FR2856150A1 - Apparatus for evaluating the concentration of at least one type of chemical in a gas phase has waveguide of porous inorganic material with variable light absorption properties - Google Patents

Abstract

The apparatus, containing a chemical detector (2) with at least one waveguide (21, 22), a system (1) for directing a light flow towards the wave guide, and an analyzer for the light flow leaving the detector, has the waveguide made from at least one inorganic porous material impregnated with at least one organic material with light absorption properties that vary according to the concentration of a predetermined type of chemical present in a gas phase (3). The apparatus, containing a chemical detector (2) with at least one waveguide (21, 22), a system (1) for directing a light flow towards the wave guide, and an analyzer for the light flow leaving the detector, has the waveguide made from at least one inorganic porous material impregnated with at least one organic material with light absorption properties that vary according to the concentration of a predetermined type of chemical present in a gas phase (3). The inorganic material is of silicon, which can be monocrystalline, oriented, boron-doped or oxidized, or it can be a chromophore material containing Congo red, bromocresol green, phenolphthalein or indigo carmine. The light emitter can be a laser or electroluminescent diode, with the light flow directed towards the chemical detector by optical fibres (11), while the analyzer contains at least one photodetector (4).

Description

Device for assessing the concentration of at least one species

chemical in a gas phase.

  The field of the invention is that of air analysis, or more generally the analysis of a gas phase. More specifically, the invention 5 relates to a device for evaluating the concentration of at least one determined chemical species present in a gas phase comprising a chemical sensor, means for emitting a light flux towards the chemical sensor and means for analysis of the luminous flux at the output of the chemical sensor.

  The invention applies in particular, but not exclusively to the detection of acid gases such as HCI or basic gases such as NH3, in the chemical and food industries, this with a view to securing production and / or storage in industrial environment.

  However, the invention can also be applied to the detection of gases such as carbon dioxide, nitrogen dioxide, etc., for the protection of public health and the environment, for example by installing a sensor according to the invention on motor vehicles.

  In the field of the invention, sensors have been proposed whose structure and operation make it possible to obtain optical detection of the chemical environment.

  There is in particular known a waveguide sensor described in the document published under the number EP - 0 575 408.

  According to this document, it is planned to add on a substrate, constituted by an optical fiber, a coating comprising a porous material forming a porous molecular structure in the form of a cage.

  To ensure optical detection of the environment, dye molecules, preferably a fluorescent dye, are trapped in the porous structure of the coating.

  The coating is provided along a portion of the optical fiber forming a waveguide to use the evanescent field of light transmitted through the fiber.

  This solution has several drawbacks.

  Firstly, such a sensor is relatively complex, time-consuming and costly to produce. Indeed, the production of the sensor notably involves successive cooking steps, at temperatures between 65 ° C. and 85 ° C., for 5 12 to 24 hours (for cooking a precursor solution) then for 18 to 30 hours (for the coating).

  The times considered are therefore very long. In addition, the temperature ranges limit the type of dyes that can be used.

  Furthermore, in the case of such a sensor, only the evanescent part of the light signal which propagates in the coating layer is affected by the environment. This has the consequence that the sensor has a fairly low sensitivity. This therefore makes it necessary to provide a relatively large length of interactions (of the order of several centimeters, or even more) in order to obtain, at the output of the sensor, significant signals or variations of signals.

  The invention particularly aims to overcome the drawbacks of the prior art.

  More specifically, the object of the invention is to propose a device for evaluating the concentration of at least one chemical species (for example acid or basic molecules) present in a gas phase using a chemical sensor, in which the chemical sensor has a significantly higher sensitivity than those of the prior art.

  The invention also aims to provide such a device in which the chemical sensor can be produced in dimensions substantially smaller than those presented by known sensors.

  The invention also aims to provide such a chemical sensor which is simple to produce and easy to implement.

  The invention also aims to provide such a chemical sensor which can be obtained more quickly than those of the prior art indicated above.

