Mode Atomic Force Microscopy Research Articles

Nano-scale characterization of nano-hydroxyapatite incorporated chitosan particles for bone repair.

In this study, injectable porous spherical particles were fabricated using chitosan (CS) biopolymer, sodium tripolyphosphate (TPP), and nano-hydroxyapatite (nHA). TPP was primarily used as an ionic crosslinker to crosslink 2% (w/v) CS droplets. 2% (w/v) nHA was used to prepare nHA incorporated particles. The surface morphological properties and nanomechanical properties such as topography, deformation, adhesion, and dissipation of CS particles with and without nHA were studied using contact mode and peakforce quantitative nanomechanical property mapping mode in atomic force microscopy. The nHA spots have higher density than CS which leads to higher forces acting on the probe tip and higher energy dissipation to lift the tip from nHA areas. The cumulative release data showed that about 87% of total BMP-2 encapsulated within the particles was released by third week of experiment period. Degradation study was conducted to understand how the particles degradation occurs in the presence of phosphate buffered saline with continues shaking in an incubator at 37° C. In addition, BMP-2 release from the 2% nHA/CS particles was studied over a three weeks period and found that BMP-2 release was governed by the simple diffusion rather than the degradation of particles.

Glycan distribution and density in native skin's stratum corneum.

BackgroundThe glycosylation of proteins on the surface of corneocytes is believed to play an important role in cellular adhesion in the stratum corneum (SC) of human skin. Mapping with accuracy the localization of glycans on the surface of corneocytes through traditional methods of immunohistochemistry and electron microscopy remains a challenging task as both approaches lack enough resolution or need to be performed in high vacuum conditions.Materials and methodsWe used an advanced mode of atomic force microscope (AFM), with simultaneous topography and recognition imaging to investigate the distribution of glycans on native (no chemical preparation) stripped samples of human SC. The AFM cantilever tips were functionalized with anti‐heparan sulfate antibody and the lectin wheat germ agglutinin (WGA) which binds specifically to N‐acetyl glucosamine and sialic acid.ResultsFrom the recognition imaging, we observed the presence of the sulfated glycosaminoglycan, heparan sulfate, and the glycans recognized by WGA on the surface of SC corneocytes in their native state. These glycans were found associated with bead‐like domains which represent corneodesmosomes in the SC layers. Glycan density was calculated to be ~1200 molecules/μm2 in lower layers of SC compared to an important decrease, (~106 molecules/μm2) closer to the surface due probably to corneodesmosome degradation.ConclusionGlycan spatial distribution and degradation is first observed on the surface of SC in native conditions and at high resolution. The method used can be extended to precisely localize the presence of other macromolecules on the surface of skin or other tissues where the maintenance of its native state is required.

Composition-Dependent Alterations in Thickness and Physical Properties of Lipid Bilayer FILM Revealed by Frequency Modulation Atomic Force Microscopy

Assembly of microdomains and membrane viscosity due to these physical properties of membrane have been highlighted as important factors supporting proper functions of membrane proteins. Although advanced imaging technologies including scanning probe microscopy and super-resolution optical microscopy have revealed dynamics of microdomains and visco-elastic properties of cell membranes, it is still difficult to link modifications of nanoscale membrane structure and specific diseases. In this study, we aimed to obtain high-resolution structural information of supported planner membrane with different fatty acid lengths using liquid dynamic-mode AFM and FM-AFM (Frequency Modulation Atomic Force Microscope). In FM-AFM, the cantilever is moved in a non-contact state, maintaining the cantilever frequency shift (Δf) is constant. This enables a highly sensitive force detection, 20 times better S/N than with existing methods, thereby markedly improving the image resolution. We used supported phosphatidylcholine membranes with fatty acids ranged from PC 12:0 to PC 22:6. For saturated PCs, membrane thickness increased along with the number of carbon from 4.32 nm (PC12:0) to 6.37nm (PC18:0), but diarachidoylphosphatidylcholine (PC20:0) showed reduced membrane thickness: PC20:0 thickness had 6.09 nm, that is ≈0.3 nm thinner than PC18:0. Unsaturation of fatty acid further reduced membrane thickness to 4.12 nm for PC20:4, suggesting that structural arrangement of PCs with fatty acid chain length and unsaturation level occurred. These data demonstrated that our imaging modality can detect membrane thickness with minimal membrane compression by scanning probe while maintaining high detection sensitivity. We will also present modulations of mechanical properties of membrane in the functions of carbon chain length and the level of unsaturation.

