U.S. patent application number 10/632947 was filed with the patent office on 2004-02-12 for antioxidant sensor.
This patent application is currently assigned to LifeScan, Inc.. Invention is credited to Chatelier, Ron, Hodges, Alastair.
Application Number | 20040026244 10/632947 |
Document ID | / |
Family ID | 29251533 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040026244 |
Kind Code |
A1 |
Hodges, Alastair ; et
al. |
February 12, 2004 |
Antioxidant sensor
Abstract
The present invention relates to a device and method for
measuring the level of an oxidant or antioxidant analyte in a fluid
sample. The device comprises a disposable electrochemical cell,
such as a thin layer electrochemical cell, containing a reagent
capable of undergoing a redox reaction with the analyte. When the
device or method is to be used with slow-reacting analytes, heat
may be applied to the sample by a resistive heating element in the
device or by an exothermic material contained within the
electrochemical cell. Application of heat will accelerate the rate
of the redox reaction between the reagent and the analyte and thus
facilitate the electrochemical measurement of slow-reacting
analytes.
Inventors: |
Hodges, Alastair; (Blackburn
South, AU) ; Chatelier, Ron; (Bayswater, AU) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Assignee: |
LifeScan, Inc.
Milpitas
CA
95035-6312
|
Family ID: |
29251533 |
Appl. No.: |
10/632947 |
Filed: |
July 31, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10632947 |
Jul 31, 2003 |
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09615691 |
Jul 14, 2000 |
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6638415 |
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09615691 |
Jul 14, 2000 |
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PCT/AU99/00152 |
Mar 11, 1999 |
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09615691 |
Jul 14, 2000 |
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09314251 |
May 18, 1999 |
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6174420 |
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09314251 |
May 18, 1999 |
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08852804 |
May 7, 1997 |
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5942102 |
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09314251 |
May 18, 1999 |
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09068828 |
Mar 15, 1999 |
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6179979 |
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Current U.S.
Class: |
204/409 ;
204/435 |
Current CPC
Class: |
G01N 33/84 20130101;
G01N 27/3272 20130101; C12Q 1/006 20130101; C12Q 1/004
20130101 |
Class at
Publication: |
204/409 ;
204/435 |
International
Class: |
G01N 027/26 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 12, 1998 |
AU |
PP 2388 |
Nov 16, 1995 |
AU |
PN 6619 |
Claims
What is claimed is:
1. A device for detecting a presence or an absence of a redox
reactive analyte in an aqueous sample, the device comprising an
electrochemical cell having a sensing chamber, a first electrode, a
second electrode, an aperture for admitting the sample into the
sensing chamber, and a reagent contained within the sensing
chamber, wherein the device contains a quantity of the reagent
sufficient for only a single test, and wherein the reagent is
capable of undergoing a redox reaction directly with the analyte to
generate an electrical signal indicative of the presence or absence
of the analyte.
2. The device of claim 1, wherein the first electrode comprises a
sensing electrode.
3. The device of claim 1, wherein the first electrode comprises a
material selected from the group consisting of platinum, palladium,
carbon, indium oxide, tin oxide, gold, iridium, copper, steel,
silver, and mixtures thereof.
4. The device of claim 1, wherein the second electrode comprises a
counter electrode.
5. The device of claim 1, wherein the second electrode comprises a
metal in contact with a metal salt.
6. The device of claim 5, wherein the metal in contact with a metal
salt is selected from the group consisting of silver in contact
with silver chloride, silver in contact with silver bromide, silver
in contact with silver iodide, mercury in contact with mercurous
chloride, and mercury in contact with mercurous sulfate.
7. The device of claim 1, the electrochemical cell further
comprising a reference electrode.
8. The device of claim 1, wherein the reagent is capable of
oxidizing an analyte comprising an antioxidant.
9. The device of claim 8, wherein the reagent is selected from the
group consisting of ferricyanide salts, dichromate salts,
permanganate salts, vanadium oxides, dichlorophenolindophenol,
osmium bipyridine complexes, and quinones.
10. The device of claim 1, wherein the reagent is capable of
reducing an analyte comprising an oxidant.
11. The device of claim 10, wherein the reagent is selected from
the group consisting of iodine, triiodide salts, ferrocyanide
salts, ferrocene, Cu(NH.sub.3).sub.4.sup.2+ salts, and
Co(NH.sub.3).sub.6.sup.3+ salts.
12. The device of claim 1, the sensing chamber further comprising a
buffer, wherein the buffer is contained within the sensing
chamber.
13. The device of claim 12, wherein the buffer is selected from the
group consisting of phosphates, carbonates, alkali metal salts of
mellitic acid, and alkali metal salts of citric acid.
14. The device of claim 1, further comprising a heating
element.
15. The device of claim 14, wherein the heating element is an
electrically resistive heating element.
16. The device of claim 14, wherein the heating element is an
exothermic substance contained within the sensing chamber.
17. The device of claim 1, wherein the second electrode is mounted
in opposing relationship a distance of less than about 150 microns
from the first electrode.
18. The device of claim 1, further comprising an interface for
communication with a meter.
