Atomic Force Microscopy Topographic Images Research Articles

Determining the energetics of vicinal perovskite oxide surfaces

The energetics of vicinal SrTiO$_3$(001) and DyScO$_3$(110), prototypical perovskite vicinal surfaces, has been studied using topographic atomic force microscopy imaging. The kink formation and strain relaxation energies are extracted from a statistical analysis of the step meandering. Both perovskite surfaces have very similar kink formation energies and exhibit a similar triangular step undulation. Our experiments suggest that the energetics of perovskite oxide surfaces is mainly governed by the local oxygen coordination.

Exploring the effects of graphene oxide concentration on properties and antifouling performance of PEES/GO ultrafiltration membranes

In this study, asymmetric polyphenylene-ether-ether-sulfone (PEES) ultrafiltration (UF) membranes containing graphene oxide (GO) were prepared via non-solvent-induced phase separation process and N-methyl pyrrolidone was used as a solvent. The synthesis of GO was confirmed by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction analysis. The morphology of the prepared GO nanosheets was observed by field emission scanning electron microscope (FESEM) and transmission electron microscope. The membranes prepared with increasing concentrations of GO nanosheets were characterized by attenuated total reflectance-FTIR, SEM, atomic force microscopy (AFM), contact angle, and UF studies. The FTIR spectra of the GO embedded membranes reveal large amounts of –OH groups present due to the existence of GO nanosheets which improved its surface hydrophilicity. The contact angle of PEES/GO membrane was significantly lower than PEES membrane. The SEM pictures showed that PEES/GO UF membranes had a sponge-like substructure with the increased porosity and pore size. An AFM topography imaging showed that roughnesses of the modified membranes were improved compared to the pristine PEES membrane. The UF studies showed that the pure water flux ( JW) and the bovine serum albumin flux ( JP) were increased with the incorporation of GO into the blend solution. For the membrane with 0.1% GO content, JW increased by 75% and JP improved twofold which correspond to the maximum values of 186 and 113 L m−2 h−1, respectively. Furthermore, the flux recovery ratio results suggested that PEES/GO membranes have better antifouling characteristics due to the changes in membrane morphology and surface hydrophilicity.

Influence of antibody immobilization strategies on the analytical performance of a magneto-elastic immunosensor for Staphylococcus aureus detection

Magneto-elastic (ME) sensors have a great advantage in microbiology due to their ability to be queried wirelessly. Staphylococcus aureus is one of the most common bacteria widespread in the environment and a major human pathogen related to numerous illnesses. Immunosensors are affinity-based assays where the analyte is highly selective. The immobilization of antibodies (Ab) is an important step in the development of such devices. This study compared the effects of two antibody immobilization strategies on the analytical performance of a magneto-elastic immunosensor: (1) random antibody covalent immobilization (CysAb) and (2) specific-oriented antibody covalent immobilization (PrGAb). Immunosensors were exposed to solutions containing S. aureus at different concentrations (104 to 108CFU/ml) and sensor resonant frequencies were measured. In order to confirm that the frequency shifts were mainly caused by the binding of S. aureus to the sensor's surface, scanning electron microscope (SEM) and indirect immunofluorescence (IIF) images were taken after bacteria exposure at 108CFU/ml. Sensor surfaces were further monitored by non-contact topographic atomic force microscopy (AFM) images. In the covalent-oriented strategy, PrG was first bound covalently to the surface, which in turn, then binds the anti-S. aureus antibody in an oriented manner. Topographic AFM images showed different surface patterns between the two antibody immobilization strategies. Specific-oriented antibody covalent immobilization (PrGAb) strategy gave the highest anti-S. aureus antibody immobilization density. Therefore, the covalent-oriented strategy presented the best performance for S. aureus capture, detecting 104CFU/ml.

