WO1995030889A1

WO1995030889A1 - Method and device for optoelectronic chemical sensing

Info

Publication number: WO1995030889A1
Application number: PCT/AU1995/000266
Authority: WO
Inventors: Michael Gal; Anatoli Alexander Chtanov; Michael Brungs
Original assignee: Unisearch Limited
Priority date: 1994-05-09
Filing date: 1995-05-09
Publication date: 1995-11-16
Also published as: AUPM551994A0

Abstract

The present invention relates to methods for the detection or assay of a gas comprising exposing an apparatus (10) including a sensor to the gas for a given period of time, the sensor (20) having a layer of gas-sensitive material (22) that changes its optical properties when contacted with the gas in a manner proportional to the concentration of the gas; maintaining the sensor at a temperature of at least 50 °C by a heater (30); and measuring any change in the optical properties of the gas-sensitive material (22) of the sensor (20). The present invention also relates to apparatus for the detection or assay of a gas and sensors (20) comprising a plurality of layers including a support layer (21) and a semi-transparent layer of gas-sensitive material (22).

Description

Method and Device for Optoelectronic Chemical Sensing Field of the Invention

The present invention relates to methods for the optical detection of gases, apparatus for the optical detection of gases and sensors for the optical detection of gases . In particular the invention relates to the optical detection of hydrogen gas . Background to the Invention

Industrial sources of air pollution are many and varied. Some sources are large and scattered, such as factories, chemical plants and electric power stations, while others create problems locally. For example, hydrogen pollution may not be a general problem, but in coal mines or in submarines concentrations of hydrogen exceeding 4% are easily ignitable and highly flammable. There is a long list of other gases, including carbon monoxide, methane, ozone, CFC's etc. with need to be carefully monitored and accurately measured in order to avoid problems or, sometimes disasters. It is not surprising therefore, that sensitive and reliable detection of atmospheric pollutants is of great concern for industry, the military and society in general. Many techniques are being used for the detection and monitoring of atmospheric pollutants . These involve optical measurement of absorption or emission as well as in situ chemical techniques. Optical detection is the favoured method in most remote sensing applications, as well as in dealing with toxic and hazardous pollutants . Not all chemical species however are equally easy to detect. Hydrogen for example, is one of the most difficult gases to monitor because the molecular structure of H2 prohibits the application of conventional analytical techniques such as optical (infrared) absorption. Non-optical methods, such as monitoring of the electrical conductivity, are considered unsafe in explosive environments. A recent study (1) has shown that no commercial sensors exist which are capable of monitoring hydrogen at percentage levels (i.e. near the explosive limit) .

To address this problem the current inventors have developed optoelectronic chemical detectors including sensors which are based on monitoring the effect of a gas on the optical properties of a specific material present in the sensors . Disclosure of Invention

Accordingly, in a first aspect the present invention consists in a method for the detection or assay of a gas comprising the following steps:

(a) exposing an apparatus including a sensor to the gas for a given period of time, the sensor having a layer of gas-sensitive material that changes its optical properties when contacted with the gas in a manner proportional to the concentration of the gas;

(b) maintaining the sensor at a temperature of at least 50°C; and

(c) measuring any change in the optical properties of the gas-sensitive material of the sensor.

In a preferred embodiment of the first aspect of the present invention the sensor comprises a plurality of layers including a support layer and a semi-transparent layer of the gas-sensitive material. Optionally, the gas- sensitive material is covered by a layer of material protective for the gas-sensitive material. The protective layer helps protect the gas-sensitive material from deterioration but which allows permeation of the gas to the gas-sensitive material. Preferably the protective material is gold.

In a further preferred embodiment of the first aspect of the present invention, a portion of the layer of gas-sensitive material is covered by a layer of transparent gas-impermeable material so that only the uncovered portion of the gas-sensitive material is changeable in its optical properties when the sensor is exposed to the gas. In this embodiment, the differential change in the optical properties of the uncovered and covered portions of the gas-sensitive material is measured. The transparent gas-impermeable material can be Siθ₂, Al₂θ₃ or similar compounds. Other materials, however, can be used for this layer including epoxy resins .

In a still further preferred embodiment of the first aspect of the present invention the gas is hydrogen and the gas-sensitive material is palladium.

When the gas-sensitive material is palladium, the layer of palladium of the sensor is preferably less than lOOnm thick. More preferably the palladium layer has a thickness of between 10 and 50nm. In a yet still further preferred embodiment of the first aspect of the present invention the sensor is maintained at a temperature of at least 60°C.

