Biomedical Engineering Reference
In-Depth Information
when a conductive probe tip is scanned across the substrate without any
applied biased voltage. The current change is recorded when the tip is
above the conductive but not the non-conductive domains of the sub-
strate. In three-electrode confi guration, a biased voltage is added and can
be applied to the substrate. In the tip detection mode, the probe tip is
brought very close to the substrate while raster-scanning the surface in an
open-loop feedback to obtain the current density data. In comparison, the
amperometric measurement can be operated at constant height, constant
current and constant impedance mode, all of which aim at maintaining
a constant distance between the probe tip and the substrates by using,
respectively, current, impedance or height as feedback signal.
136-145
Recent
advances in EC-AFM allow simultaneous measurement of current and
topographic information without substantial alteration of the instrumen-
tation.
146
Potentiometry-based measurement such as cyclic voltammetry
can also be used to probe the voltage change during the tip scanning on
the substrate.
147,148
Due to the non-contact nature of EC-AFM, the lateral
resolution is not as high as other operation modes such as scanning tunnel
microscopy. However, generally order of sub-micron level resolution can
be achieved.
146,149,150
Force spectroscopy
In single molecule AFM (SMAFM),
12,151,152
a clean non-functionalized probe
tip is lowered gradually until it contacts a layer of adsorbed proteins on
the surface. Some domains of the protein would then attach to the probe
tip through physical adsorption. Afterwards, the probe tip is pulled away
from the surface and the single protein molecule in between is stretched.
Eventually, the separating distance is so great that the protein undergoes
force-induced unfolding in order to maintain the connection. In this situ-
ation, the force for unfolding is smaller than that for the detachment of
the protein from either the probe tip or the surface. Important informa-
tion obtained from this kind of experiment is the relation between various
unfolding force peaks and the elongation distance of the protein, which is
termed single molecule force spectroscopy.
In adhesion mode AFM, the probe tip is moved up and down at each
pixel, generating a force curve.
153
The mechanism is to some extent similar
to force volume imaging. However, the focus here is the adhesion part of the
force curve, whose peak, width and area are dependent on the topography
of the sample and the strength of the adhesive force. In this way, information
on the height of the sample and the corresponding adhesive force at each
pixel can be simultaneously obtained. Obviously, the measured adhesion
force refl ects the dominating intermolecular forces. Much effort has been
devoted recently to improving the resolution of topographic measurements,
