Pull-off force measurements between rough surfaces by atomic force microscopy

Direct measurements of the pull-off (adhesion) forces between pharmaceutical particles (beclomethasone dipropionate, a peptide-type material, and lactose) with irregular geometry and rough polymeric surfaces (series of polypropylene coatings, polycarbonate, and acrylonitrile-butadiene-styrene) were carried out using the atomic force microscope. These measurements showed that roughness of the interacting surfaces is the significant factor affecting experimentally measured pull-off forces. A broad distribution of pull-off force values was noted in the measurements, caused by a varying adhesive contact area for a particle located on rough substrate. The possibility of multiple points of contact between irregularly shaped pharmaceutical particles and substrate surfaces is demonstrated with nanoindentations of the particle in a fluoro-polymer film. Force-distance curves showing the "sawtooth" pattern are additional evidence that particles make contact with substrates at more than one point. Reduced adhesion of 10- to 14-microm-diameter lactose and peptide material particles to the polypropylene coatings with a roughness of 194 nm was found in this study. Similar pull-off force versus roughness relationships are also reported for the model spherical particles, silanized glass particle with a size of 10 microm and polystyrene particle with a diameter of 9 microm, in contact with polypropylene coatings of varying roughness characteristics. It was found that the model recently proposed by Rabinovich et al. (J. Colloid Interface Sci. 232, 1-16 (2000)) closely predicts the pull-off forces for glass and lactose particles. On the other hand, the adhesion of the peptide material and polystyrene particle to polypropylene is underestimated by about an order of magnitude with the theoretical model, in which the interacting substrates are treated as rigid materials. The underestimate is attributed to the deformation of the peptide material and polystyrene particles.     

Measurement of polyamide and polystyrene adhesion with coated-tip atomic force microscopy

This work presents atomic force microscopy (AFM) measurements of adhesion forces between polyamides, polystyrene and AFM tips coated with the same materials. The polymers employed were polyamide 6 (PA6), PA66, PA12 and polystyrene (PS). All adhesion forces between the various unmodified or modified AFM tips and the polymer surfaces were in the range -1.5 to -8 nN. The weakest force was observed for an unmodified AFM tip with a PS surface and the strongest was between a PS-coated tip and PS surface. The results point to both the benefits and drawbacks of coated-tip AFM force-distance measurements. Adhesion forces between the two most dissimilar (PA6-PS and PA66-PS) materials were significantly asymmetric, e.g., the forces were different depending on the relative placement of each polymer on the AFM tip or substrate. Materials with similar chemistry and intermolecular interactions yielded forces in close agreement regardless of placement on tip or substrate. Using experimental forces, we calculated the contact radii via four models: Derjaguin, Muller, and Toporov; Johnson, Kendall, and Roberts; parametric tip-force-distance relation; and a square pyramid-flat surface (SPFS) model developed herein. The SPFS model gave the most reasonable contact tip radius estimate. Hamaker constants calculated from the SPFS model using this radius agreed in both magnitude and trends with experiment and Lifshitz theory.     

Comparison of scanning ion conductance microscopy with atomic force microscopy for cell imaging

We present the first direct comparison of scanning ion conductance microscopy (SICM) with atomic force microscopy (AFM) for cell imaging. By imaging the same fibroblast or myoblast cell with both technologies in series, we highlight their advantages and disadvantages with respect to cell imaging. The finite imaging force applied to the sample in AFM imaging results in a coupling of mechanical sample properties into the measured sample topography. For soft samples such as cells this leads to artifacts in the measured topography and to elastic deformation, which we demonstrate by imaging whole fixed cells and cell extensions at high resolution. SICM imaging, on the other hand, has a noncontact character and can provide the true topography of soft samples at a comparable resolution.     

