Atomic Force Microscopy Phase Images Research Articles

Effect of Protein Adsorption onto the Dissolution of Silicon-Substituted Hydroxyapatite

Several authors have shown the beneficial role to incorporate silicon into hydroxyapatite lattice, although, the mechanism behind the enhanced bioactivity of this Si-hydroxyapatite (Si-HA) is poorly understood. The incorporation of Si into the HA lattice alters the surface charge of HA, leading to more negative values. Due to the importance of the surface properties on the interaction between biomaterials, physiological fluids, and the host tissue, it is important to further characterize the surface of Si-HA by determining its surface energy and wettability. Our results showed that the incorporation of Si increased the hydrophilicity of HA, leading to a higher interfacial tension. Another important property for osteointegration is the formation of an apatite layer. The dissolution of Si-HA in the presence of serum-free simulated body fluid (SBF) started at early time points and using atomic force microscopy (AFM) it was possible to observe the dissolution at the grain boundaries and grains, therefore an apatite layer was formed in a short period of time. As the dissolution-precipitation process is much more complex in vivo, we tried to mimic the initial stages of the in vivo reaction by incubating the Si-HA in serum-SBF. It was shown that the dissolution kinetics in serum-SBF was slower when compared to the dissolution in serum free-SBF. At the same time point, no significant dissolution features were observed or apatite layer was visualized. The phase imaging AFM indicated the presence of a layer on top of these materials that could be a proteinaceous layer, as XPS analysis detected an increase on the concentration of nitrogen on the surface of the samples incubated in the presence of proteins.

Subcellular features revealed on unfixed rat brain sections by phase imaging

For sectioned biologic tissues, atomic force microscopy (AFM) topographic images alone hardly provide adequate information leading to revealing biological structures. We demonstrate that phase imaging in amplitude-modulation AFM is a powerful tool in mapping structures present on the surface of unfixed rat brains sections. The contrast in phase images is originated from the difference in mechanical properties between biological structures. Visualization of the native state of biological structures by way of their mechanical properties provides a complementary technique to more traditional imaging techniques such as optical and electron microscopy.

Atomic Force Microscopy (AFM) Investigation on the Surfactant Wettability Alteration Mechanism of Aged Mica Mineral Surfaces

In this study, the mica samples aged in crude oil were treated by two types of surfactants: Triton X-100 and C16TAB, to find the mechanisms of wettability alteration. The contact angle measurements were conducted stepwise to check the wettability alteration of the mica surfaces. Atomic force microscopy (AFM) topographies and phase images were obtained and used to investigate the effects of aging in crude oil and surfactant treatment of the mica surface. The results show that the solution of Triton X-100/brine changes the wettability of the mica surface aged in crude oil to water-wet; however, the solution of C16TAB/brine alters the aged mica surface to more oil-wet conditions. Fractal measurement was used to check the AFM images for quantitative assessment of the surface roughness for wettability alteration investigation. Two different mechanisms proposed here are the surface cleaning and the surfactant adsorption for Triton X-100 and C16TAB, respectively.

Characterisation and mesomorphic behaviour of new aliphatic–aromatic azomethines containing ester groups

Two series of novel symmetrical azomethines prepared by the condensation of 4,4′-(butane-1,4-diylbis(oxy))bis(butane-4,1-diyl)bis(4-aminobenzoate) (PBBA470) and 5,10,15,20,25,30,35,40,45,50,55,60-dodecaoxatetrahexacontane-1,64-diylbis(4-aminobenzoate) (PBBA 1200) with a range of aldehydes have been characterised by 1H and 13C nuclear magnetic resonance, Fourier transform infrared and UV–Vis spectroscopy. Current–voltage measurements were performed using a device comprising indium–tin oxide/compound/Alq3/Al. The effect of rod length and the nature of the terminal chains on the thermal and mesomorphic behaviour of these materials were investigated by differential scanning calorimetry, polarising optical microscopy and wide- and small-angle X-ray diffraction. Additionally, the compounds were studied using various atomic force microscopy techniques, including mode and phase imaging, and measurements based on local contrast force–distance curves and roughness, together with skew and kurtosis, are presented.

