U.S. patent application number 12/187040 was filed with the patent office on 2008-11-27 for electrochemical sensor with dry ionomer membrane.
This patent application is currently assigned to PerkinElmer LAS, Inc.. Invention is credited to Avinash Dalmia, Otto J. Prohaska.
Application Number | 20080289962 12/187040 |
Document ID | / |
Family ID | 21850187 |
Filed Date | 2008-11-27 |
United States Patent
Application |
20080289962 |
Kind Code |
A1 |
Prohaska; Otto J. ; et
al. |
November 27, 2008 |
ELECTROCHEMICAL SENSOR WITH DRY IONOMER MEMBRANE
Abstract
A miniaturized gas sensor comprised of film type electrodes, on
a non-conductive supportive substrate, and in contact with a dry
ionomer electrolyte, for detection of toxic gases, i.e., carbon
monoxide, and other oxidizable or reducible gases and vapors and
method of making same is described. The all-solid planar sensor
cell has two or more film type electrodes arranged on a
non-conductive planar surface of a supportive substrate.
Manufacturing the electrochemical sensor with dry ionomer prevents
electrode flooding and allows for improved response time upon
assembly. The sensor cell contains no liquid electrolyte and is
operated in a constant-voltage, potentiostatic or potentiodynamic
mode. A high sensitivity to a select gas or vapor is achieved by a
three-phase contact area design for a sensing electrode, which
provides contact with the solid ionomer electrolyte, as well as the
gas sample via diffusion openings or holes that penetrate through
the supportive substrate.
Inventors: |
Prohaska; Otto J.; (Beacon
Falls, CT) ; Dalmia; Avinash; (Hamden, CT) |
Correspondence
Address: |
ST. ONGE STEWARD JOHNSTON & REENS, LLC
986 BEDFORD STREET
STAMFORD
CT
06905-5619
US
|
Assignee: |
PerkinElmer LAS, Inc.
Boston
MA
|
Family ID: |
21850187 |
Appl. No.: |
12/187040 |
Filed: |
August 6, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10029659 |
Oct 22, 2001 |
7422646 |
|
|
12187040 |
|
|
|
|
Current U.S.
Class: |
204/424 ;
29/592.1 |
Current CPC
Class: |
G01N 27/4074 20130101;
G01N 27/4045 20130101; G01N 27/4071 20130101; Y10T 156/10 20150115;
Y10T 29/49002 20150115 |
Class at
Publication: |
204/424 ;
29/592.1 |
International
Class: |
G01N 27/407 20060101
G01N027/407 |
Claims
1. A sensor cell for detecting gas comprising: a substrate; a
sensing electrode in close proximity with said substrate for
detecting gas; a dry ionomer membrane in contact with said
substrate and said sensing electrode wherein said dry ionomer
membrane acts as an electrolyte; at least one opening in said
substrate for keeping gas in contact with both said dry ionomer
membrane and said sensing electrode
2. The apparatus of claim 1 wherein said dry ionomer membrane is
composed of dry sheet Nafion.RTM..
3. The apparatus of claim 1 wherein said dry ionomer membrane
contains at least one opening.
4. The apparatus of claim 3 wherein the at least one opening is a
hole.
5. The apparatus of claim 1 wherein the said substrate, sensing
electrode, and ionomer membrane are in alignment with one
another.
6. The apparatus of claim 5 wherein said ionomer membrane is dry
during alignment of said substrate, sensing electrode, and ionomer
membrane.
7. The apparatus of claim 1 wherein the sensor is assembled prior
to wetting said dry ionomer membrane.
8. The apparatus of claim 1 wherein said at least one opening in
the substrate is aligned with the electrode for defining a gas
passage.
9. The apparatus of claim 1 further including a polymer layer
positioned upon said sensing electrode for slowing inputs of gas
moving through said at least one opening in said substrate onto a
surface of said sensing electrode.
10. The apparatus of claim 1 further comprising a counter electrode
in contact with the dry ionomer membrane such that upon wetting the
dry ionomer membrane the counter electrode provides an electrical
connection.
11. The apparatus of claim 1 further comprising a reference
electrode in contact with the dry ionomer membrane such that upon
wetting the dry ionomer membrane a reference point is created
against which the potential of other electrodes can be
measured.
12. The apparatus of claim 1 wherein the dry ionomer membrane is a
perfluorosulfonic acid membrane.
13. The apparatus of claim 1 further comprising a reservoir in
contact with the dry ionomer membrane.
14. The apparatus of claim 13 wherein said reservoir does not
initially contain liquid.
15. The apparatus of claim 14 wherein said reservoir can be filled
with liquid for hydrating the ionomer membrane.
16. The apparatus of claim 1 wherein said reservoir can be filled
with liquid for hydrating the ionomer membrane.
17. The apparatus of claim 1 further including a counter electrode
in contact with said dry ionomer membrane such that upon wetting
said dry ionomer membrane said counter electrode provides an
electrical connection to said ionomer membrane so current may be
applied to said sensing electrode.
18. The apparatus of claim 1 further including a reference
electrode in contact with said dry ionomer membrane such that upon
wetting said dry ionomer membrane a reference point is created
against which the potential of other electrodes can be
measured.
19. The apparatus of claim 1 wherein said dry ionomer membrane is a
perfluorosulfonic acid membrane.
20. The apparatus of claim 1 wherein said sensing electrode is in
contact with said substrate for detecting gas.
21. The apparatus of claim 1 further comprising two or more sensing
electrodes.
22. A method of making an electrochemical sensor for the detection
of an analyte in a gas sample comprising: providing a dry ionomer
membrane; providing a substrate that has holes through its surface
and an electrode layer adjacent to said holes; punching holes in
said dry ionomer membrane; and mating said dry ionomer membrane to
said substrate such that the holes in said dry ionomer membrane
align with the holes in said substrate forming a gas inlet and
wherein said dry ionomer membrane has on its surface an electrode
layer in operative connection with said ionomer membrane which
reacts with a gas to produce mobile electrons.
23. The method of claim 22 wherein the step of providing a dry
ionomer membrane further includes selecting dry sheet
Nafion.RTM..
24. The method of claim 22 further including making said sensor
cell prior to wetting said dry ionomer membrane.
