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The atomic force microscope (AFM) is one of the family of scanning
probe microscopes, and is widely used in biological applications.
The AFM uses a flexible cantilever as a type of spring to measure
the force between the tip and the sample. The basic idea of
an AFM is that the local attractive or repulsive force between
the tip and the sample is converted into a bending, or deflection,
of the cantilever. The cantilever is attached to some form of
rigid substrate that can be held fixed, and depending whether
the interaction at the tip is attractive or repulsive, the cantilever
will deflect towards or away from the surface.
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This cantilever deflection must be detected in some way and
converted into an electrical signal to produce the images. The
detection system that has become the standard method for AFM
uses a laser beam that is reflected from the back of the cantilever
onto a detector. The optical lever principle is used,
which means that a small change in the bending angle of the
cantilever is converted to a measurably large deflection in
the position of the reflected spot.
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| The position of the laser
spot is measured by comparing the signals from different sections
of the detector. Most AFMs use a photodiode that is made of four
quadrants, so that the laser spot position can be calculated in
two directions, by comparing the signals. The vertical deflection
(measuring the interaction force) can be calculated by comparing
the amount of signal from the "top" and "bottom"
halves of the detector. The lateral twisting of the cantilever
can also be calculated by comparing the "left" and "right"
halves of the detector. |
| AFM is particularly suited
for biological applications, because the samples can be imaged
in physiological conditions. There is no need for staining or
coating, and no requirement that the sample should conduct electrons.
Therefore high resolution imaging is possible in physiological
buffer or medium, and over a range of temperatures. Living cells
can be imaged, as well as single molecules such as proteins or
DNA. The force contrast gives 3-dimensional topography information,
as well as the possibility to access other information such as
the mechanical properties or adhesion. |
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