WO2007031769A2

WO2007031769A2 - Gas sensor

Info

Publication number: WO2007031769A2
Application number: PCT/GB2006/003432
Authority: WO
Inventors: Mark Sinclair Varney; Michael Garrett; Alexandra Lindsell
Original assignee: Anaxsys Technology Limited
Priority date: 2005-09-15
Filing date: 2006-09-15
Publication date: 2007-03-22
Also published as: EP1934589A2; GB0518812D0; WO2007031769A3

Abstract

A sensor (140) for detecting a target component in a gas stream comprises a first electrode (144) ; a second electrode (146) ; an active substrate (148) extending between the first and second electrodes, the active substrate being selective to the target component, such that the presence of the target component varies the electrical conductivity of the path between the first and second electrodes. The active substrate preferably comprises a zeolite, in particular zeolite 4A or 13X. A method of detecting a target component in a gas stream comprises contacting the gas stream with an active substrate extending between first and second electrodes; determining the variation in the electrical conductivity of the path between the first and second electrodes; and providing an indication of the presence of the target component in the gas stream. The method is particularly suitable for analyzing the carbon dioxide content of breath exhaled by a subject in the assessment of respiratory function.

Description

GAS SENSOR

The present invention is related to a sensor for gases and a system for gas detection and analysis The sensor finds particular use as a sensor for detecting determining the composition of gas streams, for example the exhaled breath of a person or animal.

Gases can be detected and identified using a variety of analytical techniques and instruments. The most practical and widely used analysers use spectroscopic infra-red absorption as a method of detection, but the gas may also be detected using mass spectrometry, gas chromatography, thermal conductivity and others. Although most analytical instruments, techniques and sensors for measurement are based on the physicochemical properties of the gas, new techniques are being developed which utilise electrochemistry, and an assortment of electrochemical methods have been proposed. However, it is not possible to measure all gaseous materials directly using electrochemical techniques. An example of such a gas is carbon dioxide. Indirect methods have been devised, based on the dissolution of the gas into an electrolyte with a consequent change in the pH of the electrolyte. Other electrochemical methods use high temperature catalytic reduction of the gaseous components, such as carbon dioxide.

A more recently applied technique is to monitor a specific chemical reaction in an electrolyte that contains suitable organometallic ligands that chemically interact following the pH change induced by the dissolution of a gas, such as carbon dioxide. The pH change then disturbs a series of reactions, and the carbon dioxide concentration in the atmosphere is then estimated indirectly according to the change in the acid-base chemistry.

Acidic gases interact with water, and other (protic) solvents, and these reactions can be used as the basis for detecting and identifying the gaseous components. For example, carbon dioxide dissolves in an aqueous solution according to the following reactions:

CO₂ H- H₂O o H₂CO₃ (1)

H₂CO₃ <=> HCO₃- + H⁺ (2)

HCO₃- o CO₃ ^2" + H⁺ (3)

It will be appreciated that, as more carbon dioxide dissolves, the concentration of hydrogen ions (H ) increases.

The use of this technique for sensing gases such as carbon dioxide has the disadvantage that when used for gas analysis in the gaseous phase the liquid electrolyte must be bounded by a semi-permeable membrane. The membrane is impermeable to water but permeable to various gases, including the target gas. The membrane must reduce the evaporation of the internal electrolyte without seriously impeding the permeation of the gases being analysed. The result of this construction is an electrode which works well for a short period of time, but in which the electrolyte needs to be frequently renewed.

WO 04/001407 discloses a sensor comprising a liquid electrolyte retained by a permeable membrane, which overcomes some of these disadvantages. However, it would be very desirable to provide a sensor that does not rely on the presence and maintenance of a liquid electrolyte.

US 4,772,863 discloses a sensor for oxygen and carbon dioxide gases having a plurality of layers comprising an alumina substrate, a reference electrode source of anions, a lower electrical reference electrode of platinum coupled to the reference source of anions, a solid electrolyte containing tungsten and coupled to the lower reference electrode, a buffer layer for preventing the flow of platinum ions into the solid electrolyte and an upper electrode of catalytic platinum. GB 2,287,543 A discloses a solid electrolyte carbon monoxide sensor having a first cavity formed in a substrate, communicating with a second cavity in which a carbon monoxide adsorbent is located. An electrode detects the partial pressure of oxygen in the carbon monoxide adsorbent. The sensor of GB 2,287,543 is very sensitive to the prevailing temperature and is only able to measure low concentrations of carbon monoxide at low temperatures with any sensitivity. High temperatures are necessary in order to measure carbon monoxide concentrations that are higher, if complete saturation of the sensor is to be avoided. This renders the sensor impractical for measuring gas compositions over a wide range of concentrations.

