemblies. At very high surface coverage, perforated sheets were observed. On some samples, all of these structures were apparent, perhaps owing to concentration gradients on drying. SRFA samples were only imagined at higher concentrations. Spheres, aggregated branches, and perforated sheets were apparent. The results agree with previous work by Stevenson and Schnitzer [Soil Sci., 133(1992) 179], who applied TEM to soil FAs freeze-dried on muscovite. However, the TEM images did not detect the smaller spheres and sponge-like structures observed by AFM at low concentrations. The relevance of imaging dried samples remains questionable; hence, it is hoped in the future to use new in situ TMAFM to image FAs sorbed to surfaces in solution. Although TMAFM provided excellent images, a variety of artifacts and potential problems were encountered, as discussed.     

ArF EXCIMER LASER-INDUCED BUBBLE FORMATION DURING IRRADIATION OF NaCl SOLUTIONS

Abstract During application of the 193 nm excimer laser to vitreoretinal surgery, very deep cutting of the retina of about 100 μm/pulse was detected at the energy fluence in the range of 0.25–0.35 J/cm²/pulse. At the surface of the ablating tip insoluble bubbles were observed during the irradiation. In this paper we study production of these bubbles in aqueous solutions of sodium chloride. The yield of gaseous photoproducts was measured as a function of NaCl concentration and energy fluence. At concentrations of 100 g/L powerful water vapor bubbles and shock waves were observed. A mechanism of soft tissue cutting by the 193 nm laser in highly absorbing liquid media is suggested that explains the features of vitreoretinal ablation with this system: (1) the high cutting depth when the tip touches the tissue and (2) the absence of cutting when the tip is not in contact with the tissue. The advantages of the ArF laser for microsurgery of internal organs are discussed.     

Characterization of the surface structural,mechanical, and thermal properties of benzocyclobutene dielectric polymers using scanned probe microscopy

Scanning probe microscopy (SPM) techniques are used to characterize surfaces related to the processing of benzocyclobutene (BCB) dielectric thin films. Thermally cured resins and photodefineable resins are sold under the trade name CYLCOTENE^TM1) for electronic applications. TappingMode AFM (TMAFM) imaging is used to follow changes in adhesion promoter morphology upon baking to help explain adhesion performance. Power spectral density (PSD) analysis of TMAFM images of plasma treated BCB surfaces are unique and can be used to ‘fingerprint’ processes. Selective oxidation of the BCB surface can be used to fabricate a phase imaging standard for TMAFM. Near surface modulus of the BCB materials is 3.6 ± 0.2 GPa and the hardness is 0.38 ± 0.2 GPa measured by depth‐sensing nanoindentation. Creep recovery of indents can be used to qualitatively distinguish between thermal and photocureable materials. A heated tip in a scanning thermal microscope can induce the thermal curing of BCB over micron sized areas. Local thermal analysis with the same probe allows the measurement of the changes in the glass transition, T_g, at the surface with cure temperature.     

Nano-indentation of a room-temperature ionic liquid film on silica: a computational experiment

We investigate the structure of the [bmim][Tf(2)N]/silica interface by simulating the indentation of a thin (4 nm) [bmim][Tf(2)N] film by a hard nanometric tip. The ionic liquid/silica interface is represented in atomistic detail, while the tip is modelled by a spherical mesoscopic particle interacting via an effective short-range potential. Plots of the normal force (F(z)) on the tip as a function of its distance from the silica surface highlight the effect of weak layering in the ionic liquid structure, as well as the progressive loss of fluidity in approaching the silica surface. The simulation results for F(z) are in near-quantitative agreement with new AFM data measured on the same [bmim][Tf(2)N]/silica interface under comparable thermodynamic conditions.     

