Tapping Mode Atomic Force Microscopy Images Research Articles

(Digital Presentation) Investigation of Microstructure Dependent Inhibitor Adsorption Mechanism on Carbon Steel By in Situ Atomic Force Microscopy

In the petroleum industry, organic surfactant type inhibitors have been widely applied in the oil and gas industry considering their high efficiency. Carbon steels with various carbide contents have also been used extensively as pipeline materials during the transport and storage of crude oil due to their excellent mechanical properties and low cost. The previous investigation of inhibition mechanism on carbon steel is limited by lack of surface characterization techniques at a molecular level. Atomic force microscopy (AFM) can not only provide localized visual observation, but can also achieve the detection of mechanical properties of an inhibitor film. In this study, in situ AFM contact mode friction imaging and in situ AFM tapping mode phase imaging techniques have been applied to investigate the influence of the ferritic-pearlitic microstructure of carbon steel on inhibitor adsorption mechanisms during CO2 corrosion under different inhibitor concentrations. At concentration above surface saturation, AFM contact mode friction images show a large friction contrast between inhibitor covered cementite structures and ferrite structures, while in the absence of inhibitor, this friction contrast almost disappears, indicating the inhibitor adsorption induced this difference. AFM tapping mode phase images indicate uniform adsorption of inhibitor on both cementite and ferrite structures.The differences observed in these AFM analyses indicate that the adhesion force of inhibitor molecules on cementite could be smaller than on ferrite, or the molecular orientations of inhibitor molecules adsorbed on cementite and ferrite structures could be different. The special adsorption behavior of inhibitor molecules on cementite means the carbide component of the microstructure could directly influence inhibitor adsorption and may decrease inhibitor efficiency. Simultaneous linear polarization resistance tests in electrochemical AFM (EC-AFM) cell indicates a maximum inhibition efficiency at concentration above saturation.Keywords : Contact mode AFM, friction contrast, tapping mode AFM, phase contrast, cementite, ferrite, organic inhibitor, adhesion force.Statement of importance:Studies related to inhibitor adsorption mechanisms on ferritc-pearlitic carbon steels can be connected with the practical issues that iron carbide can impair the corrosion inhibitor performance on carbon steels. This AFM work connects the corrosion inhibition effect with the carbon steel components at a molecular level for the first time. Physical explanations regarding the interactions between inhibitor molecules and carbon steel microstructures can also be provided with the assistance of molecular simulations.Figure caption: Contact and tapping mode AFM images of inhibitor film formed on UNS G1018 steel at 2 CMC (100ppm) tetradecyldimethylbenzylammonium inhibitor + 1 wt% NaCl. The contact mode friction image indicates a large friction contrast between inhibitor covered cementite and ferrite structure, while the tapping mode phase image indicates a negligible softness contrast between cementite and ferrite structure. Figure 1

Non-covalent Immobilization of Desmoplakin Plakin Domain Molecules by Size-Selected Clusters for AFM Imaging

We exploit the immobilization of proteins by size-selected atomic gold clusters to examine the plakin domain of desmoplakin (PD), a human desmosomal protein that is mutated in severe skin diseases and cardiomyopathy, using atomic force microscopy (AFM) in the liquid phase. Contact mode AFM indicates that the PD does not establish a covalent bond size-selected gold nanocluster supported on a graphite substrate, but tapping mode AFM images indicate enhanced weak adsorption. The protein dimensions obtained are compared with predicted values based on electron microscopy, small-angle X-ray scattering (SAXS) and X-ray crystallographic studies.

Nanoscale surface characterization of aqueous copper corrosion: Effects of immersion interval and orthophosphate concentration

Morphology changes for copper surfaces exposed to different water parameters were investigated at the nanoscale with atomic force microscopy (AFM), as influenced by changes in pH and the levels of orthophosphate ions. Synthetic water samples were designed to mimic physiological chemistries for drinking water, both with and without addition of orthophosphate over a pH range 6.5–9. Copper surfaces treated with orthophosphate as a corrosion inhibitor after 6 and 24h were evaluated. Tapping mode AFM images revealed dosing of the water with 6mg/L of orthophosphate was beneficial in retarding the growth of copper by-products. The chemical composition and oxidation state of the surface deposits were characterized with X-ray diffraction (XRD), near edge X-ray absorption fine structure (NEXAFS) spectroscopy and Fourier transform infrared spectroscopy (FTIR).