GB 2,316,178 A discloses a solid electrolyte gas sensor, in which a reference electrode is mounted within a cavity in the electrolyte. A gas sensitive electrode is provided on the outside of the solid electrolyte. The sensor is said to be useful in the detection of carbon dioxide and sulphur dioxide. However, operation of the sensor requires heating to a temperature of at least 200°C, more preferably from 300 to

400°C. This represents a major drawback in the practical applications of the sensor.

Sensors for use in monitoring gas compositions in heat treatment processes are disclosed in GB 2,184,549 A. However, as with the sensors of GB 2,316,178, operation at high temperatures (up to 600°C) is disclosed and appears to be required.

Accordingly, there is a need for a sensor that does not rely on the presence of an electrolyte in the liquid phase, that is of simple construction and may be readily applied to monitor gas compositions at ambient conditions. In particular, there is a need for a sensor that can operate at ambient conditions to quickly determine the composition of a gaseous stream. The ability to be able to readily identify and quantify the components in a gas stream would be of particular use in the examination of the exhaled breath of a person or animal, as a knowledge of the composition of the exhaled gases is particularly useful in diagnosing a variety of conditions and illnesses. PCT patent application No. PCT/GB2005/003196 discloses a sensor for a range of gases, including carbon dioxide, in which a solid electrolyte precursor is used to bridge two electrodes. The electrolyte precursor is partially hydrated by the presence of water vapour in the gas stream being analysed to form an electrolyte. Dissolution of the target gases in the electrolyte establishes an electrically conductive path between the electrodes. The magnitude of the current flowing as a result of a voltage applied to the electrodes can provide a measure of the concentration of the target gas in the stream under investigation.

While the sensor disclosed in PCT/GB2005/003196 represents a significant step forward in the analysis of gaseous streams, it would be advantageous to provide a sensor have a conductance path of greater amplitude. It would also be an advantage if the sensor could be more specific to the ions being detected.

According to the present invention there is provided, in a first aspect, a sensor for detecting a target component in a gas stream, the sensor comprising: a first electrode; a second electrode; an active substrate extending between the first and second electrodes, the active substrate being selective to the target component, such that the presence of the target component varies the electrical conductivity of the path between the first and second electrodes.

The substrate, by being selective in its activity to a particular component, provides the sensor with the ability to identify particular components in the gas stream being analysed or monitored. It follows that two or more similar sensors may be combined, each having a substrate with a different activity, in order to detect a plurality of components in the gas stream. The substrate may be active by having an affinity for the target component, in particular by a chemical interaction with the target component. The composition of the substrate may be selected or modified in order to increase the affinity of the substrate layer and the sensor overall to the target component in the gas stream, thus increasing the responsiveness of the sensor to changes in the concentration of the target component in the gas stream being analysed.

The active substrate can be selected to render the sensor selective to a wide range of substances, such as simple molecules, including water, nitrogen oxides and carbon dioxide, to complex organic molecules. As will be noted hereinafter, a particular embodiment of the present invention comprises a substrate having an activity and affinity to components of the breath exhaled by a subject or patient, in particular carbon dioxide.

In addition to identifying components present in the gas stream, the sensor also enables the concentration of the target component to be determined. In a preferred embodiment, the substrate is active with respect to the target component, such that the conductance of the electrical path established between the electrodes is proportional to the concentration of the target component in the gas stream.

Suitable materials for the substrate include clays zeolites and silicalites. Suitable zeolites include the naturally occurring and synthetic zeolites. The methods of preparing suitable zeolites are well known in the art and suitable zeolites for use in the sensor of the present invention are available commercially.

The active substrate is preferably porous, with zeolites being a particularly preferred porous material. Zeolites, being highly porous materials belonging to the class of aluminosilicates have been found to be particularly suitable for use as or comprised in the active substrate of the sensor of the present invention. Zeolites are characterized by having a crystalline structure with a 3 -dimensional pore system. The pores are precisely defined in terms of their diameter. The diameter of the pores and the affinity of the zeolite to the target component may be controlled by subjecting the zeolite to ion-exchange with appropriate cations, using techniques well known in the art. It has been found that the speed of response of the sensor may be, in part, dependant upon the relationship of the pore size of the active substrate material with the diameter of the target species or molecule in the gas stream being analysed. In particular, it has been found that active substrate materials, especially zeolites, having pores with a diameter significantly greater than that of the target molecule can give rise to a sluggish or slow response of the sensor to changes in the composition of the gas stream being analysed. In contrast, the speed of response of the sensor can increase as the pore diameter approaches the diameter of the target molecule. Accordingly, it is preferred that the diameter of the pores of the active substrate are not substantially greater than the diameter of the target species or molecule. More preferably, the pore diameter is the same or less than the diameter of the target molecule or species.