AFM phase imaging of thin films of electronically conducting polymer polybithiophene prepared by electrochemical potentiodynamic deposition

Nanoscale properties of thin films of conducting polymer polybithiophene (PBT) deposited under potentiostatic and potentiodynamic conditions were compared using ex-situ atomic force microscopy (AFM) and its extension called phase imaging (PI-AFM). While the morphologies of the films prepared using the two techniques were quite similar, the phase contrast measurements revealed a profound difference in the mechanisms of potentiostatic and potentiodynamic electropolymerization, as well as in the nanoscale crystallinity and grain structure of the resulting polymer films. The overall crystallinity and degree of order were always higher for films deposited at constant potential. The differences were especially pronounced at the early deposition stages (film thicknesses of ca. 10 nm).

Hydrogel based on chitosan, itaconic acid and methacrylic acid as adsorbent of Cd 2+ ions from aqueous solution

A hydrogel has been synthesized by ionic crosslinking of chitosan (Ch) with itaconic acid (IA), followed by a free radical polymerization and crosslinking of Ch/IA network by adding methacrylic acid and the crosslinker. The resulting material (Ch/IA/MAA hydrogel) was characterized by spectral (Fourier transform infrared (FTIR)), thermal (thermogravimetric analysis (TGA)) and structural (SEM/EDX and atomic force microscopy (AFM)) analyses. The prepared hydrogel was investigated as potential adsorbent for removal of Cd 2+ ions from aqueous solution. The effect of various physico-chemical parameters such as pH, adsorbent dosage, adsorbate concentration and contact time was studied in batch experiments. The results of spectral analyses of Cd-loaded hydrogel have shown that active functional groups are –NH 2, –OH and –COOH. SEM/EDX analysis and AFM surface topography and phase images indicated that apart from the adsorption on the surface of the hydrogel, sorption takes place in the bulk, as well. The experimental kinetic and equilibrium data were better fitted by pseudo-second order kinetic model and Langmuir adsorption isotherm. The parameters obtained in thermodynamic studies showed that the adsorption of Cd 2+ on Ch/IA/MAA hydrogel was spontaneous and exothermic in nature. Desorption studies were carried out using acid leaching (HNO 3) and it has been shown that the regenerated hydrogel can be reused three times without any loss of adsorption capacity. The maximum adsorption of 285.7 mg/g has been obtained at pH 5.5 and the results of adsorption/desorption experiments implies that the Ch/IA/MAA hydrogel may be used as efficient sorbent for removal of Cd 2+ ions from aqueous solution.

Quantitative mapping of elastic moduli at the nanoscale in phase separated polyurethanes by AFM

The micro phase separated nanoscale morphology of phase separated polyurethanes (PUs) was visualized by atomic force microscopy (AFM) height and phase imaging of smooth surfaces obtained by ultramicrotonomy. PUs were obtained from 4,4′-methylenbis (phenyl isocyanate) (MDI), 1,4-butanediol (BD) and poly(tetrahydrofurane) polyether polyol (PTHF). The segmented polyether PUs with varying stoichiometric ratio of the isocyanate and hydroxyl groups were prepared to investigate the effect of molar mass, as well as the type and number of end-groups on their morphology and mechanical performance.The PU samples studied show characteristic “fingerprint” AFM phase images. Novel dynamic imaging modes of AFM, including HarmoniX material mapping and Peak Force Tapping were used to assess the mechanical performance of phase separated polyurethanes quantitatively as a function of their molecular structure. The values of surface elastic moduli were determined with nanoscale resolution and were in excellent agreement for both AFM modes. While tensile testing provides a bulk average value for the elastic modulus of the elastomers, the novel AFM based elastic moduli mappings introduced enable the study of surface stiffness with nanoscale resolution in a quantitative way.

