METHOD FOR PREPARING A CARBON DIOXIDE SENSOR, SAID SENSOR AND USES THEREOF

Abstract

The present invention relates to a method for preparing a CO2 sensor comprising the following steps: (a) providing a support (1) made of an insulating material comprising a pH-sensitive electrode (2) and a reference electrode (3); (b) providing a double-sided adhesive (4) having an acrylic-based adhesive layer (41), an opposing silicone-based adhesive layer (42) and, in its thickness, a through-recess (5); (c) applying the double-sided adhesive (4) to the support (1); (d) applying, to the double-sided adhesive (4), a silicone-based carbon dioxide-permeable membrane (6) whereby a measuring chamber (7) is obtained, and then (e) filling the measuring chamber (7) with an electrolyte (9). The present invention also relates to a CO2 sensor thus prepared, to a multi-sensor device comprising it and to their uses for measuring dissolved or gaseous CO2. Abstract figure: Fig. 1

Description

Title of the invention: METHOD FOR PREPARING A CARBON DIOXIDE SENSOR, SAID SENSOR AND ITS USES Technical field

[0001] The present invention belongs to the field of electrochemical sensors useful for the determination of chemical parameters, in particular to the field of dissolved or volatile carbon dioxide sensors.

[0002] The present invention provides a method for preparing a carbon dioxide sensor and in particular a miniaturized carbon dioxide sensor.

[0003] The present invention also relates to such a carbon dioxide sensor and its various uses. STATE OF THE PRIOR ART

[0004] The detection and / or monitoring of carbon dioxide is essential for many applications such as, for example, monitoring environmental conditions, assessing air pollution levels, understanding physiological processes or regulating biopharmaceutical production processes, etc.). Many technological approaches have been developed to detect and measure carbon dioxide, each with its own advantages and limitations.

[0005] Conventional methods for determining carbon dioxide concentration include infrared spectrometry and electrochemical sensors based on liquid or solid electrolytes. Infrared detection is a widely used technique that relies on the absorption of infrared light by carbon dioxide molecules. This method provides accurate and sensitive measurements, but may require complex instrumentation and calibration. In contrast, electrochemical sensors offer a simpler and more cost-effective approach to carbon dioxide detection.

[0006] In addition, solid electrolyte sensors operating at high temperatures have also been widely used to measure dissolved carbon dioxide. These sensors have been used in a wide variety of industries, including environmental control, biotechnology, biology, medicine, and the food industry. Beyond traditional electrochemical sensors, other methods such as non-dispersive infrared spectroscopy, photoacoustic effects, and thermal conductivity are also being explored to detect dissolved carbon dioxide.

[0007] Electrochemical sensors with liquid electrolytes are based on the principle of Se- Severinghaus, which indirectly measures the partial pressure of carbon dioxide by monitoring the pH values of an internal electrolyte. It should be noted, however, that while Severinghaus-type electrochemical sensors are inexpensive and relatively simple to use, they rely on indirect measurement of the partial pressure of carbon dioxide and can be influenced by other dissolved gases with similar behavior. However, due to their ease of integration and low cost, these sensors offer a practical solution for continuously monitoring carbon dioxide concentrations in a variety of applications.

[0008] The so-called Severinghaus principle [1] i.e. the operating principle of an electrochemical carbon dioxide sensor is based on a pH sensor immersed in a chamber containing a liquid-phase electrolyte covered with a membrane permeable and selective to CO2. After passing through the CO2-permeable membrane, the gas is dissolved in the aqueous electrolyte. The concentration in the electrolyte is then correlated to that in the medium of interest.

[0009] Indeed, the dissolution of CO2 in the electrolytic solution leads to the formation of bicarbonate ions (HCO3) and consequently a variation in pH through the equilibrium cascade as described by the following equations:

[0010] co2 (dissolved)+h2o

[0011] co2 + h2o ^h2co3

[0012] H2C(k + HCO\ JK,

[0013] HC0\ <^H+ + CO2: 3 Y

[0014] where Kh is Henry's constant, and Ki and K2 are the first and second dissociation constants of carbonic acid (H2CO3). Taking into consideration water dissociation and charge balance, the relationship between H+ ions and CO2 partial pressure is deduced as follows:

[0015] [zr]3+ Kw) [tC]-ZK^K^CO^Q

[0016] where [NaHCO3] is the sodium bicarbonate concentration in the internal electrolyte and Kw is the water dissociation constant.

[0017] The variation of pH as a function of CO2 concentration can thus be described by:

[0018] „ _ „ , / [nœ3] \ pH - pKu + log I

[0019] with pKa, the dissociation constant of H2CO3 in water whose value is known. [HCO3 ] is the concentration of bicarbonate fixed in the electrolyte. The dissolution constant of CO2 in the medium is expressed by a which is the inverse of the Henry constant of CO2.

[0020] This modification of the pH can be evaluated in real time by a potential measurement between the pH-sensitive electrode and a reference electrode also immersed in the electrolyte. However, it should be noted that the CO2 measurement is obtained indirectly through a pH sensor.

[0021] Conventional Severinghaus type sensors have certain limitations that must be taken into account. One of the main characteristics of these sensors is the use of an internal glass electrode. However, this manufacturing complexity results in high costs and relatively laborious production. In addition, due to their fragile nature, glass electrodes are prone to breakage, which can require frequent replacements, impact the reliability of measurements and cause contamination of the medium under study. The regular maintenance required to maintain the optimal performance of these sensors can also be a constraint, particularly in demanding environments.

