CA2449549C - Hybrid film type sensor - Google Patents

Abstract

A miniaturized gas sensor comprised of thick- or thin-film type electrodes, on a non-conductive supportive substrate, and in contact with a solid ionomer electrolyte, for the detection of toxic gases, i.e., carbon monoxide, and other oxidizable or reducible gases and vapors is described. The all-solid planar sensor cell has two or more film type electrodes arranged on a non-conductive planar surface of a supportive substrate. The electrodes are discrete and in intimate contact with the same solid polymer ionomer membrane. The sensor cell contains no liquid electrolyte and is operated in a constant-voltage, potentiostatic or potentiodynamic mode. A high sensitivity to a select gas or vapor is achieved by a novel three-phase contact area design for a sensing electrode which provides contact with the solid ionomer electrolyte, as well as the gas sample via diffusion openings or holes that penetrate through the supportive substrate.

Description

HYBRID FILM TYPE SENSOR
Field of the Invention [oooi] The invention is directed toward a sensor having improved repeatability and efficiency.

Background of the Invention jo0o2) Film based techniques have generally been investigated for a wide variety of sensors, as reported by Wenyi et al., 1997; Hughes et al., 1997;
Staley, 1996; Agbor et al., 1995; Tan and Tan, 1995; Menil et al., 1994;
Kunnecke et al., 1994; Creasey and Vamey, 1994; Geistlinger, 1993; Ishiji et al., 1993;
Najafi et al., 1992; Hampp et al., 1992; Nakano and Ogawa, 1994; Yamazoe and Miura, 1994; and, Madou and Otagawa, 1989. While solid-state gas sensors may have the advantage of being able to operate at elevated temperatures, they also may have the disadvantages of slow response and recovery time and a high intemal operating temperature as reported by Liu et al., 1993; and Narducci et al., 1993. More recent literature (Schwebel et af., 1997; Sheng et al., 1997;
Micocci et al., 1997) eludes to more substantial development work yet to be done.

(00031 In Kinlen et al., 1994, a Nafion -coated metal oxide pH sensor is generally characterized as having sputtered iridium oxide sensing and silver/silver chloride reference electrodes on alumina ceramic substrates. Nafion may have been used as a cation-selective ionomer coating in order to decrease the oxidation-reduction error typically affecting the performance of metal oxide pH
electrodes. In Yasuda et al., 1994, the use of Nafion as polymer-electrolyte for a thin-film CO sensor is generally described with macro-sized, sputtered Pt sensing and counter electrodes and a smaller, sputtered Au electrode as a reference electrode. A 5 wt% n-propyl alcohol solution of Nafion (DuPont, 1100 EW) may be used to form the polymer electrolyte film over the electrodes by casting.
The polymer is usually washed and protonated in aqueous sulfuric acid prior to casting. A theorized lifetime of this sensor is normally less than one month.
During this time, the CO oxidation current typically decreases steadily down to a few percent of its original value without any period of stable measurement signal.
The lifetime of the device may be extended up to three years by laminating the polymer electrolyte layer with a cast perfluorocycloether-polymer film in order to keep the CO permeability coefficient through Naflon constant. Theoretical calculations often reflect the drift rate of the signal could be significantiy reduced under these conditions.

1ooo4] A description of typical state-of-the-art hydrated solid polymer electrolyte or ionomer sensors and sensor cells is generally described by Kosek et al. U.S. Patent 5,527,446; LaConti and Griffith, U.S. Patent 4,820,386;
Shen et al., U.S. Patent 5,573,648; and, Stetter and Pan, U.S. Patent 5,331,310. These sensor cells, based on hydrated solid polymer electrolyte or ionomer technology, may have several advantages over conventional electrochemical sensor cells.
The catalytic electrodes are normally bonded directly to both sides of a proton conducting solid polymer ionomer membrane providing a stable electrode to electrolyte interface. One side of the electrolyte membrane is usually flooded with distilled water, making the sensor cell self-humidifying and independent of extemal humidity. Since no corrosive acids or bases are generally used in the sensor cell, a lifetime of over 10 years may be experienced for solid polymer ionomer sensor cells. Finally, the sensor cells may be easy to maintain and may be ideal for use in remote, unattended environments because maintenance typically entails little more than addition of water to the reservoir in the sensor housing every several months and monthly calibration checks.

