Atomic force microscopy (AFM) has great potential as a tool for structural biology, a field in which there is increasing demand to characterize larger and more complex biomolecular systems. However, ...the poorly characterized silicon and silicon nitride probe tips currently employed in AFM limit its biological applications. Carbon nanotubes represent ideal AFM tip materials due to their small diameter, high aspect ratio, large Young's modulus, mechanical robustness, well-defined structure, and unique chemical properties. Nanotube probes were first fabricated by manual assembly, but more recent methods based on chemical vapor deposition provide higher resolution probes and are geared towards mass production, including recent developments that enable quantitative preparation of individual single-walled carbon nanotube tips J. Phys. Chem. B 105 (2001) 743. The high-resolution imaging capabilities of these nanotube AFM probes have been demonstrated on gold nanoparticles and well-characterized biomolecules such as IgG and GroES. Using the nanotube probes, new biological structures have been investigated in the areas of amyloid-beta protein aggregation and chromatin remodeling, and new biotechnologies have been developed such as AFM-based haplotyping. In addition to measuring topography, chemically functionalized AFM probes can measure the spatial arrangement of chemical functional groups in a sample. However, standard silicon and silicon nitride tips, once functionalized, do not yield sufficient resolution to allow combined structural and functional imaging of biomolecules. The unique end-group chemistry of carbon nanotubes, which can be arbitrarily modified by established chemical methods, has been exploited for chemical force microscopy, allowing single-molecule measurements with well-defined functionalized tips.
•Silicon(100) surfaces with diamines followed by anchoring of copper complexes over the diamine layer, an approach that could be used for advanced functionalization of semiconducting surfaces.•Lewis ...basicity (using Fowkes–van Oss–Chaudhury–Good surface tension model) and adhesion thermodynamic work (using chemical force microscopy) were determined.•Higher basicity and thermodynamic work correlate with selective copper acetate monolayer grow. The cyclic voltammetry studies confirm the confined copper redox activity.
Silicon(100) surfaces have been modified with three different amines (aniline, benzylamine and dodecylamine) and diamines (4-aminopyridine, 4-aminomethylpyridine, 1,12-dodecyldiamine). The surface energy was measured by contact angle technique. For Si-diamine surfaces, Lewis basicity (using Fowkes–van Oss–Chaudhury–Good surface tension model) and adhesion thermodynamic work (using chemical force microscopy) were determined. We related these data, the amine/diamine nature and their geometry on the surface (via DFT calculations) with the consequent ability to coordinate copper(II) acetate. Finally, copper(II) acetate monolayers behavior was studied by cyclic voltammetry.
The XPS (ESCA) method was employed to quantitatively determine polyelectrolyte adsorption on the mica basal plane from low ionic strength solutions. Particular emphasis was given to the effect of the ...polyelectrolyte charge density. By combining the results obtained from XPS and surface force measurements it was possible to analyze the cation exchange at the surface that occurs as a result of polyelectrolyte adsorption. AFM-imaging was used to obtain information on the structure of the adsorbed layer when the polyelectrolyte coverage was low. Further, the desorption of preadsorbed polyelectrolyte layers by addition of inorganic salt and by addition of an anionic surfactant was investigated by XPS and some complementary surface force measurements. The results demonstrate that the lower the polyelectrolyte charge density is, the easier it is to remove the polyelectrolyte from the surface. The surfactant, which by itself does not adsorb to the mica surface, is more efficient in this respect than the inorganic salt. This observation can be rationalized by considering that the surfactant and polyelectrolyte form complexes with each other. Thus, the surfactant brings negative charges into the adsorbed layer that reduces the affinity to the surface. However, high-charge-density polyelectrolytes are removed to a very limited degree even when the surfactant concentration is above the critical micellar concentration, which is explainable by the poor solubility of the polyelectrolyte−surfactant complexes formed.
Clay–oil interactions play a critical role in determining the wettability of sandstone oil reservoirs, which, in turn, governs the effectiveness of enhanced oil recovery methods. In this study, we ...have measured the adhesion between –COOH functional groups and the siloxane and aluminol faces of kaolinite clay minerals by means of chemical force microscopy as a function of pH, salinity (from 0.001 M to 1 M) and cation identity (Na+ vs. Ca2+). Results from measurements on the siloxane face show that Ca2+ displays a reverse low-salinity effect (adhesion decreasing at higher concentrations) at pH 5.5, and a low salinity effect at pH 8. At a constant Ca2+ concentration of 0.001 M, however, an increase in pH leads to larger adhesion. In contrast, a variation in the Na+ concentration showed less effect in varying the adhesion of –COOH groups to the siloxane face. Measurements on the aluminol face showed a reverse low-salinity effect at pH 5.5 in the presence of Ca2+, whereas an increase in pH with constant ion concentration resulted in a decrease in adhesion for both Ca2+ and Na+. Results are explained by looking at the kaolinite’s surface complexation and the protonation state of the functional group, and highlight a more important role of the multicomponent ion exchange mechanism in controlling adhesion than the double layer expansion mechanism.
