Abstract Atomic Force Microscopy Research Articles

Spring constant of an AFM cantilever with a thin-film plasmonic waveguide formed at its end

Abstract Atomic-force-microscope (AFM)-based tip-enhanced Raman spectroscopy (TERS) is a promising analytical technique that can identify the physical and chemical properties of a sample’s surface. In the conventional TERS setup, the tip is directly irradiated by an incident light, which causes degradation of the contrast of the TERS signal due to the Raman scattered light from the surface area around the tip. We recently developed an AFM cantilever for indirect illumination AFM-TERS by milling the tip of the conventional cantilever to form a thin-film waveguide. Since the thin-film waveguide is considered as another cantilever attached at the end of the original cantilever, the waveguide cantilever can be treated as cantilevers connected in series. We then analyzed the static spring constant of the waveguide cantilever by both analytical and numerical methods and found that the static spring constant of the waveguide cantilever is lower than that of the original cantilever, which is advantageous in reducing the contact damage during the TERS measurements. We also proposed procedures to experimentally calibrate the static spring constant of the waveguide cantilever.

Quantitative Surface Morphology of Ammonia CF. Beccarii and Ammonia Parkinsoniana by Atomic Force Microscopy: Taxonomic Potential

Abstract Atomic Force Microscopy (AFM) is used to quantify the surface morphology of two benthic foraminiferal species. Scanning Electron Microscopy (SEM) is traditionally used for surface ultrastructural studies but cannot estimate three-dimensional measurements. Therefore, AFM is used to measure pore depth and surface roughness, as well as two-dimensional features such as pore density, pore diameter, and porosity. Specimens of Ammonia cf. beccarii and Ammonia parkinsoniana have been analyzed in this study. The species have been compared in this pilot study by mapping the test structure in three dimensions. The two species have a distinct topographical contrast and significant differences in pore density, diameter, porosity, depth, and surface roughness. It is demonstrated here that AFM can be used to understand the morphological differences with finer details between the species of foraminifera. The effect of the environment on the estimated surface parameters should be investigated.

Electric-Field and Mechanical Vibration-Assisted Atomic Force Microscope-Based Nanopatterning

Abstract Atomic force microscope (AFM)-based nanolithography is a cost-effective nanopatterning technique that can fabricate nanostructures with arbitrary shapes. However, existing AFM-based nanopatterning approaches have limitations in the patterning resolution and efficiency. Minimum feature size and machining performance in the mechanical force-induced nanofabrication process are limited by the radius and sharpness of the AFM tip. Electric-field-assisted atomic force microscope (E-AFM) nanolithography can fabricate nanopatterns with features smaller than the tip radius, but it is very challenging to find the appropriate input parameter window. The tip bias range in E-AFM process is typically very small and varies for each AFM tip due to the variations in tip geometry, tip end diameter, and tip conductive coating thickness. This paper demonstrates a novel electric-field and mechanical vibration-assisted AFM-based nanofabrication approach, which enables high-resolution (sub-10 nm toward sub-5 nm) and high-efficiency nanopatterning processes. The integration of in-plane vibration with the electric field increases the patterning speed, broadens the selectable ranges of applied voltages, and reduces the minimum tip bias required for nanopatterning as compared with E-AFM process, which significantly increases the versatility and capability of AFM-based nanopatterning and effectively avoids the tip damage.

AI-based atomic force microscopy image analysis allows to predict electrochemical impedance spectra of defects in tethered bilayer membranes

Atomic force microscopy (AFM) image analysis of supported bilayers, such as tethered bilayer membranes (tBLMs) can reveal the nature of the membrane damage by pore-forming proteins and predict the electrochemical impedance spectroscopy (EIS) response of such objects. However, automated analysis involving pore detection in such images is often non-trivial and can require AI-based object detection techniques. The specific object-detection algorithm we used to determine the defect coordinates in real AFM images was a convolutional neural network (CNN). Defect coordinates allow to predict the EIS response of tBLMs populated by the pore-forming toxins using finite element analysis (FEA) modeling. We tested if the accuracy of the CNN algorithm affected the EIS spectral features sensitive to defect densities and other physical parameters of tBLMs. We found that the EIS spectra can be predicted sufficiently well, however, systematic errors of characteristic spectral points were observed and need to be taken into account. Importantly, the comparison of predicted EIS curves with experimental ones allowed to estimate important physical parameters of tBLMs such as the specific resistance of submembrane reservoir. This reservoir separates phospholipid bilayer from the solid support. We found that the specific resistance of the reservoir amounts to 10^{4.25 pm 0.10}Omega cdot cm which is approximately two orders of a magnitude higher compared to the specific resistance of the buffer bathing tBLMs studied in this work. We hypothesize that such effect may be related in part due to decreased concentration of ionic carriers in the submembrane due to decreased relative dielectric permittivity in this region.