Microphase Separation of Bulk and Ultrathin Films of Polyurethane Elastomers

Surmmary: Polyurethane elastomers (PUEs) were synthesized with poly(oxytetramethylene) glycol (PTMG), 4,4′-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BD)/1,1,1-trimethylol propane (TMP) by a prepolymer method. The degree of microphase separation of bulk and ultrathin films for these PUEs was confirmed by Fourier transform infrared (FT-IR) spectroscopy, differential scanning calorimetry (DSC) and atomic force microscopy (AFM). In the bulk films, FT-IR and DSC measurements revealed that the degree of micro-phase separation strengthened with increasing BD content. AFM observation of the BD-PUE showed hard segment domains surrounded by a soft segment matrix. The domains ranged in size from 10-20 nm, while BD/TMP- and TMP PUEs did not have clear domains. On the other hand, AFM observation was carried out on thin films (200 mm in thickness) and ultrathin films (approximately 8-5 nm) prepared by spin-coating the different concentrations of PUE solutions. The microphase separated strucuture under 10 nm in thickness showed marked decreases in the size of the microphase-separated domain.     

Electrochemically controlled formation and growth of hydrogen nanobubbles

Electrogenerated microscale bubbles that are confined at the electrode surface have already been extensively studied because of their significant influence on electrochemistry. In contrast, as far as we know, whether nanoscale bubbles exist on the electrode surface has not been experimentally confirmed yet. Here, we report the observation of electrochemically controlled formation and growth of hydrogen nanobubbles on bare highly oriented pyrolytic graphite (HOPG) surface via in-situ tapping mode atomic force microscopy (TMAFM). By using TMAFM imaging, we observed that electrochemically generated hydrogen gas led to the formation of nanobubbles at the HOPG surface. We then employed a combination of techniques, including phase imaging, ex-situ degassing, and tip perturbation, to confirm the gas origin of such observed nanobubbles. We further demonstrated that the formation and growth of nanobubbles could be well controlled by tuning either the applied voltage or the reaction time. Remarkably, we could also monitor the evolution process of nanobubbles, that is, formation, growth, coalescence, as well as the eventual release of merged microbubbles from the HOPG surface.     

脂双层膜表面结构与稳定性的原子力显微镜研究

用原子力显微镜研究了1,2－二油酸甘油－3－磷酸－1甘油（DOPG）脂双层膜 的表面结构与稳定性。实验结果表明,原子力显微镜的探针与脂双层膜的相互作用 导致脂双层膜表面产生一个永久的损伤。静电相互作用对脂双层膜结构和稳定性的 影响表明,在NaCl溶液中制成的脂质体,随着NaCl浓度的增加,它们的双层膜更稳 定。在低的NaCl浓度则经常被损伤,在1 mol/L NaCl溶液中制备的指双层变得更稳 定。在KCl溶液中结果恰好相反。在高的KCl浓度中经常被损伤,随着KCl浓度的降 低,它们的双层膜更稳定。葡萄糖和蔗糖对脂双层膜结构有稳定作用。     

Sub-100 nm patterning of supported bilayers by nanoshaving lithography

Sub-100 nm wide supported phospholipid bilayers (SLBs) were patterned on a planar borosilicate substrate by AFM-based nanoshaving lithography. First, a bovine serum albumin monolayer was coated on the glass and then selectively removed in long strips by an AFM tip. The width of vacant strips could be controlled down to 15 nm. Bilayer lines could be formed within the vacant strips by vesicle fusion. It was found that stable bilayers formed by this method had a lower size limit of approximately 55 nm in width. This size limit stems from a balance between a favorable bilayer adhesion energy and an unfavorable bilayer edge energy.     

Formation and stability of a suspended biomimetic lipid bilayer on silicon submicrometer-sized pores

We report the fabrication of a thin silicon membrane with an array of micrometer and submicrometer pores that acts as a scaffold for suspending a lipid bilayer. We successfully deposited a lipid bilayer by the Langmuir-Blodgett method on a synthetic silicon membrane bearing arrays of pores with sizes of 1000, 650, and 300 nm. Topographic images obtained by AFM showed a suspended lipid film spanning the pores, whatever the pore size. Higher stability of bilayers supported on smaller pores was shown by AFM characterization. These results represent an important first step to creating a biomimetic environment to study cell membrane dynamics and/or in developing a biosensor.     

