A stable solid contact transducer and ionophore-free all-solid-state ammonium ion-selective electrode for mobile sensor application in aqueous media

Abstract

An all-solid-state ion-selective electrode (AS- NH4+ISE) for ammonium-ion-sensing based on stable conductive polymer (CPs) as a solid contact transducer and ionophore-free ion-selective membrane for mobile sensor application was fabricated. Poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) electropolymerized onto screen-printed carbon electrodes (SPCEs), and screen-printed platinum electrodes (SPPEs) as solid contact transducer was characterized for its morphology and electrochemical performance and was studied for stability – the ability of the sensing solid contact transducer to adhere to the working electrode surface and maintain electrochemical cycle stability. The stability of the solid contact transducer was studied in static measurements condition – a condition where the electrodes are submerged in aqueous solution and not moving, and there is no water flow on the electrode surface when the measurements are taken, and dynamic measurements condition – a condition where an aqueous solution flows across the electrode surface and the sensor is not moving when measurements are taken. Cyclic voltammetry (CV) showed that the electron transfer ability of SPCEs and SPPEs was significantly improved when electropolymerized PEDOT:PSS was used as the transducer. Moreover, the CVs' redox peak current showed that both electrodes could maintain the electrode’s mechanical and electrochemical functional integrity for over 30 days. The results suggest that the electropolymerized PEDOT:PSS had good adhesion to SPCEs and SPPEs working electrode surfaces. There was no significant change in the cycle stability curve in PBS, pH 7.1, after 3000 cycles conducted over 12 hours, compared to the initial cycle. Furthermore, no significant change in the cycle stability curve was observed after 30 days of undergoing CV cycles in PBS, pH 7.1, compared to the first day for both electrodes. The results suggest that electrode stability of PEDOT:PSS/SPCEs and PEDOT:PSS/SPPEs was maintained after repetitive CV cycles in aqueous media. After characterisation of solid contact transducer, the PEDOT:PSS/SPCEs were integrated into a sensing cell to investigate the electrochemical behaviour of electropolymerized PEDOT:PSS in dynamic measurement conditions. The results showed that the PEDOT:PSS/SPCEs maintained their peak potential (Ep) and peak current (Ip) after they were exposed to different flow rates of 10, 20, 30 and 40 ml/min. Furthermore, the effect of the flow rates on the Ep and Ip was investigated. The results showed that flow rates range between 0 to 40 ml/min did not affect the Ep and Ip value of the PEDOT:PSS/SPCEs. Finally, o-phenylenediamine (o-PD) as an ammonium ion-selective membrane (ISM) was electropolymerized to poly(o-phenylenediamine) and deposited simultaneously on top of the PEDOT:PSS/SPCEs solid contact transducer to fabricate AS-NH4+ISEs. The ISM's electropolymerization deposition was obtained by cyclic voltammetry (CV) with potential from 0.0 V to 0.8 V and a scan rate of 50 mV/s. The fabricated AS-NH4+ISEs can detect ammonium ions (NH4+) as low as 5.7×10-5 M with a slope of 58.49 mV/decade (R2 > 0.99) and a linear detection range from 10-3 M to 1 M. These results provide an initial insight into the applicability of the stable PEDOT:PSS/SPCE solid contact transducers for the development of AS-NH4+ISEs with high potential for scaling-up purposes and the ability for miniaturization and integration into a mobile sensor platform.