19. The device of claim 18, wherein the interface communicates a
voltage or a current.
20. The device of claim 1, wherein the electrochemical cell
comprises a thin layer electrochemical cell.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of copending application
Ser. No. 09/615,691, filed on Jul. 14, 2000, which is a
continuation-in-part, under 35 U.S.C. .sctn. 120, of copending
International Patent Application No. PCT/AU99/00152, filed on Mar.
11, 1999 under the Patent Cooperation Treaty (PCT), which was
published by the International Bureau in English on Sep. 16, 1999,
which designates the U.S. and claims the benefit of Australian
Provisional Patent Application No. PP 2388, filed Mar. 12, 1998.
Application Ser. No. 09/615,691 is also a continuation-in-part of
application Ser. No. 09/314,251, filed May 18, 1999, now U.S. Pat.
No. 6,174,420. Application Ser. No. 09/314,251 is a continuation of
application Ser. No. 08/852,804, filed May 7, 1997, now U.S. Pat.
No. 5,942,102, and a continuation of application Ser. No.
09/068,828, filed Mar. 15, 1999, now U.S. Pat. No. 6,179,979.
Application Ser. No. 08/852,804 is the national phase under 35
U.S.C. .sctn.371 of prior PCT International Application No.
PCT/AU96/00723 which has an International filing date of Nov. 15,
1996, which designated the United States of America, and which was
published by the International Bureau in English on May 22, 1997,
and claims the benefit of Australian Provisional Patent Application
No. PN 6619, filed Nov. 16, 1995. Application Ser. No. 09/068,828
is the national phase under 35 U.S.C. .sctn.371 of prior PCT
International Application No. PCT/AU96/00724 which has an
International filing date of Nov. 15, 1996, which designated the
United States of America, and which was published by the
International Bureau in English on May 22, 1997, and claims the
benefit of Australian Provisional Patent Application No. PN 6619,
filed Nov. 16, 1995.
FIELD OF THE INVENTION
[0002] The present invention relates to a device and method for
measuring the level of an oxidant or antioxidant analyte in a fluid
sample. The device comprises a disposable electrochemical cell
containing a reagent capable of directly undergoing a redox
reaction with the analyte.
BACKGROUND OF THE INVENTION
[0003] An oxidation reaction, broadly defined, involves the
transfer of one or more electrons from one molecule or atom (the
reducing agent or reductant) to another (the oxidizing agent or
oxidant). Oxidation reactions occur in a broad range of systems,
e.g., food products, living organisms, and drinking water, and may
be detrimental or beneficial. Food products exposed to oxygen may
undergo oxidative degradation, resulting in the generation of
undesirable flavors and odors, the destruction of fat-soluble
vitamins and essential fatty acids, and the production of toxic
degradation products. Beneficial oxidation reactions in food
products include those between natural or synthetic antioxidants
and oxidants, whereby the oxidant is prevented from participating
in a detrimental oxidation reaction.
[0004] Thus, it is desirable to be able to measure oxidant or
antioxidant levels in liquid samples in many fields. For example,
it is desirable in terms of manufacturing quality control as well
as health monitoring to measure the level of preservatives such as
sulfur dioxide in wine or food, the level of ascorbic acid in
fruit, vegetables, beverages, and biological fluids, and the level
of chlorine or peroxides in water. Most conveniently, these tests
are fast and easy to use and be amenable to field as well as
laboratory use.
[0005] Existing methods for measuring these components require
either expensive laboratory apparatus or skilled operators in order
for the method to be used successfully. For example, a sensor for
detecting antioxidant agents in oil is disclosed in U.S. Pat. No.
5,518,590. However, this sensor is not designed for single,
disposable use and does not use a redox agent. It is therefore
desirable to have a sensor designed for single, disposable use that
can detect oxidant or antioxidant levels in fluid samples through
the use of a redox reagent.
SUMMARY OF THE INVENTION
[0006] The present invention provides a device and method for
measuring oxidant and antioxidant analytes with a disposable
sensing element, suitable for a single use, that can be combined
with a meter to give a robust, fast, and easy to use test that is
amenable to field as well as laboratory use. In particular, the
invention relates to the use of an electrochemical sensor that
utilizes a redox agent that reacts with the analyte of interest to
produce an electrochemically detectable signal.
[0007] In one embodiment of the present invention, a device for
detecting a presence or an absence of a redox reactive analyte in
an aqueous sample is provided, the device including an
electrochemical cell having a sensing chamber, a first electrode, a
second electrode, an aperture for admitting the sample into the
sensing chamber, and a reagent contained within the sensing
chamber, wherein the electrochemical cell is designed to be
disposed of after use in a single experiment, and wherein the
reagent is capable of undergoing a redox reaction directly with the
analyte to generate an electrical signal indicative of the presence
or absence of the analyte.
[0008] In one aspect of this embodiment, the first electrode is a
sensing electrode that may consist of platinum, palladium, carbon,
indium oxide, tin oxide, gold, iridium, copper, steel, or mixtures
thereof. The first electrode may also be silver. The first
electrode may be formed by a technique such as sputtering, vapor
coating, screen printing, thermal evaporation, ink jet printing,
ultrasonic spraying, slot coating, gravure printing and
lithography.
