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:
PRIORITY APPLICATION  
       [0001]     This application is a Continuation in Part patent application of copending U.S. patent application Ser. No. 09/444,344 for a “Hybrid Film Type Sensor” filed Nov. 19,1999. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention is directed toward a sensor having improved repeatability and efficiency.  
       BACKGROUND OF THE INVENTION  
       [0003]     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 Varney, 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 internal operating temperature as reported by Liu et al., 1993; and Narducci et al., 1993. More recent literature (Schwebel et al., 1997; Sheng et al., 1997; Micocci et al., 1997) eludes to more substantial development work yet to be done.  
         [0004]     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 Nafion constant. Theoretical calculations often reflect the drift rate of the signal could be significantly reduced under these conditions.  
         [0005]     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. Pat. No. 5,527,446; LaConti and Griffith, U.S. Pat. No. 4,820,386; Shen et al., U.S. Pat. No. 5,573,648; and, Stetter and Pan, U.S. Pat. No. 5,331,310. 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 external 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.  
         [0006]     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.  
         [0007]     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.  
         [0008]     U.S. Pat. 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 repeatability 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&#39;s thickness and other physical characteriztics, which optimizes the sensor&#39;s response time. Still a further possible disadvantage is that Madou&#39;s filter is inert and does not react, if desired, with the gas that diffuses through the membrane.  
         [0009]     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  
       [0010]     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.  
         [0011]     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: polyethlene, while proton conduction takes place only through a proton conductive electrolyte layer, e.g., a Nafion® membrane.  
         [0012]     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.  
         [0013]     The invention is still further directed toward a gas sensor utilized in conjunction with a gas sensor control circuit.  
         [0014]     The invention is also directed toward a gas sensor utilized in a gas sensing instrument.  
         [0015]     The invention achieves the foregoing and other objects by providing a sensor 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 further includes a gas diffusion membrane being of electrolytic material in contact with the sensing electrode and placed within the opening and between the gases to be detected and sensing electrode. The gas diffusion membrane has a thickness determined from a permeability coefficient of the gas diffusion membrane.  
         [0016]     In another embodiment, a polymeric gas diffusion membrane is used instead of an electrolytic membrane, where the polymeric membrane may be non-electrolytic and, therefore, not reacting with the gas and/or sensing electrode.  
         [0017]     In a further embodiment, the gas diffusion membrane is made of an electrolytic material where the thickness of the membrane may be optionally determined from the permeability coefficient. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  shows a schematic top view of the non-conductive supportive substrate.  
         [0019]      FIG. 2  shows a film type electrochemical sensor cell with Pt/Air (O 2 ) reference.  
         [0020]      FIG. 3  shows a film type electrochemical sensor cell with a polymeric gas-diffusion layer over the sensing electrode membrane.  
         [0021]      FIG. 4  shows a gas sensor control circuit.  
         [0022]      FIG. 5  shows a gas sensor utilized in a gas sensing instrument. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     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.  
         [0024]      FIG. 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.  
         [0025]     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 ).  
         [0026]     A typical sensor design utilizing this method is shown in  FIG. 2 , which has reference electrode ( 4 ) and a Pt counter electrode ( 5 ). Reference electrode material includes Platinum or Pt/Air (O 2 ) 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.  
         [0027]     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&#39; distances from center to center are the same as for the holes of  FIG. 1 . The diameter of the sensing or working electrode disc is larger than the diameter of the hole in  FIG. 1  to allow for contact between the disc and the sensing electrode support ring of  FIG. 2 . Instead of a single large hole per sensor of  FIG. 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.  
         [0028]     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  FIG. 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  FIG. 3 . After all the components are unitized, the resulting structure is cut in individual sensor units.  
         [0029]     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.  
         [0030]     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 ).  
         [0031]     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 ).  
         [0032]     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.  
         [0033]     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.  
         [0034]     The formula for determining the thickness of membrane ( 9 ) based on the permeability coefficient is: 
 
 i=P   d   ×A×ΔP/d  or  d=P   d   ×A×ΔP/l  
 
 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 ΔP 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. P d  is the permeability coefficient. 
 
         [0035]     The formula for determining the volume of gas diffusing through membrane ( 9 ), per second, based on the permeability coefficient is: 
 
 F   d   =P   d   ×Δc  
        where F d  is the flow density, measured in mol/cm 2  sec, assuming a 1 cm thick membrane, standard pressure of 1 atm, and standard room temperature. P d  is the permeability coefficient, which may be measured in units of cm/sec. Δc−c 1 -c o  is the concentration difference across the membrane, measured in mol/cm 3 , where Δc is the difference between the two concentrations on both sides of the membrane. c 1  is the concentration of the material inside the membrane, which is preferably located on a surface of the sensing electrode. c 0  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 H 2 S, is contained in that gas sample).        
 
