EP1064539A1

EP1064539A1 - Heated electrochemical cell

Info

Publication number: EP1064539A1
Application number: EP99910001A
Authority: EP
Inventors: Alastair Mcindoe Hodges; Thomas William Beck
Original assignee: USF Filtration and Separations Group Inc
Current assignee: Pall Filtration and Separations Group Inc
Priority date: 1998-03-12
Filing date: 1999-03-11
Publication date: 2001-01-03
Also published as: AUPP238898A0; WO1999046585A1; CA2322757A1; JP2002506969A; TWI240071B; EP1064539A4

Abstract

The invention provides a method for determining the concentration of an analyte in a sample comprising the steps of heating the sample and measuring the concentration of the analyte or the concentration of a species representative thereof in the sample at a predetermined point on a reaction profile by means that are substantially independent of temperature. Also provided is an electrochemical cell comprising a spacer (3) pierced by an aperture which defines a cell wall, a first metal electrode (2) on one side of the spacer extending over one side of the aperture, a second metal electrode (4) on the other side of the spacer extending over the side of the aperture opposite the first electrode, means for admitting a sample to the cell volume defined between the electrodes and the cell wall, and means for heating a sample contained within the cell (10).

Description

- 1 -

HEATED ELECTROCHEMICAL CELL

TECHNICAL FIELD

This invention relates to a method and apparatus for measuring the concentration

of an analyte in solution.

The invention will be described with particular reference to the measurement of the

concentration of glucose in blood but is not limited to that use and has general

application for the measurement of analytes other than glucose and for solutions other

than blood samples.

BACKGROUND ART

Persons who suffer from diabetes routinely check their blood glucose

concentration and there is a need for simple, reliable and inexpensive means to facilitate

such routine testing.

In a common method for conducting the tests, a blood sample is combined with an

enzyme for example glucose dehydrogenase ("GDH"); the GDH oxidises glucose and in

the process becomes reduced. An oxidising mediator for example ferricyanide is

allowed to react with the reduced GDH returning the GDH to its initial form and

producing ferrocyanide in the process. The concentration of ferrocyanide produced is

then sensed for example electrochemically or spectroscopically to produce a signal

which can be interpreted to give an estimate of the glucose concentration in the sample.

In our co-pending applications PCT/AU96/00723 and PCT/AU96/00724 (the

disclosures of which are incorporated herein by reference) there are described methods

and apparatus suitable for electrochemically determining the concentration of glucose in

blood by electrochemical measurement. - 2 -

A preferred method for accurately determining the concentration of an analyte is to

react all the analyte present in the sample with reagents that produce a species that can

be sensed. This requires that the reaction of the analyte go to completion.

For reaction of GDH with glucose to go to substantial completion typically

requires several minutes. This is thought to be due to the time required for the glucose to

diffuse out from glucose-containing cells of the blood. As this length of time is

unacceptably long for the market, it is more usual to measure the glucose concentration

over a shorter period, for example 20-30 seconds and accept a less accurate response or

apply a factor to estimate the glucose concentration by kinetic extrapolation for example

as outlined in co-pending application PCT/AU96/00723. This expedient shortens the

time of the test but can lead to loss of precision of the result.

It is an object of the present invention to provide a method and apparatus which

avoids or ameliorates the above-discussed deficiencies in the prior art.

DESCRIPTION OF THE INVENTION

According to one aspect the invention consists in a method for determining the

concentration of an analyte in a sample comprising the steps of:

heating the sample in a disposable test cell; and

measuring the concentration of the analyte or the concentration of a species

representative thereof in the sample at a predetermined point on a reaction profile by

means that are substantially independent of temperature.

Those skilled in the art will understand the term "reaction profile" as used herein

to mean the relationship of one reaction variable to another. Often, for example, the

reaction profile illustrates the change of concentration of a species with respect to time. Such a profile can provide a skilled addressee with both qualitative and quantitative

information, including information as to whether a reaction system has achieved a steady

state.

Preferably, the predetermined point on the reaction profile is a steady state, and

the species representative of the concentration of the analyte is a mediator, for instance

an enzyme mediator.

In one embodiment of the invention the sample is heated by an exothermic reaction

produced upon contact of the sample with a suitable reagent or reagents.

In a second embodiment of the invention the sample is heated electrically, for

example by means of a current applied to resistive elements associated with the

measuring means.

