Continuous Manufacturing of Graphene-Contact Ion-Selective Electrodes Using Roll-to-Roll Processes: The Case for Nitrate Sensors

Abstract

Soil contains various species of nitrogen, which is crucial for plant growth. Nitrate ion (NO3 -) is essential for promoting high crop yields and is widely used in fertilizers, however groundwater can be polluted by the overuse of nitrate. “Smart agriculture” can be supported by monitoring NO3 - in ground-soil using a distributed network of disposable potentiometric sensors. We have found the performance of such sensors to be enhanced by adding graphene nanoplatelets (GrNPs) as a conductive yet hydrophobic solid contact between the electrode and ion-selective membrane. Exfoliated GrNPs can be stably dispersed in ethyl acetate by liquid-assisted ball-milling, then deposited using methods compatible with the scalable roll-to-roll (R2R) production of ion-selective electrodes (ISEs). Ultra thin layers of GrNPs were formed by spray-coating onto a moving web of screen-printed Ag electrodes on polyethylene terephthalate (PET), followed by in-line deposition of a NO3 - selective membrane and silicone sealant then oven drying. Multiple electrodes from each production run were conditioned in aqueous KNO3 solutions and potentiometric characterizations of sensitivity, selectivity and stability were completed using an 80-channel analyzer. The GrNP layer serves as a hydrophobic transducer of ion adsorption and provides a high capacitance for stable potentiometric signals while also protecting SC-ISEs from damage caused by water permeation. We find that R2R processes can successfully produce graphene-contact NO3 - ISEs for agricultural applications, with extended operational lifetimes and high levels of quality control.