Using wavelets to analyze AFM images of thin films: surface micelles and supported lipid bilayers

This paper presents micro- and nanoanalysis of thin films based on images obtained by atomic force microscopy (AFM). The analysis exploits the discrete wavelet transform and the resulting wavelet spectrum to study surface features. It is demonstrated that the wavelet technique can characterize micro- and nanosurface features and distinguish between similar surface structures. The use of a feature extraction method is shown. The method involves the separation of certain frequency content from the original AFM images and analyzing the data independently to gain quantitative information about the images. By using the feature extraction method, soft surfaces in water are analyzed and nanofeatures are measured. The packing of surface micelles of sodium dodecyl sulfate on a self-assembled monolayer is analyzed. The characteristics of pore formation, due to penetration of the antibacterial peptide protegrin, into a solid-supported lipid bilayer are quantified. The sizes of the pores are obtained, and it is observed that the line tension of the pores reduces the fluctuations of the lipid bilayer.     

PMMA/PS共混物薄膜中Voronoi结构的形成

Vomnoi结构与一些自然结构十分相象,早在17世纪,Descartes就在他的著作中采用了类似Vomnoi结构的图形来表示太阳系及其环境中的物质分布．Vomnoi图（如图1所示）的概念来自于计算几何,设有平面空间点集合A={a1,a2,……an},其中ai=（xi,yi）,对于任意一个ai,满足｜P-ai｜≤min ｜P-aj｜（j=1,2,……,n;j≠i）的点P=（x,y）的轨迹称为点集A的vomnoi图．     

Quantitative determination of surface energy using atomic force microscopy: the case of hydrophobic/hydrophobic contact and hydrophilic/hydrophilic contact

Atomic force microscopy (AFM) has been used to determine the surface energy of chemically modified surfaces at a local scale. In order to achieve this aim, it was necessary to graft both the AFM tip and the substrate with the same chemical functional groups. Two different organothiols terminated either by hydrophilic or hydrophobic chemical functionalities were used. Grafting process classically reported shows that after UV/ozone treatment for 30 min, the tip is coated by thermal deposition with 4‐5‐nm‐thick titanium layer followed by a 30‐nm‐thick gold layer. Finally, the tip is grafted by organothiols. The thickness of the layer deposited on the tip is of the same order of magnitude as the tip radius. To avoid the use of Ti and to decrease the thickness of the gold layer, we have developed a new way of grafting by using organic molecules like (3‐mercaptopropyl)triethoxysilane (MPS) as a linkage agent. Then this way of grafting was checked. Finally, AFM force‐distance curves, between grafted tips and chemically modified surface, were carried out in contact mode. Calibration of the various parts of the apparatus and especially of the cantilever (spring constant and tip radius) is of major importance to reach quantitative data. Finally, by applying a suitable theory of contact, we were able to determine the surface energy of our system. Copyright © 2005 John Wiley & Sons, Ltd.     

Flat-on lamellae in spin-coated, stable films of poly(propylene azelate)

Thin films (5-500 nm thick) of the linear aliphatic polyester (3,7) poly(propylene azelate) (PPAz) were prepared by spin-coating of CHCl(3) polymer solutions with different polymer concentrations. The morphology and structure of the spin-coated thin films were investigated by atomic force microscopy (AFM) and by grazing incidence wide-angle X-ray scattering (GIWAXS) techniques. AFM revealed the continuous nature of the flat, spherulitic films which are stable against dewetting even for polymer coatings as thin as 15 nm. GIWAXS patterns revealed a high crystal orientation of the films. A sharp reflection on the meridian whose spacing is related to the polymer chain unit length (c-axis) supports the presence of flat-on lamellae morphology in the whole range of film thicknesses investigated. The flat-on lamellae morphology is also supported by AFM images. A triclinic unit cell with the c*-axis perpendicular to the substrate is proposed for PPAz. The repulsion of the long aliphatic spacer by the Si-substrate is invoked as the main reason for the flat-on morphology observed.     

