JPS6217703B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention measures the ionic activity of an electrolytic solution by bringing the gate part of an FET sensor having an insulated gate field effect transistor (FET) structure and a liquid junction type standard electrode into contact with an electrolytic solution. A FET sensor characterized in that the temperature dependence of the conductivity of the FET's electrical conduction channel is equal to the sum of the temperature dependence of the standard electrode and the temperature dependence of the Nernst equation as an operating point. It is about how to use.

Ion sensors using FETs were introduced in 1970.
Bergveld's presentation [IEEE Trans.Bio−Med.Eng.
Vol.BME−17, 1970 (1970)], Matsuo, 1978
Esashi's presentation [IEEE Trans.Bio−Med.Eng.Vol.
BME-25, 184 (1978)], and JP-A-51-139289
It has been proposed in the No. 1 and recognized as having the following characteristics.

(b) Manufactured using IC manufacturing technology, making it smaller;
Easy to combine.

(b) It is easy to increase production, and therefore it is economically inexpensive.

(c) By changing the gate sensitive membrane, it becomes possible to measure various ions.

(d) Even if the sensor is miniaturized, the output impedance is low because the sensor itself functions as an amplifying element, and stable measurement is possible with a simple circuit.
Furthermore, packaging of the sensor is easy because the insulation between the electrodes and between the electrodes and the measurement circuit is not required to be as high as in ordinary microelectrodes.

Because of these characteristics, ion sensors made of FETs are expected to be used as sensors for medical purposes such as biological monitoring and blood analysis, for experiments such as biochemistry and physiology, or as inexpensive disposable sensors.

However, in the part of such a measurement system that is in contact with the electrolyte to be measured, the electrical conductivity of the electrical conduction channel of the FET, the interfacial potential of the standard electrode (V _R ), and the interfacial potential of the ion-sensitive surface of the FET gate part (V _N ) These three elements are sensitive to temperature changes. Here, the electrical conductivity of the channel shows different temperature dependence depending on the operating current flowing between the drain and the source, but the temperature dependence of the interfacial potential of the standard electrode (∂V _R /∂TN) and the ion Both the temperature dependence of the interfacial potential of the sensitive surface (∂V _N /∂T) have a temperature dependence that is independent of the operating current. Here, the temperature dependence of the electrical conductivity of the channel varies depending on the device and manufacturing method, but the temperature dependence of the interfacial potential of the ion-sensitive surface corresponds to the temperature peak of the so-called Nernst equation, It is a constant value depending on the temperature. The temperature dependence of the interfacial potential of standard electrodes can be found, for example, in Chemistry Handbook, Basic Edition (edited by the Chemical Society of Japan, 1966, Maruzen Co., Ltd., 9.9).
As described in detail in ``Batteries'', it depends on the electrode material and the ion concentration of the internal solution, and even if the electrode material and the ion concentration of the internal solution are specified, the temperature dependence will depend on the manufacturing method. Some differences are observed. Due to such a combination of temperature-dependent elements, it is extremely difficult to compensate for the temperature of each individual FET sensor in order to measure the ionic activity of the electrolyte. In order to solve these problems, the present inventors formed a diode on the same FET sensor, and using the fact that the temperature dependence of the diode is almost constant, used it as a temperature measurement sensor and simultaneously measured the temperature of the measurement system. We attempted to perform temperature compensation on the electric circuit itself using the temperature dependence value of the FET sensor-standard electrode system that had been measured in advance. However, there are various complications caused by incorporating diodes at the same time, namely: (1) The pattern becomes complicated and the element becomes large. (2) Increased number of lead wires. (3) Since a circuit for the temperature detection section is required, there are problems such as the sensor detection circuit becomes extremely complicated.
Furthermore, the temperature dependence of the diode is also
Although it is easier to control than , errors still occur due to the distribution of performance. In order to reduce this error to a value that can be ignored under normal measurement conditions, not only does it require strict quality control compared to diodes generally manufactured for electronic circuits, but also the operating current value of the FET sensor. The measurement system temperature dependence must be controlled to be extremely small. For example, the temperature dependence of the measurement system at a certain operating current value is 1 mV/
℃, and the normal tolerance of the temperature dependence of a diode is 0.15 mV/℃, then between room temperature and body temperature, it will give an error of Ω value of about 0.03 at most.

In order to obtain sufficient measurement accuracy with the above method, there are problems in that quality control that is practically difficult to implement and disadvantages such as unnecessarily complicated measurement circuits must be incurred. The present inventors have solved this problem by simply and easily using a simple circuit, with easier quality control, and with a more accurate FET.
The present invention was arrived at as a result of extensive research into measurements using sensors. That is, when measuring the ion activity in an electrolytic solution using an FET sensor having an insulated gate field effect transistor structure, the present invention provides that the temperature dependence of the electrical conductivity of the channel of the field effect transistor is determined by the temperature of the electrode interface of the reference electrode. This is a method of using an FET sensor characterized by using a drain current equal to the sum of the dependence and the temperature dependence of the interface of the ion sensitive part of the FET sensor as an operating point.

