WO1998017996A1

WO1998017996A1 - Electrochemical gas sensor and method for the detection of nitrogen dioxide

Info

Publication number: WO1998017996A1
Application number: PCT/US1997/019466
Authority: WO
Inventors: Glen W. Hance; Towner B. Scheffler; Alan A. Schneider
Original assignee: Mine Safety Appliances Co
Current assignee: MSA Safety Inc
Priority date: 1996-10-21
Filing date: 1997-10-20
Publication date: 1998-04-30
Anticipated expiration: 1999-04-21
Also published as: AU5001797A

Abstract

The present invention provides electrochemical sensors for the detection of nitrogen dioxide and a method of use thereof. The electrochemical gas sensors comprise a housing in which is disposed a working electrode, a reference electrode and a counter electrode. The electrochemically active surface of the working electrode preferably comprises gold. Electrical connection is maintained between the working electrode and the counter electrode via an electrolyte present within the housing. The nitrogen dioxide electrochemical sensors of the present invention are particularly well suited for use in medical environments, in part, because of their insensitivity to numerous interferent gases found in medical environments.

Description

TITLE

ELECTROCHEMICAL GAS SENSOR AND METHOD FOR THE DETECTION OF NITROGEN DIOXIDE

Field of the Invention

The present invention relates to an electrochemical gas sensor and a method of detecting a gas, and more particularly to an electrochemical gas sensor and a method for the detecting nitrogen dioxide (N0₂) .

Background of the Invention

Recently, nitric oxide has received a tremendous amount of attention in the medical community as a selective pulmonary vasodilator for use, for example, (i) in the treatment of pulmonary artery hypertension which is characteristic of severe adult respiratory distress syndrome and (ii) in certain types of surgery. See e.g., McArthur, C, "Putting NO to the Test," The Journal for Respiratory Care Practitioners, 29 (August/September 1994); Bigatello, L.M. et al . , "Prolonged Inhalation of Low Concentrations of Nitric Oxide in Patients with Severe Adult Respiratory Distress Syndrome," Anesthesiology, 80:4, 761 (1994); and Feldman, P.L., "The Surprising Life of Nitric Oxide," Chemical & Engineering News, 26 (December 1993) . Nitric oxide was first identified as an endogenous vasodilator in 1987. Inhaled nitric oxide has been shown to decrease pulmonary artery pressure in patients with pulmonary hypertension without systemic vasodilation . McArthur, supra . Unfortunately, nitric oxide (NO) is an unstable molecule and combines readily with oxygen (0₂) to form nitrogen dioxide (N0₂) , which has been shown to cause pulmonary toxicity at very low levels. McArthur, supra . Therefore, when treating patients with nitric oxide, it is recommended that nitrogen dioxide concentrations be continuously monitored inline near the patient. McArthur, supra at 30.

Nitrogen dioxide concentrations are often measured with the use of electrochemical gas sensors. In an electrochemical gas sensor, the gas to be measured typically diffuses from the test environment into the sensor housing through a gas porous or gas permeable membrane to a working electrode (sometimes called a sensing electrode) where a chemical reaction occurs. A complementary chemical reaction occurs at a second electrode known as a counter electrode (or an auxiliary electrode) . The electrochemical sensor produces an analytical signal via the generation of a current arising directly from the oxidation or reduction of the analyte gas

(that is, the gas to be detected) at the working and counter electrodes.

To be useful as an electrochemical sensor, a working and counter electrode combination must be capable of producing an electrical signal that is (1) related to the concentration of the analyte and (2) sufficiently strong to provide a signal-to-noise ratio suitable to distinguish between concentration levels of the analyte over the entire range of interest. In other words, the current flow between the working electrode and the counter electrode must be measurably proportional to the concentration of the analyte gas over the concentration range of interest.

In addition to a working electrode and a counter electrode, an electrochemical sensor often includes a third electrode, commonly referred to as a reference electrode. A reference electrode is used to maintain the working electrode at a known voltage or potential. The reference electrode should be physically and chemically stable in the electrolyte and carry the lowest possible current to maintain a constant potential.