Formation of aminosilane-functionalized mica for atomic force microscopy imaging of DNA

Factors affecting the functionalization of mica with aminosilanes, in particular, aminopropyltriethoxysilane (APTES-mica), formed from the vapor phase have been systematically studied. The relative humidity (RH) during vapor deposition has been varied, and postdeposition treatment through baking has been used, as well as the comparison of mono and trifunctionality, to investigate how optimal surfaces for AFM imaging of DNA are formed. It is found that the stability of the APTES layers is a consequence of lateral polymerization and not covalent attachment to the mica substrate. At low RH (<25%), DNA adopts an open, well-resolved conformation, whereas at >25% RH, DNA surface-induced condensation occurs. Contact mode AFM scratching experiments show that two main structures of the silane layer exist at different humidity: a monolayer exists at RH < 25%, and a bilayer structure exists at RH > 25%. Finally, structural changes that these two layer types undergo after baking at 150 degrees C were investigated by AFM and X-ray photoelectron spectroscopy (XPS), and these now prevented DNA from binding to the APTES-mica, except in the presence of Mg(II) ions.     

High resolution imaging of functional group distributions on carbon surfaces by tapping mode atomic force microscopy

Here we report, for the first time, the high resolution imaging of hydrophilic, polar functional group distributions on flat carbon surfaces by phase contrast in noncontact tapping mode AFM.     

Generation of amino-terminated surfaces by chemical lithography using atomic force microscopy

Self-assembled monolayers (SAMs) covered with nitroso end groups were reduced using an atomic force microscope. As the bias voltage become more negative (beyond -4 V), the surface potential of the scanned area become closer to that of the amino-terminated SAM. Following this chemical change, however, no change in topographic features was detected, implying retained stability of the underlying SAM layer. We then released carboxylate-modified polystyrene (PS) spheres into a pH 4 solution containing the sample. Subsequent imaging with atomic force microscopy (AFM) revealed that these PS spheres were only selectively immobilized on the regions that were originally scanned at -6 V to form amino termination. In summary, using AFM set to a specific voltage, we were able to selectively generate micropatterned regions of the SAM with amino termination.     

Shear banding in strained semicrystalline polyamide 6 films as revealed by atomic force microscopy: Role of the amorphous phase

Atomic force microscopy (AFM) has been used to visualize the plastic deformation mechanisms that are responsible for the yielding of semicrystalline polymers of low degree of crystallinity (<50%). Indeed, AFM, if operated in suitable conditions, is able to image both the amorphous and the crystalline phases. Polyamide 6 films have been drawn at temperatures T < 160 °C. Postmortem AFM observations show that, at yield, shear bands nucleate and propagate in the amorphous phase. They cross the crystalline lamellae and run over the whole surface of the sample. By crossing the lamellae, they form nanoblocks of uniform size. Neither the size of the nanoblocks nor the angle between the tensile axis and the shear bands can be explained in terms of crystal plasticity. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 687–701, 2004     

Dynamic imaging of single DNA-protein interactions using atomic force microscopy

Atomic force microscopy (AFM) imaging of static DNA-protein complexes, in air and in liquid, can be used to directly obtain quantitative and qualitative information on the structure of different complexes. For example, DNA length, the location of preferential binding sites for proteins and bending of DNA as a result of the complexation can all be measured. Recording consecutive AFM images of DNA and protein molecules under conditions that they are still able to move and interact, or dynamic AFM imaging, however, can reveal information on the dynamic aspects of the interactions between these molecules. Here, an overview is given of the technical challenges that need to be considered for successful dynamic AFM imaging studies of individual DNA-protein interactions. Necessary technical improvements to the AFM set-up and the development of new sample preparation methods are described in this paper.     

Capillary force in atomic force microscopy

Under ambient conditions, a water meniscus generally forms between a nanoscale atomic force microscope tip and a hydrophilic surface. Using a lattice gas model for water and thermodynamic integration methods, we calculate the capillary force due to the water meniscus for both hydrophobic and hydrophilic tips at various humidities. As humidity rises, the pull-off force rapidly reaches a plateau value for a hydrophobic tip but monotonically increases for a weakly hydrophilic tip. For a strongly hydrophilic tip, the force increases at low humidities (<30%) and then decreases. We show that mean-field density functional theory reproduces the simulated pull-off force very well.     

