Spatially resolved sensing devices for electrostatic potentials are extremely useful for characterisation of living cells, however many current techniques lack the speed necessary to capture spatially resolved, functional information of cells in real-time. Here an optical sensing technique is proposed based on graphene on a semiconductor stack operating in the near infra-red spectrum. By modelling coherent interference of multiply reflected beam paths within the semiconductor stack we demonstrate how the device produces a continuous reflectivity change in response to graphene Fermi energy which is ideal for sensing changes in local electrostatic fields produced by action potentials of living cells. By coupling the device with a high-speed camera we propose this platform will allow for high-speed imaging of action potentials over a large sensing area with micron scale resolution.