Next, the method of the present invention will be explained with reference to the drawings. Figure 1 shows the temperature dependence (∂V _s /〓T) of the output (V _S −source voltage) when using a FET sensor with the drain grounded and the temperature dependence of the electrode interface of the reference electrode (∂V _R FIG. 3 is a schematic diagram showing the relationship between temperature dependence (∂V _N /∂T) of the ion-sensing part interface of the FET sensor (∂V N /∂T). In FIG. 1, numerals 1, 2, and 3 indicate the temperature dependencies ∂V _s /∂T, ∂V _R /∂T, and ∂V _N /∂T, respectively. Here, V _s is the potential of the source with respect to the gate voltage, V _R is the potential of the reference electrode with respect to the liquid level, and V _N is the potential of the ion-sensitive membrane with respect to the liquid level. Here, the drain current I
When ∂V _s /∂T, ∂V _R /∂T, and ∂V _N /∂T are a, b, and c, respectively, the point where a+b+c=0, that is, the point where the temperature dependence of the three elements cancel each other out exists, and the current value will be called the triple point current value I _T . Of course, it is impossible or difficult to find such an I _T point when curve 1 is largely shifted to the (-) side or to the (+) side, but this I _T point In practice, considerable variation can be tolerated, so controlling performance to an appropriate position is extremely easy with current semiconductor manufacturing technology. By the way, the operating current I can be any value unless it is too low or too high, but from the viewpoint of stable continuous operation for a long time and safety when used in living organisms, it should be 5 μA to 1 μA.
It is preferable that it is between A, but especially preferably 20
μA to 600 μA, and if it is less than 20 μA, noise is relatively likely to enter, and if it is more than 600 μA, it can be said to be disadvantageous in terms of danger to living organisms, short life when operating on batteries, heat generation, etc.

An example of a method of measuring an FET sensor using the I _T point according to the present invention will be described with reference to FIG. Figure 2
is a circuit using the drain grounding method, and the FET in Figure 2
is a FET sensor, Ref is a standard electrode, V _D is a drain voltage, and V _R is a variable resistance, and Ref and the gate of the FET are electrically connected by an electrolyte. First, the operating current I is set to the previously measured I _T point using the ammeter A and V _R , and then V _s is measured using the voltmeter V. Here, in order to set the T _T point in advance, it is possible to use a standard electrode having the same temperature dependence as the standard electrode actually used and use the circuit shown in FIG. 3. i.e. FET sensor FET and standard electrode Ref
The dependence of the current I on the gate voltage _VGS at different temperatures, for example 15°C and 45°C, is brought into contact with a phosphate buffer solution of Ω7.
When doing this, a curve as shown in Figure 4 is obtained.The point where the relationship curves intersect is defined as the I _T point. Here, E does not need to be the same as V _D in FIG. 2, and may be any voltage that can be measured in the saturation region. In any case, this kind of measurement operation is necessary for verifying the static characteristics of the FET sensor after manufacturing.
It can be done with almost the same amount of effort compared to normal testing operations.

The method of using the FET sensor with the I _T point according to the present invention is possible not only by the drain grounding method but also by the source grounding method. However, the common source method suffers from the fact that the drain current changes during measurement and that the signal
It is inferior to the drain grounding method due to the same quantity to be measured and lack of linearity.

As described above, the method of the present invention enables accurate measurement using a simple method, and is a very useful method in practice. Also, in the above explanation, we talked about measurements using FET ion sensors, but in addition to ion sensors, there are also FET gas sensors such as carbon dioxide and ammonia gas, FET enzyme sensors that use enzymes, etc.
It utilizes the properties of FET as an ion sensor, and it is easy to understand that the method of the present invention can be applied to these.

[Brief explanation of the drawing]

Figure 1 shows the temperature dependence of the output when using a FET sensor with a drain grounded and the temperature dependence of the interfacial potential between the standard electrode and the ion-sensitive surface, and Figure 2 shows the temperature dependence of the interfacial potential between the standard electrode and the ion-sensitive surface. FIG. 3 is an example of a circuit for setting the I _T point, and FIG. 4 is a graph showing the relationship between current and gate voltage.

Claims (1)

[Claims] 1 When measuring the ion activity in an electrolyte using a FET sensor having an insulated gate field effect transistor structure, the temperature dependence of the electrical conductivity of the channel of the field effect transistor is different from the temperature dependence of the electrode interface of the reference electrode and the FET sensor. It is characterized by using the drain current equal to the sum of the temperature dependence of the ion-sensitive interface of the sensor as the operating point.
How to use FET sensors.