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.     

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.     

Surface ionization state and nanoscale chemical composition of UV-irradiated poly(dimethylsiloxane) probed by chemical force microscopy, force titration, and electrokinetic measurements

The surface chemistry and ionization state of cross-linked poly(dimethylsiloxane) (PDMS) exposed to UV/ozone were studied as a function of treatment time. Various complementary and independent experimental techniques were utilized, which yielded information on the macroscopic as well as the nanometric scale. The average chemical composition of the PDMS surface was quantitatively investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). It was found that the top 1-2 nm surface layer was dominated by silanol groups (-SiOH) for which the concentration increased with increasing treatment dose. The lateral distributions of the silanol groups were analyzed on the nanometer scale by means of atomic force microscopy (AFM) with chemically functionalized tip probes in aqueous buffer solutions at varying pHs. Spatially dependent pull-off force curves (also called "force volume" imaging) indicated the presence of strong chemical heterogeneity of the probed surface. This heterogeneity took the form of patches of silanol functionalities with high local concentration surrounded by a matrix of predominantly hydrophobic domains at low pH. The average pull-off forces for the entire surface scanned were significantly reduced for pH values larger than a characteristic pK(a) constant (in the range between 4.5 and 5.5). The extent of the decrease in the pull-off force and the particular value of pK(a) were found to be a function of treatment time and to differ from the commonly reported values for silanol functional groups on a homogeneous silica surface. These dependences were ascribed to the evoking of a protonation/deprotonation process of the surface silanol groups which was sensitive to the hydrophobic/hydrophilic balance of their close molecular environment. Intermolecular hydrogen bonding may also account for the shifts in the surface pK(a). Furthermore, depending on the nature of the electrolyte, a third effect related to double layer composition, as determined by specific ion adsorption, was quantitatively analyzed by streaming potential measurements in the presence of sodium chloride and phosphate electrolytes.     

Charge-transfer complexes interactions evidenced by chemical force microscopy

Charge-transfer complexes have been detected by chemical force microscopy (CFM) between a tip and a substrate respectively functionalized with trinitrofluorenone and 9-anthracenemethanol siloxane derivatives.     

Nano-scale mapping of mechanical and chemical surface properties of pigment coated surfaces by torsional harmonic atomic force microscopy

In this study, torsional harmonic atomic force microscopy (TH-AFM, HarmoniX mode) was applied for surface mapping of the mechanical properties of pigment-latex coated paper samples. In addition, topographic images and force maps of adhesive tip-sample interactions were captured concurrently. The spatial distribution of latex binder on the composite surface was distinguished with high resolution. The latex was found to dominate the surface chemistry of the composite coating, despite the fact that latex is a minor component in the coating color formulation. The latex resided as a thick layer between the pigments and as a thin layer on the individual pigments. In addition, the tip-sample thermodynamic work of adhesion of the composite materials on the coated surface was compared to the surface energy values obtained by contact angle measurements. A high tip-sample work of adhesion correlated to high surface energy.     

Ultra-sensitive imaging and interfacial analysis of patterned hydrophilic SAM surfaces using energy dissipation chemical force microscopy

Energy dissipation chemical force microscopy has been used to analyze the dissipative properties of chemically similar regions of hydroxyl- and carboxyl-terminated SAMs on gold with a hydroxyl-terminated tip. Energy dissipation imaging quantitatively isolates dissipative interfacial interactions from topography, producing a significantly more informative image than phase imaging. Also, energy dissipation force curves probed the rheological properties of the tip-sample interaction. Viscosity of the confined water increased slightly over that of the bulk, and SAM deformation was found to have a longer retardation time than restructuring of interfacial ions and solvent during tip-sample contact.     

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.     