Electrical connection between the working electrode and the counter electrode is maintained through an electrolyte. The primary functions of the electrolyte are: (1) to efficiently carry the ionic current; (2) to solubilize the analyte gas; (3) to support both the counter and the working electrode reactions; and (4) to form a stable reference potential with the reference electrode. The primary criteria for an electrolyte include the following: (1) electrochemical inertness; (2) ionic conductivity; (3) chemical inertness; (4) temperature stability; (5) low cost; (6) low toxicity; (7) low flammability; and (8) appropriate viscosity.

Electrochemical gas sensors of the type discussed above are generally disclosed and described in U.S. Patent Nos. 4,132,616, 4,324,632, 4,474,648; and in European Patent Application No. 0 496 527 Al . A comprehensive discussion of electrochemical gas sensors is also provided in a paper by Cao, Z. and Stetter, J.R., entitled "Amperometric Gas Sensors," the disclosure of which is incorporated herein by reference. In general, the electrodes of an electrochemical sensor provide a surface at which an oxidation or a reduction reaction occurs (that is, an electrochemically active surface) to provide a mechanism whereby the ionic conduction of the electrolyte solution is coupled with the electron conduction of the electrode to provide a complete circuit for a current. It is generally believed that the half cell reactions of the working electrode and the counter electrode, respectively, for nitrogen dioxide electrochemical gas sensors (using H₂S0₄ as the electrolyte) are as follows:

N0₂ + 2H⁺ + 2e^~ = NO + H₂0

H O = ^0? + 2H⁺ + 2 e^"

The above reactions result in the following net cell reaction:

N0₂ = NO + ^x^0₂

Although nitrogen dioxide electrochemical sensors as described above have been used in industrial settings, current sensors are generally unsuitable for use in medical environments for a number of reasons. For example, the output of such sensors is dependent upon the concentration of oxygen in the test environment. In many industrial operations, the oxygen concentration is substantially constant and thus the dependence of the output of such sensors upon oxygen concentration is unimportant. In medical environments, however, oxygen concentration in the test environment can vary substantially over time. Moreover, nitrogen dioxide electrochemical gas sensors such as the Nitrogen Dioxide CiTicel® made by City Technology Limited of Portsmouth, England are found to be sensitive to (or subject to interference from) other gases commonly used in the medical arts. Such interferent gases include, for example, nitrous oxide (N₂0) , enflurane, isoflurane and halothane, which are commonly used in anesthesia gases.

It is very desirable, therefore, to develop a nitrogen dioxide electrochemical gas sensor suitable for use in the medical arts.

Summary of the Invention

Accordingly, the present invention provides an electrochemical gas sensor for the detection of nitrogen dioxide suitable for use in the medical arts. In general, the electrochemical sensors of the present invention comprise a housing in which is disposed a working electrode, a reference electrode and a counter electrode. The electrochemically active surface of the working electrode preferably comprises gold (Au) . The electrochemically active surface of the working electrode may, for example, comprise substantially entirely gold or a combination of gold and carbon (C) . Preferably, Au comprises approximately 50% to approximately 75% in the case of a combination of Au and C.

Preferably, the electrochemically active surface of the present reference electrode is chosen to exhibit the following characteristics: (1) the absence of halide ions;

(2) a robust physical nature and (3) an appropriate operating potential, preferably approximately 0 V. As used herein, the phrase "operating potential" refers to the difference in potential between the working electrode and the reference electrode versus the normal hydrogen electrode ("NHE"). Reference electrodes suitable for use in the present sensor include silver/silver sulfate electrodes, mercury/mercurous sulfate electrodes, thallium/thallium sulfate electrodes, lead/lead sulfate electrodes and quinhydrone electrodes. Such materials and other suitable materials for the present reference electrode are identified in Ives, D.J.G. and Jang, G.J., Reference Electrodes: Theory and Practice, Academic Press, 393 (1961), the disclosure of which is incorporated herein by reference. The electrochemically active surface of the reference electrode most preferably comprises silver (Ag) and silver sulfate (Ag S0 ) (that is, a silver/silver sulfate electrode) . Electrical connection is maintained between the working electrode and the counter electrode via an electrolyte present within the housing.

