Water-enhanced guarding of polymer-coated IDE platforms as a key mechanism for achieving response immunity towards parasitic coupling events

Capacitive detection of organic pollutants present in water at low ppm and ppb levels by means of polymer-coated interdigitated electrode (IDE) structures is a challenging research and engineering task. The detection limit of a measuring system is dependent on its noise level and systematic physical nonidealties. To achieve a high detection sensitivity all parasitic impedance effects must be reduced as much as possible in order not to interfere with the information carrying signal. In comparison to ‘closed’, three-dimensional structures, the electrical sensing field of the ‘open’, planar IDE structure is geometrically difficult to guard. Direct exposure to aqueous solutions may cause unpredictable parasitic coupling to the surrounding, which results in changes in the sensitivity of response of the device to changes in environmental conditions such as salt concentration and pH. Thicker polymer layers increase the distance between the electrode plane and the water phase, thereby reducing the parasitic coupling effects but at significant cost of the sensing response time. In this work we demonstrate how water-enhanced electrical coupling to guarding electrodes is a key mechanism in achieving response immunity to parasitic electric-field bending without impairing the detection performance of the polymer-coated IDE platform. Our work contributes to the knowledge of capacitive response interpretation and mechanisms and points out important design considerations for improved sensing systems, in particular for applications in aqueous environments such as droplet and flow detectors for biomedical and environmental fields.