Single-photon atomic force microscopy

In the last few years, an array of novel technologies, especially the big family of scanning probe microscopy, now often integrated with other powerful imaging tools such as laser confocal microscopy and total internal reflection fluorescence microscopy, have been widely applied in the investigation of biomolecular interactions and dynamics. But it is still a great challenge to directly monitor the dynamics of biomolecular interactions with high spatial and temporal resolution in living cells. An innovative method termed “single-photon atomic force microscopy” (SP-AFM), superior to existing techniques in tracing biomolecular interactions and dynamics in vivo, was proposed on the basis of the combination of atomic force microscopy with the technologies of carbon nanotubes and single-photon detection. As a unique tool, SP-AFM, capable of simultaneous topography imaging and molecular identification at the subnanometer level by synchronous acquisitions and analyses of the surface topography and fluorescent optical signals while scanning the sample, could play a very important role in exploring biomolecular interactions and dynamics in living cells or in a complicated biomolecular background.     

Adsorption of humic acid onto carbonaceous surfaces: atomic force microscopy study

The adsorption of humic acid (HA) onto highly ordered pyrolytic graphite (HOPG) surfaces at different concentrations has been studied by atomic force microscopy. When HA concentration was increased from 10 to 1,000 mg/L, HA can sequentially form spherical particles, layered structures, and connected blocks on HOPG surfaces. The findings of the layer structures and small amount of fine chains have been verified and discussed. When HA was acidified by addition of acetic acid, it changed into small rigid particles. These results indicated that HA can be considered as supramolecular associations of self-assembling heterogeneous and relatively small molecules, and a small amount of polymers. The present results are important for understanding HA molecular structures and their adsorption characteristic on carbonaceous surfaces.     

Single biomolecule imaging with frequency and force modulation in tapping-mode atomic force microscopy

A new intermittent-contact atomic force microscopy (AFM) mode (frequency and force modulation AFM, FFM-AFM) has been recently proposed to characterize soft samples. This method uses excitation force frequency and amplitude modulation to eliminate bistability and reduce the tip-sample forces. This letter describes theoretical modeling of FFM-AFM applied to a single bacteriorhodopsin molecule on a substrate, showing that its cross section can be measured without damage, in contrast to conventional tapping-mode AFM. Speculations are made regarding nonideal conditions and the ability of FFM-AFM to perform quantitative nanoelasticity measurements.     

2-D array formation of genetically engineered viral cages on au surfaces and imaging by atomic force microscopy

The preparation and subsequent imaging of a two-dimensional array of a genetically and chemically modified cowpea chlorotic mottle virus (CCMV) is described. The genetic mutation provides symmetrically dispersed exposed thiol groups on the outer surface of the virus capsid. These functional groups can be used to covalently bind the capsid to smooth Au substrate. AFM imaging suggests that the genetic mutation by itself does not promote array formation but, rather, aggregation through disulfide linkages. However, breaking the symmetry of the capsid using a solid-phase approach and chemically passivating the exposed thiol groups with iodoacetic acid results in a capsid with exposed thiols only on one side of the particle. These symmetry-broken capsids were able to form self-assembled monolayers (SAM) on a Au surface.     

Molecular imaging of membrane proteins and microfilaments using atomic force microscopy

Atomic force microscopy (AFM) is an emerging technique for a variety of uses involving the analysis of cells. AFM is widely applied to obtain information about both cellular structural and subcellular events. In particular, a variety of investigations into membrane proteins and microfilaments were performed with AFM. Here, we introduce applications of AFM to molecular imaging of membrane proteins, and various approaches for observation and identification of intracellular microfilaments at the molecular level. These approaches can contribute to many applications of AFM in cell imaging.     

Direct imaging of meniscus formation in atomic force microscopy using environmental scanning electron microscopy

Environmental scanning electron microscopy was used to image meniscus formation between an AFM tip and a surface. At high relative humidity, 70%-99%, the meniscus formed is 100 to 1200 nm in height, orders of magnitude larger than predicted by the Kelvin equation using spherical geometry. The height of the meniscus also demonstrates hysteresis associated with increasing or decreasing relative humidity.     

