Ion concentration measuring apparatus

An apparatus for measuring a concentration of a specific ion contained in a test liquid held in a container with the aid of a reference electrode and an ion sensitive field effect transistor having a gate portion selectively sensitive to the specific ion, including a series circuit of a reference resistor and a constant voltage supply source connected across drain and source of the ion sensitive field effect transistor, a potential control circuit having inputs connected across the reference resistor to detect a potential difference across the reference resistor for controlling a source or drain potential of the ion sensitive field effect transistor in such a manner that the potential difference remains at a predetermined value and a voltmeter for measuring the source or drain potential as a measure of the ion concentration.

BACKGROUND OF THE INVENTION 
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 the above mentioned known ion 
concentration measuring apparatus, in which a bias voltage applied to a 
reference electrode is so controlled that a given constant current flows 
between the source and drain of an ISFET sensitive to specific ion in a 
test liquid, and then a concentration of the specific ion and an ion 
activity are detected in accordance with said bias voltage value. In FIG. 
1, a container 1 contains a test liquid 2 and an ISFET 3 is immersed in 
the test liquid 2 together with a reference electrode 5 in such a manner 
that at least a gate portion 4 of ISFET 3 is made in contact with the test 
liquid 2. Between a drain electrode 6 of ISFET 3 and the earth is 
connected a constant voltage supply source 7, and a source electrode 8 and 
a semiconductor substrate 9 of ISFET 3 are connected via a reference 
resistor 10 to the earth. The reference electrode 5 is connected to a 
potential control circuit 11 including a bias voltage source, by means of 
which the bias voltage is so controlled that the potential of the source 
electrode 8 is set to a predetermined value and thus a predetermined 
current flows between the source and drain. Then the voltage value across 
outputs 12 and 13 is measured with the aid of a voltmeter such as a valve 
voltmeter. 
In principle, the drain current of the FET is changed in accordance with 
two factors, i.e. the voltage across the drain and source and the voltage 
across the gate and source. Further, if the voltage across the drain and 
source is always maintained above a certain level, the drain current is 
substantially proportional to the voltage across the gate and source. Such 
an operating region is called a saturation region. In the ion 
concentration measuring apparatus shown in FIG. 1, the ISFET 3 is operated 
in the saturation region and thus the drain current is determined by the 
voltage across the gate and source. 
In FIG. 1, the gate potential V.sub.G of ISFET 3 is equal to a sum of a 
potential E.sub.R of the test liquid 2 set by the reference electrode 5 
and an interface potential E.sub.G generated at the chemically sensitive 
film of the gate portion 4 in response to the ion activity of specific ion 
in the test solution 2 (V.sub.G =E.sub.R +E.sub.G). Further, if it is 
assumed that a resistance value of the resistor 10 is R, the voltage of 
the constant voltage supply source is E.sub.B and the drain current is 
I.sub.D, then the voltage V.sub.DS across the drain and source may be 
expressed by V.sub.DS =E.sub.B -I.sub.D R. Then, a deviation dI.sub.D of 
the drain current I.sub.D may be represented as follows. 
##EQU1## 
As explained above, when the ISFET 3 is driven in the saturation region, 
the change in the voltage V.sub.DS across the drain and source does not 
have any influence upon the drain current I.sub.D. Therefore, in the above 
equation, it becomes 
##EQU2## 
and 
##EQU3## 
is obtained. That is to say, when E.sub.R +E.sub.G is constant, the drain 
current I.sub.D is kept also constant. 
In the saturation region, when the ion activity of the test liquid 3 
varies, the interface potential E.sub.G at the chemically selective film 
of the gate portion 4 is changed into E.sub.G +.DELTA.E.sub.G, and thus 
the drain current I.sub.D is varied. Therefore, the voltate I.sub.D R 
across the reference resistor 10 is also changed. The potential control 
circuit 11 operates to adjust the potential E.sub.R of the test liquid 2 
via the reference electrode 5 in such a manner that the drain current 
I.sub.D remains at the given value. In this case, the variation 
.DELTA.E.sub.R of the potential of the test liquid 2 serves to change from 
dI.sub.D =0 to .DELTA.(E.sub.R +E.sub.G)=0, and therefore, .DELTA.E.sub.R 
=-.DELTA.E.sub.G. By measuring the variation .DELTA.E.sub.R of the 
potential of the test liquid 2, i.e. the variation of the potential at the 
reference electrode 5 across the outputs 12 and 13, it is possible to 
detect -.DELTA.E.sub.G, so that the variation in the ion activity can be 
derived therefrom. 
In the ion concentration measuring apparatus shown in FIG. 1, since the 
ISFET 3 is driven by the constant voltage, an electrical overload 
condition does not occur and thus the operation is stable. However, since 
the variation in the ion activity is detected by changing the potential 
E.sub.R of the test liquid 2, it is absolutely necessary to electrically 
isolate the container 1 in a positive manner and it is principally 
impossible to simultaneously effect measurements with the aid of a 
plurality of ISFETs. 
SUMMARY OF THE INVENTION 
The present invention has for its object to provide an ion concentration 
measuring apparatus in which an ion concentration can be measured while a 
potential of a test liquid is maintained constant. 
It is another object of the invention to provide an ion concentration 
measuring apparatus in which simultaneous measurements with the aid of a 
plurality of ISFETs can be performed, while a potential at a test liquid 
remains constant. 
According to the invention, an ion concentration measuring apparatus 
comprises 
a container for holding a test liquid containing at least one kind of ion 
whose concentration is to be measured; 
means contacted with the test liquid for maintaining the test liquid at a 
constant potential; 
at least one ion sensitive field effect transistor having a gate portion 
which is selectively sensitive to the ion, drain and source; 
at least one measuring circuit comprising a constant voltage supply source 
which is not connected to the earth and which applies a constant voltage 
across the drain and source of the ion sensitive field effect transistor, 
a reference resistor connected across the constant voltage supply source 
and the drain or source, a potential control circuit having inputs 
connected across the reference resistor and an output connected to the 
drain or source for controlling a potential at the drain or source in such 
a manner that a voltage across the reference resistor retains a 
predetermined value, and a voltmeter for measuring a variation of the 
potential at the drain or source as a measure of the concentration of ion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
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. 
According to the invention, since the ISFET is operated in the constant 
mode, while the potential of the test liquid remains constant, stable 
operation can be attained and the small variation of the interface 
potential in accordance with the ion activity can be detected in a very 
precise manner. Further, since the test liquid is maintained at a constant 
potential, it is possible to use simultaneously a plurality of ISFETs 
which respond to the same ion or different kinds of ions. In the former 
case, it is possible to obtain an average value of ion concentrations 
measured by respective ISFETs in a simple manner. In the latter case, it 
is possible to derive a plurality of ion concentrations of different kinds 
of ions simultaneously. In either case, it is possible to shorten the 
measuring time materially. Furthermore, the reference electrode may be 
connected to the earth. In this case, if the container is made of 
conductive material and is connected to the earth, the reference electrode 
may be dispensed with. In this manner, the whole construction may be made 
much simpler.