Fitting of Atomic Force Microscopy Force Curves with a Sparse Representation Model

Atomic force microscopy (AFM) is a high-resolution scanning technology, and the measured data are a set of force curves, which can be fitted with a piecewise curve model and be analyzed further. Most methods usually follow a two-step strategy: first, the discontinuities (or breakpoints) are detected as the boundaries of two consecutive pieces; second, each piece separated by the discontinuities is fitted with a parametric model, such as the well-known worm-like chain (WLC) model. The disadvantage of this method is that the fitting (the second step) accuracy depends largely on the discontinuity detection (the first step) accuracy. In this study, a sparse representation model is proposed to jointly detect discontinuities and fit curves. The proposed model fits the curve with a linear combination of parametric functions, and the estimation of the parameters in the model can be formulated as an optimization problem with ℓ 0 -norm constraint. The performance of the proposed model is demonstrated by the fitting of AFM retraction force curves with the WLC model. Results shows that the proposed method can segment the force curve and estimate the parameter jointly with better accuracy, and hence, it is promising for automatic AFM force curve processing.

Environmental temperature effect on dimensional measurements of atomic force microscopy

Atomic force microscopy (AFM) is increasingly being used as a fundamental tool for dimensional measurements at the nanoscale in the laboratory and in industry. Since the environmental temperature is not controlled in many measurements, or is even varied on purpose, quantification of its effects on AFM dimensional measurements is needed. In this paper, the influences of the temperature in the entire environment of the AFM (excluding only the controller and computer) and that in the local environment around the tip–sample are investigated. The results show that lateral dimensional measurements are affected mainly by the entire environmental temperature. However, vertical measurements are influenced by the temperature of both the entire environment and the local environment. The effects become significant for temperatures higher than some threshold, here between 35 and 40 °C.

Simultaneous acquisition of current and lateral force signals during AFM for characterising the piezoelectric and triboelectric effects of ZnO nanorods

Atomic force microscopy (AFM) is central to investigating the piezoelectric potentials of one-dimensional nanomaterials. The AFM probe is used to deflect individual piezoelectric nanorods and to measure the resultant current. However, the torsion data of AFM probes have not been exploited to elucidate the relationship between the applied mechanical force and resultant current. In this study, the effect of the size of ZnO nanorods on the efficiency of conversion of the applied mechanical force into current was investigated by simultaneously acquiring the conductive AFM and lateral force microscopy signals. The conversion efficiency was calculated based on linear regression analysis of the scatter plot of the data. This method is suitable for determining the conversion efficiencies of all types of freestanding piezoelectric nanomaterials grown under different conditions. A pixel-wise comparison of the current and lateral force images elucidated the mechanism of current generation from dense arrays of ZnO nanorods. The current signals generated from the ZnO nanorods by the AFM probe originated from the piezoelectric and triboelectric effects. The current signals contributed by the triboelectric effect were alleviated by using an AFM probe with a smaller spring constant and reducing the normal force.

A dynamical approach to topography estimation in atomic force microscopy based on smooth orthogonal decomposition

Atomic force microscope (AFM) is one of the most versatile and powerful devices capable of producing high-resolution images of nanomaterial. Many researchers are widely investigating to improve the scanning speed and image quality of AFM by proposing different techniques. Here, we aim to present a novel approach based on the smooth orthogonal decomposition for the estimation of the surface topography in AFM. The technique proposed in this research not only eliminates the need for a closed-loop controller but also acquires the surface three-dimensional shape (topography) very quickly and accurately. The proposed technique relies on the fact that in the tapping mode of atomic force microscopy, the tip displacements are very fast compared to the topography changes, and the surface topography as a slowly varying parameter can be estimated using the smooth orthogonal decomposition algorithm. To this aim, the state space is reconstructed based on Takens’ theorem and used only the tip displacement measurement data. According to Takens’ theorem, using the delay time and embedding dimension parameters, we are able to create a system in which dynamical behaviors are similar to the original system. The results demonstrate that the proposed estimation approach is robust to noise and does not require large data or computational resources to be implemented. Also, the performance of the proposed method is appropriate for any type of force interactions between the tip and sample.