Atomistic molecular dynamics simulations of chemical force microscopy

Chemical force microscopy and related force measurement techniques have emerged as powerful tools for studying fundamental interactions central to understanding adhesion and tribology at the molecular scale. However, detailed interpretation of these interactions requires knowledge of chemical and physical processes occurring in the region of the tip-sample junction that experiments cannot provide, such as atomic-scale motions and distribution of forces. In an effort to address some of these open issues, atomistic molecular dynamics simulations were performed modeling a chemical force microscope stylus covered with a planar C12 alkylthiolate self-assembled monolayer (SAM) interacting with a solid wall. A complete loading-unloading sequence was simulated under conditions of near-constant equilibrium, approximating the case of infinitely slow tip motion. In the absence of the solid wall, the stylus film existed in a fluid state with structural and dynamic properties similar to those of the analogous planar SAM at an elevated temperature. When the wall was brought into contact with the stylus and pressed against it, a series of reversible changes occurred culminating with solidification of the SAM film at the largest compressive force. During loading, the chemical composition of the contact changed, as much of the film's interior was exposed to the wall. At all tip heights, the distribution of forces within the contact zone was uneven and subject to large local fluctuations. Analysis using the Johnson-Kendall-Roberts, Derjaguin-Muller-Toporov, and Hertz contacts mechanics models revealed significant deviations from the simulation results, with the JKR model providing best overall agreement. Some of the discrepancies found would be overlooked in an actual experiment, where, unlike the simulations, contact area is not separately known, possibly producing a misleading or incorrect interpretation of experimental results. These shortcomings may be improved upon by using a model that correctly accounts for the finite thickness of the compliant components and nonlinear elastic effects.     

Mapping the interaction forces between TAR RNA and TAT peptides on GaAs surfaces using chemical force microscopy

The complexation of the HIV transactivation response element (TAR) RNA with the viral regulatory protein TAT is of enormous interest for the design of new sensing and therapeutic strategies. In this work, we anchored TAT peptides on GaAs surfaces using microcontact printing. Atomic force microscopy was used to quantify the interaction between TAR RNA and model TAT peptide sequences. Different pH conditions were utilized in order to assess specific vs nonspecific interactions. AFM tips functionalized with TAR RNA molecules were used to collect adhesion maps that displayed stronger interaction with peptide sequences that contained a greater number of arginine residues. All of the studies consistently showed a pH dependence of the interaction between the surface bound peptides and the TAR RNA on the AFM tips. This work quantifies the TAR RNA/TAT peptide interaction after one of the molecules is anchored on a surface. The conclusions in this paper are consistent with previous work and demonstrate that cationic residues are responsible for the polyelectrolyte-like affinity of TAT peptides for TAR RNA.     

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.     

Entropic barriers in nanoscale adhesion studied by variable temperature chemical force microscopy

Intermolecular interactions drive the vast majority of condensed phase phenomena from molecular recognition to protein folding to particle adhesion. Complex energy barriers encountered in these interactions include contributions from van der Waals forces, hydrogen bonding, and solvent medium. With the spectacular exception of hydrophobic interactions, contributions from the medium are usually considered secondary. We report a variable temperature force microscopy study of the interactions between several hydrogen bonds in different solvents that challenges this point of view. Surprisingly, we observed an increase in the strength of the interaction between carboxylic acid groups in ethanol as the temperature increased. Moreover, when we switched to a nonpolar solvent we observed the opposite behavior: The binding force decreased as the temperature increased. Kinetic model of bond dissociation provided quantitative interpretation of our measurements. We attributed the observed phenomena to a large entropic contribution from the ordered solvent layers that are forming on the probe and sample surfaces upon detachment. The observed reversal in the force vs temperature trend is a manifestation of a transition between thermodynamic and kinetic regimes of unbinding predicted by the model. Our results indicate that entropic barriers dominated by the interactions of solvent molecules with the surface exist in a much wider variety of systems than previously thought.     

Local surface charge dissipation studied using force spectroscopy method of atomic force microscopy

We propose herein a method to study local surface charge dissipation in dielectric films using force spectroscopy technique of atomic force microscopy. By using a normalization procedure and considering an analytical expression of the tip‐sample interaction force, we could estimate the characteristic time decay of the dissipation process. This approach is completely independent of the atomic force microscopy tip geometry and considerably reduces the amount of experimental data needed for the calculation compared with other techniques. The feasibility of the method was demonstrated in a freshly cleaved mica surface, in which the local charge dissipation after cleavage followed approximately a first‐order exponential law with the characteristic time decay of approximately 7–8 min at 30% relative humidity (RH) and 2–3.5 min at 48% RH. Copyright © 2015 John Wiley & Sons, Ltd.     

Attachment of trianglamines to silicon wafers,chiral recognition by chemical force microscopy

The chemical attachment of enantiomerically pure trianglamines to silicon wafers through isocyanate functions was performed. The self-assembled monolayer was analyzed by ATR-FTIR, ellipsometry, AFM, contact angle measurements. An AFM tip was functionalized by enantiomerically pure trianglamine using the same pathway. Chemical force microscopy showed chiral recognition between the functionalized tip and the functionalized wafer.     

