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Results for “Atomic force microscopy (AFM)”

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PubMed2004

In situ study of nano-cracking in multilayered magnetic tapes under monotonic and fatigue loading using an AFM.

Atomic force microscopy (AFM) techniques are increasingly used for tribological studies of engineering surfaces on micro- to nano-scales. In situ surface characterization of local deformation of materials and thin coatings helps to develop a better understanding of failure mechanisms. In this study, an AFM-based technique has been developed for in situ monitoring of nano-crack formation and progression under fatigue loading. To conduct monotonic and fatigue loading tests, a tensile stage is used to mount samples on the AFM base and the same area on the sample surface is scanned intermittently during the loading process. Crack growth under monotonic and fatigue loading for multilayered magnetic tapes is studied and a crack growth mechanism for metallic magnetic tapes under monotonic loading is proposed. Fatigue strength for the metallic magnetic tapes is measured and a mathematical model based on theory of elasticity for fatigue life prediction is developed.

Journal Article↗

PubMed1992

Investigation of dialysis membranes with atomic force microscopy.

AFM was used to investigate dialysis membranes made of regenerated cellulose by the cuoxam process. The membranes were either Cuprophan or experimental samples, modified with different amounts of diethylaminoethylcellulose (DEAE). Atomic force microscopes with optical-lever detection systems were used to image the dry membranes in air as received from the manufacturer as well as wet membranes in a swollen state under water. Differences could be observed between modified and unmodified as well as between dry and wet membranes.

Cellulose↗

PubMed2001

Langmuir and Langmuir-Blodgett films of amphiphilic hexa-peri-hexabenzocoronene: new phase transitions and electronic properties controlled by pressure.

We present the synthesis as well as the structural and electronic properties of an amphiphilic derivative of hexaalkylhexa-peri-hexabenzocoronene (HBC), which contains one alkyl substituent that is terminated with a carboxylic acid group. The molecules form well-defined Langmuir films when spread from a solution at the air-water interface. Grazing-incidence X-ray diffraction (GIXD) and X-ray reflectivity studies of the Langmuir monolayer reveal two crystallographic phases at room temperature which depend on the surface pressure applied to the film. Scattering from very well-ordered (zeta = 200-400 A) pi-stacked lamellae of HBC molecules tilted approximately 45 degrees relative to the surface normal is observed in the low-pressure phase. In this phase, the HBC molecules pack in a rectangular two-dimensional unit cell with a = 22.95 A and b = 4.94 A. In the high-pressure phase, coherence from the pi stack is lost. This is a consequence of stress induced by the crystallization of the substituent alkyl chains into a hexagonal lattice, which has a trimerized superstructure in one direction: a = 3 x b = 15.78 A, b = 5.26 A, gamma = 120 degrees, A = 71.9 A2 = 3 x 23.9 A2. Thin monolayer films can be transferred to solid supports by the Langmuir-Blodgett (LB) technique. Atomic force microscopy (AFM) with atomic resolution reveals the crystalline packing of alkyl chains in the high-pressure phase. Kelvin force microscopy (KFM) shows a clear potential difference between the high- and low-pressure phases. This is discussed in terms of orbital delocalization (band formation) in the highly coherent low-pressure phase, which is in contrast to the localized molecular orbitals present in the high-pressure phase. The highly coherent pi stack is expected to sustain a very high charge-carrier mobility.

Journal Article↗

PubMed2005

Thermo-mechanical responses of a surface-coupled AFM cantilever.

Atomic force microscopy (AFM) has been widely used for measuring mechanical properties of biological specimens such as cells, DNA, and proteins. This is usually done by monitoring deformations in response to controlled applied forces, which have to be at ultralow levels due to the extreme softness of the specimens. Consequently, such experiments may be susceptible to thermal excitations, manifested as force and displacement fluctuations that could reduce the measurement accuracy. To take advantage of, rather than to be limited by, such fluctuations, we have characterized the thermomechanical responses of an arbitrarily shaped AFM cantilever with the tip coupled to an elastic spring. Our analysis shows that the cantilever and the specimen behave as springs in parallel. This provides a method for determining the elasticity of the specimen by measuring the change in the tip fluctuations in the presence and absence of coupling. For rectangular and V-shaped cantilevers, we have derived a relationship between the mean-square deflection and the mean-square inclination and an approximate expression for the specimen spring constant in terms of contributions to the mean-square inclination from the first few vibration modes.

