INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 47

2.3.1. Electrostatic and hydrophobic interactions
If the probe is strongly attracted to or repulsed from the sample surface,
it is increasingly challenging to automatically detect the initiation of
true mechanical contact. Especially in the case of proteinaceous or
glycosylated materials, electrostatic interactions with the probe can occur
to pull a like-charged probe onto the sample surface or push an
oppositely charged probe away. The manifestation of these interactions
can be a large and rapid probe deflection that pulls (or pushes) the
indenter out of a measureable range of actuation. If data acquisition is
initiated when the probe overcomes a specific load (voltage), it is
possible that the instrument will fail to display this interaction. In the
case of attractive interaction, one may then infer the material to be much
more stiff than in reality, as the indentation depth corresponding to “near
zero load” will be artifactually high. Ideally, one would seek to acquire
all load-displacement data from the initial to the final point of load train
displacement, in order to easily identify such nefarious probe-sample
interaction events that may not register with actual sample loading. With
such data, one could manually or semi-automatically identify the actual
contact point beyond which mechanical load was increased at the sample
surface.
These interactions can be buffered via immersion of the sample and
probe in ion-containing, aqueous media; this will decrease the Debye
length, or distance over which the probe “feels” an effective charge from
the sample surface, for increasing ionic strength of the solution.
Alternatively, one can chemically modify the probe to enhance or
mitigate interactions with the biomaterial surface. Here it is important to
note that certain probe materials are more amenable to chemical
modification than others. For example, diamond is a crystalline carbon
phase that is resistant to biochemical modification, whereas silica,
polystyrene, and gold are more readily functionalized with sequential
adsorption steps of molecular moities such as amine or carboxyl groups.
Chemically modified instrumented indenters have even been fabricated
by mounting AFM-cantilevered borosilicate spheres onto the indenter
shaft, such that spheres of μm-scale radii could be used to test
hypotheses of probe-surface interaction for synthetic polymers.