Tapping-mode force spectroscopy using cantilevers with interferometric high-bandwidth force sensors

We quantitatively map the surface forces and elastic modulus in tapping-mode atomic force microscopy (AFM). To achieve this, we use custom-built cantilevers with interferometric high-bandwidth force sensors that can resolve nonlinear tip-sample interaction forces, combined with a set of algorithms to process the force sensor signals in real-time. Our technique achieves quantitative mechanical measurements, while retaining nanoscale spatial resolution and minimal loading forces in tapping-mode AFM. Moreover, conventional tapping-mode AFM images are not affected and can simultaneously be acquired. As a practical demonstration, we use our technique to quantify the mechanical properties of a polystyrene and linear low-density polyethylene blend.

On the Composition of the Top Layer of Microphase Separated Thin PS-PEO Films

The topography and surface composition of thin films (ca. 100 nm) of a polystyrene-b-poly(ethylene oxide) (PS-PEO) block copolymer are investigated using a suite of complementary techniques, namely tapping mode atomic force microscopy (AFM), optical microscopy, X-ray photoelectron spectroscopy (XPS), neutron reflectometry, and wettability measurements. The copolymer films separate into lamellar structures oriented parallel to the silicon substrate, and bicontinuous and island/hole morphologies characteristic of this arrangement appear. Even though the crystalline topography of the film’s surface and its wettability properties suggest the presence of PEO on the top surface, XPS and neutron reflectometry data point undoubtedly to the presence of a top layer of PS at the air/film interface. Tapping mode AFM images unequivocally demonstrate that in air only one block is present at the air/film interface. Neutron reflectometry data identify the nature of each phase-separated layer within the film. This finding...

Adsorption of Modified Dextrins on Talc: Effect of Surface Coverage and Hydration Water on Hydrophobicity Reduction

The adsorption of three modified dextrins on the basal plane of talc has been studied using in situ tapping mode atomic force microscopy (TMAFM). The images have been used to determine the layer thickness and coverage of the adsorbed polymers. Adsorption isotherms of the polymers on talc particles were also determined using the depletion technique. Values of the adsorbed amount at equilibrium were compared with the volume of adsorbed material as determined using in situ TMAFM, revealing the presence of significant amounts of hydration water in the adsorbed layer structure. This deduction was confirmed by comparing in and ex situ TMAFM images of the adsorbed dextrins. The effect of layer thickness, coverage, and hydration water content on the contact angle of talc particles treated with polymer was investigated using the Washburn method and the equilibrium capillary pressure (ECP) method. Distinct correlations were observed between adsorbed layer properties and the measured contact angles, with the ECP measurements especially highlighting the effect of the adsorbed polymer layer hydration water. The implications for the performance of the modified dextrins in flotation are discussed.

Atomic force microscopy observation of highly arrayed phospholipid bilayer vesicle on a gold surface

Tapping mode atomic force microscopy (TM-AFM) imaging of a phospholipid bilayer vesicle (liposome) immobilized on a gold surface was performed to investigate morphologies of the electrode surfaces produced through application of three different sample preparation methods. We compared both methods from a morphological viewpoint using TM-AFM images. Liposomes, composed of zwitterionic and anionic phospholipids, were prepared by extrusion. Results indicate that the surface with immobilized L1-liposome, which was fabricated by the amino coupling method, seemed to form large amounts of aggregated or fused liposomes. In contrast, L2-liposome-containing 1-octadecanthiol that was directly attached on the gold surface using thiol-gold binding force was immobilized as a uniform surface topology without liposome aggregation. Finally, we attempted to arrange individual L3-liposome, prepared by mixing zwitterionic and anionic phospholipids, onto the gold layer by electron-beam (e-beam) lithography technique. A third method, L3-liposome formation on the sensor surface, is greatly anticipated for biosensor applications.

