AU604142B2 - Electrochemical micro sensor - Google Patents

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WOUDI INTULECXTUAL PROP1URTY()RAITO AU-Al-19680/88 INTERNATIONAL APPLICATION PUIS,0D UI&PER4II3EI Th9OOPRATION TR13ATY (PCT) (51) interntational patent ClasilflealIoa 4 tj Into ifilon~ rih don Numbert WO 88/ 09500 GO IN 27/30,27/52 Al 1(43) International Publication Datei I December 1988 (01.12,88) (21) International Application Number: PCT/US88/01772 (22) International Filing Date,.

(31) Priority Application Numbert (32) Priority Dte: (33) Priority Country: 26 May 1988 (26.05,88) 053,705 26 May 1987 (26,05,87) us (81) Designated States: AT (European patent), AU, BE (Wuropean patent), CHI (Eurpa patet). DE (Euro.

Penn patent), FR (Hu ropean patent), 003 (European patent), IT (European" patent), JP), LU (European patent), N1. (European patent), SE (European patent), Published it1h international search report, Blefore ilia expiration of tihe ime 11liijor amiendfing the clains and to be republished In the event of the reccipt of amiendmients.

(71) Apllcnt: TRANSDUCER RESEARCHl, INC. (US/ 1228 Olympus Drive, Naperville, I L 60540 (US).

(72) Inventors:. STETTER, Joseph, It. 1228 Olympus Drive, Na perville, IL 60540 MACLAY, Jordan 316 N. 4th Avenue, Maywood, I L 60153 (US).

(74) Agent: DUN LEA VY, Kevin, 727 23rd Street South, Arlington, VA 22202 (US).

X V2. T. R 2 3 F EB 89

AUSTRALIAN

2 1 DEC 1988 PATENT OFFICE This documunifl contains the a:llecildmIels 111ade unld~r "eio 49auug ise;nc 1 (54)Tltlo: HLECTRkOCFIEMICAL MICRO SENSOR (57) Abstract A micro-amperometric electrochemical sensor (10) for detecting the presence of a predetermined species In a fluid material is disclosed, The sensor includes a smooth substrate having a thin coating of solid electrolyte material (22) deposited thereon. The working (14) and counter (13) electrodes are deposited on the surface of the solid electrolyte material and adhere thereto, Electrical leads (15, 17) connect the working and counter electrodes to a potential source (19) and an apparatus for measuring the change in electrical signal (20) 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.

WO t~8/O950O PCT/US~8/O 1772 WO 880950 PCTUS8$0177 EILLCTROCHEMICAL MICRO SENSOR This invention was made with Government support under contract number ANL-61892401 awarded by the Department of Energy. The Government has certain rights in this invention.

This application is a continuation-in-part of U.S. Application SerialI No. 053,705, filed on May 26, 1987, now abandoned.

FIEL~D OF THE INVENTION The invention relates to electrochemical apparatus for sensing the presence of a s~pecies in a fluid material. More particularly the invention relates to an improved apparatus for generating an electric signal in response the the presence of a predetermined species in a fluid material.

BACKGROUND OF THE INVENTION Electrochemical sensors for the detection of the presence of a species in a fluid material have existed for quite some time. Such sensors include the Clark cell described in U.S. Patent No. 2,'913,386 issued November 17, 1959. The apparatus disclosed in that patent utilizes a dual electrode structure immersed in an electrolyte and encased at least in part in a membrane which is permeable to a predetermined species. In operation sucn a device allows the permeation of the species to be detected through the membrane and reduces said species at the 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 and WO 8095O10 PC/U88177Z reduction reactions genrate a current which is mneasurable and is proportional. to too concentration of the species being detected, Thle Clark cell is a large oulky apparatus and must include a liquid electrolytic medium in which the electrodes are immersed. The Clack apparatus suffers from several disadvantages including consumption of the species being detected during detection, slow response timoo aind alteration of the electrolyte during detection.