The measurement of the change in the optical properties of the material sensitive to the gas can be by measuring transmission of light through the sensor or by measuring reflectance of the sensor.

In a second aspect the present invention consists in an apparatus for the optical detection or assay of a gas comprising: (a) a sensor having a layer of gas-sensitive material that changes its optical properties when contacted with the gas in a manner proportional to the concentration of the gas;

(b) a heating means to maintain the temperature of the sensor to at least 50°C; and

(c) an optical measuring means to measure any change in the optical properties of the gas-sensitive material of the sensor.

In a preferred embodiment of the second aspect of the present invention, the sensor comprises a plurality of layers including a support layer and a semi-transparent layer of the gas-sensitive material. Optionally, the gas- sensitive material is covered by a layer of material protective for the gas-sensitive material. The protective layer helps protect the gas-sensitive material from deterioration but allows permeation of the gas to the gas- sensitive material. Preferably the protective material is gold.

In a still further preferred embodiment of the second aspect of the present invention, a portion of the layer of gas-sensitive material is covered by a layer of transparent gas-impermeable material so that only the uncovered portion of the gas-sensitive material is changeable in its optical properties when the sensor is exposed to the gas. In this embodiment, the differential change in the optical properties of the uncovered and covered portions of the gas-sensitive material is measured. The transparent gas-impermeable material can be Siθ₂, Al₂θ₃ or similar compounds. Other materials, however, can be used for this layer including epoxy reins. In a further preferred embodiment of the second aspect of the present invention the gas is hydrogen and the gas-sensitive material is palladium.

When the gas-sensitive material is palladium, the layer of palladium of the sensor is preferably less than lOOnm thick. More preferably the palladium layer has a thickness of between 10 and 50nm.

In a further preferred embodiment of the second aspect of the present invention, the heating means maintains the sensor at a temperature of at least 60°C. The measurement of the change in the optical properties of the material in the sensor can be by measuring transmission of light through the sensor or by measuring reflectance of the sensor.

In order to reduce the effect of thermal noise of the heating means on the optical measuring means, the apparatus optionally comprises a thermal noise reducing means. This thermal noise reducing means can comprise a transparent shield arrangement between the heating means and the optical measuring means. Other arrangements of this kind known to the art can also be used. In a third aspect the present invention consists in a sensor for the optical detection or assay of a gas comprising a plurality of layers including a support layer and a semi-transparent layer of gas-sensitive material, wherein the gas-sensitive material changes its optical properties when contacted with the gas in a manner proportional to the concentration of the gas.

Optionally, the gas-sensitive material is covered by a layer of material protective for the gas-sensitive material. The protective layer protects the gas-sensitive material from deterioration but allows permeation of the gas to the gas-sensitive material. Preferably, the protective material is gold.

In a preferred embodiment of the third aspect of the present invention, a portion of the gas-sensitive material is covered by a layer of transparent gas-impermeable material so that only the uncovered portion of the gas- sensitive material is changeable in its optical properties when the sensor is exposed to the gas . The transparent gas-impermeable material can be Siθ₂, Al₂θ₃ or similar compounds. Other materials, however, can be used for this layer including epoxy resins.

In a preferred embodiment of the third aspect of the present invention the gas is hydrogen and the gas- sensitive material is palladium. When the gas-sensitive material is palladium, the layer of palladium of the sensor is preferably less than 60nm thick. More preferably, the palladium layer has a thickness of between 10 and 50nm.

In order that the nature of the present invention may be more clearly understood, preferred forms thereof will be described with reference to the following drawings and examples .

Brief Description of Drawings

Figure 1 is a schematic representation of a sensor according to the present invention;

Figure 2 is a schematic representation of an apparatus according to the present invention wherein the optical measuring means comprises two light sources and one photodetector; Figure 3 is a schematic representation of an apparatus according to the present invention wherein the optical measuring means comprises one light source and two photodetectors;

Figure 4 is a schematic representation of an apparatus according to the present invention wherein the optical measuring means measures reflectivity of the sensor;

Figure 5 is a schematic diagram of an electronic circuitry for an apparatus according to the present invention;

Figure 6 is a graph showing the response of an apparatus according to the present invention to a burst of

4% hydrogen in air displaying approximately 20μsec response time; Figure 7 is a graph showing a response of an apparatus according to the present invention to increasing concentrations of hydrogen gas; and

Figure 8 is a graph showing a response of an apparatus according to the present invention to hydrogen gas with and without the presence of carbon monoxide gas .

