Force Curve Analysis Research Articles

Exploring the Molecular-Scale Structures at Solid/Liquid Interfaces of Li-Ion Battery Materials: A Force Spectroscopy Analysis with Sparse Modeling.

In this study, we clarify the liquid structure formed at the interface between LiCoO2 (LCO), the cathode material of Li-ion batteries, and propylene carbonate (PC), which is used as a solvent in the electrolyte, on a molecular scale. We apply sparse modeling-based modal analysis to force spectroscopy data measured by frequency modulation atomic force microscopy (FM-AFM) and show that each component in the FM-AFM force curve, such as oscillatory solvation force, background, and noise, can be automatically decomposed. Moreover, by combining detailed force curve analysis with solid/liquid interface simulations based on first-principles calculation, we have identified that there are distinct damped vibrational modes in the force curves at the LCO/PC interface with a period of about 0.57 nm and those with shorter periods, which likely correspond to the solvation forces associated with bulk-state PC molecules and those with PC molecules in "lying down" orientations.

Relationship between changes in interface characteristics and external voltage under compressing force in metal–graphene–metal stacks

Metal–graphene–metal (M–G–M) stacks have provided new specific methods for the integration of two-dimensional (2D) materials into three-dimensional (3D) electronic devices, such as transistors, supercapacitors, memristors and others. Intentional control of the local electronic properties in the stacks is the primary problem to be solved when developing hybrid structures with 2D and 3D elements. In order to do this, the physical mechanisms and processes that define the properties of the stacks must be thoroughly understood. This report introduces an approach based on the force curve analysis in terms of the interface characteristics in the M–G–M stack. In the study, the stack was produced by pressing the platinum probe of a scanning probe microscope into the surface of a gold-supported graphene monolayer, and the force curve was measured under applied direct current voltage. Based on the model of the van der Waals contact, the equilibrium interface distances and the built-in potential were obtained from the experimental results. The equilibrium state was proved to be weakly dependent on the applied voltage below a threshold level. Above this level, irreversible changes and the effect of the history of the tests were detected. The proposed method is acceptable to quantitatively describe the most essential characteristics in a local area of the M–G–M stack, essential for the construction of nano-scaled electronic devices.

Direct Visualization of Interfacial Regions between Fillers and Matrix in Rubber Composites Observed by Atomic Force Microscopy-Based Nanomechanics Assisted by Electron Tomography.

In order to explain or predict the macroscopic mechanical properties of polymer composites with complex nanostructures, atomic force microscopy (AFM)-based nanomechanics is one of the most appropriate tools because the local mechanical properties can be obtained by it. However, automatic force curve analysis based on contact mechanics would mislead us to the wrong conclusion. The purpose of this study is to elucidate this point by applying AFM nanomechanics on a carbon black (CB)-reinforced isoprene rubber (IR). The CB aggregates underneath the rubber surface prevent us from quantitatively evaluating the ratio of CB and interfacial polymer region (IPR), which is an important parameter to determine the macroscopic mechanical properties. In order to overcome this problem, transmission electron microtomography was incorporated to investigate the 3D structure in the same field of view as AFM nanomechanics. As a result, it was found that there are buried structures that do not appear in the AFM topographic image. In addition, we were able to reveal the existence of a force curve with an inflection point, which is characteristic of such "false" IPRs. To put it another way, we evidenced the existence of true IPRs for the first time by combining these state-of-the-art techniques.

Effects of Abrasive Particles on Liquid Superlubricity and Mechanisms for Their Removal.

Liquid superlubricity results in a near-frictionless lubrication state, which can greatly reduce friction and wear under aqueous conditions. However, during the running-in process, a large number of abrasive particles are generated, and because these may lead to a breakdown in superlubricity performance, they should be effectively removed. In this paper, the morphology, size, and composition of abrasive particles were verified using scanning electron microscopy with energy-dispersive X-ray spectroscopy, and their influence on liquid superlubricity was explored through friction tests. Subsequently, different solvents were used to remove the abrasive particles, and the optimal cleaning process was determined by macroscopic tribo-tests and microscopic analysis. Finally, droplet-spreading experiments and a force-curve analysis were carried out to understand the abrasive-particle removal mechanism by different solvents. We found that SiO2 was the main component in the abrasive particles, and micron-sized SiO2 particles resulted in random "wave peaks" in the coefficient of friction and, thus, the superlubricity. Absolute ethanol + ultrapure water was determined to be the optimal solvent for effectively removing abrasive particles from friction-pair surfaces and helped the lubricant in exhibiting an ultralow friction coefficient for long periods of time. We proposed a "wedge" and "wrap" model to explain the abrasive-particle removal mechanism of different solvents. The SiO2 removal mechanism outlined in this study can be applied under aqueous conditions to improve the stability and durability of liquid superlubricity in practical engineering applications.