The electrochemically active surface of the counter electrode of the electrochemical gas sensor of the present invention can comprise generally any material commonly used in forming electrodes for electrochemical sensors and suitable to carry sufficient current. In a preferred embodiment, the electrochemically active surface of the counter electrode comprises an electrically conductive carbon. As used in connection with the present invention, the phrase "electrically conductive carbon" refers generally to carbons with resistances in the range of approximately 0.2 kΩ to 180 kΩ. Such resistances were measured with an ohmmeter, using a standard two-probe technique as known in the art wherein the probes were placed approximately 1.5 cm apart upon the surface of the electrode. The carbons used in fabricating counter electrodes for sensors of the present invention preferably also have surface areas in the range of 4.6 m²/g to 1500 m²/g.

In fabricating the working electrode, reference electrode and counter electrode of the present invention, the electrochemically active material is preferably fixed upon a water resistant membrane such a GoreTex^® film.

It has been discovered that the output of nitrogen dioxide electrochemical gas sensors of the present invention is independent of the concentration of oxygen in the test environment. Moreover, it has also been discovered that the output of nitrogen dioxide electrochemical gas sensors of the present invention is insensitive to the presence of many gases commonly present in medical environments. These gases include, for example, halothane, isoflurane, enflurane, desflurane, ether, nitrous oxide, helium, cyclopropane and carbon dioxide. The present nitrogen dioxide electrochemical gas sensors thus provide a significant improvement over current electrochemical sensors designed for the detection of nitrogen dioxide which are generally unsuitable for use in medical environments. The electrochemical gas sensors of the present invention are well suited for placement inline with a source of nitric oxide (for supply of nitric oxide to a patient) to detect undesirable levels of nitrogen dioxide . Brief Description of the Drawings

Figure 1 illustrates schematically a cross-sectional view of an electrochemical gas sensor of the present invention.

Figure 2 illustrates a perspective view of an embodiment of the present counter electrode.

Figure 3 illustrates a perspective view of an embodiment of the present reference electrode.

Figure 4 illustrates an interferent gas study showing the output of several electrochemical gas sensors of the present invention and the output of a Nitrogen

Dioxide CiTicel Sensor Model 7NDH in the presence of 5% halothane .

Figure 5 illustrates an interferent gas study showing the output of several electrochemical gas sensors of the present invention and the output of a Nitrogen

Dioxide CiTicel Sensor Model 7NDH in the presence of 4% isoflurane .

Figure 6 illustrates an interferent gas study showing the output of several electrochemical gas sensors of the present invention and the output of a Nitrogen

Dioxide CiTicel Sensor Model 7NDH in the presence of 4% enflurane .

Figure 7 illustrates an interferent gas study showing the output of several electrochemical gas sensors of the present invention and the output of a Nitrogen Dioxide CiTicel Sensor Model 7NDH in the presence of 12% desflurane .

Figure 8 illustrates an interferent gas study showing the output of two electrochemical gas sensors of the present invention and the output of a Nitrogen Dioxide CiTicel Sensor Model 7NDH in the presence of 14% ether.

Figure 9 illustrates an interferent gas study showing the output of several electrochemical gas sensors of the present invention and the output of a Nitrogen Dioxide CiTicel Sensor Model 7NDH in the presence of 70% nitrous oxide.

Figure 10 illustrates an interferent gas study showing the output of several electrochemical gas sensors of the present invention and the output of a Nitrogen Dioxide CiTicel Sensor Model 7NDH in the presence of 70% helium.

Figure 11 illustrates an interferent gas study showing the output of several electrochemical gas sensors of the present invention and the output of a Nitrogen Dioxide CiTicel Sensor Model 7NDH in the presence of 12% carbon dioxide.

Figure 12 illustrates an interferent gas study showing the output of several electrochemical gas sensors of the present invention and the output of a Nitrogen Dioxide CiTicel Sensor Model 7NDH in the presence of

50% cyclopropane. Detailed Description of the Invention

As seen in Figure 1, electrochemical nitrogen dioxide sensor 1 preferably comprises a housing 5, enclosing a working electrode 10, a reference electrode 20 and a counter electrode 30. In fabricating electrochemical nitrogen dioxide sensors 1 for use in the present studies, a porous spacer or wick 35 was first placed within housing 5. Counter electrode 30 was then placed into housing 5. A porous spacer or wick 40 was preferably then placed within housing 5 followed by reference electrode 20. A porous wick 50 was subsequently placed within housing 5 followed by working electrode 10.

