What is the effective sensitivity of FluidFM probes and what do I need to consider when measuring it?
What is the sensitivity?
In the context of FluidFM, and AFM in general, the sensitivity S [m/V] describes how the bending of the cantilever [m] relates to the measured differential voltage change [V] in the photo detector of the system.
Measuring the sensitivity
The sensitivity S is typically measured by placing the cantilever against a hard surface (such as the Petri dish) and moving the vertical stage a known distance [m]. Now the change of the detector signal [V] is monitored and the sensitivity [m/V] is deduced.
It is important to note that the sensitivity is dependent on which part of the cantilever touches the substrate during the sensitivity measurement. That is why it is crucial to measure the sensitivity with the colloid already attached to the case of colloidal probes.
Generally, the same contact point is used for the sensitivity measurement as later during the experiments, for example, the pyramid of our nanopipettes.
Special case: Cell Adhesion with FluidFM
There is a special case, though. Our FluidFM micropipettes when used for adhesion:
When FluidFM micropipettes are used for single cell adhesion experiments, the sensitivity can only be measured with the very edge of the cantilever as the contact point. Later on, the cells will however interact with the cantilever opening, a bit further back.
The effective sensitivity Seff therefore is (Tourek et al. 2010. Rev.Sci.Instr. 81(7)):
where Smeas is the measured sensitivity, l is the cantilever length (200 µm), lp is the distance to the cantilever opening (193 µm), and lL is the center position of the laser on the cantilever (~180 µm).
For the FluidFM micropipette the effective sensitivity is smaller for adhesion measurements by the following factor:
Hence also the measured forces are overestimated by ~4% due to this effect.
We want to underline that for colloid probes the effective and measured sensitivity will be identical, as long as the colloid is already in place during the sensitivity measurement.