Monitor tiny variation in voltage - ESP32

We need to have a design based on ESP32 SoC. I have a microscopic membrane in my lab, this piece which we call it NMS has a variation of voltage caused by its operation. Similiar to the action potential in neurons of a nervous system. We are trying to build an electronic measurement system, which could capture the variation of voltage in -70mV to -55mV range. We want to have an accuracy of at least 1mV or possibly less in our measurements. We need some advice for building the measurement probe too, specifically, we need a way of connecting our NMS membrane which is sandwiched between a microscope slide and a cover slip to our measurement system. Thank you all.