A Novel Strategy to Construct a Flat-Lying DNA Monolayer on a Mica Surface

Flat-lying, densely packed DNA monolayers in which DNA chains are well organized have been successfully constructed on a mica surface by dropping a droplet of a DNA solution on a freshly cleaved mica surface and subsequently transferring the mica to ultrapure water for developing. The formation kinetics of such monolayers was studied by tapping mode atomic force microscopy (TMAFM) technique. A series of TMAFM images of DNA films obtained at various developing times show that before the sample was immersed into water for developing the DNA chains always seriously aggregated by contacting, crossing, or overlapping and formed large-scale networks on the mica surface. During developing, the fibers of DNA networks gradually dispersed into many smaller fibers up to single DNA chains. At the same time, the fibers or DNA chains also experienced rearrangement to decrease electrostatic repulsion and interfacial Gibbs free energy. Finally, a flat-lying, densely packed DNA monolayer was formed. A formation mechanism of the DNA monolayers was proposed that consists of aggregation, dispersion, and rearrangement. The effects of both DNA and Mg2+ concentration in the formation solution on DNA monolayer formation were also investigated in detail.

Assessment of Elasticity and Topography of Aspergillus nidulans Spores via Atomic Force Microscopy

Previous studies have described both surface morphology and adhesive properties of fungal spores, but little information is currently available on their mechanical properties. In this study, atomic force microscopy (AFM) was used to investigate both surface topography and micromechanical properties of Aspergillus nidulans spores. To assess the influence of proteins covering the spore surface, wild-type spores were compared with spores from isogenic rodA(+) and rodA(-) strains. Tapping-mode AFM images of wild-type and rodA(+) spores in air showed characteristic "rodlet" protein structures covering a granular spore surface. In comparison, rodA(-) spores were rodlet free but showed a granular surface structure similar to that of the wild-type and rodA(+) spores. Rodlets were removed from rodA(+) spores by sonication, uncovering the underlying granular layer. Both rodlet-covered and rodlet-free spores were subjected to nanoindentation measurements, conducted in air, which showed the stiffnesses to be 110 +/- 10, 120 +/- 10, and 300 +/- 20 N/m and the elastic moduli to be 6.6 +/- 0.4, 7.0 +/- 0.7, and 22 +/- 2 GPa for wild-type, rodA(+) and rodA(-) spores, respectively. These results imply the rodlet layer is significantly softer than the underlying portion of the cell wall.

A Combined Atomic Force Microscopy and Molecular Dynamics Simulation Study on a Plastocyanin Mutant Chemisorbed on a Gold Surface

A mutant of copper plastocyanin, covalently bound to an Au (111) surface through an engineered disulfide bridge, was investigated in aqueous medium by atomic force microscopy (AFM) and molecular dynamics (MD) simulations. Tapping-mode AFM images revealed adsorption of single molecules which are homogeneously distributed over the substrate and strongly bound to gold and display uniform lateral size. A statistical analysis of the height of the macromolecules on the gold substrate evidenced a distribution around a mean value consistent with that expected from the crystallographic data and with a relatively large standard deviation. A 10-ns classical MD simulation of mutated plastocyanin, hydrated by a layer of water, covalently bound to a gold surface by one or two sulfur atoms, was performed. The simulations indicate that the bound protein retains, in both cases, its overall tertiary structure during the dynamic evolution. Moreover, the macro-molecule can assume different orientations with respect to the gold substrate, which give rise to a distribution of heights on the gold substrate. Experimental and MD simulation results are compared and discussed in connection with the topological and dynamical properties of the protein system.

