Ca²⁺ detection utilising AlGaN/GaN transistors with ion-selective polymer membranes

We demonstrate highly selective and sensitive potentiometric ion sensors for calcium ion detection, operated without the use of a reference electrode. The sensors consist of AlGaN/GaN heterostructure-based transistor devices with chemical functionalisation of the gate area using poly (vinylchloride)-based (PVC) membranes having high selectivity towards calcium ions, Ca²⁺. The sensors exhibited stable and rapid responses when introduced to various concentrations of Ca²⁺. In both 0.01 M KCl and 0.01 M NaCl ionic strength buffer solutions, the sensors exhibited near Nernstian responses with detection limits of less than 10^-7 M, and a linear response range between 10^-7-10^-2 M. Also, detection limits of less than 10^-6 M were achieved for the sensors in both 0.01 M MgCl₂ and 0.01 M LiCl buffer solutions. AlGaN/GaN-based devices for Ca²⁺ detection demonstrate excellent selectivity and response range for a wide variety of applications. This work represents an important step towards multi-ion sensing using arrays of ion-selective field effect transistor (ISFET) devices.