[0022] Another major limitation of these conventional sensors is related to their size. Due to their design, these sensors can be relatively bulky, which can limit their applicability in situations where space is restricted, when discrete measurements are required, or in the context of a multi-sensor device. In addition, the presence of a glass electrode and associated components add to the overall size of the device, which can be limiting in contexts where miniaturization is essential.

[0023] Miniaturized electrochemical sensors of the Severinghaus type aim to solve some of the limitations inherent in conventional sensors. However, even in this category, several challenges remain. Thus, regarding the first “layer”, that of pH measurement, any shortcomings in the latter can compromise the overall accuracy of the sensor, thus highlighting the importance of careful selection and optimization of this component. The nature of the internal electrolyte is also a crucial point to consider in terms of price, stability over time and long-term storage. Finally, the permeable membrane that isolates the sensor from the medium studied plays a crucial role in the overall performance of the sensor since it is in direct contact with the measured medium, which makes the choice of an appropriate material depending on the specific application crucial.

[0024] Optimizing miniaturized Severinghaus-type sensors requires a balanced and careful approach in the selection and combination of these different layers. Performance, ease of industrial integration and cost are closely linked to the synergy between these elements.

[0025] The inventors therefore set themselves the goal of proposing a rapid, easy-to-implement method making it possible to prepare a miniaturized, efficient and reliable sensor for the detection and measurement of carbon dioxide, in particular dissolved carbon dioxide. Statement of the invention

[0026] The solution proposed by the inventors is a simple, rapid and easily industrializable manufacturing process making it possible to obtain a Severinghaus type carbon dioxide sensor with a specific architecture.

[0027] The method for preparing a carbon dioxide sensor according to the invention therefore involves an optimal choice and the carrying out of different steps in order to obtain the different constituent parts of the sensor, which are (i) the pH-sensitive layer, (ii) the structural material of the internal cavity, (iii) the internal electrolyte and (iv) the permeable membrane.

[0028] The specific architecture thus obtained demonstrated the ability to develop a miniaturized Severinghaus type sensor having, as illustrated in the experimental part below, all or part of the following properties: - maintaining excellent performance, particularly in terms of response times, even with low concentrations of dissolved CO2; - fully integrable into a multi-sensor system; - enabling simple and inexpensive manufacturing thanks to its compatibility with collective manufacturing processes such as screen printing, making it possible to obtain reproducible sensors at low cost; - offering strong versatility in terms of support, geometry, or even configuration; - being able, when the internal electrolyte is in the form of a hydrogel, to be stored in the open air and being in fact ready for use, thus significantly facilitating handling; and - requiring no maintenance.

[0029] Thus, the present invention relates to a method for preparing a carbon dioxide sensor, the steps of which are shown diagrammatically in [Fig.l]. This method comprises the following steps: a) providing a support 1 made of an insulating material having a first face 11 and a second face 12 opposite the first face, the first face having at least one zone comprising a pH-sensitive electrode 2 and a reference electrode 3; b) providing a double-sided adhesive 4 having a first acrylic-based adhesive layer 41, a second silicone-based adhesive layer 42 opposite the first adhesive layer and, in its thickness, a through-recess 5 having a first open end 51 and a second open end 52 opposite the first open end; c) applying the first adhesive layer 41 of the double-sided adhesive 4 to the first face 11 of the support 1 so that the first open end 51 of the recess 5 of the double-sided adhesive 4 comes opposite the area of the first face 11 of the support 1; d) applying, on the second adhesive layer 42 of the double-sided adhesive 4 and on the second open end 52 of the recess 5 of the double-sided adhesive 4, a silicone-based carbon dioxide-permeable membrane 6 whereby a measuring chamber 7 is obtained, then e) filling the measuring chamber 7 thus obtained with an electrolyte 9 comprising bicarbonate ions (HCO3) whereby the pH-sensitive electrode 2 and the reference electrode 3 are in contact with the electrolyte.

[0030] The support (or substrate) of the CO2 sensor prepared by the method according to the present invention may be any substrate made of an insulating material and in particular any substrate made of an insulating material typically used in Severinghaus type CO2 sensors. It may also be substantially flat or, on the contrary, concave or convex and of any shape such as a square, rectangular, round or oval shape.

[0031] Advantageously, the insulating material, i.e. the electrically non-conductive material of the support used in the method according to the invention, is chosen from the group consisting of non-conductive oxides, non-conductive polymers, amorphous insulating materials, crystalline insulating materials and any of their combinations.

[0032] In particular, the insulating material of the support used in the method according to the invention is chosen from the group consisting of silicon dioxide (SiO2), aluminum oxide (A12O3), magnesium oxide (MgO), a glass generally containing silicates, a silica glass, a ceramic, diamond, a polyphenylene sulfide, a polyethyleneimine, a polytetrafluoroethylene, a polyimide, a polyethylene, a polypropylene, a polystyrene, a polycarbonate, a polymethyl methacrylate, a polysulfone, a polyetherimide, a polyether ether ketone, parylene N™, Nuflon™, a silicone, an epoxy resin and any of their combinations.

[0033] The support implemented in the invention may be flexible or rigid. In a particular embodiment, this support is a substrate typically used for printed circuits (or "PCB" for "Printed Circuit Board"). Such a type of substrate is generally made up of insulating layers of epoxy resin reinforced by a weave of glass or paper fibers.

[0034] The support implemented in the context of the method according to the invention may have a single zone comprising a pH-sensitive electrode and a reference electrode. As a variant, the support implemented in the context of the method according to the invention may have several zones, identical or different and in particular of identical or different size and shape, each zone comprising a pH-sensitive electrode and a reference electrode.

