M₂O‐Al₂O₃‐SiO₂ glass gate PNA and pK ISFETs

To improve the capability of the ISFETs for measuring pNa and pK in living bodies, the ion selectivity of its sensor M₂O‐Al₂O₃‐SiO₂ (MAS, where M is an alkali metal) glass membrane was studied. The MAS membrane has been used as an ion‐sensitive membrane of pNa and pK glass electrode and it has been known that its selectivity depends on the composition of the MAS membrane. To obtain the maximum ion selectivity of the pNa and pK ISFET, the relationship between the ion selectivity and the composition of the MAS membrane was investigated by measuring the ion selectivity over various compositions. It was confirmed that the selectivity of the MAS gate ISFET between Na⁺ ions and K⁺ ions depends on both the kind of alkaline cation and the ratio of the alkali metal to aluminum in the MAS membrane. As a result, the best composition of ion‐sensitive membranes for pNa and pK ISFET was determined. A pNa ISFET was obtained which uses the LAS membrane containing Li as the alkali metal and has a selectivity of about 200 times for Na⁺ ions against K⁺ ions with a low output‐voltage drift and a long life. A pK ISFET also was obtained which uses a KAS membrane containing K and has a selectivity of about 40 times for K⁺ ions against Na⁺ ions.