Novel sulfonated poly(arylene biphenylsulfone ether) copolymers containing bisphenylsulfonyl biphenyl moiety: structural, thermal, electrochemical and morphological characteristics

AbstractA series of sulfonated poly(arylene biphenylsulfone ether) polymers containing up to two pendant sulfonic acid groups per repeat unit were successfully synthesized from 4,4′‐bis[(4‐chlorophenyl)sulfonyl]‐1,1′‐biphenyl (BCPSBP), disodium 3,3′‐disulfonate‐4,4′‐dichlorodiphenylsulfone (SDCDPS) and bisphenol A via aromatic nucleophilic displacement polycondensation. The resulting polymers were characterized by means of Fourier transform infrared and 1H NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry and thermogravimetric analysis (TGA). The number‐average molecular weight (Mn) of the synthesized polymers was in the range 15 300–22 900 g mol−1, and the polydispersity indices (Mw/Mn) varied from 2.5 to 4.4. Tough membranes with SDCDPS/BCPSBP mole ratio up to 50:50 were successfully cast using N‐methyl‐2‐pyrrolidone (NMP). An increase of sulfonic acid groups in the polymer backbone resulted in increased solubility in aprotic polar solvents and glass transition temperature. The TGA curves of all the copolymers in acid form exhibited two distinct weight‐loss profiles. The influential characteristics of the polymer electrolyte membranes, such as tensile strength, water uptake, ion‐exchange capacity and proton conductivity, were characterized with respect to the pendant sulfonic acid groups. Atomic force microscopy phase images of the acid‐form membranes clearly showed the hydrophilic domains, with sizes increasing as a function of the degree of sulfonation. Copyright © 2010 Society of Chemical Industry

Sulfonated poly(bis-A)-sulfones as proton exchange membranes for direct methanol fuel cell application

Sulfonated poly(bis-A)-sulfone (SPSF) samples were prepared by a mild postsulfonation method using trimethylsilyl chlorosulfonate as sulfonation agent, and their thermal and mechanical properties were evaluated. The serials of SPSF membranes are thermally stable up to 450°C in air. When compared with the poly(bis-A)-sulfone membrane, the hydrophilicity and water uptake of the SPSF membranes are enhanced. A microphase-separated structure comprised of hydrophilic and hydrophobic polymer backbone was observed from atomic force microscopy phase images. The hydrophilic ionic clusters become continuous to form channels when ion exchange capacity (IEC) reached 1.47 mequiv/g. Moreover, the membranes showed very good proton conductivities (20°C, 0.01–0.11 S/cm) and low-methanol permeability (0.09–3.06 × 10−6 cm2/s), and the methanol diffusion coefficients were lower than that of Nafion112 (1.35 × 10−6 cm2/s) with IEC values from 0.70 to 1.47 mequiv/g. However, the Fenton's reagent test revealed that the membranes exhibited very poor oxidation stability, which is the main defect limiting the application of SPSF for proton exchange membranes. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers

Observing the twinkling fractal nature of the glass transition

A fundamental understanding of the nature and structure of the glass transition in amorphous materials is currently seen as a major unsolved problem in solid-state physics. A new conceptual approach to understanding the glass transition temperature ( T g ) of glass-forming liquids called the twinkling fractal theory (TFT) has been proposed in order to solve this problem. The main idea underlying the TFT is the development of dynamic rigid percolating solid fractal structures near T g , which are said to be in dynamic equilibrium with the surrounding liquid. This idea is coupled with the concept of the Boltzmann population of excited vibrational states in the anharmonic intermolecular potential between atoms in the energy landscape. Solid and liquid clusters interchange or “twinkle” at a cluster size dependent frequency ω TF , which is controlled by the population of intermolecular oscillators in excited energy levels. The solid-to-liquid cluster transitions are in accord with the Orbach vibrational density of states for a particular fractal cluster g( ω) ~ ω df − 1 , where the fracton dimension d f = 4/3. To an observer, these clusters would appear to be “ twinkling.” In this paper, experimental evidence supporting the TFT is presented. The twinkling fractal characteristics of amorphous, atactic polystyrene have been captured via atomic force microscopy (AFM). Successive two-dimensional height AFM images reveal that the percolated solid fractal clusters exist for longer time scales at lower temperatures and have lifetimes that are cluster size dependent. The computed fractal dimensions, ≈ 1.88, are shown to be in excellent agreement with the theory of the fractal nature of percolating clusters in accord with the TFT. The twinkling dynamics of polystyrene within its glass transition region are captured with time-lapse one-dimensional AFM phase images. The autocorrelation cluster relaxation function was found to behave as C( t) ~ t − 4/3 and the cluster lifetimes τ versus width R were found to be in excellent agreement with the TFT via τ ~ R 1.42. This paper provides compelling new experimental evidence for the twinkling fractal nature of the glass transition.

