Hello, reference electrode is necessary for proper function of an electrochemical assay. Some ISFET sensors have integrated reference electrode so there is no need to get an external reference electrode. But if there is no integrated reference electrode, you should get there an external one. The Ag/AgCl reference electrodes are popular for sensors/biosensors construction. They are small, simply for construction, ready to be integrated, and AgCl provides immediately saturated solution because of its low solubility. But the other reference electrodes can be also used:

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