This lecture covers the principles of potentiometry, focusing on the measurement of potential difference between an indicator and a reference electrode. Topics include the Nernst equation, ISFET technology, MOSFET, DNA amplification and detection using ISFET, ion-selective electrodes, and wearable devices with ion-sensitive electrodes. The lecture also discusses the construction and applications of ion-selective electrodes, including glass and polymer membranes, and the challenges faced in developing wearable devices for monitoring ions and metabolites in biological fluids.