Computer Simulation↗

PubMed2001

Structural and functional imaging with carbon nanotube AFM probes.

Atomic force microscopy (AFM) has great potential as a tool for structural biology, a field in which there is increasing demand to characterize larger and more complex biomolecular systems. However, the poorly characterized silicon and silicon nitride probe tips currently employed in AFM limit its biological applications. Carbon nanotubes represent ideal AFM tip materials due to their small diameter, high aspect ratio, large Young's modulus, mechanical robustness, well-defined structure, and unique chemical properties. Nanotube probes were first fabricated by manual assembly, but more recent methods based on chemical vapor deposition provide higher resolution probes and are geared towards mass production, including recent developments that enable quantitative preparation of individual single-walled carbon nanotube tips [J. Phys. Chem. B 105 (2001) 743]. The high-resolution imaging capabilities of these nanotube AFM probes have been demonstrated on gold nanoparticles and well-characterized biomolecules such as IgG and GroES. Using the nanotube probes, new biological structures have been investigated in the areas of amyloid-beta protein aggregation and chromatin remodeling, and new biotechnologies have been developed such as AFM-based haplotyping. In addition to measuring topography, chemically functionalized AFM probes can measure the spatial arrangement of chemical functional groups in a sample. However, standard silicon and silicon nitride tips, once functionalized, do not yield sufficient resolution to allow combined structural and functional imaging of biomolecules. The unique end-group chemistry of carbon nanotubes, which can be arbitrarily modified by established chemical methods, has been exploited for chemical force microscopy, allowing single-molecule measurements with well-defined functionalized tips.

Animals↗

PubMed2006

Estimation of polymer-surface interfacial interaction strength by a contact AFM technique.

Atomic force microscopy (AFM) measurements were employed to assess polymer-surface interfacial interaction strength. The main feature of the measurement is the use of contact-mode AFM as a tool to scratch off the polymer monolayer adsorbed on the solid surface. Tapping-mode AFM was used to determine the depth of the scraped recess. Independent determination of the layer thickness obtained from optical phase interference microscopy (OPIM) confirmed the depth of the AFM scratch. The force required for the complete removal of the polymer layer with no apparent damage to the substrate surface was determined. Polypropylene (PP), low-density polyethylene (PE), and PP-grafted-maleic anhydride (PP-g-ma) were scraped off silane-treated glass slabs, and the strength of surface interaction of the polymer layer was determined. In all cases it was determined that the magnitude of surface interaction force is of the order of van der Waals (VDW) interactions. The interaction strength is influenced either by polymer ability to wet the surface (hydrophobic or hydrophilic interactions) or by hydrogen bonding between the polymer and the surface treatment.

Adsorption↗

PubMed2000

Citric acid etching of cervical sclerotic dentin lesions: an AFM study.

Atomic force microscopy (AFM) has been used to determine microstructural changes, etching rates of peritubular dentin, and intertubular dentin recession during demineralization in dilute acidic solutions. These studies have not included many forms of altered dentin, including noncarious sclerotic root dentin associated with Cl V (abfraction) lesions. We sought to determine microstructural changes and recession rates during demineralization in citric acid (pH 2.5, 0.018M) for the transparent/sclerotic zone. Highly polished dentin disks were prepared from teeth with noncarious C1 V lesions (n = 3) and compared with normal root dentin (n = 3). Samples were etched at 5-s intervals for 1 min and at longer intervals up to 30 min. The depth changes in various portions of the dentin with respect to the reference layer were measured and changes in microstructure observed in solution in the wet cell of the AFM. In sclerotic dentin, most tubule lumens were occluded with crystalline deposits that etched more slowly than the other dentin components, but etching rates could not be determined due to their irregular geometry. The intertubular dentin recession quickly reached a plateau after a depth change of <1 microm for all dentin types, in agreement with prior work. Mixed linear regression models indicated an important difference between the etching of sclerotic intertubular dentin and that of non-sclerotic root dentin that became apparent after 600 s (p = 0.037). The sclerotic intertubular dentin underwent less depth change at the plateau (558 nm) compared to normal root dentin (744 nm). In addition, normal root dentin underwent significantly greater recession than coronal dentin (p = 0.002). The results of this study indicate that intertubular sclerotic dentin from Cl V lesions etches differently than normal root dentin, and this may help explain the difficulties in restoring such lesions with current bonding procedures.