Activity investigation of pinostrobin towards herpes simplex virus-1 as determined by atomic force microscopy

In the present study, the antiviral activity of pinostrobin towards herpes simplex virus-1 (HSV-1) was investigated by MTT assay and atomic force microscopy. Pinostrobin can inhibit HSV-1 replication with 50% effective concentration (EC(50)) of 22.71 ± 1.72 μg/ml. MTT assay showed HSV-1 was significantly inhibited when pretreated with pinostrobin, with the inhibition of 85.69 ± 2.59%. Significant changes in morphology and size of HSV-1 were observed by atomic force microscopy (AFM) in response to pinostrobin treatment. AFM topography and phase images showed that with increasing time, the envelope was shedded and damaged, finally leading to virus inactivation. With increasing concentration, pinostrobin caused a gradual leakage, also contributing to breakage of the envelope and virus inactivation. Treatment effect of oral pinostrobin in vivo showed that pinostrobin (50mg/kg/dose) possesses definite therapeutical effect in the development of lesion score. In general, the results showed that AFM represents a powerful technique for the investigation of morphology and size of HSV-1 treated by antiviral agents. AFM is applicable to study chemically induced morphological changes at the nanometer level.

Photocatalyst TiO2–Co: the effect of doping depth profile on methylene blue degradation

In this work, we study the effect of doping depth profile on the photocatalytic and surface properties of TiO2 films. Two thin film layers of TiO2 (200 nm) and Co (5 nm), respectively, were deposited by physical evaporation on glass substrate. These films were annealed for 1 s at 100 and 400 °C and the Co layer was removed by chemical etching. Atomic force microscopy (AFM) phase images showed changes in the surface in function of thermal treatment. The grazing-incidence X-ray fluorescence (GIXRF) measurements indicated that the thermal treatment caused migration of Co atoms to below the surface, the depths found were between 19 and 29 nm. The contact angle showed distinct values in function of the doped profile or Co surface concentration. The UV–vis spectra presented a red shift with the increasing of thermal treatment. Photocatalytical assays were performed by methylene blue discoloration and the higher activity was found for TiO2–Co treated at 400 °C, the ESI-MS showed the fragments formed during the methylene blue decomposition.

Phase Segregation of Untethered Zwitterionic Model Lipid Bilayers Observed on Mercaptoundecanoic-Acid-Modified Gold by AFM Imaging and Force Mapping

Planar supported lipid bilayers (SLBs) are often studied as model cell membranes because they are accessible to a variety of surface-analytic techniques. Specifically, recent studies of lipid phase coexistence in model systems suggest that membrane lateral organization is important to a range of cellular functions and diseases. We report the formation of phase-segregated dioleoylphosphatidylcholine (DOPC)/sphingomyelin/cholesterol bilayers on mercaptoundecanoic-acid-modified (111) gold by spontaneous fusion of unilamellar vesicles, without the use of charged or chemically modified headgroups. The liquid-ordered (l(o)) and liquid-disordered (l(d)) domains are observed by atomic force microscopy (AFM) height and phase imaging. Furthermore, the mechanical properties of the bilayer were characterized by force-indentation maps. Fits of force indentation to Sneddon mechanics yields average apparent Young's moduli of the l(o) and l(d) phases of 100 +/- 2 and 59.8 +/- 0.9 MPa, respectively. The results were compared to the same lipid membrane system formed on mica with good agreement, though modulus values on mica appeared higher. Semiquantitative comparisons suggest that the mechanical properties of the l(o) phase are dominated by intermolecular van der Waals forces, while those of the fluid l(d) phase, with relatively weak van der Waals forces, are influenced appreciably by differences in surface charge density between the two substrates, which manifests as a difference in apparent Poisson ratios.

