  Modulating the electric potential on a conducting electrode is presented to
generate an optical contrast for scattering microscopy that is sensitive to
both surface charge and local topography. We dub this method
Electric-Double-Layer-Modulation microscopy. We numerically compute the change
in the local ion concentration that is the origin of this optical contrast for
three experimentally relevant geometries: nanosphere, nanowire, and nanohole.
In absence of plasmonic effects and physical absorption, the observable optical
contrast is proportional to the derivative of the ion concentration with
respect to the modulated potential. We demonstrate that this derivative depends
on the size of the object and, less intuitively, also on its surface charge.
This dependence is key to measuring the surface charge, in an absolute way,
using this method. Our results help to identify the experimental conditions
such as dynamic range and sensitivity that will be necessary for detecting the
elementary charge jumps. We conclude that the nanohole is the most suitable
geometry among these three for achieving elementary charge sensitivity.