Some of the above-mentioned disadvantages of the Clark-type electrode call are avoidedI by apparatus of the type described in U.S. Patent No. 3,260,656 issued on July 12, 1966 to James W. Rosa, Jr. Th a Ross apparatus tctilizes a sandwich comprising a cathode and an anode with a spacer thorebetwean# This sandwich is immersed in an electrolyte and is geometrically oriented so that the electrodes are parallel to a membrane which is permeable to the species being measured. The membrane combines with a housing to enTi~ose 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 sich tnat no not consumption of the species being detected occurs, Therefore the Ross sensor does not consume the species being measured as a result of the electrochemical reaction of that species with the electrodes. Whereas the Ross cell eftectively 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 shortcomings are still evident. Among them is tne fact that readings with the Ross-type cell, obtained by measuring the current flow between the electrodes, z' -1 W'O 88/09500 PCT/US88/01772 tend to stablize within a maximum of one minute in accordance with the Ross patent. It has been found tnat response times of this order are not suitable for many applications. A further disadvantage is that the diffusion layer thickness in the Ross cell is determined by the interelectrode distance whicn is oubject to variation as the assembly is stressed by forces arising from temperature and/or pressure variations. Yet another disadvantage is the cumnersome natiers of the layered structure making reliable fabrication of Ross-type devices difficult.

Yet anotner apparatus for electrolytically detecting a species in a fluid is described in U.S.

Patent 4,076,596 issued to Connery et al. on February 28, 1978. The apparatus of Connery et al. includes an insulating substrate and a plurality of fingerlike electrodes deposited on the surface of the substrate in a closely spaced interleaved geometric pattern.

Tne 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 consumed 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 disadvantage of the Connery et al. apparatus is that the solid electrolyte is deposited on top of the electrodes. The electrodes 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 electrolyte coating which will be smooth, consistent, homogeneous and adhere to the electrodes. In I 1 if I 4 addition, the aoatinq Of olootrolyto will bo ditwl0l 1by changos in humidity and tomporaturo becauso of£ tho irregular surface upon which it is coated. Another problom with the Connory ot al. apparatus is that its rooesponse times may bo too slow for some applications. This rosults because of the electrolytic roesistance of the oloctrolyto which forms a barrier between tho electrodes and tho toot fluid. As a result, the species must diffuso through tho electrolyte prior to contacting the electrodes. Since thIo Connory ot al. electrolyte is coated onto an irregular surface the electrolyte must be thicker than if it wore coated on a flat surface to accomplish a complete coating.

9 Accordingly the electrolytic resistance will be lower but "diffusion will be slower and can significantly slow a response times.

The present invention attempts to overcome some of the above disadvantages.

SUMMARY OF THE INVENTION a Y a. According to the present invention there is provided a solid electrochemical sensor which operates at temperatures below 300°C for generating an electrical signal in rosponsu to contact with a pre-determined species present in a fluid *6o* material comprising: *o a substrate having at least one surface, a solid electrolytic medium having sufficient ionic conductivity at temperatures below 100°C to sustain an ionic current flow and 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, KA 4 and C-i 9 -a cduntolr lo otrdo m1anlo In conLaot wIh and adlhordig to a ocond ourfaeo of said modium and boinq cnnoctoed to said powor courco for comploting a circuit in which a curront is capablo of flowing through both of said oloctrodo moans as a rocsult of the oloctrochomical reaction occurring at said first working olectrodo moans.

The proont invontion further provido a solid eloctrochomical conoor for gonorating an alootrical asignal in roeponoo to contact with a pro-dotormined specios a preont in a fluid matorial compricing: a subtrato comprising an insulating material and l having at loant ono surfaco, to a first layor of solid loectrolytic medium having r first and soecond surfaeoo, said first surfaco of said first layer of said modium being in contact with and adhoring to said at leat one ourfaco of said substrate, a countor oloetrodo moans in contact with and adhoring to said second surface of said first layer of oloetrolytic meodium, .oa second layor of a solid electrolytic medium S having first and seond surfaces, said first surface of said socond layor of said medium being in contact with and adhering to said counteor electrodo means, a working oloctrodo moans in contact with and adhering to said socond surface of said socond layer of oloctrolytic modium, See oan oloctrical power sourc connocted for biasing said working electrode moans at a potential at which said species will be consumed at said working electrode moans, 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 moans as a result of the electrochemical reaction occurring at said working electrode means.

i i t IIErSCITPTON OF THI'- PRPPBRRPD-],MBODTP-N'JI Preferred embodiments of tho preseont invention will now be described by way of example only, with reference to Lho accompanying drawings, in which; Fig.l is a cross-sectional view of tho amporomoMfio electrochemical apparatus.

Fig.2 is a top plan view of the amporometric oloctrochomical sensing apparatus.