Acid Etching, Dental↗

PubMed2006

Dynamic force measurements of avidin-biotin and streptavdin-biotin interactions using AFM.

Using atomic force microscopy (AFM) we performed dynamic force measurements of the adhesive forces in two model systems: avidin-biotin and streptavidin-biotin. In our experiments we used glutaraldehyde for immobilization of (strept)avidin on the tip and biotin on the sample surface. Such interface layers are more rigid than those usually reported in the literature for AFM studies, when (strept)avidin is coupled with biotinylated bovine albumin and biotin with agarose polymers. We determined the dependence of the rupture forces of avidin-biotin and streptavidin-biotin bonds in the range 300-9600 pN/s. The slope of a semilogarithmic plot of this relation changes at about 1700 pN/s. The existence of two different regimes indicates the presence of two activation barriers of these complexes during the dissociation process. The dissociation rates and activation energy barriers, calculated from the Bell model, for the avidin-biotin and streptavidin-biotin interactions are similar to each other for loading rates > 1700 pN/s but they are different from each other for loading rates < 1700 pN/s. In the latter case, the dissociation rates show a higher stability of the avidin-biotin complex than the streptavidin-biotin complex due to a larger outer activation barrier of 0.8 k(B)T. The bond-rupture force is about 20 pN higher for the avidin-biotin pair than for the streptavidin-biotin pair for loading rates < 1700 pN/s. These two experimental observations are in agreement with the known structural differences between the biotin binding pocket of avidin and of streptavidin.

Avidin↗

PubMed2002

A novel sample holder allowing atomic force microscopy on transmission electron microscopy specimen grids: repetitive, direct correlation between AFM and TEM images.

A novel sample holder that allows atomic force microscopy (AFM) to be performed on transmission electron microscope (TEM) grids is described. Consequently, AFM and TEM images were repeatedly obtained on exactly the same sample area. For both techniques, a thin carbon film was used as the imaging substrate. Although these techniques have been previously used in conjunction, AFM and TEM images on exactly the same area have not been repeatedly obtained for any system. Correlation of AFM and TEM images is useful for work where the three-dimensional topographical information provided by the AFM could be used to better interpret the two-dimensional images provided by the TEM and vice versa. To demonstrate the applicability of such correlation, new results pertaining to a fibrillar collagen system are summarized.

Fibrillar Collagens↗

PubMed2004

Examination of dentin surface using AFM (our experience).

Atomic force microscopy (AFM) as one the technique of Scanning Probe Microscopy is useful for imaging of surface structure. This method can yield three-dimensional high-resolution topographic images of sample surfaces by using a scanning technique for conductors and insulators on atomic scale. It is based upon mapping of atomic-forces on a surface of an investigated sample. The method is useful not only in physics and chemistry; it can be also applied in biological fields. Special construction of AFM scanner enables to follow biological samples in liquid environments. Artifacts caused by dehydration of samples are removed this way. Dentin of human teeth is a vital hydrated tissue. It is strongly sensitive to dehydration and drying that are commonly used in preparation of samples in examinations by Scanning Electron Microscopy (SEM). We describe our experience in examination of dentin surfaces of extracted human third molars using contact method of AFM under moist conditions.

Dentin↗

PubMedDate not supplied

STM and AFM in biotechnology.

