1. Field of the Invention
The present invention relates to a gas sensor of Severinghaus type using a hydrogen ion sensitive FET transducer of the structure of an oblong gate-insulated field effect transistor having its gate part at the front end and its electrode part at the other end.
2. Description of the Prior Art
The measurement of concentration of gas such as carbon dioxide or ammonia gas is, of course, important in industrial applications, but recently in the medical field, the measurement of partial pressure of gas in living bodies begins to be taken seriously. For example, in medicine, continuous measurements of gas partial pressure in blood of anesthetized patients, those with advanced diseases or those in the convalescent stage are lending themselves to discovery of emergent situations. For such purposes, a very small gas sensor with less than 2 mm diameter which may be inserted in any blood vessel or muscle tissue is required.
For the aforementioned purpose, heretofore in use has been a gas sensor of the Severinghaus type using a very small glass electrode. Minuaturization of the glass electrode, however, is known to involve following problems:
(a) Because the resistance of the glass film is approx. 10 M.OMEGA., an amplifier with a high input impedance is necessary. PA1 (b) The glass film, being thin, has low mechanical strength. PA1 (c) Because of the electrode area being small, the resistance of the glass film is large. PA1 The measuring instruments should be large and complex and the electrodes themselves are brittle and tend to break. Therefore, especially as a sensor to be inserted into tissues of living bodies for measuring gas partial pressure in living bodies, this instrument has posed a problem in practical applications. PA1 (a) Because of the use of the solid electrode, sensor is inflexible. PA1 (b) Its electric resistance is increased due to the miniaturization. PA1 (1) The photosensitivity of pH-ISFET exists not only on its gate part but on whole of the Si substrate including its electrode part. Accordingly, it is difficult to totally eliminate the photosensitivity by coating only the gate part with a black polymer, as above described. PA1 (2) In the Severinghaus gas sensor, the partial pressure of gas is measured by utilizing the minute change of pH of the gas absorbing liquid. Accordingly, it is necessary to avoid changes of pH of the gas absorbing liquid due to factors other than the gas partial pressure. However most of such black pigments, being chemically unstable, will become acidic due to autoxidation, etc., during a long period of storage, often inviting changes in characteristics as a gas sensor. Accordingly, using the pH-ISFET with its gate part coated with a hydrophilic polymer colored black for allaying the photosensitivity of the pH-ISFET is problematical in practical applications as a gas sensor.
On the other hand, a carbon dioxide gas sensor using a solid pH electrode of metal oxide in place of the glass electrode is disclosed in U.S. Pat. No. 3,719,576, etc. This sensor is smaller and slenderer than that using a glass electrode and is, therefore, suitable as a sensor to be inserted in living tissues, but has the following disadvantages:
On this ground, the miniaturization had its limit.
This problem has been solved by making use of a hydrogen ion sensitive FET transducer (hereinafter referred to as pH-ISFET) having the gate-insulated field effect transistor structure described in U.S. Pat. No. 4,218,298 in place of the glass electrode of the solid electrode. Such a gas sensor using the pH-ISFET is, as described in U.S. Pat. No. 4,409,980, a gas sensor composed of a pH-ISFET and a reference electrode deposited on the substrate in proximity to the gate part of the pH-ISFET, an insulator tube housing this pH-ISFET and the reference electrode with lead wires connected thereto, the gate part of the aforementioned pH-ISFET being located at an opening part provided in the insulator tube, and the lead wires extended along the tube, an electrical insulation resin closing the tube by filling the space of the part inside the tube wall housing the lead wire connecting parts, a hydrophilic polymer layer containing electrolytes which undergo change in hydrogen ion concentration, as it absorbs the gas, and which is placed around the gate part of the pH-ISFET and the reference electrode, enveloping both of them, and a gas permeable membrane coating at least whole of the aforementioned polymer layer.
The aforementioned gas sensor is preferable as a gas sensor to be inserted in living bodies. However, the pH-ISFET usually has a photo-sensitivity of the order of 10.sup.-5 -10.sup.-6 V/lux. Therefore, when a gas sensor using the pH-ISFET is used for monitoring, etc., during operation, when it receives illumination of several thousands luxes - several tens of thousands luxes, the photosensitivity of the pH-ISFET poses a serious obstacle to its practical use.
Coating the gate part of the pH-ISFET with a hydrophilic polymer colored black for allaying the photosensitivity of the pH-ISFET is described in U.S. Pat. No. 4, 273,636. However, the aforementioned proposal is to allay the photosensitivity of ion sensor using ISFET and does not relate to a gas sensor. It is in principle practicable to allay the photosensitivity of a gas sensor by utilizing a pH-ISFET with a hydrophilic polymer colored black coated on its gate part. In this instance, the gate part of the pH-ISFET and the reference electrode are enclosed by a hydrophilic polymer containing a gas absorbing liquid colored black and the outside of this polymer is coated with a gas permeable membrane. It has become evident, however, that diminishing the photosensitivity of a gas sensor by this method involves the following problems:
The present inventors have found out the improved gas sensor of this invention as a result of investigations carried out for providing a practically useful gas sensor in which the photosensitivity of the gas sensor of the Severinghaus type using pH-ISFET is allayed.