Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1998-11-23
2001-01-23
Tung, T. (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S418000, C204S415000
Reexamination Certificate
active
06176988
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a membrane electrode for measuring the glucose concentration in liquids, and an electronic circuit for operation of the membrane electrode.
BACKGROUND OF THE INVENTION
EP-A 0 141 178 discloses an arrangement for measuring the level of concentration of a substance, with which it is possible to determine levels of concentration of H
2
O
2
. The arrangement has a measuring electrode comprising noble metal which is separated from an electrolyte by a lipophilic membrane. In that case the membrane contains lipophilic ions, in particular anions, and/or carrier-bonded ions and is proton-impermeable. In a particular embodiment, contained in the electrolyte space or chamber which is separated from the electrode by the lipophilic membrane is an enzyme which converts a diffusible substance inter alia into H
2
O
2
, the concentration of which is measured by the arrangement and thus makes it possible to determine the concentration of the substance.
That arrangement for measuring the concentration of a substance suffers from the disadvantage that the measured parameter varies relatively substantially during the measurement procedure, that is to say it is subject to a certain amount of drift.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a membrane electrode for measuring the glucose concentration in a liquid, which avoids the problem encountered in the above-discussed arrangement and which can be economically produced.
A further object of the invention is to provide a membrane electrode for measuring glucose concentration in a liquid, which is of a simple structure while affording reliability in operation and operating results of enhanced accuracy.
A further object of the invention is to provide an electronic circuit for the operation of a membrane electrode for measuring the glucose concentration in a liquid, which contributes to allowing the electrode to operate in optimised fashion.
The foregoing and other objects are attained by a membrane electrode for measuring the glucose concentration in liquids, comprising: a base or main membrane with at least one noble metal electrode which is arranged on a first side of the base or main membrane; a proton-selective ion membrane which is arranged on the base or main membrane and the noble metal electrode; and a double membrane which is arranged on the ion membrane and in which glucose oxidase is contained in a suitable medium.
The foregoing and other objects of the present invention are further attained by an electronic circuit for operation of the membrane electrode comprising: a stabilised polarisation or biasing voltage source; first and second high-impedance amplifiers; a parallel resistor; an element for processing and storage of the measured parameter; and an output means.
It will be noted at this point that conventional electrodes with large noble metal surfaces exhibit a high level of sensitivity in relation to convection. That means that changes in the capillary flow which cause changes in convection within the diffusion zone of the electrode induce large changes in the electrode signal. In consideration of that fact, microelectrodes with an electrode diameter of below 50 &mgr;m with a low level of convection sensitivity are employed. Microelectrodes of that kind however suffer from the disadvantage that they have a relatively high degree of drift. It is generally in the range of between 2 and 3% per hour. The use of microelectrodes for measurement operations with an adequate degree of accuracy is therefore only possible if that drift is suitably corrected. That requires frequent calibration operations with at least two solutions or standard gases.
In contrast the electrodes according to the invention provide that the noble metal surface is covered with a protective lipophilic membrane which permits only hydrophobic and gaseous species to reach the electrode. In accordance with the invention moreover a double membrane is arranged on the ion membrane, with glucose oxidased in a suitable medium being contained in the double membrane.
The principle of potentiometric-polarographic H
2
O
2
-measurements is a combination of two different electrochemical processes, amperometry and potentiometry. The combination of those two measuring processes in one electrode is based on the observation that electrodes in which the ion membrane was brought into contact for example with platinum react on hydrogen. Investigation of that unexpected phenomenon showed that hydrogen at the platinum surface is oxidised in accordance with the following electrochemical reaction:
H
2
−2e
−
→2H
+
The protons formed are transported by means of ion carrier molecules through the ion membrane. The flux of protons through the ion membrane, which is produced by the ion carrier molecules, produces a membrane potential which can be potentiometrically determined. Precise analysis of the results showed that a lipophilic PVC-membrane with a proton-carrier contained therein acquires multifunctional properties if it is brought into contact with a noble metal surface instead of an internal electrolyte solution, for example 3- or 4-molar KCl.
It is to be noted that a crucial precondition for activation of a potentiometric-polarographic noble metal electrode of that kind is adequate hydration thereof. That is achieved by the diffusion of water vapor through the lipophilic ion membrane. The water molecules reaching the metal surface there form a dipolar layer. Subsequently a Helmholtz polarisation layer is developed with OH-anions as a charged layer if the electrode is used as an anode. Because of the absence of ions other than OH
−
and H
+
that Helmholtz layer exclusively contains water and dissociation products of water, that is to say OH
−
-ions and H
+
-ions.
The fact that an intermediate layer of between about 250 and 300 nm is formed between the noble metal surface and the ion membrane by the hydration step results in useful electrochemical interactions which do not exist in conventional electrode systems.
If H
2
O
2
diffuses through the ion membrane, it is oxidised at the polarised noble metal surface and the protons originating from that redox reaction accumulate by virtue of the diffusion resistance of the ion membrane in the intermediate space produced by hydration between the electrode surface and the ion membrane. On the other hand there is a proton flux through the membrane, that flux depending on the thickness of the membrane, the concentration and mobility of the H
+
-carriers in the lipophilic membrane, the activity of the protons in the intermediate space between the noble metal electrode and the ion membrane and the electrical field strength between the noble metal surface and the outer enzyme space. Those parameters can be suitably adjusted in accordance with the requirements involved by virtue of the configuration of the electrode according to the invention in terms of the thickness of the membrane, the applied electrical field and the other specified parameters, so that the flux of protons through the ion membrane results in the formation of two proton gradients and corresponding potential gradients. The first gradient is developed in the water-filled intermediate space between the noble metal electrode and the ion membrane while the second occurs within the membrane.
Polarographic electrodes can be used as reducing or oxidising electrochemical systems. They comprise a polarisable metal electrode, a non-polarisable reference electrode (for example Ag/AgCl) and a polarisation voltage source. The specific signal is the current which is produced by the redox reaction of the chemical species to be analysed. In principle all molecules which reach the metal surface by diffusion are completely oxidised or reduced.
Accordingly the concentration of the species to be analysed falls from its original or initial concentration value in the sample to zero at the surface of the polarised electrode. The number of molecules which diffuse to the surface
Hoffman & Baron LLP
Noguerola Alex
Tung T.
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