Intermolecular interactions between distinct chemical functionalities define a multitude of adhesion events in chemistry, biology and materials science. Modern techniques for measuring molecular ...level forces have allowed direct quantitative characterization of these interactions. In
particular, chemical force microscopy (CFM), which uses the probe tip of a force microscope covalently modified with specific organic functional groups, provides a flexible approach for studying interactions between specific chemical functionalities. In this review, we survey the progress
in CFM in recent years as it applies to adhesion of soft materials. We show how new developments in the experimental and theoretical approaches continue to build a realistic and detailed picture of adhesion interaction in condensed phases. We specifically highlight the importance of the kinetics
of the unbinding processes and solvation effects in determining the strength of intermolecular interactions. We also describe some recent new directions in CFM, such as high-throughput adhesion measurements and mapping of full intermolecular potentials.
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.
In this paper, we describe a statistical method of quantification of the number of functional groups at the contact area of a probe tip for atomic force microscopy from the result of repetitive ...pull-off force measurements. We have investigated laboratory-made carbon nanotube (CNT) probe tips to apply them for chemical force microscopy because limited number of functional groups at the tip-end is expected. Using a CNT tip, we conducted repetitive pull-off force measurements against a self-assembled monolayer terminated with carboxyl group and analyzed them in terms of the number of hydrogen bond groups at the CNT tip. The elementary hydrogen bond rupture force quantum in
n-decane medium was estimated to be 84.2
±
0.5
pN in the present system. Thus it was revealed that only a couple of hydrogen bond groups of the CNT tip were participating in hydrogen bonding with the sample on an average in this experimental system.
We have studied self-assembled monolayers (SAMs) of asymmetric dialkyldisulfide derivatives of the form CH
3–(CH
2)
11+
m
–S–S–(CH
2)
11–OH with
m
=
−4, −3, 0, +2 and +4 on gold. Sub-nanoscale ...changes in the length of the CH
3-terminated alkylchain have been used to selectively protrude one particular end group in the resulting film. The alteration of the chain length in only two methylene units already results in changes of surface properties, which have been detected with local (chemical force microscopy) and macroscopic (contact angle) techniques. In particular, advancing contact angles can be adjusted between 40° and 80°. The adhesion between a hydrophobic tip and these SAMs in water is determined by the chemical nature of the protruding end group. Chemical force microscopy, X-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy have shown that these SAMs are composed of mixed, well-packed CH
3– and OH–alkylthiolate branches. The surface composition ratio is close to 1:1 for all investigated SAMs.
Chemical force microscopy (CFM) based on tapping mode Atomic force microscopy (AFM) utilized with topographic and phase-shift analyses was used to investigate the topography and surface chemical ...properties, respectively, of the long trichoid sensilla on the antennae of male
Helicoverpa zea. AFM topographic imaging revealed regular series of step-ridges along nearly the entire length of each sensillum, except for the basal ca. 1/3 portions, which were devoid of such ridges. Inter-ridge regions were flat, with regularly spaced pores, ca. 30 nm in diameter populating these planar areas. Many pores exhibited a raised dome that often nearly completely spanned the depression, with only the edges of the depressed portion of the pore still visible. Some pores were observed also along the bases of the ridges. CFM probing of the surface for chemical interactions with the SiO
2 hydrophilic tip revealed consistently diminished hydrogen bonding of the ridge edge areas with the tip than along the flat planar inter-ridge regions. Surfaces of domes over the pores also tended to have less hydrogen bonding with the tip than the planar surfaces. Functionalizing the CFM tip by bonding octadecyl-hydrocarbon to it eliminated these surface chemical-CFM tip interactions and no differences in tip interaction with the sensillar surfaces were observed. Trichoid sensilla from the male antennae of a second species,
Utethesia ornatrix, did not exhibit similar heterogeneity between ridge edges versus planar areas with regard to hydrogen bonding with the SiO
2 hydrophilic tip. Pores on
U. ornatrix sensilla occurred only along the bases of ridges on their trichoid sensilla. We suggest that the surface lipids of the
H. zea sensilla are distributed in a chemically heterogeneous fashion to aid adsorption and transport of aldehyde pheromone component molecules through the pores into the sensillum lumen, possibly through solubilization in an epicuticular lipid layer. The trichoid sensilla of
