Abstract: The present invention relates to the development of a biosensor to determine potassium in human blood serum using dibenzo-18-crown-6 (DB18C6) as ionophore. Human blood serum contains potassium in ppm levels i.e 137 to 200 mg/litre and sodium co exists with a 30 times higher concentration. Such a high concentration tends to interfere the selectivity towards potassium, but DB18C6 proves to have an excellent selectivity towards potassium and is highly sensitive to the lowest concentration of potassium levels present in the human blood serum. So the present invention reports the fabrication and characterization of ISFET (Ion Selective Field Effect transistor) coated with a monolayer of crown ether, dissolved in chloroform, on the gate of electrode.