Mode Atomic Force Microscopy Research Articles

Formation and Structure of Perfluorosulfonated Ionomer Thin Film on a Graphite Surface

Abstract Adsorption process and structure of perfluorosulfonated acid (PFSA) polymer on a highly oriented pyrolytic graphite (HOPG) surface were elucidated by an in situ tapping mode atomic force microscope (AFM). Micellar aggregates of PFSA polymers first randomly adsorbed on a bare HOPG surface, and then the adsorbed aggregates extended under the influence of atomic arrangement of HOPG surface, to form two-dimensionally ordered domains.

Mineralogical and Chemical Investigation on the Recrystallization Process During Sintering in Phase-Interface Areas in Ancient Ceramic Matrices

In the production of ancient ceramics the manipulation of the morphology of recrystallization and crystal conversion in an inhomogeneous matrix was an important utility to get better hardened materials. The quality of an object was certainly important also in ancient times. The aim of this study was to determine the recrystallization process with respect to lattice diffusion in the ceramic matrix. This study also finally allowed the obtaining of a quantitative observation of intergranular recrystallizations at the nanoscale. According to this kind of information, it is also possible to explain the behavior of intergranular crystals growing in the matrix, which developed during sintering. The crystallization in the grain-interface-matrix was determined by polarization microscopy and scanning electron microscopy (SEM) and linked with the physical-chemical properties of the structures. The samples were first characterized by reflected light microscopy, and then atomic force microscopy (AFM) and pulse force mode (PFM) measurements were carried out at adequate points. With these investigations it was possible to obtain a model for the crystallization growth process.

Calcite [formula omitted] surface in humid environments

In this study, alternating current (AC) mode Atomic Force Microscopy (AFM) height images combined with force measurements and phase imaging were used to investigate the surface reconstruction and chemistry of the lowest energy surface, ( 1 0 1 ¯ 4 ) plane, of calcite, a stable form of calcium carbonate (CaCO 3), in the presence of relative humidity at different temperatures. At 296 K and 70% RH, calcite ( 1 0 1 ¯ 4 ) undergoes rapid restructuring during hydration forming regions on the surface that are most likely characterized as an amorphous hydrate layer similar to what forms in solution under high [Ca 2+] supersaturation conditions. This hydrate layer in turn serves as a substrate for the crystallization of another layer that possesses structural properties which differ from hydrate layer. Phase imaging reveals that these different layer structures formed in the process of water adsorption and surface reconstruction have very different energy dissipation modes. The origin of the different dissipation modes are likely due to differences in water content and hydrophobicity of these regions. The newly formed layer on top of the hydration layer is proposed to be vaterite, another polymorph of calcium carbonate. At 278 K the formation mechanism of the vaterite layer changes due to nucleation of a more crystalline hydrate layer, similar to calcium carbonate hexahydrate, instead of the amorphous hydrate layer that forms at 296 K. Force measurements corroborate the assignment of the speciation of different regions on the surface. Importantly, the AFM data show that the surface of calcite is highly inhomogeneous with regions that vary in water content. This suggests that the reactivity of calcite in humid environments will be highly spatially dependent.

Single molecular dynamic interactions between glycophorin A and lectin as probed by atomic force microscopy

Glycophorin A (GpA) is one of the most abundant transmembrane proteins in human erythrocytes and its interaction with lectins has been studied as model systems for erythrocyte related biological processes. We performed a force measurement study using the force mode of atomic force microscopy (AFM) to investigate the single molecular level biophysical mechanisms involved in GpA-lectin interactions. GpA was mounted on a mica surface or natively presented on the erythrocyte membrane and probed with an AFM tip coated with the monomeric but multivalent Psathyrella velutina lectin (PVL) through covalent crosslinkers. A dynamic force spectroscopy study revealed similar interaction properties in both cases, with the unbinding force centering around 60 pN with a weak loading rate dependence. Hence we identified the presence of one energy barrier in the unbinding process. Force profile analysis showed that more than 70% of GpAs are free of cytoskeletal associations in agreement with previous reports.