Piezoelectric immunochip coated with thin films of bacterial cellulose nanocrystals for dengue detection

Low-cost piezoelectric devices, such as simple frequency monitoring quartz crystal microbalance (QCM) devices, have good clinical utility as fast diagnostic tools for the detection of several diseases. However, unspecific antigen recognition, poor molecular probe adsorption and the need for sample dilution are still common drawbacks that hinder their use in routine diagnosis. In this work, piezoelectric sensors were previously coated with thin films of bacterial cellulose nanocrystals (CN) to provide a more sensitive and adapted interface for the attachment of monoclonal immunoglobulin G (IgGNS1) and to favor specific detection of non-structural protein 1 (NS1) of dengue fever. The assembly of the immunochip surface was analyzed by atomic force microscopy (AFM) and the NS1 detection was followed by quartz crystal microbalance with (QCM-D) and without energy dissipation monitoring (QCM). The CN surface was able to immobilize 2.30±0.5mgm−2 of IgGNS1, as confirmed by AFM topography and phase images along with QCM-D. The system was able to detect the NS1 protein in serum with only 10-fold dilution in the range of 0.01–10µgmL−1 by both QCM and QCM-D. The limits of detection of the two devices were 0.1μgmL−1 for QCM-D and 0.32μgmL−1 for QCM. As a result, QCM-D and QCM apparatuses can be used to follow NS1 recognition and have good potential for more sensitive, fast and/or less expensive diagnostic assays for dengue.

Single Molecule Fluorescence and Atomic Force Microscopy Studies of DNA Repair

DNA polymerases that are responsible for replication make approximately one error for every 107 bases copied, but the human genome contains ∼6 billion bases, which results in ∼600 errors per round of replication. The DNA mismatch repair (MMR) system corrects these DNA synthesis errors that occur during replication. MMR is initiated by the highly conserved MutS and MutL homologs, which are both dimers and contain DNA binding and ATPase activities that are essential for MMR in vivo. MutS homologs initiate repair by binding to a mismatch and undergoing an ATP-dependent conformational change that promotes its interaction with MutL homologs. This complex signals the initiation of excision and resynthesis of the newly synthesized DNA strand containing the incorrect nucleotide. We have been using a combination of atomic force microscopy (AFM) and single molecule fluorescence to characterize the stoichiometries and the conformational and dynamic properties of MutS and MutL homologs and their assembly on DNA containing a mismatch. We have also developed a new dual resonance frequency enhanced electrostatic force microscopy (DREEM), in which we simultaneously collect the AFM topographic image and an image of the electrostatic potential of the surface. The DREEM images reveal the path of DNA inside individual protein-DNA complexes, yielding unprecedented details about DNA conformations within simple and complicated complexes. I will discuss our studies on the assembly of MutS and MutL homologs on mismatches, with a focus on how AFM, DREEM, and single-molecule fluorescence can be powerful tools to study the stoichiometries, conformations, and dynamic assembly of multi-component complexes.

Effect of Modulating Spin-Coating Rate of TiO2Precursor for Mesoporous Layer onJ-VHysteresis of Solar Cells with Polar CH3NH3PbI3Perovskite Thin Film

Compared with the crystalline Si solar cells, theJ-Vcharacteristics of CH3NH3PbI3perovskite solar cells are different under forward and reverse scan, and the CH3NH3PbI3film exhibits some polarization properties. To explore those performances of the mesoporous TiO2layer based perovskite solar cells, we focus on the effect of modulating the spin-coating rate of the TiO2precursor for mesoporous layer onJ-Vhysteresis of solar cells with the polar film byJ-Vcurves, atomic force microscopy topographic images, and piezoresponse force microscopy phase images. Firstly, the AFM images illustrate that the polarization behaviors exist and the deformation scale is large at the corresponding position when the DC bias voltage increases. Secondly, it is suggested that the polar films which applied the positive DC biases voltage show a tendency to 0° phase angle, while the polar films which applied the negative DC biases voltage show a tendency to −180° phase angle. Thirdly, a weak polar hysteresis loop relation for CH3NH3PbI3film was observed. Finally, the hysteresis index for the 1500 rpm mesostructured solar cell shows relatively lowJ-Vhysteresis compared with the 3000 rpm mesostructured and the planar-structured solar cell. Our experimental results bring novel routes for reducing the hysteresis and investigating the polar nature for CH3NH3PbI3material.