[ooos] A disadvantage of the state-of-the-art sensors described above may be that the signal-to-noise ratio is not be conducive to detection of very low concentrations (parts per billion, ppb) of important environmental and biomedical gases and vapors. Also, response time may be relatively slow, and reproducibility between sensors and sensor cells may be difficult to achieve. Also, they are relatively costly.

[ooos] Recently, miniaturized thick- and thin-film type sensors have been developed where the solid ionomer membrane often acts as a conduit between the gas to be detected (sample gas) and the sensing electrode (Yasuda et al., 1994). The sample gas usually permeates through the membrane itself where a 3-phase contact area is established. A disadvantage with this configuration may be that the solid ionomer membrane water content often controls the gas permeation rate as well as proton conductivity. As the humidity increases, the membrane water content typically increases. This may cause an increase in the gas diffusion rate as well as proton conductivity and sensor signal response.
A
method for controlling or fixing the water content of the membrane may be to have a water reservoir on the back side of the membrane, directly opposite to where the film type electrodes and non-conductive supportive substrate are located. However, the back side of the membrane is often required to be free of liquid so that the sample gas can diffuse through the membrane to the sensing electrode.

[ooo7] U.S. Patent No. 4,812,221 to Madou et al. ("Madou") typically relates to a gas sensor having a porous member located in a passage adjacent and generally in contact with a sensing electrode. The pore size of the porous member may be controlled by varying the processing parameters, such as current, hydrogen fluoride concentration, and the like. In addition, Madou appears to indicate a number of other steps for providing the gas sensor, such as sizing the pores in the porous membrane, sizing the pores of the sensing electrode, and selecting the materials of the electrodes. A problem often associated with a sensor provided in accordance with Madou is difficulty in repeatabiiity and/or reproducibility due to the numerous variations from one sensor to another. Another difficulty may be problems or costs in manufacturing sensors due to the quantity of steps, where it is often believed that manufacturing becomes more expensive as the quantity of steps is increased. Another possible disadvantage is that the permeability coefficient of the filter in Madou is not disclosed to be utilized for determining the membrane's thickness and other physical characteriztics, which optimizes the sensor's response time. Still a further possible disadvantage is that Madou's filter is inert and does not react, if desired, with the gas that diffuses through the membrane.

[ooos] What is desired, therefore, is a sensor having improved repeatability. Another desire is to provide a sensor having improved response time. A further desire is to provide a sensor having an improved signal to noise ratio. A still further desire is to provide a sensor that is easy to manufacture with reduced costs while maintaining or improving its operational and manufacturing efficiency.

Summary of the Invention [ooo9] This invention is directed toward a controllable and reproducible gas sensor configuration having a three-phase contact area, whereby the sample gas diffuses to the sensing electrode and solid proton conductive membrane through openings, holes or slits that extend through the non-conductive supportive substrate as well as a solid diffusion membrane.

[oooioj This invention is further directed toward a gas sensor where the gas diffusion process is decoupled from the proton conduction process. Hence, the invention controls gas flow via the holes in the substrate and not merely through the diffusion membrane that is in contact with the sensing electrode, which is known in traditional sensors. Therefore, the invention may use, in addition to the substrate holes, a non-electrolytic or an electrolytic membrane to control gas flow. The gas diffusion is controlled by through openings of known area in the substrate or in the substrate and an additional rate limiting gas diffusion barrier film, eg: polyethiene, while proton conduction takes place only through a proton conductive electrolyte layer, e.g., a Nafion membrane.