[0009] In another aspect of this embodiment, the second electrode
is a counter electrode. The second electrode may include a metal in
contact with a metal salt, for example, silver in contact with
silver chloride, silver in contact with silver bromide, silver in
contact with silver iodide, mercury in contact with mercurous
chloride, or mercury in contact with mercurous sulfate. The second
electrode may also be a reference electrode.
[0010] In another aspect of this embodiment, the electrochemical
cell further includes a third electrode, such as a reference
electrode. The third electrode may include a metal in contact with
a metal salt, such as silver in contact with silver chloride,
silver in contact with silver bromide, silver in contact with
silver iodide, mercury in contact with mercurous chloride, and
mercury in contact with mercurous sulfate.
[0011] In another aspect of this embodiment, the reagent is capable
of oxidizing an analyte including an antioxidant. The reagent may
include ferricyanide salts, dichromate salts, permanganate salts,
vanadium oxides, dichlorophenolindophenol, osmium bipyridine
complexes, and quinones.
[0012] In another aspect of this embodiment, the reagent is capable
of reducing an analyte including an oxidant. The reagent may
include iodine, triiodide salts, ferrocyanide salts, ferrocene,
Cu(NH.sub.3).sub.4.sup.2+ salts, and Co(NH.sub.3).sub.6.sup.3+
salts.
[0013] In another aspect of this embodiment, the sensing chamber
further includes a buffer contained within the sensing chamber. The
buffer is selected from the group consisting of phosphates,
carbonates, alkali metal salts of mellitic acid, and alkali metal
salts of citric acid.
[0014] In another aspect of this embodiment, the device further
includes a heating element. The heating element may include an
electrically resistive heating element or an exothermic substance
contained within the sensing chamber, such as aluminum chloride,
lithium chloride, lithium bromide, lithium iodide, lithium sulfate,
magnesium chloride, magnesium bromide, magnesium iodide, magnesium
sulfate, and mixtures thereof.
[0015] In another aspect of this embodiment, the sensing chamber
includes a support contained within the sensing chamber. Supports
may include mesh, nonwoven sheet, fibrous filler, macroporous
membrane, sintered powder, and combinations thereof. One or both of
the reagent and buffer may be contained within or supported on the
support.
[0016] In another aspect of this embodiment, the second electrode
is mounted in opposing relationship a distance of less than about
500 microns from the first electrode, less than about 150 microns
from the first electrode, or less than about 150 microns and
greater than about 50 microns from the first electrode.
[0017] In another aspect of this embodiment, the device further
includes an interface for communication with a meter. The interface
may communicate a voltage or a current.
[0018] In another aspect of this embodiment, the electrochemical
cell includes a thin layer electrochemical cell.
[0019] In a second embodiment of the present invention, a method
for detecting a presence or an absence of a redox reactive analyte
in an aqueous sample is provided which includes providing a device
for detecting the presence or absence of an analyte in an aqueous
sample, the device including an electrochemical cell having a
sensing chamber, a first electrode, a second electrode, an aperture
for admitting the sample into the sensing chamber, and a reagent
contained within the sensing chamber, wherein the electrochemical
cell is designed to be disposed of after use in a single
experiment, and wherein the reagent is capable of undergoing a
redox reaction directly with the analyte to generate an electrical
signal indicative of the presence or absence of the analyte;
providing an aqueous sample; allowing the sample to flow through
the aperture and into the sensing chamber, such that the sensing
chamber is substantially filled; and obtaining an electrochemical
measurement indicative of the presence or absence of analyte
present in the sample.
[0020] In one aspect of this embodiment, the electrochemical
measurement is an amperometric measurement, a potentiometric
measurement, a coulometric measurement, or a quantitative
measurement.
[0021] In another aspect of this embodiment, the method includes
the further step of heating the sample, wherein the heating step
precedes the step of obtaining the electrochemical measurement.
Alternatively, the method may include the additional steps of
heating the sample, wherein the heating step follows the step of
obtaining an electrochemical measurement; and thereafter obtaining
a second electrochemical measurement indicative of the presence or
absence of a second analyte present in the sample.
[0022] In another aspect of this embodiment, the sensing chamber
further includes a buffer, for example, phosphate buffer, carbonate
buffer, alkali metal salt of mellitic acid, and alkali metal salt
of citric acid.
[0023] In a third aspect of the present invention, a method for
measuring sulfur dioxide in a sample of wine is provided, the
sulfur dioxide having a free form and a bound form and being
capable of undergoing a redox reaction with a reagent, the redox
reaction having a reaction kinetics, wherein the method includes
the steps of providing a device, the device including an
electrochemical cell having a sensing chamber, a first electrode, a
second electrode, an aperture for admitting the sample into the
sensing chamber, and a reagent capable of undergoing a redox
reaction with sulfur dioxide, wherein the electrochemical cell is
designed to be disposed of after use in a single experiment;
placing the sample of wine in the electrochemical cell, thereby
initiating the redox reaction; and obtaining a first
electrochemical measurement indicative of the level of sulfur
dioxide in free form.