Directly and quantitatively studying the interfacial interaction between SiO 2 and elastomer by using peak force AFM

Abstract The interfacial interaction between nanofillers and elastomer plays an important role in properties of elastomer nanocomposites. Up to now, most methods focus on studying the interphase of elastomer nanocomposites. The filler-elastomer interaction forces have not been directly and quantitatively characterized yet. In this study, the interaction forces at nano-Newton scale between elastomer matrix and silica (SiO 2 ) were directly and quantitatively characterized by using peak force quantitative nanomechanical mapping (PF-QNM) mode of atomic force microscopy (AFM). Three kinds of nitrile butadiene rubber (NBR) molecules with different molecular polarity were first grafted on the AFM tip by using click chemistry. The SiO 2 (or modified SiO 2 ) were pressed into flat films and worked as substrate. The interaction forces between these NBR layer grafted on AFM tip and SiO 2 (or modified SiO 2 ) substrate were directly and quantitatively obtained. The results show that as the acrylonitrile content of NBR increasing from 26 wt% to 41 wt%, the average interaction force of NBR/SiO 2 increases from 4.5 ± 1.0 nN to 7.7 ± 1.1 nN and that of NBR/m-SiO 2 increases from 6.2 ± 1.2 nN to 12.9 ± 1.0 nN. This study provides guidance in direct and quantitative characterization of filler-elastomer interactions at nano-Newton resolution.

Imaging of Soft and Biological Samples Using AFM Ringing Mode.

Ringing mode of atomic force microscopy (AFM) enables imaging the surfaces of biological samples, cells, tissue, biopolymers, etc. to obtain unique information, such as the size of molecules pulled by the AFM probe from the sample surface, heights of the sample at different load forces, etc. (up to eight different imaging channels can be recorded simultaneously, which is in addition to five channels already available in other rival modes). The imaging can be done in both air (gases) and liquid (buffers). In addition, the images obtained in ringing mode do not have several common artifacts and can be collected up to 20× faster compared to the rival imaging modes. Here we describe a step-by-step approach to collect images in ringing mode applied to biological and soft materials in general. Technical details, potential difficulties, and points of special attention are described.

AFM-Based Characterization of Electrical Properties of Materials.

Capabilities of atomic force microscopy (AFM) for characterization of local electrical properties of materials are presented in this chapter. At the beginning the probe-sample force interactions, which are employed for detection of surface topography and materials properties, are described theoretically in their application in different AFM modes and electrical techniques. The electrical techniques, which are based on detection of electrostatic probe-sample forces, are outlined in AFM contact and oscillatory resonant modes. The basic features of the detection of surface potential and capacitance gradients are explained. The applications of these techniques are illustrated on metals, surfactant compounds, semiconductors, and different polymers. Practical recommendations on use of the AFM-based electrical methods and the related challenges are given in the last section.

Influences of thickness, scanning velocity and relative humidity on the frictional properties of WS2 nanosheets

Distinguishing with the traditional cantilever mechanics method, we propose the extended cantilever mechanics method to calibrate the lateral calibration factor by using the normal spring constant obtained from atomic force microscopy (AFM) but not the Young’s modulus and the width of the cantilever, before the influences of thickness, scanning velocity and humidity on the frictional properties are investigated via friction measurement performed by the lateral force mode (LFM) of AFM. Tungsten disulfide (WS2) nanosheets were prepared through hydrothermal intercalation and exfoliation route, and AFM and Raman microscope were used to investigate the frictional properties, thickness and crystalline structure. The friction force and coefficient decrease monotonically with the increase of the nanosheet’s thickness, and the friction coefficient minimum value is close to 0.012 when the thickness larger than 5 nm. The friction property variation on the nanosheet’s thickness can be explained by the puckering effect of tip-sheet adhesion according thickness dependence of bending stiffness in the frame of continuum mechanics. The friction force is a constant value 1.7 nN when the scanning speed larger than the critical value 3.10 μm s−1, while it logarithmically increases for the scanning speed less than the critical value. It is easy to understand through the energy dissipation model and the thermally activated effect. The friction force and friction coefficient increase with the relative humidity at the range of 30%–60%, and the latter is at the range of 0.010–0.013. Influence of relative humidity is discussed via the increasing area of the water monolayer during the water adsorption process. The research can not only enrich nanotribology theory, but also prompt two dimensions materials for nanomechanical applications.

