  Single molecules that exhibit narrow optical transitions at cryogenic
temperatures can be used as local electric-field sensors. We derive the single
charge sensitivity of aromatic organic dye molecules, based on first
principles. Through numerical modeling, we demonstrate that by using currently
available technologies it is possible to optically detect charging events in a
granular network with a sensitivity better than $10^{-5}e/\sqrt{\textrm{Hz}}$
and track positions of multiple electrons, simultaneously, with nanometer
spatial resolution. Our results pave the way for minimally-invasive optical
inspection of electronic and spintronic nanodevices and building hybrid
optoelectronic interfaces that function at both single-photon and
single-electron levels.