  These objectives, as well as others which will appear subsequently, are achieved thanks to the invention which relates to a device intended to evaluate the concentration of at least one determined chemical species present in a gas phase, said device comprising a sensor chemical comprising at least one waveguide, means for emitting a light flux towards said waveguide and means for analyzing said light flux at the output of said chemical sensor, characterized in that said guide wave is constituted by at least one porous inorganic material impregnated with at least one organic material whose light absorption properties vary depending on the concentration of said at least one determined chemical species present in said gas phase.

  In this way, the sensor works by absorbing the light waves which pass through the waveguide.

  In other words, a sensor according to the invention uses and possibly affects the entire optical signal of the light wave.

  Such a sensor therefore has a significantly higher sensitivity (of the order of ppm) than that of the sensor of the prior art mentioned above.

  Consequently, such a sensor can be produced with relatively small dimensions while retaining a completely satisfactory detection capacity.

  According to a particularly advantageous characteristic, said inorganic material comprises at least porous silicon.

  Thus, the material of the waveguide is made with a material having many qualities and properties of interest in the field of the invention. Indeed, the optical properties (refractive index, absorption coefficient) are particularly suitable for guiding light.

  Preferably, said porous silicon is a monocrystalline silicon of type P or N, and is advantageously oriented (100).

  Advantageously, said porous silicon is doped with boron and preferably has a resistivity comprised in 3.10-'3.cm and 7.10-3 Q.cm. According to another characteristic, said porous silicon is oxidized.

  Of course, other embodiments can be envisaged without departing from the scope of the invention, for example by changing the crystallographic orientation of the silicon, or its resistivity.

  However, the selection of these parameters leads to a waveguide formed from oxidized porous silicon which has the advantages: of being stable over time and of not reacting with the humidity of the surrounding atmosphere; - to have a very low absorption in the visible range. According to a particular solution, said organic material is a chromophore material.

  Preferably, said chromophore material comprises Congo red.

  With Congo red, the sensor can be used to detect both acidic vapors and vapors such as ammonia, without modifying the device.

  According to a possible variant, said chromophore material comprises at least one of the materials belonging to the following group: - Bromocresol Green; - Rouge Phénol; - Phenoltphthalein; - Indigo Carmine.

  It is noted that the chromophoric materials will be particularly used for the detection of acid gases or basic gases.

  However, the organic material which permeates the chemical sensor can be of another type.

  Indeed, the detection of other types of gas can be carried out if the gas considered reacts with a molecule by moving the absorption band of this, causing a modification of the intensity of the light flux. For example, the use of phthalocyanine-based molecules with or without the addition of a metal such as cadmium or cobalt could allow the detection of gases such as NO2.

  Advantageously, said means for emitting a light flux comprise at least one of means belonging to the following group: - laser diode; light emitting diode.

  The device can therefore be produced from electronic components having a low cost. Such a device is thus inexpensive to obtain.

  According to an advantageous solution, said light flux is directed towards said chemical sensor using optical fibers.

  Such a sensor therefore has the possibility of being adapted to remote measurement cases, by moving the electrical part of the sensor by means of optical fibers placed between the source and the waveguide and between the waveguide and a photodetector. .

  Thus constituted, the detection device can then be used in an aggressive environment or in an environment presenting risks of explosion by

example.

  Preferably, said means for analyzing said light flux comprise at least one photo-detector.

  It is noted that this photo-detector can be integrated on the same silicon substrate as the waveguide.

  According to an advantageous variant, the device comprises at least one focusing lens interposed between said chemical sensor and said analysis means.

  Advantageously, said chemical sensor comprises at least two layers of porous inorganic materials, of distinct porosities, one forming a waveguide and the other forming a confinement layer.

  The invention also relates to a chemical sensor as described above, whose inorganic material constituting the waveguide preferably comprises porous silicon.

  The invention also relates to a method of manufacturing a chemical sensor as described above, characterized in that it comprises the steps of: - producing at least two porous layers in an inorganic material; - impregnation of the pores of said layers with at least one organic material whose light absorption properties vary as a function of the concentration of said molecules of a given species present in said gas phase.

