Interaction forces between talc and pitch probed by atomic force microscopy

Colloidal wood resin components present in pulp are collectively called "pitch". The presence of pitch may cause severe problems due to deposits in and on the paper machine. There is thus a need for controlling pitch aggregation and adsorption. To be able to develop more efficient pitch control systems, one needs to develop the understanding of pitch-pitch interactions and of the interactions between pitch and other materials. With this general goal in mind, we present methods for preparing geometrically well-defined pitch particles attached to atomic force microscopy tips. This has enabled us to investigate the interactions between pitch and talc, an additive commonly used for pitch control. We have used model pitch particles consisting of one component only (abietic acid), a mixture of components (collophonium), and particles prepared from real pitch deposits. We show that the forces acting between pitch and talc are attractive and, once the initial approach is made, exert this attraction out to large distances of separation. We present evidence that the formation of bridging air bubbles or cavities is responsible for this interaction.     

Electrochemistry of conductive polymers 36. pH dependence of polyaniline conductivities studied by current-sensing atomic force microscopy

We demonstrate from our current-sensing atomic force microscopic studies that both electrical and topographical properties of electrochemically prepared polyaniline (PAn) films are affected by their preparation conditions. The electrical properties of the fully doped PAn films prepared in 0.30 M nitric acid with its pH and ionic strength adjusted to 0.50 can be described as a conductor with an average conductivity of 49 (+/-13) S/cm with primarily a compact structure resulting from a relatively small growth rate. The doped PAn films prepared at pH 5.0, for example, have compact structures with large grains and lightly doped semiconducting properties with an average conductivity of about 1.54 (+/-0.09) x 10(-4) S/cm. From these data, we conclude that the degree of protonation of the monomers and the main reactions taking place during an early stage of the polymerization reaction are important factors determining the chemical structures as well as their conductivities and morphologies of the PAn films.     

Probing surface adhesion forces of Enterococcus faecalis to medical-grade polymers using atomic force microscopy

The aim of this study was to compare the initial adhesion forces of the uropathogen Enterococcus faecalis with the medical-grade polymers polyurethane (PU), polyamide (PA), and poly(tetrafluoroethylene) (PTFE). To quantify the cell-substrate adhesion forces, a method was developed using atomic force microscopy (AFM) in liquid that allows for the detachment of individual live cells from a polymeric surface through the application of increasing force using unmodified cantilever tips. Results show that the lateral force required to detach E. faecalis cells from a substrate differed depending on the nature of the polymeric surface: a force of 19 +/- 4 nN was required to detach cells from PU, 6 +/- 4 nN from PA, and 0.7 +/- 0.3 nN from PTFE. Among the unfluorinated polymers (PU and PA), surface wettability was inversely proportional to the strength of adhesion. AFM images also demonstrated qualitative differences in bacterial adhesion; PU was covered by clusters of cells with few cell singlets present, whereas PA was predominantly covered by individual cells. Moreover, extracellular material could be observed on some clusters of PU-adhered cells as well as in the adjacent region surrounding cells adhered on PA. E. faecalis adhesion to the fluorinated polymer (PTFE) showed different characteristics; only a few individual cells were found, and bacteria were easily damaged, and thus detached, by the tip. This work demonstrates the utility of AFM for measurement of cell-substrate lateral adhesion forces and the contribution these forces make toward understanding the initial stages of bacterial adhesion. Further, it suggests that initial adhesion can be controlled, through appropriate biomaterial design, to prevent subsequent formation of aggregates and biofilms.     

Adhesion forces between protein layers studied by means of atomic force microscopy

Adhesion forces between different protein layers adsorbed on different substrates in aqueous media have been measured by means of an atomic force microscope using the colloid probe technique. The effects of the loading force, the salt concentration and pH of the medium, and the electrolyte type on the strength, the pull-off distance, and the separation energy of such adhesion forces have been analyzed in depth. Two very different proteins (bovine serum albumin and apoferritin) and two dissimilar substrates (silica and polystyrene) were used in the experiments. The results clearly point out a very important contribution of the electrostatic interactions in the adhesion between protein layers.     