Fig.3 is a cross-sectional view of an alternate embodiment of the amporometric electrochomical senoing 0 apparatus.

.pa uFig.4 is a cross-sectional view of a sandwich type amporometric electrochemical sensing apparatus.

Fig.5 is a cross-sectional view of a cylindrical amperometric electrochemical sensing apparatus.

Fig.6 is a plan view of a grid pattern electrode.

Referring now to Fig.l there is shown an electrochemical sensing apparatus 10 including a substrate 11, an electrolyte 22, a counter electrode means 13 and a working electrode means 14. The sensor depicted in Fig.l is the 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.l showing the fingerlike projections of S the electrodes 13 and 14. Counter electrode 13 is connected by way of line 15 to terminal 16 and the working electrode 14 is connected by line 17 to the terminal 18.

The electrical circuit also includes a series connected electrical power source 19 for biasing the working electrode means 14 at a desired potential and an ammeter Roforrinvj flow Lo Ftq. 3 t i til~own an citWont omlbodimont of W10 oloctroociomicial ionicor of Who proluint lp'vontion. 'Jho oonsor dopictod in tF'Ji. 3 includoii aubonato 11 haing an oxido layor 21 on tho suirfaco thoroof. Dopou4 tad on tho oxido Inyor 21 and adho4rnq tho oxidc, layor 21 ia 4 fiilt layor 25 of a., a V 4.

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a a h 4, 9 a a a 4* 4* a a.

'p a a a d a a.

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I WO 88~/09500 PCT/CUS88/O 1772 ele.trolytic medium. Deposited on the first layer of electrolytic medium are the counter electrode means 13 and the working electrode means 14. Also dpouited on the first layer 25 of electrolytic medium is a reference electrode 23 having a protective coating 24 thereon. Deposited on top of the electrodes 13# 14 and the protective coating 24 io a second layer 27 of electrolytic medium.

1?inallyf on top of the second layer 27 of electrolytic medium is snfown a selectively permeable membrane 26, Referring now to Fig. 4 there is depicted another alternative embodiment of the present invention wherein the electrochemical sensing means is formed in a sandwich-type structure. This sandwich-type structure is ouilt on a layer of substrate 11. The layer of substrate 11 includes an oxide layer 21 on the surface thereof. Deposited on top of the oxide layer 21 is a first layer 25 of electrolytic medium.

Deposited on the first layer 25 of electrolytic medium is the counter electrode means 13 and the reference electrode 23. The reference electrode 23 is coated by a protective coating 24. Deposited on top of the counter electrode 13 and protective coating 24 is a second layer 27 of electrolytic 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 selectively permeable membrane 26 thereon.

Referring now to Fig. 5 there is shown yet another alternate embodiment of the present invention. Fig. 5 depicts a cross-sectional view of a cylindrical electrochemical sensor in accordance w:Ltn tne present invention. The cylindrical ~l~r i WO 88/09500 PCT/US88/01772 9 electrochemical sensor includes a substrate 31 naving 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 counter electrode means 13, a working 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 cylindricalshaped electrochemical sensor as well as other possible shapes such as spherical. Tnese alternate shapes may be desirable for specific applications of the sensing device.

The substrate 11 may be made of any suitable materials to which the electrolytic medium can be adhered. The substrate 11 is preferably an insulating material such as glass, quartz, ceramics such as alumina, etc. and silicon. Tne substrate 11 should have a thickness sufficient to assure the structural integrity of the sensor. Another important feature of the substrate 11 is that it be capable of adhering or being made to adhere to a coating material such as those used to fabricate electrodes and electrolytes. This is important because the electrodes and electrolytes must adhere to the substrate 11. There are several ways to promote adherence of a coating material to the substrate 11. One method involves oxidation of the surface of the substrate 11 to form an oxide layer thereon. Many electrolytic materials adhere well to oxides. An additional oxide layer may also be coated on the surface of the substrate 11 to promote adherence of an electrolyte thereto. Also, adhesion promotors for improving adhesion of Nafion to glass and other silacious substrates may be used. Such WO 88/09500 PCT/US88/01772 10 adhesion promoters include but are not limited to rimethoxysilylpropyl) -N,N,N-trimethyl-ammonium -hloride, octadecyltrichlorosilane, and 8-hydroxy-l,3,6-pyrenetrisulfonic acid trisodium salt. These adhesion promoters chemically bond the electrolyte to the substrate 11 Lo give additional bonding strength. The adhesion promoters are applied to the substrate 11 just prior to spin coating of the electrolyte onto the substrate. Sucn promoters are described in Szentirmay, Campbell, and Martin, Silane Coupling Agents for Attacning Nafion to Glass and Silica, Anal. Cnem., Vol. 58 pp, 661-662, March 1986, which is hereby incorporated by reference..