25. The method of claim 22 further including positioning a polymer
layer upon said sensing electrode for slowing inputs of gas moving
through said inlet onto a surface of said sensing electrode.
26. The method of claim 22 where the step of providing a substrate
further includes positioning a counter electrode in contact with
said dry ionomer membrane such that upon wetting said dry ionomer
membrane said counter electrode provides an electrical connection
to said ionomer membrane so current may be applied to said sensing
electrode.
27. The method of claim 22 where the step of providing a substrate
further includes positioning a reference electrode in contact with
said dry ionomer membrane such that upon wetting said dry ionomer
membrane a reference point is created against which the potential
of other electrodes can be measured.
28. The method of claim 22 where the step of obtaining a dry
ionomer membrane further includes obtaining a perfluorosulfonic
acid membrane.
29. A method of making an electrochemical sensor for the detection
of an analyte in a gas sample comprising: a) providing a dry
ionomer membrane free from liquid droplets; b) providing a
substrate with at least one opening through its surface and a first
electrode layer adjacent to said opening; c) connecting said dry
ionomer membrane to said substrate; d) aligning said at least one
opening in said substrate with the first electrode for defining a
gas passage; and e) depositing a second electrode on said substrate
for operatively connecting said ionomer membrane, said first
electrode, and said second electrode.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is a continuation of currently
pending U.S. patent application Ser. No. 10/029,659, filed Oct. 22,
2001, the content of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention is directed toward an electrochemical sensor
manufactured using a dry ionomer membrane, and a method of
manufacturing the same.
BACKGROUND OF THE INVENTION
[0003] Solid, ionic conductive elements are known and have been
used in hydrogen-oxygen fuel cells, as is well known to those
skilled in the fuel cell art. The use of such solid, ionic
conductive electrolyte elements in an electrochemical gas sensor
has also been demonstrated, however, has not been heretofore
proposed or used in such electrochemical gas sensors to solve the
problems of electrode flooding in the typical prior art gas sensor,
as we presently understand the prior art.
[0004] The present invention provides an improved, less expensive
and simpler construction for an electrochemical gas sensor as well
as a simplified operation without the prior problems of flooding
the electrodes caused by the use of wet ionomer membrane or resins
during fabrication. The electrochemical gas-sensing cell of the
present invention is capable of sensing concentrations of
electrochemically active gases in gas mixtures in the parts per
billion range. The use of a dry ionomer membrane in the gas sensor
fabrication eliminates the problem of flooding of the electrode
surface in sensors manufactured utilizing a wet or pre-equilibrated
ionomer membrane. The present invention utilizes a solid dry
ionomer membrane to manufacture an electrochemical sensor. At a
desired time after assembly, the ionomer membrane can be
equilibrated with water so that the membrane obtains significant
ionic conductivity at room temperatures. By postponing hydration of
the ionomer membrane, calibration time of electrochemical sensors
is unexpectedly reduced. The dry ionomer membrane can be utilized
with film-based techniques, which have been widely investigated in
electrochemical sensor microfabrication technology.
[0005] Film based techniques in microfabrication technology are
known for a wide variety of sensors. Solid-state gas sensors have
demonstrated the advantage of being able to operate at elevated
temperatures, however they have the disadvantages of slow response
and recovery time and a high internal operating temperature. The
disadvantages and limitations of the state-of-the-art sensors
prevent efficient usage of such sensors in battery-powered
instruments.
[0006] Nafion.RTM.-coated metal oxide pH sensor with sputtered
iridium oxide sensing and silver/silver chloride reference
electrodes on alumina ceramic substrates are also known in the art.
Nafion.RTM. has been used as a cation-selective ionomer coating in
order to decrease the oxidation-reduction error generally affecting
the performance of metal oxide pH electrodes. The use of
Nafion.RTM. as polymer-electrolyte for a thin-film CO sensor with
macro-sized, sputtered Pt sensing and counter electrodes and a
smaller, sputtered Au electrode as reference electrode is also
known in the art. A 5 wt % n-propyl alcohol solution of Nafion.RTM.
(DuPont, 1100 EW) is used to form the polymer electrolyte film over
the electrodes by casting. The polymer is washed and protonated in
aqueous sulfuric acid prior to casting. The reported lifetime of
this sensor is reported to be less than one month. During this
time, the CO oxidation current decreases steadily down to a few
percent of its original value without any period of stable
measurement signal. The lifetime of the device may be extended up
to three years by laminating the polymer electrolyte layer with a
cast perfluorocycloether-polymer film in order to keep the CO
permeability coefficient through Nafion.RTM. constant; theoretical
calculations showed that the drift rate of the signal could be
significantly reduced under these conditions.
[0007] Nafion.RTM. is a copolymer of
perfluoro-3,6-dioxa-4-methyl-7octene-sulfonic acid and
tetrafluoroethylene (Teflon). Nafion.RTM. can be described as
having a Teflon backbone with occasional side chains added of
another fluorocarbon. The side chain terminates in a sulfonic acid
(--SO.sub.3H). With the exception of the sulfonic acid group, all
of Nafion.RTM. is a fluorocarbon polymer. Like most fluoropolymers,
it is extremely resistant to chemical attack (corrosion resistant).
The sulfonic acid group is immobilized within the bulk fluorocarbon
matrix and cannot be removed, but unlike the fluorocarbon matrix
the sulfonic acid groups do participate in chemical reactions. The
presence of the sulfonic acid adds three important properties to
Nafion.RTM.: 1) Nafion.RTM. functions as an acid catalyst due to
the strongly acid properties of the sulfonic acid group; 2)
Nafion.RTM. functions as an ion exchange resin when exposed to
solutions; 3) Nafion.RTM. very readily absorbs water, from the
vapor phase or from the liquid phase. Each sulfonic acid group will
absorb up to 13 molecules of water. The sulfonic acid groups form
ionic channels through the bulk hydrophobic polymer, and water is
very readily transported through these channels. Nafion.RTM.
functions like a very selective, semi-permeable membrane to water
vapor.