In a highly preferred embodiment the measuring means is an electrochemical cell

of the kind described in co-pending applications PCT/AU96/00723 and

PCT/AU96/00724 and the sample is heated by application of an alternating voltage

signal between electrodes of the sensor.

According to a second aspect the invention consists in an electrochemical cell

comprising a spacer pierced by an aperture which defines a cell wall, a first metal

electrode on one side of the spacer extending over one side of the aperture, a second

metal electrode on the other side of the spacer extending over the side of the aperture

opposite the first electrode, means for admitting a sample to the cell volume defined

between the electrodes and the cell wall, and means for heating a sample contained

within the cell. - 4 -

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be more particularly described by way of example only

with reference to the accompanying drawings wherein:

Figure 1 shows schematically a sensor strip according to the invention in a cross-

section taken longitudinally through the midline of the sensor strip.

Figure 2 shows the results of tests conducted in accordance with one embodiment

of the present invention for blood samples with varying haematocrits and glucose

concentrations.

Figure 3 shows the results of tests conducted in accordance with another

embodiment of the present invention for blood samples with varying haematocrits and

glucose concentrations.

BEST MODE FOR CARRYING OUT THE INVENTION

In preferred embodiments of the method of the invention, glucose concentration is

measured using an electrochemical cell of the kind described in PCT/AU96/00723

and/or PCT/AU96/00724 (our co-pending applications). The method of measurement

described in those applications utilises an algorithm which enables the value of the

diffusion coefficient of the redox mediator to be calculated and the concentration of

reduced mediator to be determined in a manner which is substantially independent of

sample temperature. The method therein described is different from prior art methods

which measure Cottrell current at known times after application of a potential. The

present invention differs in that the sample is heated.

In a first embodiment of the present method the blood sample is heated prior to

and/or during conduct of the electrochemical measurement by means of an exothermic - 5 -

reaction. In the first embodiment a reagent that liberates heat on contact with blood is

contained within the sensor cell. Examples of such reagents are salts which give out heat

when they dissolve such as aluminium chloride, lithium halide salts, lithium sulphate,

magnesium halide salts and magnesium sulphate. Another class of reagents which

would be suitable are those with two components which liberate heat upon mixing.

These two components would be placed in separate locations in the sensor during

fabrication, for example on coatings upon opposite internal cell walls and are deployed

such that when a sample is introduced into the sensor at least one of the components

dissolves and then comes into contact with the second component. Upon contact the two

components react to liberate heat. The reagents used to generate the heat must not

adversely effect the function of the other active elements in the sensor. For instance,

they must not corrode the electrode materials, denature an enzyme if present, or

adversely interact with any mediator that may be present. Upon introducing a sample of

blood into the sensor heat is liberated and the temperature of the blood sample is raised.

This facilitates reaction of the glucose with the GDH and since the measurement of

ferrocyanide concentration is temperature independent an accurate assessment of glucose

concentration can be made in a much shorter time than would otherwise be possible.

Less preferably, the heat generating reagent can be added after the sample is

admitted to the cell.

Preferably the sample temperature is raised by from 5 to 15°C, for example from

20°C to 30°C or 35°C within a period of 2 to 10 seconds. The temperature peak is

desirably reached within 2-5 seconds. - 6 -

A second embodiment of the invention employs a cell in which an electrically

resistive element is incorporated. The sample may then be electrically heated by passing

a current through the resistive element. For example, with reference to figure 1 there is

shown an electrochemical sensor comprising a plastic substrate 1 bearing a first

electrode 2 (for example a sputtered layer of gold), a separator layer 3 having a circular

aperture punched out which defines a cell volume 10 bounded on one cylindrical face by

first electrode 2. The opposite face of cylindrical cell 10 is covered by a second

electrode layer 4 (for example a sputter coating of palladium) which in this case is

carried by a rubber or plastic layer 5. A metal foil layer 6 provides electrical contact to a

resistive bridge 9 formed in the rubber or plastic layer 5. An insulating layer 7 for

example of plastic provides insulation against heat loss through the metal foil. An

aperture 8 in layer 7 provides for electrical contact with metal foil layer 6. Resistive

bridge 9 is formed for example from carbon particles impregnated into the rubber or

plastic of layer 5 at a loading and of a geometry such as to give a suitable electrical

resistance between metal foil 6 and electrode layer 4. This method has the advantage of

concentrating the heating effect adjacent the cell. Resistive heating elements may be

fabricated by other means for example by coating an electrically conducting substrate

with an electrically insulating layer which can be made partially conductive in particular

regions if desired for example by exposure to particular chemicals and light.

