Monitor tiny variation in voltage
The ESP32 doesn't measure pure voltage (meaning, its input doesn't look like an infinite resistor.) I don't find ohms value for the input pins of ESP32 A/D in their spec sheets.  It might be 1Meg, it might be 100K.

What's voltmeter-resistance tolerated by your micropipette setup?  If the A/D input needs to look like a 100meg resistor, then you'll definitely need an op-amp to serve as an input buffer.  (Also an op-amp can handle both pos and neg inputs over a range of many volts.)

Also this:  


Many patch clamp amplifiers do not use true voltage clamp circuitry,
but instead are differential amplifiers that use the bath electrode
to set the zero current (ground) level. This allows a researcher to
keep the voltage constant while observing changes in current. To make
these recordings, the patch pipette is compared to the ground
electrode. Current is then injected into the system to maintain a
constant, set voltage. However much current is needed to clamp the
voltage is opposite in sign and equal in magnitude to the current
through the membrane. See: https://en.wikipedia.org/wiki/Patch_clamp#Recording


For micropipette membrane probes, to attain low enough system noise, usually the voltage is fixed or "commanded," while only the nanoamps are being measured.  We use a circuit called a "headstage amp:" an op-amp wired to act as a transimpedance amplifier.  This is placed between the pipette electrode and the rest of your circuitry (the ESP32 input.)   So, the membrane generates a 10nA pulse, and the amplifier gives a 1.0V output.   The amplifier has two inputs (diff amp,) so we can compare potentials between the clamped cell and the fluid environment.

Search words: patch-clamp headstage schematic

I just replaced the fried FETs on one of these last month: an old classic version (not one of the expensive, cutting-edge products being used today.)  I sketched out a crude schematic.

This one uses an external dual-JFET to attain extreme low-noise operation, with the FET feeding an inexpensive LF356 op-amp.  The 100meg resistor gives 0.1V-per-nanoamp operation.

PS

The fancy and expensive version is Axopatch, Axon corp.  Here's one of their headstage amplifiers.  Rather than buying dual FETs from Siliconix, they built their own!  They mounted them on a peltier cooler, so they can chill it down to -20C, reducing the noise even more.  This thing can detect the pulses from a single ion channel opening/closing.