Suitable zeolites for use in the sensor of the present invention include the Type A, Type P, Type X and Type Y zeolites. Preferred zeolites include Type A and Type X zeolites, in particular zeolite 4A and 13X. Zeolite 4A, having a pore diameter of about 4 Angstrom, is a particularly preferred zeolite for use when the target molecule in the gas stream is carbon dioxide, the measurement of which is particularly important in the analysis of respiratory disorders in humans and animals, as discussed hereinafter.

Suitable zeolites for use in the sensor of the present invention are known in the art and available commercially, or may be prepared using techniques and methods well known in the art.

Synthetic aluminium-magnesium hydroxy carbonates, in particular hydrotalcite, are also suitable for use as the active substrate material.

The sensor is particularly suitable for the detection of carbon dioxide, in particular carbon dioxide present in the exhaled breath of a person or animal. In addition, the sensor provides a fast and accurate response to changes in the composition of the gas stream being analysed. These features make the sensor of the present invention particularly suitable for use as a capnographic sensor in the analysis of exhaled breath of a subject.

The present invention provides a sensor that is particularly compact and of very simple construction. In addition, the sensor may be used at ambient temperature conditions, without the need for any heating or cooling, while at the same time producing an accurate measurement of the target substance concentration in the gas being analysed.

The sensor preferably comprises a housing or other protective body to enclose and protect the electrodes. The sensor may comprise a passage or conduit to direct the stream of gas onto the electrodes. In a very simple arrangement, the sensor comprises a conduit or tube into which the two electrodes extend, so as to be contacted directly by the gaseous stream passing through the conduit or tube. When the sensor is intended for use in the analysis of the breath of a patient, the conduit may comprise a mouthpiece, into which the patient may exhale. Alternatively, the sensor may be formed to have the electrodes in an exposed position on or in the housing, for direct measurement of a bulk gas stream. The precise form of the housing, passage or conduit is not critical to the operation or performance of the sensor and may take any desired form. It is preferred that the body or housing of the sensor is prepared from a non-conductive material, such as a suitable plastic.

In some applications, the presence of water vapour in the gas stream being analysed may be required. If insufficient water vapour is present in the gaseous stream, the sensor may be provided with a means for increasing the water vapour content of the gas stream. Such means may include a reservoir of water and a dispenser, such as a spray, nebuliser or aerosol.

The sensor comprises a first or working electrode and a second or counter electrode. Such a two-electrode construction is known in the art. The electrodes may comprise any suitable metal or alloy of metals, with the proviso that the electrode does not react with the electrolyte or any of the substances present in the gas stream. Preference is given to metals in Group VIII of the Periodic Table of the Elements (as provided in the Handbook of Chemistry and Physics, 62^nd edition, 1981 to 1982, Chemical Rubber Company). Other suitable metals include copper, silver and gold. Preferably, each electrode is prepared from gold or platinum. Carbon or carbon- containing materials may also be used to form the electrodes.

The electrodes may have any suitable shape and configuration. Suitable forms of electrode include points, lines, rings and flat planar surfaces. The effectiveness of the sensor can depend upon the particular arrangement of the electrodes and may be enhanced in certain embodiments by having a very small path length between the adjacent electrodes. This may be achieved, for example, by having each of the first and second electrodes comprise a plurality of electrode portions arranged in the form of an array of interdigitated electrode portions, in particular arranged in a concentric pattern.

The electrodes are preferably oriented as close as possible to each other, to within the resolution of the manufacturing technology. The first and second electrode can be between 10 to 1000 microns in width, preferably from 50 to 500 microns. The gap between the first and second electrodes can be between 20 and 1000 microns, more preferably from 50 to 500 microns. The optimum track-gap distances are found by routine experiment for the particular electrode material, geometry, configuration, and substrate under consideration. In a preferred embodiment the optimum first or working electrode track widths are from 50 to 250 microns, preferably about 100 microns, and the second or counter electrode track widths are from 50 to 750 microns, preferably about 500 microns. The gaps between the first and second electrodes are preferably about 100 microns.

The first electrode and second electrode may be of equal size. However, in one preferred embodiment, the surface area of the second or counter electrode is greater than that of the first or working electrode to avoid restriction of the current transfer. Preferably, the counter electrode has a surface area at least twice that of the working electrode. Higher ratios of the surface area of the counter electrode and working electrode, such as at least 3:1, preferably at least 5:1 and up to 10:1 may also be employed. The thickness of the electrodes is determined by the manufacturing technology, but has no direct influence on the electrochemistry. The magnitude of the resultant electrochemical signal is determined principally by exposed surface area, that is the surface area of the electrodes directly exposed to and in contact with the gaseous stream. Generally, an increase in the surface area of the electrodes will result in a higher signal, but may also result in increased susceptibility to noise and electrical interference. However, the signals from smaller electrodes may be more difficult to detect.

In its simplest form, the sensor consists of the electrodes in combination with the active substrate. For example, the electrodes may be applied to the surface of the substrate or encapsulated within the substrate material.