When using a cell according to the second embodiment the sample is admitted to the

cell, a potential is applied across the resistive element, and after the required amount of

heat has been generated the potential across the resistive element is interrupted and after - 7 - an optional wait time a potential is applied between the first electrode and second

electrode to perform the electrochemical assay of the analyte.

Alternatively the potential across the resistive element can be maintained during

the assay of the analyte at its initial level or at a lower level sufficient to substantially

maintain the sample temperature at the desired level.

In another embodiment, the means for applying the potential to the resistive

element is such that the current flowing through the resistive element is monitored and

the potential automatically adjusted so as to maintain the required power output. This

heats the sample in a reproducible fashion, even if the resistance of the resistive element

varies from one sensor to the next. Furthermore, the power level required can be

adjusted on the basis of the ambient temperature measured by a separate sensor. The

leads to a more reproducible sample temperature being reached over a range of ambient

temperature at which the sensor is being used.

In a third embodiment of the invention the sample is heated simply by applying an

alternating voltage signal between the working and counter-electrodes of a sensor, for

example, of the kind described in our co-pending applications. If this alternating voltage

signal has a correct frequency and amplitude it will heat the sample while still allowing

an accurate determination of the analyte to be subsequently made by the sensor. Because

the voltage signal is alternating any reaction that occurs during one half voltage cycle is

reversed during the second half of that cycle, resulting in no net change but in the

dissipation of energy that will appear as heat in the sample. This is particularly

applicable to sensors of the type disclosed in our abovementioned co-pending patent - 8 -

applications where any small changes that may occur in the cell are quickly removed

after interruption of the alternating potential as the cell relaxes back to its initial stage.

When using cells such as described in our co-pending applications the sample

volumes are very small and heating can be achieved with low energy input.

EXAMPLES OF HEATED STRIP EXPERIMENTS

Example 1

Disposable test strips of the type described in PCT/AU96/00724 were heated by

placing a metal bar, heated to 50°C, in contact with the sample receiving area of the

strip. Whole blood samples were introduced into the sample receiving area of the strip

and 13 seconds allowed for the glucose present in the sample to react with the sensor

reagents. Current was then collected for ten seconds and analyzed according to the

methods described in PCT/AU96/00723. The results of these tests for blood samples

with haematocrits of 67.5%, 49.5% and 20% and glucose concentrations between 2.5

mM and 30 mM are shown in figure 2.

Example 2

Disposable test strips of the type described in PCT/AU96/00724 were modified by

adhering a heater element to the base of the strip, beneath the sample receiving area. The

heater element was fabricated by sputtering two parallel low resistance metallic tracks

onto a polyester substrate and then sputtering a thin, resistive metallic track at right

angles to the low resistance metallic tracks, such that the resistive metallic track

contacted both of the parallel low resistance tracks. This heater was then glued to the

base of the disposable test strip using an adhesive, such that the resistive track was

positioned directly beneath and facing the sample receiving area on the strip. - 9 -

The parallel low resistance tracks protruded from the end of the strip and provided

electrical contacts for a power supply to power the heater. The power supply for the

heater consisted of a battery and a variable resistor, which could be adjusted to vary the

rate of heating. Whole blood samples were introduced into the sample receiving area of

the strip and 20 seconds allowed for the glucose present in the sample to react with the

sensor reagents. Current was then collected for ten seconds and analyzed according to

the methods described in PCT/AU96/00723. The results of these tests for blood samples

with haematocrits of 65%, 46% and 20% and glucose concentrations between 2.8 mM

and 32.5 mM are shown in figure 3.

Although the invention has been herein described with reference to

electrochemical methods for measuring glucose concentration in blood it will be

appreciated that the method may also be applied utilising suitable spectroscopic or other

measuring methods and to samples other than blood and to analytes other than glucose.

Claims

-10- THE CLAIMS OF THE INVENTIONARE AS FOLLOWS:

1. A method for determining the concentration of an analyte in a sample

comprising the steps of:

heating the sample in a disposable test cell; and

measuring the concentration of the analyte or the concentration of a species

representative thereof in the sample at a predetermined point on a reaction profile

by means that are substantially independent of temperature.

