Biology Reference
In-Depth Information
force is attained by non-contact imaging. True non-contact-AFM (nc-AFM)
has only been realized in a vacuum environment by utilizing a cantilever
with a signiicantly large quality factor in vacuum.
33
If high-speed nc-AFM is
realized in liquid conditions, we can use stiffer cantilevers with much higher
resonant frequencies, which will promise markedly higher imaging rates.
The non-contact imaging capability in liquids has already been achieved
by ion-conductance scanning probe microscopy (ICSPM).
34
Owing to progress
in fabrication techniques for producing very sharp glass capillaries with a
small pore at the apex, the spatial resolution of ICSPM has reached a few
nanometre.
35
Immobile protein molecules with ~14 nm in size on living cell
membranes have been successfully imaged.
36
However, it seems dificult
to increase the imaging rate of ICSPM; the bandwidth of ion-conductance
detection cannot be easily increased, because the ionic current through the
small pore of the capillary electrode is very low.
Although not for high-speed nc-AFM, control algorithms to reconcile
a large quality factor of the cantilever with high-speed imaging have been
proposed.
37,38
The position and velocity of the oscillating cantilever are
continuously monitored (or discretely monitored with small time-bins).
From these measured quantities, an estimator calculates the tip-sample
interaction force of each tapping cycle. A model-based predictor uses the
estimated force to control the tip-sample distance in the next tapping cycle.
Experiments with conventional AFMs implemented with the new controllers
demonstrated regulation of the tip-sample interaction force at each tapping
cycle, irrespective of the time delay of the cantilever's response. However, to
apply this method to a real high-speed AFM, extremely fast digitization and
calculations are required.
8.4.2 High-Speed AFM Combined with Opcal Microscopy
The size of a cell is generally over a few tens of micrometers in width and a
few micrometers in height. On the other hand, the extension ranges of the
high-speed scanner we normally use are approximately 3 μm
s
1 μm
s
2 μm
in
-directions, which are too small to be used for imaging a cell.
In practice, such a small imaging area makes it dificult to ind cells to be
imaged.
One of the solutions is combining a high-speed scanner with a conventional
low-speed scanner for wide area imaging. Another solution is combining high-
speed AFM with an optical microscope. Since optical microscopy and high-
speed AFM have advantages and disadvantages over each other, combining
these techniques into a single instrument would therefore be useful. From
the optical image covering a wide area of the sample, we can quickly ind a
x
-
s
y
-
s
z
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