[0035] The preparation of the support used in the context of the method according to the invention with at least one zone comprising a pH-sensitive electrode and a reference electrode is a conventional method in the preparation of Se-veringhaus type CO2 sensors. Examples of techniques conventionally used for this preparation include collective manufacturing by screen printing or by silicon process or even inkjet printing techniques.

[0036] The pH-sensitive electrode that the support used in the context of the method according to the invention has has (i) at least one electrically conductive layer arranged on the support made of an insulating material, i.e. the lower face of the conductive layer is in direct contact with the first face of the support as previously defined and (ii) at least one pH-sensitive layer comprising pH-sensitive elements, directly or indirectly, arranged on this electrically conductive layer.

[0037] The conductive layer of the pH-sensitive electrode presented by the support used in the context of the method according to the invention makes it possible to electrically connect this electrode to the electronic measuring part. It can be made of any electrically conductive material and in particular any electrically conductive material typically used in Severinghaus type CO2 sensors.

[0038] Advantageously, the conductive layer is made of an electrically conductive material chosen from the group consisting of metals such as noble metals, carbons and electrically conductive polymers or copolymers.

[0039] More particularly, the conductive layer of the pH-sensitive electrode presented by the support used in the context of the method according to the invention is made of a metal chosen from the group consisting of gold, copper, stainless steel, silver, nickel, aluminum, titanium, platinum, palladium, ruthenium, iridium, molybdenum or one of their alloys.

[0040] When the pH-sensitive layer is indirectly disposed on the electrically conductive layer, a layer comprising an electrochemical transducer is disposed between these two layers. Alternatively, when the pH-sensitive layer is directly disposed on the electrically conductive layer, the pH-sensitive layer comprises not only pH-sensitive elements but also an electrochemical transducer.

[0041] The electrochemical transducer in the pH-sensitive electrode presented by the support used in the context of the method according to the invention is the element of this electrode whose function is to transcribe the chemical recognition of the pH-sensitive layer into an electrical signal, i.e. a measurable, sensitive and easily exploitable physical signal.

[0042] Any electrochemical transducer conventionally used in Severinghaus type CO2 sensors can be used in the context of the present invention. Typically, the electrochemical transducer is chosen from carbon-based nanomaterials such as, for example, single-walled or multi-walled carbon nanotubes, carbon nanowires and graphene; carbon inks such as, for example, graphite-based ink C10903P14 (Gwent Electronic. Materials Ltd, Montypool, UK) or carbon-based ink BQ242 (DuPont, Bristol, UK); and semiconducting polymers such as, for example, poly(3,4-ethylenedioxythiophene) coupled to sodium poly(styrene sulfonate) (PEDOT:PSS) or poly(3-octylthiophene-2,5-diyl).

[0043] Those skilled in the art know various pH-sensitive elements that can be used for the pH-sensitive electrode implemented in the invention. Advantageously, the pH-sensitive elements of the pH-sensitive electrode that the support implemented in the context of the method according to the invention has are made of a material chosen from transition metal oxides.

[0044] In particular, the pH-sensitive elements of the pH-sensitive electrode presented by the support used in the context of the method according to the invention are in nanometric form and in particular in the form of powder, grains, flakes, particles or one of their mixtures and are made of a material chosen from the group consisting of titanium oxide (TiO2), ruthenium oxide (RuO2), palladium oxide (PdO2), platinum oxide (PtO2), iridium oxide (IrO2), tungsten oxide (WO2) and their mixtures.

[0045] More particularly, the pH-sensitive elements of the pH-sensitive electrode presented by the support used in the context of the method according to the invention are made of iridium oxide (IrO2).

[0046] The support of the CO2 sensor prepared by the method according to the invention further comprises a reference electrode. Any reference electrode or pseudo-reference electrode conventionally used in Severinghaus type CO2 sensors can be used in the context of the present invention. Advantageously, the reference electrode presented by the support used in the context of the method according to the invention is a silver chloride (AgCl / Ag) pseudo-reference electrode.

[0047] In the preparation process according to the invention, a double-sided adhesive is used. The three essential elements of this double-sided adhesive are a first acrylic-based adhesive layer, a second silicone-based adhesive layer and a through-cut recess. The double-sided adhesive used in the process according to the invention has adhesive layers of different chemical compositions: it is therefore an asymmetrical double-sided adhesive.

[0048] The first acrylic-based adhesive layer of this double-sided adhesive is intended to be applied to the first face of the support as previously described, which allows for strong adhesion and effective sealing at the support level. Similarly, the second silicone-based adhesive layer has a very good affinity with silicones and in particular the silicone of the CO2-permeable membrane intended to be applied to this second adhesive layer.

[0049] Finally, the through-cavity that the double-sided adhesive used has is intended to form the walls of the measuring chamber or cavity of the CO2 sensor in which the pH-sensitive electrode, the reference electrode and the electrolyte are located. Typically, the shape of the first open end of this recess is substantially identical to the shape of the area of the first face of the support made of an insulating material as previously defined. Advantageously, this recess has a substantially parallelepipedal shape. The machining of such a recess is routine work for those skilled in the art, either manually using a punch or a cutter, or in collective manufacturing by mechanical or laser cutting.

[0050] Typically, the double-sided adhesive used in the context of the present invention comprises, between the first acrylic-based adhesive layer and the second silicone-based adhesive layer, a polymeric substrate and in particular a polyester substrate. Advantageously, the double-sided adhesive used in the invention has a thickness of between 50 μm and 585 μm and in particular a thickness of the order of 85 μm (i.e. 85 μm ± 10 μm).

