CA1279896C - Eletrochemical micro sensor - Google Patents
Eletrochemical micro sensorInfo
- Publication number
- CA1279896C CA1279896C CA000567772A CA567772A CA1279896C CA 1279896 C CA1279896 C CA 1279896C CA 000567772 A CA000567772 A CA 000567772A CA 567772 A CA567772 A CA 567772A CA 1279896 C CA1279896 C CA 1279896C
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/404—Cells with anode, cathode and cell electrolyte on the same side of a permeable membrane which separates them from the sample fluid, e.g. Clark-type oxygen sensors
- G01N27/4045—Cells with anode, cathode and cell electrolyte on the same side of a permeable membrane which separates them from the sample fluid, e.g. Clark-type oxygen sensors for gases other than oxygen
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- Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
- Measurement Of The Respiration, Hearing Ability, Form, And Blood Characteristics Of Living Organisms (AREA)
- Electric Double-Layer Capacitors Or The Like (AREA)
- Primary Cells (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A micro-amperometric electrochemical sensor for detecting the presence of a pre-determined species in a fluid material is disclosed. The sensor includes, a smooth substrate having a thin coating of solid electrolytic material deposited thereon. The working and counter electrodes are deposited on the surface of the solid electrolytic material and adhere thereto. Electrical leads connect the working and counter electrodes to a potential source and an apparatus for measuring the change in an electrical signal caused by the electrochemical oxidation or reduction of the species. Alternatively, the sensor may be fabricated in a sandwich structure and also may be cylindrical, spherical or other shapes.
A micro-amperometric electrochemical sensor for detecting the presence of a pre-determined species in a fluid material is disclosed. The sensor includes, a smooth substrate having a thin coating of solid electrolytic material deposited thereon. The working and counter electrodes are deposited on the surface of the solid electrolytic material and adhere thereto. Electrical leads connect the working and counter electrodes to a potential source and an apparatus for measuring the change in an electrical signal caused by the electrochemical oxidation or reduction of the species. Alternatively, the sensor may be fabricated in a sandwich structure and also may be cylindrical, spherical or other shapes.
Description
~2''7~6 EL~clr~cr~ L~
FIELD ~F TH~ VENrIVI~l Tne invention relates to electroci~elnic~l apparatus for sensing the presence of a species ln fluid material. More particularly the invelltion relates to an improved apparatus for yeneratillg a electric siynal in response the the presellce of a predetermined species in a fluid mdterial.
BACKJ~OUND OF THE INVENTION
Electrocl~emical sensors for the detection ~f tne presence of a species in a fluid material have existed for quite some tirne. Sucn sensors include the Clark cell described in ~.S. Patent No. 2,913,38~
issued Noveln~er 17, 19590 The apparatus disclosed in tha~ patent utilizes a dual electrode structure immersed in an electrolyte and encase~ at least in part in a membrane whicn is permeable to a predeter~nined species. In operation such a device allows the permeation of tne species to be detected tl~rough the memDrane and reduces said species at tne cathode. At the same time the anode is oxidized as a result of the electrical and ionic connections between the anode and cathode. These oxidation an~
reduction reactions generate a current Wi~iCh is measurable and is proportional to the concentration of the species 3eing detected. The Clark cell is a large bulky apparatus and rnust include a liquid electrolytic medium in wnicn the electrodes are i~nrnersed. The Clark apparatus suffers ~roln sever~L
disadvantages including consumption of the species being detected during detection, slow response tilnes and alterdtion of the electrolyte duriny detection.
_._ Some of the above-mentioned disadvantages of the Clark-type electrode cell are avoided by apparatus of the type described in UOS. Patent No. 3,260,656 issued on July 12, 1966 to James W. Ross, Jr. The Ross apparatus utilizes a sandwich comprising a cathode and an anode with a spacer therebetween.
This sandwich is immersed in an electrolyte and is geometrically oriented so that the electrodes are parallel to a membrane whlch is permeable to the species being measured. The membrane combines with a housing to enclose the cathode-anode combination in an electrolyte. In the Ross-type cell the species being measured is consumed at one electrode and regenerated at the other electrode such that no net consumption of the species being detected oc~curs. Therefore -the Ross sensor does not consume the species being measured as a result of the electro-chemical reaction of that species with the electrodes. rihereas the Ross cell effectively overcomes the problems of alteration of the electrodes and/or electrolyte, depletion of the species from the test fluid, and extension of the depletion layer into the test fluid causing stirring and fouling dependence, certain other shortcomlngs are still evident. Among them is the fact that readings with the Ross-type cell, obtained by measuring the current flow between the electrodes, tend to stabilize within a maximum of one minute in accordance with the Ross patent. It has been found that response times of this order are not suit-able for many applications. A further disadvantage is that the diffusion layer thickness in the Ross cell is determined by the interelectrode distance which is subject to variation as the assembly is stressed by forces arising from temperature and/or pressure variations. Yet another disadvantage is the cumbersome nature of the layered structure making reliable fabrication of Ross-type devices difficult.
Yet another apparatus for electrolytically detecting a species in a fluid is described in ~.S. Patent 4,076,596 issued to Connery et al. on February 28, 1978. The apparatus of Connery et alO inc]udes an insulating substrate and a plurality of fingerlike electrodes deposited on the surface of the sub-strate in a closely spaced interleaved geometric pattern. The electrodes are covered with a -thin film of electrolyte and a permeable membrane. The electrolyte is selected so that the species being measured is generated at one electrode and con-sumed at the other with no net consumption of the species being detected. The Connery et al. apparatus may include a solid electrolyte deposited on the electrodes. While the Connery et al. apparatus eliminates some of the problems of the Ross-type cell it has several disadvantages of its own. One primary dis-advantage of the Connery et al. apparatus is that the solid electrolyte is deposited on top of the electrodes. The elec-trodes form an irregular surface having high points where the electrodes are present and valleys at the spaces between the electrodes. This makes it difficult to deposit a solid electro-lyte coating which will be smooth, consistent, homogeneous and adhere to the electrodes. In addi-tion, the coating of electro-lyte will be distorted by changes in numidity and temperature because of the irregular surface upon which it is coated.
Another problem with the Connery et al. apparatus is that its response times may be too slow for some applications. This results because of the electrolytic resistance of the electro-lyte which forms a barrier between the electrodes and the test ~2798~
fluid. As a result, the species must ~if~use throu~h the electrolyte prior to contacting tne electtodes.
Since the Connery et al. electrolyte is coate~ onto an irregular surface tne electrolyte Inust De thicker than if it were coated on a flat suLface to accomplish d complete coating. Accordingly the electrolytic resistance ~ e lo~er ~ut diffusioo will be slower and can significantly slow response times.
It is tne primary ooject of tne present invention to provide an apparatus for electrolytically detecting a species in a fluid materidl~ whici) nas smooth electrolyte coatings with yood repeatability.
It is a further object of tne uresent invention to provi~e a solid electrolyte layer having excellent adherence to the electrodes.
It is a still further object of t~le present invention to provide a tninner electrolyte layer to thereby reduce the diffusion resistance of the ap~?aratus.
It is a still furtner object of the present invention to provide an apparatus having a structure which ~ninimizes stresses on the electrolyte and tnereby decreases distortion of the electrolyte as a result of temperature and/or humi~ity variations.
It is a still further object of the present invention to provide an a~paratus for electrolytically detecting a species in a fluid material with a response time which is fast enough for use in application~ requiring a very fast response.
Tnese and other ob~ects of the present invention will be apparent to one of ordinary skill in the art from the summary an~ detailed descriptions whicl follow.
~LZ~g~9~
SU~MA~Y OF T~E INVENTION
The present invention relates to a solid electrochemical sensor for generatin~ an electcical signal in response to contact witll a predetermined species pr~sent in a fluid materidl comp~ising a substrate naving at least one surface, a solid electrolytic medium having first and second surfaces, said first surface of said medium being in contact with dnd adhering to said at least one surface of said substrate, a working electrode means in contact with and adhering to said second surface of said medium, an electrical power source connected for biasiny said workiny electrode means at a potential at which said species will ~e consulned at said wor~ing electrode, and a counter electrode means in contact with and adhering to said second surface of said medium and being connected to said ~ower source for completing a circuit in which a current is ca~a~le of flo~ing tnrou~h Dotn of ~ai~ elec~r~ e means as a result of the electrochemical reaction occurring at said working electrode means.