As mentioned previously, the substrate 11 preferably includes an oxide layer on 21 on the surface thereof to promote the adherence of the electrolytic medium to the substrate 11. Such an oxide layer 21 may be created by simple oxidation of the surface of the substrate 11. For instance, a substrate 11 such as silicon can be surface oxidized to produce a silicon dioxide surface coating.

Alternatively, tne oxide layer 21 may be deposited on or attached to the surface of the suostrate 11 in any suitable manner.

The substrate 11 should have a smooth surface before and after oxidation. Such a smooth surface will promote smooth coatings of electrolytic medium on the substrate 11. Moreover, a smooth surface will lead to consistent and repeatable coatings of electrolytic medium enabling mass production of consistent sensors. Further, the smooth surface of the substrate 11 promotes adhesion of the elJectrolytic medium to the substrate and thereby prevents the electrolytic medium from peeling off the substrate 11. Finally, the existence of a smooth W.O 88/09500 Pcr/US88/01772 11 surface on the substrate 11 minimizes the stresses applied to the electrolytic medium by the substrate 11 upon exposure to varying temperature and/or humidity conditions. This, in turn, will minimize the distortion of the electrolytic medium 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 exchange of the measured species with the test fluid. The electrolytic medium must also have satisfactory chemical, thermal and dimensional stability. Such polymer electrolytes such as poly-sulfonic acids, typically polystyrene sulfonic acid or perfluoro linear polymers such as those marketed under the name "Nafion" by Du Pont are suitable for use as the electrolytic medium of the present invention. In addition, the electrolytic medium must be capable of adhering not only to the substrate 11, but also to the electrodes 13, 14 or the electrolytic medium must be capable of oeing adhered to the substrate 11 and the electrodes 13, 14 by adhesives, adhesion promoters or the like.

Electrolytes of the type employed in the electrolytic medium of present invention demonstrate excellent electrolytic and electronic compatibility with oxides such as silicon dioxide. As a result, it is preferable to coat such 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 obtain smooth, repeatable layers of electrolytic medium, the i electrolyte may be spin coated using, onto the L i I I--I WO 88/09500 PCT/US88/01772 12 surface of the substrate 11. 4--ir-p oce-s-s-s- Sdesor-i-ed-i-n-ou- oo-pend-ing=-application Serial No id-on-M ay-26-r19S-7- This spin coating technique produces a very thin, smooth and homogeneous coating of the electrolytic medium on the substrate 11. Other methods of coating the electrolytic medium onto the substrate 11 may be used if they produce a coating having the desired properties of smoothness, homogeneity, thickness and structural stability.

The electrodes of the present invention are preferably metal. These electrodes may be deposited on the surface of the electrolytic medium through the use of thick film, or thin film techniques. Such methods include sputtering and/or eva,,oration onto the electrolytic surface of a thin film of metal to form the electrodes with the definition of the surface areas being accomplished by photo-etching processes. Other thin film techniques such as deposition of a metal layer and photo-etching of that layer are also acceptable.

Tne metals used to fabricate the electrodes of the present invention may include one or more of the following: platinum, palladium, rhodium, lead, silver, gold and iridium. It will be understood that other materials may be used as long as they satisfy the requirements of the present invention. These other materials must be capable of reacting with the species to be detected, as well as adhering or being adhered to the electrolytic medium. Selection of the proper electrode material for a particular reaction will depend on the species wnich is to be detected, as well as the ability to adhere the electrode material to the electrolytic medium.

The analysis or identification of a gas using these electrodes may be accomplished in any of a i C~ I rrrrr~--rarco; WO 88/09500 PCT/US88/01772 13 number of ways, Fo: instance, such electronic variables as resistance, impedance, electrolytic reactions, oxidation-reduction reactions and polarization may be monitored during exposure of the sensor to a gas. Data obtained by monitoring any of these electronic variables can be used to analyze or identify a gas or components tnereof.