  In this way, the waveguide is produced in a single overall step, the technical principle thereof remaining unchanged whatever the coloring molecule used.

  Preferably, the method comprises a step of producing two porous layers formed consecutively in a silicon substrate.

  Advantageously, said step of producing at least two porous layers is obtained by electrochemical attack.

  According to a particular embodiment, a layer intended to form a waveguide is obtained by electrochemical attack with a current density of approximately 50 mA / cm 2, a second layer intended to form an insulating layer being obtained by electrochemical attack with a current density of about 100 mA / cm2.

  However, other embodiments can be envisaged to modify the porosity of the layers, this for example by varying the current density of the electrochemical attack.

  Preferably, the process comprises, prior to said impregnation step, a step of dissolving Congo red in a solution of N, N-dymethylformamide, with a concentration of between 0.01 g / 1 and 0.5 g / 1.

  Advantageously, said impregnation step is carried out by depositing approximately 7 gl / cm 2 of surface of said chemical sensor.

  In the case for example of a planar sensor, we will therefore take into consideration the planar surface that the sensor occupies.

  Thus, the waveguide is impregnated so that the surface of the pores is covered in a relatively homogeneous manner.

  According to an advantageous solution, said impregnation step is carried out under vacuum at a temperature of around 25 C.

  It is therefore possible to produce the sensor under operating conditions that are simple to obtain.

  Preferably, the method comprises, prior to said impregnation step 10, a step of thermal oxidation of said at least one porous layer at a temperature of approximately 900 ° C. for approximately one hour.

  Other characteristics and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment of the invention given by way of illustrative and nonlimiting example, and from the appended drawings among which: Figure 1 schematically shows a detection device incorporating a chemical sensor according to the invention; - Figures 2 to 4 are curves representing absorption spectra of the Congo red chromophore. With reference to FIG. 1, a detection device intended to evaluate the concentration of acidic or basic molecules present in a gas phase comprises: - an emitting light source 1, such as a laser diode or a light-emitting diode; - a chemical sensor 2 intended to be placed in a medium 3 containing a gaseous phase to be analyzed; - a photo-detector 4.

  It is noted that, according to a possible variant, a focusing lens 5 is positioned between the chemical sensor 2 and the photodetector 4.

  The operation of the device illustrated in FIG. 1 is as follows: a light wave is emitted by a source 1 and is sent to the sensor 2 preferably using an optical fiber 11. At the output of the sensor, the photodetector 4 measures the flux transmitted by the sensor 2 (after focusing by a lens 5 according to the present embodiment) and gives an electrical signal proportional to this flux.

  When the sensor 2 is brought into contact with an environment rich in H + or OH- ions, the transmitted light flux is modified by optical absorption of the sensor. The measurement of the electrical signal makes it possible to indicate an analysis result.

  According to the present embodiment, the chemical sensor is produced so as to consist of a reactive planar waveguide, having two layers 21, 22 of oxidized porous silicon.

  As will become clear later, the surface of the pores is covered with the reactive dye.

  Such a planar waveguide is produced from a P-type monocrystalline silicon substrate 15, oriented (100) heavily doped with boron, with a resistivity of between 3.10-3 Q.cm and 7.10-'3 Q.cm .

  The porosity of these layers is obtained by an electrochemical attack in a 20% hydrofluoric acid solution, by a technique known to those skilled in the art.

  To obtain the waveguide, the two porous layers 21, 22 are produced consecutively with different current densities, this in order to obtain distinct porosities. As an indication, a current density of 50 mA / cm 2 is used for the guiding layer 21 and a current density of 100 mA / cm 2 for the production of the confinement layer 22.

  The layer 21 therefore forms a layer which guides the light waves because it has a higher optical refractive index than that of the lower layer 22.

  Note that the thickness of the layers 21 and 22 is defined as a function of the dimension of the optical fiber with which they are associated. For example, for a fiber having a diameter of 10 μm, layers 21, 22 are produced, each having a thickness of 5 Jtm.