Fidelity Measurement on Signal Detection Algorithms With Application to AFM Imaging

Many signal processing and decision making algorithms reported in contemporary literature characterize the performance of the proposed methods utilizing decision error rates. However there is significant need for strategies that provide quantitative assessment of the fidelity of decisions made by the algorithms. This paper develops novel techniques utilizing which fidelity measures can be assigned quantitatively on some prevalent signal detection algorithms. The developed fidelity measurement methods with the detection algorithms are employed for topography imaging utilizing dynamic mode atomic force microscope (AFM). The AFM is a versatile metrology tool for interrogating material at the nano-scale. In spite of its remarkable achievements, a key issue that remains largely unaddressed is the assessment of fidelity of the measurement data. The developed paradigm facilitates user specific priority for either detection of sample features with high decision confidence or on not missing detection of true features. The fidelity measures presented here are suitable for real-time implementation. A detailed comparative study is presented to characterize the proposed signal detection algorithms and fidelity measurement techniques under practical AFM applications. Comprehensive simulation and experimental data corroborate the effectiveness of proposed methods.

Atomic force microscopy of cells as a method for the study of the pathogenesis and AS THE basis for the development of methods of DISEASE DIAGNOSIS

The application of atomic force microscopy (AFM) for the study of micro- and nanoscale areas of the cell surface allows researchers to introduce new cell parameters and to obtain qualitatively new notions about the causes and mechanisms of changes of the cell properties. The aim of the work was to assess the prospects of AFM of cells using the example of blood cells for the study and development of new methods of disease diagnosis based on the specificity of AFM modes of operation and the recent AFM data on the cell surface properties.

Broad modulus range nanomechanical mapping by magnetic-drive soft probes

Stiffness matching between the probe and deformed portion of the sample in piezo-drive peak force modulation atomic force microscopy (AFM) limits the modulus measurement range of single probes. Here we develop a magnetic drive peak force modulation AFM to broaden the dynamic range of the probe with direct cantilever excitation. This approach not only successfully drives the softest commercial probe (6 pN nm−1) for mapping extremely soft samples in liquid but also provides an indentation force of hundreds of nanonewtons for stiff samples with a soft probe. Features of direct measurements of the indentation force and depth can unify the elastic modulus range up to four orders of magnitude, from 1 kPa to 10 MPa (in liquid) and 1 MPa to 20 GPa (in air or liquid) using a single probe. This approach can be particularly useful for analysing heterogeneous samples with large elastic modulus variations in multi-environments.

High harmonic exploring on different materials in dynamic atomic force microscopy

In atomic force microscopy (AFM), high-frequency components consisted in dynamic tip-sample interaction have been recently demonstrated as a promising technique for exploring more extensive material properties. Here we present an exploratory study of high harmonic atomic force microscopy by force-spectroscopy and high harmonic imaging. Since these components are very weak compared to the fundamental response, we firstly designed a high harmonic cantilever by tuning the second order flexural resonance frequency to an integer 6 times of its fundamental mode (i.e. ω 2=6 ω 1). Moreover, it is verified that high harmonic can discern extra features than topographies on different samples with amplitude/frequency modulation (AM/FM) dynamic AFM mode. In AM mode, the first resonance amplitude and 6th harmonic amplitude were discussed. The 6th harmonic is more sensitive than the first order response. In FM mode, it is noted that the decaying rate of the 6th harmonic frequency is approximately 6 multiples to the slope of the fundamental frequency shift when the tip approaches to the surface of sample. This non-destructive method was also adopted to investigate the local interlayer coupling and intercalation in the two-dimensional graphene films tentatively.

An atomic force microscopy mode for nondestructive electromechanical studies and its application to diphenylalanine peptide nanotubes

Nondestructive scanning probe microscopy of fragile nanoscale objects is currently in increasing need. In this paper, we report a novel atomic force microscopy mode, HybriD Piezoresponse Force Microscopy (HD-PFM), for simultaneous nondestructive analysis of piezoresponse as well as of mechanical and dielectric properties of nanoscale objects. We demonstrate this mode in application to self-assembled diphenylalanine peptide micro- and nanotubes formed on a gold-covered substrate. Nondestructive in- and out-of-plane piezoresponse measurements of tubes of less than 100 nm in diameter are demonstrated for the first time. High-resolution maps of tube elastic properties were obtained simultaneously with HD-PFM. Analysis of the measurement data combined with the finite-elements simulations allowed quantification of tube Young's modulus. The obtained value of 29 ± 1 GPa agrees well with the data obtained with other methods and reported in the literature.