Local conductance mapping of water-intercalated graphene on mica

We report that the conductance of graphene is influenced by intercalated water layers using current sensing atomic force microscopy (AFM). We obtained a confined water layer between chemical vapor deposition graphene and mica by transferring graphene onto mica in a liquid water bath. Atomic force microscopy topographic images confirm high coverage by a single water layer, and scanning tunneling microscopy (STM) verifies a clean surface without contamination by measuring the honeycomb lattice structure of the graphene. We show that the surface conductance is perturbed by the presence of a water layer between the graphene and mica, which is not found in the STM topographic image. We found that the graphene on the edge and at pinholes of the water layer exhibits lower conductance, compared with that of graphene on the water terrace. We attribute the perturbation of conductance to structural defects from the water film and a variation of interaction between the edge of the water and graphene.

In-vivo high resolution AFM topographic imaging of Caenorhabditis elegans reveals previously unreported surface structures of cuticle mutants

Atomic force microscopy (AFM) is a powerful method for topographic imaging of surfaces with nanometer resolution. AFM offers significant advantages over scanning electron microscopy (SEM) including the acquisition of quantitative 3D-images and biomechanical information. More importantly, for in-vivo biological imaging, AFM does not require sample dehydration/labeling. We show for the first time high-resolution topographical images of the cuticle of the model organism C. elegans under physiological conditions using AFM. C. elegans is used extensively for drug screening and to study pathogen adherence in innate immunity; both applications highly depend on the integrity of the nematode's cuticle. Mutations affecting both drug adsorption and pathogen clearance have been proposed to relate to changes in the cuticle structure, but never visually examined in high resolution. In this study we use AFM to visualize the topography of wild-type adult C. elegans as well as several cuticle collagen mutants and describe previously unseen anatomical differences.

Complementary studies of NMR spin diffusion and atomic force microscopy – Structural characterization of diblock copolymers

Abstract Due to their unique microphase structure copolymer systems have become important potential candidates for nanolitographic purposes. In order to properly design copolymeric templates a complete characterization of the phase separated system is required. Due to the fact that structural analysis of organic compounds remains demanding, especially when it comes to nanometer scale features such as interfacial spaces, there are very often at least two complementary experimental techniques employed to fully characterize observed morphologies. In this work diblock copolymers of poly (styrene)-b-poly (ethylene oxide) (PS-PEO) with three different molecular weights were investigated. Two different experimental methods were used: nuclear magnetic resonance spectroscopy (NMR) and atomic force microscopy (AFM). AFM topographic images of self-assembled polymer films enabled estimation of average interdomain distances, whereas the NMR spin diffusion experiment provided structural parameters related to stiff PS domains and interfacial space thickness. An application of two complementary methods enabled complete description of the studied morphologies and revealed substantial differences in polymer chain mobility between two distinct phases.

Direct DNA Immobilization onto a Carbon Nanotube Modified Electrode: Study on the Influence of pH and Ionic Strength

Over the past years, DNA biosensors have been developed to analyze DNA interaction and damage that have important applications in biotechnological researches. The immobilization of DNA onto a substrate is one key step for construction of DNA electrochemical biosensors. In this report, a direct approach has been described for immobilization of single strand DNA onto carboxylic acid-functionalized carbon nanotubes modified glassy carbon electrode. To do this, we first modified the glassy carbon electrode surface with MWCNT-COOH. The immersion of MWCNT-COOH/GCE in ss-DNA probe solution, with different pH and ionic strength, was followed by suitable interaction between amine group of ss-DNA bases and carboxylic groups of MWCNT-COOH. This interaction leads to successful ss-DNA immobilization on MWCNT-COOH that was confirmed by cyclic voltammetry, electrochemical impedance spectroscopy and atomic force microscopy. Immobilization of ss-DNA on the modified electrode increased the charge transfer resistant but decreased the peak current of redox probe ([Fe(CN)6]3-/4-). The result of cyclic voltammograms implicates that enhancements in the DNA immobilization are possible by adroit choice of low pH and high ionic strength. The standard free-energy of adsorption (ΔG°ads) was calculated from electrochemical impedance spectroscopy data (-47.75 kJ mol-1) and was confirmed covalent bond formation. atomic force microscopy topographic images demonstrate increased surface roughness after ss-DNA immobilization. Results offer a simple, rapid and low-cost of DNA immobilization strategy can be opportunities to design of novel nucleic acid biosensors.

