Anodization treatment of flexible graphite sheet electrodes for Nernstian pH sensing

The ability to accurately determine pH in various complex matrix samples is important for understanding and managing various chemical environments. While glass pH probes are the standard for pH measurements, they present several drawbacks, including the need of frequent calibrations, and high susceptibility to interference from alkali metals. Herein, we demonstrate a voltammetric Nernstian pH sensing approach utilizing low-cost, flexible graphite sheet (GS) electrodes. A rapid electrochemical treatment (10 s) of GS electrodes introduces quinone functional groups immobilized onto the electrode surface, whose redox behavior is subsequently monitored. Due to the pH-sensitive nature of quinones present on the GS sites, we established the capability of GS electrodes to measure pH. The quinoid peak position shifts by 56 ± 1 mV pH^-1 at 298 K across an aqueous pH range of 2.0 to 11.0. Interestingly, the proposed sensor does not require any degassing step before analysis. We also investigated the effect of common interfering species (Cu²⁺, Pb²⁺, Cd²⁺, Zn²⁺, Cr²⁺, and Mn²⁺ and Na⁺) and did find no significant impact (<1.7 %) on the GS-based pH sensor's response. We subsequently analyzed different undiluted complex samples (vinegar, cola, antacid tablet slurry, cleaning products, milk, and seawater) yielding excellent results comparable to those obtained with a standard glass pH electrode. Given the widespread application of GS electrodes in electroanalysis, this approach offers a versatile method for the simultaneous measurement of other electroactive compounds and the monitoring of pH during such analyses.