Calibration-free potassium quantification via a stoichiometric enzyme cascade on laser-induced graphene

Potassium (K⁺) monitoring is essential for clinical management, particularly for point-of-care testing (POCT) that requires rapid and reliable analysis. Conventional potentiometric ion-selective electrodes rely on equilibrium potential responses and frequent calibration, which limits their applicability in decentralized testing. Here, we present an amperometric K⁺ biosensor based on a pyruvate kinase/oxidase (PK/POx) enzyme cascade integrated with a laser-induced graphene (LIG) electrode. K⁺-dependent enzymatic activity is converted into a direct current signal via 4-nitrosoaniline-mediated redox cycling. The porous, defect-rich LIG architecture acts as a reaction-accelerating electrochemical interface that facilitates enzymatic turnover and mediator-assisted electron transfer. Under optimized conditions, the sensor exhibited a linear response over the physiological range of 2-8 mM, a detection limit of 1.56 mM, high selectivity against interfering ions, excellent inter-sensor reproducibility (RSD = 2.41-5.20%), and reliable performance in spiked human serum samples. By operating in a rate-governed, current-mode sensing framework rather than an equilibrium-based potential measurement, the PK/POx/LIG platform enables point-of-use operation without requiring sensor-specific or user-performed calibration at the time of measurement, enabling rapid potassium monitoring in POCT applications.