Membrane for an Electrochemical Measuring Electrode Device

The invention will now be further described with reference to the drawings, in which

FIG. 1 is a schematical view of a first, presently preferred embodiment of an electrochemical measuring electrode device of the CLARK type and according to the invention for measuring the partial pressure of oxygen,

FIG. 2 is a perspective and partly broken away view of a front part of the electrochemical measuring electrode device shown in FIG. 1 also illustrating a first embodiment of a membrane according to the invention,

FIG. 3 is a perspective and partly broken away view corresponding to the view of FIG. 2 also illustrating a second and presently preferred embodiment of the membrane according to the invention,

FIG. 4 is a perspective and partly broken away view of a second embodiment of the electrochemical measuring electrode device according to the invention, the electrode device being a transcutaneous, polarographic and potentiometric electrode device,

FIG. 5 is an elevational view of the first embodiment of the membrane according to the invention shown in FIG. 2,

FIG. 6 is an elevational and partly broken away view of the second and presently preferred embodiment of the membrane according to the invention shown in FIG. 3,

FIGS. 7, 8 and 9 are schematical views illustrating separate steps of a first method according to the invention of preparing the first embodiment of the membrane according to the invention,

FIGS. 10 and 11 are schematical views of two alternative embodiments of the method according to the invention of preparing the second and presently preferred embodiment of the membrane according to the invention,

FIG. 12 is a diagrammatical view illustrating the responses of an electrode device of the CLARK type according to the invention and the response of a conventional electrode device of the CLARK type, the electrode devices being identical to one another except for the catalytic means characteristic of the present invention, and

FIG. 13 is a diagrammatical view illustrating the responses of the electrode devices mentioned above with reference to FIG. 12 in a different test set-up.

In FIG. 1, a first embodiment of an electrochemical measuring electrode device of the CLARK type according to the invention for measuring the partial pressure of oxygen in a blood sample is shown. The electrode device is designated by the reference numeral 10 in its entirety and comprises a plastic housing 12, a platinum cathode 14 which is constituted by a platinum wire of a diameter of 20 .mu.m, which is sealed into a lead glass tube 16, and an anode 18. The platinum cathode 14 defines an active, exposed oxygen-reducing cathode surface 20, which is in contact with an electrolyte solution, which is confined in an electrolyte chamber 22 defined by an oxygen-permeable plastic membrane 24. As is evident from FIG. 1, the plastic housing 12 encircles the lead glass tube 16 so that an annular chamber 26 is defined between the plastic housing 12 and the lead glass tube 16. The annular chamber 26 communicates with the electrolyte chamber 22 through an electrolyte solution passage 28. The anode 18 is constituted by a silver/silver chloride coating of the lead glass tube 16 and communicates chemically with the electrolyte solution of the annular chamber 26 and further with the electrolyte solution of the electrolyte chamber 22 through the electrolyte solution passage 28.

On the membrane 24, platinum black particles of a particle size of less than 1 um are applied, as shown in FIG. 2 or, alternatively as shown in FIG. 3. The membrane 24 is maintained in position in front of the end surace of the electrode device 10 by means of an O-ring 30 and covers the exposed oxygen-reducing surface 20 of the Pt-wire cathode 14. The electrolyte solution is an aqueous solution of the composition: 191 mmole KH.sub.2 PO.sub.4, 298 mmole Na.sub.2 HPO.sub.4 . 2H.sub.2 O, 139 mmole KC1, 0.26 mmole AgCl and thymol as a germicide. In use, a polarization voltage of -630 mV is supplied to the electrode device 10 from an external measuring apparatus 32 through a cable 34.

The electrode device is fitted in a measuring cell 36 of the type described in U.S. Pat. No. 4,160,714. In the measuring apparatus 32, the current generated by the electrode device by the reduction of oxygen is amplified and converted from analogue to digital form in an A/D converter for calculating the partial pressure of oxygen, which is displayed on a first display 38 of the measuring apparatus 32. The measuring apparatus 32, further comprises a second display 40 for displaying the current generated by the electrode device 10 in digital representation.