In a preferred embodiment, the sensor comprises an inert support, upon which is deposited the active substrate. The inert support may be any suitable material, with the proviso that it does react or interact with the substrate, the electrodes or the components in the gas stream to be analysed. Suitable inert supports include glass, polymers, ceramics and the like. The use of an inert support offers the advantage of providing strength and rigidity to the sensor assembly. In addition, the inert support allows the thickness of the active substrate to be reduced and the path length of the gas components entering the substrate and the conductive path between the electrodes more closely controlled. This in turn provides for a sensor that is more accurate and more responsive.

In one arrangement, the active substrate is deposited on the inert support and the electrodes applied to the surface or encapsulated in the substrate layer. An alternative arrangement is to have at least one of the electrodes applied directly to the surface of the inert support and the active substrate applied as a layer over both the electrode and support. In this way at least one or both of the electrodes is disposed between the substrate and the inert support.

To improve the electrical insulation of the electrodes, the portions of the electrodes that are not disposed to be in contact with the gaseous stream (that is the non-operational portions of the electrodes) may be coated with a dielectric material, patterned in such a way as to leave exposed the active portions of the electrodes.

While the sensor operates well with two electrodes, as hereinbefore described, arrangements with more than two electrodes, for example including a third or reference electrode, as is well known in the art. The use of a reference electrode provides for better potentiostatic control of the applied voltage, or the galvanostatic control of current, when the "iR drop" between the counter and working electrodes is substantial. Dual 2-electrode and 3-electrode cells may also be employed.

A further electrode, disposed between the counter and working electrodes, may also be employed. The temperature of the gas stream may be calculated by measuring the end-to-end resistance of the electrode. Such techniques are known in the art.

The electrodes of the sensor of the present invention may be formed by printing the electrode material in the form of a thick film screen printing ink onto the active substrate or the inert support, if used. The ink consists of four components, namely the functional component, a binder, a vehicle and one or more modifiers. In the case of the present invention, the functional component forms the conductive component of the electrode and comprises a powder of one or more of the aforementioned metals used to form the electrode.

The binder holds the ink to the substrate and merges with the substrate during high temperature firing. The vehicle acts as the carrier for the powders and comprises both volatile components, such as solvents and non-volatile components, such as polymers. These materials evaporate during the early stages of drying and firing respectively. The modifiers comprise small amounts of additives, which are active in controlling the behaviour of the inks before and after processing.

Screen printing requires the ink viscosity to be controlled within limits determined by rheological properties, such as the amount of vehicle components and powders in the ink, as well as aspects of the environment, such as ambient temperature.

The printing screen may be prepared by stretching stainless steel wire mesh cloth across the screen frame, while maintaining high tension. An emulsion is then spread over the entire mesh, filling all open areas of the mesh. A common practice is to add an excess of the emulsion to the mesh. The area to be screen printed is then patterned on the screen using the desired electrode design template.

The squeegee is used to spread the ink over the screen. The shearing action of the squeegee results in a reduction in the viscosity of the ink, allowing the ink to pass through the patterned areas onto the substrate. The screen peels away as the squeegee passes. The ink viscosity recovers to its original state and results in a well defined print. The screen mesh is critical when determining the desired thick film print thickness, and hence the thickness of the completed electrodes.

The mechanical limit to downward travel of the squeegee (downstop) should be set to allow the limit of print stroke to be 75 - 125um below the substrate surface. This will allow a consistent print thickness to be achieved across the substrate whilst simultaneously protecting the screen mesh from distortion and possible plastic deformation due to excessive pressure.

To determine the print thickness the following equation can be used: Tw = (Tm x Ao) + Te

Where Tw = Wet thickness (um);

Tm = mesh weave thickness (um); Ao = % open area;

Te = Emulsion thickness (um).

After the printing process the sensor element needs to be levelled before firing. The levelling permits mesh marks to fill and some of the more volatile solvents to evaporate slowly at room temperature. If all of the solvent is not removed in this drying process, the remaining amount may cause problems in the firing process by polluting the atmosphere surrounding the sensor element. Most of the solvents used in thick film technology can be completely removed in an oven at 150 °C when held there for 10 minutes.

Firing is typically accomplished in a belt furnace. Firing temperatures vary according to the ink chemistry. Most commercially available systems fire at 850 ⁰C peak for 10 minutes. Total furnace time is 30 to 45 minutes, including the time taken to heat the furnace and cool to room temperature. Purity of the firing atmosphere is critical to successful processing. The air should be clean of particulates, hydrocarbons, halogen-containing vapours and water vapour.