A second embodiment of the invention relates to a solid electrochemical sensor for generating an electrical signal in response to contact with a predeterlnined species present in a rluid Inaterial comprising a substrate having at least one surface, a first layer of a solid electrolytic ~nedium having first and second surfaces, said first surface of said first layer of mediuln being in contact with and adhering to said at least one surface of said substrate, a counter electrode ~nean.s in contact ~ith and adhering to said second surface of said first layer of electrolytic ~nedium, a second layer of a solid electrolytic ~ediuln having first and secon~3 surfaces, said first surface of .~aid second layer of __ ~2~9~96 medium Deing in contact with and adilering to said counter electrode means, a workiny electrvde means in contact with and adhering to said second surface o~
said second layer of electrolytic me(liu~n, an electrical power source connected for biasing said wor~iny electrode means at a pote~ ial at ~ icl~ r;ai~
species will De consumed at said working electro~D, and means for connecting said counter electrode means to said power source for completing a circuit in w~lich a curren~ is capa~le of flowing through ~oth of said electrode means as a result of the electrocllelnical reaction occurring at said workiny electrode means.
BRIEF DESCRIPTION OF T~2 DRAWI~GS
Fig. 1 is a cross-sectional view of tMe amperometric electrochemical apparatus o~ the present invention.
Fig. 2 is a top plan view of tllP ~perol,letric electrochemical sensing apparatus of the present invention.
Fig. 3 is a cross sectional view of an alternate embodiment of the amperometric electrochemical sensing apparatus of the present invention.
Fig. 4 is a cross-sectional view of a sandwich-type amperometric electrochemical sensing apparatus in accordance with the present invention.
Fig. 5 is a cross-sectional view of a cylindrical amperometric electrochemical sensing apparatus in accordance with the present invention.
DETAILED DESCRIPTION ~F TIIE P~FER ED EI~ODIMENT~
Referring now to Fig. 1 there is showll an electrochemical sensing apparatus 1~ inc~udin~ a ~279~
su~strate 11, an electrolyce 22, a counter electrode means 13 and a working electrode mearls 14. The sensor depicted in Fig. 1 is tne simplest, least expensive, as well as one of the most efficient sensors in accordance with the present invention.
Referring now to Fig. 2 which is a top plan view of the apparatus of Fig. 1 showing the fingerlike projections of the electrodes 13 and 14. Courlter electrode 13 is connected ~y way of line 15 to terminal 16 and tne working electrode 14 is connected by line J7 to the terminal 18. The electrical circuit also includes a series connected electrical power source 19 for Diasing the working electrode means 14 at a desired potential and an ammeter 20.
Referring now to Fig. 3 there is shown an alternate emDodiment of the electrochemical sensor of the present invention. Tne sensor depicted in Fig. 3 includes substrate 11 having an oxiae layer 21 on the surface thereof. Deposited on the oxide layer 21 and aaheriny to the oxide lay~r 21 is a first laye~ 25 o~
electrolytic rnedium. Deposited on the first layer 25 of electrolytic medium are tne counter electrode means 13 and the working electrvde means 14. Also deposited on the first layer 25 of electrolytic medium is a reference electrode 23 having a protective coating 24 tnereon~ Deposited on top o tne electrodes 13, 14 and the protective coating 24 is a second layer 27 of electrolytic medium.
Finally, on top of the second layer 27 of electrolytic medium is snown a selectively permea~)le memDrane 26.
Referring no~ to Fig. 4 there is depicted anotl~er alternative embodiment of the present invelltioll wherein the electrochemical sensing means is forlned in a sand~ich-type structure. This sarldwich-type structure is Duilt on a layer ~f sllDstra~e 11. rne 7g~
layer of suDstrate 11 includes an oxide layer 21 on the surface tnereof. Deposited on top of the oxide layer 21 is a first layer 25 of electrolytic medium.
Deposited on the first layer 25 of electro1ytic medium is the counter electrode mealls 13 and the reference electrode 23. ~rne referellce electrode 2~
is coated vy a protective coating 24. ~eposited on top of the counter electrode 13 and protective coating 24 is a second layer 27 of electro1ytic medium. Then, deposited on the second layer 27 of electrolytic medium is the working electrode 14 of the electrochemical sensor. Deposited on top of the working electrode means 14 is a third layer 28 of electrolytic medium which includes a selectiv~ly permeable membrane 26 thereon.
Refe~ring now to Fig. 5 there is shown yet another alternate embodiment of the present invention. Fig. 5 depicts d cross-sectional view of a cylindrical electrochemical sensor in accordance with the present invention. Tne cyli~drical electrochemical sensor includes a su~strate 31 having an oxide layer 32 on the surface thereof. On top of the oxide layer 32 is deposited a first layer 33 of electrolytic medium. On the first layer 33 of electrolytic medium is deposited a cbunter electrode means 13, a workin~ electrode means 14 and a reference electrode 23. The reference electrode 23 is coated with a protective coating 24. It will be understood that any of the alternate embodiments shown in Figs. 1-4 may be adapted to the cylindrical-shaped electrochemical sensor as well as other possible shapes such as spherical. These alternate shapes may be desirable fvr specific app1ications of the sensing device.
The substrate 11 may be made of any suitable materials to which the electrolytic medium can be ` ~2~9~
_9_ adnered. The substrate 11 is preferably an insulating Inaterial such as glass, ~uaetz, ceralllics such as alulnina, etc. and silicon. rlle su~stra~e lL
should have a thickness sufficient to assure the structural integrity o~ the sensor. Another important feature of the su~strdte 11 is that it ~--capable of adhering or being made to adhere to a coating Inaterial such as those used to fabricate electrodes and electrolytes. This is ilnportan~
because the electrodes and electrolytes must adllece to the substrate llo There are several ways to promote adl~erence of a coatin~ material to the substrate ]1. One Inethod involves oxidation of the surface of the substrate 11 to forln an oxide layer thereon. Many electrolytic materials adhere well to oxides. An additional oxide layer ~nay also be coated on the suriace of tne substrate 11 to prolnote adherence of an electrolyte thereto. Also, adhesio promotors for improviny adhesion of Nafion to glass and other silacious substrates rnay be used. Such adhesion promoters include but are not lilnited to N-(trimethoxysilylpropyl)-N,N,N-trimethyl-a~nlnoniul, chloride, octadecyltrichlorosilane, and 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt. These adhesion promoters chemically bond the electrolyte to the substrate 11 to give additional bonding strengtn. The adhesion promoters are appLied to the substrate 11 just prior to spin ooating of the electrolyte onto the substrate. Such promoters are described in Szentirmay, M.N., Calnpbell, L.F., and Martin, C.R., Silane Coupling Agents for Attaching Nafion to Glass and Silica, Anal. Chem., Vol. 5B pp.
661-66~, Mdrch 1986.
As mentioned previously, the suL~strate 11 preferably includes an oxide layer on 21 oll the ~2~7~8'~
surface tilereof to promote the adherence vf the electrolytic metiiu~n to tt~e su~strat~ ucl~ ~n oxide layer 21 Inay be treated by simple oxidation of the surface of tne suDstrate 11. For instance, a substrate 11 such as silicon can be surface oxidi~ed to produce a silicon dioxide surface COdting.
Alternatively, the oxide layer 21 may be deposited on or attached to the surface of tlle substrate 11 in any suitable manner.