[0024] In one aspect of this embodiment, the method further
includes the steps of heating the sample of wine for a period of
time sufficient for sulfur dioxide in bound form to react with the
reagent, wherein the heating step is conducted after the step of
obtaining a first electrochemical measurement; and thereafter
obtaining a second electrochemical measurement indicative of the
level sulfur dioxide in free form and in bound form combined.
Alternatively, the method may include the further steps of
obtaining a second electrochemical measurement indicative of the
kinetics of reaction of the sulfur dioxide in bound form with the
reagent, wherein the second electrochemical measurement is obtained
after the step of obtaining a first electrochemical measurement;
and calculating the level of bound sulfur dioxide using the
kinetics of reaction.
[0025] In a fourth aspect of the present invention, a method of
manufacture of a device for detecting the presence or absence of a
redox reactive analyte in an aqueous sample is provided, the device
including an electrochemical cell having a sensing chamber, a first
electrode, a second electrode, an aperture for admitting the sample
into the sensing chamber, and a reagent contained within the
sensing chamber, wherein the electrochemical cell is designed to be
disposed of after use in a single experiment, and wherein the
reagent is capable of undergoing a redox reaction directly with the
analyte to generate an electrical signal indicative of the presence
or absence of the analyte, the method including forming an aperture
extending through a sheet of electrically resistive material, the
aperture defining a side wall of the sensing chamber; mounting a
first layer having a first electrode to a first side of the sheet
and extending over the aperture, defining a first sensing chamber
end wall, the first electrode facing the first side of the sheet;
mounting a second layer having a second electrode to a second side
of the sheet and extending over the aperture defining a second
sensing chamber end wall in substantial overlying registration with
the first layer, the second electrode facing the second side of the
sheet, whereby the sheet and layers form a strip; forming an
aperture in the strip to permit entry of a sample into the sensing
chamber; and providing a reagent capable of undergoing a redox
reaction directly with the analyte, wherein the reagent is
contained within the sensing chamber.
[0026] In one aspect of this embodiment, the method includes the
further step of providing a vent in the strip to permit escape of
air displaced from the sensing chamber when sample fills the
sensing chamber. Another further step includes mounting an
electrically resistive heating element to the strip.
[0027] In a further aspect of this embodiment, the aperture is of a
rectangular cross-section.
[0028] In a further aspect of this embodiment, at least one of the
electrodes includes a noble metal, for example, palladium,
platinum, and silver. At least one of the electrodes may be a
sputter coated metal deposit. The electrodes may be adhered to the
sheet, for example, by an adhesive such as a heat activated
adhesive, pressure sensitive adhesive, heat cured adhesive,
chemically cured adhesive, hot melt adhesive, or hot flow
adhesive.
[0029] In a further aspect of this embodiment, the method includes
further steps such as providing an exothermic substance or buffer
contained within the sensing chamber; printing the reagent or
buffer onto at least one wall of the sensing chamber; or providing
a support such as mesh, fibrous filler, macroporous membrane,
sintered powder, and combinations thereof contained within the
sensing chamber. The reagent may be supported on or contained
within the support.
[0030] In a further aspect of this embodiment, at least the sheet
or one of the layers of the device manufactured according to the
method is a polymeric material selected from the group consisting
of polyester, polystyrene, polycarbonate, polyolefin, and mixtures
thereof. Alternatively, at least the sheet or one of the layers is
polyethylene terephthalate.
[0031] In a further aspect of this embodiment, the second electrode
is mounted in opposing relationship a distance of less than about
500 microns from the first electrode; less than about 150 microns
from the first electrode; or less than about 150 microns and
greater than about 50 microns from the first electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 shows a plan view of an electrochemical cell.
[0033] FIG. 2 shows a cross-section view on line 10-10 of FIG.
1.
[0034] FIG. 3 shows an end-section view on line 11-11 of FIG.
1.
[0035] FIG. 4 shows schematically a heated electrochemical cell in
a cross section taken longitudinally through the midline of the
cell.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0036] The following description and examples illustrate a
preferred embodiment of the present invention in detail. Those of
skill in the art will recognize that there are numerous variations
and modifications of this invention that are encompassed by its
scope. Accordingly, the description of a preferred embodiment
should not be deemed to limit the scope of the present
invention.
[0037] The Sample and Analyte
[0038] In preferred embodiments, a method and device for measuring
oxidant or antioxidant levels in fluid samples is provided. The
method and device are applicable to any oxidant or antioxidant that
exists in a usefully representative concentration in a fluid
sample. Antioxidants that may be analyzed include, for example,
sulfur dioxide and ascorbic acid. Oxidants that may be analyzed
include, for example, chlorine, bromine iodine, peroxides,
hypochlorite, and ozone. Water insoluble oxidants or antioxidants
may also be analyzed if an aqueous form can be prepared, e.g., by
using a detergent to prepare an emulsion of the water insoluble
redox reactive analyte.
