Development of potassium-selective ion-sensitive field-effect transistor (ISFET) by depositing ionophoric crown ether membrane on the gate dielectric, and its application to the determination of K⁺-ion concentrations in blood serum

112-118This paper reports the fabrication and characterization of potassium ISFET (ion-sensitive field-effect transistor) by deposition of a crown ether layer on the gate. This ISFET is referred to as potassium MEMFET (membrane field-effect transistor) or potassium-selective MEMFET device. Calibration of ISFET in standard KCl solutions has shown that the sensitivity of crown ether-ISFET towards potassium (2.3 mV/mg/L) was approximately double that of the nitride-gate ISFET (1 mV/mg/L) in the 100-400 mg/L range. The mechanism of ISFET potassium sensitivity enhancement by the ionophoric crown ether layer on the gate has been explained. Use of the device for potassium concentration measurements in human blood serum has been demonstrated. The distribution of potassium ion concentrations (151-210 mg/L) in a series of human blood serum samples has been determined from the reference value obtained by atomic absorption spectroscopy and the ISFET sensitivity measured with KCl solutions. These diluted samples showed a constant pH ~ 7.0-7.1, confirming the maintenance of pH within close tolerance by the human body; therefore any errors due to pH variation amongst the samples were eliminated. Sensitivity of the ISFET with/without crown ether coating on the gate towards sodium ions (0.34 mV/mg/L) has been found to be comparatively less than that of the crown ether ISFET for potassium, showing that sodium ions in blood serum do not interfere with the measurements