Force Curve Mapping Research Articles

Mapping of endoglucanases displayed on yeast cell surface using atomic force microscopy

The surface of yeast cells has been an attractive interface for the effective use of cellulose. Surface enzymes, however, are difficult to visualize and evaluate. In this study, two kinds of unique anchoring regions were used to display the cellulase, endoglucanase (EG), on a yeast cell surface. Differences in the display level and the localization of EG were observed by atomic force microscopy. By surveying the yeast cell surface with a chemically modified cantilever, the interactive force between the cellulose and EG was measured. Force curve mapping revealed differences in the display levels and the localization of EG according to anchoring regions. The proposed methodology enables visualization of displayed enzymes such as EG on the yeast cell surface.

Atomic force microscopy on phase-control pulsed force mode in water: Imaging and force analysis on a rhodium-octaethylporphyrin layer on highly oriented pyrolytic graphite

Abstract We developed phase-control pulsed force mode (p-PFM) as the operation mode for atomic force microscopy (AFM). The p-PFM allowed us to observe soft or weakly adsorbed materials in a liquid environment using a conventional AFM apparatus, and allowed for force curve mapping (FCM) after offline data processing. We applied the p-PFM to a rhodium-octaethylporphyrin (RhOEP) layer on highly oriented pyrolytic graphite (HOPG), which is applicable to anode catalysts of fuel cells. The RhOEP/HOPG system was stably observed in water by this mode. In the p-PFM image, we found both large and small protrusions, which were not observed in the dynamic force mode, in air. The detailed force analysis suggested that these protrusions are nanobubbles located on the HOPG substrate exposed in holes or pits of the RhOEP layer.

AFM indentation method used for elastic modulus characterization of interfaces and thin layers

Atomic force microscopy (AFM) is increasingly being used as a nanoindentation tool to measure local elastic properties of surfaces. In this article, a method based on AFM in force volume (force curve mapping) mode is employed to measure the elastic modulus distribution at the interface of a glass flake-reinforced polypropylene sample and at a lead-free Cu–solder joint. Indentation arrays are performed using a diamond AFM tip. The processing of experimental AFM indentation data is automated by customized software that can analyse and calibrate multiple force curves. The analysis algorithm corrects the obtained force curves by selecting the contact point, discarding the non-contact region and subtracting the cantilever deflection from the measured force curve in order to obtain true indentation curves. A Hertzian model is then applied to the resulting AFM indentation data. Reference materials are used to estimate the tip radius needed to extract the elastic modulus values. With the proposed AFM measurement method, we are able to obtain high-resolution maps showing elastic modulus variations around a composite interface and a Cu–solder joint. No distinct interphase region is detected in the composite case, whereas a separate intermetallic layer (1–2 μm thick) of much higher Young’s modulus (~131 GPa) than Cu and solder material is identified in the Cu–solder joint. Elastic modulus results obtained for the Cu (~72 GPa), solder (~50 GPa) and glass (~65 GPa) materials are comparable to the results obtained by instrumented indentation [~73, ~46 and ~61 GPa], which accentuates the potential of this method for applications requiring high lateral resolution.

Atomic force microscopy studies of living bacterial cells in native soil and permafrost

The most complicated problem in microbiology is to study microorganisms in the native environment. Native biotopes (soils etc.) are complex mixture of many environments that are very heterogenic and have different physicochemical, mineralogical and other properties. Microorganisms form and change the environmental during their life, as well as the environment act on cells. These interactions reflect on cell morphology and physiology. In our experiments we demonstrated the possibility to distinguish cells from mineral particles in active soil samples using multimode atomic force microscope. Wide variety of atomic force microscopy modes might help in this complicated task, namely: force curve mapping, measuring adhesion properties, study in liquid experiments as well as the treatment of cells and samples by various medical and chemical reagents. Some model investigations with soil bacteria strains in different physiological state should be done for adequate recognition and interpretation of microbial cells in native mineral environment using scanning probe microscopy.

AFM probing in aqueous environment of Staphylococcus epidermidis cells naturally immobilised on glass: Physico-chemistry behind the successful immobilisation

AFM probing of microbial cells in liquid environments usually requires them to be physically or chemically attached to a solid surface. The fixation mechanisms may influence the nanomechanical characterization done by force curve mapping using an AFM. To study the response of a microbial cell surface to this kind of local measurement this study attempts to overcome the problem associated to the uncertainties introduced by the different fixation treatments by analysing the surface of Staphylococcus epidermidis cells naturally (non-artificially mediated) immobilised on a glass support surface. The particularities of this natural bacterial fixation process for AFM surface analysis are discussed in terms of theoretical predictions of the XDLVO model applied to the systems bacteria/support substratum and bacteria/AFM tip immersed in water. In this sense, in the first part of this study the conditions for adequate natural fixation of three S. epidermidis strains have been analyzed by taking into account the geometries of the bacterium, substrate and tip. In the second part, bacteria are probed without the risk of any possible artefacts due to the mechanical or chemical fixation procedures. Forces measured over the successfully adhered cells have (directly) shown that the untreated bacterial surface suffers from a combination of both reversible and non-reversible deformations during acquisition of force curves all taken under the same operational conditions. This is revealed directly through high-resolution tapping-mode imaging of the bacterial surface immediately following force curve mapping. The results agree with the two different types of force curves that were repeatedly obtained. Interestingly, one type of these force curves suggests that the AFM tip is breaking (rather than pushing) the cell surface during acquisition of the force curve. In this case, adhesive peaks were always observed, suggesting a mechanical origin of the measured pull-off forces. The other type of force curves shows no adhesive peaks and exhibits juxtaposing of approaching and retraction curves, reflecting elastic deformations.

