pH Sensors Using 3d-Block Metal Oxide-Coated Stainless Steel Electrodes

pH response and mechanical properties of Fe2O3-TeO2-based glass/stainless steel enamel electrodes for pH sensors

Glass pH sensors are unsuitable for in vivo biomedical, clinical, or food applications because of the brittleness of glass and the difficulty in measuring small volumes. Enamel structures such as glass/stainless steel are candidates for glass-based pH electrodes. In this study, new enamel electrodes for pH sensors using Fe₂O₃-TeO₂-based glass/stainless steel were developed. The effect of NiO addition to Fe₂O₃-TeO₂ glass on the pH sensitivity and the three-point bending strength of enamels were investigated. The effect of NiO addition to Fe₂O₃-TeO₂ glass/stainless steel on the pH sensitivity was negligible. Fe₂O₃-TeO₂-based glass/stainless steel showed pH sensitivity appropriate to a working electrode. Enameling at a lower temperature under an air atmosphere was desirable for narrowing the gap between pH 4-7 and pH 7-9 sensitivities. The NiO addition to Fe₂O₃-TeO₂ glass/stainless steel decreased the three-point bending strength. Therefore, NiO did not serve as an adhesion oxide in the Fe₂O₃-TeO₂ glass. Fe₂O₃-TeO₂ glass/stainless steel possessed the highest three-point bending strength among all samples when prepared at 670 °C under an air atmosphere. Therefore, no NiO addition and enameling at a lower temperature under an air atmosphere are desirable for obtaining more robust Fe₂O₃-TeO₂ glass/stainless steel than Li₂O-SiO₂-based glass electrodes for pH sensors.

Development of Tungsten Oxide Nanoparticle Modified Carbon Fibre Cloth as Flexible pH Sensor

A reagent-less pH sensor based on disposable and low cost carbon fibre cloth (CFC) is demonstrated for the first time, where tungsten oxide nanoparticles were grown directly onto the CFC substrate. For comparison purpose, tungsten oxide nanoparticle modified glassy carbon electrode (GCE) was also fabricated as a pH sensor, where hydrothermally synthesized tungsten oxide nanoparticles were drop casted onto the GCE surface. The corresponding equilibrium potential using tungsten oxide/CFC as a pH sensor was measured using open circuit potential (OCP), and was found to be linear over the pH range of 3-10, with a sensitivity of 41.38 mVpH^-1, and response time of 150 s. In the case of tungsten oxide/GCE as a pH sensor, square wave voltammetry (SWV) was used to measure the shifts in peak potential and was found to be linear with a pH range of 3-11, and a sensitivity of 60 mVpH^-1 with a potential drift of 2.4-5.0% after 3 hour of continuous use. The advantages of tungsten oxide/CFC and tungsten oxide/GCE as pH sensing electrode have been directly compared with the commercial glass probe based electrode, and validated in real un-buffered samples. Thereby, tungsten oxide nanoparticles with good sensitivity and long term stability could be potentially implemented as a low cost and robust pH sensor in numerous applications for the Internet of Things (IoT).

Effect of heat-treatment on the pH sensitivity of stainless-steel electrodes as pH sensors

Effect of heat-treatment on the pH sensitivity of uncoated stainless-steel electrodes was investigated to comprehend the pH sensitivity of metal-oxide coated stainless-steel electrodes as novel pH sensors. The pH sensitivity of stainless-steel electrodes as-received and heat-treated at 500 °C, 600 °C and 700 °C for 24 h were 91 %, 94 %, 102 % and 91 %, respectively. The pH sensitivity tended to increase with increasing heat-treatment time at a given temperature. Thus, the most suitable heat-treatment condition for the stainless-steel electrodes was 600 °C for 24 h. The austenite phase (fcc) was the main phase on the surface of the heat-treated stainless-steel electrodes. Unexpectedly, the change in the martensite phase (bcc) as the second phase with heat-treatment temperature was similar to the pH sensitivity, with the martensite phase affecting the pH sensitivity. Therefore, it appeared that the pH sensitivity of the metal-oxide coated stainless-steel electrodes was affected by the underlying stainless-steel as well as the outer metal-oxide film coating. A prototype stainless-steel tube electrode was used as a working electrode for demonstrating the depth profiling of pH. The stainless-steel tube electrode showed good performance for measuring pH depth profiles compared to commercially available glass electrodes.