I recently wrote about the noise in nanopores, as a thinking-out-loud exercise to clarify for myself what the noise sources were and what the consequences of changing the concentration of salt ions would be. Today I did another, smaller exercise, thinking about how much capacitance the membrane of a nanopore device has and whether it would have any effect on the measurements.
I was going to try to use the standard parallel-plate capacitance formula, using the area, the separation, and the dielectric constant. The nanopore devices used at UCSC have a lipid bilayer closing a 1 µm hole between the two wells of the device, giving a plate area of about 785E-15 m2, but I don’t know the thickness nor the dielectric constant for the lipid bilayer, both of which undoubtedly depend on what lipid is used.
Since I’m mainly interested in a ballpark estimate, I looked up the capacitance of lipid bilayers on the web and found
O Alvarez and R Latorre
Voltage-dependent capacitance in lipid bilayers made from monolayers.
Biophys J. 1978 January; 21(1): 1–17.
which gives a measurement of capacitance (at 0 volts) of 0.65 to 0.81 µF/(cm)2 for various lipids. For the nanopore, that would give a capacitance of 5–6fF, far less than the capacitance of the wires connecting to the electrodes in the wells, which are probably in the pF range. So for practical purposes, we can mostly ignore the capacitance of the lipid bilayer, and regard the nanopore as a variable resistor, with a resistance of about 10GΩ.