Study of the surface morphology of polyisobutylene-based block copolymers by atomic force microscopy

This paper reports the investigation of the nanostructured surface morphology of linear polystyrene-block-polyisobutylene-block-polystyrene (SIBS) triblock copolymers and novel arborescent SIBS block copolymers by Atomic Force Microscopy (AFM) in the tapping mode. Thin films spin coated from toluene onto silicon wafers were studied. The nanostructured morphology of the block copolymers varied with the hard polystyrene (PS) and soft polyisobutylene (PIB) segment composition, ranging from spherical to lamellar nanometer-sized discreet PS phases dispersed in a continuous PIB matrix. Annealing the samples resulted in well developed/ordered structures. The arborescent blocks had irregularly distributed PS phases in the PIB matrix. Annealing had a dramatic effect on the morphology which still remained irregular. Three-dimensional AFM image and section analysis indicated the presence of a height difference between PIB (high-lying plateaus or hills) and PS (low-lying plateaus or valleys) in the block copolymers, which became more prominent during annealing. It is theorized that the rubbery PIB chains are able to relax, thereby protruding from the surface, anchored by the physically crosslinked PS phases.     

In-situ investigation of surface reactions on alkali halides by atomic force microscopy

In-situ AFM images have been produced of dissolution processes on NaCl and KBr single crystals under different organic solvents. Using a home-built sample holder stable imaging conditions have been achieved and atomic resolution could be obtained on both materials. Dissolution rates have been determined quantitatively from the AFM images and compared with empirical solvent parameters such as acceptor number AN and E_T(30) value. The AFM results are in agreement with the expected behaviour of the liquids. Dissolution rates could be controlled using mixtures of polar and non polar solvents. The results are a basis for further investigations of other systems and technologically important processes such as corrosion and surface modification.     

Antigen binding forces of single antilysozyme Fv fragments explored by atomic force microscopy

We used atomic force microscopy (AFM) to explore the antigen binding forces of individual Fv fragments of antilysozyme antibodies (Fv). To detect single molecular recognition events, genetically engineered histidine-tagged Fv fragments were coupled onto AFM tips modified with mixed self-assembled monolayers (SAMs) of nitrilotriacetic acid- and tri(ethylene glycol)-terminated alkanethiols while lysozyme (Lyso) was covalently immobilized onto mixed SAMs of carboxyl- and hydroxyl-terminated alkanethiols. The quality of the functionalization procedure was validated using X-ray photoelectron spectroscopy (surface chemical composition), AFM imaging (surface morphology in aqueous solution), and surface plasmon resonance (SPR, specific binding in aqueous solution). AFM force-distance curves recorded at a loading rate of 5000 pN/s between Fv- and Lyso-modified surfaces yielded a distribution of unbinding forces composed of integer multiples of an elementary force quantum of approximately 50 pN that we attribute to the rupture of a single antibody-antigen pair. Injection of a solution containing free Lyso caused a dramatic reduction of adhesion probability, indicating that the measured 50 pN unbinding forces are due to the specific antibody-antigen interaction. To investigate the dynamics of the interaction, force-distance curves were recorded at various loading rates. Plots of unbinding force vs log(loading rate) revealed two distinct linear regimes with ascending slopes, indicating multiple barriers were present in the energy landscape. The kinetic off-rate constant of dissociation (k(off) approximately = 1 x 10(-3) s(-1)) obtained by extrapolating the data of the low-strength regime to zero force was in the range of the k(off) estimated by SPR.     