[OOo11j The invention is also directed toward utilizing a method of mass producing film type gas sensors by stacking a number of component layers to form a series of adjacent sensors which are subsequently separated into individual sensors.

[00012] The invention is still further directed toward a gs sensor utilized in conjunction with a gas sensor control circuit.

[00013] The invention is also directed toward a gas sensor utilized in a gas sensing instrument.

[00014] The invention achieves the foregoing and other objects by providing a sensor cell having a substrate with a first surface and a second surface, a sensing electrode in contact with the substrate, and an opening extending from the first surface to the second surface proximate to the sensing electrode for controlling a gas flow. The invention also includes a gas diffusion membrane being of electrolytic material in contact with the sensing electrode and placed within the opening and between the gasses to be detected and the sensing electrode.

[00015] In a preferred embodiment, the thickness of the gas diffusion membrane is calculated based upon a parameter selected from the group consisting of a material of said gas diffusion membrane, a difference in partial pressure of the gas across said gas diffusion membrane, a permeability coefficient of said gas diffusion membrane, and combinations thereof. More preferably, the gas diffusion membrane has a thickness determined from a permeability coefficient of the gas diffusion membrane.

[00016] In a further preferred embodiment, the gas diffusion membrane is proton exchange membrane, especially an anion, hydroxide ion exchange membrane. Alternatively, it can be an electrolytic material.

Brief Description of the Drawings [00017] Figure 1 shows a schematic top view of the non-conductive supportive substrate.

-g-[ooo1s] Figure 2 shows a film type electrochemical sensor cell with Pt/Air (02) reference.

[o0019] Figure 3 shows a film type electrochemical sensor cell with a polymeric gas-diffusion layer over the sensing electrode membrane.

[0002o] Figure 4 shows a gas sensor control circuit.

[00021] Figure 5 shows a gas sensor utilized in a gas sensing instrument.
Detailed Description of the Invention [00022] The present invention overcomes the limitations of the sensors mentioned above by uniquely combining an advanced solid polymer ionomer membrane configuration with a film type electrode on a non-conductive supportive substrate. The substrate has diffusion openings or holes having a known area which permit easy access of the sample gas, through a diffusion membrane, to a sensing electrode contact area. The sensor configuration provides a three phase contact area at the interface of the membrane, the sensing electrode, and the gas being detected. This design utilizes the precision of solid-state device fabrication techniques to yield inexpensive, low maintenance, highly sensitive, rapidly responsive, and reproducible sensor devices for environmental, industrial, and biomedical monitoring.

[00023] Figure 1 shows the top view of a substrate (1), which is preferably non-conductive and may be a sheet or film of ceramic or alumina, having holes (2). Holes (2) are shown to be distributed in generally parallel rows but such an arrangement of uniformity is not required for proper operation of the sensor.
The distance between holes (2) in the parallel rows and the distance between the rows determine the dimensions of the sensor. In a preferred embodiment, holes (2) are ideally punched in a single step, while the alumina plate is still soft, in the "green"
stage of substrate fabrication, prior to high-temperature sintering. In other embodiments, holes (2) are created using laser ablation or use of soluble fillers.
In further embodiments, any manner for providing holes (2) is envisioned.

1000241 Conducting leads 32, 33, and 34 and thick- and thin-film electrodes are formed on substrate (1) for multiple electrodes. Leads 32-34 may be provided using screen printing or lithographic techniques or any other manner for providing leads 32-34 on substrate (1).

[00025] A typical sensor design utilizing this method is shown in Figure 2, which has reference electrode (4) and a Pt counter electrode (5): Reference electrode material includes Platinum or Pt/Air (02) and counter electrode material includes Platinum. The contact for the sensing electrode (6) is a ring concentric to the hole. This ring can be made of smooth, rough or platinized platinum. In other embodiments, this ring may be Platinum, Gold, Titanium, Tantalum, Zirconium, Tungsten, or Niobium. Some platinization of the ring may provide better contact. Simultaneous platinization of electrodes can be performed by customized electrolytic plating on properly masked multi-sensor plates.

