An investigation of thick-film electrodes for environmental monitoring

Abstract

Rivers and streams are one of the most fragile elements of the whole environmental structure that provides fresh water-useful to humans. Therefore, it is necessary to detect the contamination of watercourses rapidly. To monitor the pollution at various watercourses points, a network of upstream early warning systems consisting of in-situ miniaturized electrochemical sensors is required. The electrochemical sensors are a fast responding, sensitive, and low-maintained alternative to the current methods of downstream detection of pollution in watercourses.

Thick-film (TF) technologies together with electrochemical methods adopt those requirements and offer new types of portable devices that are particularly well suited to on-line monitoring and analysis outside laboratories. Additionally, thick-film technology offers a competitive solution to the current expensive environmental monitoring devices by manufacturing multi-parameter sensors in the thick-film process and employs electrochemical technology in the sensor design. Ideally, those sensing devices will consist of multi-sensing elements and will be used for measurements of pH, ions, gases and organics in solution.

Thus, this project work, collectively sponsored by the Environmental Agency and Faculty of Engineering and the Environment of the University of Southampton, describes the development of low-cost, robust, and miniaturised environmental and chemical sensor arrays that can be used for pH water quality monitoring. Different sensory materials of commercial and in-house origin have been employed to produce various potentiometric solid-state thick-film (TF) reference electrodes andTF working ion-selective electrodes. The novel thick-film devices described herein are an advantageous alternative to most widely used commercial liquid electrolyte Ag/AgCl reference electrodes and glass bulb pH electrode. These small (50.8 mm x 8.5 mm) devices are mimicking the structure of the conventional available electrodes that are bulky, fragile and expensive. The responses of these novel sensors - such as the hydration time, the potential drift trend and the potential stability to chloride ion concentration - have been experimentally evaluated in the laboratory setup, compared and discussed. The performance of the TF screen-printed silver-silver chloride (Ag/AgCl) reference electrodes were examined with variations of the potassium chloride (KCl) concentration in the final (top) polymer (ESL 242 SB) salt matrix layer of the electrode. Additionally, different types of binder (glass and polymer) were tested for the underlying Ag/AgCl layer. The choice of binders and the amount of the potassium chloride in the salt matrix layer were crucial in manufacturing the Ag/AgCl reference electrode. The results showed a trade-off effect between the potential drift and sensitivity of the electrodes to chloride ions activity related to the porosity of the salt matrix layer. The reference electrodes with different layer compositions converge to a roughly equivalent common potential. It was due to the chloridisation of Ag and dissolution of AgCl in the Ag/AgCl layer to reach equilibrium layer. The addition of another layer on top of the KCl-containing salt matrix layer provided a better stability in varying concentrations of KCl test solutions. The chloride susceptibilities of the electrodes decreased to +2 mV/decade[Cl^-]that made the electrodes stable enough to be used at any chloride ion concentration. The screen-printed Ag/AgCl reference electrodes were also used in combination with a screen-printed metal oxide (RuO₂) ion-selective electrode. The pH sensor exhibited a sensitivity of approximately 50 mV/pH at ambient temperature. The reported results help to explain better the behaviour of thick-film electrodes and contribute towards the optimisation of their design and fabrication for use in solid-state chemical sensors.

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Identifiers

ORCID for John Atkinson: orcid.org/0000-0003-3411-8034

ORCID for Martyn Hill: orcid.org/0000-0001-6448-9448

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