In FIG. 2, the end part of the electrode device 10 according to the present invention is shown in greater detail also disclosing the structure of the membrane 24 which comprises a non-biological catalytic means constituted by platinum black particles effective for promoting the decomposition of H.sub.2 O.sub.2 generated in the electrolyte solution by the reduction of oxygen. Although the catalytic means is preferably constituted by a covering of at least part of the membrane of the electrode device, the catalytic means may, alternatively, e.g., be a layer in an annular configuration or an annular groove arranged at the front end of the lead glass tube 16, e.g., encircling the cathode.

The membrane 24 is a circular piece of biaxially oriented polypropylene foil of a thickness of 20 .mu.m. The diameter of the membrane 24 is 13 mm and the central part of the membrane 24 contacting a sample is approximately 5 mm. In a central region 42 of the membrane 24, platinum black particles are pressed into the foil of the membrane. The central region 42 measures approximately 7 mm.sup.2.

In FIG. 3, which is a partly broken away view of the end portion of the electrode device 10 basically corresponding to the view of FIG. 2, an alternative and presently preferred embodiment of the membrane 24 according to the invention is shown. The membrane 24 shown in FIG. 3 is a two-layer membrane a first or outer layer of which is designated by the reference numeral 44 and constituted by the above-described biaxially oriented polypropylene foil. An inner layer 46 of the membrane 24 is constituted by a solidified plastic material including platinum black particles constituting a stable, non-biological catalytic means effective for promoting the decomposition of H.sub.2 O.sub.2 generated in the reduction of oxygen.

The preparation of the first embodiment of the membrane 24 shown in FIG. 2 and the preparation of the second embodiment of the membrane 24 shown in FIG. 3 will be described in greater detail below with reference to FIGS. 7-9 and 10-11, respectively. In FIGS. 5 and 6, the first and the second embodiment, respectively, of the membrane 24 are shown in elevational views and as far as FIG. 6 is concerned also in partly broken away view.

In FIG. 4, a second or alternative embodiment of the electrode device 10 according to the invention is shown. The electrode device shown in FIG. 4 is a transcutaneous and combined polarographic and potentiometric electrode device of the type described in the applicant's published international patent application PCT/DK81/00035, publication No. WO 81/02831 to which reference is made. In FIG. 4, components similar to the components described above with refernece to FIGS. 1-3 are designated by reference numerals identical to the reference numerals of FIGS. 1-3. In the transcutaneous, polarographic and potentiometric electrode device 10 shown in FIG. 4, the anode 18 is constituted by a solid silver body. Centrally within the solid silver body, a pH-electrode 48 is arranged. At the lower side surface of the silver body 18, recesses 50 are provided which are filled by the electrolyte solution and constitute electrolyte reservoirs. In two further recesses of the silver body 18, a Zener diode 52 and an NTC resistor 54 are arranged. The Zener diode 52 and the NTC resistor constitute a heating element and a temperature detector element, respectively, of the transcutaneous polarographic and potentiometric electrode device, as is well-known in the art. In front of the pH-electrode 48 and further in front of the cathode of the electrode device, not shown in FIG. 4, a spacer structure 56 is arranged. The spacer structure is constituted by a water and H.sub.2 O.sub.2 and carbon dioxide permeable plastic element further comprising platinum black particles constituting a catalytic means effective for promoting the decomposition of H.sub.2 O.sub.2 formed by the reduction of O.sub.2. As is evident from FIG. 4, the spacer structure 56 is of a basically annular configuration.

The platinum black particles of the membrane 24 or of the spacer structure 56 are prepared by grinding commercially available platinum black of the type FLUKA No. 81110. The commercially available platinum black has a variable grain size of approx. 5 .mu.m.

When preparing the ground platinum black particles, 1 g is weighed out and suspended in benzene at a concentration of approx. 20 mg/ml. The suspension is transferred to a beaker filled with small glass beads, and stirred on a magnetic stirrer of a conventional laboratory type for 24 hours. Thus a particularly finely distributed platinum black (dust) is obtained with a comparatively uniform particle size distribution and with an average particle diameter <0.5 um.