[000261 The sensing or working electrode (6) may be a disc of Teflon -bonded or Nafion-bonded platinum or other electrocatalyst. A number of discs are deposited on an ionomer film, such as Nafion electrolyte membrane (8) at uniform distances from each other, for instance, by decal transfer, silk printing, spray painting, artist brush lettering, or by any approach which lends itself to uniform deposition of a design on a transfer substrate without waste. The discs' distances from center to center are the same as for the holes of Figure 1. The diameter of the sensing or working electrode disc is larger than the diameter of the hole in Figure 1 to allow for contact between the disc and the sensing electrode support ring of Figure 2. Instead of a single large hole per sensor of Figure 1(which requires the use of the substrate to control diffusion of the analyte), a series of smaller openings may be used, with small enough diameters to control diffusion independently of the analyte flow. The areas of the openings are chosen so as to control diffusion of the sample gas toward the sensor and to maintain a constant diffusion rate independent of any changes in the sample gas flow rate. By using a number of these diffusion-controlling orifices, a reasonably large signal may be obtained. The range for the length to diameter ratio for these orifices is 2/20 with a preferred ratio of 3/10.

[000271 Over the empty alumina surface (the surface with no printed leads and electrodes) a gas-permeable diffusion film (9) is deposited. Film (9) is made to conform to the sensing electrode over holes (2), as shown in Figure 3, or hangs loose over sensing electrode (6). The substrate (with multiple arrays of printed conductors), the Nafion membrane (with multiple sensing electrode discs), and the gas-permeable film are arranged as shown in the schematic representation of Figure 3. After all the components are unitized, the resulting structure is cut in individual sensor units.

[0002al Because holes (2) control the diffusion, or flow, of gas that diffuse toward sensing electrode (6), membrane (9) is of a polymeric material.
Polymeric is defined to be either electrolytic, such as an ionomeric film, or non-electrolytic.
Depending upon the application or experiment, membrane (9) may be of a non-electrolytic material when no reaction, or interference, is desired between membrane (9) and sensing electrode (6). In other embodiments, membrane (9) may be of an electrolytic material when reaction between membrane (9) and sensing electrode (6) is desired. In further embodiments, membrane (9), whether electrolytic or non-electrolytic, may, in addition to holes (2), provide a second structure to control gas diffusion. However, it is understood that holes (2) provide the primary gas diffusion structure to the invention.

[000291 Permeability of membrane (9), which is an indication of the diffusion rate of a gas through membrane (9), may be expressed as a coefficient, which is a function of the gas to be diffused through membrane (9) and the material selected for membrane (9).

[000301 In other words, the permeability coefficient is an indication of the amount of molecules that may move through membrane (9) with a certain thickness, at a certain temperature, under a certain pressure, and in a given time.
Hence, a user selects membrane (9) for use with the invention. A first step is for the user to determine the gas to be diffused and the type of material, based on reactive properties with the gas, to be used in the sensor. Then, based on the permeability coefficient of the material and gas to be diffused, the user determines the thickness, surface area of membrane (9), and other physical characteristics of membrane (9).

[00031] By selecting membrane (9), the sensor is completed quickly and easily, thereby reducing manufacturing costs. Further, the sensor has enhanced repeatability because membrane (9) is generally of a similar material, thickness, surface, area, and permeability coefficient from sensor to sensor. Hence, membrane (9) is controlled during manufacturing in an improved manner over the prior art.

[000321 It is understood that the thickness of membrane (9) is merely one physical characteristic that may be determined from the permeability coefficient and that other characteristics may be calculated.

