Highly stable direct-printed polyazulene-based miniaturized electrode for pH analysis in human body fluids

• Solid-contact flexible pH electrode using direct printing enables scalable sensor production. • Electropolymerized polyazulene transducing layer ensures efficient ion-to-electron conversion. • Miniaturized solid-state Ag/AgCl/PVA-KCl reference electrode shows high stability comparable to standard ones. • pH electrode enables reliable measurements in biological fluids: urine, saliva, serum, and drainage fluid. pH monitoring in biological fluids plays a critical role in clinical diagnostics and therapeutic management. This study presents a novel solid-contact pH electrode fabricated using a direct-printed (DP) graphite (Gr) electrode on a flexible substrate, followed by an electropolymerized hydrophobic polyazulene (pAz) transducing layer, and an ion-selective membrane (ISM). The pH electrode was paired with a miniaturized solid-state Ag/AgCl reference electrode incorporating a photopolymerized PVA-KCl matrix. The miniaturized reference electrode exhibited excellent potential stability (±2.5 mV across pH 2–11), and minimal signal drift (10 µV/h). The miniaturized pH electrode exhibited a sensitivity of 55.7 mV/dec, with a rapid response time of 6 s (vs. Orion™ ROSS Ultra™ reference electrode) or 42 s (vs. miniaturized solid-state reference electrode) and a linear response over the pH range of 2–10. The pH electrode demonstrated excellent analytical performance in diverse biological fluids, including urine, serum, saliva, and surgical drain fluid, closely matching the performance of a laboratory-grade combined glass pH electrode. These results underscore the potential of the proposed platform as a reliable and technologically scalable tool for real-time pH assessment in biomedical applications.