Adsorption of Dextrin on Hydrophobic Minerals

The adsorption of dextrin on talc, molybdenite, and graphite (three naturally hydrophobic minerals) has been compared. Adsorption isotherms and in situ tapping mode atomic force microscope (TMAFM) imaging have enabled polymer adsorbed amount and morphology of the adsorbed layer (area coverage and polymer domain size) to be determined and also the amount of hydration water in the structure of the adsorbed layer. The effect of the polymer on the mineral contact angles, measured by the captive bubble method on cleaved mineral surfaces, indicates clear correlations between the hydrophobicity reduction of the minerals, the adsorbed amount, and the surface coverage of the adsorbed polymer. Predictions of the flotation recovery of the treated mineral phases have been confirmed by performing batch flotation experiments. The influence of the polymer surface coverage on flotation recovery has highlighted the importance of this key parameter in the predictions of depressant efficiency. The roles of the initial hydrophobicity and the surface structure of the mineral basal plane in determining adsorption parameters and flotation response of the polymer-treated minerals are also discussed.

Detecting the Magnetic Response of Iron Oxide Capped Organosilane Nanostructures Using Magnetic Sample Modulation and Atomic Force Microscopy

A new imaging strategy using atomic force microscopy (AFM) is demonstrated for mapping magnetic domains at size regimes below 100 nm. The AFM-based imaging mode is referred to as magnetic sample modulation (MSM), since the flux of an AC-generated electromagnetic field is used to induce physical movement of magnetic nanomaterials on surfaces during imaging. The AFM is operated in contact mode using a soft, nonmagnetic tip to detect the physical motion of the sample. By slowly scanning an AFM probe across a vibrating area of the sample, the frequency and amplitude of vibration induced by the magnetic field is tracked by changes in tip deflection. Thus, the AFM tip serves as a force and motion sensor for mapping the vibrational response of magnetic nanomaterials. Essentially, MSM is a hybrid of contact mode AFM combined with selective modulation of magnetic domains. The positional feedback loop for MSM imaging is the same as that used for force modulation and contact mode AFM; however, the vibration of the sample is analyzed using channels of a lock-in amplifier. The investigations are facilitated by nanofabrication methods combining particle lithography with organic vapor deposition and electroless deposition of iron oxide, to prepare designed test platforms of magnetic materials at nanometer length scales. Custom test platforms furnished suitable surfaces for MSM characterizations at the level of individual metal nanostructures.

Learning about modes in atomic force microscopy by means of hands-on activities based ona simple apparatus

This study was conducted to examine the results of using a low-cost hands-on setup incombination with accompanying activities to promote understanding of the contact modeof atomic force microscopy (AFM). This contact mode setup enabled learners to study howAFM works by hand scanning using probing cantilevers with different characteristics ontwo types of artificial surface. They also learnt how to select the various probing cantileverson the basis of their understanding of the advantages and disadvantages of using them.

Development of self‐vibration/detection AFM probe using quartz tuning fork

AbstractWe present a novel type of quartz tuning‐fork probe, which oscillates and detects its own probe deformation, for atomic force microscopy (AFM) systems. The quartz tuning‐fork structure was fabricated using anisotropic wet etching, and a sharp tip was formed on a beam using a focused ion beam (FIB) system. We evaluated the vibration properties of the fabricated tuning fork with the tip structure in an anti‐phase vibration mode and obtained a Q‐factor value of 2.5 to 2.8 × 103. This value was one order larger than that of a conventional Si probe. We experimentally clarified that the tuning‐fork probe measured a 100‐nm step of the Si surface, as well as the debris with a thickness of a few tens of nanometers on the surface by self‐vibration and self‐detection in cyclic‐contact mode AFM. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Probing the intrinsically oil-wet surfaces of pores in North Sea chalk at subpore resolution

