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Measuring the behavior of redox-active molecules in space and time is crucial for understanding chemical and biological systems and for developing new technologies. Optical schemes are noninvasive and scalable, 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 monolayer MoS 2 can image spatial and temporal changes in redox molecule concentration. Because of the strong dependence of MoS 2 photoluminescence on doping, changes in the local chemical potential substantially modulate the photoluminescence of MoS 2 , with a sensitivity of 0.9 mV / Hz on a 5 μm × 5 μm pixel, corresponding to better than parts-per-hundred changes in redox molecule concentration down to nanomolar concentrations at 100-ms frame rates. This provides a new strategy for visualizing chemical reactions and biomolecules with a two-dimensional material screen.