Metal oxides based electrochemical pH sensors: Current progress and future perspectives

Flexible potentiometric pH sensors for wearable systems

“…MO x have recently gained increasing interest for electrochemical and biosensing applications due to their unique electrical, electrochemical and highly sensitive properties which assert their utility in healthcare and water and food quality monitoring applications. 32,[187][188][189] Due to high surface to volume ratio, the morphology of nano structured MO x enhances the sensitivity, response time, selectivity and catalytic activity of sensitive electrodes. 47 For example, in comparison to ZnO nanorods (sensitivity 28.4 mV pH À1 ) ZnO nanotubes show high sensitivity (45.9 mV pH À1 ) due to their low dimensionality and higher levels of surface and subsurface oxygen vacancies.…”

“…Among various MO x used for the fabrication of pH sensors, RuO 2 and IrO 2 allow pH measurements over wide ranges, with fast responses, high accuracy and high durability. 32 However, for exible and wearable pH sensors, the application of these oxides, especially RuO 2 , is limited due to the lack of biocompatibility, exibility, high cost and high temperature processing required. 32,188,191 The sensitivity of MO x based pH sensors largely depends on the material composition and the method used for material deposition because the microstructure, porosity, surface homogeneity and crystalline structure of the material inuences the sensing performance (Table 3).…”

“…32 However, for exible and wearable pH sensors, the application of these oxides, especially RuO 2 , is limited due to the lack of biocompatibility, exibility, high cost and high temperature processing required. 32,188,191 The sensitivity of MO x based pH sensors largely depends on the material composition and the method used for material deposition because the microstructure, porosity, surface homogeneity and crystalline structure of the material inuences the sensing performance (Table 3). 32,191,192 It has been demonstrated that IrO 2 has very good biocompatibility and proved its cell viability close to 100% which is higher than indium tin oxide (ITO).…”

“…Conventional glass pH electrodes are not suitable for wearable systems due to their lack of bending capability and the fact that glass can easily crack during user movement. 32 In addition, glass pH sensitive electrodes are large in size, are difficult to be miniaturized, and require regular topping up of the reference buffer solution. [33][34][35] As a result, alternative technologies such as solid-state pH sensors are being explored.…”

“…A summary of the design and components of state-of-the-art exible potentiometric pH sensors are presented along with the gaps or challenges to be addressed. Unlike other reviews articles on polymer-based pH sensors, 45 microfabricated electrochemical pH sensors 46 and MO x based sensors, 32 this article focuses on exible potentiometric pH sensors for wearable, their design and components, applications, challenges and the future outlook. A substantial part of the discussion focuses on the design of exible potentiometric pH sensors, with emphasis on exible substrates, the materials for the reference and sensitive electrodes, sensitivity and biocompatibilitywhich is paramount to the success of wearable pH sensors for health applications.…”