Attenuation of Redox Interference in RuO₂ pH Sensor Using a Ta₂O₅/Nafion Multilayer Architecture

Despite the growing preference for solid-state pH sensors due to their advantages over glass pH electrodes, their practical application is still limited by challenges, particularly in metal-oxide based systems. The main issue is their susceptibility to redox-active species such as dissolved oxygen, ascorbic acid, sulfides, and transition-metal ions. In this work, the effects of representative redox interferents, with particular emphasis on ascorbic acid, together with dissolved oxygen and potassium permanganate, were investigated and mitigated through the application of Ta₂O₅/Nafion overlayers. The solid-state metal-oxide pH-sensitive electrodes were fabricated with laser micro-etching of radio-frequency (RF) sputtered RuO₂ thin films deposited on ceramic Al₂O₃ substrates. The resulting RuO₂ electrodes exhibited excellent potentiometric pH sensing characteristics, including near-Nernstian sensitivity (58.8 mV pH^-1 at 25 °C), highly linear response over a wide pH range (pH 2-12, R² > 0.9999), minimal hysteresis (1.3 mV), low potential drift (2.9 mV h^-1), and fast response times (<30 s). Following on from our previous work, which demonstrated that the Ta₂O₅/Nafion solid-state sensor could measure beverage pH accurately, here we explain why that architecture works by systematically investigating the role of Ta₂O₅ and Nafion in suppressing redox interference and improving measurement stability in complex sample matrices. The results demonstrate that RuO₂ electrodes modified with combined Ta₂O₅/Nafion thin films exhibit significantly enhanced measurement stability compared to unmodified RuO₂ electrodes, owing to effective suppression of potential fluctuations induced by dissolved oxygen and representative redox couples, particularly those associated with ascorbic acid interference. These findings could give a promising strategy for improving the robustness and reliability of solid-state RuO₂-based pH sensors in complex electrochemical environments.