Кинетика взаимодействия зонда атомно-силового микроскопа с поверхностью неоднородного полимера в режиме быстрой наноиндентации

Atomic force microscopy (AFM) is a powerful tool for studying the structural and physical-mechanical properties of surfaces. The AFM experiment consists in detecting the interaction be-tween the probe and the material. The probe is an elastic cantilever fixed horizontally at one end; a tip is located at the free end of the cantilever. The interaction of the tip with the surface causes de-flection of the cantilever, which is interpreted depending on the experimental conditions. High-speed indentation atomic force microscopy methods allow obtaining maps of three-dimensional re-lief and physical-mechanical properties of the material in high resolution at micro- and nanoscale levels. In this case, the probe is rapidly pressed on the material and the interaction is recorded at each point of the surface of the studied area. The user sets the speed at which the probe approaches the material surface. This value is assumed to be constant and is called the indentation rate. How-ever, this is not true: the tip speed at the loose end of the cantilever depends in general on many factors. As an example of investigation of polyurethane surface (heterogeneous two-phase poly-mer) it is shown that the probe-material interaction speed nonlinearly depends on the surface area, indentation depth and direction, nominal speed of experiment, cantilever properties. This affects the measurement of relief topography, stiffness, Van der Waals forces and adhesion between the probe and the material. Thus, for a correct quantitative interpretation of the results, the interaction speed of the tip with the material must be taken into account; this statement is especially important when working with soft polymers. The presented methods can be useful in the study of a wide class of soft heterogeneous materials.

Probing fibronectin adsorption on chemically defined surfaces by means of single molecule force microscopy

Atomic force microscope (AFM) based single molecule force spectroscopy (SMFS) and a quartz crystal microbalance (QCM) were respectively employed to probe interfacial characteristics of fibronectin fragment FNIII8–14 and full-length fibronectin (FN) on CH3–, OH–, COOH–, and NH2-terminated alkane-thiol self-assembled monolayers (SAMs). Force-distance curves acquired between hexahistidine-tagged FNIII8–14 immobilised on trisNTA-Ni2+ functionalized AFM cantilevers and the OH and COOH SAM surfaces were predominantly ‘loop-like’ (76% and 94% respectively), suggesting domain unfolding and preference for ‘end-on’ oriented binding, while those generated with NH2 and CH3 SAMs were largely ‘mixed type’ (81% and 86%, respectively) commensurate with unravelling and desorption, and ‘side-on’ binding. Time-dependent binding of FN to SAM-coated QCM crystals occurred in at least two phases: initial rapid coverage over the first 5 min; and variably diminishing adsorption thereafter (5–70 min). Loading profiles and the final hydrated surface concentrations reached (~ 950, ~ 1200, ~ 1400, ~ 1500 ng cm−2 for CH3, OH, COOH and NH2 SAMs) were consistent with: space-filling ‘side-on’ orientation and unfolding on CH3 SAM; greater numbers of FN molecules arranged ‘end-on’ on OH and especially COOH SAMs; and initial ‘side-on’ contact, followed by either (1) gradual tilting to a space-saving ‘end-on’ configuration, or (2) bi-/multi-layer adsorption on NH2 SAM.

On the frequency dependence of viscoelastic material characterization with intermittent-contact dynamic atomic force microscopy: avoiding mischaracterization across large frequency ranges.

Atomic force microscopy (AFM) is a widely use technique to acquire topographical, mechanical, or electromagnetic properties of surfaces, as well as to induce surface modifications at the micrometer and nanometer scale. Viscoelastic materials, examples of which include many polymers and biological materials, are an important class of systems, the mechanical response of which depends on the rate of application of the stresses imparted by the AFM tip. The mechanical response of these materials thus depends strongly on the frequency at which the characterization is performed, so much so that important aspects of behavior may be missed if one chooses an arbitrary characterization frequency regardless of the materials properties. In this paper we present a linear viscoelastic analysis of intermittent-contact, nearly resonant dynamic AFM characterization of such materials, considering the possibility of multiple characteristic times. We describe some of the intricacies observed in their mechanical response and alert the reader about situations where mischaracterization may occur as a result of probing the material at frequency ranges or with probes that preclude observation of its viscoelastic behavior. While we do not offer a solution to the formidable problem of inverting the frequency-dependent viscoelastic behavior of a material from dynamic AFM observables, we suggest that a partial solution is offered by recently developed quasi-static force–distance characterization techniques, which incorporate viscoelastic models with multiple characteristic times and can help inform dynamic AFM characterization.