  The layers thus obtained are then thermally oxidized at 900 ° C. for 1 hour, before being impregnated with a chromophore.

  According to the present embodiment, the chromophore and Congo red which is a dye known as a pH indicator in the range 3 - 5, 2.

  As indicated by the diagrams of Figures 2 to 4, the optical absorption band of Congo red moves in the visible range from blue to red depending on the protonation and deprotonation of the molecule when it is present of an acidic or basic atmosphere.

  The layer 21 impregnated with Congo red therefore reacts differently depending on the nature of the contaminating gas in the medium 3.

  Figure 2 illustrates the absorption spectrum of Congo red in a neutral atmosphere.

  In the presence of an acid gas such as HCl, the guide layer 21 absorbs red light, as illustrated by the absorption spectrum of FIG. 3.

  In the presence of a basic gas such as NH3, the guide layer 21 does not absorb the red light, as illustrated by the absorption spectrum of FIG. 4.

  It is noted that the reactions of Congo red are stable over time and reversible. There is therefore no destruction of the sensor during the measurement of the contaminating gas, the regeneration of the sensor can be obtained by a reverse chemical reaction.

  It is further noted that such a sensor does not disturb the environment insofar as the chemical reactions are confined in the pores of the sensor.

  The impregnation of the layers 21, 22 with the dye, in this case Congo red, is carried out in the following manner.

  Congo red one is dissolved in N, Ndymethylformamide, with a concentration of between 0.01 g / l and 0.5 g / l.

  The impregnation step as such is carried out by depositing the solution thus obtained at the rate of approximately 7 [tl / cm 2 of the chemical sensor. O10

  Experimentation with such a chemical sensor shows that the specific surface of the pores is very large and that the dye is fixed on the walls of the pores in a homogeneous manner over the entire thickness of the porous layer. Thus, a low concentration of dye in the layer is sufficient.

  With a laser source (, = 633 nm), the optical propagation results show the tilting of the sensor as a function of the acidity of the gas: - in a neutral atmosphere, the sensor propagates the light and the voltage obtained at the output of the photo -detector is 1.3 V, this voltage being proportional to the light flux at the output 10 of the sensor; - in the presence of acid vapors, the sensor absorbs the radiation and the measured voltage is no more than 0.07 V; - in the presence of ammonia vapors, the propagation of the optical radiation is again observed and the voltage measured is 1.3 V. Whatever the concentration of H + or OH- ions present in the reaction, the voltage values are always the same. Only the time required for the chemical reaction of all Congo red molecules varies.

  Thus, the time for a complete reaction increases if the concentration of toxic gas is low.

  On the other hand, in the presence of an atmosphere saturated with gas, the response time is very short: - for acid vapors, the response time is less than 500 ms; - for basic vapors, the response time is less than ms.

  Other chromophores could be used in other conceivable embodiments, these chromophores having to present a displacement of their absorption band linked to the electronic transitions produced by chemical reactions with the atmosphere in which the sensor is immersed.

  As an indication, we can in particular use: - Bromocresol Green, for a pH range between 3.8 (transmitting in yellow) and 5.4 (transmitting in blue) - Phenol Red, for a pH range between 6.8 (transmitting in yellow) and 8.2 (transmitting in red) - Phenoltphthalein, for a pH range between 8 (colorless) and 10 (transmitting in red); 10 - Carmin Indigo, for a pH range of between 11.5 (transmitting in yellow) and 14 (transmitting in blue).