Hierarchical Assembly of β 2-Microglobulin Amyloid In Vitro Revealed by Atomic Force Microscopy

The kinetics of spontaneous assembly of amyloid fibrils of wild-type β 2-microglobulin (β 2M) in vitro, under acid conditions (pH 2.5) and low ionic strength, has been followed using thioflavin-T (ThT) binding. In parallel experiments, the morphology of the different fibrillar species present at different time-points during the growth process were characterised using tapping-mode atomic force microscopy (TM-AFM) in air and negative stain electron microscopy (EM). The thioflavin-T assay shows a characteristic lag phase during which the nucleation of fibrils occurs before a rapid growth in fibril density. The volume of fibrils deposited on mica measured from TM-AFM images at each time-point correlates well with the fluorescence data. TM-AFM and negative-stain EM revealed the presence of various kinds of protein aggregates in the lag phase that disappear concomitantly with a rise in the density of amyloid fibrils, suggesting that these aggregates precede fibril growth and may act as nucleation sites. Three distinct morphologies of mature amyloid fibrils were observed within a single growth experiment, as observed previously for the wild-type protein and the variant N17D. Additional supercoiled morphologies of the lower-order fibrils were observed. Comparative height analysis from the TM-AFM data allows each of the mature fibril types and single protofilaments to be identified unambiguously, and reveals that the assembly occurs via a hierarchy of morphological states.

Large fluctuations in the disassembly rate of microtubules revealed by atomic force microscopy

Atomic force microscopy (AFM) in situ has been used to observe the cold disassembly dynamics of microtubules at a previously unrealised spatial resolution. Microtubules either electrostatically or covalently bound to aminosilane surfaces disassembled at room temperature under buffer solutions with no free tubulin present. This process was followed by taking sequential tapping-mode AFM images and measuring the change in the microtubule end position as a function of time, with an spatial accuracy down to ±20 nm and a temporal accuracy of ±1 s. As well as giving average disassembly rates on the order of 1–10 tubulin monomers per second, large fluctuations in the disassembly rate were revealed, indicating that the process is far from smooth and linear under these experimental conditions. The surface bound rates measured here are comparable to the rates for GMPCPP-tubulin microtubules free in solution, suggesting that inhibition of tubulin curvature through steric hindrance controls the average, relatively low disassembly rate. The large fluctuations in this rate are thought to be due to multiple pathways in the kinetics of disassembly with differing rate constants and/or stalling due to defects in the microtubule lattice. Microtubules that were covalently bound to the surface left behind the protofilaments covalently cross-linked to the aminosilane via glutaraldehyde during the disassembly process. Further work is needed to quantitatively assess the effects of surface binding on protofibril disassembly rates, reveal any differences in disassembly rates between the plus and minus ends and to enable assembly as well as disassembly to be imaged in the microscope fluid cell in real-time.

Spontaneous Formation of Air Nanobubbles on Hydrocarbons Deposited on the Au(111)/Water Interface

We have observed surface features on hydrocarbons deposited on Au(111) from the laboratory atmosphere in water and in a sodium chloride aqueous solution, using tapping mode atomic force microscopy (TMAFM). TMAFM images of the Au(111)/water interface reveal that the features are pancake-shaped structures with the range of measured dimensions of 20–150 nm in width and 0.5–20 nm in height, and that many features reappear at almost the same sites immediately after the disruption of the features by contact mode AFM imaging. The results of our TMAFM observations imply that the features are nanosized bubbles as those reported previously, and that dissolved gas nucleates at specific sites of the hydrophobized Au(111) surface at low, temporal gas supersaturations under atmospheric pressure.

Surface characterization, microstructure, and wetting of networks from ,-dihydroxy(polydimethylsiloxane) and 1,1,2,2-tetrahydrotridecafluoro octyltriethoxysilane