Probing small unilamellar EggPC vesicles on mica surface by atomic force microscopy

Sonicated small unilamellar egg yolk phosphatidylcholine (EggPC) vesicles were investigated using atomic force microscopy (AFM) imaging and force measurements. Three different topographies (convex, planar, and concave shape) of the EggPC vesicles on the mica surface were observed by tapping mode in fluid, respectively. It was found that the topography change of the vesicles could be attributed to the interaction force between the AFM tip and vesicles. Force curves between an AFM tip and an unruptured vesicle were obtained in contact mode. During approach, two breaks corresponding to the abrupt penetration of upper and lower bilayer of vesicle were exhibited in the force curve. Both breaks spanned a distance of around 4 nm close to the EggPC bilayer thickness. Based on Hertz analysis of AFM approach force curves, the Young's modulus (E) and the bending modulus (kc) for pure EggPC vesicles were measured to be (1.97 +/- 0.75) x 10(6)Pa and (0.21 +/- 0.08) x 10(-19)J, respectively. The results show that the AFM can be used to obtain good images of intact and deformed vesicles by tapping mode, as well as to probe the integrity and bilayer structure of the vesicles. AFM force curve compare favorably with other methods to measure mechanical properties of soft samples with higher spatial resolution.     

Interaction forces between metal-chelating lipid monolayers measured by colloidal probe atomic force microscopy

Surface forces between LB films of metal-chelating lipids in water have been studied using colloidal probe atomic force microscopy. The LB films of an amphiphile functionalized by the iminodiacetic acid group were prepared on hydrophobic glass substrates. The electric double layer repulsion operated between these LB film surfaces changed depending on pH reflecting the different protonation states of the iminodiacetic acid groups. The titration curve of the iminodiacetic acid monolayer was obtained from the force profiles. The Cu²⁺ complexation process was also monitored by measuring the force profiles at various Cu²⁺ ion concentrations.     

SEBS aggregate patterning at a surface studied by atomic force microscopy

The morphologies of films spin coated from dilute block copolymer solution onto a mica substrate were studied by atomic force microscopy (AFM). Variables of interest were the polymer concentration, solvent, heating temperature, aging, and ultrasonic effect. It is shown that the solution concentration is the predominant factor in determining the shape of the aggregates displayed from spheres and rods to irregular patches with increasing concentration. The solubility parameter of the solvent plays an important role in modifying the distribution and the size of clusters at the surface. The structures of the aggregates at the surface are metastable, which could evolve with temperature from rodlike aggregates into regular stripes when annealed at a temperature higher than the order-disorder transition temperature of SEBS, whereas those in solution could evolve with aging and ultrasonic treatment into a more stable network structure.     

Measuring the size dependence of Young's modulus using force modulation atomic force microscopy

The dependence of the local Young's modulus of organic thin films on the size of the domains at the nanometer scale is systematically investigated. Using atomic force microscopy (AFM) based imaging and lithography, nanostructures with designed size, shape, and functionality are preengineered, e.g., nanostructures of octadecanethiols inlaid in decanethiol self-assembled monolayers (SAMs). These nanostructures are characterized using AFM, followed by force modulation spectroscopy and microscopy measurements. Young's modulus is then extracted from these measurements using a continuum mechanics model. The apparent Young's modulus is found to decrease nonlinearly with the decreasing size of these nanostructures. This systematic study presents conclusive evidence of the size dependence of elasticity in the nanoregime. The approach utilized may be applied to study the size-dependent behavior of various materials and other mechanical properties.     

Evaluation of interaction forces between profilin and designed peptide probes by atomic force microscopy

We evaluated the binding affinity of peptide probes for profilin (protein) using force curve measurement techniques and atomic force microscopy (AFM). The peptide probes designed and synthesized for this investigation were H-A3GP5GP5GP5G-OH (1), H-A3GP5G-OH (2), H-A3G7-OH (3), and H-A3G-OH (4). Each peptide probe was immobilized on a cantilever tip, and the interaction force to profilin, immobilized on a mica substrate, was examined by force curve measurements. The retraction forces obtained showed a sequence-dependent affinity of the peptide probe for profilin. The retraction force for peptide probe 1 was the largest of the four probes examined, and it confirmed that peptide probe 1 has high affinity for profilin. The single molecular retraction force between peptide probe 1 and profilin was estimated to be 96 pN, as determined by Gaussian fitting to the histogram of the retraction forces.     