         [0037]     The formula for determining the concentration of gas within membrane ( 9 ) as a function of time based on the permeability coefficient is: 
 
 c   1 ( t )= c   0 ×(1- e   −t/T ) 
         T =V/(A×P d ) . . . 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     P d  is the permeability coefficient     In this example c 1  is a function of time and changes from zero at c 1  (o) to c 1 (for t=infinity)=c 0          
 
         [0043]     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 sulphide   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 × cm × 10 10 /sq cm × sec × Δcm Hg at Standard Temperature and Pressure             
 
         [0044]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
               
               
                   
                   
                   
                 Ratio 
                   
               
               
                 Material 
                   P O 2   
                   P CO 2   
                 (CO 2 /O 2 ) 
                   P H 2 O 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 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 
                 0.035 
                 0.17 
                 4.9 
                 175 
               
               
                 terephthalate 
               
               
                 Nylon 6 
                 0.038 
                 0.16 
                 4.2 
                 275 
               
               
                 Polyvinyl chloride 
                 0.045 
                 0.16 
                 3.6 
                 275 
               
               
                 (unplasticized) 
               
               
                 Polyethylene 
                 0.40 
                 1.80 
                 4.5 
                 12 
               
               
                 (0.964 density) 
               
               
                 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 
                 2.20 
                 9.2 
                 4.2 
                 65 
               
               
                 (0.907 density) 
               
               
                 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 
                 15.8 
                 75.7 
                 4.8 
                 4,060 
               
               
                 (0.922 density) 
               
               
                 Poly (2,6 dimethyl 
               
               
                 phenylene oxide) 
               
               
                 Natural rubber 
                 23.3 
                 153 
                 6.6 
                 2,600 
               
               
                 Poly (4 methyl 
                 32.3 
                 92.6 
                 2.9 
                 — 
               
               
                 pentene 1) 
               
               
                 Poly dimethyl siloxane 
                 605 
                 3,240 
                 5.3 
                 40,000 
               
               
                   
               
               
                   Temperature is 30° C. for all of the above    
               
               
                   *High acrylonitrile copolymer    
               
               
                   Units cc × cm × 10 −10 /sq cm × sec × Δcm Hg at Standard Temperature and Pressure    
               
             
          
         
       
     
         [0045]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                   
                 O 2  Permeability 
               
               
                   
                 Polymer Membrane 
                 10 −10  [mol m/s m 2  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 
               
               
                   
                 PDMS 
                 167 
               
               
                   
                   
               
             
          
         
       
     
         [0046]                                                                        TABLE 4                                       Permeability × 10 10  moles/sec                Polymer   O 2     CO 2     H 2 O                            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 cm 2 , at room temperature with a differential pressure of 760 mm across the membrane. 
 
         [0047]     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                       Time                   Output   (Air→       Electrode   Membrane   Thickness   (air)   Nitrogen)               Galvanic   Teflon    0.5 mil    4.40 μa    2.0 sec       (55-mil cathode)   ″    1.0 mil    1.63 μa    8.0 sec           Polypropylene    0.5 mil    0.80 μa   10.0 sec           ″    1.0 mil   0.222 μa   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       with 10-mil   Composite       cathode)                  
 