Modes for Carrying out the Invention

In one form the present invention consists in a detector 10 for measuring hydrogen by optical means. Since the molecular structure of H2 prohibits optical (infrared) absorption, the present method relies on the permeation of hydrogen from the area surrounding the detector 10 into a specially designed sensor 20 having a thin film 22 of a material sensitive to hydrogen and optically" detecting the hydrogen in this material by the change in the optical properties of the material. The sensor 20 consists of a multi-element layer structure where its optical properties change as a function of the concentration hydrogen. The key component in this structure is a thin film layer of palladium (Pd) 22 which is well known to preferentially absorb hydrogen (2). When the sensor 20 is exposed to hydrogen, the complex dielectric constants of the palladium film 22 change, which alter the optical properties of the material, allowing the detection and measurement of hydrogen.

The schematics of the structure of the multi-layer sensor 20 are shown in Fig.l. A support 21, made from a dielectric material, is covered by a semi-transparent thin film of Pd 22 and followed by a protective gold layer 23. The gold layer 23 is used to minimise the oxidation and deterioration of the Pd layer 22 so as to ensure that the Pd layer 22 is able to absorb hydrogen. The thickness of the Pd layer 22 is determined by the required sensitivity of the sensor 20 while the gold layer 23 thickness is determined by its optical transparency. A portion 27 of this structure can be covered by layer 25 of another transparent gas-impermeable material, such as Siθ₂, or Al₂θ₃, while the other portion 28 of the sensor 20 is uncovered. The thin gold film 23 is permeable to hydrogen but the Siθ₂ (or Al₂θ₃) film 25 is not. Thus upon exposure to hydrogen, only the palladium not covered by Siθ₂ will be able to absorb hydrogen, the other portion 27 will be unaffected by it. The sensor is heated to a temperature of 50°C or higher. Preferably, a temperature of at least 60°C is used. The measurement of hydrogen is based on the difference in the transparency of the two portions 27, 28 of the sensor.

SUBSTTΓUTE SHEET(Rule26) The present inventors found that using glass quartz or sapphire as the support 21 material alone resulted in poor sensor 20 stability and problems of long term drift (due to impurity diffusion from the substrate into the Pd film 22). To overcome this problem, a "buffer" layer 24 was applied between the support 21 and the Pd film 22. Several metal layers, such as chromium or aluminium, and also with dielectric materials, such as Al₂θ₃ (by oxidising Al) , Siθ₂, SiO_x and MgF₂ were tried. Metal films were soon abandoned due to the opaqueness of the thicker films. Of the dielectric materials tried, MgF₂ was found to be the best. It forms the necessary "screen" to suppress the impurity diffusion from the support 21 into the metal, has the correct optical transparency and also proved to have the required mechanical properties .

The sensor 20 was insensitive to the thickness of the MgF₂ film. An approximately 0.5μm thick layer 24 between the glass support 21 and the Pd film 22 was used.

Another component in the sensor 20 which required considerable attention was the protective layer 23 which covers the Pd film 22. This layer had to protect the active layer (Pd) 22 from deteriorating with time due to oxygen, moisture etc, but at the same time had to be optically transparent, stable and, most importantly, had to be permeable to hydrogen. A number of films such as Ag, In, Bi, Siθ₂, polyamide, etc were tested and several of them satisfied one or other of the requirements but only an approximately 35nm thick gold film fulfilled all the requirements adequately. Therefore, in the sensors, the "active" Pd film 22 is covered by an approximately 35nm thick gold film 23.

The question of the Pd film 22 thickness is important as it significantly affects the key device parameters: mechanical stability, time constant and sensitivity. At a very early stage of the research, it was found that thermally evaporated thick films (d > lOOnm) tend to "flake" and "peel off" the support 21 after exposure to hydrogen. This is probably due to the large stress which develops in the film 22 after it absorbs hydrogen. Thin films (d < 50nm) , on the other hand, show good mechanical stability even after being saturated by (100%) hydrogen, whereas very thin films have low sensitivity to hydrogen detection. Using thermally evaporated films, the optimum layer thicknesses were found to be in the lOn < d < 50nm thickness range. (The mechanical stability of thicker layers could be improved by depositing the films onto other supports such as chromium on glass, or by using other deposition methods, such as sputtering.) The film 22 thickness was also found to affect the sensor 10 time constant. As expected from diffusion considerations, the time constant increases monotonically with the Pd film 22 thickness. In the range lOnm < d < 50nm the time constant is less than 20/s as can be seen in Fig. 6.

