Blade Coated Polyvinylidene Fluoride-Carbon Black Composites for Printed Solid Contact Ion Selective Electrodes

Solid contact ion-selective electrodes (SC-ISEs) have a high potential to enable continuous ion sensing in health and environmental applications due to their low readout power and complexity requirements. Despite this, their usage in distributed sensor nodes remains practically limited by the need for periodic recalibration. To address this challenge carbon nanomaterials have been effectively utilized in SC-ISEs to reduce calibration drift due to their high effective surface area, intrinsic hydrophobicity to resist interfacial water layer formation and electrochemical inertness. To translate these advancements to scalable sensor nodes, the processibility of these materials for high throughput solid contact fabrication across a range of device geometries needs to be considered. In this work, we have optimized an ink based on carbon black nanoparticles (CB) and polyvinylidene fluoride binder (PVDF) with desirable properties for blade coating at roll-to-roll compatible coating speed. This approach also gives control of solid contact geometry using a polyimide stencil during the coating process. The cured films have good flexibility and adhesion to PET substrates. By varying the blade height and speed this approach allows for control over the thickness and area of the resultant solid contact film in a single blade coating step, improving processibility over previous multilayer drop casting techniques. We find that by increasing the loading of carbon black, a maximum water contact angle of 154.7° ± 4.3° is achieved, which supports the superhydrophobic nature of the solid contact. SC-ISEs selective to nitrate utilizing these solid contacts exhibited expected Nernstian sensitivity (>50 mV/Decade) and selectivity coefficients that match previously published empirical values. The porous PDVF-CB solid contact forms a maximum specific electrical double-layer capacitance of 2.54 ± 0.21 F/g after drop casting the ion-selective membrane and resists water layer formation during extended ion sensing. Further, to eliminate the need for drop-casting of the ion-selective membrane, we demonstrate a modified ink composition to enable a simplified single-step blade-coated electrode for SC-ISEs. We use this approach to increase the electrode sizing and achieve flexible SC-ISEs with >1 mF electrode capacitances as a step towards long-term, calibration-free sensing.