[0051] Any asymmetrical double-sided adhesive, particularly a commercial one, which has the above characteristics can be used in the preparation process according to the invention. As a particular example, mention may be made of the double-sided adhesive with reference 5302A and marketed by the company NITTO DENKO.

[0052] Once the double-sided adhesive is placed on the first side of the support, a silicone-based CO2-permeable membrane is placed on the second silicone-based adhesive layer of the double-sided adhesive and on the second open end of the recess of the double-sided adhesive. Typically, this CO2-permeable membrane is a crosslinked silicone film.

[0053] Advantageously, the CO2-permeable membrane used in the method according to the invention has a thickness of between 20 μm and 400 μm and in particular a thickness of the order of 50 μm (i.e. 50 μm ± 10 μm).

[0054] Any CO2-permeable membrane having the above characteristics can be used in the preparation process according to the invention. However, permeable membranes manufactured from a two-part commercial formulation (MED-6010, Nusil) spin-coated onto a silicon wafer in order to obtain the thinnest possible membrane and then followed of a transfer on double-sided adhesive have shown reproducibility problems mainly due to difficulties in transferring the silicones, thus generating asperities and / or cracks on the membrane once transferred. With the aim of solving this problem and simplifying future industrialization of the process, an optimal process was determined with commercial silicone films. The integration of these silicones into the manufacturing process of carbon dioxide sensors has shown excellent reproducibility of transfers while facilitating manufacturing.

[0055] Thus, the CO2-permeable membrane used in the process according to the invention is a commercial membrane and in particular a crosslinked silicone film of reference SILPURAN® Film 2030 marketed by the company Wacker Chemie AG.

[0056] The side walls of the measuring chamber or cavity of the CO2 sensor prepared by the method according to the present invention correspond to the lateral edges of the recess of the double-sided adhesive. The lower part of this measuring chamber corresponds to the support made of an insulating material at the area as previously defined and the upper part of this measuring chamber corresponds to the CO2-permeable membrane covering the second open end of the recess of the double-sided adhesive.

[0057] The method according to the present invention makes it possible to obtain measuring chambers whose volume is small, which makes it possible to obtain high-performance CO2 sensors (see experimental section below). The measuring chamber obtained during the preparation method according to the invention has a volume of between 1 cm3 and 12 cm3, in particular between 1.1 cm3 and 9 cm3 and, in particular, of the order of 1.275 cm3 (i.e. 1.275 cm3 ± 0.05 cm3). As particular examples, the measuring chamber obtained during the preparation method according to the invention may have a volume of the order of 1.275 cm3, of the order of 5.025 cm3 (i.e. 5.025 cm3 ± 0.05 cm3) and of the order of 8.775 cm3 (i.e. 8.775 cm3 ± 0.05 cm3).

[0058] Step e) of the preparation method according to the invention consists in filling the measuring chamber with an electrolyte. This filling is carried out by means of at least two fluid channels 81, 82 opening into the measuring chamber and adapted to bring, into the measuring chamber, an electrolyte or a precursor thereof and to extract the air contained in the measuring chamber. These fluid channels are typically fluid microchannels, pierced at the level of the CO2-permeable membrane and / or machined at the level of the double-sided adhesive, in particular at the level of the polymeric substrate and, in particular, at the level of the polyester substrate that this double-sided adhesive comprises. In a particular embodiment, the permeable membrane is pierced in particular using a needle associated with a syringe for the injection of the electrolyte or its precursor. The silicone of the permeable membrane, and in particular of the commercial permeable membrane as previously defined, is “self-regenerating”. However, to ensure that the measuring chamber or internal cavity is properly sealed, a second adhesive or a second permeable membrane may be added over the assembly.

[0059] In a first embodiment, the electrolyte used in the preparation method according to the present invention is a liquid electrolyte. The fluidic channel(s) as previously defined make it possible to bring such a liquid electrolyte into the measuring chamber.

[0060] In a second embodiment, the electrolyte used in the preparation method according to the present invention is an electrolyte in the form of a hydrogel. An internal electrolyte in the form of a hydrogel makes it possible to limit or even eliminate evaporation during the storage and use phases of the CO2 sensor, as well as to simplify manufacturing.

[0061] In this second embodiment, to avoid any blockage of the fluidic channel(s) as previously defined, it is a solution comprising the precursor compounds of this hydrogel which will be brought, via this or these fluidic channels, into the measuring chamber.

[0062] Typically, a hydrogel designates a material formed from at least two constituents: a solution, also called a gelled solution, which is a liquid "trapped" by a second compound which forms a three-dimensional network throughout the solution, the solution of a hydrogel being water or an aqueous solution i.e. a solution whose solvent is water.

[0063] In the present invention, all precursor compounds generally used to prepare a hydrogel can be used. Advantageously, these precursor compounds are chosen from the group consisting of chitosan, xanthan, carrageenan, dextran, agar, alginate, gelatin, collagen, fibrin, polyethylene glycol, hyaluronic acid, their (meth)acrylated derivatives and their mixtures.

[0064] In a particular embodiment, the electrolyte in the form of a hydrogel used in the invention is a hydrogel obtained from precursors chosen from the group consisting of agar, gelatin, chitosan and methacrylated dextran. In this particular embodiment, the solution comprising the precursor compounds of such a hydrogel typically comprises 1% by mass of agar, 25% by mass of gelatin, 10% by mass of chitosan or 5% by mass of methacrylated dextran.