Alternative techniques for preparing the electrodes and applying them to the substrate or inert support, if present, include spin/sputter coating and visible/ultraviolet/laser photolithography. In order to avoid impurities being present in the electrodes, which may alter the electrochemical performance of the sensor, the electrodes may be prepared by electrochemical plating. In particular, each electrode may be comprised of a plurality of layers applied by different techniques, with the lower layers be prepared using one of the aforementioned techniques, such as printing, and the uppermost or outer layer or layers being applied by electrochemical plating using a pure electrode material, such as a pure metal. In a further aspect, the present invention provides a method of detecting a target component in a gas stream, the method comprising: contacting the gas stream with an active substrate extending between first and second electrodes; determining the variation in the electrical conductivity of the path between the first and second electrodes; and providing an indication of the presence of the target component in the gas stream.

The variation in the electrical conductivity of the substrate may be determined by applying a voltage to the first and second electrodes. The voltage may be applied in a continuous manner or in an intermittent or pulsed form. The voltage, when applied, may be a constant voltage or may cycle between a lower voltage and a higher voltage.

The method requires that an electric potential is applied across the electrodes. In one simple configuration, a voltage is applied to the counter electrode, while the working electrode is connected to earth (grounded). In its simplest form, the method applies a single, constant potential difference across the working and counter electrodes. Alternatively, the potential difference may be varied against time, for example being pulsed or swept between a series of potentials. In one embodiment, the electric potential is pulsed between a so-called 'rest' potential, at which no reaction occurs, and a reaction potential.

In operation, a linear potential scan, multiple voltage steps or one discrete potential pulse are applied to the working electrode, and the resultant Faradaic reduction current is monitored as a direct function of the dissolution of target molecules in the water bridging the electrodes. The measured current in the sensor element is usually small. The current is converted to a voltage using a resistor, R. As a result of the small current flow, careful attention to electronic design and detail may be necessary. In particular, special "guarding" techniques may be employed. Ground loops need to be avoided in the system. This can be achieved using techniques known in the art.

The current that passes between the counter and working electrodes is converted to a voltage and recorded as a function of the carbon dioxide concentration in the gaseous stream. The sensor responds faster by pulsing the potential between two voltages, a technique known in the art as 'Square Wave Voltammetry'. Measuring the response several times during a pulse may be used to assess the impedance of the sensor.

The shape of the transient response can be simply related to the electrical characteristics (impedance) of the sensor in terms of simple electronic resistance and capacitance elements. By careful analysis of the shape, the individual contributions of resistance and capacitance may be calculated. Such mathematical techniques are well known in the art. Capacitance is an unwanted noisy component resulting from electronic artifacts, such as charging, etc. The capacitive signal can be reduced by selection of the design and layout of the electrodes in the sensor. Increasing the surface area of the electrodes and increasing the distance between the electrodes are two major parameters that affect the resultant capacitance. The desired Faradaic signal resulting from the passage of current due to reaction between the electrodes may be optimized, by experiment. Measurement of the response at increasing periods within the pulse is one technique that can preferentially select between the capacitive and Faradaic components, for instance. Such practical techniques are well known in the art.

The potential difference applied to the electrodes of the sensor element may be alternately or be periodically pulsed between a rest potential and a reaction potential, as noted above. Figure 3 shows examples of voltage waveforms that may be applied. Figure 3 a is a representation of a pulsed voltage signal, alternating between a rest potential, V₀, and a reaction potential VR. The voltage may be pulsed at a range of frequencies, typically from sub-Hertz frequencies, that is from 0.1 Hz, up to 1OkHz. A preferred pulse frequency is in the range of from 1 to 500 Hz. Alternatively, the potential waveform applied to the counter electrode may consist of a "swept" series of frequencies, represented in Figure 3b. A further alternative waveform shown in Figure 3c is a so-called "white noise" set of frequencies. The complex frequency response obtained from such a waveform will have to be deconvoluted after signal acquisition using techniques such as Fourier Transform analysis. Again, such techniques are known in the art.

One preferred voltage regime is OV ("rest" potential), 25OmV ("reaction" potential), and 20Hz pulse frequency.

It is an advantage of the present invention that the electrochemical reaction potential is approximately +0.2 volts, which avoids many if not all of the possible competing reactions that would interfere with the measurements, such as the reduction of metal ions and the dissolution of oxygen.

The method of the present invention is particularly suitable for use in the analysis of the exhaled breath of a person or animal. From the results of this analysis, an indication of the respiratory condition of the patient may be obtained.

In a further aspect, the present invention provides a method of analyzing the exhaled breath of a person or animal, the method comprising: causing the exhaled breath to contact an active substrate extending between first and second electrodes; determining the variation in the electrical conductivity of the path between the first and second electrodes; and providing an indication of the composition of the exhaled breath. The sensor and method of the present invention are of use in monitoring and determining the lung function of a patient or subject. The method and sensor are particularly suitable for analyzing tidal concentrations of substances, such as carbon dioxide, in the exhaled breath of a person or animal, to diagnose or monitor a variety of respiratory conditions. The sensor is particularly useful for applications requiring fast response times, for example personal respiratory monitoring of tidal breathing (capnography). Capnographic measurements can be applied generally in the field of respiratory medicine, airway diseases, both restrictive and obstructive, airway tract disease management, and airway inflammation. The present invention finds particular application in the field of capnography and asthma diagnosis, monitoring and management, where the shape of the capnogram changes as a function of the extent of the disease. In particular, due to the high rate of response that may be achieved using the sensor and method of the present invention, the results may be used to provide an early alert to the onset of an asthma attack in an asthmatic patient.