[0008] The physical properties of Nafion.RTM. are similar to other
fluoropolymers. It is a translucent plastic, with reasonable
flexibility. When used as an ion exchange membrane, it is specified
by its manufacturer, DuPont, to operate at temperatures up to
190.degree. C. An unusual property of Nafion.RTM. is its propensity
to change in physical size. As Nafion.RTM. absorbs water, it will
swell (increase in size) by up to 22%. When exposed to alcohols it
will swell up to 88%.
[0009] Table 1 shows readily available types of Nafion.RTM.
membranes. All measurements were taken with membranes conditioned
to 23.degree. C., and 50% Relative Humidity (RH).
TABLE-US-00001 Membrane Type Nominal Thickness (mm) Weight Caliper
(g/dm.sup.2) N-111 0.025 0.5 N-112 0.051 1.0 N-1135, N-1035 0.089
1.9 N-115, N-105 0.127 2.5 N-117 0.183 3.6
[0010] Dry ionomer membranes can also be defined as those ionomer
membranes that are hygroscopic. Hygroscopic membranes are those
membranes that readily absorb or attract moisture from the air; or
membranes having an affinity for moisture. One such example is
Nafion.RTM. 117 perflourinated membrane manufactured and sold by
E.I. du Pont de Nemours and Co. Dry ionomer membranes do not
include membranes that have been soaked in any solution such as
water, or acidic solution.
[0011] The present invention relates to the manufacture of
electrochemical sensors using hygroscopic Nafion.RTM., or
Nafion.RTM. sold in dry sheets to form the ionomer membrane, which
has had the unexpected result of facilitating the manufacturing
process and the development of sensors with improved start-up times
after assembly of the sensor.
[0012] Table 2 compares mechanical and electrical properties of dry
sheet Nafion.RTM. at 50% RH and 23.degree. C. to wet Nafion.RTM.
soaked in water.
TABLE-US-00002 NAFION .RTM. Mechanical and Electrical Properties
Property Typical Value Test Method Tensile Modulus, MPa (kpsi) 50%
RH, 23 C. 249 (36) ASTM D 882 water soaked, 23 C. 114 (16) water
soaked, 100 C. 64 (9.4) Tensile Strength (max), MPa (kpsi) 50% RH,
23 C. 43 (6.2) in MD, 32 (4.6) in TD ASTM D 882 watersoaked, 23 C.
34 (4.9) in MD, 26 (3.8) in TD water soaked, 100 C. 25 (3.6) in MD,
24 (3.5) in TD Elongation at Break, % 50% RH, 23 C. 225 in MD, 310
in TD ASTM D 882 water soaked, 23 C. 200 in MD, 275 in TD water
soaked, 100 C. 180 in MD, 240 in TD Tear Resistance - Initial, g/mm
50% RH, 23 C. 6000 in MD, TD ASTM D 10004 water soaked, 23 C. 3500
in MD, TD water soaked, 100 C. 3000 in MD, TD Tear Resistance -
Propagating, g/mm 50% RH, 23 C. >100 in MD, >150 in TD ASTM D
1922 water soaked, 23 C. 92 in MD, 104 in TD water soaked, 100 C.
74 in MD, 85 in TD Density, g/cm.sup.3 2.0 -- Conductivity,, S/cm
0.083
[0013] Table 2 shows that tear resistance (g/mm) of dry membrane
increases with thickness. These values for tear resistance are
typical of N-112 0.051 mm membrane.
[0014] Where specified in table 2, "MD" means machine direction,
and "TD" means transverse direction. Also, conductivity
measurements made for 1100 EW membranes utilizing membrane
conditioned at 100 C water for 1 hour. The conductivity measurement
cell was submersed in 25 C water during experiment, and membrane
impedance (real) taken at zero imaginary impedance.
[0015] Table 4 compares water uptake from dry Nafion.RTM. membrane
(dry weight basis) to water soaked Nafion.RTM. membrane at
100.degree. C. for 1 hour.
TABLE-US-00003 NAFION .RTM. Hydrolytic Properties Typical Property
Value Test Method Water Uptake, % water 35 ASTM D 570 Thickness
Change, % Increase from 50% RH, 23 C. to water soaked, 23 C. 10%
ASTM D 756 from 50% RH, 23 C. to water soaked, 100 C. 14% Linear
Expansion, % Increase from 50% RH, 23 C. to water soaked, 23 C. 10%
ASTM D 756 from 50% RH, 23 C. to water soaked, 100 C. 15%
[0016] A description of typical state-of-the-art hydrated solid
polymer electrolyte or ionomer sensors and sensor cells is
described by Kosek et al. U.S. Pat. No. 5,527,446; LaConti and
Griffith, U.S. Pat. No. 4,820,386; Shen et al., U.S. Pat. No.
5,573,648; and, Stetter and Pan, U.S. Pat. No. 5,331,310 all of
which are herein incorporated by reference. These sensor cells,
based on hydrated solid polymer electrolyte or ionomer technology,
have several advantages over conventional electrochemical sensor
cells. The catalytic electrodes are bonded directly to both sides
of a proton conducting solid polymer ionomer membrane providing a
stable electrode to electrolyte interface. One side of the
electrolyte membrane is flooded with distilled water, making the
sensor cell self-humidifying and independent of external humidity.
Since no corrosive acids or bases are used in the sensor cell, a
lifetime of over 10 years has been demonstrated for solid polymer
ionomer sensor cells. Finally, the sensor cells are easy to
maintain, and so are ideal for use in remote, unattended
environments. Regular addition of water to the reservoir in the
sensor housing every several months and monthly calibration checks
are the only requirements.
[0017] One of the concerns with the state-of-the-art sensors
described above is that the signal-to-noise ratio is not conducive
to detection of very low concentrations (parts per billion, ppb) of
important environmental and biomedical gases and vapors. Response
time is relatively slow, and reproducibility between sensors and
sensor cells is not high. Also, they are relatively costly.
