A novel pH sensor based on hydroquinone monosulfonate-doped conducting polypyrrole

Functionalized polypyrrole (PPy) with hydroquinone monosulfonate (HQS) incorporated as the dopant has been prepared by a simple one-step electropolymerization of pyrrole at a stainless steel electrode from aqueous solution. Potentiometric pH responses of the HQS-doped PPy electrodes showed a response slope of −50.54 ± 1.67 mV/pH (28 °C), a linear working range of pH 2–12 and a response time less than 100 s. The electrode stability was maintained over the period of a month. Compared with other PPy-based pH electrodes reported previously, the HQS-doped PPy electrode shows significantly improved potentiometric response slope, response time, reproducibility and stability. Interference studies with several ions showed minimal effects on the potentiometric response of the modified electrode. A combination of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectrometry (ToF-SIMS) was performed to investigate the surface composition and characteristics of the electrodes, including the chemical basis of the electrode performance. Cyclic voltammetry revealed the expected electrochemical response of the HQS, which was electroactive in response to pH changes. HQS-doped PPy shows excellent potential as a novel pH sensor, incorporating both the electroactive species and conducting support in an integrated form, for a variety of applications requiring pH monitoring.