Quantitative mapping of high modulus materials at the nanoscale: comparative study between atomic force microscopy and nanoindentation.

Local mechanical properties of submicron features are of particular interest due to their influence on macroscopic material performance and behaviour. This study is focused on local nanomechanical measurements, based on the latest Atomic Force Microscopy (AFM) mode, where the peak force set point is finely controlled at each pixel. After probe calibration, we evaluate the impact of spring constant of two AFM hand-crafted natural full diamond tips with steel cantilevers, used for mapping. Based on the fast capture of the cantilever deflection at each pixel and real time force curve analysis in the elastic region, AFM local measured contact moduli mappings of the silica beads (>50GPa) incorporated in an epoxy resin matrix, are compared with those determined using classical instrumented nanoindentation tests. Our analyses show that with the two AFM probes, without local residual deformation, the high moduli of the silica beads measured with this advanced AFM mode are within the standard deviation of the values determined by classical nanoindentation. LAY DESCRIPTION: The knowledge of material properties at the nanometer scale is a key parameter for well understanding and determining the behavior of material at macroscopic scale. In this paper, we compare two methods (an advanced mode and a classical one) based on the analysis of probes in interaction with the surface of studied material. We focus on a latest developed mode for determining local mechanical properties with a very high spatial resolution. For the advanced mode, we also consider two different hand-crafted probes. Our analyses show that with the high spatial resolution advanced mode, local mechanical properties are well determined. We also highlight the impact of the properties of the used probes for this advanced mode. In a final step, the power of the presented investigation lies in the fact that it does not modify the topography of the surface.

Scanning Probe-Directed Assembly and Rapid Chemical Writing Using Nanoscopic Flow of Phospholipids.

Nanofluidic systems offer a huge potential for discovery of new molecular transport and chemical phenomena that can be employed for future technologies. Herein, we report on the transport behavior of surface-reactive compounds in a nanometer-scale flow of phospholipids from a scanning probe. We have investigated microscopic deposit formation on polycrystalline gold by lithographic printing and writing of 1,2-dioleoyl-sn-glycero-3-phosphocholine and eicosanethiol mixtures, with the latter compound being a model case for self-assembled monolayers (SAMs). By analyzing the ink transport rates, we found that the transfer of thiols was fully controlled by the fluid lipid matrix allowing to achieve a certain jetting regime, i.e., transport rates previously not reported in dip-pen nanolithography (DPN) studies on surface-reactive, SAM-forming molecules. Such a transport behavior deviated significantly from the so-called molecular diffusion models, and it was most obvious at the high writing speeds, close to 100 μm s-1. Moreover, the combined data from imaging ellipsometry, scanning electron microscopy, atomic force microscopy (AFM), and spectroscopy revealed a rapid and efficient ink phase separation occurring in the AFM tip-gold contact zone. The force curve analysis indicated formation of a mixed ink meniscus behaving as a self-organizing liquid. Based on our data, it has to be considered as one of the co-acting mechanisms driving the surface reactions and self-assembly under such highly nonequilibrium, crowded environment conditions. The results of the present study significantly extend the capabilities of DPN using standard AFM instrumentation: in the writing regime, the patterning speed was already comparable to that achievable by using electron beam systems. We demonstrate that lipid flow-controlled chemical patterning process is directly applicable for rapid prototyping of solid-state devices having mesoscopic features as well as for biomolecular architectures.

Adhesion properties of polyacrylic block copolymer pressure‐sensitive adhesives and analysis by pulse NMR and AFM force curve

ABSTRACTThe adhesion strength of a pressure‐sensitive adhesive (PSA) is influenced by two factors, the interfacial adhesion and the cohesive strength. A suitable method for the estimation of these two factors was investigated. Blends of triblock and diblock copolymers consisting of poly(methyl methacrylate) (hard) and poly(n‐butyl acrylate) (soft) blocks (A) and blends of triblock copolymer and poly(n‐butyl acrylate) oligomer (B), both with different blend ratios, were prepared as model PSAs. The peel strength decreased with an increase in the hard block content for B, whereas it was independent for A. The tack increased with a decrease in the hard block content for A, whereas it was independent for B. The influence of the hard block content on the peel strength and tack was thus different. The 1H pulse nuclear magnetic resonance analysis and force curve analysis showed that the molecular mobility was higher for B than for A. The Young's modulus and adhesive energy calculated by the Johnson–Kendall–Roberts two‐point method using the atomic force microscopy (AFM) force curve qualitatively reflected the cohesive strength and the interfacial adhesion, respectively. The Young's modulus and adhesive energy are found to be useful parameters to investigate the adhesion mechanism. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47791.