The sensitivity of the sensor 20 is also a function of the Pd film 22 thickness. Thinner films were more sensitive to lower concentrations of hydrogen, while thicker films were more sensitive to higher concentrations of hydrogen. The thickness range lOnm < d < 50nm was found to be optimal for the 1%-10% hydrogen range. To utilise a differential detection technique by comparing the optical properties of a hydrogen impregnated and "virgin" film, a portion 27 of the layer structure is covered with a hydrogen impenetrable layer 25 and left the portion 28 half "bare". To achieve this, half of the glass MgF₂ d/Au layer structure 27 was covered with either Siθ₂, Al₂θ₃ or a thin epoxy layer, while leaving the other half 28 uncovered. Upon exposure to hydrogen, only the part not covered absorbs hydrogen, the other part 27 (the covered half) is unaffected by the hydrogen. The optical properties of the two portions 27 and 28 will be different because one contains the hydrogen impregnated Pd while the other contains the pure Pd. In this form of the invention, the measurement of hydrogen is based on the difference in the transparency (reflectivity) of the two portions 27 and 28 of the palladium film 22.

During the deposition of the different layers, the in situ monitoring of the layer thickness is important. This is conventionally done with a built-in thickness monitor which is calibrated for the different materials and systems . The inventors developed and used an inexpensive optical method of monitoring the layer thickness during growth using the transmission of the film itself as a measure of the film thickness. This involved a light emitting diode and a photodiode built into the evaporation equipment. After accurate calibration, this method proved to be suitable for the requirements .

In one example, the optical measurement is based on a technique called differential transmission, shown in Fig. 2. This technique is capable of measuring very small differences in optical constants. The sensor 20 is illuminated alternatively by a light source 41 in the form of two light emitting diodes (LED's) and the light transmitted through the sensor 20 is detected by a photodetector 42. If there is no hydrogen in the area surrounding the sensor 20, the two portions 27, 28 of the palladium film 22 will have identical transparencies and the photodetector 42 will only generate a DC (constant) signal. Upon exposure to hydrogen, the transparencies of the two portions 27, 28 of the palladium film 22 will differ (because only one portion 28 of the Pd films "sees" the hydrogen) and therefore the photodetector 42 will generate two different signal levels depending which LED is on. The resulting AC signal is proportional to the amount of hydrogen in the Pd film 22, and thus proportional to the amount of hydrogen in the area surrounding the sensor 20. This AC signal is carefully measured by an ensuing electronic circuitry as set out in Fig. 5.

The electronic circuitry of the apparatus was designed to have the following general properties: (*) high sensitivity and low noise

(*) reliable performance (*) wide dynamic range

(*) high stability and low dependence on the power supply parameters (*) low operating voltage and low power consumption

(*) relative simplicity and low cost The primary modules of an apparatus having two photodetectors is shown in Fig. 5 are a low noise pre¬ amplifier, a follow-on amplifier, a phase-locked detector, an oscillator and a voltmeter. A separate unit was used to control and monitor the temperature of the sensor. The role of the low noise pre-amplifier is to amplify the difference signal generated by the two photodetectors. It is this signal which is proportional to the hydrogen concentration. This was achieved by using two low-noise instrumental amplifiers (XR 5533) and 100% negative feedback loops for AC and DC components of the signal. The photodetectors are included in the negative feedback loops so that the whole input stage is stabilised by the high quality amplifiers. The AC component of the signal is first amplified by an adjustable gain amplifier (XR 5533) then rectified by the phase-locked detector. The ensuing DC signal is amplified by the buffer amplifier, measured by a voltmeter and is displayed on a liquid crystal display. The signal may be converted to hydrogen percentage by examining the calibration curve of the given sensor. An op-amp generates the sine current for the LED's. The phase-locked detector, oscillator and output buffer-filter are all components of the same integrated circuit (SE 5521). The reference signal for the phase- sensitive detector and the LED power supply are generated by the oscillator which operates at audio frequencies (few hundred HZ)

In the visible or near infrared spectral region, the method is independent of the angle of incidence, the polarisation and the wavelength of light and provides great flexibility in the optical alignment of the system. For example, a different version of the optical arrangement is shown in Fig. 3, where only one LED is used as the light source 41 which illuminates both portions 27 and 28 of the palladium film 22, and the differential nature of the technique is achieved by using two photodetectors 42 which are alternately turned on and off. Figs . 3 and 4 also shows the placement of the heating means 30 for the sensor 20. The differences in optical transmission of the two portions of the sensor are measured and amount of hydrogen present is determined.

