Biomedical Engineering Reference
In-Depth Information
Figure 8.17
Image of bacterial aquaporin, a membrane protein still inserted in it native membrane
[33]. Individual tetramers are clearly resolved. Lateral size of the image is 70 nm. (Image courtesy of
Dr. Simon Scheuring.)
with the surface can then be too strong and the surface is scratched, or the mol-
ecules of interest are damaged or wiped out. To minimize these problems, other
modes have been developed in which the spring sustaining the tip oscillates and
periodically “taps” the surface. The signal used for the feedback is then not the
average position of the tip but the amplitude of oscillation measured by a lock-in
detection. Although the tip still interacts rather strongly with the sample when it
touches it, there is no transverse force (friction) applied to it and a lot of damage
is avoided.
The AFM cantilever then behaves as a damped oscillator: it is characterized
by a characteristic resonance frequency and a quality factor Q that witnesses the
viscosity of the medium. In air (even more so in vacuum) the quality factor is high
(easily 100-1,000) meaning that the resonance is well defined. In water, Q is much
lower (1-10) and consequently, because the resonance frequency is not well defined,
the feedback is less sensitive and more damage is brought to the sample. Recent
electronic methods aimed at electronically increasing the quality factor may become
a good alternative to these problems.
With the microscopic sizes dealt with in this topic, it is illustrative to describe
the motion of a free cantilever subjected to thermal agitation. In that case, one
can use the equipartition theorem on the potential elastic energy of this harmonic
oscillator.