The platinum black dust is easily suspended in the suspending agent.

In accordance with a first method of preparing a membrane according to the invention, 10 .mu.l of a benzene or ethanol suspension of the above-described platinum particles is, as shown in FIG. 7, taken out by means of a Cartsberg pipette 58 and transferred to a circular polypropylene foil 60. As is evident from FIG. 7, the foil 60 may be supported on a support structure 64 which further includes a heating element 66. In FIG. 7,the 10 .mu.l suspension including the platinum black particles is designated the reference numeral 62. The suspending agent is evaporated by standing for about 5 minutes. Alternatively, the evaporation could be performed by applying heat to the support structure 64 from the heating element 66. Thereafter, as is evident from FIG. 8, a TEFLON.RTM. foil 68, the melting point of which exceeds 175.degree. C., is arranged on top of the foil 60 with the layer of platinum black particles 62 from which the dispensing agent has been evaporated. A pressing tool 70 which also includes a heating element 72 is brought into contact with the upper side surface of the TEFLON.RTM. foil 68 as indicated by an arrow 73 and pressed against the foil 60 with a pressure of approximately 300 kg/cm.sup.2 (29.4.times.10.sup.6 N/m.sup.2 ), while heat is applied to the pressing assembly from the heating elements 66 and 72 in order to heat the foils 60 and 68 to a temperature of approximately 130.degree. C. The above pressure is maintained for a period of time of approximately 10 min., whereupon the platinum black particles have been pressed into the foil material 60.

After the above 10 min. period of time, the heating elements 66 and 72 are disconnected from their supply. When the system has cooled to room temperature, the pressing tool is elevated and, as is evident from FIG. 9, the TEFLON.RTM. foil 68 is removed. The platinum black particles adhere completely to the polypropylene foil 60.

From an electrode response time point of view, the above-described first method of preparing a membrane according to the invention may be the most advantageous as the membrane prepared in accordance with the above-described method basically has characteristics as to strength and permeability to oxygen which are identical to the characteristics of a polypropylene membrane which does not include platinum black particles. However, it has been found that this method may sometimes result in a membrane which may give rise to a quiescent or zero current of the electrode device when the membrane is arranged on the electrode device and maintained in position by means of its O-ring as shown in FIGS. 1-3. This is believed to be due to the fact that some of the platinum black praticles which should have been pressed into the membrane material protrude from the inner side surface of the membrane and are consequently brought into contact with the exposed cathode surface so that a galvanic current is generated by the contact between the exposed cathode surface and the platinum black particles. Thus, in this method, it seems important to ensure that substantially all of the platinum black particles are pressed into the membrane material.

In accordance with a second and presently preferred embodiment of preparing the membrane according to the invention, the platinum black particles are suspended in a plastic material which is in a liquid state, e.g., in solution, whereupon the suspension is applied to the polypropylene foil and solidified, e.g., by evaporation. Two alternatives of applying the suspension to the foil in accordance with the above-described presently preferred embodiment of the method according to the invention are shown in FIGS. 10 and 11.

In FIG. 10, the suspension of the platinum black particles in the liquid plastic material is poured to an outer or upper side surface of the polypropylene foil 60 which is arranged in an oblique position, whereupon the suspension which is designated by the reference numeral 74 in FIG. 10 flows down the oblique foil whereby a homogeneous suspension layer is provided and is solidified, e.g., by evaporation.

The suspension 74 may, as is shown in FIG. 11 alternatively be applied to the outer or upper side surface of the foil 60 in a screening process by employing a squeegee or spatula 76.

The liquid plastic material may, e.g., be prepared as a solution of 0.35 g PUR (polyurethane) in 5 ml DMF (dimethylformamide), and 20 ml THF (tetrahydrofuran) which may be prepared within a few hours. In 3 ml of this solution, 5 mg platinum black particles are suspended and ground by stirring with a magnetic stirrer for about two hours in a beaker of diameter 20 mm filled with about 20 glass beads of a diameter of 3 mm.

