All-solid-state ion-selective sensors for measurement of potassium ions in aqueous media

Abstract

Ion-selective electrode (ISE) sensors have been widely used in diagnostics, food security, and environmental monitoring applications owing to their low-cost and straightforward fabrication processes. However, monitoring physiological ions in real-time remains a challenge where conventional ISEs suffer poor sensitivity, limit of detection, and lifetime due to the sensing membranes of conventional ISEs, where ions were not efficiently converted into electrons. Therefore, this project aims to utilize nanomaterials of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate as all-solid-state transducers to improve the sensitivity, limit of detection, and lifetime of the sensor. Moreover, this work aims to develop all-solid-state potassium ion-selective electrode (AS-K+ISE) sensors for detection and quantification of potassium ions (K+) in aqueous media. This research is also to evaluate and validate the sensor performance of the AS-K+ISE sensors by monitoring dynamic K+ changes in aedes mosquito larvae. The effects of modifying low-cost screen-printed carbon electrodes (SPCEs) with poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), reduced graphene oxide stabilized in polystyrenesulfonate (rGO:PSS), and their composite (rGO:PSS-PEDOT:PSS) on the electrical conductivity and electrochemical reversibility of electrochemical sensor performance were characterised in terms of peak current (Ip), peak-to-peak potential separation (ΔEp), shift in peak potentials (Ep_shift), and effective surface area (Ae). Cyclic voltammetry (CV) results revealed transducer rGO:PSS-PEDOT:PSS has synergistic effect of PEDOT:PSS and rGO:PSS, where PEDOT:PSS has high peak current (Ip) suggesting fast electron transfer kinetics at the electrode-electrolyte interface, and rGO:PSS is reversible as evidenced by small ΔEp and Ep_shift. Owing to PEDOT:PSS having the highest peak current of 1.637 mA which is 23 folds higher than rGO:PSS, AS-K+ISE sensors were fabricated by drop-casting potassium-selective membrane (KISM) onto PEDOT:PSS/SPCEs. Potentiometry measurements were used to determine limit of detection, sensitivity, linear range, lifetime, response time, and selectivity of AS-K+ISE sensors of varying K+ concentrations. The AS-K+ISE sensors can detect as low as 0.01 µM of K+, with a near-Nernstian slope of 59.6 mV per decade within a linear range between 0.1 mM and 100 mM. The lifespan extends to at least 24 weeks, with instantaneous response of 3-10 s toward increasing K+ concentrations. The AS-K+ISE sensors showed a superior selectivity toward K+ over interfering ions Na+, and was successfully evaluated to detect varying K+ concentrations in aqueous media. Finally, the ability of the developed AS-K+ISE sensors was validated by monitoring dynamic K+ changes in aedes mosquito larvae. The results demonstrated performance of all-solid-state sensors based on screen printed electrodes that have performance of sensitivity, selectivity, and lifetime compared to microfabricated electrodes, paving efforts towards a low cost with high performance sensors for biomedical and environmental applications.