The substrate 11 should have a smooth surface before and after oxidation. Such a smooth surface will prolnote smootn coatings of electrolytic medium on the substrate 11. ~oreover, a smooth surface will lead to c:onsistent and repeata~le coatings of electrolytic medium enaoling mass production of consistent sensors. Further, the smooth surface of tlle subs~.rate 11 promotes adhesion of the electrolytic mediuln to the substrate and tllereby prevents tne electrolytic medium from peeling of~ tt~e substrate 11. ~inally, the existence of a slnoott~
surface on the substrate 11 minimizes the stresses applied to the electrolytic mediuln by the substrate 11 upon exposure to varying temperature and/or humidity conditions. Tnis, in turn, will minimize the distortion of the electrolytic Inediurn as a result of these temperature and/or humidity variations The electrolytic medium of the present invention is preferably a solid material. The electrolytic medium must be capable of allowing diffusion of all reactants and products between the cathodes and anodes as well as allowing exchant~e of the measured spc~cies with the test fluid. The electrolytic mediuln must also nave satisfactory chemical, thermal and dimensional stability. Such polymer electrolytes such as poly-sulfonic acids, typically polystyrene sulfonic acid or perfluoro linear polymers sucll as --ll--those marketed under ~^a~ie-l~rk ~Nafion~ ~y Du Pont are suitable for use as the electrolytic medium of the present invelltion. In ad~ition, the electrolytic medium must be capable of a~hering not only to the substrate 11, but also to tne electrodes 13, 14 oe the electrolytic medium must be capable of Deing adhered to the substrate 11 and the electrodes 13, 14 by adhesives, adnesion prolnoters or the like.
Electrolytes of the ty~e employed in the electrolytic medium of present invention dernollstrate excellent electrolytic an~ electronic colnpati~ility with oxides such as silicon dioxide, As a result ! i t is preferable to coat SUCII electrolytes onto an oxide covered surface, The oxide layer 21 can be obtained by thermal oxidation of the semiconductor wafer substrate 11. Other substrates 11 that are already oxides may also be used, such as alumina,.sapphire, glass and polymers. In order to o~tain smooth, repeatable layers of electrolytic mediuln, the electrolyte may be spin coated using a plloto-resist spinner, onto the surface of the sunstrate 1~. Tllis process is described in Unl-t~ S-tates Patent No.
FIELD ~F TH~ VENrIVI~l Tne invention relates to electroci~elnic~l apparatus for sensing the presence of a species ln fluid material. More particularly the invelltion relates to an improved apparatus for yeneratillg a electric siynal in response the the presellce of a predetermined species in a fluid mdterial.
BACKJ~OUND OF THE INVENTION
Electrocl~emical sensors for the detection ~f tne presence of a species in a fluid material have existed for quite some tirne. Sucn sensors include the Clark cell described in ~.S. Patent No. 2,913,38~
issued Noveln~er 17, 19590 The apparatus disclosed in tha~ patent utilizes a dual electrode structure immersed in an electrolyte and encase~ at least in part in a membrane whicn is permeable to a predeter~nined species. In operation such a device allows the permeation of tne species to be detected tl~rough the memDrane and reduces said species at tne cathode. At the same time the anode is oxidized as a result of the electrical and ionic connections between the anode and cathode. These oxidation an~
reduction reactions generate a current Wi~iCh is measurable and is proportional to the concentration of the species 3eing detected. The Clark cell is a large bulky apparatus and rnust include a liquid electrolytic medium in wnicn the electrodes are i~nrnersed. The Clark apparatus suffers ~roln sever~L
disadvantages including consumption of the species being detected during detection, slow response tilnes and alterdtion of the electrolyte duriny detection.
_._ Some of the above-mentioned disadvantages of the Clark-type electrode cell are avoided by apparatus of the type described in UOS. Patent No. 3,260,656 issued on July 12, 1966 to James W. Ross, Jr. The Ross apparatus utilizes a sandwich comprising a cathode and an anode with a spacer therebetween.
This sandwich is immersed in an electrolyte and is geometrically oriented so that the electrodes are parallel to a membrane whlch is permeable to the species being measured. The membrane combines with a housing to enclose the cathode-anode combination in an electrolyte. In the Ross-type cell the species being measured is consumed at one electrode and regenerated at the other electrode such that no net consumption of the species being detected oc~curs. Therefore -the Ross sensor does not consume the species being measured as a result of the electro-chemical reaction of that species with the electrodes. rihereas the Ross cell effectively overcomes the problems of alteration of the electrodes and/or electrolyte, depletion of the species from the test fluid, and extension of the depletion layer into the test fluid causing stirring and fouling dependence, certain other shortcomlngs are still evident. Among them is the fact that readings with the Ross-type cell, obtained by measuring the current flow between the electrodes, tend to stabilize within a maximum of one minute in accordance with the Ross patent. It has been found that response times of this order are not suit-able for many applications. A further disadvantage is that the diffusion layer thickness in the Ross cell is determined by the interelectrode distance which is subject to variation as the assembly is stressed by forces arising from temperature and/or pressure variations. Yet another disadvantage is the cumbersome nature of the layered structure making reliable fabrication of Ross-type devices difficult.
Yet another apparatus for electrolytically detecting a species in a fluid is described in ~.S. Patent 4,076,596 issued to Connery et al. on February 28, 1978. The apparatus of Connery et alO inc]udes an insulating substrate and a plurality of fingerlike electrodes deposited on the surface of the sub-strate in a closely spaced interleaved geometric pattern. The electrodes are covered with a -thin film of electrolyte and a permeable membrane. The electrolyte is selected so that the species being measured is generated at one electrode and con-sumed at the other with no net consumption of the species being detected. The Connery et al. apparatus may include a solid electrolyte deposited on the electrodes. While the Connery et al. apparatus eliminates some of the problems of the Ross-type cell it has several disadvantages of its own. One primary dis-advantage of the Connery et al. apparatus is that the solid electrolyte is deposited on top of the electrodes. The elec-trodes form an irregular surface having high points where the electrodes are present and valleys at the spaces between the electrodes. This makes it difficult to deposit a solid electro-lyte coating which will be smooth, consistent, homogeneous and adhere to the electrodes. In addi-tion, the coating of electro-lyte will be distorted by changes in numidity and temperature because of the irregular surface upon which it is coated.
Another problem with the Connery et al. apparatus is that its response times may be too slow for some applications. This results because of the electrolytic resistance of the electro-lyte which forms a barrier between the electrodes and the test ~2798~
fluid. As a result, the species must ~if~use throu~h the electrolyte prior to contacting tne electtodes.
Since the Connery et al. electrolyte is coate~ onto an irregular surface tne electrolyte Inust De thicker than if it were coated on a flat suLface to accomplish d complete coating. Accordingly the electrolytic resistance ~ e lo~er ~ut diffusioo will be slower and can significantly slow response times.
It is tne primary ooject of tne present invention to provide an apparatus for electrolytically detecting a species in a fluid materidl~ whici) nas smooth electrolyte coatings with yood repeatability.
It is a further object of tne uresent invention to provi~e a solid electrolyte layer having excellent adherence to the electrodes.
It is a still further object of t~le present invention to provide a tninner electrolyte layer to thereby reduce the diffusion resistance of the ap~?aratus.
It is a still furtner object of the present invention to provide an apparatus having a structure which ~ninimizes stresses on the electrolyte and tnereby decreases distortion of the electrolyte as a result of temperature and/or humi~ity variations.
It is a still further object of the present invention to provide an a~paratus for electrolytically detecting a species in a fluid material with a response time which is fast enough for use in application~ requiring a very fast response.
Tnese and other ob~ects of the present invention will be apparent to one of ordinary skill in the art from the summary an~ detailed descriptions whicl follow.
~LZ~g~9~
SU~MA~Y OF T~E INVENTION
The present invention relates to a solid electrochemical sensor for generatin~ an electcical signal in response to contact witll a predetermined species pr~sent in a fluid materidl comp~ising a substrate naving at least one surface, a solid electrolytic medium having first and second surfaces, said first surface of said medium being in contact with dnd adhering to said at least one surface of said substrate, a working electrode means in contact with and adhering to said second surface of said medium, an electrical power source connected for biasiny said workiny electrode means at a potential at which said species will ~e consulned at said wor~ing electrode, and a counter electrode means in contact with and adhering to said second surface of said medium and being connected to said ~ower source for completing a circuit in which a current is ca~a~le of flo~ing tnrou~h Dotn of ~ai~ elec~r~ e means as a result of the electrochemical reaction occurring at said working electrode means.
