Morphology of Pt-doped nanofabricated RuO2 sensing electrodes and their properties in water quality monitoring sensors

Integrated water quality monitoring sensors were fabricated using micro-fabrication technologies, nanostructured Pt-doped RuO2 sensing electrode (SE) and an Ag/AgCl reference electrode (RE). The properties of RuO2–SE were evaluated for potentiometric detection of pH and dissolved oxygen (DO) at temperatures from 9 to 30 °C. Pt-doped nanostructured RuO2–SEs were prepared on alumina sensor substrates. Their morphology was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), electron dispersion spectroscopy (EDX), thermo-gravimetric/differential thermal analysis (TGA/DTA) and Fourier transform infrared spectroscopy (FTIR). It was found that morphology and surface compositions of nanostructured RuO2–SEs are sensitive to superoxide oxygen ions (O2−). Sensors with Pt-doped RuO2–SE showed Nernstian response to pH from 2.0 to 13.0 and were also capable of measuring DO in the range of 0.6–8.0 ppm. The measurement results show very good linearity and an excellent reproducibility was obtained during the 8 months of testing so far. Nernstian slope was 58 mV/pH at a temperature of 23 °C. Although RuO2–SEs have exhibited very good response time to pH changes, which was within 1–2 s at a temperature of 23 °C, as water temperature cooled down, the sensor response time increased and was about 8–10 min at working temperature of 9 °C. Influence of the hydrogen ion (H+) diffusion in RuO2 films on the output emf drift at pH measurements was also investigated.