Multi-frequency data analysis in AFM by wavelet transform

Interacting cantilevers in AFM experiments generate non-stationary, multi-frequency signals consisting of numerous excited flexural and torsional modes and their harmonics. The analysis of such signals is challenging, requiring special methodological approaches and a powerful mathematical apparatus. The most common approach to the signal analysis is to apply Fourier transform analysis. However, FT gives accurate spectra for stationary signals, and for signals changing their spectral content over time, FT provides only an averaged spectrum. Hence, for non-stationary and rapidly varying signals, such as those from interacting cantilevers, a method that shows the spectral evolution in time is needed. One of the most powerful techniques, allowing detailed time-frequency representation of signals, is the wavelet transform. It is a method of analysis that allows representation of energy associated to the signal at a particular frequency and time, providing correlation between the spectral and temporal features of the signal, unlike FT. This is particularly important in AFM experiments because signals nonlinearities contains valuable information about tip-sample interactions and consequently surfaces properties. The present work is aimed to show the advantages of wavelet transform in comparison with FT using as an example the force curve analysis in dynamic force spectroscopy.

Measuring the viscoelastic creep of soft samples by step response AFM.

We have measured the creep response of soft gels and cells after applying a step in loading force with atomic force microscopy (AFM). By analysing the creep response data using the standard linear solid model, we can quantify the viscous and elastic properties of these soft samples independently. Cells, in comparison with gels of similar softness, are much more viscous, as has been qualitatively observed in conventional force curve data before. Here, we quantify the spring constant and the viscous damping coefficient from the creep response data. We propose two different modes for applying a force step: (1) indirectly by increasing the sample height or (2) directly by employing magnetic cantilevers. Both lead to similar results, whereas the latter seems to be better defined since it resembles closely a constant strain mode. The former is easier to implement in most instruments, and thus may be preferable from a practical point of view. Creep analysis by step response is much more appropriate to analyse the viscoelastic response of soft samples like cells than the usually used force curve analysis.

Decoding Cytoskeleton-Anchored and Non-Anchored Receptors from Single-Cell Adhesion Force Data

Complementary to parameters established for cell-adhesion force curve analysis, we evaluated the slope before a force step together with the distance from the surface at which the step occurs and visualized the result in a two-dimensional density plot. This new tool allows detachment steps of long membrane tethers to be distinguished from shorter jumplike force steps, which are typical for cytoskeleton-anchored bonds. A prostate cancer cell line (PC3) immobilized on an atomic-force-microscopy sensor interacted with three different substrates: collagen-I (Col-I), bovine serum albumin, and a monolayer of bone marrow-derived stem cells (SCP1). To address PC3 cells’ predominant Col-I binding molecules, an antibody-blocking β1-integrin was used. Untreated PC3 cells on Col-I or SCP1 cells, which express Col-I, predominantly showed jumps in their force curves, while PC3 cells on bovine-serum-albumin- and antibody-treated PC3 cells showed long membrane tethers. The probability density plots thus revealed that β1-integrin-specific interactions are predominately anchored to the cytoskeleton, while the nonspecific interactions are mainly membrane-anchored. Experiments with latrunculin-A-treated PC3 cells corroborated these observations. The plots thus reveal details of the anchoring of bonds to the cell and provide a better understanding of receptor-ligand interactions.

Elastic modulus of low-density lipoprotein as potential indicator of its oxidation.

Evaluation of low-density lipoprotein oxidation is important in the risk assessment of cardiovascular disease. Atomic force microscope is widely used to evaluate the physical properties including stiffness on a single-particle scale. In this study, the effect of low-density lipoprotein oxidation on the low-density lipoprotein stiffness was investigated using an atomic force microscope. Isolated low-density lipoprotein particles with or without oxidation were densely bound to an Au substrate on mica, and then pressed and deformed by the atomic force microscope tip. The stiffness of each low-density lipoprotein particle was estimated as the elastic modulus obtained by the force curve analysis. Biochemical change of low-density lipoprotein due to oxidation was studied by electrophoresis. The elastic modulus of low-density lipoprotein particles ranged between 0.1 and 2 MPa. The oxidation of low-density lipoprotein increased the number of low-density lipoprotein particles with smaller elastic moduli, indicating the decrease in low-density lipoprotein stiffness. The elastic modulus of low-density lipoprotein might be potentially useful to evaluate low-density lipoprotein oxidation.

