  Measuring the behavior of redox-active molecules in space and time is crucial
for better understanding of chemical and biological systems and for the
development of new technologies. Optical schemes are non-invasive, scalable and
can be applied to many different systems, but usually have a slow response
compared to electrical detection methods. Furthermore, many fluorescent
molecules for redox detection degrade in brightness over long exposure times.
Here we show that the photoluminescence of pixel arrays of an atomically thin
two-dimensional (2D) material, a monolayer of MoS$_{2}$, can image spatial and
temporal changes in redox molecule concentration in real time. Because of the
strong dependence of MoS$_{2}$ photoluminescence on doping and sensitivity to
surface changes characteristic of 2D materials, changes in the local chemical
potential significantly modulate the photoluminescence of MoS$_{2}$, with a
sensitivity of 0.9 mV/$\sqrt{Hz}$ on a 5 $\mu$m by 5 $\mu$m pixel,
corresponding to better than parts-per-hundred changes in redox molecule
concentration down to nanomolar concentrations at 100 ms frame rates. The
real-time imaging of electrochemical potentials with a fast response time
provides a new strategy for visualizing chemical reactions and biomolecules
with a 2D material screen.