As will be understood, the response time of the electrode device comprising the above-described second and presently preferred embodiment of the membrane according to the invention may be slightly slower than the response time of the electrode device comprising the above-described first embodiment of the membrane according to the invention if the membrane thickness is not reduced to compensate for the added layer. However, the increase in the response time by applying the inner layer 46 to the outer layer 44 of the membrane 24 shown in FIG. 3 only amounts to approximately 50%, and the response time of the electrode device shown in FIG. 3 may obviously be reduced by simply decreasing the thickness of the outer membrane layer 44. The membrane 24 shown in FIGS. 3 and 6 has the advantage that the platinum black particles are all embedded in the layer 46 and thus cannot give rise to a quiescent current or zero current.

In FIG. 12, a diagram is shown illustrating the response A of an aged oxygen electrode device as shown in FIGS. 1 and 2 according to the invention comprising a membrane of the type described above with reference to FIGS. 2 and 5 and prepared as described above with reference to FIGS. 7-9 and the response B of the same electrode device comprising a polypropylene membrane of a conventional type, i.e. not including platinum black particles.

The responses were recorded in an experiment in which the electrode devices were mounted in a set-up of the type shown in FIG. 1 for measuring the oxygen partial pressure on gas samples. Atmospheric air saturated with aqueous vapour, i.e. air of an oxygen partial pressure of 145 mm Hg (19.3 kPa) was introduced into the measuring cell 36 every 6 minutes and conducted past the electrode device.

After the oxygen electrode device had been mounted in the measuring cell for approx. 3 days, a sample of the composition of 95% argon, 5% CO.sub.2, i.e. of an oxygen partial pressure of zero, was introduced into the measuring cell. The digitized response of the electrode device was recorded every 2 seconds and converted to mm Hg on the basis of calibration data at 145 mm Hg. The two response curves A and B shown in FIG. 3 were plotted on the basis of the response data thus recorded.

It is seen that after 90 seconds the electrode current and/or the response B of the conventional electrode device reaches a value corresponding to an oxygen partial pressure of approximately 9 mm Hg, whereas the electrode current of the response A of the electrode device according to the invention already after 45 seconds reaches a stable value corresponding to an oxygen partial pressure of 2 mm Hg. Thus it can be seen that the response of the electrode device of the present invention reaches its stable level considerably faster than the conventional electrode device.

Based on the theoretical reflections above, it is evident that the respect curve B of the conventional aged electrode device will be positioned above the response curve A of the electrode device according to the invention, when the oxygen level in the sample is lower than the oxygen level at which the electrode device has stabilized. This is due to the fact that the H.sub.2 O.sub.2 level in the electrolyte solution while the response curve is recorded is higher than the level of H.sub.2 O.sub.2 originating from the present oxygen level and thus provides a higher current than the current to be expected from the present oxygen level.

The rate of the response is usually expressed by the time constant of the electrode device, which is approximately 5 seconds for a non-aged conventional oxygen electrode device. The same value is ascertained for the electrode devices according to the invention. Aged conventional oxygen electrode devices have a small time constant during the first part of the response and a large time constant during the last part of the response.

In connection with measurements on samples with an oxygen level lower than the oxygen level at which the electrode device has stabilized (145 mm Hg), the time constant during the last part of the response for aged conventional oxygen electrode devices was experimentally found to be more than 15 seconds.

On the basis of the response curves, it is estimated that the analysing time may be reduced by more than 50% in an analysing apparatus in which the electrode device according to the invention replaces the conventional device.

In FIG. 13, the response D of an aged electrode device according to the invention and of the type described above with reference to FIG. 12 and the response C of the conventional electrode device also described above with reference to FIG. 12 are shown.

With the one exception that a sample of a high oxygen level (80% O.sub.2, 5% CO.sub.2, 15% N.sub.2) was used instead of a sample of an oxygen level of zero, the experimental work which led to the plotting of FIG. 13 was carried out in the same manner as described in connection with FIG. 12.

In this case a lower oxygen content was recorded (as expected) when employing the conventional electrode device than when employing the electrode device according to the invention.