Effect of Statins on the Nanomechanical Properties of Supported Lipid Bilayers

Many drugs and other xenobiotics may reach systemic concentrations where they interact not only with the proteins that are their therapeutic targets but also modify the physicochemical properties of the cell membrane, which may lead to altered function of many transmembrane proteins beyond the intended targets. These changes in bilayer properties may contribute to nonspecific, promiscuous changes in membrane protein and cell function because membrane proteins are energetically coupled to their host lipid bilayer. It is thus important, for both pharmaceutical and biophysical reasons, to understand the bilayer-modifying effect of amphiphiles (including therapeutic agents). Here we use atomic force microscopy topography imaging and nanomechanical mapping to monitor the effect of statins, a family of hypolipidemic drugs, on synthetic lipid membranes. Our results reveal that statins alter the nanomechanical stability of the bilayers and increase their elastic moduli depending on the lipid bilayer order. Our results also suggest that statins increase bilayer heterogeneity, which may indicate that statins form nanometer-sized aggregates in the membrane. This is further evidence that changes in bilayer nanoscale mechanical properties may be a signature of lipid bilayer-mediated effects of amphiphilic drugs.

Atomic-Scale Variations of the Mechanical Response of 2D Materials Detected by Noncontact Atomic Force Microscopy.

We show that noncontact atomic force microscopy (AFM) is sensitive to the local stiffness in the atomic-scale limit on weakly coupled 2D materials, as graphene on metals. Our large amplitude AFM topography and dissipation images under ultrahigh vacuum and low temperature resolve the atomic and moiré patterns in graphene on Pt(111), despite its extremely low geometric corrugation. The imaging mechanisms are identified with a multiscale model based on density-functional theory calculations, where the energy cost of global and local deformations of graphene competes with short-range chemical and long-range van der Waals interactions. Atomic contrast is related with short-range tip-sample interactions, while the dissipation can be understood in terms of global deformations in the weakly coupled graphene layer. Remarkably, the observed moiré modulation is linked with the subtle variations of the local interplanar graphene-substrate interaction, opening a new route to explore the local mechanical properties of 2D materials at the atomic scale.

Surface Characterization of Alkali Treated Kenaf Fibers by XPS and AFM

Kenaf fibers were immersed in 6% solution of sodium hydroxide (NaOH) for 1, 2, 3, 4 and 5 hours. The treated fibers were characterized with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The XPS results showed that the alkali treatment increased the proportion of oxygen atoms on the fiber surface as indicated by the increase in oxygen-carbon ratio. An increase in C2 peak (C-OH bonds) was also observed suggesting the present of cellulose on the fiber surface. The decrease in C1 peak (C-C bonds) suggested that lignin was removed from the fiber surface following the alkali treatment. Scale-like structures were observed from the AFM topographic images to indicate the presence of cellulose microfibrils in the primary cell wall of the treated fiber. Alkali treated fiber surface showed lower mean surface roughness than the untreated fiber indicating smoother fiber surface. The fiber surface became relatively smooth after the alkali treatment due to the removal of non-cellulosic layer.

Real-time single molecular study of a pretreated cellulose hydrolysis mode and individual enzyme movement.