Embodiments of the present invention will now be described, by way of example only, having reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional view of a sensor according to a first embodiment of the present invention;

Figure 2 is a cross-sectional view of a sensor according to a second embodiment of the present invention;

Figures 3a, 3b and 3c are voltage versus time representations of possible voltage waveforms that may be applied to the electrodes in the method of the present invention, as discussed hereinbefore;

Figure 4 is a cross-sectional representation of one embodiment of the sensor of the present invention; Figure 5 is an isometric schematic view of a face of one embodiment of the sensor element according to the present invention;

Figure 6 is an isometric schematic view of an alternative embodiment of the sensor element of the sensor of the present invention;

Figure 7 is a schematic view of a potentiostat electronic circuit that may be used to excite the electrodes of the sensor element;

Figure 8 is a schematic view of a galvanostat electronic circuit that may be used to excite the electrodes;

Figure 9 is a schematic representation of a breathing tube adaptor for use in the sensor of the present invention;

Figure 10 is a flow-diagram providing an overview of the inter-connection of sensor elements and their connection into a suitable measuring instrument of an embodiment of the present invention; and

Figures 11 to 14 are graphical representations of the response of a sensor of the present invention to changes in the composition of a carbon-dioxide containing gas stream.

Referring to Figure 1, there is shown a sensor assembly, generally indicated as 2, comprising a support 4, to which has been applied an active substrate layer 6. A working electrode 8 is applied to the surface of the substrate layer 6. The sensor further comprises a counter electrode (not shown in Figure 1).

The active substrate may be applied to the support by any suitable technique known in the art. One suitable technique is by preparing the active substrate as a slurry or solution in a suitable solvent, such as water, applying the slurry or solution to the support and removing the solvent by evaporation and drying.

The electrodes may be applied to the surface of the substrate by any suitable technique. One preferred technique is thick film screen printing, details of which are known in the art.

An alternative embodiment is shown in Figure 2. The components of the sensor of Figure 2 corresponding to those of Figure 1 have been indicated using the same reference numerals. Accordingly, the sensor in Figure 2 comprises a support 4, a substrate 6 and electrodes 8. However, in the arrangement shown in Figure 2, the electrodes 6 have been applied directly to the surface of the support 4 and the substrate layer applied to both the support and electrodes. The techniques for applying the substrate and electrodes are as mentioned above.

Referring to Figure 4, there is shown a sensor according to the present invention. The sensor is for analyzing the carbon dioxide content and humidity of exhaled breath. The sensor, generally indicated as 102, comprises a conduit 104, through which a stream of exhaled breath may be passed. The conduit 104 comprises a mouthpiece 106, into which the patient may breathe.

A sensing element, generally indicated as 108, is located within the conduit 104, such that a stream of gas passing through the conduit from the mouthpiece 106 is caused to impinge upon the sensing element 108. The sensing element 108 comprises a support substrate 110 of an inert material, onto which is mounted a working electrode 112 and a reference electrode 114. The working electrode 112 and reference electrode 114 each comprise a plurality of electrode portions, 112a and 114a, arranged in concentric circles, so as to provide an interwoven pattern minimizing the distance between adjacent portions of the working electrode 112 and reference electrode 114. In this way, the current path between the two electrodes is kept to a minimum. A layer 116 of insulating or dielectric material extends over a portion of both the working and counter electrodes 112 and 114, leaving the portions 112a and 114a of each electrode exposed to be in direct contact with a stream of gas passing through the conduit 104. The arrangement of the support, electrodes 112 and 114, and the solid electrolyte precursor is shown in more detail in Figures 5 and 6.

Referring to Figure 5, there is shown an exploded view of a sensor element, generally indicated as 140, comprising a support layer 142 of inert material. A working electrode 144 is mounted on the substrate layer 142 from which extend a series of elongated electrode portions 144a. Similarly, a reference electrode 146 is mounted on the substrate layer 142 from which extends a series of electrode portions 146a. As will be seen in Figure 5, the working electrode portions 144a and the reference electrode portions 146a extend one between the other in an intimate, interdigitated array, providing a large surface area of exposed electrode with minimum separation between adjacent portions of the working and reference electrodes. A layer of active substrate material 148 overlies the working and reference electrodes 144, 146.

The active substrate material comprises a zeolite.

