Biology Reference
In-Depth Information
applied by the probe on the sample must be precisely controlled: biomolecules
are of soft nature and get damaged easily when an excessive force (typically
higher than 300 pN
90
) is applied. To obtain this precise force control, two
strategies have been used in conventional slow AFMs: (1) the contact mode
imaging using soft probes of spring constant of 0.01-0.1 N/m—these probes
provide high delection-to-applied-force ratio and thus allow contouring
the sample while maintaining the applied forces within a small range of
several tens of piconewtons; (2) the oscillatory mode imaging using probes
of spring constant of 0.1 N/m—the oscillation minimizes the contact time of
the probe and the sample, reducing the friction between probe and sample.
Both strategies have yielded similar image quality with a lateral resolution of
about 1 nm. Yet, they differ in that the contact mode is around 5 to 10 times
faster than the oscillatory mode but limited to samples with low corrugation,
while the oscillatory mode can image samples of higher corrugations but at
a slower rate.
(a)
(b)
Figure 2.9.
High-speed contact mode AFM imaging of purple membrane. (a) Topograph
at submolecular resolution at an imaging rate of 10 frames per second. (b) Monitoring
association and dissociation of bR trimers from and to the edge of the bR array.
From the two strategies used for the high-resolution imaging of
biomolecules with the conventional AFM, only the oscillatory mode has
been implemented in the high-speed and high-resolution AFM imaging of
Search WWH ::
Custom Search