Trial manufacture of a miniaturized calcium-ion electrode, and its practical application to determine critical pH and to develop a new evaluation method of food cariogenicity

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By covering the PVC membrane including calcium-ionophore to the ion-sensitive field-effect transistor (ISFET) electrode directly, we made the calcium ISFET electrode. Though we tried to change the thickness of PVC membrane and the kind of ionophore, we were unable to make the calcium-ion electrode that is not influenced by pH.Therefore, the silicon nitride membrane (the hydrogen-ion reaction membrane) was removed from the surface of the ISFET electrode, and Parylene C was deposited instead of the silicon nitride, and then the PVC membrane was covered on it. When the calcium ion of various concentrations were measured by using the improved electrode the potential response of the straight line was observed the range of 2 mmol/L from 1μmol/L of calcium ion. Next, we examined the influence of pH to calcium ion measurement. As calcium-ion concentration becomes high, pH hardly affected the potential response to calcium ion. And it became clear to hardly receive the influence of pH in 1μmol/L as calcium-ion concentration. It is considered that calcium ion is able to measured without receiving the influence of pH in the dental plaque where calcium ion concentration could be higher than 1μmol/L.When calcium ion was measured with the electrode of silicon nitride membrane covering, the hindrance of the sodium ion was hardly observed (less than 40 mmol/L). However, it was hindered by the sodium ion of low concentration (less than 10 mmol/L), when we used the electrode of Parylene C covering. The manufacture of the practical electrode that excels in the potential response to calcium ion without receiving the influence of sodium ion, is necessary, to measure the calcium ion in dental plaque precisely. It is because sodium-ion concentration is about 8 mmol/L in unstimulated saliva and about 30 mmol/L in stimulated saliva.