Miniature reference electrode

Details Miniature reference electrode Miniature reference electrode

2000-03-24

2002-07-16

Tung, T. (Department: 1743)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S402000, C204S420000

Reexamination Certificate

active

06419809

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The disclosed embodiments of the invention pertain to a miniature reference electrode for use in the chemical sensor or biosensor based on the electrochemical principle in many electrochemical experiments, and in particular, to a miniature silver/silver chloride reference electrode with high stability and general applicability as a reference electrode.
BACKGROUND OF THE INVENTION
A reference electrode is an electrode used as a reference in a potential setting or potential measurement. It is an important structural element indispensable not only in the basic studies of electrochemistry, but also in the application research of chemical sensors. For example, in cyclic voltammogram and other experiments with the electrode potential included as an information, there is a demand on a reference electrode that can provide a stable reference potential. Also, for the chemical sensors and biosensors, in particular, for the potentiometric sensors for potential measurement, the potential of the indicator electrode that measures the chemical substance is measured using the reference electrode as a standard, and the concentration of the chemical substance is measured with the potential as an index. Consequently, if a highly stable and reliable reference electrode is not used, the deviation in the potential leads directly to a measurement error.
Consequently, the reference electrode is a basic and important element for the electrochemical sensor. At present, the silver/silver chloride electrode is most widely used as the reference electrode. The structure of the silver/silver chloride electrode is shown in FIG.
1
. In container (
4
) made of glass, rod-shaped “silver/silver chloride” (
1
) is set. In the end portion of glass container (
4
), a liquid junction (
3
) made of a ceramic plug or the like is formed. The interior of the container (
4
) is filled with saturated KCl solution or saturated NaCl solution.
In recent years, there has been progress achieved in miniaturizing the chemical sensors and biosensors. Consequently, there has been a demand for development of a miniature reference electrode that can provide a stable potential. The miniature silver/silver chloride electrode conventionally used to meet this demand is shown in FIG.
2
. On substrate (
10
), base metal layer (
9
), silver layer (
6
) and silver chloride layer (
7
) are sequentially laminated, and “silver/silver chloride” is only in a simple thin film form.
For the thin-film silver/silver chloride electrode (
FIG. 2
) as the miniature reference electrode, the “silver/silver chloride” portion (the portion of silver layer (
6
) and silver chloride layer (
7
)) is prepared as follows. For example, silver layer (
6
) is formed on a layer of chromium or titanium as the adhesion layer (
9
). Its surface is partially converted to silver chloride. In certain cases, in order to further improve the adhesion property of silver, an intermediate layer made of gold or platinum or the like is included between adhesion layers (
9
) and silver [layer] (
6
) in consideration of the structure or process of the sensor. (Also, this structural body is usually formed by patterning with the aid of photolithography).
The thin-film silver/silver chloride electrode (
FIG. 2
) is usually used by placing it in direct contact with the sample solution. However, for the silver/silver chloride reference electrode, the electrode potential varies as a function of the active amount of chlorine ions. Consequently, in order to perform an appropriate measurement, it is necessary to use it in a solution having a predetermined active amount of chlorine ions. Consequently, when it is used in combination with a certain micro indicator electrode, a solution with a predetermined active amount of chlorine ions is prepared separately and brought into contact with the “silver/silver chloride” portion, and an appropriate amount of sample solution is added to perform the measurement.
However, in this prior method, each time it is used, one has to prepare a solution with a predetermined active amount of chlorine ions. Also, when the sample is injected, the active amount of chlorine ions varies a little, leading to unwanted variations in the potential. Also, in the case when a sample is measured without dilution as the level best for the practical application, it is very difficult to measure the active amount of chlorine ions beforehand, and the measurement value itself is unreliable.
Consequently, when measurement with a high reliability is to be carried out, as shown in
FIG. 1
, one must make the “silver/silver chloride” portion in contact with a saturated solution of KCl and make the connection to the outside through a liquid junction.
However, for the structural body of the thin-film silver/silver chloride reference electrode shown in
FIG. 2
, as it is a thin film, the silver chloride layer (
7
) is very thin. Consequently, the durability is low, and it is damaged in a time as short as a few minutes in a saturated solution of KCl. Consequently, there is yet no commercially available product using this structure directly (that is, the structure in which solution of KCl or NaCl in a saturated or nearly saturated concentration is filled as the internal electrolyte, and connection to the outside is performed by means of a liquid junction).
SUMMARY OF THE INVENTION
The disclosed embodiments of the present invention provide a miniature reference electrode with improved stability and durability. In particular, the embodiments of the invention provides a miniature silver/silver chloride reference electrode with high stability and general applicability as a reference electrode.
A “silver/silver chloride” structural body is shown in FIG.
3
. It differs from a conventional product in that the surface of silver thin film (
6
) is covered with a water-repellent polymer layer (
8
) (for ease of reference, the same part numbers as in
FIG. 2
are adopted here for similar elements), and silver chloride layer (
7
) is formed from the end portion of silver thin film (
6
) instead of from the surface of the thin film. It was found that the lifetime of the thin-film “silver/silver chloride” structural body in the saturated aqueous solution of KCl or AgCl was prolonged by nearly 300-fold. Thus, the thin-film “silver/silver chloride” structural body is used as the reference electrode having the liquid junction as explained above.
The thin-film “silver/silver chloride” structural body can be manufactured as a reference electrode having a liquid junction (
15
) (FIG.
4
). In this case, the container portion (
16
) for storing the electrolyte (saturated aqueous solution of KCl or AgCl) and liquid junction (
15
) arc formed on a silicon substrate (
14
), and it is bonded to a glass substrate (
17
). The thin-film “silver/silver chloride” structure (
18
) is thus formed.
For the reference electrode with a liquid junction, when it is in use, an electrolyte is injected from outside (
11
) of the silicon substrate through an electrolyte feeding port (
13
), the end portion of liquid junction (
12
) is recessed into a predetermined solution, and the portion of pad (
19
) is connected to a terminal for measurement. The reference electrode having the liquid junction shown in
FIG. 4
can display the desired stable potential.
However, for the miniature reference electrode of the aforementioned prior art, there were several problems. The problems of the miniature reference electrode of the aforementioned prior art and the methods for solving them, more specifically, the problems that take place in company with setting of the aforementioned liquid junction and the methods for solving them, are as follows.
The first problem is as follows: When the electrolyte (KCl) is introduced as an aqueous solution, due to flow-out of KCl inside it and flow-in of water outside it, the concentration of KCl of the internal solution significantly decreases to a measurement error, and the lifetime becomes as short as a few hours. On the other hand, when more KCl is stored in

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