In this work we demonstrate the use of Fully Depleted Silicon On Insulator (FDSOI) transistors as pH sensors with a 23 nm silicon nitride sensing layer built in the Back-End-Of-Line (BEOL). The back end process to deposit the sensing layer and fabricate the electrical structures needed for testing is detailed. A series of tests employing different pH buffer solutions has been performed on transistors of different geometries, controlled via the back gate. The main findings show a shift of the drain current (I-D) as a function of the back gate voltage (V-B) when different pH buffer solutions are probed in the range of pH 6 to pH 8. This shift is observed at V-B voltages swept from 0 V to 3 V, demonstrating the sensor operation at low voltage. A high sensitivity of up to 250 mV/pH unit (more than 4-fold larger than Nernstian response) is observed on FDSOI MOS transistors of 0.06 mu m gate length and 0.08 mu m gate width. (C) 2017 Elsevier Ltd. All rights reserved.