Microfabricated Tips and Colloidal Probes

Tip wear is one of the major problems in force measurements. The assumption that the surfaces of tip and sample do not change during a force experiment is probably in most cases wrong. Computer simulations showed that a transfer of atoms between the two surfaces is likely as they get into contact [247]. Chung et al. 248] could show that during the first approach significant structural changes of the tip apex can occur, even at low forces. 

Higher resolution can be achieved with transmission electron microscopy (TEM), where even the crystalline shape of the tip could be resolved [248], but this is achieved at the price of much higher experimental effort and is not done routinely. An alternative method for tip shape characterization is to image a sharp structure and reconstruct the tip shape from the image [249-251]. In this case, one relies on a defined structure of the sharp object and the characterization itself might cause damage to the tip. Therefore, characterizing the tip size and shape for routine applications is an unsolved problem, neither is the resolution satisfactory nor are the methods noninvasive. 

However, the colloid probe technique is not limited to particles. For biological studies, strategies to attach single spores [264], bacteria-coated beads [265], or single cells [266, 267] have been developed. To study forces in emulsions [268] or flotation cells such as oil drops [269, 270, 696] and bubbles have been attached to cantilevers [271]. 

A limitation of the colloid probe technique, however, is the minimum particle size that can be reproducibly attached by using optical microscopy. For spheres smaller than 1 J.m, it becomes difficult to correctly position the particle at the very end of the cantilever to avoid touching the substrate with the edge of the cantilever. In this respect, the name colloid probe is somewhat misleading since colloidal particles are usually smaller than 1 pm. Recently, there have been attempts to attach nanoparticles to the end of AFM tips either by wet chemistry [272] or by epoxy-coating of tips and dipping them into a powder [273]. 

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