Controlling conducting channels of single-walled carbon nanotube array with atomic force microscopy

In this paper, we report a technique of controlling the number of single-walled carbon nanotubes (SWNTs) between two electrodes. The SWNT devices consist of an array of SWNTs between two silver/palladium electrodes, which are fabricated with electron beam lithography and silver. Using the tip-sample interaction in contact mode of atomic force microscopy (AFM), the number of individual SWNTs can be controllably decreased. The current–voltage measurements indicate that the resistance increases when the conducting channels of SWNTs are reduced. This simple and controllable approach could be useful for the assembly of high performance devices with a desired number of channels for future electronic investigations.

Peculiarities of living cell response to the external stimuli revealed via quasistatic mode of atomic force microscopy

The technique of atomic force microscopy allows revealing living cell morphology and mechanical properties characterization under physiologically relevant conditions. Here, we review our recent results on living cell reaction to different external influences obtained by this technique. The Bruker PeakForce QNM quasistatic mode was used to study living fibroblasts, erythrocytes, sensory neurons, and endothelial cells.

Custom-Designed Glassy Carbon Tips for Atomic Force Microscopy.

Glassy carbon is a graphenic form of elemental carbon obtained from pyrolysis of carbon-rich precursor polymers that can be patterned using various lithographic techniques. It is electrically and thermally conductive, mechanically strong, light, corrosion resistant and easy to functionalize. These properties render it very suitable for Carbon-microelectromechanical systems (Carbon-MEMS) and nanoelectromechanical systems (Carbon-NEMS) applications. Here we report on the fabrication and characterization of fully operational, microfabricated glassy carbon nano-tips for Atomic Force Microscopy (AFM). These tips are 3D-printed on to micro-machined silicon cantilevers by Two-Photon Polymerization (2PP) of acrylate-based photopolymers (commercially known as IP-series resists), followed by their carbonization employing controlled pyrolysis, which shrinks the patterned structure by ≥98% in volume. Tip performance and robustness during contact and dynamic AFM modes are validated by morphology and wear tests. The design and pyrolysis process optimization performed for this work indicate which parameters require special attention when IP-series polymers are used for the fabrication of Carbon-MEMS and NEMS. Microstructural characterization of the resulting material confirms that it features a frozen percolated network of graphene sheets accompanied by disordered carbon and voids, similar to typical glassy carbons. The presented facile fabrication method can be employed for obtaining a variety of 3D glassy carbon nanostructures starting from the stereolithographic designs provided by the user.

Nanoscale compositional mapping of cells, tissues, and polymers with ringing mode of atomic force microscopy

Recently developed sub-resonance tapping modes (such as Digital Pulse, Peak Force Tapping, HybriD, etc.) of atomic force microscopy (AFM) allow imaging of compositional contrast of (bio)materials and biological cells down to the nanoscale. Here we report on a powerful extension of those modes, “ringing” mode, which more than doubles the number of non-trivial physical channels that can be collected with a regular sub-resonance tapping. It can simultaneously record five new additional compositional parameters related to adhesive and viscoelastic properties of the sample surface: the restored (averaged) adhesion, adhesion height, pull-off neck height, detachment distance, and detachment energy losses. Ringing mode can be up to 20 times faster and showing fewer artifacts compared to the existing sub-resonance tapping modes. Ringing mode is based on an analysis of ringing signal of the AFM cantilever after detaching the AFM probe from the sample surface (this signal is currently treated as noise, and typically filtered out in the existing modes). We demonstrate that this new mode allows recording robust and unique information on fixed human epithelial cells, corneocyte skin flakes, and polymers used for bioimplants.

Silica-Coated Core-Shell Structured Polystyrene Nanospheres and Their Size-Dependent Mechanical Properties.

The core-shell structured PS/SiO2 composite nanospheres were synthesized on the basis of a modified Stöber method. The mechanical properties of monodisperse nanospheres were characterized with nanoindentation on the basis of the atomic force microscopy (AFM). The surface morphologies of PS/SiO2 composite nanospheres was scanned with the tapping mode of AFM, and the force-distance curves were measured with the contact mode of AFM. Different contact models were compared for the analyses of experimental data. The elastic moduli of PS/SiO2 composite nanosphere (4-40 GPa) and PS nanosphere (∼3.4 GPa) were obtained with the Hertz and Johnson-Kendall-Roberts (JKR) models, respectively, and the JKR model was proven to be more appropriate for calculating the elastic modulus of PS/SiO2 nanospheres. The elastic modulus of SiO2 shell gradually approached a constant value (∼46 GPa) with the increase of SiO2 shell thickness. A core-shell model was proposed for describing the relationship between PS/SiO2 composite nanosphere's elastic modulus and shell thickness. The mechanical properties of the composite nanospheres were reasonably explained on the basis of the growth mechanism of PS/SiO2 composite nanospheres, in particular the SiO2 shell's formation process. Available research data of PS/SiO2 composite nanospheres in this work can provide valuable guidance for their effective application in surface engineering, micro/nanomanufacturing, lubrication, and so on.