The electrodes may be characterized as a working electrode 14, a counter electrode 13 and a reference electrode 23. The working electrode 14 is the electrode at which the species is consumed by an electrochemical reaction. The counter electrode 13 is the electrode at which the species being detected is preferably regenerated by an electrochemical reaction. However, counter electrodes 13 which do not regenerate the species being detected, such as those of a Clark cell may also be used though they are not preferred. The reference electrode 23 does not participate in tne chemical reactions but does serve to provide a potential reference for tne 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 of the reference electrode 23 to the species. SUcn coatings may include epoxies and any other coatings 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 the system monitoring means in order to compensate for the electrocnemical reaction occurring at the 4 _.ii ii r WO 8?/095OO PaT/US88/01772 14 reference electrode 23, The reaction occurring at the reference electrode 23 will cause a change in potential between the reference electrode 23 and the working electrode 14. This potential change can be accounted for through the use of the 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 calc'ulating such change using the Nerns~t equation.

h~ preferred embodiment og'the present invention also includes a selectively permeable memJorane 26 which may be deposited over the top of the electrodes 13, 14 or omver the top of a second layer of electrolytic medium. This selectively permeable membrane 26 serves to allow the diffusion of the species to be detected through to thui working electrode 14 and the electrolytic medium. However, it does not allow diffusion of certain other materials which may be present in the fluid material being sensed. Tnerefore, the membjrane 26 can be used to improve species specificity of the sensing apparatus. The membrane 26 can also be used to prevent harmful components of tne fluid material fro, reaching the electrodes 13,14 and the electrolytic medium and altering their properties in some way.

The membrane 26 may be composed of any material which is selectively permeable to the species being detected. Such materials include rubbers 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 the most preferred embodiment. since it requires the least number of components, minimizes the electrolytic interference and simplifies the construction. Further, the planar sensor allows for WO 88/09500 PCT/US88/01772 15 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 of manufacture, and consistency.

The device of Figs. 1 and 2 offers many advantages over prior art devices. In this embodiment the electrodes 13 and 14 are in direct contact with the fluid material thereby eliminating the need for the fluid material to diffuse across membranes or electrolytes. This direct contact results in a shorter response time because 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 substrate 11 rather than onto the electrodes 13 and 14. Since the substrate 11 has a smooth surface the electrolytic medium will form a smooth, tnin, homogeneous coating on the substrate 11. Prior art devices coated the electrolytic medium over the electrodes 13,14 thus forming a non-homogeneous coating due to the roughness of the surface onto which the electrolytic medium had to be coated. The coating of the invention also minimizes the stresses placed on the electrolyte by the surface onto which it is coated since the electrolytic medium 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 of electrolytic medium between the substrate 11 and the counter electrode means 13. However, in tne sandwich-type structure there is also a second WO 88/09500 PCT/US8$/O 1772 layer 27 Of electrolytic medium coated atop the counter electrode means 13, Theo sandwich-type structure has several advantages over the planar structure. The main advantage of the sandwich-type structure is the increased rigidity of the sensor otructure due to the extra layers of niaterial applied tnereto. This increased rigidity will minimize the distortion of the electrodes 13 and 14 and electrolytic medium which usually results from theormal 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 the counter electrode 13 and the reference electrode 23. Excellent sandwich structures are possible as a result of the coating techniques developed in our co-pending application Serial No. 053,722, filed on May 26, 1987 which is hereby incorporated by reference. These coating techniques allow for 'snooth, relatively homogeneous coatings of the electrolytic medium to be applied over the electrodes 13, 14.

In the preferred embodiment of the present invention, the perimeter of the working electrode 14 is maximized with respect to the area of contact of the working electrode 14 with the electrolytic medium. This maximization of the perimeter to area ratio results in a corresponding maximization of the signal to noise ratio of the sensor. The theoretical basis for this result is that the area of contact between the working electrode 14 and the electrolytic medium appears to be responsible for the noise in the sensor. Whereas, the electrochemical reaction between the working electrode 14 and the species s; I Ir i I i r llllllllll~" WO 88/09500 t PCT/US88/01772 17 appears to be catalyzed by the electrolytic medium.