[0065] In an even more particular embodiment, the electrolyte in the form of a hydrogel used in the invention is an agar hydrogel. In this even more particular embodiment, the solution comprising the precursor compounds of such a hydrogel comprises 1% by mass of agar.

[0066] Whether the electrolyte used in the context of the present invention is in liquid form or in the form of a hydrogel, it comprises bicarbonate ions in an amount of between 1 mM and 20 mM, in particular between 3 mM and 10 mM and, in particular, of the order of 5 mM (i.e. 5 mM ± 1 mM). This concentration of bicarbonate ions is understood to be in the liquid electrolyte or in the solution comprising the precursor compounds of the hydrogel forming the electrolyte. The bicarbonate ions are typically present in the form of sodium bicarbonate (NaHCO3).

[0067] Advantageously, in the context of the method according to the invention, the liquid electrolyte and the solution comprising the precursor compounds of the hydrogel forming the electrolyte further comprise a salt in which the anion is a chloride (Cl ). Advantageously, the cation of this salt is an alkali metal cation such as Li+, Na+, K+; an alkaline-earth metal cation such as Mg2+, Ca2+; or a metal cation of a transition metal such as Cu2+, Zn2+ and Al3+. In a particular embodiment, the salt used in the liquid electrolyte and the solution comprising the precursor compounds of the hydrogel forming the electrolyte is KC1. More particularly, this salt is present in the liquid electrolyte and the solution comprising the precursor compounds of the hydrogel forming the electrolyte in an amount of between 25 mM and 250 mM, in particular between 50 mM and 150 mM and, in particular, of the order of 100 mM (i.e. 100 mM ± 10 mM).

[0068] Finally, the solution comprising the precursor compounds of the hydrogel forming the electrolyte used in the preparation process according to the invention may comprise at least one element necessary for the formation of a hydrogel from these precursor compounds. Such an element may be, for example, a crosslinking agent such as polyethylene glycol diglycidyl ether (or PEGDE) or a photoinitiator such as lithium phenyl-2,4,6-trimethylbenzoylphosphinate (or LAP).

[0069] It should be noted that it is possible, before step e) of the method of the invention, i.e. before filling the measuring chamber of the sensor with the electrolyte, to subject the assembly obtained at the end of step d), assembly comprising the support made of an insulating material, the pH-sensitive electrode, the reference electrode, the double-sided adhesive and the CO2-permeable membrane as previously defined, i.e. the sensors without internal electrolyte, to a sterilization step, in particular by autoclave or by gamma irradiation. Indeed, no deterioration of the structure of the sensors following such sterilization has been observed.

[0070] The present invention also relates to a CO2 sensor obtained by the preparation method as previously defined. This sensor comprises: - a support 1 made of an insulating material having a first face 11 and a second face 12 opposite the first face, the first face having at least one zone comprising a pH-sensitive electrode 2 and a reference electrode 3; - a double-sided adhesive 4 having a first acrylic-based adhesive layer 41, a second silicone-based adhesive layer 42 opposite the first adhesive layer and, in its thickness, a through-recess 5 having a first open end 51 and a second open end 52 opposite the first open end; the first adhesive layer 41 of the double-sided adhesive 4 being in contact with (i.e. applied to) the first face 11 of the support 1 so that the first open end 51 of the recess 5 of the double-sided adhesive 4 is opposite the area of the first face 11 of the support 1; - a silicone-based carbon dioxide permeable membrane 6 in contact with (i.e. applied to) the second adhesive layer 42 of the double-sided adhesive 4 and in contact with (i.e. applied to) the second open end 52 of the recess 5 of the double-sided adhesive 4; - a measuring chamber 7 whose side walls correspond to the side edges of the recess 5 of the double-sided adhesive 4, whose lower part corresponds to the support 1 made of an insulating material at the level of the zone as previously defined and whose upper part corresponds to the CO2-permeable membrane 6 covering the second open end 52 of the recess 5 of the double-sided adhesive 4; said measuring chamber being filled with an electrolyte 9 comprising bicarbonate ions (HCO3).

[0071] The various elements that comprise the sensor according to the present invention are as previously defined within the framework of the preparation method according to the present invention.

[0072] The present invention also relates to a multi-sensor device comprising at least one CO2 sensor as previously defined. In such a multi-sensor device, the CO2 sensor may be associated, for example, with one or more sensors chosen from the group consisting of temperature sensors, relative humidity sensors and electrochemical sensors such as, for example, metabolic sensors, pH sensors and ionic sensors.

[0073] The present invention finally relates to the use of a CO2 sensor as previously defined or of a multi-sensor device as previously defined for measuring (i) the CO2 dissolved in a liquid medium in contact with said sensor or said multi-sensor device such as, for example, water or a biological fluid or (ii) the gaseous CO2 present in a gaseous fluid in contact with said sensor or said multi-sensor device.

[0074] As examples of applications of such use, we can cite:

[0075] - in the chemical industry and in particular the pharmaceutical industry: For example, CO2 Dissolved is a critical parameter in biopharmaceutical production processes according to PAT guidelines. By influencing other parameters such as extracellular and intracellular pH, it has an effect on different metabolic pathways that are involved in growth or in the formation and quality of products;

[0076] - in physiological analysis and medical diagnosis: For example, sensors worn on the person for monitoring the blood carbon dioxide pressure (PaCO2) of patients with respiratory failure;

[0077] - in the food industry: For example, for brewing or manufacturing wine, in a brewery, dissolved CO2 is measured to monitor quality attributes to ensure an identical taste sensation;

[0078] - in the industrial field: For example, the measurement of dissolved CO2 is a very important parameter for the optimization of industrial processes;

[0079] - in environmental analysis and in particular for water quality: By For example, the measurement of dissolved CO2 is a very important parameter for assessing the water quality of fish ponds and monitoring marine and freshwater waters.