Collagen adsorption and structure on polymer surfaces observed by atomic force microscopy

The structure and adsorption patterns of type I and type III collagen were imaged on various polymer substrates with atomic force microscopy. Type I collagen had higher adsorption on polystyrene than on a series of polymethacrylates and formed a network of tightly, interwoven strands. Upon adsorption to different polymethacrylates, with varying side chain lengths, the collagen molecules formed long, branching fibrils. Types I and III collagen had different adsorption patterns, in some cases, on the identical substrate material. For example, instead of forming a tightly packed network, type III forms long, branching fibers on the polystyrene surface. On other materials, such as poly(n-butyl methacrylate), the two types of collagen showed similar adsorption pattern and structure. Adsorbed collagen was also imaged on various blends of polystyrene and polymethacrylates to determine how the polymer surface chemical structure and surface topography mediates protein adsorption.     

Nanomanipulation of extended single-DNA molecules on modified mica surfaces using the atomic force microscopy

The technique of nanometer scale manipulation is very important in constructing nano-structures and nano-devices. By using atomic force microscope, three kinds of controllable manipulation on single-DNA molecules were introduced in the paper. DNA molecules deposited and extended on modified mica surface were first imaged by tapping mode, then cutting, bending, and pushing were respectively performed on single-DNA molecules. The results of the manipulation depend on the interaction between tip and DNA as well as between substrate and DNA.     

In-situ atomic force microscopy (AFM) imaging: influence of AFM probe geometry on diffusion to microscopic surfaces

The effect of AFM probe geometry on diffusion to micrometer-scale reactive (electrode) interfaces is considered. A disk-shaped substrate electrode was held at a potential to reduce a species of interest (aqueous Ru(NH 3) 6 (3+)) at a diffusion-controlled rate and the current response during AFM imaging provided information on local mass transport to the interface. This approach reveals how the AFM probe influences diffusion to a reactive surface, which is of importance in more clearly delineating the conditions under which in-situ AFM can be treated as a noninvasive probe of surface processes involving mass transport (e.g., electrode reactions and crystal dissolution and growth). An assessment has been made of three types of probes: V-shaped silicon nitride contact mode probes; single beam silicon probes; and batch-fabricated scanning electrochemical-atomic force microscopy (SECM-AFM) probes. Two disk electrodes, (6.1 microm and 1.6 microm diameter) have been considered as substrates. The results indicate that conventional V-shaped contact mode probes are the most invasive and that the batch-fabricated SECM-AFM probes are the least invasive to diffusion at both of the substrates used herein. The experimental data are complemented by the development of simulations based on a simple 2D model of the AFM probe and active surface site. The importance of probe parameters such as the cantilever size, tip cone height, and cone angle is discussed, and the implications of the results for studies in other areas, such as growth and dissolution processes, are considered briefly.     

Interaction between solid surfaces in a polymer melt studied by atomic force microscopy

Using an atomic force microscope (AFM) the interaction between an AFM tip and a planar silicon oxide surface has been measured across poly(dimethylsiloxane) (PDMS, M_W = 18 000). Due to the small radius of curvature of the AFM tip the hydrodynamic repulsion of the tip was negligible and forces could be measured in equilibrium. This is confirmed by the fact that force-versus-distance curves measured at different approaching velocities were indistinguishable. In equilibrium a repulsive force was observed which could best be described by a power law, F ∝ 1/d^2.5 where d is the distance.     

Tip–surface interactions in noncontact atomic force microscopy on reactive surfaces

The imaging process in noncontact atomic force microscopy (AFM) is studied on a number of reactive surfaces, namely, the Takayanagi reconstructed Si(111), InP(110), and GaAs(110). We show that on these surfaces, the short-range dangling-bond type of interaction between the tip and the surface is decisive in achieving atomic resolution. The short-range tip–surface interaction is modeled in the density functional theory within the GGA approximation. We show that we can achieve quantitative agreement with the experimental data in the commonly used frequency modulation technique for AFM surface corrugation with a very simple model for the tip geometry treating the tip–surface interaction in the perturbation theory. The nature of the short-range tip–surface interaction on the three surfaces is considered and the consequences thereof for the experiments is discussed.