Surface-modified siloxane networks were prepared using the dibutyltin diacetate catalyzed reaction of α,ω-dihydroxy(polydimethylsiloxane), HO(Me2SiO)nH, with tridecafluoro-1,1,2,2-tetrahydrooctyl triethoxysilane, [CF3(CF2)5(CH2)2Si(OC2H5)3, FTEOS]. Surface characterization of these elastomers was carried out using electron spectroscopy for chemical analysis (ESCA) and atomic force microscopy (AFM). Surface phase separation occurs as a function of increasing ratio of FTEOS to HO(Me2SiO)nH. Dynamic contact angle (DCA) analysis with water as the interrogating fluid showed that water contamination affects DCA data in the compositional range 4x–8x, where “x” is the ratio of ethoxy groups from FTEOS to OH groups in HO(Me2SiO)nH. Diffusion of polydimethylsiloxane oil species from the coatings is blocked in compositions of 10x or greater. Chemical stability is also enhanced for 10x and 12x compositions. (A) Tapping mode AFM images. Image area 30 × 30 × 1 μm. (B) DCA data with arrows showing collapse due to water surface contamination, 4x-0.8 cycle before collapse, 6x-1 cycle, 8x-1.6 cycles, 10x and 12x-stable. Stage speed 100 μm · s−1 and dwell times between sample immersion and withdrawal 10 s.

AFM observation of small surface pores of hollow-fiber dialysis membrane using highly sharpened probe

Determining pore size distribution is important for characterization of a dialysis membrane. However, conventional microscopic techniques cannot present a sufficient image for determining pore size distribution. In the present study, tapping mode atomic force microscopy (TMAFM) has been shown to be a powerful tool for observing and evaluating the small surface pores of a hollow-fiber dialysis membrane. Sample fixing technique described below and a highly sharpened probe have made it possible to observe small pores on a soft and undulant surface of a dialysis membrane. This is the first time that clear TMAFM images of surface pores of a hollow-fiber dialysis membrane at such high resolution have been presented. Pore diameter was determined by image analysis. Average pore diameter of APS-150 (Asahi-kasei, Japan) determined by TMAFM was compared with those by field emission scanning electron microscopy (FESEM) and by the Hagen–Poiseuille equation. The average pore diameter of APS-150 determined by TMAFM was slightly higher than that by FESEM. The average pore diameter determined by the Hagen–Poiseuille equation was intermediate between values for that of inside and outside surfaces determined by TMAFM.

The Influence of the Underlying Gold Substrate on Glucose Oxidase Electrodes Fabricated Using Self-Assembled Monolayers

The influence of the underlying gold surface topography on the response of enzyme electrodes fabricated by covalent attachment of glucose oxidase to a 3-mercaptopropionic acid self-assembled monolayer was assessed. Six different gold substrates were prepared. The bare substrates were evaluated for roughness and defects using atomic force microscopy (AFM) and scanning tunneling microscopy (STM). Despite large variations in surface roughness, the shapes of the enzyme electrode calibration plots indicate that approximately the same amount of MPA was adsorbed on the surface and from electrochemical stripping experiments the same number of active enzyme molecules were attached. However, compared with the smoother surfaces, enzyme electrodes fabricated on surfaces with high microscopic roughness had a smaller linear range but a similar current sensitivity in the linear portion of the calibration plot. Tapping-mode AFM images of the enzyme modified surfaces showed that the enzyme adsorbed onto the surface as clusters of five or six enzyme molecules. The formation of these clusters did not appear to be influenced by the surface roughness.

Aspects of Mineral Structure in Normally Calcifying Avian Tendon

Structural characteristics of normally calcifying leg tendons of the domestic turkey Meleagris gallopavo have been observed for the first time by tapping mode atomic force microscopy (TMAFM), and phase as well as corresponding topographic images were acquired to gain insight into the features of mineralizing collagen fibrils and fibers. Analysis of different regions of the tendon has yielded new information concerning the structural interrelationships in vivo between collagen fibrils and fibers and mineral crystals appearing in the form of plates and plate aggregates. TMAFM images show numerous mineralized collagen structures exhibiting characteristic periodicity (54–70 nm), organized with their respective long axes parallel to each other. In some instances, mineral plates (30–40 nm thick) are found interspersed between and in intimate contact with the mineralized collagen. The edges of such plates lie parallel to the neighboring collagen. Many of these plates appear to be aligned to form larger aggregates (475–600 nm long × 75–90 nm thick) that also retain collagen periodicity along their exposed edges. Intrinsic structural properties of the mineralizing avian tendon have not previously been described on the scale reported in this study. These data provide the first visual evidence supporting the concept that larger plates form from parallel association of smaller ones, and the data fill a gap in knowledge between macromolecular- and anatomic-scale studies of the mineralization of avian tendon and connective tissues in general. The observed organization of mineralized collagen, plates, and plate aggregates maintaining a consistently parallel nature demonstrates the means by which increasing structural complexity may be achieved in a calcified tissue over greater levels of hierarchical order.