[0080] Other characteristics and advantages of the present invention will become apparent to those skilled in the art upon reading the examples below given for illustrative and non-limiting purposes, with reference to the appended figures. Brief description of the drawings

[0081] [Fig. 1] already presented corresponds to the diagram of the process for preparing a CO2 sensor according to the invention.

[0082] [Fig.2] shows an example of responses of dissolved CO2 sensors according to the invention. Responses to different concentrations of dissolved CO2 of 4 sensors according to the invention (INV-1) with a thickness of 5302A adhesive (Nitto) and a permeable membrane made of Silpuran 2030 (Wacker).

[0083] [Fig.3A] shows the characterization of the impact of the internal electrolyte volume on the response time of the dissolved CO2 sensors according to the invention.

[0084] [Fig.3B] shows the indicated response times corresponding to T99% for different volumes of liquid internal electrolytes (5 mM NaHCO3 and 0.1 M KC1).

[0085] [Fig.4] shows the responses of 3 dissolved CO2 sensors according to the invention (INV-1) during changes of PBS solutions (250 mM) having different dissolved CO2 concentrations. The dotted line curve corresponds to the average of the responses of the dissolved sensors according to the invention (INV-1) and the transparent area to the standard deviation. The solid line curve corresponds to the response of a commercial optical sensor (CE-1) and the transparent area represents the accuracy range. expected, ±10% of the reference value.

[0086] [Fig.5] shows the interferences of conductivity, pH and oxygen dissolved on 4 CO2 sensors according to the invention (INV-1) compared to a commercial sensor (CE-1).

[0087] [Fig.6] shows the response of 4 dissolved CO2 sensors according to the invention (INV-1) with an internal liquid electrolyte (5 mM NaHCO3, 100 mM KC1) in a continuous line and 4 dissolved CO2 sensors according to the invention (INV-2) with an AGAR-based electrolyte (1%) in a dotted line.

[0088] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS I. Preparation of CO2 sensors.

[0089] 1.1. CO2 sensors according to the invention.

[0090] The proof of concept of the manufacturing method and the integration of the dissolved carbon dioxide sensors of the present invention was carried out with two examples INV-1 and INV-2 whose substrate is made of epoxy resin reinforced by a screen of glass or paper fibers (PCB) and the reference electrode is a pseudo-reference electrode made of Ag / AgCl. The other elements constituting these sensors are as defined in Table 1 below.

[0091] [Tables 1] pH Sensitive Electrode Cavity Forming Film Electrolyte CO2 Permeable Membrane INV-1 IrOx Nitto 5302A Adhesive NaHCO3 (5 mM) KC1 (100 mM) DI Water Silpuran® 2030 50 pm INV-2 IrOx Nitto 5302A Adhesive Agar (1%) NaHCO3 (5 mM) KC1 (100 mM) DI Water Silpuran® 2030 50 pm

[0092] Table 1. The pH-sensitive electrodes used in the sensors according to the invention are prepared as follows:

[0093] - Formulation and mixing step: use of commercial ink BQ242 (DuPont, Bristol, UK) unmodified and mixed with iridium oxide (IrO2) powder, the dry matter mass ratio of IrO2 to BQ242 being 6%

[0094] - Deposition on a conductive gold layer: screen printing process

[0095] - Drying: 2-3 hours at 80°C.

[0096] The generic preparation method of a CO2 sensor and consists, chronolo- logically, in: - manufacturing of a pH sensor based on collective manufacturing (by screen printing or by silicon process), on the substrate 1 in epoxy resin reinforced by a frame of glass or paper fibers (PCB) with gold conductive tracks, of the pH-sensitive electrode 2 based on a metal oxide and of the reference electrode 3 in Ag / AgCl; - machining and depositing a double-sided adhesive 4 to the geometries of the internal cavity of the sensor to obtain the desired volume; - transfer by gluing of the CO2-permeable membrane 5; - injection of the internal electrolyte 9 or its precursor using micro fluid channels 81, 82 drilled at the level of the CO2 permeable membrane 5 or machined at the level of the double-sided adhesive 4.

[0097] 1.2. CO2 sensor of the prior art.

[0098] In order to highlight the functionality and performance of the sensors according to the invention, a counter-example was carried out and studied under the same conditions with a commercial optical sensor of the Presens brand (CE-1). II. Characterization and tests.

[0099] II. 1. Methods.

[0100] For both examples, the sensors are immersed in solutions, connected to potentiostats which make it possible to measure the values of the open circuit potentials for different solutions (changes in solution of known NaHCO3 concentration or metered additions of gas (CO2 or N2)).

[0101] II.2. Results.

[0102] Validation of principle carried out

[0103] [Fig.2] illustrates a typical response during solution changes with different NaHCO3 concentrations of 4 dissolved CO2 sensors (INV-1) manufactured simultaneously on the same substrate according to the method as previously defined (raw data without processing). Good inter-sensor reproducibility can be observed.

