PH sensors are widely used in chemical and biological applications. Metal oxides-based pH sensors have many attractive features including insolubility, stability, mechanical strength, electrocatalyst and manufacturing technology. Various metal oxide thin films prepared by radio frequency (R.F.) magnetron sputtering have attractive features, including high pH sensitivity, fast response, high resolution, good stability and reversibility as well as potential for measuring pH under conditions that are not favourable for the commonly used glass electrodes-based pH sensors. In addition, thin film pH sensors prepared by R.F. magnetron sputtering offer many advantages, such as ease of packaging, low cost through the use of standard microfabrication processes, miniaturisation, capability of measuring pH at high temperatures, ruggedness and disposability. In this paper, recent development of R.F. magnetron sputtered thin films for pH sensing applications are reviewed.

The influence of the Ar/O₂ gas ratio during radio frequency (RF) sputtering of the RuO₂ sensing electrode on the pH sensing performance is investigated. The developed pH sensor consists in an RF sputtered ruthenium oxide thin-film sensing electrode, in conjunction with an electroplated Ag/AgCl reference electrode. The performance and characterization of the developed pH sensors in terms of sensitivity, response time, stability, reversibility, and hysteresis are investigated. Experimental results show that the pH sensor exhibits super-Nernstian slopes in the range of 64.33–73.83 mV/pH for Ar/O₂ gas ratio between 10/0–7/3. In particular, the best pH sensing performance, in terms of sensitivity, response time, reversibility and hysteresis, is achieved when the Ar/O₂ gas ratio is 8/2, at which a high sensitivity, a low hysteresis and a short response time are attained simultaneously.