[0039] Methods and devices for obtaining electrochemical
measurements of fluid samples are discussed further in copending
U.S. patent application Ser. No. 09/616,433, filed on Jul. 14,
2000, entitled "IMMUNOSENSOR," copending U.S. patent application
Ser. No. 09/616,512, filed on Jul. 14, 2000, entitled "HEMOGLOBIN
SENSOR," and U.S. Pat. No. 6,444,115, issued on Sep. 3, 2002,
entitled "ELECTROCHEMICAL METHOD FOR MEASURING CHEMICAL REACTION
RATES," each of which is incorporated herein by reference in its
entirety.
[0040] The device and method may be used with any
analyte-containing sample which is fluid and which is capable of
solubilizing the redox reagent to a sufficient extent. Typical
samples include beverages such as fruit and vegetable juice,
carbonated beverages, drinking water, beer, wine, and spirits.
However, it is not intended that the method be limited to
comestible samples. If the sample is not in fluid form or is not
capable of solubilizing the redox reagent to a sufficient extent,
the analyte contained within the sample may be extracted into a
suitable fluid using extraction techniques well-known in the art.
The sample may be pre-treated prior to its introduction into the
electrochemical cell. For example, pH may be adjusted to a desired
level by means of a buffer or neutralizing agent, or a substance
that renders interfering oxidants or antioxidants nonreactive may
be added. The sample may also be preheated before introduction into
the cell so as to accelerate the rate at which the redox reaction
takes place.
[0041] The Electrochemical Cell
[0042] The electrochemical cell of present invention is disposable
and designed for use in a single experiment. In a preferred
embodiment, the electrochemical cell is a thin layer sensor such as
that disclosed in U.S. Pat. No. 5,942,102 (incorporated herein by
reference in its entirety). As herein used, the term "thin layer
electrochemical cell" refers to a cell having closely spaced
electrodes such that reaction products from the counter electrode
arrive at the working electrode. In practice, the separation of
electrodes in such a cell for measuring glucose in blood will be
less than 500 microns, and preferably less than 200 microns. A
preferred embodiment of such an electrochemical cell is illustrated
in FIGS. 1, 2, and 3. The cell illustrated in FIGS. 1, 2, and 3
includes a polyester core 4 having a circular aperture 8. Aperture
8 defines a cylindrical cell side wall 12. Adhered to one side of
core 4 is a polyester sheet 1 having a sputter coating of palladium
2. The sheet is adhered by means of an adhesive 3 to core 4 with
palladium 2 adjacent core 4 and covering aperture 8. A second
polyester sheet 7 having a second sputter coating of palladium 6 is
adhered by means of contact adhesive 5 to the other side of core 4
and covering aperture 8. There is thereby defined a cell having
cylindrical side wall 12 closed on each end by palladium metal 2,
6. The assembly is notched at 9 to provide for a solution to be
admitted to the cell or to be drawn in by wicking or capillary
action and to allow air to escape. The metal films 2, 6 are
connected with suitable electrical connections or formations
whereby potentials may be applied and currently measured.
[0043] Such a thin layer electrochemical cell is prepared by first
forming an aperture extending through a sheet of electrically
resistive material, the aperture defining a side wall of the
electrochemical cell. Suitable electrically resistive materials,
which may be used in the sheet containing the aperture, or in other
layers in the cell, include, for example, materials such as
polyesters, polystyrenes, polycarbonates, polyolefins, polyethylene
terephthalate, mixtures thereof, and the like. In a preferred
embodiment, the aperture in the sheet is rectangular, however other
shapes, e.g., circular, may be used as well.
[0044] After the aperture is formed, a first thin electrode layer
is then mounted on one side of the sheet of electrically resistive
material, extending over the aperture and forming an end wall. The
layer may be adhered to the sheet, for example, by means of an
adhesive. Suitable adhesives include, for example, heat activated
adhesives, pressure sensitive adhesives, heat cured adhesives,
chemically cured adhesives, hot melt adhesives, hot flow adhesives,
and the like. The electrode layer is prepared by coating (e.g., by
sputter coating) a sheet of electrically resistive material with a
suitable metal, for example, palladium.
[0045] A second thin electrode layer is then mounted on the
opposite side of the electrically resistive material, also
extending over the aperture, so as to form a second end wall. In a
preferred embodiment, the electrode layers are mounted in opposing
relationship at a distance of less than about 1 millimeter,
desirably less than about 800 microns, more desirably less that
about 600, or preferably less than about 500 microns, more
preferably less than about 300 to 150 microns, more preferably less
than 150 microns, and most preferably between 25, 40, 50, 100 and
150 microns. A second aperture or ingress is then provided for
liquid to enter the cell. Such an ingress can be provided by
forming a notch along one edge of the device which extends through
the electrode layers and aperture. The electrode layers are
provided with connection means allowing the sensors to be placed in
a measuring circuit.