[0104] Effect of electrolyte volume

[0105] The response time of sensors to variations in dissolved CO2 is a crucial criterion. As a reminder, adding a membrane to the sensor will slow down the diffusion of chemical species in contact with the electrode, thus increasing the response time. This phenomenon is even more accentuated when the volume of the electrolyte increases since the distance traveled by the analyte after passing the membrane also increases. Added to this is the fact that there is no mechanical convection in the enclosure.

[0106] The influence of the internal electrolyte volume was studied by varying the thickness internal chambers thanks to the manufacturing flexibility (machining and stacking) allowed by the choice of adhesives to manufacture them. Thus, three different internal chamber volumes for INV-1 type sensors were characterized, the three volumes tested being 1.275 cm3, 5.025 cm3 and 8.775 cm3.

[0107] A significant impact was observed, thus, as described upstream, the response time varies with the volume of the electrolyte: as the volume increases, so does the response time ([Fig.3A]). The devices with the lowest volumes that could be manufactured showed excellent average response times to 99% of the signal of 1.34 min (± 0.55 SD) over a range of 0 to 5% CO2, a data that can be compared to that indicated by Presens of: t90 < 3 min for a change of 2 to 5% (or 15 mmHg - 38 mm Hg in pCO2) ([Fig.3B]).

[0108] The internal electrolyte with the smallest volume made it possible to obtain response times at 99% of the order of a minute. At the date of the invention, such rapid response times at 99% of the signal have never been obtained with a similar Severinghaus type configuration.

[0109] Performance characterization

[0110] The characterization of the performances was carried out by measuring the accuracy of the sensors from a commercial Presens sensor (CE-1) and calculating the relative bias. If we consider, as a reference value, the response of CE-1, the accuracy of the sensors developed according to the invention can be characterized by measuring the absolute distance of their responses from that of the commercial sensor.

[0111] [Fig.4] illustrates the results obtained. The dotted line curve corresponds to the average of the responses of 3 sensors developed according to the architecture and manufacturing method of the present invention and the transparent area at the calculated standard deviation. This curve is put into perspective with the response of the commercial optical sensor (CE-1, solid line) and the required accuracy area of ±10% of the reference value (transparent area). Two points can be observed: - the dissolved CO2 sensors according to the invention are accurate to at least ±10% (compared to the commercial optical sensor) because their responses overlap the area of the commercial optical sensor; - the fidelity of the sensors is satisfactory because the standard deviation (transparent area) is relatively low.

[0112] Interferences induced by pH, dissolved oxygen and temperature

[0113] The interferences induced by pH, dissolved oxygen and temperature were then characterized with different dissolved CO2 sensors of the INV-1 type.

[0114] After performing a calibration against the commercial Presens sensor (CE-1), the conductivity values of the solution were modified by adding KC1, the pH by adding HCl and NaOH, and the dissolved oxygen by bubbling ultra-pure O2.

[0115] [Fig.5] illustrates the response to these parameter changes of 4 dissolved CO2 sensors (INV-1) in parallel with a commercial optical sensor (CE-1). The results showed that the dissolved CO2 sensors are not affected by variations in the conductivity of the medium from 7.6 mS / cm to 31.6 mS / cm, nor by variations in pH from 3.64 to 8.42 pH units, and neither by variations in the dissolved oxygen concentration from 0% (argon) to 100% (ultra pure O2).

[0116] Finally, these results highlight a significantly better resolution for the dissolved CO2 sensors according to the present invention compared to the commercial sensors of the Presens brand on very low concentrations of dissolved CO2.

[0117] The results set forth in this section show that the dissolved CO2 sensors of the present invention are capable of providing accurate and faithful measurements to at least ±10% over a measurement range of 0 to 25% of dissolved CO2. Also, the response times obtained are very satisfactory under variable conditions of conductivity, dissolved oxygen and pH.

[0118] Influence of hydrogel

[0119] A feature of some of the dissolved CO2 sensors of the present invention is the presence of an internal electrolyte in the form of a hydrogel in order to limit or even eliminate evaporation during storage and use phases, as well as to simplify manufacturing.

[0120] In this sense, several electrolyte alternatives in hydrogel form have been evaluated: AGAR (1% by mass), 5 mM NaHCO3, 100 mM KC1, - Gelatin (25% by mass), 5 mM NaHCO3, 100 mM KC1, - Chitosan (10% by mass), polyethylene glycol diglycidyl ether (or PEGDE) (40 mM), 5 mM NaHCO3, 100 mM KC1, - Methacrylated dextran (5% by mass), phenyl- Lithium 2,4,6-trimethylbenzoylphosphinate (or LAP) (0.5% by mass), 5 mM NaHCO3, 100 mM KC1.

[0121] The formulation composed with AGAR hydrogel (1% by mass) showed the best results in terms of 99% response time as well as ease of preparation. As detailed by [Fig.6], such a formulation shows a response to dissolved CO2 (sensitivity and 99% response time) similar to liquid electrolytes.

[0122] In addition, a first evaluation of ambient air storage of these sensors was carried out. The sensors were left in the open air for 24 hours, then rehydrated with water vapor. By carrying out an identical calibration following this rehydration phase, the dissolved CO2 sensors manufactured from a “solid” AGAR electrolyte retained a similar sensitivity (Table 2 below).