Structural, optical and electrical properties of GaN films grown by metalorganic chemical vapor deposition on sapphire.

ABSTRACTProperties of GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on c-plane of sapphire have been investigated using atomic force microscopy (AFM), wet etching for defect investigation, transmission electron microscopy (TEM), high-resolution X-ray diffraction, Hall effect measurements and low-temperature photoluminescence (PL). Tapping-mode AFM images of the as-grown samples showed atomically smooth surfaces (rms roughness ≍ 0.2 nm) consisting of terraces separated by about 3Å bi-layer steps. Hot H3PO4 chemical etching was used to produce hexagonal-shaped etch pits at the surface defect sites as revealed by AFM imaging. The obtained etch pit densities (9×108 - 2 ×109 cm−2) were in agreement with the dislocation density found by plan-view and cross-sectional TEM observations. The full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve was about 4.8 and 3.9 arcmin for the symmetric (002) and asymmetric (104) directions, respectively. PL spectrum at 15 K demonstrated sharp peaks (FWHM ≍ 4 meV) in the excitonic region, which were attributed to free and bound excitons. The spectrum contained also weak PL bands with maxima at about 2.2, 2.9 and 3.27 eV, which have been attributed to three different acceptors.

Tapping-Mode Atomic Force Microscopy Produces Faithful High-Resolution Images of Protein Surfaces

Compared to contact-mode atomic force microscopy (CMAFM), tapping-mode atomic force microscopy (TMAFM) has the advantage of allowing imaging surfaces of macromolecules, even when they are only weakly attached to the support. In this study, TMAFM is applied to two different regular protein layers whose structures are known to great detail, the purple membrane from Halobacterium salinarum and the hexagonally packed intermediate (HPI) layer from Deinococcus radiodurans, to assess the faithfulness of high-resolution TMAFM images. Topographs exhibited a lateral resolution between 1.1 and 1.5 nm and a vertical resolution of ∼0.1 nm. For all protein surfaces, TMAFM and CMAFM topographs were in excellent agreement. TMAFM was capable of imaging the fragile polypeptide loop connecting the transmembrane α-helices E and F of bacteriorhodopsin in its native extended conformation. The standard deviation (SD) of averages calculated from TMAFM topographs exhibited an enhanced minimum (between 0.1 and 0.9 nm) that can be assigned to the higher noise of the raw data. However, the SD difference, indicating the flexibility of protein subunits, exhibited an excellent agreement between the two imaging modes. This demonstrates that the recently invented imaging-mode TMAFM has the ability to faithfully record high-resolution images and has sufficient sensitivity to contour individual peptide loops without detectable deformations.

Direct Visualization of Gram-Negative Bacterial Lipopolysaccharide Self-Assembly

Bacterial lipopolysaccharides (LPS) are outer cell wall components of gram-negative bacteria that may cause septic shock in mammals. The exact morphology of LPS when interacting with macromolecular complexes of the septic shock pathway in blood is still uncertain. Here, the geometry and morphology of hydrated bacterial LPS, dispersed in solution, at and below its the critical aggregate concentration, were directly examined by tapping mode atomic force microscopy (TMAFM) and dynamic light scattering. High-resolution phase-shift TMAFM images of hydrated LPS of Salmonella minnesota Re595, Salmonella typhimurium, and Escherichia coli 0111:B4 adsorbed on mica surfaces unveiled nanosized lipidic particles with a species-specific organization. The complex hydrodynamic geometry exhibited by LPS in dilute suspensions may have consequences for the interpretation of LPS biological activity in the host immune response.