After placement of working electrode 10 within housing 5, the perimeter of working electrode 10 was sealed, preferably via heat sealing, to housing 5. The interior of housing 5 was then filled with an electrolyte such as H₂S0₄ via opening 70. Upon filling of the interior of housing 5 with electrolyte, opening 70 was sealed, preferably via heat sealing using a water resistant membrane such as a GoreTex film (not shown) . In the present studies, housing 5 was also placed within an outer housing (not shown) . The electrical leads of working electrode 10 and reference electrode 20 were shorted with a "snorting-clip" . A detailed discussion of a preferred assembly for electrochemical gas sensor 1 is set forth in U.S. Patent No. 5,338,429, the disclosure of which is incorporated herein by reference.

Wicks 40 and 50 operate to prevent physical contact of the electrodes but allow the liquid electrolyte to contact the electrodes and thereby provide ionic connection between working electrode 10 and counter electrode 30. Preferably, the electrolyte used in electrochemical nitrogen dioxide sensor 1 is H₂S0₄, although many other electrolytes can be used therein.

Reference electrodes 20 for use in electrochemical sensors 1 for the present studies were preferably fabricated via silk screen deposition of an ink comprising silver powder and silver sulfate. This ink was preferably deposited via silk screening upon a GoreTex film as known in the art. As also known in the art, GoreTex films provide a very good support for an electrochemically active material and also provide a good diffusion barrier, allowing analyte gas to diffuse into the electrochemical sensor while preventing escape of electrolyte. The silver/silver sulfate ink may also be deposited using hand painting techniques as known in the art. Preferably, a film of silver/silver sulfate having a thickness in the range of approximately 1 to 10 mil is deposited. The ratio of silver to silver sulfate in reference electrode 20 is not important to the operation thereof.

Working electrodes 10 for use in electrochemical sensors 1 for the present studies were preferably fabricated via silk screen deposition of gold or gold/carbon upon a GoreTex film. The gold or gold/carbon may also be deposited via hand painting. Preferably, a film having a thickness of approximately 1 to 10 mil was deposited. Similarly, counter electrodes 30 for use in electrochemical sensors 1 for the present studies were preferably fabricated via silk screen deposition of a carbon ink upon a GoreTex film. The carbon may also be deposited using hand painting techniques as known in the art. Preferably, a film having a thickness in the range of approximately 1 to 10 mil is deposited for the counter electrode. More preferably, a film having a thickness in the range of approximately 3 to 6 mil is deposited.

Counter electrodes for use in electrochemical sensors of the present invention are preferably fabricated from carbons having relatively low resistance and relatively high surface area. Preferably, such sensors are fabricated from carbons with resistances in the range of approximately .2 kΩ to 180 kΩ. Preferably, the carbons have surface areas in the range of approximately 4.6 m²/g to 1500 m²/g. Suitable carbons for use in the counter electrodes of the present invention are set forth in United States Patent Application Serial No. 08/426,271, filed April 21, 1995, the disclosure of which is incorporated herein by reference. The carbon used in the present studies was Johnson Matthey JMAC carbon.

After deposition of the films for each of the reference electrode, working electrode and counter electrode as described above, the films were sintered to fix the electrochemically active material upon the substrate GoreTex such as is described in U.S. Patent No. 4,790,925, the disclosure of which is incorporated herein by reference.

As illustrated in Figures 1 and 2, counter electrode 30 is preferably shaped in the general form of an annulus or ring. As illustrated in Figures 1 and 3, reference electrode 20 is preferably shaped in a generally circular form (that is, in the general shape of a disk) . As clear to those skilled in the art, however, counter electrode 30, reference electrode 20 and working electrode 10 of electrochemical sensor 1 can be fabricated in many different shapes. Preferably, electrochemical nitrogen dioxide sensor 1 is subjected to a "cook-down" or "equilibration" period before use thereof to provide an adequately stable and low baseline. During the cook-down or equilibration period, electrochemical sensor 1 is stored at ambient conditions for a defined period of time. As common in the art, electrochemical sensor 1 is preferably maintained at operating potential during the cook-down period. As the operating potential of the electrochemical sensor 1 is preferably approximately zero (0) volts, working electrode 10 and reference electrode 20 are preferably electrically shorted during the cook-down period.