[000331 The formula for determining the thickness of membrane (9) based on the permeability coefficient is:

1000341 i=PdxAxaP/d or d=PdxAxaP/I

The flux of gas through the gas permeable membrane (9), and thus the measured amount of current i, depends on the difference in partial pressure AOP of the gas across the membrane (9). The area A is equal to the geometric sensing electrode area and d is the membrane (9) thickness. Pd is the permeability coefficient.

[00035] The formula for determining the volume of gas diffusing through membrane (9), per second, based on the permeability coefficient is:

[00036] Fd = Pd x Ac where Fd is the flow density, measured in mol/cm2sec, assuming a 1 cm thick membrane, standard pressure of I atm, and standard room temperature. Pd is the permeability coefficient, which may be measured in units of cm/sec. Ac - c1- co is the concentration difference across the membrane, measured in mol/cm3, where Ac is the difference between the two concentrations on both sides of the membrane. cl is the concentration of the material inside the membrane, which is preferably located on a surface of the sensing electrode. co is the concentration outside of the membrane, which is preferably the gas sample, surrounding air outside the chamber, or the gas flowing through a GC (in which we want to measure how much of a specific gas, for example H2S, is contained in that gas sample).

[00037] The formula for determining the concentration of gas within membrane (9) as a function of time based on the permeability coefficient is:
[00038] cl (t) = ca X (1 - e"u) T= V/(A x Pd)...is the time constant of the equation above A is the geometrical area of the membrane V is the volume of the material in front of the membrane Pd is the permeability coefficient In this example cl is a function of time and changes from zero at c, (o)tocl (fort=inflnity) = co [00o3s] The following is a non-exhaustive list of possible materials and permeability coefficients that may be used for membrane (9):

Table 1.

Temperature Permeability coefficient Gas deg C Glassine Polyethylene Carbon dioxide 30 9.7 28.1 Hydrogen suiphide 30 9.7 43.0 Oxygen 30 10.5 6.9 Nitrogen 0 11.2 0.25 30 9.4 2.1 50 9.3 7.4 70 8.4 22.0 Units cc x cm x 1010/sq cm x sec x dcm Hg at Standard Temperature and Pressure Table 2.
Katio Material P02 PC02 (CO2/02) PH2O
Polyacrylonitrile 0.0003 0.0018 6.0 300 Polymethacrylonitrile 0.0012 0.0032 2.7 410 Polyvinylidene chloride 0.0053 0.029 5.5 1 Barex (Sohio Co.)* 0.0054 0.018 3.3 660 Polyethylene terephthalate 0.035 0.17 4.9 175 Nylon 6 0.038 0.16 4.2 275 Polyvinyl chloride (unplasticized) 0.045 0.16 3.6 275 Polyethylene (0.964 density) 0.40 1.80 4.5 12 Cellulose acetate 0.80 2.40 3.0 6,800 (unplasticized) Butyl rubber 1.30 5.18 4.0 120 Polycarbonate 1.40 8.0 5.7 1,400 Polypropylene (0 907 density) 2.20 9.2 4.2 65 Polystyrene 2.63 10.5 3.8 1,200 Neoprene 4.0 25.8 6.5 910 Teflon 4.9 12.7 2.6 33 Polyethylene (0.922 density) Poly (2,6 dimethyl 15.8 75.7 4.8 4,060 phenylene oxide) Natural rubber 23.3 153 6.6 2,600 Poly (4 methyl pentene 1) 32.3 92.6 2.9 ---Poly dimethyl siloxane 605 3,240 5.3 40,000 Temperature is 30 C for all of the above *High acrylonitri{e copolymer Units cc x cm x 10'10/sq cm x sec x dcm Hg at Standard Temperature and Pressure Table 3.
0, Permeabilit~
Polymer Membrane 10' [mol m/s m Pa]
PTFE. 0.25 Mylar 0.0063 Nylon 6 0.013 PVA10'10 0.033 PVC 0.047 Methyl cellulose 0.233 Cellulose acetate 0.267 PE, high density 0.33 Polystyrene 0.4 Natural rubber 8 Fluorosilicone 36.6 Table 4.