Therefore, the triple-pnase boundary between the working electrode 14, electrolytic medium, and the species is the preferred location for the electrochemical reaction between the species and the working electrode 14. As a result, the signal generated by the electrochemical reaction appears to oe directly proportional to the perimeter of the working electrode 14 since the perimeter is a measure of the triple-pnase boundary. The preferred perimeter to area ratio (perimeter /area) is from about 0.4 to 500 and more preferably is from about 2 to about 500.

When the electrodes are located atop the electrolyte, the most desirable electrode geometry is long, thin electrodes whereby the surface area of the electrode in contact with the electrolyte is minimized while the triple-phase boundary at the electrode edges in contact with the electrolyte is maximized. Thus, the optimum configuration for a rectangular electrode occur" wnen one side is much longer than the other side, in which rectangular electrode the perimeter to area ratio and signal to noise ratio is proportional to the ratio of the sides of the rectangle, which is much greater than 1. When the electrolyte is coated over the surface of the electrodes, the length, width and height all become important to the perimeter to area ratio.

Accordingly, in three dimensions it is highly advantageous to maximize one dimension of the electrode while minimizing the other two dimensions to thereby obtain the greatest perimeter to area ratio.

Referring now to Fig. 6, there is shown an electrode configuration which may be adopted in order to maximize the perimeter to area ratio. It should i _j c i II WO 88/09500 PCT/US88/01772 18 be noted that these type of grid pattern electrodes provide a high perimeter to area ratio since both the external perimeter 42 of tne electrode 41 and the internal perimeter 43 around the holes 44 of the electrode 41 both contribute to the signal strengtn of the electrode. Further, when employing the grid pattern, the holes in the electrode serve to help minimize the area of the electrode in contact with the electrolyte and thus minimize the background noise created by the surface area contact of the electrode with the electrolyte. The geometry shown in Figure 6 is the preferred embodiment for obtaining a high and beneficial signal to noise ratio from these microfabricated electrode structures having the electrolyte coated atop the electrodes.

These grid electrodes are easily fabricated by etching uniformly spaced parallel rectangles onto thin copper foil masks. Then, the masks are used to evaporate gold electrodes. A first evaporation of 3500 angstroms of gold is done using the masks and then the masks are rotated 900 and a second evaporation of 3500 angstroms of gold is performed to obtain working electrodes in a grid pattern as shown in Fig. 6.

The preferred grid width is less than 125 micrometers. The preferred grid spacing is less than 200 micrometers. Finally, the preferred number of holes in the grid is greater than 300. These values are somewhat limited by the available microfabrication techniques and improvements in the art of microfabrication should provide the capability of fabricating electrodes having smaller grid widths and grid spacing and larger numbers of holes in these grid pattern electrodes.

It is important to note that the sensing apparatus of the present invention, if it uses a -1 1' W~O $8/09500 'PCT/US8/01772 -19 Nafion electrolyte, must be operated in an environment having at least some humidity, The abaence of water in the environment will prevent the successful operation of the apparatus by hindering 4 the role of the electrolytic medium. Thnis is oacauo the per fluoro memnbrane requires water to activate free protons. Other solid electrolytes, such as polyvinylalcohol and polyethylene oxide, may not require the presence of humidity.

one of ttie primary advantages~ of the micro sensor of the present invention is that it is capable of operation at much lower temperatures than prior art sensor devices. This device is capable of operating at temperatures of from about -400 to about 300 0

C

and more preferably the device is operated at between 0 C and 100 0 C. The most preferred operating temperatures for the device are from about -5 0 C to about 35 0 C. N'o heating means is required to operate the sensor since it can be operated at room temperature if desired. many materials can be used as substrates and membranes over the electrolytes which could not be used in the prior art since extremely high temperature operation is not required for operation of the microsensors of this invention.

Thus, the electrolytic medium of the present invention is characterized by the capability to conduct ions in sufficient quantity at room temperature to allow operation of the microsensor at lower temperatues than were possible with prior art microsensor s.

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 WO 88/09500. PCT/US88/O1772 preferably fed to a microcomputer for normalization of the signal as well as any otner mathematical manipulations such as calibration which may be necessary. The response time for the sensor is usually less than five seconds.

rTne following examples are provided to illustrate certain embodiments of the present invention.