Structure of Composite Based on Polyheteroarylene Matrix and ZrO₂ Nanostars Investigated by Quantitative Nanomechanical Mapping.

It is known that structure of the interface between inorganic nanoparticles and polymers significantly influences properties of a polymer–inorganic composite. At the same time, amount of experimental researches on the structure and properties of material near the inorganic-polymer interface is low. In this work, we report for the first time the investigation of nanomechanical properties and maps of adhesion of material near the inorganic-polymer interface for the polyheteroarylene nanocomposites based on semi-crystalline poly[4,4′-bis (4″-aminophenoxy)diphenyl]imide 1,3-bis (3′,4-dicarboxyphenoxy) benzene, modified by ZrO2 nanostars. Experiments were conducted using quantitative nanomechanical mapping (QNM) mode of atomic force microscopy (AFM) at the surface areas where holes were formed after falling out of inorganic particles. It was found that adhesion of AFM cantilever to the polymer surface is higher inside the hole than outside. This can be attributed to the presence of polar groups near ZrO2 nanoparticle. QNM measurements revealed that polymer matrix has increased rigidity in the vicinity of the nanoparticles. Influence of ZrO2 nanoparticles on the structure and thermal properties of semi-crystalline polyheteroarylene matrix was studied with wide-angle X-ray scattering, scanning electron microscopy, and differential scanning calorimetry.

Dissolution Processes at Step Edges of Calcite in Water Investigated by High-Speed Frequency Modulation Atomic Force Microscopy and Simulation.

The microscopic understanding of the crystal growth and dissolution processes have been greatly advanced by the direct imaging of nanoscale step flows by atomic force microscopy (AFM), optical interferometry, and X-ray microscopy. However, one of the most fundamental events that govern their kinetics, namely, atomistic events at the step edges, have not been well understood. In this study, we have developed high-speed frequency modulation AFM (FM-AFM) and enabled true atomic-resolution imaging in liquid at ∼1 s/frame, which is ∼50 times faster than the conventional FM-AFM. With the developed AFM, we have directly imaged subnanometer-scale surface structures around the moving step edges of calcite during its dissolution in water. The obtained images reveal that the transition region with typical width of a few nanometers is formed along the step edges. Building upon insight in previous studies, our simulations suggest that the transition region is most likely to be a Ca(OH)2 monolayer formed as an intermediate state in the dissolution process. On the basis of this finding, we improve our understanding of the atomistic dissolution model of calcite in water. These results open up a wide range of future applications of the high-speed FM-AFM to the studies on various dynamic processes at solid-liquid interfaces with true atomic resolution.

Atomic force microscopy for two-dimensional materials: A tutorial review

Low dimensional materials exhibit distinct properties compared to their bulk counterparts. A plethora of examples have been demonstrated in two-dimensional (2-D) materials, including graphene and transition metal dichalcogenides (TMDCs). These novel and intriguing properties at the nano-, molecular- and even monatomic scales have triggered tremendous interest and research, from fundamental studies to practical applications and even device fabrication. The unique behaviors of 2-D materials result from the special structure–property relationships that exist between surface topographical variations and mechanical responses, electronic structures, optical characteristics, and electrochemical properties. These relationships are generally convoluted and sensitive to ambient and external perturbations. Characterizing these systems thus requires techniques capable of providing multidimensional information under controlled environments, such as atomic force microscopy (AFM). Today, AFM plays a key role in exploring the basic principles underlying the functionality of 2-D materials. In this tutorial review, we provide a brief introduction to some of the unique properties of 2-D materials, followed by a summary of the basic principles of AFM and the various AFM modes most appropriate for studying these systems. Following that, we will focus on five important properties of 2-D materials and their characterization in more detail, including recent literature examples. These properties include nanomechanics, nanoelectromechanics, nanoelectrics, nanospectroscopy, and nanoelectrochemistry.