[0018] Recently, miniaturized thick and thin film type sensors have
been developed where the solid ionomer membrane acts as a conduit
between the gas to be detected (sample gas) and the sensing
electrode. The sample gas permeates through the membrane itself
where a 3-phase contact area is established. The concern with this
configuration is that the solid ionomer membrane water content
controls the gas permeation rate as well as proton conductivity. As
the humidity increases, the membrane water content increases. This
causes an increase in the gas diffusion rate as well as proton
conductivity and sensor signal response. The best method of
controlling or fixing the water content of the membrane is to have
a water reservoir on the back side of the membrane, directly
opposite to where the film type electrodes and non-conductive
supportive substrate are located, however other configurations
positioning the water reservoir on the front side of the membrane
are possible. Unfortunately in the back side configuration the back
side of the membrane is required to be free of liquid so that the
sample gas can diffuse through the membrane to the sensing
electrode.
[0019] Another concern of the state-of-the-art sensors is flooding
of the electrode surfaces caused during the fabrication of the
sensors. Flooding causes the formation of liquid droplets on the
electrode surface and results in decreased sensor sensitivity after
assembly. Electrochemical sensor arrangements where an electrode
lies immediately adjacent to a hydrated ionomer membrane are prone
to flooding. The propensity to flood is further increased with the
thickening of the electrode; hence thick film electrodes are more
prone to flooding than thin film electrodes.
[0020] The propensity of electrode flooding is further increased by
the common use of Nafion.RTM. as the ionomer membrane of choice. A
perfluorosulfonic acid membrane is defined as a polymer that
contains small proportions of sulfonic or carboxylic ion functional
groups. Nafion.RTM. is typically cleaned extensively by boiling in
water to remove impurities. The use of wet Nafion.RTM. in the
manufacturing process results in the formation of liquid droplets
on the electrode; hence sensors are formed with decreased
sensitivity.
[0021] Typically, sensors with flooded electrodes need to be
flushed with dry gas for extended periods greater than 24 hours in
order to regain their optimal response rate. One typical embodiment
of this invention solves the problems associated with wet
Nafion.RTM. use by using a dry Nafion.RTM. sheet in the production
process. This dry Nafion.RTM. sheet is obtained in hygroscopic form
and has not been boiled, soaked in any liquid, or otherwise treated
(i.e. equilibrated in an acidic solution).
[0022] The best method for hydrating the ionomer membrane such as
Nafion.RTM. would be to have a water reservoir located adjacent to
the membrane, and opposite to where the film type electrodes are
located. These reservoirs can contain a water seal, which may be
broken anytime after assembly in order to release water and hydrate
the ionomer membrane. Providing an orifice in the sensor housing
with a cap enables refillable reservoirs.
[0023] Another problem associated with the use of wet Nafion.RTM.
in electrochemical sensors is that wet parts are difficult to work
with. Therefore, making electrochemical sensors with dry
Nafion.RTM. decreases the difficulty of handling materials during
the manufacturing process.
[0024] Yet another problem associated with the use of wet
Nafion.RTM. in the manufacturing of electrochemical sensors is that
the wet parts may result in sensors with a varying amount of
sensitivity from one another. Hence, using dry Nafion.RTM. provides
a means of obtaining more uniform results in sensor
reproducibility.
[0025] The present invention overcomes the limitations of the
state-of-the-art in miniaturized electrochemical sensors stated
above by uniquely combining a dry ionomer membrane configuration
with a thick or thin film type electrode on a non-conductive
supportive substrate. The substrate may have diffusion openings or
holes having a known area, which permit easy access of the sample
gas to a sensing electrode contact area. The sensor configuration
provides a three phase contact area that serves as an interface for
the ionomer membrane, the electrodes, and the gas being detected.
This design utilizes the precision of solid-state device
fabrication techniques to yield inexpensive, low maintenance,
highly sensitive, rapidly responsive, and reproducible sensor
devices for environmental, industrial, and biomedical
monitoring.
SUMMARY OF THE INVENTION
[0026] This invention is directed toward a controllable and
reproducible gas sensor configuration having a three-phase contact
area, whereby the sample gas diffuses to the sensing electrode and
membrane through openings, holes or slits that extend through the
non-conductive supportive substrate.
[0027] This invention is further directed toward a gas sensor where
the gas diffusion process is decoupled from the proton conduction
process. The gas diffusion is controlled only through openings of
known area in the substrate or in the substrate and an additional
rate limiting gas diffusion barrier film or polymer layer, eg:
polyethylene or Nafion.RTM. film, while proton conduction takes
place only through an electrolyte layer, e.g., a Nafion.RTM.
membrane. An important distinction to be made in this invention is
that the gas diffusion barrier film or polymer layer may be
manufactured using wet or dry Nafion.RTM. film, however, the novel
Nafion.RTM. ionomer membrane must be dry hygroscopic Nafion.RTM. in
order to effectively prevent flooding of the electrode.
[0028] The invention is also directed toward utilizing a method of
mass-producing film type gas sensors by stacking a number of
component layers to form a series of adjacent sensors which are
subsequently separated into individual sensors.
[0029] The invention is still further directed toward a gas sensor
utilized in conjunction with a gas sensor control circuit.
[0030] The invention is also directed toward a gas sensor utilized
in a gas-sensing instrument.
[0031] The invention is still further directed toward a gas sensor
that combines a dry form solid polymer ionomer membrane
configuration with a thick or thin film type electrode on a
non-conductive supportive substrate.
[0032] The invention is also directed toward a gas sensor where a
few hours of storage time are required to make the sensor
operational upon complete assembly.
[0033] The invention is still further directed toward a sensor with
increased consistency in reproducibility of sensitivity levels
within the manufacture of a batch of sensors.
[0034] The invention is still further directed towards
manufacturing a sensor where upon completion of assembly the
electrodes remain dry.
[0035] The invention is still directed towards a method for making
electrochemical gas sensors by providing a dry ionomer membrane.
This dry ionomer membrane may be Nafion.RTM..
[0036] This invention is directed towards a method for making
electrochemical sensors by providing a dry ionomer membrane, and a
substrate that has holes through its surface and an electrode layer
adjacent to the holes. Holes may be punched in the ionomer membrane
so that the ionomer membrane can be mated to the substrate such
that the holes of the ionomer membrane and holes in the substrate
line up to form a gas inlet where the gas is able to contact the
working electrode.