Investigating the Interfacial Properties of Single-Liquid Nanodroplets by Atomic Force Microscopy

The interaction of a liquid with a surface is of fundamental importance to many areas of scientific research. Information on the properties of liquids at surfaces at the microscopic scale is also required to elucidate the mechanisms behind macroscopic observations. In this study, the atomic force microscope (AFM) was employed in various complementary modes of operation to investigate the properties of nanometer-scale oil droplets existing on a polystyrene surface. Force curve mapping was used to gently probe the surface of the fluid droplets, and through automated analysis of the force curves the true topography and microscopic contact angle of the droplets were extracted. The contact angle was found to be 13 ± 2°, which is in excellent agreement with values obtained from macroscopic measurements. A discrepancy between these values and contact angle values obtained from tapping mode images were attributed to deformation of the soft interface. The resultant phase contrast mechanisms in the liquid tapping m...

Mapping the Surface Characteristics of Polystyrene Microtiter Wells by a Multimode Scanning Force Microscopy Approach

Polystyrene microtiter wells are commonly employed as substrates for immunoassays and for molecular screening. The full characterization of such substrates is therefore critical for their optimal use in such settings. In this study, various scanning force microscopy analysis modes are employed for their surface characterization. This is achieved through the spatial mapping of probe–sample interactions via lateral force microscopy, force–curve mapping, phase imaging, and pulsed force microscopy. Previous surface analytical studies had indicated the presence of a volatile hydrocarbon contaminant on well surfaces, which resulted in areas resistant to protein adsorption. These studies, however, were unable to directly determine the contaminant's dimensions or its distribution over the substrate. The obtained results confirm the presence of discrete patches of a hydrophobic material on the well surface, which additionally are found to behave as a viscous fluid. Such findings concur with the predictions made in the previous study and illustrate the potential benefits of a combined surface analytical approach.

Artifacts in Adhesion Force Images Obtained by Force Curve Mapping

We applied force curve mapping to chromosomes. Two types of artifacts were observed in the adhesion force images thus obtained. Location shifting between the adhesion and topographic images and deformation of the measured adhesive structure were seen. These artifacts are explained by theoretical analysis to be the result of cantilever tip sliding due to cantilever deflection during the adhesion force measurements. Analysis also showed that this tip sliding caused a small, highly adhesive region to escape measurement. The artifacts observed are not small enough to be negligible for molecular-size applications, and therefore, possible solutions to eliminate them are discussed.

Data processing in force curve mapping

Force curve mapping is expected to become a useful technique for material analysis. In order to analyze the huge amount of force curve data involved in such mapping, we have developed specialized data processing software. This software can extract three types of surface properties: adhesion force, topographic height and elasticity. We applied our data processing technique to force curve mapping of, first, particles on a substrate and, second, a silicon nitride membrane window. Our data processing successfully extracted those surface properties for these samples, demonstrating its effectiveness.

Data Processing in the Extraction of Properties from Force Curves for Mapping

Properties extracted from force curves obtained by atomic force microscopy (AFM) conducted at various locations on a sample can be mapped to determine their distribution over the sample surface, that is, force curve mapping. Since a huge amount of force curve data must be handled in such mapping, automatic force curve analysis is required. We have developed an algorithm which makes use of a differentiated force curve having peaks at the snap-in and snap-out points of the original force curve and negative values when the AFM tip is in contact with the sample surface. This algorithm was applied to various samples.

Scanning Probe Microscope Study of Mixed Chain-Length Phase-Segregated Langmuir-Blodgett Monolayers

ABSTRACTUsing a scanning probe microscope, the influence of adhesion on measured film height of mixed-chain-length fatty acid Langmuir-Blodgett monolayers was investigated. A 1:1 mixture of CH3[CH2]22COOH and CH3[CH2]14COOH, was deposited in which the long chain-length acid, C24, was segregated from the short chain-length acid, C16. Two experiments were performed, contact scanning and force curve mapping. From both experiments, the film height difference measured between the C16 and C24 regions was larger than expected. Adhesion of the tip was 20% larger over the C16 acid regions. Using a compressional modulus derived from film pressure vs. area isotherms of the pure components, the adhesion difference and apparent filmheight difference over the two regions can be understood.

Design and calibration of a scanning force microscope for friction, adhesion, and contact potential studies

We present the design and calibration of a scanning force microscope which can be used to study friction, adhesion, and contact potential differences between the cantilever tip and surface. The microscope uses a modular design where the laser, cantilever/sample holder, reflecting mirror, and detector are mounted directly on an optical table. The laser, reflecting mirror, and detector are mounted on translation and rotation stages. With this design the components can be rearranged to calibrate the Z piezo motion as a function of applied voltage. Using the detector micrometers, the detector response (voltage-to-distance relationship) can be determined after each series of measurements. The cantilever/sample holder is constructed such that the components are material matched and thermally compensated from a common reference point. This design feature minimizes thermal drift of the instrument. The instrument can be used in a contact scanning mode where both normal and lateral deflections of the cantilever are measured. In addition, the instrument can be used in frictional force studies, force curve mapping of the surface, and contact potential measurements. We present examples of each, including a detailed account of the instrument design and calibration.