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

TLDR

Electrochemical pH sensors are in high demand across food processing, health monitoring, agriculture, nuclear sectors, and water quality monitoring due to their fast response (<10 s), wide pH range (2–12), near‑Nernstian sensitivity (~59 mV/pH), easy integration on flexible substrates, biocompatibility, and low fabrication cost. This review aims to survey metal‑oxide materials used for pH sensing, evaluate composite designs, and assess their suitability for flexible/wearable biosensor applications. The review examines potentiometric, conductimetric, chemi‑resistor, ISFET, extended‑gate FET, and electrochemical impedance spectroscopy techniques used to characterize metal‑oxide pH sensors.

Abstract

Electrochemical pH sensors are on high demand in numerous applications such as food processing, health monitoring, agriculture and nuclear sectors, and water quality monitoring etc., owing to their fast response (<10 s), wide pH sensing range (2–12), superior sensitivity (close to Nernstian response of 59.12 mV/pH), easy integration on wearable/flexible substrates, excellent biocompatibility and low cost of fabrication. This article presents an in-depth review of the wide range of MOx materials that have been utilized to develop pH sensors, based on various mechanisms (e.g. potentiometric, conductimetric, chemi-resistors, ion sensitive field effect transistor (ISFET) and extended-gate field effect transistor etc.). The tools and techniques such as potentiometric and electrochemical impedance spectroscopic that are commonly adopted to characterize these metal oxide-based pH sensors are also discussed in detail. Concerning materials and design of sensors for various practical application, the major challenges are toxicity of materials, interfernce of other ions or analytes, cost, and flexibility of materials. In this regard, this review also discusses the metal oxide-based composite sensing (active) material, designs of pH sensors and their applications in flexible/wearable biosensors for medical application are examined to present their suitability for these futuristic applications.