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are capable of providing atomic-scale images, as well as insights into functional characteristics, of surfaces. Biological materials can be studied by depositing them on appropriate substrates, and samples have been imaged with varying degrees of resolution. In this review, the principles of STM and AFM are summarized, and practical aspects of implementing them for studies relevant to biotechnology are discussed, with the emphasis on investigations of biological macromolecules such as DNA and proteins. Items covered include samples, surfaces, medium, tips, and typical operating conditions. Finally, prospects are discussed for potential future applications to biotechnology.

Biotechnology↗

PubMed2001

Progressive accretion of amelogenin molecules during nanospheres assembly revealed by atomic force microscopy.

Amelogenin proteins, the principal components of the developing dental enamel matrix, self-assemble to form nanosphere structures that are believed to function as structural components directly involved in the matrix mediated enamel biomineralization. The self-assembly behavior of a recombinant murine amelogenin (rM179) was investigated by atomic force microscopy (AFM) for further understanding the roles of amelogenin proteins in dental enamel biomineralization. Recombinant rM179 amelogenin was dissolved in a pH 7.4 Tris-HCl buffer at concentrations ranging from 12.5 to 300 microg/ml. The solutions were adsorbed on mica, fixed with Karnovsky fixative and rinsed thoroughly with water for atomic force microscopy (AFM). At low concentrations (12.5-50 microg/ml), nanospheres with diameters varying from 7 to 53 nm were identified while at concentrations ranging between 100-300 microg/ml the size distribution was significantly narrowed to be steadily between 10 and 25 nm in diameter. These nanospheres were observed to be the basic building blocks of both engineered rM179 gels and of the developing enamel extracellular matrix. The stable 15-20-nm nanosphere structures generated in the presence of high concentrations of amelogenins were postulated to be of great importance in facilitating the highly organized ultrastructural microenvironment required for the formation of initial enamel apatite crystallites.

Amelogenin↗

PubMed2001

Spin-column isolation of DNA-protein interactions from complex protein mixtures for AFM imaging.

Applications of atomic force microscopy (AFM) to investigate structural-functional interactions between DNA and proteins, at the molecular level, should prove valuable for gaining a better understanding of gene expression. Specific genomic DNA-protein interactions occur within a sea of intracellular proteins. Successful AFM imaging requires isolating the specific DNA-protein complex free of background protein contamination. Using spin-column chromatography, we report the successful isolation and AFM imaging of transcription factor DNA complexes from DNA molecules incubated with crude cell lysates. This method should be applicable for the isolation and imaging of other specific DNA-protein complexes pertinent to functional genomic research.

Chromatography, Gel↗

PubMed2005

Branched coordination multilayers on gold.

A C3-symmetric tridentate hexahydroxamate ligand molecule was specially synthesized and used for coordination self-assembly of branched multilayers on Au surfaces precoated with a self-assembled monolayer (SAM) of ligand anchors. Layer-by-layer (LbL) growth of multilayers via metal-organic coordination using Zr4+ ions proceeds with high regularity, adding one molecular layer in each step, as shown by ellipsometry, wettability, UV-vis spectroscopy, and atomic force microscopy (AFM). The branched multilayer films display improved stiffness, as well as a unique defect self-repair capability, attributed to cross-linking in the layers and lateral expansion over defects during multilayer growth. Transmetalation, i.e., exposure of Zr4+-based assemblies to Hf4+ ions, was used to evaluate the cross-linking. Conductive atomic force microscopy (AFM) was used to probe the electrical properties of the multilayers, revealing excellent dielectric behavior. The special properties of the branched layers were emphasized by comparison with analogous multilayers prepared similarly using linear (tetrahydroxamate) ligand molecules. The process of defect annihilation by bridging over defective areas, attributed to lateral expansion via the excess bishydroxamate groups, was demonstrated by introduction of artificial defects in the anchor monolayer, followed by assembly of two layers of either the linear or the branched molecule. Analysis of selective binding of Au nanoparticles (NPs) to unblocked defects emphasized the superior repair mechanism in the branched layers with respect to the linear ones.

Journal Article↗

PubMed1997

Reversible unfolding of individual titin immunoglobulin domains by AFM.