Chemical force microscopy (CFM) is an extension of atomic force microscopy (AFM) that employs a chemically-functionalized tip, which makes it possible to control chemical interactions between tip and ...sample, so CFM can be used to probe local chemical information on the surfaces of materials and biological samples under near-native environments at nanoscale spatial resolution. We describe applications of CFM for materials characterization, including measurements of single intermolecular interaction forces and investigations of nanoscale heterogeneity of surface-chemical properties of polymeric materials.
Atomic force microscopy with chemically modified cantilever tips (chemical force microscopy) was used to study the pull-off forces (adhesion forces) on cellulose model surfaces and bleached softwood ...kraft pulp fibers in aqueous media. It was found that for the –COOH terminated tips, the adhesion forces are dependent on pH, whereas for the –CH
3 and –OH terminated tips adhesion is not strongly affected by pH. Comparison between the cellulose model surfaces and cellulosic fibers under our experimental conditions reveal that surface roughness does not affect adhesion strongly. X-ray photoelectron spectroscopy (XPS) and Fourier Transformed Infrared (FTIR) spectroscopy reveal that both substrate surfaces have homogeneous chemical composition. The results show that chemical force microscopy can be used for the chemical characterization of cellulose surfaces at a nano-level.
Chemical force microscopy (CFM) was used to characterize the chemical heterogeneity of two commercially available nanofiltration and reverse osmosis membranes. CFM probes were modified with three ...different terminal functionalities: methyl (CH
3), carboxyl (COOH), and hydroxyl (OH). Chemically distinct information about the membrane surfaces was deduced based on differences in adhesion between the CFM probes and the membrane surfaces using both traditional atomic force microscopy (AFM) force measurements and spatially resolved friction images. Contact angle titration and streaming potential measurements provided general information about surface chemistry and potential, which largely complemented the CFM analyses, but could not match the accuracy of CFM on the atomic level. Using CFM it was found that both membranes were characterized as chemically heterogeneous. Specifically, membrane chemical heterogeneity became more significant as the scan size approached colloidal or micron-sized dimensions. In many instances, the chemically unique regions, contributing to the overall chemical heterogeneity of the membrane surface, were substantially different in chemistry (e.g., hydrophobicity) from that determined for the surface at large from contact angel and streaming potential analyses. Topographical and corresponding CFM images supports previous adhesion studies finding a correlation between surface roughness and the magnitude of adhesion measured with AFM. However, chemical specificity was also significant and in turn measurable with CFM. The implication of these findings for future membrane development is discussed.
A method for direct functionalization of silicon and silicon nitride cantilevers with bifunctional silanes was tested with model surfaces to determine adhesive forces for different hydrogen-bonding ...chemistries. Application for biomass surface characterization was tested by mapping switchgrass and isolated switchgrass cellulose in topographic and force-volume mode using a cellulose-specific cantilever.
Adhesive and frictional forces between surfaces modified with self-assembled monolayers (SAMs) and immersed in solvents were measured with chemical force microscopy as functions of surface ...functionality and solvent. Si/SiO2substrates were modified with SAMs of alkylsiloxanes (SiCl3(CH2)n-X), and gold-coated AFM tips were modified with SAMs of alkylthiolates (HS-(CH2)n-X). SAMs of alkylsiloxanes terminated in a methyl or oxidized vinyl group; SAMs of alkanethiolates terminated in a methyl or carboxyl group. Adhesive and frictional forces were measured in hexadecane, ethanol, 1,2-propanediol, 1,3-propanediol, and water. The work of adhesion (W) was calculated with the Johnson–Kendall–Roberts theory of adhesive contact. The JKR values agreed well with values derived from the Fowkes–van Oss–Chaudhury–Good surface tension model and from contact angle results. Calculated values ofWfor all combinations of contacting surfaces and solvents spanned two orders of magnitude.Wcorrelated with the surface tension of the solvent for hydrophobic/hydrophobic interactions; hydrophilic/hydrophilic and hydrophobic/hydrophilic interactions were more complex. Friction forces were fit to a modified form of Amonton's law. For any solvent, friction coefficients were largest for the hydrophilic/hydrophilic contacting surfaces. The friction coefficient for any contacting pair was largest in hexadecane. In polar solvents, friction coefficients scaled with solvent polarity only for hydrophobic/hydrophobic contacting pairs.
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