单晶硅片负极界面形貌的原位AFM探测

利用原子力显微镜原位研究单晶硅片负极在首次充放电循环中的界面形貌变化。硅负极表面固体电解质界面(SEI)膜的形成过程为：初始SEI膜从1.5 V开始形成,在1.25–1.0 V之间生长快速,0.6 V左右生长缓慢。初始SEI膜具有层状结构的特征,表层薄膜较软,下层呈颗粒状,机械稳定性较好。在锂化电位下,硅负极表面的单晶结构逐渐变得颗粒化,发生不可逆的结构变化。经过首个充放电循环后,硅负极表面被厚度不均一的SEI膜所覆盖,SEI膜的厚度大约为10–40 nm。     

Supported bilayer lipid membrane arrays on photopatterned self-assembled monolayers

This work demonstrates the use of photocleavable cholesterol derivatives to create supported bilayer lipid membrane arrays on silica. The photocleavable cholesteryl tether is attached to the surface by using the reaction of an amine-functionalized self-assembled monolayer (SAM) and the N-hydroxysuccinimide-based reagent 9. The resultant SAM contains an ortho-nitrobenzyl residue that can be cleaved by photolysis by using soft (365 nm) UV light regenerating the original amine surface, and which can be patterned using a mask. The photoreaction yield was approximately 75 % which was significantly higher than previously found for related ortho-nitrobenzyl photochemistry on gold substrates. The SAMs were characterized by means of contact angle measurements, ellipsometry and X-ray photoelectron spectroscopy. Patterned surfaces were characterized with SEM and AFM. After immersing the patterned surface into a solution containing small unilamellar vesicles of egg phosphatidylcholine (PC), supported lipid membranes were formed comprised of lipid bilayer over the amine functionalized "hydrophilic" regions and lipid monolayer over the cholesteryl "hydrophobic" regions. This was confirmed by fluorescence microscopy and AFM. FRAP studies yielded a lateral diffusion coefficient for the probe molecule of 0.14+/-0.05 microm(2) s(-1) in the bilayer regions and approximately 0.01 microm(2) s(-1) in the monolayer regions. This order of magnitude difference in diffusion coefficients effectively serves to isolate the bilayer regions from one another, thus creating a bilayer array.     

Effect of aminoglycoside on mechanical properties of zwitterionic phospholipid vesicles

The effect of the aminoglycoside (streptomycin) incorporation on the nanomechanical properties of pure dipalmitoylphosphatidylcholine (DPPC) vesicles was studied using atomic force microscope (AFM) on mica surface. The vesicles were prepared by extrusion and adsorbed on the mica surface. The forces, measured between an AFM tip and the vesicle, presented that the breakthrough of the tip into the vesicles occurred two times. Each breakthrough represented each penetration of the tip into each bilayer. Force data prior to the first breakthrough were fitted well with the Hertzian model to estimate Young's modulus and bending modulus of the vesicles. It was found that the Young's modulus and bending modulus were not varied with the incorporation of AGs (streptomycins) up to the 1:1 AG/DPPC vesicle system. This result may suggest that the AGs do not lead to the disruption of DPPC packing.     

Tip penetration through lipid bilayers in atomic force microscopy

In studies of solid supported lipid bilayers with atomic force microscopes (AFM) the force between tip and bilayer is routinely measured. During the approach of the AFM tip in aqueous electrolyte first a short-range repulsive force is observed. For many solid-like and some liquid-like lipid bilayers a subsequent break-through is observed. We observe such a break-through also for dioleoyloxypropyl-trimethylammonium chloride (DOTAP) which is expected to be liquid-like. Here we describe a model which assumes that the jump reflects the penetration of the AFM tip through the lipid bilayer. The model predicts a logarithmical dependence of the break-through force on the approaching velocity of the AFM tip. Two parameters are introduced: The ratio A/αV, α being a geometric factor, A being the area over which pressure is exerted on the bilayer, V the activation volume, and k₀, the rate of spontaneous formation of a hole in the lipid bilayer that is big enough to allow the break-through of the tip. Experiments with bilayers consisting of DOTAP and dioleoylphosphatidylserine (DOPS) show that the break-through forces behave in the predicted way. For DOTAP we obtain ratios A/αV of about 58 nm⁻¹ and rates k₀ ranging from 1.9×10⁻⁸ to 2.5×10⁻⁴ s⁻¹. For DOPS the corresponding values are 162 nm⁻¹ and 2.0 s⁻¹.