Preferably, a substantially stable baseline in the range of approximately 0.10 to 0.20 μA is achieved during the cook-down period. It has been found that a cook-down period of approximately sixteen (16) hours is sufficient to provide an adequate baseline for electrochemical nitrogen dioxide sensor 1, however, briefer cook-down periods have not yet been investigated. Electrochemical nitrogen dioxide sensors 1 used in the studies discussed below were subjected to a cook-down period of greater than sixteen (16) hours.

Studies of sensors 1 were performed under computer control in which sixteen (16) sensors could be tested simultaneously. A baseline reading for each sensor was established as the sensor output after a ten-minute exposure to air (0 ppm nitrogen dioxide) . In testing for nitrogen dioxide concentration, air was first applied to electrochemical sensors 1 for ten (10) minutes followed by application of air having a known concentration of nitrogen dioxide (for example, 20 ppm nitrogen dioxide) for 10 minutes . Response time and response time ratio (RTR) are empirical measures of the speed of response of a sensor and are critically dependent on the manner in which the test is performed (for example, the length of time the experiment lasts and/or the time at which the sensor reaches 100% of its final output) . In the present studies, both response time and RTR were based upon a ten (10) minute exposure to test gas. RTR was calculated by dividing (i) the sensor output after one (1) minute of exposure to nitrogen dioxide test gas by (ii) the sensor output after ten (10) minutes of exposure to nitrogen dioxide test gas. Based upon a ten-minute test, RTR is also the percentage of final response (that is, response or output obtained after ten minutes) obtained in one minute. Response time was generally recorded as the 90% response time (t₉₀) . The t₉₀ response time is the time, in seconds, required for the sensor to reach 90% of the response or output obtained after ten minutes of exposure to test gas . The sensitivity (in units of μA/ppm N0₂) was established as the sensor output after ten (10) minutes of exposure to nitrogen dioxide .

All the sensor cells studied had a pattern of five (5) 7/64 inch diameter inlet holes to allow the test gas to enter the sensor cells. The test gas had a nitrogen dioxide concentration of approximately 10 to 20 ppm. An average output of approximately 0.55 ± 0.1 μA/ppm was obtained under these experimental conditions. As clear to one of ordinary skill in the art, however, sensitivity can generally be increased by increasing the total surface area of such inlet holes to allow more gas to enter the sensor cell . The electrochemical sensors of the present invention were found to provide a substantially linear signal over at least the range of approximately 0 to 300 ppm N0₂. Linearity studies at higher concentrations of N0₂ were not possible because of limitations of the available instrumentation. The response time of the present sensors was found to be approximately 30 seconds to 90% and was independent on the age of the sensor. The RTR of the present sensors was found to be approximately 95%.

The sensitivity of the present sensors was found to be affected by humidity, however. In that regard, sensitivity was found to decrease if the sensor was stored in low humidity, whereas sensitivity was found to increase if the sensor was stored in a humid environment. In general, sensitivity was found to decrease if the sensors were stored in an environment having a relative humidity of less than approximately 15%. Preferably, therefore, the sensors of the present invention are stored in an environment having a relative humidity in the range of approximately 15 to 90%. It is believed that the drop in sensor sensitivity at low humidity is a result of loss of solution contact. This "drying" and the resultant sensitivity loss at low humidity are reversible upon exposure of the sensor to ambient conditions in which the relative humidity is preferably in the range of approximately 15 to 90%.

The results of several interferent studies are set forth in Table 1. The data provided for each interferent gas correspond to the sensor output (that is, the indicated concentration of nitrogen dioxide in ppm) upon exposure of the sensor to 100 ppm of the interferent gas. In Table 1, the results achieved with the present sensor are compared to the results achieved with Nitrogen Dioxide CiTicel sensors available from City Technology. The data provided for the City Technology sensors were taken from the corresponding City Technology technical manual. The results indicate that the present sensor is generally less susceptible to erroneous results arising from the presence of the interferent gases studied than the City Technology sensor. Although the output of the present invention is shown to be somewhat sensitive to the presence of H₂S and Cl₂, such interferent gases are not expected to be present in medical environments.