[0037] It is the object of the present invention to provide a
method of making an electrochemical sensor for the detection of an
analyte in a gas sample by providing a dry ionomer membrane free
from liquid droplets; providing a substrate with at least one
opening through its surface and a first electrode layer adjacent to
the opening; connecting the dry ionomer membrane to the substrate;
aligning the hole in the dry ionomer membrane with the opening in
the substrate for defining a gas passage; and depositing a second
electrode on the substrate for operatively connecting the ionomer
membrane, the first electrode, the second electrode, and an analyte
of interest. In certain embodiments, the dry ionomer membrane
further includes a hole aligned with the sensing electrode for
defining a gas passage.
[0038] It is the object of the present invention to provide a
method of making an electrochemical sensor for the detection of an
analyte in a gas sample by providing a dry ionomer membrane which
is a dry sheet of Nafion.RTM..
[0039] It is the object of the present invention to provide a
method of making an electrochemical sensor for the detection of an
analyte in a gas sample by making the sensor cell prior to wetting
said dry ionomer membrane.
[0040] It is the object of the present invention to provide a
method of making an electrochemical sensor for the detection of an
analyte in a gas sample by positioning a polymer layer upon the
sensing electrode for defining the diffusion rate at which the gas
is moving through the inlet onto a surface of the sensing
electrode.
[0041] It is the object of the present invention to provide a
method of making an electrochemical sensor for the detection of an
analyte in a gas sample wherein providing a substrate further
includes positioning a counter electrode in contact with the dry
ionomer membrane such that upon wetting the dry ionomer membrane
the counter electrode provides an electrical connection to the
ionomer membrane so current may be applied to said sensing
electrode.
[0042] It is the object of the present invention to provide a
method of making an electrochemical sensor for the detection of an
analyte in a gas sample wherein the step of providing a substrate
further includes positioning a reference electrode in contact with
the dry ionomer membrane such that upon wetting the dry ionomer
membrane a reference point is created against which the potential
of other electrodes can be measured.
[0043] It is the object of the present invention to provide a
method of making an electrochemical sensor for the detection of an
analyte in a gas sample wherein the step of obtaining a dry ionomer
membrane further includes obtaining a perfluorosulfonic acid
membrane.
[0044] It is the object of the present invention to provide a
method of making an electrochemical sensor for the detection of an
analyte in a gas sample by providing a reservoir in contact with
the dry ionomer membrane.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] FIG. 1 shows a cross-sectional view in accordance with one
preferred embodiment of the sensor cell assembly utilizing thick
film type electrodes and a hole in the ionomer membrane.
[0046] FIG. 2 shows a schematic top view of a non-conductive
supportive substrate with openings in the substrate.
[0047] FIG. 3 shows a hybrid film type electrochemical sensor cell
with Pt/Air (O.sub.2) reference, and sensing and counter
electrodes.
[0048] FIG. 4 shows a film type electrochemical sensor cell with a
polymeric gas-diffusion layer over the sensing electrode
membrane.
[0049] FIG. 5 shows a top view of a thick-film type electrochemical
sensor cell.
[0050] FIG. 6 shows a cross-section of the thick-film type
electrochemical sensor cell.
[0051] FIG. 7 shows a cross-sectional view of a hybrid sensor cell
assembly.
[0052] FIG. 8 shows a gas sensor control circuit.
[0053] FIG. 9 shows a gas sensor utilized in a gas-sensing
instrument.
[0054] FIG. 10 is a plot of current (.mu.A) vs. time (seconds)
illustrating the real time response of a sensor subjected to 1 ppm
H.sub.2S after assembly using dry Nafion.RTM. in one preferred
embodiment.
[0055] FIG. 11 is a plot of current (.mu.A) vs. time (minutes)
illustrating various real time responses and decreased
stabilization time for thin film sensors subjected to 100 ppb
H.sub.2S after assembly using dry Nafion.RTM. in one preferred
embodiment.
[0056] FIG. 12 is a plot of current (.mu.A) vs. time (seconds)
illustrating the real time response of a sensor subjected to 1 ppm
H.sub.2S at 100% relative humidity after assembly using dry
Nafion.RTM. and a one-hour storage period in one preferred
embodiment.
[0057] FIG. 13 is a flow chart illustrating the manufacturing
process for sensors of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0058] FIG. 1 shows a cross-sectional view of one preferred
embodiment of the sensor cell assembly (25). The sensor housing
(10) contains a substrate (1) and sensor chip (21). The sensor chip
is incorporated into the substrate so that it acts as a natural
extension of the substrate upon assembly. A reference electrode
(4), a counter electrode (5), and a sensing electrode (7) are
disposed on the sensor chip (21). An ionomer membrane (24), i.e.
Nafion.RTM.117 about 0.183 mm. thick, is positioned upon the sensor
chip (21) so that it is in contact with each electrode and the
substrate (1). A three phase contact area (26) is formed by gas
inlet (6), i.e. of a circular shape, about 1.0 mm in diameter
through the substrate (1) and ionomer membrane (24) immediately
adjacent the sensing electrode (7). When a sample gas enters the
housing at gas entrance (22) it travels to gas inlet (6), in order
to pass through the substrate (1) and ionomer membrane (24). At
which point the gas can enter a three-phase contact area (26) where
it contacts the ionomer membrane (24) and the sensing electrode
(7). Water reservoir (11) provides means for hydrating the dry
ionomer membrane (24) after sensor assembly. Upon hydrating the
ionomer membrane after assembly the sensor exhibits a fast response
time because the ionomer membrane (24) acts as a proton-conducting
element between the film type sensing electrode (7), reference (4),
and counter electrode (5). In certain embodiments, hydrating
ionomer membrane (24) after assembly prevents the problem of long
calibration periods as found in the state-of-the-art
electrochemical sensors due to flooding of electrodes (4), (5), and
(7).
[0059] The sensor chip (21) can be made out of any material that
one of ordinary skill in the art would use to make a sensor chip.
For example, nonconductive examples of such materials include
ceramic material, or glass. Sensor chip (21) is integrated and
incorporated into the substrate such that it acts as an extension
or continued piece of the substrate within the sensor housing (10).
Water reservoirs (11) are positioned opposite the electrodes. In
certain embodiments, the reservoirs contain a cap or seal that is
desirably broken after the manufacture of the sensor so that water
may hydrate the dry ionomer membrane (24). A reference electrode
(4) and counter electrode (5) are positioned on the substrate such
that they are in contact with the dry ionomer membrane (24), but
are not in contact with one another or the sensing electrode (7).