The active substrate material 148 is applied by the repeated immersion in a suspension or slurry of the mesoporous material in a suitable solvent, in particular water. The sensor element is dried to evaporate the solvent after each immersion and before the subsequent immersion. Other materials may be incorporated into the active substrate layer by subsequent immersion in additional solutions or suspensions. The number of immersions is determined by the required thickness of the active substrate layer, and the chemical composition is determined by the number and variety of additional solutions that the sensor is dipped into. It will be obvious that there are a number of other means whereby the thickeness and composition of the coating may be similarly achieved, such as: pad, spray, screen and other mechanical methods of printing. Such techniques are well known in the field.

An alternative electrode arrangement is shown in Figure 6, in which components common to the sensor element of Figure 5 are identified with the same reference numerals. It will be noted that the working electrode portions 144a and the reference electrode portions 146a are arranged in an intimate circular array. The electrodes and substrate are coated in a layer of active substrate material, as described above in relation to Figure 5.

Referring to Figure 7, there is shown a potentiostat electronic circuit that may be employed to provide the voltage applied across the working and reference electrodes of the sensor of the present invention. The circuit, generally indicated as 200, comprises an amplifier 202, identified as 'OpAmpl', acting as a control amplifier to accept an externally applied voltage signal Vj_n. The output from OpAmpl is applied to the control (counter) electrode 204. A second amplifier 206, identified as '0pAmp2' converts the passage of current from the counter electrode 204 to the working electrode 208 into a measurable voltage (V_out). Resistors Rl , R2 and R3 are selected according to the input voltage, and measured current.

An alternative galvanostat circuit for exciting the electrodes of the sensor is shown in Figure 8. The control and working electrodes 204 and 208 are connected between the input and output of a single amplifier 212, indicated as 'OpAmpl'. Again, resistor Rl is selected according to the desired current.

Turning to Figure 9, an adaptor for monitoring the breath of a patient is shown. A sensor element is mounted within the adaptor and oriented directly into the air stream flowing through the adaptor, in a similar manner to that shown in Figure 4 and described hereinbefore. The preferred embodiment illustrated in Figure 9 comprises and adaptor, generally indicated as 300, having a cylindrical housing 302 having a male-shaped (push-fit) cone coupling 304 at one end and a female-shaped (push-fit) cone coupling 306 at the other. A side inlet 308 is provided in the form of an orifice in the cylindrical housing 302, allowing for the adaptor to be used in the monitoring of the tidal breathing of a patient. The side inlet 308 directs gas onto the sensor element during inhalation by a patient through the device. The monitoring of tidal breathing may be improved by the provision of a one-way valve on the outlet of the housing 302.

With reference to Figure 10 there is shown in schematic form the general layout of a sensor system according to the present invention. The system, generally indicated as 500, comprises a sensor element having a counter electrode 502 and a working electrode 504. The counter electrode 502 is supplied with a voltage by a control potentiostat 506, for example of the form shown in Figure 7 and described hereinbefore. The input signal for the control potentiostat 506 is provided by a digital-to-analog converter (D/ A) 508, itself being provided with a digital input signal from a microcontroller 510. The output signal generated by the sensing element is in the form of a current at the working electrode 504, which is fed to a current-to-voltage converter 512, the output of which is in turn fed to an analog-to-digital converter (A/D) 514. The microcontroller 510 receives the output of the A/D converter 514, which it employs to generate a display indicating the concentration of the target substance in the gas stream being monitored. The display (not shown in Figure 10 for reasons of clarity) may be any suitable form of display, for example an audio display or visual display. In one preferred embodiment, the microcontroller 510 generates a continuous display of the concentration of the target substance, this arrangement being particularly useful in the monitoring of the tidal breathing of a patient.

The sensors of the present invention may be employed individually, or as a series of sensor elements connected sequentially together in-line to measure a series of gases from a single gas stream. For example, a series of sensors may be employed to analyse the exhaled breath of a patient. In addition, two or more sensors may be used to compare the composition of the inhaled and exhaled breath of a patient.

EXAMPLES

The sensor and method of the present invention are further illustrated by the following working examples.

Example 1

A sensor element was prepared comprising gold working and reference electrodes supported on an alumina support layer. The electrodes were applied to the substrate using the screen printing method detailed hereinbefore. The electrodes were arranged as shown in Figure 6.

A layer of active substrate material comprising zeolite 13X (ex Ineos Silicas

Limited, pore diameter approximately 8 Angstrom) was applied to the electrodes and alumina support, so as to overlie the electrodes. The active substrate was applied by spray-coating with a dilute aqueous suspension.

The sensor so prepared was used in a experiment to measure the change in concentration of carbon dioxide in a gas stream using the following procedure.

The sensor was supported by a clamp stand and was exposed on all sides to the ambient atmosphere. 5% Carbon dioxide gas (ex. BOC Limited) was bubbled at a flow rate of 10 litres per minute through deionised water retained in a vertically- mounted column of 10cm diameter and 300cm length at a temperature of 38⁰C to saturate and equilibrate the gas.

