  Electrochemical techniques conventionally lack spatial resolution and average
local information over an entire electrode. While advancements in spatial
resolution have been made through scanning probe methods, monitoring dynamics
over large areas is still challenging, and it would be beneficial to be able to
decouple the probe from the electrode itself. In this work, we leverage single
molecule microscopy to spatiotemporally monitor analyte surface concentrations
over a wide area using unmodified hexagonal boron nitride (hBN) in organic
solvents. Through a sensing scheme based on redox-active species interactions
with fluorescent emitters at the surface of hBN, we observe a linear decrease
in the number of emitters under positive voltages applied to a nearby
electrode. We find consistent trends in electrode reaction kinetics vs
overpotentials between potentiostat-reported currents and optically-read
emitter dynamics, showing Tafel slopes greater than 290 mV per decade. Finally,
we draw on the capabilities of spectral single molecule localization microscopy
(SMLM) to monitor the fluorescent species identity, enabling multiplexed
readout. Overall, we show dynamic measurements of analyte concentration
gradients at a micrometer-length scale with nanometer-scale depth and
precision. Considering the many scalable options for engineering fluorescent
emitters with 2D materials, our method holds promise for optically detecting a
range of interacting species with unprecedented localization precision.
