Hydrothermally reduced graphene oxide as a sensing material for electrically transduced pH sensors

•The electrochemical nature of oxygen functional groups present on the hydrothermally reduced graphene oxide is investigated by cyclic voltammetry techniques.•Hydrothermally reduced graphene oxide based potentiometric and chemiresistive pH sensor is fabricated as a reagent less sensing probe.•Chemiresistive pH sensor is prepared by a drop casting method on the interdigitated gold electrodes with 100 µm interfinger spacing.•Stable sensor response of 55 mV/pH is obtained in potentiometric sensing mode and a sensitivity of 44 Ω / pH is achieved in chemiresistive sensing mode. A reagent-less sensing probe has been developed using hydrothermally reduced graphene oxide (HRGO) and investigated as potentiometric and chemiresistive pH sensor. The HRGO modified electrode, used as a potentiometric sensing probe, showed a sensitivity of 66 mV / pH for a freshly prepared electrode. After a few exposures to different buffers, a stable sensitivity of 55 mV / pH was obtained in a pH range from 2 to 12. In the chemiresistive sensing mode, the HRGO electrode showed a sensitivity of around 44 Ω / pH in a pH range from 4 to 7. The results showed that the HRGO modified electrode could be used as a potentiometric pH sensor and as a chemiresistive sensor. However, the chemiresistive sensor has a narrow working pH range of 4–7 compared to the potentiometric sensing method. The chemiresistive sensor has a good sensitivity between pH 4 and 7 and may be used to measure pH in some biological processes. The electrochemical nature of HRGO surface functionalities was explored by cyclic voltammetry. The presence of a quinone-like redox couple was identified on the HRGO surface. The redox activity of these functional groups was studied at different pH. Further, the quinone-like moieties were confirmed by their electrocatalytic activity towards dissolved oxygen.