Electrochemical sensor

Details Electrochemical sensor Electrochemical sensor

2000-03-03

2003-09-02

Nguyen, Nam (Department: 1744)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S403150, C204S433000

Reexamination Certificate

active

06613205

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an electrochemical sensor having in at least one region of an electrically insulating, planar substrate an electrochemically active sensor layer (sensor spot), which is applied by means of a thick film technique and the surface of which can be brought into contact in a measuring area with the aqueous sample to be determined, at least one conductive path being provided for signal pick-up which is also applied on the substrate by means of a thick film technique, and the sensor layer containing at least one oxide of a metal from subgroups
7
and
8
of the Periodic Table as sensor component.
DESCRIPTION OF THE PRIOR ART
In the following the term thick film techniques will include coating techniques such as screen printing, dispenser coating, tampon printing and similar such methods, where layer thicknesses of greater than 1 &mgr;m are obtained, whereas techniques such as vacuum depositing, sputtering, or photo-lithography, which are usually employed to obtain thicknesses of less than 500 nm, are considered as thin film techniques and will thus be excluded.
Due to their electrochemically active sensor layers the electrochemical sensors mentioned above, which may be operated amperometrically or potentiometrically, are primarily suitable for determining the pH value or H
2
O
2
concentration, for example. By variations in the sensor configuration other quantities, which will influence or alter the pH or H
2
O
2
concentration, may be determined in addition to these primary quantities. For example, a pH sensitive layer of a platinum metal oxide may be covered with a hydrophobic layer that is CO
2
-permeable, and the CO
2
concentration may be determined from the resulting change in pH.
Incorporating MnO
2
into a suitable electrode material for anodic reaction of hydrogen peroxide also is standard practice. EP 0 603 154, for example, describes an amperometric enzyme electrode containing an enzyme immobilized or adsorbed in the porous electrode material. Depending on the concentration of the corresponding enzyme substrate in a sample there will be a variation in the concentration of hydrogen peroxide, which may be determined via the anodic reaction.
As regards the term “electrochemical sensor” to be used in the following it should be noted that this will essentially refer only to the electrochemically active base layer of the sensor and the elements picking up the electric signal. Any reference to further additives, or additional layers for transforming one chemical variable into another, is given only as an example without intending an exhaustive description. In the amperometric measuring process disclosed in EP 0 603 154, for instance, the enzyme need not necessarily be immobilized on the porous base electrode, as the enzymatic reaction could also take place outside of the sensor.
The pH sensitivity of the platinum metal oxides, which has been known in the art for some time, will permit the development of potentiometric pH sensors (or rather, sensors which are essentially based on pH measurement). Miniaturized planar sensors on the basis of metal oxide, which are intended for pH determination, are manufactured using thick film techniques, i.e. both conventional methods and, increasingly, polymer thick film processes.
DE 195 06 863 A1, for instance, discloses a pH sensor made by means of a thick film technique, and the corresponding manufacturing process. The sensor employs ruthenium oxide as an electrochemically active component, the layer containing the electrochemically active component being applied on a substrate using a polymer thick film technique. The individual layers are applied as following: First of all, a conductive path of silver is applied on the substrate, which is followed by the electrochemically active layer consisting of a mixture of ruthenium dioxide and a paste that is commonly used in polymer thick film technology for preparing insulating layers. In a final step, a non-conductive insulating layer is applied. The layers are tempered and cured in an oven.
In DE 44 30 662 A1 an iridium oxide electrode for pH measurement and a method of fabricating such an electrode are described. On a substrate of organic or inorganic material a pH sensitive layer is applied, which contains oxidized iridium powder. For preparation of the electrode iridium oxide powder is mixed with organic and/or inorganic binders; the resulting paste is applied to the substrate using a screen printing technique. The iridium pH electrodes obtained in this way are designed for pH measurement and as basic sensors for biosensors and gas sensors. The substrate consists of a ceramic material in this case, on which a conductive path is applied in a first printing step. In a second printing step the conductive path is covered with a polymer insulating layer with the exception of a contact area, which remains uncovered. In a third printing step the thick film paste containing iridium oxide is applied, after the addition of an intermediate layer if required.
With the metal oxide sensors known in the art the sequence of the printing steps is strictly prescribed, at least as far as the electrically conductive path and the metal oxide layer are concerned. This may be be undesirable, for example in the instance of cracks occurring at the interface of different materials, which may impair the conductivity and thus functionality of the sensor.
Another problem is due to the fact that electrochemically active sensor layers containing an oxide of the metals Mn, Ru, Ir, or Pt, exhibit a surface structure or contain a chemical substance which will permit the sample solution to penetrate the sensor layer and reach the conductor. This may lead to phenomena such as drifting (for example, due to swelling processes) and/or corrosion (forming of galvanic elements), as exemplified by the system based on ruthenium oxide and silver. Further negative effects are to be expected with working electrodes containing MnO
2
as electrochemically active component. Due to material swelling in the aqueous environment galvanic elements will be formed, which will impair the electrochemical characteristics of the sensor (distortion of the H
2
O
2
signal, susceptibility to interferences, pH effects)
In “Sensors and Actuators” B 48, 1998, 505-511 the weakening of the pH signal during storage of a planar thick film pH electrode based on ruthenium dioxide is described in this context. Upon close examination of the storage conditions it becomes obvious that the sensor is being destroyed by corrosion. In the acid pH range (i.e. pH 2, approx.) ruthenium oxide has a standard potential of 0.86 V for the transition from Ru(IV) to Ru(III), and is thus capable of oxidizing silver, which will explain the corrosive destruction observed. As a possible solution to this problem use of a printed, spot-shaped intermediate layer is proposed between the electrochemically active sensor layer and the conductor. Apart from the disadvantage of an additional printing step, which will increase manufacturing costs, it has been found that such intermediate layers are susceptible to the diffusion of Ag ions after having been in use for some time (about 1 day), which will also impair the electrochemically active sensor layer.
Further interferences due to the diffusion of Ag ions have been found in sensors containing MnO
2
as sensor component for H
2
O
2
detection. Such interferences include sudden short-term current flows produced by a discharge of the polarized, amperometric working electrodes acting as capacitors. In addition, longer polarization times are encountered on initial operation of the sensor, and a change in electrode behavior vis-a-vis electrochemical interferents such as uric acid or solutions with differing pH values is experienced throughout the service life of the electrode.
U.S. Pat. No. 5,507,936 shows a sensor configuration, where the electrochemically active sensor layer of iridium oxide is applied on a metal layer consisting of iridium, this metal layer providing the electrical conn

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