Each of the electrodes (4), (5), and (7) may each be manufactured
out of platinum or of any material that one of ordinary skill in
the art would use to manufacture thick or thin film type
electrodes. This illustrates one preferred embodiment of the
present invention. Other embodiments utilizing a two-electrode
structure are certainly possible. Also, in certain embodiments,
housing (10) includes an orifice or hole such that water reservoir
(11) can be refilled with solution.
[0060] In FIG. 1, thick film type electrodes (4), (5) and (7) are
deposited on a sensor chip (21). These electrodes may be arranged
as one of skill in the art typically deposits electrodes on a
substrates or sensor chips. For example, flag-type, dot-type, and
band-type film embodiments known in the art are all possible.
[0061] Making the opening in the ionomer membrane can be performed
by any method one of ordinary skill in the art would use to make a
hole or slit in the membrane. For example, a simple punch apparatus
may be used to punch a hole into a dry Nafion.RTM. sheet. Next, the
punched ionomer sheet will be positioned on sensor chip (21) such
that the punched holes are disposed on the sensing electrode
(7).
[0062] FIG. 2 shows the top view of another preferred embodiment of
the invention having a ceramic film type substrate (1) (e.g.,
alumina) having holes (2) uniformly distributed in parallel rows.
This embodiment relates to hybrid film type sensors. The distance
between the holes in the parallel rows and the distance between the
rows determine the dimensions of the sensor. The holes are ideally
punched in a single step, while the alumina plate is still soft, in
the "green" stage of substrate fabrication, prior to
high-temperature sintering. Other techniques to create the holes
include laser ablation or use of soluble fillers.
[0063] Using screen-printing or lithographic techniques, conducting
leads (3) and thick- and thin-film electrodes are formed on the
non-conductive substrate (1) for multiple electrodes. A typical
hybrid sensor design utilizing this method is shown in FIG. 3,
which has a single reference electrode (4) (e.g., Pt/Air (O.sub.2)
electrode) and a Pt counter electrode (5). The contact for the
sensing electrode (27) is a ring concentric to the hole. This ring
can be made of smooth, rough or platinized platinum. Some
platinization may provide better contact. Simultaneous
platinization of electrodes can be performed by customized
electrolytic plating on properly masked multi-sensor plates.
[0064] The sensing or working electrode (7) may be a disc of
Teflon.RTM.-bonded or Nafion.RTM.-bonded platinum or other
electrocatalyst. In one preferred hybrid sensor embodiment a number
of discs are deposited on an ionomer film, such as Nafion.RTM.
electrolyte membrane (8) at uniform distances from each other, for
instance, by decal transfer, silk printing, spray painting, artist
brush lettering, or by any approach which lends itself to uniform
deposition of a design on a transfer substrate without waste. The
discs' distances from center to center are the same as for the
holes of FIG. 2. The diameter of the sensing or working electrode
disc is somewhat larger than the diameter of the hole in FIG. 2 to
allow for contact between the disc and the sensing electrode
support ring (27) of FIG. 3. Instead of a single large hole per
sensor of FIG. 2 (which requires the use of the substrate to
control diffusion of the analyte), a series of smaller openings may
be used, with small enough diameters to control diffusion
independently of the analyte flow. The areas of the openings are
chosen so as to control diffusion of the sample gas toward the
sensor and to maintain a constant diffusion rate independent of any
changes in the sample gas flow rate. By using a number of these
diffusion-controlling orifices, a reasonably large signal may be
maintained.
[0065] Over the empty alumina surface (the surface with no printed
leads and electrodes) a gas-permeable diffusion film (9) is
deposited in one configuration of the invention. This film is made
to conform to the sensor electrode over the holes (as shown in FIG.
4), or hangs loose over the (sensor) sensing electrode (7). The
substrate (with multiple arrays of printed conductors), the
Nafion.RTM. membrane (with multiple sensing electrode discs), and
the gas-permeable film are arranged as shown in the schematic
representation of FIG. 4. After all the components are unitized,
the resulting structure is cut in individual sensor units. This
gas-permeable diffusion film (9) can also be disposed upon
electrode (7) as shown in FIG. 1 as another embodiment of the
present invention.
[0066] An additional advantage of this structure as shown in FIG.
(4) is that it allows for a water reservoir (11) over the
Nafion.RTM. membrane (8) on the opposite, or back side from where
the sensing electrode is located as shown in FIG. 7.
[0067] A schematic drawing of the sensor cell assembly of this
invention is shown in FIG. 5. In a preferred embodiment of this
invention a hole of approximately 80 mils (0.080 in) is formed in a
film type substrate (1) and sensing electrode contacts (27), and Pt
counter (5) and reference electrodes (4) are then deposited on the
substrate (1) surface as shown in FIG. 5. In an alternative
embodiment of this invention the hole (2) is drilled directly
through the non-conductive substrate and integral sensing electrode
contact structure. As a result the sample gas has direct contact
through the substrate hole (2) with the sensing electrode as shown
in FIG. 6. This film type substrate structure is mounted in a
sensor housing (10) as shown in FIG. 7 with a solid ionomer
membrane (Nafion.RTM. 117). The Pt sensing electrode support ring
(27) (with hole in center) and solid counter (5) and reference (4)
electrodes are compressed tightly against the Nafion.RTM. membrane
(8). The fixture as shown in FIG. 7 has a water reservoir (11) on
the opposite side of the membrane from where the electrodes are
located. The reservoir (11) is filled with distilled water and wets
the membrane, thus fixing and controlling the water content of the
membrane and electrode assemblies. The reservoir (11) is sealed
with a seal or cap (20).
[0068] The hybrid film type sensor configuration from above is
integrated with a potentiostat and a voltage of approximately +0.1
V is applied to the Pt sensing electrode with respect to a Pt/Air
(O.sub.2) reference. This corresponds to an applied potentiostatic
voltage of approximately 1.16 V with respect to a normal hydrogen
electrode (NHE).
[0069] Gas samples of air and 7.4 ppm SO.sub.2 in air are
introduced into the sampling port of the fixture described above.