The hydrogen concentration is determined from the measured optical (electrical) signal by examining the calibration curve of the given sensor 20. The calibration of each sensor 20 was achieved by the use of known concentrates of hydrogen/air mixtures . A typical calibration for a 20nm thick Pd film 22 is shown in Fig.7. The chosen concentration range of hydrogen includes the explosive limit (4%). The sensitivity of the detector 10 to other gases was also tested. There was no detectable response from the detector to the following species: N2, He, Ar, CO2 , H2O, CH4. The chemical selectivity of this detector 10 for hydrogen is obtained by the use of palladium as the active material owing to its capacity to selectively absorb hydrogen. The simplicity of the design ensures either a portable unit or a fibre optic system capable of monitoring hydrogen in hazardous environments .

In some applications, differential reflectance may be desirable. In this form, the change in reflectivity of the two portions 27 and 28 of the Pd film 22 is measured. As this measurement is also a function of the optical constants it is therefore just as sensitive to hydrogen as the differential absorption method described above.

The sensor 20 may be used in the reflectivity mode, as shown in Fig. 4. In this mode, the support layer 21 has a layer 26 having a reflective characteristic (for example a mirror) . This reflective layer 26 reflects light coming from the light source 41. This method has additional advantages as compared to the transmission method as light propagates through the gas-sensitive material film 22 twice thereby increasing the sensitivity of the detector 10. It also makes the heating of the sensor simple as all the optical measuring means 40 is on one side of the sensor 20. Carbon monoxide (CO) is a gas which in coal mines often coexists with hydrogen. To selectively detect hydrogen the sensor should not be affected in any way by CO or any other gas . The inventors found that at room temperature, in the presence of even small amount of CO (less than 1% in air), the time constant of the palladium/hydrogen interaction is too long for an accurate measurement to be made. Therefore the optical properties of the palladium film 22 do not change within a reasonable period of time when both hydrogen and CO are present. Having the sensor at room temperature restricts the use of such a device only to CO-free atmospheres.

To overcome this problem the inventors tried a number of different methods including combining the sensor with a catalytic CO converter which aimed to convert CO to CO2 , a gas which does not interfere with the measurement by the detector. This method was found not to work because hydrogen interacted with the catalytic converters . A number of different membranes which are selectively permeable to hydrogen were also tried unsuccessfully. Several possible materials (such as indium, bismuth) were tried, however, these were found to degrade with time due to oxidation, moisture, etc. Several porous oxides, such as AI2O3, Siθ2 were also tried, but these did^'not give complete protection against CO.

The inventors have found that by using a heating means 30 and heating the sensor 20 containing the palladium film 22 to at least 50°C the deleterious effect of CO can be eliminated. A temperature of 60°C or greater was also found to overcome the effect of CO. Furthermore, only the sensor 20 containing the Pd film 22 needs to be heated and the measuring means 40 may be at room temperature. The results of the device operating at 60°C detecting 1% hydrogen in air with the presence of 5% of CO in given in Fig. 8. This would be an extreme case occurring in a coal mine and demonstrates that the detector and sensor works under these extreme conditions . In order to minimise any thermal noise from heating means 30 to the optical measuring means 40, the present inventors found that this can be achieved by placing between the heating means 30 and the optical measuring means 40. One form of the means to reduce thermal noise found suitable was a transparent shield arrangement in the form of glass plate or plates between the heating means and the optical measuring means 40. Other insulating means known to the art would also be suitable for the present invention.

References

(1) M.P. Brungs, C. Maunchausse, D. Stroescύ and D.L. Trimm, "Evaluation of Hydrogen Detectors for Coal Mines", J. Inst. Energy 65, 66 (1992). (2) F. Lewis: The palladium hydrogen system, (Academic Press, New York, 1967).

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS :

1. A method for the detection or assay of a gas comprising the following steps:

(b) maintaining the sensor at a temperature of at least 50°C; and

2. The method of claim 1 wherein the sensor comprises a plurality of layers including a support layer and a semi- transparent layer of the gas-sensitive material.

