Biology Reference
In-Depth Information
monovalent and divalent ions) conditions are adjusted in such a way as to
obtain a mild electrostatic repulsion of the silicon nitride tip by a negatively
charged sample.
32
The contact mode is suitable for imaging lat or weakly corrugated
surfaces. During scanning, the tip is always in contact with the sample surface
and the force applied by the tip is kept constant (<100 pN) using a z-feedback
loop. The highest lateral resolution is actually obtained with this mode (below
the nanometer range), as shown below for protein-enriched membranes.
33
A
scanning rate of 2-5 Hz is easily achieved with artiicial membranes. However,
when the sample is weakly adsorbed or corrugated, or when it is dificult
to properly tune the tip-sample interaction, the tapping mode (also called
intermittent contact mode) is more appropriate and prevents sample damage
and tip contamination by reducing shear forces. In this mode, oscillation of the
tip can be obtained using acoustic or magnetic excitation, and the oscillation
amplitude is used as a feedback signal. Although less performant than the
contact mode, the tapping mode yields good lateral and vertical resolutions
on artiicial membranes.
10
By using this mode the tip can also probe visco-
elastic properties of the sample by measuring the phase signal lag, which was
successfully used to differentiate luid and gel phases in SLBs
(
Fig. 1.2c,d
)
.
Generally, the scan rate is ~1 Hz and the oscillation amplitude ~10-100 nm,
and the setpoint is adjusted to achieve less than 10% oscillation damping.
The group of Hinterdorfer demonstrated that the use of the second harmonic
oscillation amplitude as a feedback signal can increase the lateral resolution
and the sensitivity to local variations in elasticity.
34
Improvements in temporal
resolution and material properties mapping have also been obtained using a
torsional harmonic cantilever with an off-axis tip.
35
It is important to notice
that tapping mode imaging should be further developed in the coming years
because most current AFM developments rely on this mode (see Section 1.7).
1.4 LATERAL MEMBRANE ORGANIZATION
AFM has been extensively used to address the problem of lateral heterogeneity
and segregation of lipids and proteins in biological membranes. It has been
especially used to characterize rafts microdomains, a subset category of
liquid-ordered lipid domains enriched in sphingolipids and cholesterol (Chl),
which work as functional platforms in cells. Using lipid mixtures mimicking
the composition of the plasma membrane's outer lealet, essentially mixtures
of phosphatidylcholine (PC), sphingomyelin (SM) and Chl, numerous papers
have demonstrated the coexistence of the luid- or liquid-disordered (
ld
)
phase with the liquid-ordered (
lo
) or gel phase and tried to rely the physical
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