Example 1 A 211 silicon wafer was oxidized to provide an insulating silicon dioxide surface. Tnen the wafer was spin-coated with a 5% tafion solution (Aldrich Chemical Co., Milwaukee, WI) to make a planat electrolytic structure. A two-step evaporation procedure was used to create grid electrode patterns on the surface of tne Nafion layer. An evaporation system containing both e-beam and thermal evaporation capability was used to deposit the electrode structures. A photolitnograpnically etched evaporation mask was prepared from thin copper foil in which a number of parallel rectangles were etched, each 6 mm long and 125 microns wide. After the first evaporation deposition the mask was rotated 900 and a second evaporation was performed. Gold wire (99.9%, Engelhard Minerals and Chemicals Co., NJ) was used as the evaporation source. The electrodes were electrically connected to a power source.

Example 2 A sensor fabricated as in Example 1. was exposed to various gas mixtures at a room temperature of about 70-75OF with the following results. The sensor was operated at a constant potential of +300 m illivolts versus the Platinum/air reference electrode. S/N is the signal to noise ratio of the sensor. The signals are given as normalized values with the signal for H 2 S being taken as 1.0 and all other responses being scaled accordingly.

aRNSOR SIGNALT '~iO VARIjrUS GAS MLXTUR iAS Y, .gAGNAS) 9NNISJiL){ J31N.

92 ppm NO/NO 2 1 2.30 0.28 83 ppm iHS/N 2 1 27.10 0.28 27.1 92 ppm NO/NO 2 21 0.24 0.02 12.0 83 ppm H 2

S/N

2 21 2.72 0.02 13.6 49 ppm N0 2 /air 21 0.02 0.02 ppm SO 2 /air 21 0.036 0.02 1.8 200 ppm CO/air 21 0.00 0.02 0.0 100 ppm HCN/air 21 spiko 0.02 spike to The examples of the present invention described above provide an apparatus for electrolytically detecting a species in a fluid material, which has smooth electolyte coatings with good repeatability. It further provides a solid electrolyte layer having excellent adherence to the electrodes and a thinner electrolyte layer to thereby reduce the diffusion resistance of the apparatus.

Further advantages include the provision of an apparatus having a structure which minimizes stresses on the electrolyte and thereby decreases distortion of the electrolyte as a result of temperature and/or humidity variations.

Furthermore, it provides an apparatus for electrolytically detecting a species in a fluid material which is capable of operating at room temperature or temperatures significantly lower than prior art electrochemical sensors.

I' Llo- 21A It also provides an apparatus for electrolytically detecting a species in a fluid material with a response time which is fast enough for use in applications requiring a very fast response.

The foregoing description of embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications and variations will be obvious to one of ordinary skill in the art in light of the above teachings.

Accordingly, the scope of the invention is to be defined by the claims appended hereto.

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Claims (17)

1. A solid electrochemical sensor which operates at temperatures below 300 0 C for generating an electrical signal in response to contact with a pre-determined species present in a fluid material comprising: a substrate naving at least one surface, a solid electrolytic medium having sufficient ionic conductivity at temperatures below 100°C to sustain an ionic current flow and having first and second surfaces, said first surface of said medium being in contact with and adnering 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 comprising a coating over said reference electrode to prevent exposure of said reference electrode to said species. i, 1. WO 88/09500 PCT/US88/01772 23

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. 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 aoout 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 which form a grid pattern having a perimeter 2 to area ratio of between 2 and about 500.

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. 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 WO 88/09500 PCT/US8/017s72 24 contact with a pre-determined species present in a fluid material comprising a substrate comprising an insulating material and 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 secund 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 sa I 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 contac, with and adhe_.-.ig to said second surface of said first layer of electrolytic medium. 11 WO 88/09500 PCT/US88/01772 25

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. 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 adhlesion promoter on said at least one surface of said substrate to promote adherence of said electrolytic medium and said substrate. S19. An apparatus in accordance with Claim 11 wherein said working electrode means comprises thin strips of metal which form a grid pattern having has a perimeter 2 to area ratio of between about 0.4 and about 500. L- A- 26f 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 microns.

21. A solid electrochemical sensor according to claim 1 and substantially as described with reference to and as illustrated in the accom~panying drawings.

22. A solid oloctrochomical sonsor according to claim 11 and substantially as described with roforonco to and as illustrated In tho accompanyinq drawings, S. DATED THIS 15THT DAY OF AUGUST, 1990, TRANSDUCER RESEARCH, INC. By Its Patent Attorneys Ogg& GRIFFITH HACK CO. Fellows Institute of Paton; Attorneys of Australia.