A sensitive and highly stable polypyrrole-based pH sensor with hydroquinone monosulfonate and oxalate co-doping

A polymer-based pH electrode has been successfully fabricated via a simple electropolymerization of pyrrole on stainless steel using a co-doping system. Hydroquinone monosulfonate (HQS) was selected as a functional dopant while oxalic acid was used as a co-dopant. A combination of cyclic voltammetry and X-ray photoelectron spectroscopy revealed that passive layers of iron oxalate and chromium oxalate formed on the stainless steel surface at the initial stage of electropolymerization. The potentiometric characteristics of the co-doped polypyrrole (PPy) electrode exhibited a response slope of -54.67 ± 0.70 mV/pH at 28 °C, a linearity range from pH 2 to 12 and correlation coefficient greater than 0.995. The open-circuit potential was stable in buffer solutions with a response time less than 10 s, regardless of the age of electrode. The co-doped PPy electrode could be used as a multi-use pH sensor up to 60 days without any effect to the potentiometric response. Interferences from most of common ions are acceptably small. In comparison with a HQS-doped PPy electrode, the co-doped PPy electrode provides a superior pH measurement in solution due to the higher pH sensitivity and longer lifetime. Co-doping results in an improvement in the adhesion strength of the co-doped PPy film with the stainless steel electrode. Easy fabrication and the low production cost of the co-doped PPy electrodes offer an alternative to pH sensors having a comparable potentiometric performance with commercial glass electrodes.