A second embodiment of the invention relates to a solid electrochemical sensor for generating an electrical signal in response to contact with a predeterlnined species present in a rluid Inaterial comprising a substrate having at least one surface, a first layer of a solid electrolytic ~nedium having first and second surfaces, said first surface of said first layer of mediuln being in contact with and adhering to said at least one surface of said substrate, a counter electrode ~nean.s in contact ~ith and adhering to said second surface of said first layer of electrolytic ~nedium, a second layer of a solid electrolytic ~ediuln having first and secon~3 surfaces, said first surface of .~aid second layer of __ ~2~9~96 medium Deing in contact with and adilering to said counter electrode means, a workiny electrvde means in contact with and adhering to said second surface o~
said second layer of electrolytic me(liu~n, an electrical power source connected for biasing said wor~iny electrode means at a pote~ ial at ~ icl~ r;ai~
species will De consumed at said working electro~D, and means for connecting said counter electrode means to said power source for completing a circuit in w~lich a curren~ is capa~le of flowing through ~oth of said electrode means as a result of the electrocllelnical reaction occurring at said workiny electrode means.
BRIEF DESCRIPTION OF T~2 DRAWI~GS
Fig. 1 is a cross-sectional view of tMe amperometric electrochemical apparatus o~ the present invention.
Fig. 2 is a top plan view of tllP ~perol,letric electrochemical sensing apparatus of the present invention.
Fig. 3 is a cross sectional view of an alternate embodiment of the amperometric electrochemical sensing apparatus of the present invention.
Fig. 4 is a cross-sectional view of a sandwich-type amperometric electrochemical sensing apparatus in accordance with the present invention.
Fig. 5 is a cross-sectional view of a cylindrical amperometric electrochemical sensing apparatus in accordance with the present invention.
DETAILED DESCRIPTION ~F TIIE P~FER ED EI~ODIMENT~
Referring now to Fig. 1 there is showll an electrochemical sensing apparatus 1~ inc~udin~ a ~279~
su~strate 11, an electrolyce 22, a counter electrode means 13 and a working electrode mearls 14. The sensor depicted in Fig. 1 is tne simplest, least expensive, as well as one of the most efficient sensors in accordance with the present invention.
Referring now to Fig. 2 which is a top plan view of the apparatus of Fig. 1 showing the fingerlike projections of the electrodes 13 and 14. Courlter electrode 13 is connected ~y way of line 15 to terminal 16 and tne working electrode 14 is connected by line J7 to the terminal 18. The electrical circuit also includes a series connected electrical power source 19 for Diasing the working electrode means 14 at a desired potential and an ammeter 20.
Referring now to Fig. 3 there is shown an alternate emDodiment of the electrochemical sensor of the present invention. Tne sensor depicted in Fig. 3 includes substrate 11 having an oxiae layer 21 on the surface thereof. Deposited on the oxide layer 21 and aaheriny to the oxide lay~r 21 is a first laye~ 25 o~
electrolytic rnedium. Deposited on the first layer 25 of electrolytic medium are tne counter electrode means 13 and the working electrvde means 14. Also deposited on the first layer 25 of electrolytic medium is a reference electrode 23 having a protective coating 24 tnereon~ Deposited on top o tne electrodes 13, 14 and the protective coating 24 is a second layer 27 of electrolytic medium.
Finally, on top of the second layer 27 of electrolytic medium is snown a selectively permea~)le memDrane 26.
Referring no~ to Fig. 4 there is depicted anotl~er alternative embodiment of the present invelltioll wherein the electrochemical sensing means is forlned in a sand~ich-type structure. This sarldwich-type structure is Duilt on a layer ~f sllDstra~e 11. rne 7g~
layer of suDstrate 11 includes an oxide layer 21 on the surface tnereof. Deposited on top of the oxide layer 21 is a first layer 25 of electrolytic medium.
Deposited on the first layer 25 of electro1ytic medium is the counter electrode mealls 13 and the reference electrode 23. ~rne referellce electrode 2~
is coated vy a protective coating 24. ~eposited on top of the counter electrode 13 and protective coating 24 is a second layer 27 of electro1ytic medium. Then, deposited on the second layer 27 of electrolytic medium is the working electrode 14 of the electrochemical sensor. Deposited on top of the working electrode means 14 is a third layer 28 of electrolytic medium which includes a selectiv~ly permeable membrane 26 thereon.
Refe~ring now to Fig. 5 there is shown yet another alternate embodiment of the present invention. Fig. 5 depicts d cross-sectional view of a cylindrical electrochemical sensor in accordance with the present invention. Tne cyli~drical electrochemical sensor includes a su~strate 31 having an oxide layer 32 on the surface thereof. On top of the oxide layer 32 is deposited a first layer 33 of electrolytic medium. On the first layer 33 of electrolytic medium is deposited a cbunter electrode means 13, a workin~ electrode means 14 and a reference electrode 23. The reference electrode 23 is coated with a protective coating 24. It will be understood that any of the alternate embodiments shown in Figs. 1-4 may be adapted to the cylindrical-shaped electrochemical sensor as well as other possible shapes such as spherical. These alternate shapes may be desirable fvr specific app1ications of the sensing device.
The substrate 11 may be made of any suitable materials to which the electrolytic medium can be ` ~2~9~
_9_ adnered. The substrate 11 is preferably an insulating Inaterial such as glass, ~uaetz, ceralllics such as alulnina, etc. and silicon. rlle su~stra~e lL
should have a thickness sufficient to assure the structural integrity o~ the sensor. Another important feature of the su~strdte 11 is that it ~--capable of adhering or being made to adhere to a coating Inaterial such as those used to fabricate electrodes and electrolytes. This is ilnportan~
because the electrodes and electrolytes must adllece to the substrate llo There are several ways to promote adl~erence of a coatin~ material to the substrate ]1. One Inethod involves oxidation of the surface of the substrate 11 to forln an oxide layer thereon. Many electrolytic materials adhere well to oxides. An additional oxide layer ~nay also be coated on the suriace of tne substrate 11 to prolnote adherence of an electrolyte thereto. Also, adhesio promotors for improviny adhesion of Nafion to glass and other silacious substrates rnay be used. Such adhesion promoters include but are not lilnited to N-(trimethoxysilylpropyl)-N,N,N-trimethyl-a~nlnoniul, chloride, octadecyltrichlorosilane, and 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt. These adhesion promoters chemically bond the electrolyte to the substrate 11 to give additional bonding strengtn. The adhesion promoters are appLied to the substrate 11 just prior to spin ooating of the electrolyte onto the substrate. Such promoters are described in Szentirmay, M.N., Calnpbell, L.F., and Martin, C.R., Silane Coupling Agents for Attaching Nafion to Glass and Silica, Anal. Chem., Vol. 5B pp.
661-66~, Mdrch 1986.
As mentioned previously, the suL~strate 11 preferably includes an oxide layer on 21 oll the ~2~7~8'~
surface tilereof to promote the adherence vf the electrolytic metiiu~n to tt~e su~strat~ ucl~ ~n oxide layer 21 Inay be treated by simple oxidation of the surface of tne suDstrate 11. For instance, a substrate 11 such as silicon can be surface oxidi~ed to produce a silicon dioxide surface COdting.
Alternatively, the oxide layer 21 may be deposited on or attached to the surface of tlle substrate 11 in any suitable manner.
The substrate 11 should have a smooth surface before and after oxidation. Such a smooth surface will prolnote smootn coatings of electrolytic medium on the substrate 11. ~oreover, a smooth surface will lead to c:onsistent and repeata~le coatings of electrolytic medium enaoling mass production of consistent sensors. Further, the smooth surface of tlle subs~.rate 11 promotes adhesion of the electrolytic mediuln to the substrate and tllereby prevents tne electrolytic medium from peeling of~ tt~e substrate 11. ~inally, the existence of a slnoott~
surface on the substrate 11 minimizes the stresses applied to the electrolytic mediuln by the substrate 11 upon exposure to varying temperature and/or humidity conditions. Tnis, in turn, will minimize the distortion of the electrolytic Inediurn as a result of these temperature and/or humidity variations The electrolytic medium of the present invention is preferably a solid material. The electrolytic medium must be capable of allowing diffusion of all reactants and products between the cathodes and anodes as well as allowing exchant~e of the measured spc~cies with the test fluid. The electrolytic mediuln must also nave satisfactory chemical, thermal and dimensional stability. Such polymer electrolytes such as poly-sulfonic acids, typically polystyrene sulfonic acid or perfluoro linear polymers sucll as --ll--those marketed under ~^a~ie-l~rk ~Nafion~ ~y Du Pont are suitable for use as the electrolytic medium of the present invelltion. In ad~ition, the electrolytic medium must be capable of a~hering not only to the substrate 11, but also to tne electrodes 13, 14 oe the electrolytic medium must be capable of Deing adhered to the substrate 11 and the electrodes 13, 14 by adhesives, adnesion prolnoters or the like.