[0123] [Tables2] Liquid electrolyte Electrolyte Agar 1% Electrolyte Agar 1% Dried then rehydrated 66.37 ± 0.68 (n=4) 70.63 ± 0.32 (n=4) 61.54 + 6.8 (n=4)

[0124] Table 2: Slope values (mean ± SD) of linear regressions for these sensors with liquid electrolyte, AGAR (1% by mass) and AGAR (1% by mass) rehydrated after 24 hours of evaporation in ambient air (Sensitivity [Log(%CO2) / mV]) Bibliographic reference

[0125] [1] Severinghaus, John W.; Bradley, A. Freeman (1958). "Electrodes for Blood pO2 and pCO2 Détermination". Journal of Applied Physiology. American Physiological Society, vol. 13, pages 515-520.

Claims

Claims

1. A method for preparing a carbon dioxide sensor comprising the following steps: a) providing a support (1) made of an insulating material having a first face (11) and a second face (12) opposite the first face, the first face having at least one area comprising a pH-sensitive electrode (2) and a reference electrode (3); b) providing a double-sided adhesive (4) having a first acrylic-based adhesive layer (41), a second silicone-based adhesive layer (42) opposite the first adhesive layer and, in its thickness, a through-recess (5) having a first open end (51) and a second open end (52) opposite the first open end;c) applying the first adhesive layer (41) of the double-sided adhesive (4) to the first face (11) of the support (1) so that the first open end (51) of the recess (5) of the double-sided adhesive (4) comes opposite the area of the first face (11) of the support (1); d) applying, on the second adhesive layer (42) of the double-sided adhesive (4) and on the second open end (52) of the recess (5) of the double-sided adhesive (4), a carbon dioxide-permeable membrane (6) based on silicone whereby a measuring chamber (7) is obtained; then e) filling the measuring chamber (7) thus obtained with an electrolyte (9) comprising bicarbonate ions (HCO3) whereby the pH-sensitive electrode (2) and the reference electrode (3) are in contact with the electrolyte.;

2. Preparation process according to claim 1, characterized in that the pH-sensitive elements of said pH-sensitive electrode are made of a material among the transition metal oxides and, advantageously, are made of iridium oxide (IrO2).

3. Preparation process according to claim 1 or 2, characterized in that said reference electrode presented by the support used in the context of the process according to the invention is a silver chloride pseudoreference electrode (AgCl / Ag).

4. Preparation process according to any one of claims 1 to 3, characterized in that said double-sided adhesive has a thickness of between 50 μm and 585 μm and in particular a thickness of the order of 85 pm (ie 85 pm ± 10 pm).

5. Preparation process according to any one of claims 1 to 4, characterized in that said CO2-permeable membrane has a thickness of between 20 pm and 400 pm and in particular a thickness of the order of 50 pm (i.e. 50 pm ± 10 pm).

6. Preparation process according to any one of claims 1 to 5, characterized in that said measuring chamber has a volume of between 1 cm3 and 12 cm3, in particular between 1.1 cm3 and 9 cm3 and, in particular, of the order of 1.275 cm3 (i.e. 1.275 cm3 ± 0.05 cm3).

7. Preparation method according to any one of claims 1 to 6, characterized in that the filling during said step e) is carried out by means of at least two fluid channels (81, 82) opening into said measuring chamber and adapted to bring, into said measuring chamber, an electrolyte or a precursor of the latter and to extract the air contained in said measuring chamber, said fluid channels being pierced at the level of the CO2-permeable membrane and / or machined at the level of the double-sided adhesive.

8. Preparation process according to any one of claims 1 to 7, characterized in that said electrolyte is a liquid electrolyte.

9. Preparation process according to any one of claims 1 to 7, characterized in that said electrolyte is an electrolyte in the form of a hydrogel and, advantageously, an agar hydrogel.

10. Preparation process according to any one of claims 1 to 9, characterized in that said bicarbonate ions are present, in the liquid electrolyte or in the solution comprising the precursor compounds of the hydrogel forming the electrolyte, in an amount of between 1 mM and 20 mM, in particular between 3 mM and 10 mM and, in particular, of the order of 5 mM (i.e. 5 mM ± 1 mM).

11. CO2 sensor obtained by the preparation method according to any one of claims 1 to 10, comprising - a support 1 made of an insulating material having a first face 11 and a second face 12 opposite the first face, the first face having at least one area comprising a pH-sensitive electrode 2 and a reference electrode 3; - a double-sided adhesive 4 having a first acrylic-based adhesive layer 41, a second silicone-based adhesive layer 42 opposite the first adhesive layer and, in its thickness, a through-cavity 5 having a first open end 51 and a second open end 52 opposite the first open end; the first adhesive layer 41 of the double-sided adhesive 4 being in contact with the first face 11 of the support 1 so that the first open end 51 of the recess 5 of the double-sided adhesive 4 is opposite the area of the first face 11 of the support 1; - a silicone-based carbon dioxide permeable membrane 6 in contact with the second adhesive layer 42 of the double-sided adhesive 4 and with the second open end 52 of the recess 5 of the double-sided adhesive 4; - a measuring chamber 7 whose side walls correspond to the side edges of the recess 5 of the double-sided adhesive 4, the lower part of which corresponds to the support 1 made of an insulating material at the level of the zone as previously defined and the upper part of which corresponds to the CO2-permeable membrane 6 covering the second open end 52 of the recess 5 of the double-sided adhesive 4; said measuring chamber being filled with an electrolyte 9 comprising bicarbonate ions (HCO3).

12. Multi-sensor device comprising at least one CO2 sensor according to claim 11.

13. Use of a CO2 sensor according to claim 11 or a multi-sensor device according to claim 12 for measuring (i) dissolved CO2 in a liquid medium in contact with said sensor or said multi-sensor device or (ii) gaseous CO2 present in a gaseous fluid in contact with said sensor or said multi-sensor device.

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