Permeability X1010 moles/sec Polymer 02 CO2 H20 Dimethyl silicone rubber 804 4200 50,000 Natural rubber 32.0 Polyethylene (low density) 10.7 33.0 444 Teflon 7.6 21.0 46 Polystyrene 1.6 4.0 -----Polypropylene 1.34 4.5 100 Mylar 0.025 0.046 70 For 1-mil thick membranes, 1 cm2, at room temperature with a differential pressure of 760 mm across the membrane.

[oooao] The following is a non-exhaustive list of possible materials and thicknesses that may be used for membrane (9):

Table 5. Electrode output and response time as a function of cathode area, membrane, and membrane thickness 97.5% Response Output Time Electrode Membrane Thickness (air) (Air-- Nitrogen) Galvanic Teflon 0.5 mil 4.40 a 2.0 sec (55-mil cathode) " 1.0 mil 1.63 pa 8.0 sec Polypropylene 0.5 mil 0.80 ya 10.0 sec " 1.0 mil 0.222 pa 30.0 sec Polarographic Polypropylene 0.5 mil 0.80 na 18.0 sec (1.0-mil cathode) " 1.0 mil 0.41 na 45.0 sec Teflon 0.125 mil 8.3 na 0.3 sec Teflon 0.25 mil 4.2 na 0.6 sec Teflon 0.375 mil 3.75 na 1.0 sec Teflon 0.5 mil 2.70 na 2.2 sec Teflon 0.75 mil 2.34 na 7.0 sec Teflon 1.0 mil 1.78 na 17.0 sec Fermentation Special 110 na 45.0 sec (Polarographic Silicone withl0-mil Composite cathode) [00041] The film type sensor configuration described above is integrated with a potentiostat and a voltage of approximately +0.1 V is applied to the Pt sensing electrode with respect to a Pt/Air (02) reference. This corresponds to an applied potentiostatic voltage of approximately 1.16 V with respect to a normal hydrogen electrode (NHE).

[00042] Gas samples of air and 7.4 ppm SO2 in air are introduced into the sampling port of the fixture described above. The gas flow is approximately _ 60 cm3/min and temperature is approximately 25 C. The sample gas diffuses through the 80-mil hole in the non-conductive substrate and electrochemically reacts at the exposed sensing electrode/solid ionomer electrolyte surface.
Humidification is provided by the liquid water in the reservoir which soaks the opposite, or back side of the membrane where the electrode structures are located.

[00043] The background response signal with air is 30 nanoamps (nA).
The response signal with 7.4 ppm SO2 in air is 135 nA. This corresponds to a net response signal for 7.4 ppm S02 in air of 105 nA or 14.2 nA/ppm per 80-mil hole.
It is possible to increase the magnitude of signal and signal-to-noise ratio by increasing the number of holes in the substrate above the integral sensing electrode structure.

[00044] It is also possible, with this configuration, to detect other oxidizable or reducible gases such as CO, NO, NO2, H2S, ozone, C02, hydrogen, hydrazine, ammonia, HCI, alcohols and acetone.

[00045] Referring to Figures 4 and 5, a block diagram of the sensor control circuit (13) is shown. The sensor control circuit (13) is designed to:
1) control the potential of the sensing electrode (6) at a predetermined voltage (the "potentiostatic voltage", or "Epot'); 2) measure the temperature; 3) convert the gas concentration-related current to a temperature-compensated voltage signal;
and 4) provide properly amplified voltage to the data acquisition/storage microprocessor (14). An on-board micro power-regulated power supply (16) uses the microprocessor's (14) power supply to provide the required 1-3.9 volts for the sensor circuitry. The DC power can be supplied by a 6-V battery (16d) or an AC
adaptor (16e).

