Laser-Induced Graphene Electrodes for Direct and Reagentless Voltammetric pH Sensing

Laser-induced graphene (LIG) electrodes were developed as fully reagentless voltammetric pH sensors for direct analysis of complex aqueous matrices. By systematically varying laser power and scan speed, the morphology, wettability, and surface chemistry of the LIG were investigated. The resulting electrodes exhibited quinone groups identified by electrochemical characterization, which mediate a well-defined proton-coupled electron transfer process. The optimized sensors displayed a linear response (R2 > 0.99) over the pH range 2–10, with a Nernstian slope of −60 ± 0.5 mV pH–1, reproducible across independently fabricated batches and requiring no chemical or electrochemical pretreatment. Accurate pH determination was achieved in unmodified commercial beverages (cola, vinegar, beer, orange juice), pharmaceuticals samples (antiacid) and seawater, with relative errors <6% compared to standard glass membrane electrodes. The sensors operated reliably in the presence of dissolved oxygen and other matrix constituents. Stability tests over 100 SWV cycles and 30 days of storage confirmed consistent response and minimal potential drift, while repeated reuse produced gradual slope drifting, without compromising linearity. The results demonstrate that laser processing parameters alone can be used to tune the electrochemical properties of LIG toward intrinsic pH sensitivity, without functionalisation, highlighting the potential for disposable and field-deployable analytical systems capable of direct pH determination in unaltered samples.