Pore surface properties control oil recovery. This is especially true for chalk reservoirs, where pores are particularly small. Wettability, the tendency for a surface to cover itself with fluid, is traditionally defined by the angle a droplet makes with a surface, but this macroscopic definition is meaningless when the particles are smaller than even the smallest droplet. Understanding surface wetting, at the pore scale, will provide clues for more effective oil recovery. We used a special mode of atomic force microscopy and a hydrophobic tip to collect matrices of 10,000 force curves over 5- x 5-mum(2) areas on internal pore surfaces and constructed maps of topography, adhesion, and elasticity. We investigated chalk samples from a water-bearing formation in the Danish North Sea oil fields that had never seen oil. Wettability and elasticity were inhomogeneous over scales of 10s of nanometers, smaller than individual chalk particles. Some areas were soft and hydrophobic, whereas others showed no correlation between hardness and adhesion. We conclude that the macroscopic parameter, "wetting," averages the nanoscopic behavior along fluid pathways, and "mixed-wet" samples have patches with vastly different properties. Development of reservoir hydrophobicity has been attributed to infiltrating oil, but these new results prove that wettability and elasticity are inherent properties of chalk. Their variability, even on single particles, must result from material originally present during sedimentation or material sorbed from the pore fluid some time later.

Temperature and pH-responsive properties of poly(styrene- co-maleic anhydride)-grafting poly(oxypropylene)-amines

A series of temperature- and pH-responsive copolymers were prepared by grafting poly(oxypropylene)-diamines (POP-amine) onto poly(styrene- co-maleic anhydride) (SMA) copolymers. Owing to the presence of poly(oxypropylene)-segments in the pendants and their expressive hydrogen bonding properties, the copolymers exhibited lower critical solution temperature (LCST) behavior in water with a range of 11–49 °C depending on the POP-segmental weight and the SMA backbone structure. Furthermore, the co-existence of NH 2 termini in the pendants and amidoacids in the linking sites in the structures rendered the copolymers also responsive to environmental pH changes. Structural variations in the SMA backbones, particularly the ratio of styrene/maleic anhydride monomers, POP lengths, and amine functionalities largely influenced the copolymer aggregation in the medium with different pH or thermal conditions. The responsiveness was correlated to the surface morphologies observed by tapping/phase mode atomic force microscopy (TM-AFM) on Mica film surface and also fluorescent properties in aqueous solution.

Surface nanostructure formation by the interaction of slow xenon ions on HOPG surfaces

Experiments on nanostructure formation on highly oriented pyrolytic graphite (HOPG) surfaces irradiated by slow highly charged Xeq+ (q = 23-29) ions have been carried out at a new experimental terminal for surface physics on the 320 kV ECR platform at the Institute of Modern Physics (IMP). In the tapping mode atomic force microscope (AFM) image, the nano-sized hillocks protruding from the surfaces are probed. The height and diameter of the nanostructures increase with the projectile charge state. The present results reveal a similarity between the nanostructure formation produced by slow highly charged ions and the track formation induced by swift heavy ions.

Immobilization of biomaterials on nanopatterned surface using nanoporous alumina for biodevices

Nanobiodevices have been required to improve their performance based on their small size. Nanopatterning itself is a very important process in the fabrication of nanobiodevices. Furthermore, there are many challenges in immobilization of biomaterials on a nanopatterned surface, since the performance of biodevices is highly affected by the state of immobilized biomaterials on the substrate. In this paper, we introduce a simple and versatile approach for nanopatterning biomaterials using nanoporous alumina. Uniform nanoconcave patterns were revealed on the surface of aluminum after the removal of nanoporous alumina formed during anodization. Au was deposited on the nanopatterned surface by sputtering. Cysteine-modified azurin was then immobilized on the Au nanopatterned surface by self-assembled layer formation without any organic linker materials. The samples were investigated by phase contrast imaging in tapping mode of atomic force microscopy. These methods can be used for nanopatterning various biomaterials for their application to biochip and other bioelectronic devices.

Study on elastic modulus of individual ferritin

The mechanical property of individual ferritin was measured with force-volume mapping (FV) under contact mode of atomic force microscopy (AFM) in this work. The elastic modulus of individual ferritin was estimated by the Hertz mode. The estimated value of the elastic modulus of individual ferritin was about 250–800 MPa under a small deformation. In addition, the elastic modulus of individual ferritin was compared with that of the colloid gold nanoparticle.

