Figure 1 : Schematic of the experiment.

From: Mapping surface charge density of lipid bilayers by quantitative surface conductivity microscopy

Figure 1

(a) Schematic of the SICM setup and QSCM mode. A bias is applied between the Ag/AgCl electrodes inside the pipette and in the bath. The resulting current is affected by the distance between the pipette tip and sample surface and by the sample surface charge. The feedback keeps this current constant at 99% of the unperturbed current. This leads to a charge dependent tip–sample distance. In situation 1, the sample is negatively charged mica, while in situation 2, it is the cationic lipid DPTAP. The red (blue) line represents the path of the pipette tip at +100 mV (−100 mV) bias potential. The black line represents the path taken had the sample been uncharged. The dashed box indicates the tip–sample region investigated with FEM simulations. The simulation geometry (b) is created with rotational symmetry along the pipette axis to reduce the three-dimensional problem to 2D. One electrode, a is positioned at the top of the pipette, the other, b is the outer edge of the water bath. The charged sample, c is positioned at a variable distance d from the tip of the pipette. The tip itself is characterized by its inner radius ri and an opening angle θ. (c) The lipids used in the experiments are DPTAP (+1 charge) DPPE (neutral) and DPPG (−1 charge).