Water quality assessment by an integrated multi-sensor based on semiconductor RuO2 nanostructures

A multi-sensor based on a nanostructured semiconductor ruthenium oxide (RuO2) sensing electrode (RuO2-SE) deposited on an alumina substrate and capable of being coupled with a simple turbidity sensor has been evaluated for long-term pH stability during a 12-month non-stop trial. The multi-sensor is designed to detect the main parameters of water quality: pH, dissolved oxygen (DO), temperature, conductivity and turbidity over a temperature range of 9-30 degrees C. The morphology of the film SE used in the sensor structure was investigated by scanning electron microscopy and energy dispersive x-ray-analysis at the beginning of the trial and after 12 months of service. It was found that both morphology and surface compositions of nanostructured RuO2-SEs did not change significantly. They keep their high sensitivity to adsorption of superoxide ions (O-2(-)) despite heavy depositions of bio-fouling. The sensors with a RuO2-SE have demonstrated a stable 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 excellent reproducibility was obtained during the trial. The Nernstian slope was approximately 58 mV pH(-1) at a temperature of 23 degrees C. Although RuO2-SEs have been shown to exhibit very good response time for pH changes, within a few seconds at a temperature of 23 degrees C, as the water temperature cooled down, the sensor response time increased significantly and was about 8-10 min or longer at a temperature of 9 degrees C. The influence of hydrogen ion (H+) diffusion in nanostructured RuO2 films on the output emf drift during pH measurements was also investigated. Additional turbidity and conductivity measurements revealed that the multi-sensor is capable of measuring both high and low ranges at different temperatures, exhibiting a high linearity of characteristics.