Defective vs high-quality graphene for solid-contact ion-selective electrodes: Effects of capacitance and hydrophobicity

Abstract Solid-contact ion-selective electrodes (SC-ISEs) are a promising type of miniaturized analytical devices which rely on solid contacts (SC) for ion detection. For example, an electrical double-layer (EDL) capacitance-based SC-ISEs involves carbon-based SC materials. A crucial challenge is potential stability, which depends on the capacitance and hydrophobicity of the SC materials. Herein we examine representative graphene-based SC-ISEs to explore the effects of these two factors. Six different types of graphenes were used to fabricate SC-ISEs, including four defective graphenes (reduced graphene oxides, RGO) with different defect levels, and two high-quality graphenes produced by different methods. A series of physical and chemical methods were used to investigate the amount of defects and the hydrophobicity. The capacitances of the SC materials and SC-ISEs were also examined in detail. It was found that the hydrophobicity of graphene had a negative correlation with its capacitance. The reduced RGO with a medium level of defects had the highest potential stability among the six types of graphene studied, due to its balance between hydrophobicity and capacitance. This work offers some guidelines for the design of carbon-based SC-ISEs and recommends that the SC material should have a high capacitance with a certain degree of hydrophobicity. Finally, we demonstrate a practical application of ion analysis in soil leaching solutions using RGO-based SC-ISEs.