BackgroundThe main challenges of large-scale biochemical conversion involve the high costs of cellulolytic enzymes and the inefficiency in enzymatic deconstruction of polysaccharides embedded in the complex structure of the plant cell wall, leading to ongoing interests in studying the predominant mode of enzymatic hydrolysis. In this study, complete enzymatic hydrolysis of pretreated biomass substrates was visualized in situ and in real time by atomic force microscopy (AFM) topography and recognition imaging. Throughout the entire hydrolytic process, a hydrolysis mode for exoglucanase (CBH I) consisting of a peeling action, wherein cellulose microfibrils are peeled from sites on the pretreated cellulose substrate that have cracks sufficiently large for CBH I to immobilize.ResultsWe quantitatively monitored the complete hydrolytic process on pretreated cellulose. The synergetic effect among the different enzymes can accelerate the cellulose hydrolysis rate dramatically. However, the combination of CBH I and β-glucosidases (β-G) exhibited a similar degradation capacity as did whole enzyme (contains the cellobiohydrolases and endoglucanase as its major enzyme components). We developed a comprehensive dynamic analysis for individual cellulase acting on single pretreated cellulose through use of functional AFM topography and recognition imaging. The single crystalline cellulose was divided into different regions based on the cracks on the substrate surface and was observed to either depolymerize or to peel away by the jammed enzyme molecules. After the exfoliation of one region, new cracks were produced for the enzyme molecules to immobilize. The fiber width may have a relationship with the peeling mode of the fibers. We performed a statistical height measure of the generated peaks of the peeled fibers. The height values range from 11 to 24 nm. We assume that the CBH I enzymes stop progressing along the cellulose microfibril when the peeled microfibril height exceeds 11 nm.ConclusionThe combination of CBH I and β-G can achieve an effective hydrolysis of the pretreated biomass substrates. The single-molecule study of the complete hydrolytic process indicates that the hydrolytic mode involves the peeling of the microfibrils and progressive depolymerization, which depend on the size of the cracks on the surface of the pretreated cellulose microfibrils.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0498-x) contains supplementary material, which is available to authorized users.

Investigation of the magnetostrictive effect in a terfenol-D plate under a non-uniform magnetic field by atomic force microscopy

We present a novel high resolution approach for the characterization of the evolution of magnetic domain structures and magnetostrictive shifts in a Terfenol-D plate under different magnetic fields distribution using atomic force microscopy (AFM). Investigations are made to examine the magnetostrictive effect in the material under similar conditions as it is used in magnetoelectric vibration energy converters. The surface topography of the sample was imaged as it changes with induced magnetization for different relative height positions between the magnets and the plate. The magnetic field distribution was analyzed based on finite element analysis while both longitudinal and transversal magnetostrictive strains were determined based on cross sections analysis performed on the AFM topography images. In addition, AFM amplitude images are presented to visualize the evolution of magnetic domain structures. Results demonstrate that the maximum longitudinal elongation of the plate is 266ppm when it is placed just above the magnets. On the other hand, magnetostriction close to zero are observed when the sample was placed in the farthest measured position from the magnets which confirm the validity of the proposed approach.

Tribochemistry of bearing steels: A new AFM method to study the material–tribofilm correlation

A new AFM method for relating microstructure to tribofilm formation is presented. Three aspects of this method distinguish it. Firstly, the new method maps the same region employing atomic force microscopy (AFM) in three sequential steps: preEDTA and postEDTA, before and after the tribofilm removal; and as-etched, after revealing the steel microstructure. Secondly, a new software was written for correcting the displacement between the sequential AFM topography images, and for analysing the data. Thirdly, the method allows to correlate the tribofilm with the steel microstructure, and relating it with protrusion of residual carbides and tribofilm roughness for regions as large as 50μm×50μm. This allows to performing statistical analyses to correlate tribofilm thickness with different microstructural constituents.

Evaluation of Structure and Assembly of Xyloglucan from Tamarind Seed (Tamarindus indica L.) with Atomic Force Microscopy.

The role of xyloglucan (XG) in the cell wall of plants and its technological usability depends on several factors, pertaining to molecular structure. Therefore, the goal of this study was to evaluate the nano-structure and self-assembly of XG by atomic force microscopy (AFM). As the model, a non-modified xyloglucan from a tamarind seed (Tamarindus indica L.) was used. Samples were minimally processed, i.e., treated with low-power ultrasound and studied on the surface of mica in ambient butanol. AFM topographic images revealed rod-like nanomolecules of xyloglucan with a mean height of 2.3 ± 0.5 nm and mean length of 640 ± 360 nm. The AFM study also showed that XG chains possessed a helical structure with a period of 115.8 ± 29.2 nm. This study showed possible-bending of molecules with a mean angle of 127.8 ± 25.6°. The xyloglucan molecules were able to aggregate as cross-like and a parallel like assemblies, and possibly as rope-like structures. The self-assembled bundles of xyloglucan chains were often complexed at an angle of 114.2 ± 36.3°.