Phase imaging atomic force microscopy in the characterization of biomaterials

The phase imaging atomic force microscopy is a powerful tool in surface characterization of the biomaterials, and the resulting phase image is able to detect chemical variation and reveal more detailed surface properties than the morphological image. However, the chemical- and morphological-dependent phase images were still not distinguished well. In order to better understand actual occurring phase images, we examined non-carious human maxillary incisor, microphase separated polyurethane and self-assembling peptide nanofibres. We herein reported that phase image mainly plotted the morphological change: the phase peak corresponding to the morphological valley, and the morphological peak to the phase valley, and exhibited fine surface structures of materials. The chemical-dependent phase contrast was generally masked by their inherent roughness. For the sample being very rough and having great phase separation, its chemical-dependent phase contrast could be detected at the hard tapping mode ('Amp. Ref. "set point ratio"': -0.4 to -0.8), for the sample with medium roughness only at the light tapping mode ('Amp. Ref.': -0.1 to -0.4). These results will help us understand and determine actual occurring phase images of natural or fabricated biomaterials, even, other materials.

Chemical surface composition of the polyethylene implanted by Ag + ions studied by phase imaging atomic force microscopy

High density polyethylene (HDPE) has been modified by Ag + ion implantation with the energy of 60 keV. The total amount of implanted silver ions was 1, 5 and 12 × 10 15 ions/cm 2. The surface topography was observed by atomic force microscopy (AFM), while the surface composition changes were detected using phase imaging AFM. Surface topography changes were studied in detail using 3D surface parameters analyses. The average roughness decreased for the implanted HDPE indicating the flattening of the surface. Phase AFM images indicated the homogenization of the polyethylene during ion implantation, while histogram analyses confirmed the change in surface composition.

Ambipolar all-polymer bulk heterojunction field-effect transistors

We demonstrate solution processable all-polymer based field-effect transistors (FETs) exhibiting comparable electron and hole mobilities. The semiconducting layer is a bulk heterojunction of poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (n-type polymer) and regioregular poly(3-hexylthiophene) (p-type polymer). These polymers form a type-II heterojunction as revealed by the faster photoluminescence dynamics of the blend compared to the pristine materials. An electron mobiliy of 4 × 10−3 cm2/V s and a hole mobility of 2 × 10−3 cm2/V s were extracted from the transfer characteristics of bottom contact FETs. The balanced mobilities suggest that the active layer is a fine network of the two components, as confirmed by atomic force microscopy phase images.

Dynamics of hydrated polyurethane biomaterials: Surface microphase restructuring, protein activity and platelet adhesion

Microphase separation is a central feature of segmented polyurethane biomaterials and contributes to the biological response to these materials. In this study we utilized atomic force microscopy (AFM) to study the dynamic restructuring of three polyurethanes having soft segment chemistries of interest in biomedical applications. For each of the materials we followed the changes in near surface mechanical properties during hydration, as well as fibrinogen activity and platelet adhesion on these surfaces. Both AFM phase imaging and force mode analysis demonstrated that these polyurethane biomaterials underwent reorientation and rearrangement resulting in a net enrichment of hard domains at the surface. Fibrinogen activity and platelet adhesion on the polyurethane surfaces were found to decrease with increasing hydration time. The findings suggest that water-induced enrichment of hydrophilic hard domains at the surface changes the local surface physical and chemical properties in a way that influences the conformation of fibrinogen, changing the availability of the platelet-binding sites in the protein. This work demonstrates that the hydrated polyurethane biomaterial interface is a complex and dynamic environment where the surface chemistry is changing, altering the activity of fibrinogen and affecting blood platelet adhesion.