High Resolution AFM of KcsA Structure and Clustering in a Lipid Bilayer

KcsA, a potassium channel found in Streptomyces lividans, is an important prototype for all other K+ channels which share the highly conserved signature sequence TVGYG. It oligomerizes to form a tetrameric channel which is intracellular pH-activated in the range pH < 5.5. The crystal structure of the transmembrane section in the closed state has been resolved at 2.0 A which has provided an insight into the mechanisms of ion selectivity and gating. Its biological function remains poorly understood, but protein clustering observed in-vivo and in-vitro may be significant.Amplitude Modulation Atomic Force Microscopy (AM-AFM) has been used to achieve sub nm single molecule structural information of the topology of membrane protruding segments of KcsA reconstituted in lipid bilayers under conditions approaching physiological ideal and to monitor changes induced by different pH conditions. The spatial resolution of AFM also allowed investigation of the supramolecular organization of KcsA within the membrane.We have imaged KcsA reconstituted in 1-Palmitoyl-2-Oleoyl-sn-Glycero-3-[Phospho-rac-(1-glycerol)] supported bilayers under neutral and acidic conditions (pH 3-7). We obtained high resolution images showing well-defined subunit structure with marked heterogeneity at low pH. Under neutral conditions, a change in channel height occurs, which is likely to be due to a conformation change and is accompanied by a loss of subunit detail. We also found the distribution of tetrameric KcsA in the lipid bilayer changed with pH. We infer that protein-protein interactions are responsible for lowering the 4-fold symmetry of the tetramer, and that pH-mediated clustering of KcsA and conformational change are governed by protein-protein and lipid-protein interactions. The observed dependence of KcsA spatial organization on pH has possible implications for channel activation (cooperative gating) and biological function.

Different Individual Amyloid Fibrils Exhibit Different Beta Sheet Secondary Structures via Near-field Infrared Spectroscopy

Morphology and secondary structure of individual amyloid fibrils synthesized from the #21-31 peptide fragment of B2-microglobulin were obtained using apertureless near-field scanning infrared microscopy (ANSIM). The sample exhibits a heterogeneous population of fibrils: some fibrils exhibit parallel and some with anti-parallel beta sheet structures. ANSIM is an optical technique which uses a carbon monoxide IR laser coupled with a tapping mode atomic force microscope using homodyne detection demodulated at the tip frequency. The experimental near-field spectra correlate strongly to the calculated near-field spectrum as well as the far-field spectrum. Near-field images exhibit high attenuation of the amyloid fibrils at approximately 1630 cm−1. Strong attenuation of the incident IR radiation at 1691 cm−1 corresponding to the presence of anti-parallel beta sheet structure is observed for a fraction of the fibrils. The observation of the anti-parallel beta sheet conformation is linked to the synthesis method used to produce these amyloid fibrils. It is shown that the addition of trimethylamine N-oxide (TMAO) can accelerate the formation of amyloid fibrils and induce anti-parallel beta sheet structure.

Indirect Modulation of Nonmagnetic Probes for Force Modulation Atomic Force Microscopy

Frequency-dependent changes for phase and amplitude images are demonstrated with test platforms of organosilane ring patterns, using force modulation atomic force microscopy (FM-AFM) with an alternate instrument configuration. The imaging setup using indirect magnetic modulation (IMM) is based on indirect oscillation of soft, nonmagnetic cantilevers, with spring constants <1 N m(-1). The tip is driven to vibrate by the motion of a tip holder assembly which contains ferromagnetic materials. The entire tip assembly is induced to vibrate with the flux of an external ac electromagnetic field, supplied by a wire coil solenoid placed underneath the sample plate. With the use of IMM, dynamic parameters of the driving frequencies and amplitude of the tip motion can be optimized to sensitively map the elastic response of samples. An advantage of this instrument setup is that a magnetic coating is not required to drive the periodic oscillation of the tip. The instrument configuration for IMM may not be practical for intermittent imaging modes, which often work best with stiff cantilevers. However, indirect actuation provides an effective approach for imaging with low force setpoints and is well-suited for dynamic AFM modes using continuous contact imaging.