U. ornatrix do not exhibit such surface chemical heterogeneity, and this species-difference may be due to the usage by
U. ornatrix of hydrocarbon molecules rather than aldehydes for their sex pheromone components.
Surface chemical microstructure of hydrochloric acid hydrolyzed tapioca starch producing different amylose:amylopectin (Am:Ap) ratios were studied with scanning chemical force microscopy (CFM). The ...chemical force probes were functionalized of two types with –OH (phosphate specific) and –CH
3 (carbon specific). Lateral force trace-minus-retrace (TMR) images from –OH and –CH
3 probes revealed changes in the phosphate domains and the carbon backbone for the varying acid hydrolyzed tapioca starch compared to that of the native tapioca starch. Scanning electron micrographs (SEM) showed different degree of the granule surface disruption before and after hydrolysis. The exterior structures of the acid hydrolyzed starch granules were chemically investigated with CFM to study the relationships of the surface molecular structures and the Am:Ap ratios.
A novel chemically sensitive imaging mode based on adhesive force detection by previously developed pulsed-force-mode atomic force microscopy (PFM-AFM) is presented. PFM-AFM enables simultaneous ...imaging of surface topography and adhesive force between tip and sample surfaces. Since the adhesive forces are directly related to interaction between chemical functional groups on tip and sample surfaces, we combined the adhesive force mapping by PFM-AFM with chemically modified tips to accomplish imaging of a sample surface with chemical sensitivity. The adhesive force mapping by PFM-AFM both in air and pure water with CH
3- and COOH-modified tips clearly discriminated the chemical functional groups on the patterned self-assembled monolayers (SAMs) consisting of COOH- and CH
3-terminated regions prepared by microcontact printing (μCP). These results indicate that the adhesive force mapping by PFM-AFM can be used to image distribution of different chemical functional groups on a sample surface. The discrimination mechanism based upon adhesive forces measured by PFM-AFM was compared with that based upon friction forces measured by friction force microscopy. The former is related to observed difference in interactions between tip and sample surfaces when the different interfaces are detached, while the latter depends on difference in periodic corrugated interfacial potentials due to Pauli repulsive forces between the outermost functional groups facing each other and also difference in shear moduli of elasticities between different SAMs.
We compared the chemical composition of wood fibres and fibre surfaces of several eucalypt species and hybrids originating from various growth sites in South Africa. The objective was to test for ...differences in chemical surface composition due to genetics or site with the ultimate aim to facilitate a tailor-made supply of wood for pulping that results in an optimal blend of fibres that can be pulped together with similar yields. This, however, requires a sound knowledge of the fibre properties. The surface functionality on the single fibre level is a key property, because it determines how good inter-fibre bonding will be when paper is formed, which depends amongst other fibre properties on the amount of free hydroxyl groups that are available and therefore on the cellulose content on the fibre surface. The cellulose and lignin content on the fibre surface were determined with chemical force microscopy, a variation of atomic force microscopy. Since the general bulk composition of the fibre and the surface composition might differ, both parameters were determined. We found significant differences in the cellulose and lignin content on fibre surfaces, with regard to genotype and site, respectively. In some, but not all, cases, the surface composition of wood fibres followed the bulk composition, and differences were generally more pronounced. Differences due to genotype were significant, especially with regard to the surface lignin content—but variation due to site was also distinctly recognisable. This variation in surface functionality could be the reason why some pulpwood blends result in a lower pulp yield and different quality.
We show that the theory of the surface energy of condensed phases, which is based on its separation into London–van der Waals and acid–base components, can be used for the interpretation of adhesion ...force and work of adhesion measurements with the chemical force microscope. However, it can be done in a straightforward manner only for surfaces and liquids with the same type of solvation. Even in this case, some complicating factors like dissimilarity of the interacting hydro- and fluorocarbon groups has to be considered. At lyophilic surfaces liquid tends to form layered solvation structures at the interface and the presence of these structures may influence interfacial energies. We show that this type of effect can be very significant for hydrophilic surfaces in water. For instance, we found the surface free energy of the OH or COOH terminated monolayers in water is 20÷40 mJ m
−2 higher than interfacial energy in hexadecane. Additionally, we predict that if liquid forms a layered structure at the interface, the observed adhesion force depends on the load applied to the CFM cantilever. The experimental evidence of such a behavior is presented.
PDF