The Influence of Sintering Temperature and Mn-Ti Doped BaFe<sub>12</sub>O<sub>19</sub> on the Microwave Absorbing Properties

The effect of sintering temperature and Mn-Ti (x = 0.1 and 0.5 % mol) doped in Barium hexaferrite (BaFe12O19) on the microwave absorption properties have been investigated. The BaFe(12-2x)MnxTixO19samples were prepared by using a solid state reaction method from technical grade materials of BaCO3, Fe2O3, MnO2, and TiO2. The permagraph and vector network analyzer (VNA) measurement with frequency of 4 – 10 GHz were used to determine the magnetic properties and microwave absorbing properties, respectively. The magnetic induction and magnetizing force (B-H) curve analysis of BaFe(12-2x)MnxTixO19revealed that by increasing the Mn-Ti dopant concentrations, the remanence (Br) and BH(max) increases, meanwhile, the HCBvalue decreases for both 1100 and 1150oC. The frequency dependence of reflection loss (RL) measurement given anoptimum value of-26.15 dB at frequency of 9.18 GHz which obtained by x = 0.1% mol Mn-Ti (1100oC). The maximum absorption peak was achieved for 0.5 % mol Mn-Ti sample with ~98.3 % at 9.18 GHz. By controlling the sintering temperature and Mn-Ti dopant concentration, the RL and absorption behavior can be modified to desirable values and also indicates that higher Mn-Ti dopant concentration (0.5% mol) possess better absorbing characteristics. Furthermore, it was found that complex permitivitty and permeability values of BaFe12O19can be modified by controlling the Mn-Ti dopant concentration.

M710 分子間相互作用測定におけるAFMフォースカーブ解析手法の検討(GS2,3 材料力学(9),修士研究発表セッション)

M710 分子間相互作用測定におけるAFMフォースカーブ解析手法の検討(GS2,3 材料力学(9),修士研究発表セッション)

Automated AFM force curve analysis for determining elastic modulus of biomaterials and biological samples

The analysis of atomic force microscopy (AFM) force data requires the selection of a contact point (CP) and is often time consuming and subjective due to influence from intermolecular forces and low signal-to-noise ratios (SNR). In this report, we present an automated algorithm for the selection of CPs in AFM force data and the evaluation of elastic moduli. We propose that CP may be algorithmically easier to detect by identifying a linear elastic indentation region of data (high SNR) rather than the contact point itself (low SNR). Utilizing Hertzian mechanics, the data are fitted for the CP. We first detail the algorithm and then evaluate it on sample polymeric and biological materials. As a demonstration of automation, 64 × 64 force maps were analyzed to yield spatially varying topographical and mechanical information of cells. Finally, we compared manually selected CPs to automatically identified CPs and demonstrated that our automated approach is both accurate (< 10nm difference between manual and automatic) and precise for non-interacting polymeric materials. Our data show that the algorithm is useful for analysis of both biomaterials and biological samples.

Nanoscale Mechano–Electronic Behavior of a Metalloprotein as a Variable of Metal Content

In this work, we have explored an approach to finding a correlation between the mechanical response of a metalloprotein against a range of applied force (by force curve analysis) and its electrical response under pressure stimulation (by current sensing atomic force spectroscopy) at the nanoscale. Iron-storage protein ferritin has been chosen as an experimental model system because it naturally contains a semiconducting iron core. This core consists of a large number of iron atoms and is therefore expected to exert a clear influence on the overall mechanical response of the protein structure. Four different ferritins (apoferritin, Fe(III)-ferritins containing ~750 and ~1400 iron atoms, and holoferritin containing ~2600 iron atoms) were chosen in order to identify any relation between the mechano-electronic behavior of the ferritins and their metal content. We report the measurement of Young's modulus values of the ferritin proteins as applicable in a nanoscale environment, for the first time, and show that these values are directly linked to the iron content of the individual ferritin type. The greater the iron content, the greater the Young's modulus and in general the slower the rate of deformation against the application of force. When compressed, all the four ferritins exhibited increased electronic conductivity. A correlation between the iron content of the ferritins and the current values observed at certain bias voltages could be made at higher bias values (beyond 0.7 V), but no such discrimination among the four compressed ferritins could be made at the lower voltages. We propose that only at higher voltages can the iron atoms that reside deeper inside the core of the ferritins be accessed. The iron atoms that could be situated at the inner wall of the protein shell appear to make a general contribution to the electronic conductivity of the four ferritin systems.