Intermolecular forces between acetylcholine and acetylcholinesterases studied with atomic force microscopy

With the aid of atomic force microscopy, the intermolecular forces between acetyleholinesterases (AChE) and its natural substrate acetylcholine (ACh) have been studied. Through force spectrum measurement based on imaging of AChE molecules it was found that the attraction force between individual molecule pairs of ACh and AChE was (10±1) pN just before the quaternary ammonium head of ACh got into contact with the negative end of AChE and the decaying distance of attraction was (4±1) nm from the surface of ACHE. The adhesion force between individual ACh and AChE molecule pairs was (25±2) pN, which had a decaying feature of fast-slow-fast (FSF). The attraction forces between AChE and choline (Ch), the quaternary ammonium moiety and hydrolysate of ACh molecule, were similar to those between AChE and ACh. The adhesion forces between AChE and Ch were (20±2) pN, a little weaker than that between ACh and ACHE. These results indicated that AChE had a steering role for the diffusion of ACh toward it and had r     

Single-molecule atomic force microscopy on live cells compares aptamer and antibody rupture forces

Some researchers have questioned whether synthetic aptamers bind as robustly as natural antibodies. To address this issue, we used single-molecule atomic force microscopy to measure the rupture force between a protein and both its aptamer and its antibody. The rupture force on live cell membranes between the aptamer and protein was 46 ± 26 pN; the force with the antibody was 68 ± 33 pN, we conclude that the binding forces are about equal.     

Microscopic origin of the humidity dependence of the adhesion force in atomic force microscopy

Water condenses between an atomic force microscope (AFM) tip and a surface to form a nanoscale bridge that produces a significant adhesion force on the tip. As humidity increases, the water bridge always becomes wider but the adhesion force sometimes decreases. The authors show that the humidity dependence of the adhesion force is intimately related to the structural properties of the underlying water bridge. A wide bridge whose width does not vary much with tip-surface distance can increase its volume as distance is increased. In this case, the adhesion force decreases as humidity rises. Narrow bridges whose width decreases rapidly with increasing tip-surface distance give the opposite result. This connection between humidity dependence of the adhesion force and the structural susceptibility of the water bridge is illustrated by performing Monte Carlo simulations for AFM tips with various hydrophilicities.     

Investigation of surface changes on mica induced by atomic force microscopy imaging under liquids

Surface changes on muscovite mica induced by tip-surface interactions in atomic force microscopy (AFM) experiments under liquids are described. Investigations have been performed with AFM operated both in contact mode (CM-AFM) and in tapping mode (TM-AFM). Additionally, force-distance measurements have been carried out. In contrast to CM-AFM pronounced surface changes can be observed in TM-AFM experiments. However, TM-AFM images of areas previously scanned in contact mode show that imaging in contact mode changes the surface, too. An evaluation of force-distance measurements reveals that these changes depend on the adhesive interaction between tip and sample, which in turn strongly depends on the surrounding medium. The artefact can be avoided by changing the pH-value of the medium or by working with mixtures of ethanol and water. This greatly enhances the applicability of TM-AFM for in-situ investigation of surface processes on mica, which is a frequently used substrate for many technological and biological applications.     

Probing protein-peptide-protein molecular architecture by atomic force microscopy and surface plasmon resonance

We demonstrate the creation of a protein multilayer which utilises the high affinity interaction between streptavidin and biotin and incorporates a peptidic spacer. Surface plasmon resonance measurements enabled us to monitor the construction of the multilayer in real time. Atomic force microscopy was utilised to determine surface functionality at each stage of the multilayer construction, allowing us to investigate the associated mechanical properties. In this context we observed an increase in biomolecular stretching on the formation of the multilayer. We demonstrate, utilising circular dichroism, that variations in the solvent can affect the secondary structure of the peptide linker and hence its mechanical properties. Trifluoroethanol titrations on the assembled system indicate that the multilayer properties are also stimuli responsive with regard to solvent conditions. These results indicate that the multilayer stretch before cleavage is increased in the presence of trifluoroethanol. This was not expected from the study of the individual linker alone, indicating the need to study the system as a whole as opposed to the isolated components.