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All-solid-state potassium ion-selective electrodes (K+ ISEs) using ordered mesoporous carbon (OMC) sphere as the solid contact (SC) are fabricated in this work, which exhibit excellent K+-sensing capability including the stable Nernstian response in a wide linear range of 10−4.19–10−0.21 M, a low detection limit of 5.4 μM, short response time, and good stability. The excellent performances of the K+ ISEs are attributable to the OMC spheres with interconnected mesopore structure, uniform spherical morphology, high surface area, high conductivity, and high capacitance, which provide the highly efficient ion-to-electron transduction between the ion-selective membrane and conductive component in the electrodes. More importantly, the OMC sphere-based SC layer can be deposited in the flexible ISEs by solution processing, which thus enables the construction of ion-sensitive field-effect transistor-based handheld K+-sensing system with high sensitivity and reproducibility, demonstrating the potentials of the OMC spheres in practical ion detection devices. [ABSTRACT FROM AUTHOR]