Electrolytes of the ty~e employed in the electrolytic medium of present invention dernollstrate excellent electrolytic an~ electronic colnpati~ility with oxides such as silicon dioxide, As a result ! i t is preferable to coat SUCII electrolytes onto an oxide covered surface, The oxide layer 21 can be obtained by thermal oxidation of the semiconductor wafer substrate 11. Other substrates 11 that are already oxides may also be used, such as alumina,.sapphire, glass and polymers. In order to o~tain smooth, repeatable layers of electrolytic mediuln, the electrolyte may be spin coated using a plloto-resist spinner, onto the surface of the sunstrate 1~. Tllis process is described in Unl-t~ S-tates Patent No.
4,795,543. This spin coating technique produces a very thin, smootn and homo~eneous coating of the electrolytic mediuln on the substrate 11. Other Inethods of coating the electrolytic Inedium onto the substrate 1l may be used if they produce a coating having the desired properties of ~moothness~ homogeneity, thickness and structural stabilityO
Tlle electrodes of the present invention are preferably Inetal. These electrodes rnay ~e deposited on the surface of the electrolytic Inediuln through the use of thick film, or thin film techni~lues. Sucll methods include sputteriny and/or evaporation onto the electrolytic surface ~f a thin filln o~ metal to ~Z7~
form the electrodes witn tne definition of tlle surface areas ~eing accomp1ished by photo-etclling processes. Otner thin film tecnni~lues such as deposition of a metal layer and pnoto-etching of tnat layer are also accepta~le.
The metals used to fa~ricate the e1ec~rodes oE
the present invention rnay inc1ude one or more of the following: platinum, palladium, rhodium, lead, silver, gold and iridium. It will ~e understood that other ,naterials may ~e used as long ~s they satisy the re~uirements of the present invention. Tnese other materials must be capable of reacting with the species to be detectedl as well as adhering oe bein~
adhered to the electrolytic medium. Selection of the proper electrode materia' for a particular reaction will depe.1d on tne species which is to De detected, as well as t~e ability to adhere t~e electrode material ~o tne electrolytic medium.
Tne analysis or identification of a gas using tnese electro~es m~y De aCCui;lpiisi~ed ill al~y of a number of ways. ~or instance, sucn electronic variables as resistance, impedance, electrolytic reactions, oxidation-reductioll reactions and polarization may be monitored duriny exposure of the sensor to a gas. Data ootained by monitoring any of these electronic variables can be used to analyze or identify a gas or components thereof.
Tne eiectrodes may be characterized as a working electrode 14, a counter electrode 13 and a reference electrode 23. The wor~ing electrode 14 is the electrode at which the species is consumed by an electroche~nical reaction. The counter electrod~ 13 is the electrode at whicil the species being detected is preferably regenerated by an electroc~lemical reaction. However, counter electrodes 13 wtlich do not regenerate the species being detected, such as ~2~g89~
those of a Clark cell may also be used though t~ey are not preferred. The reference electrode 23 ~o~s not participate in the chemical reactions but does serve to provide a potential teference for the working electrode 14. Normally, a potential is applied between the reference electrode 23 and the working electrode 14.
Since it is often desirable to prevent electrochemical reaction from occurring at the reference electrode 23, the reference electrode 23 is often coated to prevent exposure o the reference electrod~ 23 to tne species. Such coatings may include epoxies and any other coatinys which do not allow the diffusion of the species to the surface of the reference electrode 23. Alternatively, the reference electrode 23 may be left uncoated and thereby be exposed to the species. In this instance it is necessary to include a correction factor in tne syste~n monitoring means in order to colnpensate for the electrvchemical reaction occurring at the reference electrode 23. The reaction vccurring at the reference electrode 23 will cause a change in potential between the reference electrode 23 and the working electrode 14. This ~otential change can be accounted for through the use of tne Nernst equation. Therefore, the reference electrode 23 may be left exposed to the species if the monitoring means is programmed to compensate for the change in potential by calculating such change using the Nernst e~uation.
A preferred embodiment of the yresent invention also includes a selectively permeahle membrane 26 Wllich may ~e deposited over the top of the electrvdes 13, 14 or over the top of a second layer of electrolytic Inedium. This selectively permeable membrane 26 serves to allow the diffusion of the lZ*9B96 ~14-species to be detected through to ti)e ~orking electrode 14 and the electrolytic Inedium. However, it does not allow diffusion of certain other materials which .nay De present in the fluid materia1 being sensed. Therefore, the mern~rane 26 can be use~
to improve species s~ecificity of the sensing apparatus. The membrane 26 can also be used to prevent harlnful components of the ~luid material froln reaching the electrodes 13,14 and the electrolytic medium and altering tneir ~roperties in solne way.
~he mem~rane 26 may be composed of any material Wt)i is selectively permeable to the species ~eing detected Such materials include ru~bers and synthetic polymers among other materials.
Figs. 1-3 depict a planar sensor structure in accordance with the present invention. Such a planar structure is tne most ~referred embodiment since it requires the least number of components, minimizes the electrolytic interference and simplifies the construction. ~urtl;er, the planar sensor allo~s for smoother and more homogeneous coatings of the electrolytic medium since these coatings, with the exception of the second layer of electrolytic material in Fig. 3, are being applied to smooth surfaces. This type of sensor geometry gives excellent results because of its simplicity of design, ease o manufacture, and consistency.
The device of ~igs. 1 and 2 offers many advantages over prior art devices. In this embodinnent the electrodes 13 and 14 are in direct contact with the fluid material thereby eliminating the need for tne fluid material to di~use across meMoranes or electrolytes. This direct contact results in a shorter response time beca~se of the elimination of the diffusion resistance of electrolytic or membrane layers. Anotner important advantage of this embodiment results from the coating of the electrolytic medium directly onto the su~strate 11 rather than onto tne electrodes 13 and 14. Since the substrate 11 has a smoot~ sur~ace t~e electrolytic medium will form a smootn, thil), homogeneous coating on tne substrate 11. Prior art devices coated the electrolytic medium over the electrodes 13,14 thus forming a non-holno~elleous coating due to the rougnness of ti~e surface onto wnich the electrolytic medium had to be coated. T~e coating of the invention also minirnizes tne stresses placed on the electrolyte by the surface onto which it is coated since the electrolytic mediun is coated onto a smooth surface.
Another embodiment of the present invention is shown in Fig. 4. This sensor has a sandwich-type structure. Again, there is a thin coating of a first layer 25 electrolytic medium between tne su~strate 11 and the counter electrode means 13. ~o~ er, in the sandwich-type structure there is also a second layer 27 of electrolytic medium coated atop the counter electrode means 13. The sandwich-type structure nas several advantages over the planar structure. The main advantage of the sandwich-type structure is tne increased rigidity of the sensor structure due to the extra layers of material applied thereto. This increased rigidity will minimize the distortion of the electrodes 13 and 14 and electrolytic mediuln whici~ usually results froln thermal and physical stresses placed on the sensor apparatus. Another advantage of the sandwich-type structure is that the counter electrode 13 and reference electrode 23 are partially shielded from the fluid material by an additional layer of electrolytic medium. This will minimize undesirable reactions at tl~e counter electrode 13 and the reference electr~de 2~.
Excellent sandwich structures are possible as a result o~ the coating tect~ni~ues deve1l~ped in United States Patent No. 4,795 543.
These coating tec}lni~lles a1lo~ foe smooth, relatively nomogeneous coatings o~ tlle electrolytic r,ledium to ~e dpp1ied over the el~ctrodes 13, 140 ' The preferred ernbodiment of the present inventio will maximize the perimeter of tile working electrode 14 with respect to the area of contact of the working electrode l4 with the electrolytic medium. This maximization of the perimeter to area ratio results in a corresponding maximization of the sigllal to noise ratio of the sensor. The theoretica1 ~asis for this result is that the area of contact between the working electrode 14 and the electro1ytic medium appears to be responsible for the noise in the sensor. Whereas, the electrochemica1 reactioll between the working electrode 14 and the species appears to ~e catalyzed ~y the electrolytic medium.