Single-molecule anatomy by atomic force microscopy and recognition imaging

Atomic force microscopy (AFM) has been a useful technique to visualize cellular and molecular structures at single-molecule resolution. The combination of imaging and force modes has also allowed the characterization of physical properties of biological macromolecules in relation to their structures. Furthermore, recognition imaging, which is obtained under the TREC(TM) (Topography and RECognition) mode of AFM, can map a specific protein of interest within an AFM image. In this study, we first demonstrated structural properties of purified α Actinin-4 by conventional AFM. Since this molecule is an actin binding protein that cross-bridges actin filaments and anchors it to integrin via tailin-vinculin-α actinin adaptor-interaction, we investigated their structural properties using the recognition mode of AFM. For this purpose, we attached an anti-α Actinin-4 monoclonal antibody to the AFM cantilever and performed recognition imaging against α Actinin-4. We finally succeeded in mapping the epitopic region within the α Actinin-4 molecule. Thus, recognition imaging using an antibody coupled AFM cantilever will be useful for single-molecule anatomy of biological macromolecules and structures.

Size to density coupling of supported metallic clusters

One of the difficulties in standard growth of metallic nano-clusters on oxide substrates as model catalysts is the strong coupling between clusters size and density. Employing multiple cycles, amorphous solid water-buffer layer assisted growth (ASW-BLAG) procedure, we demonstrate how the size to density coupling can be eliminated under certain conditions. In this study, gold clusters were deposited on a SiO2/Si(100) substrate in UHV, using ASW as a buffer layer assisting aggregation and growth. The clusters were imaged ex situ by tapping mode atomic force microscope (AFM) and high-resolution scanning electron microscope (HR-SEM). In situ Auger electron spectroscopy (AES) measurements have led to independent evaluation of the gold covered area. In order to increase the clusters density we have introduced a multiple BLAG procedure, in which, a BALG cycle is repeated up to 10 times. The cluster density can be increased this way by more than five fold without changing their size. Above a specific number of cycles, however, the cluster density reaches saturation and a gradual increase in clusters size is observed. Larger clusters correlate with lower saturation density following multiple BLAG cycles. This observation is explained in terms of long range cluster-cluster attraction between clusters already on the substrate and those approaching in the next BLAG cycle. This attraction is more pronounced as the clusters become larger. We have shown that at saturation density, inter-cluster distance can not be smaller than 20 nm for clusters 4 nm in diameter or larger. Employing two consecutive BLAG cycles, characterized by different parameters (metal dosage and buffer layer thickness) result in a bi-modal size distribution. Moreover, it is demonstrated that one can prepare this way co-adsorbed bi-metallic film of e.g. Au and Pd clusters, with specific density and size on the same substrate. The ASW-BLAG procedure is thus expected to introduce a new pathway for tailor made, versatile model catalysts.

Measurement of adhesion strengths between various milk products on glass surfaces using contact angle measurement and atomic force microscopy

The adhesion strength of milk products (whole, 2% reduced fat, skim and chocolate milk) on glass surfaces was analyzed before and after manual washing. When measured, whole and chocolate milk had the lowest contact angle (21.0°). The work of adhesion was calculated using the Young and Dupré Equation and the results illustrated that chocolate milk had the strongest and the 2% reduced fat milk had the weakest adhesion force. There was a 2.2% difference in these forces. After the manual washing, atomic force microscopy (AFM) was used to test the adhesion potential of the samples. Contact and tapping AFM modes were used to determine the thicknesses of the films from each sample. The greatest thickness was found for the whole milk. Chocolate milk had the highest variability in thickness measurement. AFM also confirmed that whole and chocolate milk had significantly higher adhesive strengths to glass surfaces. Thus, whole and chocolate milk would be more difficult to clean when compared with the other products.

Amplitude and frequency modulation torsional resonance mode atomic force microscopy of a mineral surface

Scanning probe imaging in a shear force mode allows for the characterization of in-plane surface properties. In a standard AFM, shear force imaging can be realized by the torsional resonance mode. In order to investigate the imaging conditions on mineral surfaces, a torsional resonance mode atomic force microscope was operated in amplitude (AM) and frequency modulation (FM) feedback. Freshly cleaved chlorite was investigated, which showed brucite-like and talc-like surface areas. In constant amplitude FM mode, a slight variation in energy dissipation was observed between both surfaces. Amplitude and frequency vs. distance curves revealed that the tip was in repulsive contact with the specimen during imaging.