[ooo46] The control amplifier portion (17b) of the sensor control circuit (13) consists of a micro power operational amplifier (e.g., MAX407 or LM6062). The sensing (6), counter (5) and reference (4) electrode portions of the sensor -assembly (1) are in the feedback loop of the control amplifier (1 7b) as shown in Figure 4, a standard configuration for potentiostat circuits. An adjustable voltage divider (17a) allows the polarizing voltage (Epot) to be set at a predetermined voltage range such as 0 to 50 W. This signal is compared to the reference electrode (4) voltage (which appears with it at the summing junction) by the control amplifier (1 7b) of the sensor control circuit (13). The latter adjusts the current through the sensor cell (10) to minimize the difference between the Epot and the reference electrode (4) voltages.

[00047] The resulting sensor cell assembly (19) current (flow of electrons from sensing electrode (6) to counter electrode (5)), which is linearly related to the concentration of gas, is transformed into a voltage signal by the current-to-voltage converter (15a). Temperature compensation of the sensor signal is effected in the next stage of amplification (15b) using a thermistor (18a) which is positioned in the gas sensor housing (10). The last stage of amplification (15c) provides, if required, inversion of the voltage signal as well as gain adjustment, to permit calibration for normal variations in sensitivity among sensors. The same type of micro power operational amplifier is used for these stages (15a), (15b), (15c) as for the control amplifier (15b). The transformed current signal is directed to an A/D channel on the data acquisition board of the microprocessor (14).

lo0048] Power for the sensor control circuit (13) is provided by a Duracell 6-V battery (16d) (PX 28A or 28L) through a micro power-regulated power supply (16). The power supply (16) utilizes a voltage inverter (e.g., ICL 7660) (16a) to convert the positive battery voltage to a negative voltage of the same magnitude, and a positive voltage regulator (e.g., MAX663) (16c) and negative voltage regulator (e.g., MAX 664) (16b) to provide a stable t3.9 volts.

[00049] The film type gas or vapor sensing instrument (12), as shown in Figure 5, includes the sensor cell assembly (19), potential-control circuitry (13), power supply (16), and the microprocessor (14) with the data acquisition-recording unit. The sensing instrument (12) is preferably battery operated, and has the ability to sample the gas or vapor and temperature signals at intervals and store in the random access memory (RAM) on the data acquisition board days to weeks of data. The data acquisition circuit microprocessor is programmed to sample and store the gas concentration signals at preset intervals. Data are off-loaded to a personal computer by accessing the microprocessor through an RS232 port.

[oo050j The sensor cell assembly (19) and its potential-control circuit (13) are integrated with a battery-operated microprocessor (14) of 32K memory, which samples the sensor signal as well as temperature and other signals at 10-, 20-, or 30-second intervals and stores an average value at intervals of 2, 5, or minutes according to a programmable protocol. The data acquisition/storage unit in the microprocessor (14) can record 8 days of data, storing at 2-minute intervals, or up to 40 days storing at 10-minute intervals. In clinical testing to date, a 2-minute interval is suitable for one-day clinical studies and a 10-minute interval is appropriate for extended use. The microprocessor (14) with data acquisition/logic circuit can be programmed to sample more than one analog signal from the control circuit (13), and to convert these to digital signals and store them (i.e., gas concentration and temperature) at preset intervais together with real-time data. Data are off-loaded to a personal computer by accessing the microprocessor (14) through an RS232 port. After downloading, the digital data are converted to engineering units of gas concentration and temperature, and can be graphed by a menu-driven Lotus 123 spreadsheet. Through a potentiometer in the gain amplifier circuit (15c), the device can be calibrated with calibrated gas samples, to indicate gas concentrations in the ambient. The potential-control circuit (13) shown in Figure 4 is powered, in a preferred embodiment, by six volt AA-size batteries (16d). A typical microprocessor (14) with data acquisition-recording capability that has been successfully used is sold by ONSET
Computers, Falmouth, MA, under the product name of "Tattletale Lite ." The sensor cell assembly (19) with its control circuit (13) is also designed to yield a current or voltage signal proportional to gas flux that could be used to continuously transmit the data to a remote receiving device or central monitoring station or unit.