Therefore, tne triple-phase boundary between the working electrode 14, electrolytic mediuln, and the species is the preferred location for the electrochemical reaction between the species and tlle working electrode 14. As a result, the signal generated by the electrochemical reaction appears to be directly proportional co the perimeter of the working electrode 14 since the peri~neter is a medsuee of the triple-phase boundary.
It is important to note that the sensillg apparatus of the present lnvention, if it uses a Nafion electrolyte, rnust be operated in an environment having at least some hulrlidity. Tl~e absence of water in the environment ~ill prevellt the 7~396 successful operation of tn- appdratus by hit~dering the role of the electrolytic mediuln. rnis is ~ca~l;e the yer fl~oro melnbrane re~3ires water to activate free protons. Ocher solid electrolytQs, s~3ch as polyvinylalcohol and pol~ethylene oxid~, may not re~uire the presence of nulnidity.
In operation, the assembly is contacted with a fluid material including the species to be detected.
the species will diffuse to the working electrode 14 and there an electrochemical reaction will take place generating a measurable signal. The signal is measured by the ammeter 20 and the measured signal is preferably fed to a Microcomputer for normalization of the signal as well as any other mathe3natica1 manipulations SUCh as calibration Whicll may be necessary. The response time for the sensor is usually less than five seconds.
The following examples are provided to illustrate certain embodiments of the present invention.
Example 1 A 2~ silicon wafer wa~ oxidized to provide an insulating silicon dioxide surface. Then the wafer was spin-coated with a 5% Nafion solution (Aldrich Chemical Co., Milwaukee, WI) to Inake a planar electrolytic structure. A two-step evaporation procedure was used to create grid electrode patterns r on the surface of the Nafion layer. An evaporation system containing both e-bea3n and therlnal evaporation capability was used to deposit the electrode structures. A photolitho~raphically etched evaporation mask was prepared from thin copper foiL
in which a number of parallel rectangles ~.~ere etcl)ed, each 6 mln long and 125 microns wide. After the first evaporation deposition the mask was rotated 90 and a second evaporation was performed. Gold wire (99.9~, Engelhard Minerals anc Chemicals ~o., NJ) was us~d as ~79~96 -l8-the evaporation source. The electrodes were electrically connected to a power source.
~xample 2 A sensor fa~ricated as in ~xalnple 1 was exposed to various gas mixtures ~ith tne ~ollo~ing results.
The sensor was operated at a constant potential of +300 millivolts versus the Platinuln/air reference electrode. S/N is the signal to noise ratio of the sensor. The signals are glven as normalized values with the signal for H2S being taken as 1.0 and all other responses being scaled accordingly.
_ensor Sigrlal to Various Gas !~ixtures Gas Mi~ture Day _gnal(S) Noi~e(N) S/N
92 ppm NC)/NO2 1 2.380.28 8.5 83 ppi~ H~S/N2 1 27.10.28 27.1 92 ppm NO/NO221 0.24 0.0212.0 83 ppm H2S/N221 2.72 0.0213.6 4~ pp.~ 2~ir 21 0.02 O.U2 l.U
80 ppm SO2/air 21 0.036 U.02 1.8 200 ppm C0/air21 0.00 O.U2 0.0 100 ppm HCN/air 21 spike0.02 spike ;
__
Tlle electrodes of the present invention are preferably Inetal. These electrodes rnay ~e deposited on the surface of the electrolytic Inediuln through the use of thick film, or thin film techni~lues. Sucll methods include sputteriny and/or evaporation onto the electrolytic surface ~f a thin filln o~ metal to ~Z7~
form the electrodes witn tne definition of tlle surface areas ~eing accomp1ished by photo-etclling processes. Otner thin film tecnni~lues such as deposition of a metal layer and pnoto-etching of tnat layer are also accepta~le.
The metals used to fa~ricate the e1ec~rodes oE
the present invention rnay inc1ude one or more of the following: platinum, palladium, rhodium, lead, silver, gold and iridium. It will ~e understood that other ,naterials may ~e used as long ~s they satisy the re~uirements of the present invention. Tnese other materials must be capable of reacting with the species to be detectedl as well as adhering oe bein~
adhered to the electrolytic medium. Selection of the proper electrode materia' for a particular reaction will depe.1d on tne species which is to De detected, as well as t~e ability to adhere t~e electrode material ~o tne electrolytic medium.
Tne analysis or identification of a gas using tnese electro~es m~y De aCCui;lpiisi~ed ill al~y of a number of ways. ~or instance, sucn electronic variables as resistance, impedance, electrolytic reactions, oxidation-reductioll reactions and polarization may be monitored duriny exposure of the sensor to a gas. Data ootained by monitoring any of these electronic variables can be used to analyze or identify a gas or components thereof.
Tne eiectrodes may be characterized as a working electrode 14, a counter electrode 13 and a reference electrode 23. The wor~ing electrode 14 is the electrode at which the species is consumed by an electroche~nical reaction. The counter electrod~ 13 is the electrode at whicil the species being detected is preferably regenerated by an electroc~lemical reaction. However, counter electrodes 13 wtlich do not regenerate the species being detected, such as ~2~g89~
those of a Clark cell may also be used though t~ey are not preferred. The reference electrode 23 ~o~s not participate in the chemical reactions but does serve to provide a potential teference for the working electrode 14. Normally, a potential is applied between the reference electrode 23 and the working electrode 14.
Since it is often desirable to prevent electrochemical reaction from occurring at the reference electrode 23, the reference electrode 23 is often coated to prevent exposure o the reference electrod~ 23 to tne species. Such coatings may include epoxies and any other coatinys which do not allow the diffusion of the species to the surface of the reference electrode 23. Alternatively, the reference electrode 23 may be left uncoated and thereby be exposed to the species. In this instance it is necessary to include a correction factor in tne syste~n monitoring means in order to colnpensate for the electrvchemical reaction occurring at the reference electrode 23. The reaction vccurring at the reference electrode 23 will cause a change in potential between the reference electrode 23 and the working electrode 14. This ~otential change can be accounted for through the use of tne Nernst equation. Therefore, the reference electrode 23 may be left exposed to the species if the monitoring means is programmed to compensate for the change in potential by calculating such change using the Nernst e~uation.
A preferred embodiment of the yresent invention also includes a selectively permeahle membrane 26 Wllich may ~e deposited over the top of the electrvdes 13, 14 or over the top of a second layer of electrolytic Inedium. This selectively permeable membrane 26 serves to allow the diffusion of the lZ*9B96 ~14-species to be detected through to ti)e ~orking electrode 14 and the electrolytic Inedium. However, it does not allow diffusion of certain other materials which .nay De present in the fluid materia1 being sensed. Therefore, the mern~rane 26 can be use~
to improve species s~ecificity of the sensing apparatus. The membrane 26 can also be used to prevent harlnful components of the ~luid material froln reaching the electrodes 13,14 and the electrolytic medium and altering tneir ~roperties in solne way.
~he mem~rane 26 may be composed of any material Wt)i is selectively permeable to the species ~eing detected Such materials include ru~bers and synthetic polymers among other materials.
Figs. 1-3 depict a planar sensor structure in accordance with the present invention. Such a planar structure is tne most ~referred embodiment since it requires the least number of components, minimizes the electrolytic interference and simplifies the construction. ~urtl;er, the planar sensor allo~s for smoother and more homogeneous coatings of the electrolytic medium since these coatings, with the exception of the second layer of electrolytic material in Fig. 3, are being applied to smooth surfaces. This type of sensor geometry gives excellent results because of its simplicity of design, ease o manufacture, and consistency.
The device of ~igs. 1 and 2 offers many advantages over prior art devices. In this embodinnent the electrodes 13 and 14 are in direct contact with the fluid material thereby eliminating the need for tne fluid material to di~use across meMoranes or electrolytes. This direct contact results in a shorter response time beca~se of the elimination of the diffusion resistance of electrolytic or membrane layers. Anotner important advantage of this embodiment results from the coating of the electrolytic medium directly onto the su~strate 11 rather than onto tne electrodes 13 and 14. Since the substrate 11 has a smoot~ sur~ace t~e electrolytic medium will form a smootn, thil), homogeneous coating on tne substrate 11. Prior art devices coated the electrolytic medium over the electrodes 13,14 thus forming a non-holno~elleous coating due to the rougnness of ti~e surface onto wnich the electrolytic medium had to be coated. T~e coating of the invention also minirnizes tne stresses placed on the electrolyte by the surface onto which it is coated since the electrolytic mediun is coated onto a smooth surface.