Effect of latex on surface structure and wetting of pigment coatings

The amount and type of latex binder has a significant effect on both the structure and chemical properties of pigment coatings, which affects critical surface properties, e.g., gloss, ink setting rate via liquid absorption, and spreading properties. Increased knowledge is required for improved optimization in practical applications of pigment coating layers. To this end, the effect of binder amount for two different styrene–butadiene latices in kaolin pigment coatings on a base paper was studied, by measuring the surface topography, surface chemistry, and liquid absorption of these coatings. The topography was thoroughly analyzed from nanometer to millimeter length scales by atomic force microscopy (AFM), confocal optical microscopy (COM), and X-ray tomography. The area fractions of latex and pigments on the surface of the coating were determined from AFM phase contrast images, and the contact angles for pure latex and kaolin fractions were determined by using the Cassie equation. An explicit value for the effective surface area of the coatings was determined from the topographic data, which was used for determining the Wenzel correction for the contact angles. Work of adhesion and surface energy components were determined for the samples using three different methods (Owens–Wendt, Wu, vOCG). The latex was found to be evenly distributed over the surfaces, based on AFM phase imaging and ToF–SIMS mapping. The latex binder amount significantly affected the wetting behavior and interfacial properties of the coating layer. The results of this fundamental study can be used for optimization of functional properties of pigment coating systems by regulation of the amount and properties of their latex binder.

Improving the lifetime of white polymeric organic light-emitting diodes

We report on efficient polymeric white organic light-emitting diodes with unprecedented stability. The investigated devices are based on an electroluminescent copolymer of electron and hole-transporting units and red-, green-, and blue-emitting chromophores. We find that the glass transition of the polymer (Tg=182.5 °C) is the process determining the relation between thermal annealing during fabrication and device lifetime. For devices annealed below Tg, the device lifetime significantly increases with increasing annealing temperature. For annealing temperatures above Tg, however, the current density in the devices rapidly increases while their lifetime slightly decreases. Insight into the underlying processes is provided by atomic force microscopy phase imaging and by UV/visible and fluorescence spectroscopy. We also investigated the influence of the operating temperature of the device: besides the commonly known fact that elevated operating temperatures reduce the lifetime, we discovered that the acceleration coefficient, which determines the scaling of the device lifetime with applied current density, was reduced. At the glass transition, the device lifetime no longer depended on the current density. The device lifetime was improved even further by introducing an additional cross-linkable hole-transport layer. Optimized devices achieve a half-luminance lifetime of 1860 h when operated at room temperature and at an initial luminance of 500 cd m−2. As a result of the relatively balanced stability of the three chromophores, the emission spectrum remains virtually unchanged over the entire device lifetime. Finally, to reduce the time required for the lifetime measurements, we propose to analyze the voltage increase over the first 10–50 h of the lifetime test and find that this allows precisely estimating the lifetime of our devices.

Visualization of enzymatic hydrolysis of cellulose using AFM phase imaging

Complete cellulase, an endoglucanase (EGV) with cellulose-binding domain (CBD) and a mutant endoglucanase without CBD (EGI) were utilized for the hydrolysis of a fully bleached reed Kraft pulp sample. The changes of microfibrils on the fiber surface were examined with tapping mode atomic force microscopy (TM–AFM) phase imaging. The results indicated that complete cellulase could either peel the fibrillar bundles along the microfibrils (peeling) or cut microfibrils into short length across the length direction (cutting) during the process. After 24 h treatment, most orientated microfibrils on the cellulose fiber surface were degraded into fragments by the complete cellulase. Incubation with endoglucanase (EGV or EGI) also caused peeling action. But no significant size reduction of microfibrils length was observed, which was probably due to the absence of cellobiohydrolase. The AFM phase imaging clearly revealed that individual EGV particles were adsorbed onto the surface of a cellulose fiber and may be bound to several microfibrils.