Nanowire and microband arrays for enhanced electrochemical sensing

Abstract

Water quality monitoring is crucial to ensure that (i) water is safe to be consumed by humans, and (ii) that it is of adequate quality for aquatic life. The European Union has dedicated two of its seventeen sustainable development goals for 2030 to water quality. While many methods are available to monitor water quality, these typically involve the use of additional reagents, or require specialist equipment or personnel to carry out the required analysis. In this thesis, electrochemical methods are explored and developed such that they allow for reagent-free, point-of-care testing without the need for specialist training. The main goal of this thesis was to develop interference free measurements that could be applied to remote analysis. To do this, interdigitated micro-electrode arrays were designed and fabricated wherein each comb of electrodes could be biased at different potentials allowing for unique analysis opportunities. The first section deals with the detection of residual free chlorine, the most common disinfectant in water. To do this a pH control method using the interdigitated electrodes was developed that forced the free chlorine in solution to convert to hypochlorous acid, which can be easily detected. This approach prevents loss of signal due to presence of hypochlorite, which is less active in the potential window investigated. Concentrations as low as 0.35 ppm were measured and this method was applied to both buffered water samples and tap water samples. Following from this, the pH control method was applied to the detection of monochloramine. This is the second most common disinfectant for water, typically used in North America. It is also a by-product of water chlorination, so it also needs to be monitored in systems where it is not used as the primary disinfectant. The pH control method was refined in this section and resulted in a sensor that could detect monochloramine by converting it to dichloramine. Monochloramine detection is hindered by the presence of dissolved oxygen, while dichloramine is free from oxygen interference. A detection limit of 0.03 ppm was calculated and this method was applied to water distribution networks even those with high conductivity and alkalinity. Finally, a reagent free method of dissolved oxygen quantification was developed. Dissolved oxygen is a measure of water quality typically used to determine how well the water supports aquatic life. Low dissolved oxygen concentrations can also indicate the presence of bacteria or contaminants like nitrates and nitrites. The detection method developed herein involves reduction of dissolved oxygen to produce hydrogen peroxide, which is subsequently detected at the other comb of electrodes free from interference. Gold and platinum mixed arrays were used to take advantage of both materials properties for oxygen reduction and peroxide oxidation. Oxygen concentrations between 0 and 10 ppm were detected using this method (benchmarked against commercial gold standards), and this approach was applied to real water samples taken from farm run off. This water typically contains high concentrations of fertilizer, yet accurate oxygen measurements were obtained.