A D/ A converter was used to apply successive voltages of OV and 25OmV at a frequency of 0.055 seconds per pulse (18 Hz square wave cycle) across the working and counter electrodes of the sensor. The current response was converted to a measurable voltage by an A/D converter, controlled by a microcontroller.

The humidified stream of carbon dioxide gas was directed at the sensor from a nozzle placed 1 cm from the sensor element. The gas stream was applied to the sensor element for a period of 3 seconds, and then deflected away for a period of 3 seconds, in order to determine the response of the sensor element and the change in the signal.

The response of the sensor element is shown graphically in Figure 11, in which the measured output current (micro Amps) is plotted against time (seconds). It will be noted that the sensor responded very rapidly to the change in carbon dioxide concentration.

Example 2

A sensor was prepared and tested as described in Example 1, with the exception that the active substrate layer comprised zeolite 4A (ex Ineos Silicas Limited, pore diameter 4 Angstrom).

The response of the sensor element was recorded and is shown graphically in Figure 12, in which the measured current (micro Amps) is plotted against time. It will be noted that the sensor responded very rapidly to the change in carbon dioxide concentration in the gas stream. It will be further noted that the speed of response of the sensor using zeolite 4 A as the active substrate was significantly faster than that of the sensor employing zeolite 13X, as evidenced by a comparison of the gradient of the portion of the curve A-A in each of Figures 11 and 12.

Example 3

A sensor was prepared and tested as described in Example 1 , with the exception that the active substrate layer comprised hydrotalcite (ex Ineos Silicas Limited).

The response of the sensor element was recorded and is shown graphically in

Figure 13, in which the measured current (micro Amps) is plotted against time. It will be noted that the sensor responded very rapidly to the change in carbon dioxide concentration in the gas stream. It will be further noted that the speed of response of the sensor using hydrotalcite as the active substrate was comparable to that of the zeolite 13X and slower than that of the sensor employing zeolite 4 A, as evidenced by a comparison of the gradient of the portion of the curve A-A in each of Figures 11, 12 and 13.

Example 4

A sensor was prepared and tested as described in Example 1, with the exception that the active substrate layer comprised zeolite P (ex Ineos Silicas Limited, pore diameter 3-5 Angstrom).

The response of the sensor element was recorded and is shown graphically in

Figure 14, in which the measured current (micro Amps) is plotted against time. It will be noted that the sensor responded very rapidly to the change in carbon dioxide concentration in the gas stream. It will be further noted that the speed of response of the sensor using zeolite P as the active substrate is intermediate to that of the sensors employing zeolite 13X and Zeolite 4 A, as evidenced by a comparison of the gradient of the portion of the curve A-A in each of Figures 11, 12 and 14.

Claims

1. A sensor for detecting a target component in a gas stream, the sensor comprising: a first electrode; a second electrode; an active substrate extending between the first and second electrodes, the active substrate being selective to the target component, such that the presence of the target component varies the electrical conductivity of the path between the first and second electrodes.

2. The sensor according to claim 1 , wherein the conductivity of the path between the first and second electrodes provides an indication of the concentration of the target component in the gas stream.

3. The sensor according to claim 1 or 2, wherein the active substrate comprises a zeolite or a silicate.

4. The sensor according to claim 3, wherein the active substrate comprises a Type A or Type X zeolite, preferably zeolite 4A or zeolite 13X.

5. The sensor according to any preceding claim, wherein the active substrate comprises a porous material, having a pore diameter substantially the same as the target component or less.

6. The sensor according to any preceding claim, wherein the first and second electrodes comprise a metal selected from Group VIII, gold, copper and silver.

7. The sensor according to any preceding claim, wherein the sensor further comprises an inert support.

8. The sensor according to claim.7, wherein the active substrate is disposed on a surface of the inert support and the first and second electrodes are disposed on the surface of the substrate.

9. The sensor according to claim 7, wherein at least one of the first and second electrodes is disposed between the substrate and the inert support.

10. A method of detecting a target component in a gas stream, the method comprising: contacting the gas stream with an active substrate extending between first and second electrodes; determining the variation in the electrical conductivity of the path between the first and second electrodes; and providing an indication of the presence of the target component in the gas stream.

11. The method of claim 10, wherein the conductance of the path between the first and second electrodes is determined and provides an indication of the concentration of the target component in the gas stream.

12. The method of claim 10 or 11 , wherein a voltage is applied to the first and second electrodes.

13. The method of claim 12, wherein the voltage is constant or varied over time.

14. A method of analyzing the exhaled breath of a person or animal, the method comprising: causing the exhaled breath to contact an active substrate extending between first and second electrodes; determining the variation in the electrical conductivity of the path between the first and second electrodes; and providing an indication of the composition of the exhaled breath.