[0046] Chemicals for use in the cell, such as redox reagents,
buffers, and other substances, may be supported on the cell
electrodes or walls, on one or more independent supports contained
within cell, or may be self supporting. If the chemicals are to be
supported on the cell electrodes or walls, the chemicals may be
applied by use of application techniques well known in the art,
such as ink jet printing, screen printing, lithography, ultrasonic
spraying, slot coating, gravure printing, and the like. Suitable
independent supports may include, but are not limited to, meshs,
nonwoven sheets, fibrous fillers, macroporous membranes, and
sintered powders. The chemicals for use in the cell may be
supported on or contained within a support.
[0047] In a preferred embodiment, the materials used within the
cell as well as the materials used to construct the cell are in a
form amenable to mass production, and the cells themselves are
designed to be able to be used for a single experiment then
disposed of.
[0048] According to the present invention a disposable cell is one
that is inexpensive enough to produce that it is economically
acceptable to be used only for a single test. Secondly, that the
cell may conveniently only be used for a single test.
Inconveniently in this context means that steps such as washing
and/or reloading of reagents would need to be taken to process the
cell after a single use to render it suitable for a subsequent
use.
[0049] Economically acceptable in this context means that the
perceived value of the result of the test to the user is the same
or greater than the cost of the cell to purchase and use, the cell
purchase price being set by the cost of supplying the cell to the
user plus an appropriate mark up. For many applications, this
requires that the cells have relatively low materials costs and
simple fabrication processes. For example, the electrode materials
of the cells should be inexpensive, such as carbon, or be used in
sufficiently small amounts such that expensive materials may be
used. Screen printing carbon or silver ink is a process suitable
for forming electrodes with relatively inexpensive materials.
However, if it is desired to use electrode materials such as
platinum, palladium, gold or iridium, methods with better material
utilization, such as sputtering or evaporative vapor coating, are
more suitable as they may give extremely thin films. The substrate
materials for the disposable cells also need to be inexpensive.
Examples of such inexpensive materials are polymers such as
polyvinylchloride, polyimide, polyester and coated papers and
cardboard.
[0050] Cell assembly methods also need to be amenable to mass
production. These methods include fabricating multiple cells on
cards and separating the card into individual strips subsequent to
the main assembly steps, and web fabrication where the cells are
produced on a continuous web, which is subsequently separated into
individual strips. Card processes are most suitable when close
spatial registration of multiple features is required for the
fabrication and/or when stiff cell substrate materials are to be
used. Web processes are most suitable when the down web
registration of features is not as critical and flexible webs may
be used.
[0051] The convenient single use requirement for the disposable
cell is desirable so that users are not tempted to try to reuse the
cell and possibly obtain an inaccurate test result. The single use
requirement for the cell may be stated in user instructions
accompanying the cell. More preferably, the cell may also be
fabricated such that using the cell more than once is difficult or
not possible. This may be accomplished, for example, by including
reagents that are washed away or consumed during the first test and
so are not functional in a second test. Alternatively, the signal
of the test may be examined for indications that reagents in the
cell have already reacted, such as an abnormally high initial
signal, and the test aborted. Another method includes providing a
means for breaking electrical connections in the cell after the
first test in a cell has been completed.
[0052] Cells for measuring antioxidants in the prior art do not
satisfy these requirements for disposability. The cell disclosed by
Richard J. Price et al. in Analyst, November 1991, Vol. 116, pages
1121-1123 uses a silver wire, a platinum wire and a platinum disc
as the electrodes for a cell measuring antioxidants in oil.
Platinum wires are too expensive to be used in a single use device
in this application, and the cell is designed for continuous
monitoring, not a single test. In U.S. Pat. No.5,518,590, Fang
discloses another cell for measuring antioxidants in oil. This cell
also uses platinum wire as an electrode and is also designed for
continuous use, namely, effectively conducting multiple tests over
time. This cell also requires a liquid or gel layer containing a
polar solvent. Such a device is not conducive to mass fabrication
and storage due to the need to contain the liquid components,
possibly over long periods, prior to use.
[0053] The Electrodes
[0054] At least one of the electrodes in the cell is a sensing
electrode, defined as an electrode sensitive to the amount of
reduced redox agent in the antioxidant case or oxidized redox agent
in the oxidant case. In the case of a potentiometric sensor wherein
the potential of the sensing electrode is indicative of the level
of analyte present, a second electrode acting as reference
electrode is present which acts to provide a reference
potential.
[0055] In the case of an amperometric sensor wherein the sensing
electrode current is indicative of the level of analyte in the
sample, at least one other electrode is present which functions as
a counter electrode to complete the electrical circuit. This second
electrode may also function as a reference electrode.
Alternatively, a separate electrode may perform the function of a
reference electrode.
[0056] Materials suitable for the sensing, counter, and reference
electrodes must be compatible with the redox reagents present in
the device. Compatible materials will not react chemically with the
redox reagent or any other substance present in the cell. Examples
of such suitable materials include, but are not limited to,
platinum, palladium, carbon, indium oxide, tin oxide, mixed
indium/tin oxides, gold, silver, iridium and mixtures thereof.
These materials may be formed into electrode structures by any
suitable method, for example, by sputtering, vapor coating, screen
printing, thermal evaporation or lithography. In preferred
embodiments, the material is sputtered or screen printed to form
the electrode structures.
