Chemical force microscopy
With AFM, structural morphology is probed using simple tapping or contact modes that utilize van der Waals interactions between tip and sample to maintain a constant probe deflection amplitude (constant force mode) or maintain height while measuring tip deflection (constant height mode).
CFM, on the other hand, uses chemical interactions between functionalized probe tip and sample.
Choice chemistry is typically gold-coated tip and surface with R−SH thiols attached, R being the functional groups of interest.
CFM enables the ability to determine the chemical nature of surfaces, irrespective of their specific morphology, and facilitates studies of basic chemical bonding enthalpy and surface energy.
Typically, CFM is limited by thermal vibrations within the cantilever holding the probe.
[1][2] Hydrophobicity is used as the primary example throughout this consideration of CFM, but certainly any type of bonding can be probed with this method.
[1] The method was demonstrated using hydrophobicity where polar molecules (e.g. COOH) tend to have the strongest binding to each other, followed by nonpolar (e.g. CH3-CH3) bonding, and a combination being the weakest.
All combinations of functionalization were tested, both by tip contact and removal as well as spatial mapping of substrates patterned with both moieties and observing the complementarity in image contrast.
This is the simpler mode of CFM operation where a functionalized tip is brought in contact with the surface and is pulled to observe the force at which separation occurs,
being various surface energies between the tip, sample, and the medium each is in (liquids discussed below).
(e.g. from quantum chemistry simulations), the total number of ligands participating in tension can be estimated as
As stated earlier, the force resolution of CFM does allow one to probe individual bonds of even the weakest variety, but tip curvature typically prevents this.
< 10 nm has been determined as the requirement to conduct tensile testing of individual linear moieties.
[2] A quick note to mention is the work corresponding to the hysteresis in the force profile (Figure 2) does not correlate to the bond energy.
Even if surface passivation with EtOH were considered (discussed below), the large error seems irrecoverable.
[3] This strongly implies that the cantilever has a force constant smaller than or on the order of that for bond interactions and, therefore, it cannot be treated as perfectly rigid.
Chemical interactions can also be used to map prepatterned substrates with varying functionalities (see Figure 3).
This is detected by laser deflection in a position sensitive detector, thereby producing a chemical profile image of the surface.
When the cantilever functionalization is switched such that the tip is bent when encountering hydrophobic areas of the substrate instead, the complementary image is observed.
Frictional force response to the amount of perpendicular load applied by the tip on to the substrate is shown in Figure 4.
Of experimental importance is the fact that contrast between different functionalities on the surface may be enhanced with an application of greater perpendicular force.
When the solvent is immiscible with functional groups, larger than usual tip-surface bonding exists.
Therefore, organic solvents are appropriate for studying van der Waals and hydrogen bonding, while electrolytes are best for probing hydrophobic and electrostatic forces.
A biological implementation of CFM at the nanoscale level is the unfolding of proteins with functionalized tip and surface (see Figure 5).
[4] Due to the increased contact area, the tip and the surface act as anchors holding protein bundles while they separate.
As uncoiling ensues, the force required jumps, indicating various stages of uncoiling: (1) separation into bundles, (2) bundle separation into domains of crystalline protein held together by van der Waals forces, and (3) linearization of the protein upon overcoming the secondary bonding.
Information on the internal structure of these complex proteins, as well as a better understanding of constituent interactions are provided with this method.
The high aspect ratio of carbon nanotubes (easily >1000) is exploited to image surfaces with deep features.
[5] The use of the carbon material broadens the functionalization chemistry since there are countless routes to chemical modification of nanotube sidewalls (e.g. with diazonium, simple alkyls, hydrogen, ozone/oxygen, and amines).
Because of their approximately planar ends, one can estimate the number of functional groups that are in contact with the substrate knowing tube diameter and number of walls, which helps in determining single moiety tensile properties.
