Open circuit ground multiplexing?
Rather than leaving the unconnected ground electrodes open you can short them to ground - there will not be any potential on them then.

Since the current is so low I would not use a semiconductor multiplexer - the leakage currents will swamp the signal being measured. I would recommend mechanical relays (possibly reed relays). The voltage across the relay will never be more than the input to the pico ammeter (&lt;1V).

Use a single-pole changeover relay per ground electrode with the common going the electrode, the NC terminal to ground and the NO terminal tied to all the others you can select between the multiple experiments with the unused ones being grounded.

Another way that avoids mechanical relays but with lower accuracy is to use diodes as the switching element to combine the electrode signals and short to ground all but the sensor being measured before the diode. The active element shorting the unwanted signals to ground needs to be selected to have very low leakage (&lt;1nA) with a diode drop across it. The diodes also need to be low leakage parts with &lt;1nA lead with a reverse bias of 1 diode drop.