[0057] Non-limiting examples of materials suitable for use in the
reference electrode include metal/metal salt systems such as silver
in contact with silver chloride, silver bromide or silver iodide,
and mercury in contact mercurous chloride or mercurous sulfate. The
metal may be deposited by any suitable method and then brought into
contact with the appropriate metal salt. Suitable methods include,
for example, electrolysis in a suitable salt solution or chemical
oxidation. Such metal/metal salt systems provide better potential
control in potentiometric measurement methods than do single metal
component systems. In a preferred embodiment, the metal/metal salt
electrode systems are used as a separate reference electrode in an
amperometric sensor.
[0058] The Redox Reagent
[0059] Suitable redox reagents include those which are capable of
undergoing a redox reaction with the analyte of interest. Examples
of redox reagents suitable for use in analyzing antioxidant
analytes include, but are not limited, to salts of ferricyanide,
dichromate, osmium bipyridine complexes, vanadium oxides, and
permanganate. Organic redox reagents such as
dichlorophenolindophenol, and quinones are also suitable. In a
preferred embodiment, the redox reagent for analyzing an
antioxidant is ferricyanide. Examples of reagents suitable for use
in analyzing oxidant analytes include iodine and salts of
triiodide, ferrocyanide, ferrocene, Cu(NH.sub.3).sub.4.sup.2+, and
Co(NH.sub.3).sub.6.sup.3+. In a preferred embodiment, the redox
reagent for measuring an oxidant is ferrocyanide.
[0060] The Buffer
[0061] Optionally, a buffer may be present along with the redox
reagent in dried form in the electrochemical cell. If a buffer is
used, it is present in an amount such that the resulting pH level
is suitable for adjusting the oxidizing (or reducing) potential of
the redox reagent to a level suitable for oxidizing (or reducing)
the analytes of interest but not other species that it is not
desired to detect. The buffer is present in a sufficient amount so
as to substantially maintain the pH of the sample at the desired
level during the test. Examples of buffers suitable for use include
phosphates, carbonates, alkali metal salts of mellitic acid, and
alkali metal salts of citric acid. The choice of buffer will depend
on the desired pH. The buffer is selected so as not to react with
the redox reagent. Alkali buffers are preferred for use in
conjunction with carbonated beverages.
[0062] Other Substances Present Within The Cell
[0063] In addition to redox reagents and buffers, other substances
may also be present within the cell. Such substances include, for
example, viscosity enhancers and low molecular weight polymers.
Hydrophilic substances may also be contained within the cell, such
as polyethylene glycol, polyacrylic acid, dextran, and surfactants
such as those marketed by Rohm & Haas Company of Philadelphia,
Pa., under the trade name Triton.TM. or by ICI Americas Inc. of
Wilmington, Del., under the trade name Tween.TM.. Such substances
may enhance the fill rate of the cell, provide a more stable
measurement, and inhibit evaporation in small volume samples.
[0064] Method for Measuring Analyte Concentration
[0065] In measuring an antioxidant or oxidant analyte present in a
sample, the sample is introduced into the sensor cell, whereupon
the sample dissolves the dried reagents present in the cell. The
redox reagent then reacts with any antioxidants or oxidants of
interest present in the sample to form the reduced or oxidized form
of the redox reagent. In the case of a potentiometric sensor, the
resulting ratio of oxidized to reduced form of the redox reagent
fixes the potential of the sensing electrode relative to the
reference electrode. This potential is then used as a measure of
the concentration of the analyte originally in the sample.
[0066] In a preferred embodiment, the sensing cell is operated as
an amperometric sensor. According to this embodiment, the reduced
(or oxidized) redox reagent formed by reaction with the analytes of
choice is electrochemically oxidized (or reduced) at the sensing
electrode. The current resulting from this electrochemical reaction
is then used to measure the concentration of analytes originally in
the sample. In other embodiments, the sensor is operated in
potentiometric or coulometric mode.
[0067] The cell's electrodes are used to produce an electrical
signal, i.e., a voltage or current, readable by an attached meter.
In a preferred embodiment, an interface for connecting the cell to
the meter is provided. The meter may display the measurement in a
visual, audio or other form, or may store the measurement in
electronic form.
[0068] Heating the Sample
[0069] Certain oxidant or antioxidant analytes are slow to react
with the redox reagent. To accelerate the reaction, and thus reduce
the time required to obtain the measurement, the sample may be
heated. In a preferred embodiment, a means for heating the sample
is provided in the disposable electrochemical sensor device.
[0070] Two suitable means of heating the cell are described in
WO99/46585 (incorporated herein by reference in its entirety).
WO99/46585 discloses a method for determining the concentration of
an analyte in a sample wherein the sample is heated and the
concentration of the analyte (or species representative of the
analyte) is measured at a predetermined point on a reaction profile
(defined as the relationship of one reaction variable to another)
by temperature independent means. The sample may be heated either
by an exothermic reaction produced upon contact of the sample with
a suitable reagent or reagents or the sample may be heated
electrically by means of a current applied to resistive elements
associated with the cell.
