On the encapsulation materials for ion-selective electrodes

ABSTRACT Ion-selective electrodes (ISEs) built with chalcogenide glass membranes are sensors capable of detecting heavy metal ions in electrolytic solutions. Using ISEs in-situ and repeatedly can be challenging not only to the sensitive membrane, but to the material used to encapsulate the electrical contact between the membrane and the metallic wire. If the encapsulation fails, the sensor must be discarded. Despite how critical these issues are on this sensor component, no studies on the requirements and selection of the encapsulation material have been reported so far. This material must isolate the electrical contact of the surrounding solution, have a low level of absorption of the solution and the ions in the solution, as well as keep its properties stable in the long-term. The material requirements will also depend on the geometry of the membrane, either bulk or thin film. To stimulate further research and development in the field of ISEs, we investigated five encapsulating materials commercially available in the Latin American region (transparent Poxipol, Vaicel, Wilpox APV 3C, Wilpox Doming P-5 and Araldite CY-248 + HY-956). We analyzed the encapsulation requirements to enable stable and reliable ISEs by evaluating the following properties: viscosity, curing time, hardness, surface quality, formation of bubbles, effect of mechanical polishing, reaction to Ag paint, effect of water, and cleaning and storage solutions. We found that, under the proper conditions, Wilpox APV 3C and Araldite CY-248 + HY-956 epoxy resins are the most suitable materials to encapsulate bulk chalcogenide glass membranes, while Wilpox APV 3C epoxy resin and Vaicel acrylic resin turn out to be the best candidates for thin film membranes.