Solution-processable all-solid-state chloride-selective electrode: Enhanced sensitivity from anion dopant exchange

Ion-selective electrodes (ISEs) are widely used in many industries, with recent research focusing on their miniaturization by replacing the liquid filling solutions with solid-contacts. Solution-processing is the preferred method for preparing solid-contact ISEs (SC-ISEs) due to its ease and scalability. However, while there are many solution-processable cationic SC-ISEs, this remains a challenge for anionic SC-ISEs due to poorer compatibility with the solid-contacts. Many anionic SC-ISEs are still prepared by complex techniques, such as electropolymerisation of the solid-contact. Thus, strategies for solution-processable solid-contacts which can interface well with anionic ion-selective membrane (ISMs) are required. Here, we report the fabrication of a fully-solution-processable chloride (Cl^-) SC-ISE by anion exchange of poly(3,4-ethylenedioxythiophene)-polyethylene glycol (PEDOT-PEG) solid-contact before drop-casting the ISM. Significant improvement in sensitivity was observed after PEDOT-PEG anion exchange, with the optimal SC-ISE exhibiting near-Nernstian response (-53.3 ± 0.5 mV/decade, versus -33.4 ± 1.8 mV/decade for the unexchanged SC-ISE) across a wide dynamic range (0.05 M-6.03 μM). Our SC-ISE also exhibited excellent selectivity against phosphate (H₂PO₄^-), bicarbonate (HCO₃^-) and acetate (CH₃CO₂^-) and could be utilized with minimal conditioning time and for prolonged usage. Finally, given the importance of Cl^- sensing in healthcare, we also demonstrated the potential of our Cl^- SC-ISE in sensing multiple synthetic biological samples such as sweat, urine and blood, and real human sweat (forearm samples). This work not only demonstrates the versatility of our anion exchange protocol, but also furthers the understanding of the different enhancement mechanisms - sensitivity or selectivity - depending on whether an ionophore was present in the ISM. We showed that regardless of the mechanism, our simple and efficient protocol could mitigate the issue of the original underperformance and can thus be readily extended to the scalable preparation of multiple types of anion-selective SC-ISEs.