[0071] One method of heating the sample via exothermic reaction
involves placing in the electrochemical cell a reagent that
liberates heat on contact with the sample. Examples of such
reagents include salts which give out heat when they dissolve, such
as aluminum chloride, lithium halide salts, lithium sulfate,
magnesium halide salts and magnesium sulfate. The reagent or
reagents used to liberate heat must not adversely affect the
function of the other active elements in the cell, such as by
corroding electrode materials, reacting with the analyte so as to
affect its response, or adversely interacting with other reagents
present.
[0072] When the sample is to be heated electrically, the
electrochemical cell may be equipped with an electrically resistive
element. FIG. 4 shows a preferred embodiment of an electrochemical
sensor as described in WO99/46585. The sensor comprises a
nonconducting substrate 21, bearing a first electrode 22, a
separator layer 23 having a circular aperture 30 punched out which
defines a circular cell wall 30. The first electrode 22 defines one
end of the cell, the other end being defined by the second
electrode layer 24, which is carried by a second nonconducting
layer 25. A metal foil layer 26, provides electrical contact to a
resistive bridge 29 formed in the second nonconducting layer 25. An
insulating layer 27 provides insulation against heat loss through
the metal foil layer 26. An aperture 28 is formed in insulating
layer 27 to allow access for electrical connection to foil 26.
[0073] In preferred embodiments, resistive elements may be prepared
by impregnating one or more of the nonconducting layers carrying an
electrode layer with a substance such as carbon particles. The
nonconducting layers may include such materials as plastic or
rubber. The impregnated rubber or plastic layer forms a resistive
bridge between the electrode of the electrochemical cell and the
metal foil layer. When a potential is applied across the resistive
element, heat is generated in the impregnated rubber or plastic
layer, which in turn heats the sample in the electrochemical cell.
Alternatively, at least two low resistance tracks joined by a high
resistance track can be formed on an external face of the sensor.
In such an embodiment, the low resistance tracks serve to make
contact with the meter and the high resistance track forms the
electrically resistive element.
[0074] Multiple Cell Devices
[0075] In certain situations, it may be desirable to measure more
than one oxidant and/or antioxidant analyte in a sample. This may
be accomplished by using an array of two or more electrochemical
cells as described above. Each cell contains a redox reagent suited
for use with one of the analytes present in the sample. Each cell
is also equipped with buffers or heating means, if required for
that particular analyte. Such an array of cells may be used not
only to determine the concentration of known analytes of interest,
but may also be used to screen a sample of unknown analyte
composition for the presence or absence of a variety of
analytes.
[0076] Various embodiments of a cell array are contemplated. In one
embodiment, cell construction techniques as described above are
used to fabricate a device having multiple sensing chambers and
electrodes but sharing one or more layers of insulating material.
In another embodiment, two or more electrochemical cells as
described above are adhered together, either directly to each other
or to a separate support material. Alternatively, two or more cells
as described above, but containing different reagents, may be
packaged together in a kit suitable for use in a particular
application, i.e., a analysis of a sample containing multiple
analytes or different forms of the same analyte.
[0077] Analysis of Sulfur Dioxide in Wine
[0078] One example of an analysis wherein it is useful to heat the
sample is the measurement of sulfur dioxide in wine. Sulfur dioxide
in wine functions as an antioxidant and is typically present in two
forms: the free form and the bound form. The free form is more
quickly oxidized by the redox reagent in the sensor than is the
bound form. It is normally desirable to measure both the free and
bound forms of sulfur dioxide in wine. To measure both forms, a
heating means is included in the electrochemical cell. A sample of
the wine is placed in the sensing cavity, whereupon the redox
reagent present reacts quickly with the free sulfur dioxide to
produce a sensor signal. This signal is analyzed and then heat is
applied to the sample via the heating means. In a preferred
embodiment, heating is applied with a slow rise in temperature so
as to avoid excessive evaporation of the sample. After a suitable
period of time at elevated temperature, the bound sulfur dioxide
reacts with the redox reagent, thereby producing a second sensor
signal. From these two signals the free concentration and total
concentration of sulfur dioxide in the sample are obtained, and
thus, by difference, are the free and bound form concentrations
obtained. While this two-step method is beneficial for obtaining
the concentration of the free and bound forms of sulfur dioxide in
wine, the invention also contemplates other uses for such a method.
For example, a two (or more) step method may be used for analyzing
suitable samples containing an analyte having two or more forms
with different reaction kinetics, or samples containing two or more
different analytes each having different reaction kinetics.
[0079] The above description discloses several methods and
materials of the present invention. This invention is susceptible
to modifications in the methods and materials, as well as
alterations in the fabrication methods and equipment. Such
modifications will become apparent to those skilled in the art from
a consideration of this disclosure or practice of the invention
disclosed herein. Consequently, it is not intended that this
invention be limited to the specific embodiments disclosed herein,
but that it cover all modifications and alternatives coming within
the true scope and spirit of the invention as embodied in the
attached claims.
* * * * *