Chemical Force Microscopy

Abstract

Atomic force microscopy is an imaging tool used widely in fundamental research, although it has, like other scanned probe microscopies, provided only limited information about the chemical nature of systems studied. Modification of force microscope probe tips by covalent linking of organic monolayers that terminate in well-defined functional groups enables direct probing of molecular interactions and imaging with chemical sensitivity. This new chemical force microscopy technique has been used to probe adhesion and frictional forces between distinct chemical groups in organic and aqueous solvents. Contact mechanics provide a framework to model the adhesive forces and to estimate the number of interacting molecular groups. In general, measured adhesive and frictional forces follow trends expected from the strengths of the molecular interactions, although solvation also plays an important role. Knowledge of these forces provides a basis for rationally interpretable mapping of a variety of chemical functionalities and processes such as protonation and ionization.

Peer Reviewed: Atomic Force Microscopy Probes Go Electrochemical

“…The substrate UME was biased at a potential sufficient to generate an electroactive species (IrCl 6 2-) from electrolysis of a solution mediator (IrCl 6 3-). The zone over which the tip collects IrCl 6 2-is clearly evident in the top part of Figure 5b (as a current flow at the SECM/AFM probe) and correlates well with the underlying location of the electrode, identified by topographical imaging in the bottom part of Figure 5b. The magnitude of the current measured at the tip electrode as a function of tip-substrate separation is quantitatively determined on the basis of models for diffusion from a diskshaped source (38).…”

“…In conventional form, AFM lacks chemical specificity, but molecular recognition is possible by tailoring the chemical functionality of the probe and substrate (6). Often termed chemical force microscopy (CFM), this genre measures the interaction between the modified tip and the substrate by recording force versus tip separation curves: as the tip is brought toward, into contact with, and then away from the surface.…”

“…High-resolution electrochemical imaging studies with SECM/AFM provided further evidence. An electroactive solute (IrCl 6 3-) was added to the solution, and the DSA was held at a potential to oxidize the solute while the SECM/AFM probe was biased to collect the product (IrCl 6 2-) just above the surface. The resulting current images demonstrated that only a few of the platinum deposits were capable of facile electron transport.…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…[114,115]). Force measurements with modified (''functionalized'') microfabricated tips or modified colloidal probes are called ''chemical force microscopy'' (for reviews see [116,117]). Before discussing modification of silicon and silicon nitride tips we mention other tips, prepared by special procedures for particular purposes.…”

Cantilever-Based Optical Interfacial Force Microscopy

“…However, AFM usage has been limited to passive applications (e.g., pull-off force measurement in the force-distance curve) and can only be applied to the measurement of friction while the tip is touching the sample surface because of an intrinsic mechanical instability of the tip-sample assembly near a sample surface called the "snap-to-contact problem" (Burnham, 1989;Lodge, 1983). During measurements, the mechanical instability occurs when the force derivative (i.e., dFa/dz), in respect to the tip position (z), exceeds the stiffness of the cantilever (spring constant k) (Greenwood, 1997;Israelachvili & Adams, 1978;Noy et al, 1997;Sarid, 1991), causing data points to be missed near the sample surface (Cappella & Dietler, 1999). This has been a significant barrier to understanding the nanoscopic water junction between the tip and the surface in ambient conditions, which makes it difficult, with AFM data, to directly reveal the interfacial water structure and/or analyze it with existing theories.…”