Another embodiment of the present invention is shown in Fig. 4. This sensor has a sandwich-type structure. Again, there is a thin coating of a first layer 25 electrolytic medium between tne su~strate 11 and the counter electrode means 13. ~o~ er, in the sandwich-type structure there is also a second layer 27 of electrolytic medium coated atop the counter electrode means 13. The sandwich-type structure nas several advantages over the planar structure. The main advantage of the sandwich-type structure is tne increased rigidity of the sensor structure due to the extra layers of material applied thereto. This increased rigidity will minimize the distortion of the electrodes 13 and 14 and electrolytic mediuln whici~ usually results froln thermal and physical stresses placed on the sensor apparatus. Another advantage of the sandwich-type structure is that the counter electrode 13 and reference electrode 23 are partially shielded from the fluid material by an additional layer of electrolytic medium. This will minimize undesirable reactions at tl~e counter electrode 13 and the reference electr~de 2~.
Excellent sandwich structures are possible as a result o~ the coating tect~ni~ues deve1l~ped in United States Patent No. 4,795 543.
These coating tec}lni~lles a1lo~ foe smooth, relatively nomogeneous coatings o~ tlle electrolytic r,ledium to ~e dpp1ied over the el~ctrodes 13, 140 ' The preferred ernbodiment of the present inventio will maximize the perimeter of tile working electrode 14 with respect to the area of contact of the working electrode l4 with the electrolytic medium. This maximization of the perimeter to area ratio results in a corresponding maximization of the sigllal to noise ratio of the sensor. The theoretica1 ~asis for this result is that the area of contact between the working electrode 14 and the electro1ytic medium appears to be responsible for the noise in the sensor. Whereas, the electrochemica1 reactioll between the working electrode 14 and the species appears to ~e catalyzed ~y the electrolytic medium.
Therefore, tne triple-phase boundary between the working electrode 14, electrolytic mediuln, and the species is the preferred location for the electrochemical reaction between the species and tlle working electrode 14. As a result, the signal generated by the electrochemical reaction appears to be directly proportional co the perimeter of the working electrode 14 since the peri~neter is a medsuee of the triple-phase boundary.
It is important to note that the sensillg apparatus of the present lnvention, if it uses a Nafion electrolyte, rnust be operated in an environment having at least some hulrlidity. Tl~e absence of water in the environment ~ill prevellt the 7~396 successful operation of tn- appdratus by hit~dering the role of the electrolytic mediuln. rnis is ~ca~l;e the yer fl~oro melnbrane re~3ires water to activate free protons. Ocher solid electrolytQs, s~3ch as polyvinylalcohol and pol~ethylene oxid~, may not re~uire the presence of nulnidity.
In operation, the assembly is contacted with a fluid material including the species to be detected.
the species will diffuse to the working electrode 14 and there an electrochemical reaction will take place generating a measurable signal. The signal is measured by the ammeter 20 and the measured signal is preferably fed to a Microcomputer for normalization of the signal as well as any other mathe3natica1 manipulations SUCh as calibration Whicll may be necessary. The response time for the sensor is usually less than five seconds.
The following examples are provided to illustrate certain embodiments of the present invention.
Example 1 A 2~ silicon wafer wa~ oxidized to provide an insulating silicon dioxide surface. Then the wafer was spin-coated with a 5% Nafion solution (Aldrich Chemical Co., Milwaukee, WI) to Inake a planar electrolytic structure. A two-step evaporation procedure was used to create grid electrode patterns r on the surface of the Nafion layer. An evaporation system containing both e-bea3n and therlnal evaporation capability was used to deposit the electrode structures. A photolitho~raphically etched evaporation mask was prepared from thin copper foiL
in which a number of parallel rectangles ~.~ere etcl)ed, each 6 mln long and 125 microns wide. After the first evaporation deposition the mask was rotated 90 and a second evaporation was performed. Gold wire (99.9~, Engelhard Minerals anc Chemicals ~o., NJ) was us~d as ~79~96 -l8-the evaporation source. The electrodes were electrically connected to a power source.
~xample 2 A sensor fa~ricated as in ~xalnple 1 was exposed to various gas mixtures ~ith tne ~ollo~ing results.
The sensor was operated at a constant potential of +300 millivolts versus the Platinuln/air reference electrode. S/N is the signal to noise ratio of the sensor. The signals are glven as normalized values with the signal for H2S being taken as 1.0 and all other responses being scaled accordingly.
_ensor Sigrlal to Various Gas !~ixtures Gas Mi~ture Day _gnal(S) Noi~e(N) S/N
92 ppm NC)/NO2 1 2.380.28 8.5 83 ppi~ H~S/N2 1 27.10.28 27.1 92 ppm NO/NO221 0.24 0.0212.0 83 ppm H2S/N221 2.72 0.0213.6 4~ pp.~ 2~ir 21 0.02 O.U2 l.U
80 ppm SO2/air 21 0.036 U.02 1.8 200 ppm C0/air21 0.00 O.U2 0.0 100 ppm HCN/air 21 spike0.02 spike ;
__
Claims (21)
1. A solid electrochemical sensor for generating an electrical signal in resonse to contact with a pre-determined species present in a fluid material comprising:
a substrate having at least one surface, a solid electrolyte medium having first and second surfaces, said first surface of said medium being in contact with and adhering to said at least one surface of said substrate, a working electrode means in contact with and adhering to said second surface of said medium, an electrical power source connected for biasing said working electrode means at a potential at which said species will be consumed at said working electrode means, and a counter electrode means in contact with and adhering to said second surface of said medium and being connected to said power source for completing a circuit in which a current is capable of flowing through both of said electrode means as a result of the electrochemical reaction occurring at said first working electrode means.
a substrate having at least one surface, a solid electrolyte medium having first and second surfaces, said first surface of said medium being in contact with and adhering to said at least one surface of said substrate, a working electrode means in contact with and adhering to said second surface of said medium, an electrical power source connected for biasing said working electrode means at a potential at which said species will be consumed at said working electrode means, and a counter electrode means in contact with and adhering to said second surface of said medium and being connected to said power source for completing a circuit in which a current is capable of flowing through both of said electrode means as a result of the electrochemical reaction occurring at said first working electrode means.
2. An apparatus in accordance with Claim 1 further comprising a reference electrode having at least one surface in contact with and adhering to said second surface of said medium.
3. An apparatus in accordance with claim 2 further comprosing a coating over said reference electrode to prevent exposure of said reference electrode to said species.
4. An apparatus in accordance with Claim 2 further comprising a selectively permeable membrane means separating all of said electrode means from said fluid material.
5. An apparatus in accordance with Claim 1 wherein said substrate comprises an insulating material.
6. An apparatus in accordance with Claim 1 wherein said electrolytic medium comprises a layer of between about 0.1 and about 4.0 microns in thickness.
7. An apparatus in accordance with Claim 6 wherein said working electrode means comprises thin strips of metal having a perimeter to area ratio of between 0.4 cm and 200 microns.
8. An apparatus in accordance with Claim 1 wherein said substrate comprises an oxide layer on said at least one surface of said substrate.
9. An apparatus in accordance with Claim 1 wherein said substrate further comprises at least one adhesion promoter on said at least one surface of said substrate to promote adherence between said electrolytic medium and said substrate.
10. An apparatus in accordance with Claim 1 further comprising a second layer of a solid electrolytic medium having a first surface in contact with and adhering to said working and counter electrode means.
11. A solid electrochemical sensor for generating an electrical signal in response to contact with a pre-determined species present in a fluid material comprising a substrate having at least one surface, a first layer of solid electrolytic medium having first and second surfaces, said first surface of said first layer of said medium being in contact with and adhering to said at least one surface of said substrate, a counter electrode means in contact with and adhering to said second surface of said first layer of electrolytic medium, a second layer of a solid electrolytic medium having first and second surfaces, said first surface of said second layer of said medium being in contact with and adhering to said counter electrode means, a working electrode means in contact with and adhering to said second surface of said second layer of electrolytic medium, an electrical power source connected for biasing said working electrode means at a potential at which said species will be consumed at said working electrode means, and means for connecting said counter electrode means to said power source for completing a circuit in which a current is capable of flowing through both of said electrode means as a result of the electrochemical reaction occurring at said working electrode means.