A Novel Topography and Elasticity Synchronous Measure Approach With an AFM

Atomic force microscopy (AFM) is a powerful tool to measure the topography or the mechanical properties of samples at the nanoscale. Currently, the method of force–distance curve analysis is usually adopted to obtain the mechanical properties by an AFM, which presents the drawbacks of long time consuming and impossibility to measure the topography and the mechanical properties simultaneously. To address these problems, this article proposes a novel topography/elasticity measurement approach, which efficiently obtains the topography and the mechanical properties of detected samples with a conventional AFM. To be specific, the set points for forward and backward scanning are carefully selected to be different, so as to yield different sample deformations and piezoactuator displacement, for the same point with forward and backward scanning. On this basis, a mathematical model is set up to simultaneously calculate the topography and Young’s modulus of the sample surface by carefully analyzing the obtained data from the scanning processes. In addition, considering the effect of $z$ -axis drift during the measurement process, an information fusion algorithm based on the least-squares method and the Kalman filter is proposed to successfully compensate for the drift and thus further improve the imaging quality. Experimental results are included to demonstrate the good performance of the proposed approach for topography and mechanical properties’ measurement.

Atomic force microscopy for cell mechanics and diseases

Abstract Atomic Force Microscopy (AFM) is a powerful technique widely employed in biophysics, for instance to study topography of living cells and cell mechanics. Cell mechanics is a very interesting, biophysical parameter of cells, because it is strongly changed by various cellular processes, for example during cell division, cell movement, differentiation, aging, and also various diseases. Since cancer is a prominent example of changes in mechanical properties of diseases, the concept of the mechanical fingerprint has developed, which makes it possible to distinguish between healthy and diseased cells. In this article we report on various studies of cell mechanics with the AFM. We will first give a brief introduction on AFM principles and operational modes and then we will report on some applications of AFM in the field of cellular biophysics, like discriminating between healthy and cancer cells, as well as distinguishing cancer cells at different stages of malignancy. Overall, we will show that AFM has made a significant contribution in studying the biophysics of cancer and the concept of mechanical fingerprints could find a wide variety of applications in biomedicine and medical diagnostics.

Current measurements in the intermittent-contact mode of atomic force microscopy using the Fourier method: a feasibility analysis.

Atomic force microscopy (AFM) is an important tool for measuring a variety of nanoscale surface properties, such as topography, viscoelasticity, electrical potential and conductivity. Some of these properties are measured using contact methods (static contact or intermittent contact), while others are measured using noncontact methods. Some properties can be measured using different approaches. Conductivity, in particular, is mapped using the contact-mode method. However, this modality can be destructive to delicate samples, since it involves continuously dragging the cantilever tip on the surface during the raster scan, while a constant tip–sample force is applied. In this paper we discuss a possible approach to develop an intermittent-contact conductive AFM mode based on Fourier analysis, whereby the measured current response consists of higher harmonics of the cantilever oscillation frequency. Such an approach may enable the characterization of soft samples with less damage than contact-mode imaging. To explore its feasibility, we derive the analytical form of the tip–sample current that would be obtained for attractive (noncontact) and repulsive (intermittent-contact) dynamic AFM characterization, and compare it with results obtained from numerical simulations. Although significant instrumentation challenges are anticipated, the modelling results are promising and suggest that Fourier-based higher-harmonics current measurement may enable the development of a reliable intermittent-contact conductive AFM method.

Characterizing AFM Tip Lateral Positioning Variability Through Non-Vector Space Control-Based Nanometrology

Atomic force microscopy (AFM) based nanotechnology has been widely implemented in various fields for decades in light of its overwhelming advantages, such as nanometer spatial resolution, adaptability to liquid ambient, and various nanomechanical/electrical metrological approaches. It is noted that though AFM possesses imaging capability up to nanometer resolution, it is hard to achieve nanometer level positioning precision due to the existing system variability, especially the thermal drift, which distorts AFM images through relatively long capturing time. Since an AFM image is typically utilized as a global reference map to navigate its tip to the desired locations for precise measurement and manipulation, the system variability distorted image will definitely diversify the experimental results. Therefore, it is necessary to characterize the positioning variability for better experimental results evaluation and decision-making. Although various approaches were proposed to evaluate AFM positioning error, to our best knowledge, there is little research about characterizing its positioning variability precisely and systematically. In this study, we present a universal metrological approach to quantitatively measure AFM tip locating variability by developing a featureless spiral local scan strategy together with the non-vector space (NVS) navigation approach. As a demonstration, the proposed nanometrology was conducted on a specific AFM platform to unravel its positioning property.

