Ion-Selective Sensors Based on Laser-Induced Graphene for Evaluating Human Hydration Levels Using Urine Samples

Complex graphene electrode fabrication protocols including conventional chemical vapor deposition and graphene transfer techniques as well as more recent solution-phase printing and postprint annealing methods have hindered the wide-scale implementation of electrochemical devices including solid-state ion-selective electrodes (ISEs). Herein, a facile graphene ISE fabrication technique that utilizes laser induced graphene (LIG), formed by converting polyimide into graphene by a CO₂ laser and functionalization with ammonium ion (NH₄⁺) and potassium ion (K⁺) ion-selective membranes, is demonstrated. The electrochemical LIG ISEs exhibit a wide sensing range (0.1 × 10⁻³–150 × 10⁻³ m for NH₄⁺ and 0.3 × 10⁻³–150 × 10⁻³ m for K⁺) with high stability (minimal drop in signal after 3 months of storage) across a wide pH range (3.5–9.0). The LIG ISEs are also able to monitor the concentrations of NH₄⁺ and K⁺ in urine samples (29–51% and 17–61% increase for the younger and older patient; respectively, after dehydration induction), which correlate well with conventional hydration status measurements. Hence, these results demonstrate a facile method to perform in-field ion sensing and are the first steps in creating a protocol for quantifying hydration levels through urine testing in human subjects.