Single-molecule atomic force microscopy (AFM) was used to investigate the mechanical properties of titin, the giant sarcomeric protein of striated muscle. Individual titin molecules were repeatedly stretched, and the applied force was recorded as a function of the elongation. At large extensions, the restoring force exhibited a sawtoothlike pattern, with a periodicity that varied between 25 and 28 nanometers. Measurements of recombinant titin immunoglobulin segments of two different lengths exhibited the same pattern and allowed attribution of the discontinuities to the unfolding of individual immunoglobulin domains. The forces required to unfold individual domains ranged from 150 to 300 piconewtons and depended on the pulling speed. Upon relaxation, refolding of immunoglobulin domains was observed.

Adsorption↗

PubMed2005

Influence of bulk elasticity and interfacial tension on the deformation of gelled water-in-oil emulsion droplets: an AFM study.

We used atomic force microscopy (AFM) to study the deformation and wetting behavior of large (50-250 microm) emulsion droplets upon mechanical loading with a colloidal glass probe. Our droplets were obtained from water-in-oil emulsions. By adding gelatin to the water prior to emulsification, also droplets with a bulk elasticity were prepared. Systematic variations of surfactant and gelatin concentrations were made, to investigate their effect on the deformation and wetting behavior of the droplets and to identify the contributions of interfacial tension, bulk elasticity, and expelled water. The AFM experiments were performed in force--distance mode and showed on approach a repulsive regime which in many cases was terminated by a jump-in of the probe. In the case of pure water (i.e. gelatin-free) droplets, the repulsive part of the curve showed a good linearity, thus allowing the extraction of an effective droplet spring constant. This quantity was found to decrease on raising the surfactant concentration from below the critical micelle concentration (cmc) to well above the cmc, and its numerical values were found to correspond remarkably well to literature values for the interfacial tension. Our findings indicate that, on gelatin increase inside the droplets, the bulk elasticity gradually becomes dominant and the droplets' stiffness does not depend anymore on surfactant concentration. Also the stability of the droplet interface against wetting, as measured by the force at which the jump-in instability occurs, was enhanced by gelatin. For gelatin concentrations of > or =15 wt %, the droplets were found to behave like purely elastic bodies. Both gelatin and surfactant contribute positively to the stability against interface breakup.

Journal Article↗

PubMed2005

Comparison of resistance to protein adsorption and stability of thin films derived from alpha-hepta-(ethylene glycol) methyl omega-undecenyl ether on HSi(111) and HSi(100) surfaces.

Oligo(ethylene glycol)-terminated thin films were prepared by photo-induced hydrosilylation of alpha-hepta-(ethylene glycol) methyl omega-undecenyl ether (EG(7)) on hydrogen-terminated silicon (111) and (100) surfaces. Their resistance to protein adsorption, and stabilities (from hours to days) under a wide variety of conditions, such as air, water, biological buffer, acid, and base, were investigated using contact-angle goniometry and ellipsometry techniques. Results indicated higher stability of the films chemisorbed on Si(111) than on Si(100). Furthermore, micron-sized patterns were fabricated on the films via AFM anodization lithography. Using atomic force microscopy (AFM) and fluorescence microscopy, we demonstrated that various proteins including fibrinogen, avidin, and bovine serum albumin (BSA) predominately adsorbed onto the patterns, but not the rest of the film surfaces.

Adsorption↗

PubMed2005

Local spectroscopy and atomic imaging of tunneling current, forces, and dissipation on graphite.

Theory predicts that the currents in scanning tunneling microscopy (STM) and the attractive forces measured in atomic force microscopy (AFM) are directly related. Atomic images obtained in an attractive AFM mode should therefore be redundant because they should be similar to STM. Here, we show that while the distance dependence of current and force is similar for graphite, constant-height AFM and STM images differ substantially depending on the distance and bias voltage. We perform spectroscopy of the tunneling current, the frequency shift, and the damping signal at high-symmetry lattice sites of the graphite (0001) surface. The dissipation signal is about twice as sensitive to distance as the frequency shift, explained by the Prandtl-Tomlinson model of atomic friction.

Journal Article↗