Prospects for resolving chemical structure by atomic force microscopy: a first-principles study

In a recent paper, the chemical structure of a molecule was resolved by means of atomic force microscopy (AFM): using a metal tip terminated in a CO molecule, the authors could image the internal bonding arrangement of a pentacene molecule with remarkable spatial resolution (notably better than with other tip terminations), as verified by their first-principles calculations. Here we further explore with first-principles calculations the mechanisms, applicability, and capabilities of this approach for a wider range of situations, by varying the imaged molecule and the tip beyond the experimental cases. In our simulations, a high atomic resolution is found to be dominated by the electronic structure of the last two atoms on the tip apex which are set perpendicularly to the sample molecule. For example, tips terminated in CH(4) or pentacene itself (both having a C-H apex) yield similar images, while tips terminated in O(2) or CO give quite different images. While using a CO-terminated tip successfully resolves the chemical structure of pentacene and of other extended planar networks based on C(6) rings, this tip fails to resolve the structures of benzene (with its single C(6) ring) or nonplanar C(6) networks, such as C(60) or small-diameter carbon nanotubes. Defects (such as N substitution for a C-H group) were also found to significantly influence the image resolution. Our findings indicate that further application of this approach requires, for each sample, careful selection of a suitable "imaging" molecule as tip termination.     

Local morphological and dimensional changes of enzyme-degraded cellulose materials measured by atomic force microscopy

A study of the degradation effects of enzyme treatment on the dimensional changes of cellulose aggregate fibrils (CAFs) with dimensions of ∼100,000 × 3,000 × 300 nm from fully bleached kraft fiber was performed. CAFs were incubated with cellulase for up to 32 h. The insoluble CAFs fragments remaining after enzymatic hydrolysis were then subjected to variable relative humidity (RH). Each sample was imaged by an atomic force microscope (AFM) in tapping mode. The images were analyzed to determine the dimensional changes of the insoluble CAFs. Enzymatic hydrolysis continuously depolymerized the CAFs over 32 h, ultimately causing 20% of the CAFs to become soluble. Compared to initial dimensions of the reference CAFs with no enzymatic treatment, the dimensions of the enzyme treated CAFs were generally more responsive to humidity and exhibited an increased frequency of plastic deformations.     

Dynamic simulations of adhesion and friction in chemical force microscopy

A hybrid molecular simulation approach has been applied to investigate dynamic adhesion and friction between a chemical force microscope (CFM) tip and a substrate, both modified by self-assembled monolayers (SAMs) with hydrophobic methyl (CH(3)) or hydrophilic hydroxyl (OH) terminal groups. The method combines a dynamic model for the CFM tip-cantilever system and a molecular dynamics (MD) relaxation technique for SAMs on Au(111) at room temperature. The hybrid simulation method allows one to simulate force-distance curves (or adhesion) and friction loops (or friction coefficient) in the CFM on the experimental time scale for the first time. The simulation results also provide valuable molecular information at the interface that is not accessible in CFM experiments, such as the actual tip position with respect to the cantilever support position, molecular and hydrogen-bonding structures at the interface, and load distributions among different molecular chains (or single-molecule forces). Results show that the adhesion force and friction coefficient for the OH/OH contact pair are much larger than those for the CH(3)/CH(3) pair due to the formation of hydrogen bonds. During the retraction of a CFM tip from a surface, the CFM tip is away from the sample surface slightly while the spring undergoes dramatic elongation in the normal direction before rupture occurs. Single-molecule forces are distributed unevenly at the contact area. Surface energies calculated for functionalized surfaces compare well with those determined by experiments.     

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.     

Quantitative investigation by atomic force microscopy of supported phospholipid layers and nanostructures on cholesterol-functionalized glass surfaces

Understanding the interaction mechanisms of phospholipids with surfaces is crucial for the exploitation of lipid bilayers as models of the cell membrane as well as templates for biosensors. Moreover, controlling and manipulating lipid nanoparticles for the investigation of their properties by means of single-particle sensitive surface techniques require the ability to tailor the chemical properties of surfaces to achieve a stable and sparse binding of lipid particles, while keeping them from aggregating, or denaturing. Here we present a quantitative morphological and structural investigation by atomic force microscopy of supported phospholipid layers and nanostructures on cholesterol-functionalized glass surfaces, in comparison with other surfaces with different interfacial properties. We show that the functionalization of glass coverslips with cholesterol groups is a viable route for the production of optically transparent, scanning probe microscopy-compatible clean substrates for the effective immobilization of both extended single lipid bilayers and lipid nanoparticles.