Carbon Black and PEDOT:PSS in a Synergistic Solid Contact for Reliable Printed Potentiometric Sensors

Printed potentiometric sensors require reliable solid contacts to provide stable and reproducible ion-selective electrodes. However, hampering fabrications and unfavorable maintenance often hinder their breakthrough on a commercial scale. Herein, we develop a solid-contact calcium-selective screen-printed electrode harnessing straightforward manufacturing based on the combination of carbon black (CB) and poly(3,4-ethylene dioxythiophene) doped with poly(sodium 4-styrenesulfonate) (PEDOT:PSS) as an ion-to-electron transducer. Drop-casting was used to simply deposit the CB layer, while PEDOT:PSS was rapidly electropolymerized (71 s) onto the working electrode to deliver affordable manufacturing of the printed electrode while ensuring valid potentiometric performance. Subsequently, the ion-selective membrane (ISM) and the polyvinyl butyral (PVB)-based reference membrane were, respectively, drop-cast onto the working electrode and reference electrode (RE). Upon the optimization of solid-contact layers, the solid-contact Ca²⁺-ISEs were investigated by evaluating their potentiometric performance versus a conventional RE. CB/PEDOT:PSS-modified screen-printed electrodes demonstrated a low-frequency capacitance of 55 μF, and an outstanding standard potential interelectrode reproducibility (±1 mV). The combination of both types of ion-to-electron transducers provided calcium detection in the linear range 10^-1-10^-7 M with a Nernstian sensitivity (28.3 ± 0.3 mV/decade), ensured over 28 days under dry storage. Furthermore, the absence of the water layer effect was also demonstrated. Lastly, the fully printed platform was assembled to achieve a miniaturized and easily field-deployable potentiometric device, consisting of CB/PEDOT:PSS/ISM configuration and PVB-based reference membrane on the working electrode and the RE, respectively. The resulted all-solid-state sensor revealed a Nernstian sensitivity with a 29.0 ± 0.5 mV/decade slope and a stable signal up to 72 h (drift = -0.2 mV/h). Analysis carried out using commercially available bottled water also demonstrated the sensor successful performance in the determination of calcium ion in real samples.