590
J. Jaisawal et al.
3.1 Forces on the Approach Curve
During approaching of two hard surfaces in the absence of long-range interactions
van der Waals interactions plays a major role [11] which is visualized in terms of tiny
deflection of the cantilever, exactly prior to the jump into contact point in approach
curve. The position of the cantilever became unstable due to such jumps. To see this
jump to contact, the cantilever stiffness must exceed the gradient of force between
the tip and sample.
Appearance of the repulsive force, which is the smooth and increasing exponentially in the approach curve indicates the presence of electrostatic or polymer-brush
forces [8, 9]. These force curves can be used with the indentation depth to study the
properties of samples. When the tip is pressed opposite to the samples during the
approach cycle, the indentation depth appears. These AFM force curves have also
been previously used to determine viscoelastic properties of some biological systems
like cartilage, glial cells, epithelial cells and gelatin gels [12].
3.2 Forces on the Retraction Curve
The very common feature on the retraction curve is the appearance of an adhesion
force due to the hysteresis characteristics. Below the zero-deflection line, the adhesion
force observed as the cantilever deflection and depends on type of the sample. The
connection between the sample surface and the tip due to capillary bridge reveals
the appearance of adhesion forces. This capillary force relies in the medium (air or
liquid) in which AFM-FS is performed [11]. For samples in air, several nanometers
of water molecules are present in the form of adsorbed molecules to the surface.
These adsorbed water molecules are responsible for forming the contacting the tip
and sample, thus it is needed to surpass the surface tension and drag the tip totally
out from surface of sample. In presence of liquid medium (Could be buffer, salt
solution or media) the adhesion force relies on factors like the type of solution used
as well as the interaction energies between tip and sample. The presence of polymer
between the sample and the tip reveals the negative deflection away from the sample
surface at a given distance. This difference in observed deflection is because of the
polymer extension. The polymer stretched until it breaks the bond from the surface
or the tip. Subsequently, the cantilever came back to the initial position, i.e. at the
zero-deflection line [8, 10, 11]. AFM-FS can also be performed for determining the
unbinding events between specific receptor–ligand pairs [10]. The force generated
due to this multiple events of binding/unbinding generally decided from cumulative
AFM cantilever deflection, further, coming back to the initial point i.e. zero-deflection
line. Moreover, a single (un)binding phenomena or the consecutive unbinding of
numerous interaction between receptor and ligand leads to the force curves. The
combination of polymer extension and adhesion phenomena reveals the complexity
of such curves to analyze. In previous literature, particular examples of various force
J. Jaisawal et al.
3.1 Forces on the Approach Curve
During approaching of two hard surfaces in the absence of long-range interactions
van der Waals interactions plays a major role [11] which is visualized in terms of tiny
deflection of the cantilever, exactly prior to the jump into contact point in approach
curve. The position of the cantilever became unstable due to such jumps. To see this
jump to contact, the cantilever stiffness must exceed the gradient of force between
the tip and sample.
Appearance of the repulsive force, which is the smooth and increasing exponentially in the approach curve indicates the presence of electrostatic or polymer-brush
forces [8, 9]. These force curves can be used with the indentation depth to study the
properties of samples. When the tip is pressed opposite to the samples during the
approach cycle, the indentation depth appears. These AFM force curves have also
been previously used to determine viscoelastic properties of some biological systems
like cartilage, glial cells, epithelial cells and gelatin gels [12].
3.2 Forces on the Retraction Curve
The very common feature on the retraction curve is the appearance of an adhesion
force due to the hysteresis characteristics. Below the zero-deflection line, the adhesion
force observed as the cantilever deflection and depends on type of the sample. The
connection between the sample surface and the tip due to capillary bridge reveals
the appearance of adhesion forces. This capillary force relies in the medium (air or
liquid) in which AFM-FS is performed [11]. For samples in air, several nanometers
of water molecules are present in the form of adsorbed molecules to the surface.
These adsorbed water molecules are responsible for forming the contacting the tip
and sample, thus it is needed to surpass the surface tension and drag the tip totally
out from surface of sample. In presence of liquid medium (Could be buffer, salt
solution or media) the adhesion force relies on factors like the type of solution used
as well as the interaction energies between tip and sample. The presence of polymer
between the sample and the tip reveals the negative deflection away from the sample
surface at a given distance. This difference in observed deflection is because of the
polymer extension. The polymer stretched until it breaks the bond from the surface
or the tip. Subsequently, the cantilever came back to the initial position, i.e. at the
zero-deflection line [8, 10, 11]. AFM-FS can also be performed for determining the
unbinding events between specific receptor–ligand pairs [10]. The force generated
due to this multiple events of binding/unbinding generally decided from cumulative
AFM cantilever deflection, further, coming back to the initial point i.e. zero-deflection
line. Moreover, a single (un)binding phenomena or the consecutive unbinding of
numerous interaction between receptor and ligand leads to the force curves. The
combination of polymer extension and adhesion phenomena reveals the complexity
of such curves to analyze. In previous literature, particular examples of various force