Nitrate ion detection using AlGaN/GaN heterostructure-based devices without a reference electrode

AlGaN/GaN heterostructure-based devices can be engineered through heterostructure design to have a high transconductance near zero gate–drain voltage, potentially enabling high sensitivity, reference electrode free, ion sensing. As a proof of concept, these devices were coated with a PVC-based membrane containing a plasticizer and an ionophore to detect nitrate ions in solution. The sensor response is measured as a change in conductivity across two contacts using a Kelvin probe (or four-contact) geometry, with the current between the two outer contacts kept constant. We show that this sensor for nitrate is sensitive and stable with a rapid response time (i.e. less than 60 s). The detection limit remains consistently low over multiple runs/days. In a 0.1 M KH2PO4 ion buffer, a detection limit of less than 1 × 10−6 M and a linear response range of 10−6–10−3 M were achieved. Furthermore, detection limits of approximately 10−6 M and 10−4 M in 0.1 M K2SO4 and 0.1 M KCl ion buffers, respectively, were demonstrated. In a 0.1 M KH2PO4 ion buffer, there was minimal change in sensor response upon addition of KOH increasing the pH from approximately 4–11. As a control, devices without a PVC membrane coating were tested under identical conditions and exhibited negligible response to nitrate ion exposure. Furthermore, using transistor theory, we show that the apparent gate response is near-Nenrstian under a variety of conditions. The success of this study paves the way for extending this technology to selectively sensing multiple ions in water through incorporation of the appropriate polymer based membranes on arrays of devices.