         [0048]     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 (O 2 ) reference. This corresponds to an applied potentiostatic voltage of approximately 1.16 V with respect to a normal hydrogen electrode (NHE).  
         [0049]     Gas samples of air and 7.4 ppm SO 2  in air are introduced into the sampling port of the fixture described above. The gas flow is approximately 60 cm 3 /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.  
         [0050]     The background response signal with air is 30 nanoamps (nA). The response signal with 7.4 ppm SO 2  in air is 135 nA. This corresponds to a net response signal for 7.4 ppm SO 2  in air of 105 nA or 14.2 nA/ppm per 80-mil hole. It is possible to increase the magnitude of signal and signal-to-noise ratio by increasing the number of holes in the substrate above the integral sensing electrode structure.  
         [0051]     It is also possible, with this configuration, to detect other oxidizable or reducible gases such as CO, NO, NO 2 , H 2 S, ozone, CO 2 , hydrogen, hydrazine, ammonia, HCl, alcohols and acetone.  
         [0052]     Referring to  FIGS. 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 “E pot ”); 2) measure the temperature; 3) convert the gas concentration-related current to a temperature-compensated voltage signal; and 4) provide properly amplified voltage to the data acquisition/storage microprocessor ( 14 ). An on-board micro power-regulated power supply ( 16 ) uses the microprocessor&#39;s ( 14 ) power supply to provide the required ±3.9 volts for the sensor circuitry. The DC power can be supplied by a 6-V battery ( 16   d ) or an AC adaptor ( 16   e ).  
         [0053]     The control amplifier portion ( 17   b ) 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 ( 17   b ) as shown in  FIG. 4 , a standard configuration for potentiostat circuits. An adjustable voltage divider ( 17   a ) allows the polarizing voltage (E pot ) to be set at a predetermined voltage range such as 0 to 50 mV. This signal is compared to the reference electrode ( 4 ) voltage (which appears with it at the summing junction) by the control amplifier ( 17   b ) of the sensor control circuit ( 13 ). The latter adjusts the current through the sensor cell ( 10 ) to minimize the difference between the E pot  and the reference electrode ( 4 ) voltages.  
         [0054]     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 ( 15   a ). Temperature compensation of the sensor signal is effected in the next stage of amplification ( 15   b ) using a thermistor ( 18   a ) which is positioned in the gas sensor housing ( 10 ). The last stage of amplification ( 15   c ) 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 ( 15   a ), ( 15   b ), ( 15   c ) as for the control amplifier ( 15   b ). The transformed current signal is directed to an A/D channel on the data acquisition board of the microprocessor ( 14 ).  
         [0055]     Power for the sensor control circuit ( 13 ) is provided by a Duracell 6-V battery ( 16   d ) (PX 28A or 28L) through a micro power-regulated power supply ( 16 ). The power supply ( 16 ) utilizes a voltage inverter (e.g., ICL 7660) ( 16   a ) to convert the positive battery voltage to a negative voltage of the same magnitude, and a positive voltage regulator (e.g., MAX663) ( 16   c ) and negative voltage regulator (e.g., MAX 664) ( 16   b ) to provide a stable ±3.9 volts.  
         [0056]     The film type gas or vapor sensing instrument ( 12 ), as shown in  FIG. 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.  
         [0057]     The sensor cell assembly ( 19 ) and its potential-control circuit ( 13 ) are integrated with a battery-operated microprocessor ( 14 ) of 32 K memory, which samples the sensor signal as well as temperature and other signals at 10-, 20-, or 30-second intervals and stores an average value at intervals of 2, 5, or 10 minutes according to a programmable protocol. The data acquisition/storage unit in the microprocessor ( 14 ) can record 8 days of data, storing at 2-minute intervals, or up to 40 days storing at 10-minute intervals. In clinical testing to date, a 2-minute interval is suitable for one-day clinical studies and a 10-minute interval is appropriate for extended use. The microprocessor ( 14 ) with data acquisition/logic circuit can be programmed to sample more than one analog signal from the control circuit ( 13 ), and to convert these to digital signals and store them (i.e., gas concentration and temperature) at preset intervals together with real-time data. Data are off-loaded to a personal computer by accessing the microprocessor ( 14 ) through an RS232 port. After downloading, the digital data are converted to engineering units of gas concentration and temperature, and can be graphed by a menu-driven Lotus® 123 spreadsheet. Through a potentiometer in the gain amplifier circuit ( 15   c ), the device can be calibrated with calibrated gas samples, to indicate gas concentrations in the ambient. The potential-control circuit ( 13 ) shown in  FIG. 4  is powered, in a preferred embodiment, by six 1½ volt AA-size batteries ( 16   d ). A typical microprocessor ( 14 ) with data acquisition-recording capability that has been successfully used is sold by ONSET Computers, Falmouth, Mass., under the product name of “Tattletale Lite®.” 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.  
         [0058]     The sensing electrodes can be organized in multiple arrays or sets containing a necessary number of counter or reference electrodes. Reference electrodes such as Pt/air (O 2 ), PtO 2 , 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 (O 2 ), PtO 2  or Pt/H 2  which has a higher BET (Brunauer, Emmett, Teller) surface area (25 m 2 /g or larger) and/or larger geometric surface areas than the sensing electrode.  
         [0059]     Electrochemically reversible electrodes may be used in 3 or 2 electrode configurations, but especially in a 2 electrode arrangement where the counter electrode also acts as a reference electrode. Electrochemically reversible electrodes are constructed of stable catalyst materials and usually have a relatively large electrochemical active surface area so that they remain stable and their potential is not perturbed by small current flow. Examples include PtO 2  and Ag/AgCl electrodes.