Differences in Nanoscale Elasticity of Planar and Nanofibrillar Tissue Cultures

ABSTRACTAstrocytes are cellular bridges between the neurons and capillaries in the blood brain barrier. It was recently suggested that the nanophysical properties of the basement membrane of the blood brain barrier can influence astrocyte and neuron responses. In this work, cerebral cortical astrocytes were cultured on standard poly-L-Lysine coated glass substrates, Aclar substrates, and electrospun polyamide nanofibers whose properties may recapitulate those of the basement membrane. The nanoscale elasticity of each culture environment was investigated by force curve analysis and compared. The elasticity of the individual nanofibers on nanofibrillar surfaces was also investigated. Finally, variations in elasticity of scaffolds were correlated with astrocyte responses.

Adsorption of bovine serum albumin on amorphous carbon surfaces studied with dip pen nanolithography

This article reports the use of dip pen nanolithography (DPN) for the study of adsorption of bovine serum albumin (BSA) proteins on amorphous carbon surfaces; tetrahedral amorphous carbon (t-aC) and silicon doped hydrogenated amorphous carbon (a-C:H:Si). Contact angle study shows that the BSA proteins reduce the contact angle on both carbon materials. We also noticed that the drop volume dependence is consistent with a negative line tension, i.e. due to an attractive protein/surface interaction. The DPN technique was used to write short-spaced (100 nm) BSA line patterns on both samples. We found a line merging effect, stronger in the case of the a-C:H:Si material. We discuss possible contributions from tip blunting, scratching, cross-talk between lever torsion and bending and nano-shaving of the patterns. We conclude that the observed effect is caused in large measure by the diffusion of BSA proteins on the amorphous carbon surfaces. This interpretation of the result is consistent with the contact angle data and AFM force curve analysis indicating larger tip/surface adhesion and spreading for the a-C:H:Si material. We conclude by discussing the advantages and limitations of DPN lithography to study biomolecular adsorption in nanoscale wetting environments.

Atomic Force Microscopy Force Mapping in the Study of Supported Lipid Bilayers

Investigating the structural and mechanical properties of lipid bilayer membrane systems is vital in elucidating their biological function. One route to directly correlate the morphology of phase-segregated membranes with their indentation and rupture mechanics is the collection of atomic force microscopy (AFM) force maps. These force maps, while containing rich mechanical information, require lengthy processing time due to the large number of force curves needed to attain a high spatial resolution. A force curve analysis toolset was created to perform data extraction, calculation and reporting specifically in studying lipid membrane morphology and mechanical stability. The procedure was automated to allow for high-throughput processing of force maps with greatly reduced processing time. The resulting program was successfully used in systematically analyzing a number of supported lipid membrane systems in the investigation of their structure and nanomechanics.

Using Grip Strength Force Curves to Detect Simulation: A Preliminary Investigation

Analysis of grip strength force curves has successfully detected suboptimal effort in industrial rehabilitation research. This study examined force curve analysis as an effort measure when grip strength was administered according to standardized neuropsychological procedures in a sample without reported neurologic and upper extremity injury. Eighty-two undergraduates were randomized to control (n = 26), naïve simulator (n = 28), and coached simulator (n = 28) conditions. Outcome measures included grip strength in kilograms, variables calculated from grip strength force curves, and the Word Memory Test (WMT). While average force in kilograms was not significantly different between groups, significant differences were found on the average to peak force ratio as calculated from grip strength force curves. The classification accuracy of average to peak force ratio was lower than the WMT, but comparable to other effort measures. Force curve analysis may warrant further study in a clinical sample.

A closer look at the complex hydrophilic/hydrophobic interactions forces at the human hair surface

The complex chemical structure of the hair surface is far from being completely understood. Current understanding is based on Rivett's model1 that was proposed to explain the macroscopic hydrophobic nature of the surface of natural hair. In this model covalently-linked fatty acids are chemically grafted to the amorphous protein (keratin) through a thio-ester linkage2,3. Nevertheless, experience like wetting and electrical properties of human hair surface4 shows that the complexity of the hair surface is not fully understand based on this model in literature. Recent studies in our laboratory show for the first time microscopic evidence of the heterogeneous physico-chemical character of the hair surface. By using Chemical Force Microscopy, the presence of hydrophobic and ionic species are detected and localized, before and after a cosmetic treatment (bleaching). Based on force curve analysis the mapping of the local distribution of hydrophilic and hydrophobic groups of hair surface is obtained. A discussion on a more plausible hair model and its implications will be presented based on these new results.