3. The method of claim 1 or 2 wherein the layer of gas- sensitive material is covered by a layer of material protective for the gas-sensitive material and permeable to the gas .

4. The method of claim 3 wherein the protective layer is gold.

5. The method of any one of claims 1 to 4 wherein a portion of the layer of the gas-sensitive material is covered by a layer of transparent gas-impermeable material so that only the uncovered portion of the gas-sensitive material is changeable in its optical properties when the sensor is exposed to the gas, and wherein the differential change in the optical properties of the uncovered and covered portions of the gas-sensitive material is measured.

6. The method of claim 5 wherein the transparent gas- impermeable material Siθ₂ or Al₂θ₃.

7. The method of any one of claim 1 to 6 wherein the gas is hydrogen and the gas-sensitive material is palladium.

8. The method of claim 7 wherein the layer of palladium has a thickness of less than lOOnm.

9. The method of claim 8 wherein the layer of palladium has a thickness between 10 and 50n .

10. The method of any one of claims 1 to 9 wherein the sensor is maintained at a temperature of at least 60°C.

11. The method of any one of claims 1 to 10 wherein the measuring of the change in the optical properties of the gas-sensitive material is by measuring transmission of light through the sensor or by measuring reflectance of the sensor.

12. An apparatus for the optical detection or assay of a gas comprising:

(a) a sensor having a layer of a gas-sensitive material that changes its optical properties when contacted with the gas in a manner proportional to the concentration of the gas;

(b) a heating means to maintain the temperature of the sensor to at least 50 C; and (c) an optical measuring means to measure any change in the optical properties of the gas-sensitive material of the sensor.

13. The apparatus of claim 12 wherein the sensor comprises a plurality of layers including a support layer and a semi-transparent layer of the gas-sensitive material .

14. The apparatus of claim 12 or 13 wherein the layer of gas-sensitive material is covered by a layer of material protective for the gas-sensitive material and permeable to the gas .

15. The apparatus of claim 14 wherein the protective layer is gold.

16. The apparatus of any one of claims 12 to 15 wherein a portion of the layer of the gas-sensitive material is covered by a layer of transparent gas-impermeable material so that only the uncovered portion of the gas-sensitive material is changeable in its optical properties when the sensor is in contact with the gas, and wherein the optical measuring means measures the differential change in the optical properties of the uncovered and covered portions of the gas-sensitive material.

17. The apparatus of claim 16 wherein the transparent gas-impermeable material is Siθ₂ or Al₂θ₃.

18. The apparatus of any one of claims 12 to 17 wherein the gas is hydrogen and the gas-sensitive material is palladium.

19. The apparatus of claim 18 wherein the layer of palladium has a thickness of less than lOOnm.

20. The apparatus of claim 19 wherein the layer of palladium has a thickness of between 10 and 50nm.

21. The apparatus of any one of claims 12 to 20 wherein the heating means maintains the sensor at a temperature of at least 60°C.

22. The apparatus of any one of claims 12 to 21 wherein the measuring means measures the transmission of light through the sensor or measures reflectance of the sensor.

23. The apparatus of any one of claims 12 to 22 further comprising a means to reduce the effect of thermal noise of the heating means on the optical measuring means.

24. The apparatus of claim 23 wherein the means to reduce thermal noise comprises a transparent shield arrangement between the heating means and optical measuring means .

25. A sensor for the optical detection or assay of a gas comprising a plurality of layers including a support layer and a semi-transparent layer of gas-sensitive material, wherein the gas-sensitive material changes its optical properties when contacted with the gas in a manner proportional to the concentration of the gas .

26. The sensor of claim 25 wherein the layer of gas- sensitive material is further covered by a layer of material protective for the gas-sensitive material and permeable to the gas .

27. The sensor of claim 26 wherein the protective layer is gold.

28. The sensor of any one of claims 25 to 27 wherein a portion of the layer of gas-sensitive material is covered by a layer of transparent gas-impermeable material so that only the uncovered portion of the gas-sensitive material is changeable in its optical properties when the sensor is in contact with the gas .

29. The sensor of claim 28 wherein the transparent gas- impermeable material is S θ₂ or AI₂O₃.

30. The sensor of any one of claims 25 to 29 wherein gas is hydrogen and the gas-sensitive material is palladium.

31. The sensor of claim 30 wherein the layer of palladium has a thickness of less than lOOnm.

32. The sensor of claim 31 wherein the layer of palladium has a thickness between 10 and 50nm.