  Local electrochemical measurements and imaging at the nanoscale are crucial
for the future development of molecular devices, sensors, materials
engineering, electrophysiology and various energy applications from artificial
photosynthesis to batteries. The ultimate step towards single-molecule
sensitivity requires the measurement of aA currents, which is three orders of
magnitude below that of current state-of-the-art measurement abilities. Here,
we show electrochemical measurements at the sub aA level and <80 nm spatial
resolution, that we reach by exploiting the ultra-high sensitivity of our GHz
microscope for local faradaic interface capacitances. We demonstrate this for a
well-known surface-bound ferrocene alkyl monolayer, a system that cannot be
studied at the nanoscale unless large nanoarrays are used11. We report the
simultaneous measurement of local cyclic voltammograms (CV) which provide
atomistic information on the respective electron transfer reaction and reveal
two molecular configurations with a similar redox energy potential -- insights
inaccessible by electrochemical ensemble measurements.