Large area graphene and graphene oxide patterning and nanographene fabrication by one-step lithography

A cheap and green technology based on electro-erosion, performed at room conditions and scalable to large area (cm2) is demonstrated for structuring graphene and graphene oxide films on any substrate. This one-step technique based on electrical discharges produced by a direct current voltage source competes favorably with laser patterning. The threshold voltage for complete graphene elimination as determined by Raman mapping is around 20V. At low relative humidity conditions (30%) the transformation to graphene oxide is also detected for operating voltages above 40V, the oxidation being probably mediated by the residual adsorbed water at the surface. We also show the close correlation of atomic force microscopy (AFM) phase images with the modified graphene characteristics while AFM topographic images are dominated by extrinsic aspects. The use of a spring probe relaxes the requirement of precision for the tip-sample distance and sample flatness to around 10μm which is very convenient for large scale applications. Furthermore, this technique allows the formation of high quality nanographene, with size around 18nm, and graphene micro-ribbon lattices in a very fast way and very well defined edges, with dimensions down to 1μm width and mm length, very promising for terahertz graphene plasmonic applications.

Corrosion protection of copper by silane sol–gel coatings

The corrosion protection of three silane-based coatings on copper substrate was investigated in 3.5 % NaCl solution. For this purpose, tetraethoxysilane (TEOS), 3-tri-methoxysilyl-1-propanethiol (TPS) and a hybrid TEOS + TPS were coated onto a Cu substrate by the sol–gel deposition method. The corrosion behavior of uncoated and coated copper specimens was evaluated using potentiodynamic polarization, electrochemical impedance spectroscopy and immersion tests. Furthermore, the surface morphologies and chemical compositions of the coatings were assessed using atomic force microscopy (AFM), scanning electron microscopy and Fourier transform infrared spectrometer (ATR-FTIR). The results indicated that hybrid TEOS + TPS coating exhibited the best corrosion performance as compared with the copper substrate and single TEOS and TPS coatings. This behavior was ascribed to the simultaneous formation of strong Cu–S and cross-link Si–O–Si bonds according to the ATR-FTIR observations. The surface topography studies by AFM after 120-h immersion in 3.5 % NaCl demonstrated the heavy degradation of the coating in the case of TEOS. In contrast, hybrid TEOS + TPS coating produced a homogenous and highly adherent protective nanostructure coating (75–250 nm) on the copper substrates. The three-dimensional AFM topography images of TEOS-, TPS- and TEOS + TPS-coated copper surface (a, c and e) before and (b, d and f) after immersion in 3.5 % (w) NaCl solution for 120 h.

Cerebral ischemia-induced mitochondrial changes in a global ischemic rat model by AFM

Mitochondria play a central role in cell survival, and apoptotic cell death is associated with morphological changes in mitochondria. Quantification of the morphological and mechanical property changes in brain mitochondria is useful for evaluating the degree of ischemic injury and the neuroprotective effects of various drugs. This study was performed to investigate the changes in brain mitochondria in an 11-vessel occlusion ischemic model treated with magnesium sulfate (MgSO4), utilizing atomic force microscopy (AFM). Rats were randomly divided into three groups consisting of sham (n=6), global ischemia (GI, n=6), and MgSO4-treated global ischemia (MgSO4, n=6). The biophysical properties of brain mitochondria determined from AFM topographic images and adhesion force from force–distance measurements. The mean perimeter of ischemic mitochondria significantly increased to 2,396±541nm (vs. 1,006±318nm in control group, P<0.001). The MgSO4 treatment during global ischemia reduced the perimeter of ischemic mitochondria (1,127±399nm, P<0.001). The other parameters including length, width and area were significantly different than the GI group. Besides, the adhesion force (23.2±3.9nN) of isolated mitochondria from the MgSO4 group was close to normal levels (28.5±2.5nN), compared with that of ischemic ones (17.7±3.3nN, P<0.001). To confirm the neuroprotective effects of MgSO4, we performed Nissl staining. This study suggested that quantitative analysis of mitochondrial changes utilizing AFM could be effective for evaluating neuronal injury and drug effects.