12. An apparatus in accordance with Claim 11 further comprising a reference electrode in contact with and adhering to said second surface of said first layer of electrolytic medium.
13. An apparatus in accordance with Claim 12 wherein said reference electrode is also in contact with an adhering to said first surface of said second layer of electrolytic medium.
14. An apparatus in accordance with Claim 12 wherein said reference electrode further comprises a coating to prevent exposure of said reference electrode to said species.
15. An apparatus in accordance with Claim 11 further comprising a third layer of electrolytic medium in contact with and adhering to said working electrode means.
16. An apparatus in accordance with Claim 11 further comprising a selectively permeable membrane means separating said working electrode means from said fluid material.
17. An apparatus in accordance with Claim 11 wherein said substrate comprises an oxide layer on said at least one surface.
18. An apparatus in accordance with Claim 11 wherein said substrate comprises at least one adhesion promoter on said at least one surface of said substrate to promote adherence of said electrolytic medium and said substrate.
19. An apparatus in accordance with Claim 11 wherein said working electrode means has a perimeter to area ratio of between about 0.4 and about 500.
20. An apparatus in accordance with Claim 11 wherein said first and second layers of electrolytic medium each have a thickness of between about 0.1 and about 4.0 microns.
21. An apparatus in accordance with Claim 11 wherein said substrate comprises an insulating material.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US5370587A | 1987-05-26 | 1987-05-26 | |
| US07/053,705 | 1987-05-26 |
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|---|---|
| CA1279896C true CA1279896C (en) | 1991-02-05 |
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ID=21985990
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000567772A Expired - Fee Related CA1279896C (en) | 1987-05-26 | 1988-05-26 | Eletrochemical micro sensor |
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| Country | Link |
|---|---|
| EP (1) | EP0363427A4 (en) |
| AU (1) | AU604142B2 (en) |
| CA (1) | CA1279896C (en) |
| WO (1) | WO1988009500A1 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4795543A (en) * | 1987-05-26 | 1989-01-03 | Transducer Research, Inc. | Spin coating of electrolytes |
| FR2695481B1 (en) * | 1992-09-07 | 1994-12-02 | Cylergie Gie | Amperometric measurement device comprising an electrochemical sensor. |
| GB2290382A (en) * | 1994-03-25 | 1995-12-20 | Univ Cranfield | Electrochemical measurement and reactions in highly-resistive solvents |
| US5841021A (en) * | 1995-09-05 | 1998-11-24 | De Castro; Emory S. | Solid state gas sensor and filter assembly |
| GB9625464D0 (en) * | 1996-12-07 | 1997-01-22 | Central Research Lab Ltd | Gas sensor |
| DE102004037312B4 (en) | 2004-07-31 | 2015-02-05 | Dräger Safety AG & Co. KGaA | Electrochemical gas sensor and method for its production |
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| US751897A (en) * | 1904-02-09 | Gitido bodlander | ||
| US2913386A (en) * | 1956-03-21 | 1959-11-17 | Jr Leland C Clark | Electrochemical device for chemical analysis |
| US3260656A (en) * | 1962-09-27 | 1966-07-12 | Corning Glass Works | Method and apparatus for electrolytically determining a species in a fluid |
| US3305457A (en) * | 1965-08-09 | 1967-02-21 | Hyman Edward Sidney | Hydrocarbon detection |
| US3701632A (en) * | 1970-03-05 | 1972-10-31 | California Inst Of Techn | Vapor phase detectors |
| US3836449A (en) * | 1970-06-01 | 1974-09-17 | California Inst Of Techn | Electrolytic cell for use with vapor phase detectors |
| US3776832A (en) * | 1970-11-10 | 1973-12-04 | Energetics Science | Electrochemical detection cell |
| US3824168A (en) * | 1970-11-10 | 1974-07-16 | Energetics Science | Gas detecting and quantitative measuring device |
| US3909386A (en) * | 1970-11-10 | 1975-09-30 | Energetics Science | Gas detector unit |
| US3764269A (en) * | 1971-12-28 | 1973-10-09 | North American Rockwell | Sensor for fluid components |
| US4076596A (en) * | 1976-10-07 | 1978-02-28 | Leeds & Northrup Company | Apparatus for electrolytically determining a species in a fluid and method of use |
| US4267023A (en) * | 1977-10-17 | 1981-05-12 | Orion Research Incorporated | Chemically integrating dosimeter and gas analysis methods |
| JPS5467497A (en) * | 1977-11-09 | 1979-05-30 | Nippon Soken | Gas detector |
| NL7915013A (en) * | 1978-03-08 | 1980-06-30 | British Gas Corp | A GAS SENSITIVE ELEMENT AND A METHOD FOR MANUFACTURING IT. |
| US4228400A (en) * | 1978-03-28 | 1980-10-14 | Research Corporation | Conductometric gas analysis cell |
| US4169779A (en) * | 1978-12-26 | 1979-10-02 | Catalyst Research Corporation | Electrochemical cell for the detection of hydrogen sulfide |
| US4184937A (en) * | 1978-12-26 | 1980-01-22 | Catalyst Research Corporation | Electrochemical cell for the detection of chlorine |
| US4227984A (en) * | 1979-03-01 | 1980-10-14 | General Electric Company | Potentiostated, three-electrode, solid polymer electrolyte (SPE) gas sensor having highly invariant background current characteristics with temperature during zero-air operation |
| JPS55154450A (en) * | 1979-05-19 | 1980-12-02 | Nissan Motor Co Ltd | Air-fuel-ratio detector |
| JPS55166039A (en) * | 1979-06-12 | 1980-12-24 | Nissan Motor Co Ltd | Air fuel ratio detector |
| US4329214A (en) * | 1980-07-21 | 1982-05-11 | Becton, Dickinson And Company | Gas detection unit |
| US4347732A (en) * | 1980-08-18 | 1982-09-07 | Leary David J | Gas monitoring apparatus |
| GB2094005B (en) * | 1981-02-03 | 1985-05-30 | Coal Industry Patents Ltd | Electrochemical gas sensor |
| JPS57137850A (en) * | 1981-02-20 | 1982-08-25 | Nissan Motor Co Ltd | Oxygen concentration measuring element |
| US4423407A (en) * | 1981-02-27 | 1983-12-27 | Dart Industries Inc. | Apparatus and method for measuring the concentration of gases |
| US4387165A (en) * | 1982-04-22 | 1983-06-07 | Youngblood James L | H2 S Detector having semiconductor and noncontinuous inert film deposited thereon |
| US4571292A (en) * | 1982-08-12 | 1986-02-18 | Case Western Reserve University | Apparatus for electrochemical measurements |
| US4477541A (en) * | 1982-12-22 | 1984-10-16 | The United States Of America As Represented By The United States Department Of Energy | Solid electrolyte structure |
| US4521290A (en) * | 1984-03-16 | 1985-06-04 | Honeywell Inc. | Thin layer electrochemical cell for rapid detection of toxic chemicals |
| US4587105A (en) * | 1984-05-17 | 1986-05-06 | Honeywell Inc. | Integratable oxygen sensor |
| US4795543A (en) * | 1987-05-26 | 1989-01-03 | Transducer Research, Inc. | Spin coating of electrolytes |
-
1988
- 1988-05-26 CA CA000567772A patent/CA1279896C/en not_active Expired - Fee Related
- 1988-05-26 EP EP19880906286 patent/EP0363427A4/en not_active Withdrawn
- 1988-05-26 WO PCT/US1988/001772 patent/WO1988009500A1/en not_active Application Discontinuation
- 1988-05-26 AU AU19580/88A patent/AU604142B2/en not_active Ceased
Also Published As
| Publication number | Publication date |
|---|---|
| EP0363427A1 (en) | 1990-04-18 |
| EP0363427A4 (en) | 1991-01-16 |
| AU1958088A (en) | 1988-12-21 |
| AU604142B2 (en) | 1990-12-06 |
| WO1988009500A1 (en) | 1988-12-01 |
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