A Self-Cleanable Microelectromechanical Systems pH Sensor Based on Three-Dimensional Truncated Pyramid Microstructured IrO x /TiO Composite Electrodes

Precise pH monitoring is essential for water quality regulation and ecological risk assessment, yet long-term operation of electrochemical pH sensors is often compromised by surface fouling, resulting in signal instability and performance degradation. Here, we developed a microelectromechanical (MEMS) pH sensor featuring a three-dimensional truncated-pyramid architecture that spatially separates an IrOx sensing unit from a TiO2 photocatalytic cleaning unit on a monolithic silicon substrate. Rather than simply integrating the two functional materials on a planar interface, the three-dimensional architecture physically decouples pH sensing from photocatalytic cleaning, thereby reducing direct exposure of the IrOx sensing layer to highly reactive species generated near TiO2. The electrodeposited IrOx layer at the top of the truncated pyramids serves as the pH-sensitive interface, whereas the TiO2 layer located at the bottom is activated under UV irradiation to generate reactive oxygen species for degradation of adsorbed organic contaminants and in situ regeneration of the sensing interface. This UV-triggered photocatalytic response confirms the photoactivation capability of TiO2 and provides mechanistic support for excitation by UV irradiation. The applied UV irradiation intensity was selected to be comparable to the UV component of natural sunlight, providing an environmentally relevant excitation condition for evaluating the photocatalytic regeneration capability. However, the long-term self-cleaning performance under actual natural solar illumination requires further validation. Integrated with a packaged Ag/AgCl reference electrode, the sensor exhibits a highly linear response over pH 2-12 (R2 = 0.9998), a measurement error of ±0.05 pH, and a response time of approximately 1 s. Following organic fouling, UV-induced photocatalytic regeneration achieves an interfacial charge-transfer recovery efficiency of approximately 95% and substantially restores the potentiometric response. Further tests in three real water samples with different contamination levels demonstrate matrix-dependent fouling behavior and effective photocatalytic regeneration. Periodic UV cleaning enables continuous operation for more than 160 h while introducing only a small transient potential fluctuation of approximately 5 mV. These results demonstrate that spatially decoupling the sensing and photocatalytic cleaning functions through three-dimensional microstructural engineering provides a promising strategy for improving the fouling tolerance, regenerability, and long-term applicability of electrochemical pH sensors in complex aqueous environments.