Claims (25)

  1. Device for evaluating the concentration of at least one determined chemical species present in a gas phase, said device comprising a chemical sensor comprising at least one waveguide, means for emitting a light flux towards said waveguide and means for analyzing said light flux at the output of said chemical sensor, characterized in that said waveguide consists of at least one porous inorganic material impregnated with at least one organic material whose properties light absorption vary depending on the concentration of said at least one specific chemical species present in said gas phase.   2. Device according to claim 1, characterized in that said inorganic material comprises at least porous silicon.   3. Device according to claim 2, characterized in that said porous silicon is a P-type or N-type monocrystalline silicon. 4. Device according to any one of claims 2 and 3, characterized in that said porous silicon is oriented (100).   5. Device according to any one of claims 2 to 4, characterized in that said porous silicon is doped with boron.   6. Device according to any one of claims 2 to 5, characterized in that said porous silicon has a resistivity comprised in 3.10-'3 Q.cm and 7.10-3 Q.cm.   7. Device according to any one of claims 2 to 6, characterized in that said porous silicon is oxidized.   8. Device according to any one of claims 1 to 7, characterized in that said organic material is a chromophore material.   9. Device according to claim 7, characterized in that said chromophore material comprises Congo red.   10. Device according to claim 7, characterized in that said chromophore material comprises at least one of the materials belonging to the following group: - Bromocresol Green; - Rouge Phénol; - Phenoltphthalein; - Indigo Carmine.   11. Device according to any one of claims 1 to 10, characterized in that said means for emitting a light flux comprise at least one of means belonging to the following group: laser diode; - light emitting diode.   12. Device according to any one of claims 1 to 11, characterized in that said light flux is directed towards said chemical sensor using optical fibers.   13. Device according to any one of claims 1 to 12, characterized in that said means for analyzing said light flux comprise at least one photo-detector.   14. Device according to any one of claims 1 to 13, characterized in that it comprises at least one focusing lens interposed between said chemical sensor and said analysis means.   15. Device according to any one of claims 1 to 14, characterized in that said chemical sensor comprises at least two layers of porous inorganic materials, of distinct porosities, one forming a waveguide and the other forming a containment layer.   16. Chemical sensor intended to be integrated in a device for evaluating the concentration of at least one determined chemical species present in a gas phase comprising means for emitting a light flux towards said chemical sensor and means for analyzing said luminous flux at the output of said chemical sensor, said chemical sensor comprising at least one waveguide, characterized in that said waveguide consists of at least one porous inorganic material impregnated with at least one organic material whose properties of light absorption vary depending on the concentration of said molecules of a given species present in said gas phase. 5 17. Chemical sensor according to claim 16, characterized in that said inorganic material comprises at least porous silicon.   18. A method of manufacturing a chemical sensor according to any one of claims 16 and 17, characterized in that it comprises the steps of: - producing at least two porous layers in an inorganic material; - impregnation of the pores of said layers with at least one organic material, the light absorption properties of which vary as a function of the concentration of said molecules of a given species present in said gas phase.   19. A method of manufacturing a chemical sensor according to claim 18, characterized in that it comprises a step of producing two porous layers formed consecutively in a silicon substrate.   20. Method of manufacturing a chemical sensor according to either of claims 18 and 19, characterized in that said step of producing at least two porous layers is obtained by electrochemical attack.   21. A method of manufacturing a chemical sensor according to claims 19 and 20, characterized in that a layer intended to form a waveguide is obtained by electrochemical attack with a current density of about 50 25 mA / cm , a second layer intended to form an insulating layer being obtained by electrochemical attack with a current density of approximately 100 mA / cm 2.   22. Method for manufacturing a chemical sensor according to any one of claims 18 to 21, characterized in that it comprises, prior to said impregnation step, a step of dissolving Congo red in a solution of N , N-dymethylformamide, with a concentration of between 0.01 g / l and 0.5 g / 1.   23. A method of manufacturing a chemical sensor according to claim 22, characterized in that said impregnation step is carried out by depositing about 7 dl / cm2 of surface of said chemical sensor.   24. A method of manufacturing a chemical sensor according to any one of claims 18 to 23, characterized in that said impregnation step is carried out under vacuum at a temperature of about 25 C.   25. Method of manufacturing a chemical sensor according to any one of claims 10 to 24, characterized in that it comprises, prior to said impregnation step, a step of thermal oxidation of said at least one porous layer at a temperature of about 900 C for about an hour.