The gas flow is approximately 60 cm.sup.3/min and temperature is
approximately 25.degree. C. The sample gas diffuses through the
80-mil hole in the non-conductive substrate and electrochemically
reacts at the exposed sensing electrode/solid ionomer electrolyte
surface. Humidification is provided by the liquid water in the
reservoir which soaks the opposite, or back side of the membrane as
to where the electrode structures are located.
[0070] The background response signal with air is 30 nanoamps (nA).
The response signal with 7.4 ppm SO.sub.2 in air is 135 nA. This
corresponds to a net response signal for 7.4 ppm SO.sub.2 in air of
105 nA or 14.2 nA/ppm per 80-mil hole. It is possible to increase
the magnitude of signal and signal-to-noise ratio by increasing the
number of holes in the substrate above the integral sensing
electrode structure.
[0071] It is also possible, with this configuration, to detect
other oxidizable or reducible gases such as CO, NO, NO.sub.2,
H.sub.2S, ozone, CO.sub.2, hydrogen, hydrazine, ammonia, HCl,
alcohols and acetone.
[0072] Referring to FIGS. 8 and 9, a block diagram of the sensor
control circuit (13), which can be used with any embodiment of the
present invention is shown. The sensor control circuit (13) is
designed to: 1) control the potential of the sensing electrode (7)
at a predetermined voltage (the "potentiostatic voltage", or
"E.sub.pot"); 2) measure the temperature; 3) convert the gas
concentration-related current to a temperature-compensated voltage
signal; and 4) provide properly amplified voltage to the data
acquisition/storage microprocessor (14). An on-board micro
power-regulated power supply (16) uses the microprocessor's (14)
power supply to provide the required .+-.3.9 volts for the sensor
circuitry. The DC power can be supplied by a 6-V battery (16d) or
an AC adaptor (16e).
[0073] The control amplifier portion (17b) of the sensor control
circuit (13) consists of a micro power operational amplifier (e.g.,
MAX407 or LM6062). The sensing (7), counter (5) and reference (4)
electrode portions of the sensor assembly (25) are in the feedback
loop of the control amplifier (17b) as shown in FIG. 8, a standard
configuration for potentiostat circuits. An adjustable voltage
divider (17a) allows the polarizing voltage (E.sub.pot) to be set
at a predetermined voltage range such as 0 to 50 mV. This signal is
compared to the reference electrode (4) voltage (which appears with
it at the summing junction) by the control amplifier (17b) of the
sensor control circuit (13). The latter adjusts the current through
the sensor cell (10) to minimize the difference between the
E.sub.pot and the reference electrode (4) voltages.
[0074] The resulting sensor cell assembly (19) current (flow of
electrons from sensing electrode (7) to counting electrode (5),
which is linearly related to the concentration of gas, is
transformed into a voltage signal by the current-to-voltage
converter (15a). Temperature compensation of the sensor signal is
effected in the next stage of amplification (15b) using a
thermistor (18a) which is positioned in the gas sensor housing
(10). The last stage of amplification (15c) provides the necessary
inversion of the voltage signal as well as gain adjustment, to
permit calibration for normal variations in sensitivity among
sensors. The same type of micro power operational amplifier is used
for these stages (15a), (15b), (15c) as for the control amplifier
(15b). The transformed current signal is directed to an A/D channel
on the data acquisition board of the microprocessor (14).
[0075] Power for the sensor control circuit (13) is provided by a
Duracell 6-V battery (16d) (PX 28A or 28L) through a micro
power-regulated power supply (16). The power supply (16) utilizes a
voltage inverter (e.g., ICL 7660) (16a) to convert the positive
battery voltage to a negative voltage of the same magnitude, and a
positive voltage regulator (e.g., MAX663) (16c) and negative
voltage regulator (e.g., MAX 664) (16b) to provide a stable .+-.3.9
volts.
[0076] The film type gas or vapor-sensing instrument (12), as shown
in FIG. 9, includes the sensor cell assembly (19),
potential-control circuitry (13), and the microprocessor (14) with
the data acquisition-recording unit. The sensing instrument (12) is
preferably battery operated, and has the ability to sample the gas
or vapor and temperature signals at intervals and store in the
random access memory (RAM) on the data acquisition board days to
weeks of data. The data acquisition circuit microprocessor is
programmed to sample and store the gas concentration signals at
preset intervals. Data are off-loaded to a personal computer by
accessing the microprocessor through an RS232 port.
[0077] The sensor cell assembly (19) and its potential-control
circuit (13) are integrated with a battery-operated microprocessor
(14) of 32K memory, which samples the sensor signal as well as
temperature and other signals at 10-, 20-, or 30-second intervals
and stores an average value at intervals of 2, 5, or 10 minutes
according to a programmable protocol. The data acquisition/storage
unit in the microprocessor (14) can record 8 days of data, storing
at 2-minute intervals, or up to 40 days storing at 10-minute
intervals. In clinical testing to date, a 2-minute interval is
suitable for one-day clinical studies and a 10-minute interval is
appropriate for extended use. The microprocessor (14) with data
acquisition/logic circuit can be programmed to sample more than one
analog signal from the control circuit (13), and to convert these
to digital signals and store them (i.e., gas concentration and
temperature) at preset intervals together with real-time data. Data
are off-loaded to a personal computer by accessing the
microprocessor (14) through an RS232 port. After downloading, the
digital data are converted to engineering units of gas
concentration and temperature, and can be graphed by a menu-driven
Lotus.RTM. 123 spreadsheet. Through a potentiometer in the gain
amplifier circuit (15c), the device can be calibrated with
calibrated gas samples, to indicate gas concentrations in the
ambient. The potential-control circuit (13) shown in FIG. 8 is
powered, in a preferred embodiment, by six 11/2-volt AA-size
batteries (16d). Atypical microprocessor (14) with data
acquisition-recording capability that has been successfully used is
sold by ONSET Computers, Falmouth, Mass., under the product name of
"Tattletale Lite.RTM.." The sensor cell assembly (19) with its
control circuit (13) is also designed to yield a current or voltage
signal proportional to gas flux that could be used to continuously
transmit the data to a remote receiving device or central
monitoring station or unit. It should be known that the embodiment
depicted in FIG. 8 is for exemplary purposes and is not a sole
limitation of the invention.