[ooo51] The sensing electrodes can be organized in multiple arrays or sets containing a necessary number of counter or reference electrodes. Reference electrodes such as Pt/air (02), Pt02, or dynamic hydrogen electrode as described by Giner (1964) may be employed. Electrically driven 3- or 2-electrode film type configurations may be employed using potentiostatic or potentiodynamic control of the potential of the sensing electrode (6). While the potentiostatic technique is a steady-state measurement, the potentiodynamic technique is a transient-type measurement method. In practice, the potentiodynamic process consists of four basic steps: a) hold sensing electrode potential constant with respect to the reference electrode to form a well-defined oxide layer; b) place the sensing electrode at open circuit to react the oxide layer with the gas for a pre-determined time, during which the sensing electrode rest potential shifts, depending on reactant concentration; c) resume potentiostatic control of the sensing electrode, changing the potential back to the initial potential and electrochemically regenerating the oxide layer; and d) measure the current (or charge) necessary to regenerate the oxide layer. The primary advantages of the potentiodynamic method are increased sensitivity, specificity, and stability. Two-electrode configurations require a reversible or stable counter-reference electrode such as Pt/air (02), Pt02 or Pt/H2 which has a higher BET (Brunauer, Emmett, Teller) surface area (25 m2/g or larger) and/or larger geometric surface areas than the sensing electrode.

[00052] Electrochemically reversible electrodes may be used in 3 or 2 electrode configurations, but especially in a 2 electrode arrangement where the counter electrode also acts as a reference electrode. Electrochemically reversible electrodes are constructed of stable catalyst materials and usually have a relatively large electrochemical active surface area so that they remain stable and their potential is not perturbed by small current flow. Examples include Pt02 and Ag/AgCI electrodes.

Claims (5)

1. A sensor cell for detecting gases comprising:
a substrate having a first surface and a second surface;
a sensing electrode; said sensing electrode in contact with said substrate;
an opening extending from said first surface to said second surface proximate to said sensing electrode for controlling a gas flow;
a gas diffusion membrane in contact with said sensing electrode and placed within said opening and between the gases to be detected and said sensing electrode;
a thickness of said gas diffusion membrane calculated based upon a parameter selected from the group consisting of a material of said gas diffusion membrane, a difference in partial pressure of the gas across said gas diffusion membrane, a permeability coefficient of said gas diffusion membrane, and combinations thereof; and said gas diffusion membrane is a proton exchange membrane.

2. The sensor cell in claim 1, whereby said gas diffusion membrane is an anion, hydroxide ion exchange membrane.

3. A sensor cell for detecting gases comprising:
a substrate having a first surface and a second surface;
a sensing electrode; said sensing electrode in contact with said substrate;
an opening extending from said first surface to said second surface proximate to said sensing electrode for controlling a gas flow; a gas diffusion membrane being of polymeric material in contact with said sensing electrode and placed within said opening and between the gases to be detected and said sensing electrode;
a thickness of said gas diffusion membrane calculated based upon a parameter selected from the group consisting of a material of said gas diffusion membrane, a difference in partial pressure of the gas across said gas diffusion membrane, a permeability coefficient of said gas diffusion membrane, and combinations thereof; and wherein said gas diffusion membrane is an electrolytic material.

4. A sensor cell for detecting gases comprising:
a substrate having a first surface and a second surface;
a sensing electrode; said sensing electrode in contact with said substrate;
an opening extending from said first surface to said second surface proximate to said sensing electrode for controlling a gas flow;
a gas diffusion membrane being of electrolytic material in contact with said sensing electrode and placed within said opening and between the gases to be detected and said sensing electrode.

5. The sensor cell of claim 4, wherein a thickness of said gas diffusion membrane is determined from a permeability coefficient of said gas diffusion membrane.