Ion-Selective All-Solid-State Printed Sensors: A Systematic Review

The recent advancements in technologies related to printed electronics brought significant innovation in the design and fabrication of electrochemical miniaturized sensors. One of the most exciting categories that can benefit from the transition from macro to micro enabled by novel printing techniques is ion-selective electrodes (ISEs). Miniaturized ISEs are also called "all-solid-state ISEs" (ASS-ISEs) since they rely on a solid contact (SC) transduction layer, enabling easier miniaturization and integration. This is particularly interesting for a wide variety of applications ranging from medical to industrial. In all these fields, printed ASS-ISEs have been proven to reach significant results in terms of metrological properties, obtaining near-Nernstian sensitivities, limits of detection down to 10^{{-{10}}} , best selectivity coefficients around 10^{{-{8}}} , shorter response time lower than 5 s, stability reaching six months and optimal variabilities (with best repeatability (RP) lower than 1% and reproducibility (RPR) lower than 2%). The use of innovative printing techniques could further enhance these properties, as well as improve aspects such as flexibility, material waste, resolution improvement, and the finest control of surface functionalization. Although a wide range of examples has multiplied in the literature over the past decade, to date, most of them still lack uniformity or detailed guidance regarding both the fabrication process and the metrological characterization procedure. With this in mind, this review aims to serve as a guideline for the identification of project specifications for the design, fabrication, and test of ion-selective ASS printed sensors, as well as to propose a common metric in characterizing these devices.