Biomedical Engineering Reference
In-Depth Information
Fig. 3.15. A model force-distance curve. At point A, the probe is far from the surface, at B 'snap-in'
occurs as attractive forces pull the probe onto the surface. The force becomes repulsive as the probe
continues to be driven towards the sample. At some user-defined point C, the direction of travel
reverses. At point D 'pull-off' occurs as the force applied to the cantilever overcomes tip-sample
adhesion. Adhesion data is used for force spectroscopy while slope data is used for nanoindentation
(Section 3.2.2).
(i) The number of interacting molecules. Depending on the tip radius, a large number
of molecules are likely to be able to interact with the surface at one time.
(ii) Orientation and accessibility of interacting molecules. Typically, the investigator
would like to make comparisons between the molecular interactions measured at
the surface, and results from solution studies, but the grafting of molecules to the
tip may affect the results.
(iii) The speed of approach and withdrawal of the tip for the surface will affect the
results.
(iv) Experimental environment. One advantage of AFM is that it may be carried out in
almost any environment. For most chemical and biological work it is useful to
carry out the experiments in liquid. It is simple then to change the composition of
the liquid to see how it affects the results. For example, to prove antibody/antigen
interactions, commonly blocking antibodies are injected into solution, after which
forces may disappear to zero [163].
(v) Statistical variation in results is typically very large. This means increased experi-
mental time, which is not normally a problem, as each force curve typically takes
less than 1 second to acquire, but in addition a very large dataset is typically
generated, and a lot of data analysis is likely to be required.
In reality, the results from force spectroscopy between molecules rarely look much like the
cartoon in Figure 3.15. Usually, specific forces between molecules lead to much
more complicated results. An example is shown in Figure 3.16. In the blue (retract)
curve, several typical features can be seen. One is the almost-flat region labelled a. In
this region, polymer chains linking the molecules to the AFM tip were unfolding. During