Effect of Osteoblasts Cell Adhesion Behavior on Biomaterial Surfaces by Atomic Force Microscope

Atomic force microscopy (AFM) is a great scientific invention that can visualize cell morphology in aqueous environment and provide information to investigate cell biomechanics at a high spatial resolution in a controlled environment with force sensitivity. Contemporary AFM techniques permit solving a number of problems of cell biomechanics due to synchronized evaluation of the local mechanical properties. For characterizing mechanical properties force spectroscopy is used that provides information on cellular structures including cytoskeleton structure and morphology. For the success of biomedical implant, the most crucial factor is biocompatibility and osteo-conductivity of the implant material that can be characterized by the change in mechanical properties of cellular filaments and nucleus determined by exploiting atomic force microscope techniques. Hydroxyapatite is a bioactive material in bio-ceramics, hence used for fillers, bone grafts and metallic implant coating. Recently developed HAp/amino acid fluorescent complexes could be a significant candidate to be used for dental implant, had shown antibacterial properties with visible light irradiation. This study aims at revealing murine osteoblasts cell (MC3T3) adhesion behavior on the HAp coating and HAp/amino acid complexes. The AFM revealed that no significant changes were observed in mechanical properties of the osteoblasts cells when adhered on electrochemically deposited HAp coating and HAp/amino acid ligands complex coating. SEM and EDX analysis revealed cell morphology were identical for HAp and HAp/amino acid ligands complex coating. Such characteristics are desirable for the success of implant biomaterial coating that can preserve both antibacterial property and cell adhesion behavior.

Probing Mechanical Characteristics of Living Fibroblasts via Atomic Force Microscopy

Atomic force microscopy (AFM) measurements have revealed hardening of intact fibroblasts after treatment with colchicine. The reliability of AFM measurements was confirmed by the identification of cells the lateral response of which to indentation did not disturb the normal force contribution. AFM data on the mechanical characteristics of such cells admit an unambiguous interpretation.

A Comparative Study of Force Measurements in Solution Using Micron and Nano Size Probe

Atomic force microscopy (AFM) is a device that is used for not only high-resolution imaging but also used for measuring forces. It is possible to quantify the surface density change for both colloid and nano probe as well as silica surface. By changing the quantity of ions within a potassium chloride solution, it then becomes possible to evaluate the quantity of ions that attach themselves to AFM colloid probe, nano probe and silica samples. In this study, the force was measured between AFM probes and silica surface in different ionic concentrations. Two different types of AFM probe were used: a colloid probe with a radius of 500 nano-meters and a nano probe with a radius of 10 nano-meters. This study is focused on measuring how the force magnitude, especially electrical double layer force, varied between the two types of probes by changing ionic concentrations. For all test trials, the results agreed with the electrical double layer theory. Although the micron probe was almost an exact match for all ranges, the nano probe was closest within its short-range forces. This is attributed to the formula use when analyzing the electrical double layer force. Because the formula was originally calculated for the micron probe, the shape and size of the nano probe created too many variables for an exact match. Along with quantifying the forces, this experiment allowed for an observation of Van der Waals force making it possible to calculate the Hamaker constant. Conclusively, all results show that the obtained surface charge density increases as the ionic concentration increases. In addition, through the comparison of the results obtained from the nano-sized probe and the micron-sized probe, it was concluded that nano size probe mapped higher surface charge density above the silica surface than the micron-sized probe under the same conditions.

Atomic Force Microscopy for Cell Membrane Investigation.

Atomic force microscopy (AFM) is a very versatile tool for studying biological samples at nanometer-scale resolution. The cell membrane plays a key role in compartmentalization, nutrient transportation, and signal transduction, while the structural feature of both sides of the membrane remains elusive. Here we describe our methods for the preparation of the cell membrane from the red blood cells and nucleated cells. High-resolution AFM topographs reveal substructural details of both sides of the cell membrane. The structure composition of cell membrane can be directly observed by time-lapse AFM and the positional information of membrane proteins can be located by molecular recognition.

Atomic-Force Microscopy Studies of Polyfluoroalkylacrylate Latex Particles

Atomic-force microscopy was used to visualize latex particles of polyfluoroalkylacrylates and to determine their sizes and particle-size distributions. The conditions for preparing samples for the studies and the operating mode of an NTEGRA Prima (NT-MDT) scanning probe microscope were determined.