2. A method according to Claim 1 wherein the predetermined point on the reaction

profile is a steady state.

3. A method according to Claim 1 or Claim 2 wherein the species representative of

the concentration of the analyte is a mediator.

4. A method according to Claim 3 wherein the mediator is an enzyme mediator.

5. A method according to any one of the preceding claims wherein the sample is

heated by an exothermic reaction produced upon contact of said sample with at

least one suitable reagent.

6. A method according to Claim 5 wherein the at least one suitable reagent is a salt

which liberates heat on dissolution.

7. A method according to Claim 6 wherein the salts are selected from the group

consisting of aluminium chloride, lithium halides, lithium sulfate, magnesium

halides, and magnesium sulfate.

8. A method according to Claim 5 wherein the at least one suitable reagent

is a two component system which liberates heat upon mixing. - 11 -

9. A method according to Claim 8 wherein each of the two components are placed

in separate locations in a sensor during fabrication.

10. A method according to Claim 9 wherein said two components are placed as

coatings upon opposite internal cell walls of a sensor.

11. A method according to Claim 1 wherein the sample is heated electrically.

12. A method according to Claim 11 wherein said sample is heated by means of a

current applied to resistive elements associated with said measuring means.

13. A method according to any one of the preceding Claims wherein the

concentration of the analyte is measured by an electrochemical measurement.

14. A method according to Claim 13 wherein the sample is heated prior to and/or

during conduct of the electrochemical measurement.

15. A method according to any one of the preceding Claims wherein the sample

temperature is raised by from 5 to 15┬░C.

16. A method according to any one of the preceding Claims wherein the sample

temperature is raised within a period of 2- 10 seconds.

17. A method according to any one of the preceding Claims wherein a peak

temperature is reached within 2-5 seconds.

18. A method according to any one of the preceding Claims wherein the analyte is

glucose.

19. A method according to any one of the preceding Claims wherein the sample is

blood.

20. A method according to Claim 19 wherein the blood sample is combined with an

enzyme. - 12 -

21 A method according to Claim 20 wherein the enzyme is glucose dehydrogenase

(GDH) which oxidises glucose and is converted to reduced GDH.

22. A method according to Claim 21 wherein an oxidising mediator is present.

23. A method according to Claim 22 wherein said oxidising mediator is ferricyanide.

24. A method according to Claim 23 wherein said ferricyanide reacts with said

produced GDH to produce ferrocyanide.

25. A method according to Claim 24 wherein the ferrocyanide produced is sensed to

produce a signal representative of the glucose concentration of the sample.

26. A method according to Claim 25 wherein the sensing is by electrochemical

means.

27. A method according to Claim 25 wherein the sensing is by a spectroscopic

means.

28. An electrochemical cell comprising a spacer pierced by an aperture which

defines a cell wall, a first metal electrode on one side of the spacer extending

over one side of the aperture, a second metal electrode on the other side of the

spacer extending over the side of the aperture opposite the first electrode, means

for admitting a sample to the cell volume defined between the electrodes and the

cell wall, and means for heating a sample contained within the cell.

29. An electrochemical cell according to Claim 28 wherein the means for heating a

sample is an electrically resistive element.

30. A method of heating an electrochemical cell as defined in Claim 28 including the

step of applying a potential across the resistive element to regenerate the

required amount of heat, interrupting the potential across the resistive element - 13 -

and applying a potential between the first electrode and second electrode to

perform the electrochemical assay of the analyte.

31. A method according to Claim 30 wherein a potential across the resistive element

is maintained during the assay of the analyte at an initial level or at a lower level

sufficient to substantially maintain the sample temperature of the desired level.

32. A method according to Claim 30 or 31 wherein means for applying potential to

the resistive element is such that the current flowing through the resistive element

is monitored and the potential automatically adjusted so as to maintain the

required power output.

33. A method according to Claim 32 wherein the power output can be adjusted on

the basis of ambient temperature measured by a separate sensor.

34. A method of determining the concentration of an analyte in a sample

substantially as herein described with reference to any one of the examples.

35. An electrochemical cell substantially as herein described with reference to figure

1 or any one of the examples.

36. A method of heating an electrochemical cell substantially as herein described

with reference to any one of the examples.