[0078] The sensing electrodes can be organized in multiple arrays
or sets containing a necessary number of counter or reference
electrodes. Reference electrodes such as Pt/air (O.sub.2),
PtO.sub.2, or dynamic hydrogen electrode known in the art may be
employed. Electrically driven 3- or 2-electrode film type
configurations may be employed using potentiostatic,
potentiodynamic or potential control. Two-electrode configurations
require a reversible or stable counter-reference electrode such as
Pt/air (O.sub.2), PtO.sub.2 or Pt/H.sub.2 which has a higher BET
(Brunauer, Emmett, Teller) surface area (25 m.sup.2/g or larger)
and/or larger geometric surface areas than the sensing
electrode.
[0079] Electrochemically reversible electrodes may be used in 3 or
2 electrode configurations, but especially in a 2-electrode
arrangement where the counter electrode also acts as a reference
electrode. Electrochemically reversible electrodes are constructed
of stable catalyst materials and usually have a relatively large
electrochemical active surface area so that they remain stable and
their potential is not perturbed by small current flow. Examples
include PtO.sub.2 and Ag/AgCl electrodes.
[0080] A real time plot of current (.mu.A) vs. time (seconds)
illustrating the response of a sensor subjected to 1 ppm H.sub.2S
after assembly using dry Nafion.RTM. as shown in FIG. 10,
demonstrates that the sensor prepared with dry Nafion.RTM. responds
within two minutes to 90% of a stable response. The background
response signal with air is 10 nanoamps (nA). The response signal
with 1 ppm H.sub.2S in air is 35 nA. This corresponds to a net
response signal for 1 ppm H.sub.2S in air of 25 nA.
[0081] FIG. 11 shows a plot of current (.mu.A) vs. time (minutes)
illustrating various real time responses and decreased
stabilization time for thin film sensors subjected to 100 ppb
H.sub.2S after assembly using dry Nafion.RTM.. The plot
demonstrates that the sensor prepared with dry Nafion.RTM. responds
within two minutes to 90% of a stable response.
[0082] FIG. 12, is a plot of current (.mu.A) vs. time (seconds)
illustrating the real time response of a sensor subjected to 1 ppm
H.sub.2S at 100% relative humidity after assembly using dry
Nafion.RTM. and a one-hour storage period. The graph shows that
sensors prepared using dry Nafion.RTM. respond to 90% of a stable
response, which is reached within a few minutes of assembly.
[0083] FIG. 13 is a flow chart illustrating the manufacturing
process for sensors of the present invention. The sensors of the
present invention are manufactured using known techniques for
manufacturing electrochemical sensors, including method of
mass-producing film type gas sensors by stacking a number of
component layers to form a series of adjacent sensors which are
subsequently separated into individual sensors. The component
layers are stacked by depositing a second substrate (28) on top of
a completely manufactured sensor.
[0084] FIG. 13 depicts a method of making an electrochemical sensor
for the detection of an analyte (36) in a gas sample (37) by
providing (29) a dry ionomer membrane free from liquid droplets;
providing (38) a substrate with at least one opening through its
surface and a first electrode layer adjacent to the opening;
forming (40) at least one opening in the substrate; forming (30) at
least one hole in the dry ionomer membrane; connecting (31) the dry
ionomer membrane to the substrate; aligning (32) the hole in the
dry ionomer membrane with the opening in the substrate for defining
(39) a gas passage; and depositing (33) a second electrode on the
substrate for operatively connecting the ionomer membrane, the
first electrode, the second electrode, and an analyte of interest.
The figure also illustrates providing (34) at least one reservoir
into the sensor, and the housing of the sensor. The reservoir is
filled (35) with a liquid prior to initial use of the sensor.
Preferably the liquid used in the reservoir of the present
invention is water. FIG. 13 also illustrates the step of optionally
depositing (41) an additional electrode on the sensor substrate.
The additional electrode can be a reference electrode.
[0085] FIG. 13 illustrates the novel step of providing (29) a
membrane. In particular the membrane provided is a dry ionomer
membrane, which is free from liquid droplets. Although Nafion.RTM.
is a preferred ionomer for use in such sensors, it is not the only
solid electrolyte polymer available. For instance, both anionic and
cationic polymer membranes can be incorporated into electrochemical
sensors of this type. Similar ionomers that are part of a class of
solid polymeric ion exchangers that conduct ions upon exposure to
water are available. Known examples include membranes made from
polystyrene with fixed negative sites (sulfonate, carboxylate or
phosphonate) or fixed positive sites (quaternary ammonium or
quaternary phosphonium). Other examples include Neosepta
manufactured by the Tokuyama Corporation, Selemion.TM. of Asahi
Glass, Sybron Ionac, Morgane, PCA, RAI, Gor-Tex.RTM. in ionic form
and others.
[0086] As described above, the dry ionomer membranes can also be
defined as those ionomer membranes that are hygroscopic.
Hygroscopic membranes are those membranes that readily absorb or
attract moisture from the air; or membranes having an affinity for
moisture. One such example is Nafion.RTM. 117 perflourinated
membrane manufactured and sold by E.I. du Pont de Nemours and
Co.
[0087] FIG. 13 also illustrates the step of forming (30) at least
one hole in the ionomer membrane. Making the opening in the ionomer
membrane can be performed by any method one of ordinary skill in
the art would use to make a hole or slit in the membrane. For
example, a simple punch apparatus may be used to punch a hole into
a dry Nafion.RTM. sheet. The diameter of the punch can easily be
changed to change the hole size. Preferably the hole would be about
1.0 mm in diameter. A substrate as described above is also
provided. Forming (30) one or more openings in the substrate can be
performed, as one of ordinary skill in the art would form an
opening. The substrate may be punched or cut to contain holes.
These holes are typically about the same size as the hole in the
ionomer membrane.
[0088] The foregoing description of various embodiments of the
invention have been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and obviously many
modifications and variations are possible in light of the above
teaching. The embodiments were chosen and described in order to
best explain the principles of the invention and its practical
application to thereby enable others skilled in the art to best
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto.
* * * * *