Ion concentration measuring apparatus

#The present invention relates to an ion concentration measuring apparatus comprising a semiconductor ion sensor.

#A semiconductor ion sensor utilizing a field effect in a semiconductor body has been developed and is called Ion Sensitive Field Effect Transistor (ISFET). Such an ISFET has been disclosed in Japanese Patent Application Laid-open Publications Nos. 139,289/76 and 26,292/77. In ISFET, on a gate of an insulated gate field effect transistor manufactured by a well developed IC technique, is formed a chemical selective film containing ion-exchange substances or enzymes, and a potential at an interface between the chemical selective film and an electrolyte is detected to measure specific ion concentration and substances acting upon the enzymes in the electrolyte.

#The ion concentration measuring apparatus comprising the above mentioned ISFET has been disclosed in, for instance, Japanese Patent Application Laid-open Publication No. 136,396/79.

#FIG. 1 is a schematic view showing a known ion concentration measuring apparatus;

#FIG. 2 is a schematic view illustrating a construction of an embodiment of the ion concentration measuring apparatus according to the invention; and

#FIG. 3 is a schematic view depicting another embodiment of the ion concentration measuring apparatus according to the invention.

#FIG. 2 is a schematic view showing an embodiment of the ion concentration measuring apparatus according to the invention. In the present embodiment, a container 21 contains a test liquid 22 including a specific ion whose concentration is to be measured. An ISFET 23 sensitive to the specific ion is immersed into the test liquid 22 together with a reference electrode 25 in such a manner that at least a gate portion 24 thereof is made in contact with the test liquid 22. The reference electrode 25 is connected to a constant voltage supply source 26 for maintaining the test liquid 22 at a predetermined potential. Across a drain (D) and a source (S) of ISFET 23 is connected a series circuit of a reference resistor 27 and a constant voltage supply source 28 so as to apply such a constant voltage that ISFET 23 operates in a saturation region. A potential difference across the reference resistor 27 is detected by a potential control circuit 29. In the present embodiment, the potential control circuit 29 controls a potential at the source S of ISFET 23 in such a manner that the potential difference, i.e. the drain current is maintained at a predetermined value. In this manner, a variation in an interface potential at the gate portion 24 of ISFET 23 due to the ion activity can be detected as a variation in the source potential. Therefore, by detecting the variation in the source potential with the aid of a voltmeter 30, it is possible to measure the ion concentration.

#FIG. 3 is a schematic view illustrating another embodiment of the ion concentration measuring apparatus according to the invention. In the present embodiment, concentrations of three different kinds of ions in a test liquid 32 contained in a container 31 are to be simultaneously measured. For this purpose, in the test liquid 32 are immersed three ISFETs 33a, 33b and 33c having gate portions 34a, 34b and 34c, respectively which are selectively sensitive to respective one of the three kinds of ions. These ISFETs may be formed on the same semiconductor substrate. Further a reference electrode 35 is also immersed in the test liquid 32. The reference electrode 35 is connected to a constant voltage supply source 36 so as to maintain the test liquid 32 at a given potential. Across drains and sources of respective ISFETs 33a, 33b and 33c are connected series circuits of reference resistors 37a, 37b and 37c and constant voltage supply sources 38a, 38b and 38c, respectively, so that the ISFETs 33a, 33b and 33c are biased to operate in the saturation condition. Across respective reference resistors 37a, 37b and 37c are connected potential control circuits 39a, 39b and 39c, respectively, which control source potentials of respective ISFETs 33a, 33b and 33c in such a manner that the potential differences across the reference resistors 37a, 37b and 37c are maintained at predetermined values. Then, the concentrations of specific ions can be measured simultaneously by detecting the variations of the source potentials with the aid of voltmeters 40a, 40b and 40c, respectively.

#The present invention is not limited to the embodiments explained above, but various modifications and alternations may be conceived by those skilled in the art within the scope of the invention. For instance, the reference resistor may be connected to the source instead of the drain side. Further, the control for maintaining the potential difference across the reference resistor, i.e. the current passing between the drain and source to the predetermined value may be effected by changing the drain potential instead of the source potential. In such a case, it is matter of course to detect the change of the drain potential to measure the ion concentration. Moreover, in case of using a plurality of ISFETs, they may be formed on separate substrates or they may respond to the same ion. In the latter case, the ion concentration may be derived as an average of a plurality of measured values.