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Introduction
The development of microscopes is important for a better understanding of biological processes.
Visualization techniques are mainly based on optical imaging utilizing electromagnetic waves.
The atomic force microscope (AFM), or scanning force microscope (SFM) was invented in 1986 by
Binnig, Quate and Gerber. The AFM utilises a sharp probe moving over the surface of a sample
in a raster scan.
In the case of the AFM, the probe is a tip on the end of a cantilever
(length of about 200 µm) with a low spring constant (of the order of 1 Newton/m) which bends in response to
the force between the tip and the sample.
Most AFMs employ an optical lever technique: as the cantilever flexes, light from a laser is
reflected onto a split photo-diode. By measuring the difference signal, changes in the
bending of the cantilever can be measured.
The movement of the tip or sample is performed by a precise positioning device made from
piezo-electric ceramics. Thus, in contrast to optical microscopes, AFMs measure surfaces in
all three dimensions, with x- and y-resolutions in a typical range of 20 Å (under special
conditions it could be better than 1 Å) and a z-resolution in a typical range of better than
1 Å.
Two AFM-modes can be distinguished: In the "constant force mode" (also called "height mode")
the tip is in contact with the probe and measures the three-dimensional surface. The user
could modify (consciously or unconsciously) the structure of the probe.
The second mode is called "dynamic mode". Approaching the sample with an oscillating
cantilever results in tapping of the sample by the tip. When the tip encounters elevation
changes of the surface, the amplitude changes in consequence of interactions on the atomic
scale (e.g. van der Waals Forces). In this case, the feedback system responds to keep the
amplitude of the cantilever on a constant level.
In the past years AFM has been increasingly used to study biological samples, e.g. cell-surface
morphology, proteins, and DNA.
Another possible application is the detection of ultralow forces. This has been used to
observe receptor-ligand interactions on single molecule level: the ligand is bound to an AFM
tip and receptors to a substrate or vice versa. Tip and substrate have to be brought into
contact and the force required for receptor-ligand dissociation is measured. This application
can be used for e.g. antibody-antigen recognition.
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