Silicon-Based Ion-Sensitive Field-Effect Transistor Shows Negligible Dependence on Salt Concentration at Constant pH

wherethe interface between the tran-sistor channel and the solutionwas made from silica, we demonstrate here that FETs coveredwith a thin alumina layer are almost insensitive to ions at dif-ferent concentrations except for hydrogen ions. Such FETs aretherefore ideal pH sensors, only responding to protons andnot to other ions. We also demonstrate that this result canonly be obtained if the liquid gate potential is carefully appliedthrough a counter electrode and potentiometrically sensed bya reference electrode.Sensing silicon FETs were produced according to the previ-ously reported protocol