AFM Probe Functionalization

Atomic force microscopy (AFM) is a surface characterization instrument which utilizes a tip to “feel” the surface. AFM has great potential as a tool for materials science studies in that it not only is a tool to image the topography of solid surfaces at high resolution, but also characterizes the surface mechanical, electrical and other properties. Recently there is an increasing demand to characterize larger and more complex biomolecular systems, modern AFM techniques have experienced a great deal of progress in the evaluation of local mechanical properties and topography of the living cells at a high spatial resolution and force sensitivity. Besides surface characterization, AFM based local anodic oxidation (LAO) lithography has been used to fabricate micro- and nano-structures on organic or inorganic surfaces. For example, AFM has been used to cut carbon nanotubes or etch holes in highly oriented pyrolytic graphite (HOPG). The advantages of LAO include the ability to pattern surfaces with nanometer resolution and to examine devices during the lithography process, and the ease of fabrication tuning.

The most important working principle of various AFM-based applications is the interactions between AFM probe and surface. Quantifying the tip-surface interactions provides the information of surface physical and chemical properties. An ideal high-resolution probe should also be accessible to chemical modification to enable molecular-scale sensing and manipulation of matter. It has been shown that the AFM tip can be modified to present well-defined chemical or biological functionality, which provides specific chemical contrast as well as topography. For example, single walled carbon nanotube (SWNT) tips provide significantly better lateral resolution compared with commercial silicon and silicon nitrile AFM tips. Carboxylic groups at the SWNT probe end have been modified with amine groups to study the force titrations between tip and self-assemble monolayers.

SEM images of CNT fiber modified AFM probe at low (left) and high (right) resolution. Reprinted from Ref 1.*

Although several techniques have been utilized for AFM probe modifications, a number of issues must be taken into consideration to achieve optimal results: the selection of a suitable AFM probe is critical, i.e., the sharpness of the tip and the cantilever’s spring constant; the selection of tip functionalization chemistry, since the ligand molecule must be connected to the tip so that the binding strength between the tip and the molecule is more than the interaction between the surface and the ligand; control of ligand’s surface density, which is crucial for single binding events measurements. Other factors such as temperature, buffer composition, and pH during the measurement must also be considered. At Matexcel, our scientists can help you with the AFM probe functionalization based on your specific research needs.

Reference:
Slattery, Ashley D. "Solution based methods for the fabrication of carbon nanotube modified atomic force microscopy probes." Nanomaterials. 2017.

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