c

DO

m

CΛ m rπ TABLE 1

rπ 10

.03

Figures 4 through 12 illustrate further interferent studies in which a number of gases commonly found in a medical environment (that is, halothane, isoflurane, enflurane, desflurane, ether, nitrous oxide, helium, carbon dioxide and cyclopropane, respectively) were introduced into a flowstream to study the interferent effect thereof on a number of electrochemical gas sensors of the present invention and on a Nitrogen Dioxide CiTicel Model 7NDH sensor. In general, the interferent gas under study was introduced at the concentrations indicated in Figures 4 through 11 into a flowstream that initially comprised approximately 30% oxygen and approximately 70% nitrous oxide (N₂0) with approximately 2 ppm nitrogen dioxide and approximately 10 ppm nitric oxide. The flow rate of the flowstream was approximately 1 L/min.

In general, electrochemical gas sensors of the present invention in which working electrode 10 comprised (1) Au and (2) Au/C (75/25) were studied. The outputs of a number of such sensors are set forth in each of Figures 4 through 12. The point of time at which the interferent gas was introduced into the flowstream is marked with a downward arrow in each of Figures 4 through 12.

As illustrated, the outputs of the electrochemical gas sensors of the present invention are generally less susceptible to interference from gases commonly found in a medical environment than the Nitrogen Dioxide CiTicel sensor. In several experiments, a downward oriented spike was experienced in the output of electrochemical gas sensors under the present invention when the interferent gas was introduced into the flowstream. Such downward oriented spikes are believed to result from a decrease in output caused by dilution of the concentration of nitrogen dioxide of the flowstream. The effect is more pronounced for highly volatile gases such as halothane, isoflurane, enflurane and desflurane.

Although the present invention has been described in detail in connection with the above examples, it is to be understood that such detail is solely for that purpose and that variations can be made by those skilled in the art without departing from the spirit of the invention except as it may be limited by the following claims.

Claims

WHAT IS CLAIMED IS:

1. An electrochemical gas sensor for the detection of nitrogen dioxide, comprising: a housing, the housing having disposed therein a working electrode, a reference electrode and a counter electrode, the working electrode comprising gold, electrical connection being maintained between the working electrode and the counter electrode via an electrolyte present within the housing.

2. The electrochemical gas sensor of Claim 1 wherein the counter electrode comprises an electrically conductive carbon.

3. The electrochemical gas sensor of Claim 1 wherein the reference electrode comprises silver and silver sulfate .

4. The electrochemical gas sensor of Claim 1 wherein the working electrode comprises gold and carbon.

5. The electrochemical gas sensor of Claim 1 further comprising circuitry adapted to maintain the potential difference between the working electrode and the reference electrode at approximately 0 volts with respect to the normal hydrogen electrode .

6. The electrochemical gas sensor of Claim 1 wherein the reference electrode comprises an electrochemically active substance selected from the group consisting of silver/silver sulfate, mercury/mercurous sulfate, thallium/thallium sulfate, lead/lead sulfate and quinhydrone .

7. A method for the detection of nitrogen dioxide in an environment, the method comprising the step of placing an electrochemical gas sensor in contact with the environment, the electrochemical gas sensor, comprising a housing, the housing having disposed therein a working electrode, a reference electrode and a counter electrode, the working electrode comprising gold, electrical connection being maintained between the working electrode and the counter electrode via an electrolyte present within the housing.

8. The method of Claim 7 wherein the counter electrode comprises an electrically conductive carbon.

9. The method of Claim 7 wherein the reference electrode comprises silver and silver sulfate.

10. The method of Claim 7 further comprising the step of maintaining the potential difference between the working electrode and the reference electrode at approximately 0 volts with respect to the normal hydrogen electrode.

11. The method of Claim 7 wherein the working electrode comprises gold and carbon.

12. The method of Claim 7 wherein the reference electrode comprises an electrochemically active substance selected from the group consisting of silver/silver sulfate, mercury/mercurous sulfate, thallium/thallium sulfate, lead/lead sulfate and quinhydrone.

13. The method of Claim 7 wherein the environment comprises a nitric oxide source for supplying nitric oxide to a patient.

14. The method of Claim 13 wherein electrochemical gas sensor is placed inline with the source for supplying nitric oxide to the patient.