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trigger threshold on each path, and the lower peak of each oscillation period
is determined by means of sample and hold analysis. Subsequent peaks form
a staircase function, which is then iltered and fed into the AFM controller,
where
U
up
provides the information to establish the corresponding recognition image.
Moreover, the utilization of cantilevers with low
U
down
drives the feedback loop to record the topography image and
in
combination with a proper chosen driving frequency and amplitude regime
enables that both types of information are unrelated.
Q
factor (~1 in liquid)
Generally, the ideal
amplitude regime for the observation of recognition events differs from one
functionalized cantilever to another. It depends on the length of the linker
molecule, on the exact location of the linker molecule on the tip apex and on
the size of the attached molecule. It typically lies in the range of 10-20 nm.
To summarize, the topography and recognition images can be
simultaneously and independently obtained using a specially designed
electronic circuit (PicoTREC, Agilent Technologies, Chandler, Arizona), which
splits the cantilever oscillation amplitude into the lower and upper parts (with
respect to the cantilever resting position) and contains speciic information
4,12
Figure 7.2.
Schematic of TREC functioning. The raw cantilever delection signal is fed
into the TREC box, where the maxima (
U
down
) of each oscillation
period are used for the recognition and the topography image, respectively.
U
up
) and the minima (
7.4 APPLICATIONS OF TREC TO CELLS
Mapping receptor-binding sites on cellular surfaces is a challenging task in
molecular cell biology. This information can usually be obtained from the
extensive exploitation of common optical techniques such as immunostaining
(or immunocytochemistry) or even by somewhat sophisticated techniques
such as single-molecule optical microscopy,
13
near-ield scanning optical
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