Sensor for detecting an analyte concentration

Abstract

Sensor (1) for detecting an analyte concentration with: - a housing (3); - a sensor element (2) arranged within the housing (3), which has a front surface (6) pressed against a housing wall (8) of the housing (3), wherein the housing wall (8) has an opening (9) leaving an analyte-sensitive region of the front surface (6) free, and wherein the sensor element (2) has at least one contact element for electrical contacting; - an internal electrolyte (15) accommodated within the housing (3), which is in contact with a medium surrounding the housing (3) via an electrochemical transfer (16) arranged in the housing wall; and - a reference electrode (14) immersed in the internal electrolyte (15), characterized in that a flexible printed circuit board (12) is arranged within the housing (3), which comprises at least one first conductor track which is connected to the at least one contact element of the sensor element (2), and at least one second conductor track which serves as a potential derivation of the reference electrode (14).

Description

[0001] The invention relates to a sensor for detecting an analyte concentration in a measuring medium.

[0002] Determining the concentration of an analyte in a measuring medium plays an important role in many industrial applications, for example, in chemical or pharmaceutical engineering, food technology, biotechnology, but also in non-industrial analytical applications, such as environmental measurement technology. To determine ion concentrations, sensors are often used in laboratories as well as in industrial process plants. Sensors containing a sensor element with an analyte-sensitive component are used. An analyte-sensitive membrane, for example, can be used as an analyte-sensitive component. For example, the glass membrane of the well-known pH glass electrode is sensitive to the concentration of H + or H 3 O + -ions in a measuring medium.

[0003] Alternatively, a semiconductor element can serve as the analyte-sensitive component, for example a component comprising an EIS structure, such as an ion-sensitive field-effect transistor (ISFET) or a capacitor with an EIS structure, whose capacitance depends on the concentration of the substance to be determined. The acronym "EIS" stands for "electrolyte insulator-semiconductor," i.e., the sensor comprises a layer structure with at least one insulator coating applied to a semiconductor layer or a semiconductor substrate, which, during measuring operation of the sensor, is in contact with an electrolyte, namely the measuring medium. A voltage drop occurs at the interface between the insulator layer and the measuring medium.By appropriately selecting the insulator coating, particularly by providing an analyte-sensitive coating on or as part of the insulator coating, the sensitivity of the sensor can be adjusted such that the voltage drop can serve as a measure of the analyte concentration. For example, the voltage drop at the interface between a tantalum(V) oxide (Ta) layer and a crystalline silicon layer depends on the crystalline silicon layer. 2 O 5 ) layer and an aqueous measurement solution depends essentially on the pH of the measurement solution. By using other layer structures, EIS sensor elements can be formed that are sensitive to other ions in a corresponding manner. By immobilizing suitable detector structures, which may include enzymes, for example, on the EIS structure, it is also possible to measure concentrations of non-ionic substances, such as glucose or penicillin, using such a sensor.

[0004] The previously mentioned ion-sensitive field-effect transistor also includes such an EIS structure. The transistor gate of a pH-sensitive ISFET is protected, for example, by a pH-sensitive insulator coating, the Ta 2 O 5 The charge carrier density in the semiconductor channel between the source and drain of the ISFET then depends accordingly on the pH value of the medium in contact with the gate. For example, DE 198 57 953 A1 describes a sensor for measuring ion concentrations or the pH value of a liquid using an ISFET.

[0005] Such sensor elements with analyte-sensitive components, particularly semiconductor-based ones, are often designed in the form of a plate or disk, for example as a chip or chip array, with front or rear contact elements for electrically contacting the analyte-sensitive component. Sensors with such sensor elements are often designed as rod-shaped measuring probes comprising a housing immersible in a medium, in which the sensor element is arranged such that its analyte-sensitive component comes into contact with the measuring medium. The contact elements for electrically contacting the analyte-sensitive component, via which the sensor element is connected to sensor electronics, and the sensor electronics themselves are arranged in a protected manner within the housing. The measuring probe can be connected to a higher-level unit, for example a measuring transducer or a bus coupler, via a cable or wirelessly.The higher-level unit can supply the measuring probe with energy, receive and further process measurement signals output by the sensor electronics, or output signals to the sensor electronics. Examples of such sensors can be found in US 6 117 292 A, US 6 153 070 A, or EP 1 396 718 A1.

[0006] EP 1 396 718 A1 describes a sensor with an ISFET as the sensor element. By means of a pressure part, the ISFET is pressed against the end face of a sensor housing with a rear surface facing away from its ion-sensitive surface area. The pressure part has a central opening that leaves the ion-sensitive surface area of ​​the ISFET free, whereas the source terminal and the drain terminal of the ISFET are arranged in an inner region of the sensor that is protected against the ingress of the measuring medium. The pressure part is connected to the sensor housing by a medium-tight circumferential ultrasonic weld. The interior of the sensor housing is divided into an inner sensor chamber and a sensor intermediate chamber by an inner tube running within the housing. The intermediate sensor chamber serves as the reference electrode chamber, i.e. it contains a reference electrolyte into which a reference electrode is immersed.Lead wires connected to the front or rear contact elements of the ISFET are routed through the sensor interior to contact the source and drain of the ISFET. The lead wires connect the ISFET to a display device on the sensor. The sensor interior is backfilled from the inner tube to the rear surface of the ISFET with a potting compound, for example, based on an epoxy adhesive, which provides mechanical support for the ISFET.

[0007] EP 1 396 718 A1 exemplifies the principle consistently followed in the prior art of separating a sensor interior, which serves to contact and guide the sensor element's lead wires, from an electrolyte-filled intermediate space in which the reference electrode, usually designed as a second-type reference electrode, is arranged. The production of the sensors known from the prior art and designed according to this principle requires a large number of individual work steps and is therefore very complex and costly.

[0008] In addition, the sensor housing of such sensors often contains a sensor circuit, often electronic, connected to the lead wires, also referred to as sensor electronics, which performs initial processing, particularly amplification and, if necessary, digitization of the measurement signal. The ingress of moisture into the sensor electronics can lead to sensor failure. Therefore, great efforts must be made to ensure the tightness of the electrolyte-filled reference electrode chamber with respect to the sensor electronics, which are usually also housed in the sensor housing.

[0009] It is therefore the object of the present invention to provide an electrochemical sensor of the type mentioned above that overcomes the aforementioned disadvantages. In particular, the sensor should be manufactured with less effort and ensure improved sealing of the region of the sensor housing containing the sensor circuit with respect to the internal electrolyte contained within the housing.

[0010] This object is achieved according to the invention by a sensor according to claim 1.

[0011] The sensor according to the invention for detecting an analyte concentration comprises: - a housing; - a sensor element arranged within the housing, which has a front surface pressed against a housing wall of the housing, wherein the housing wall has an opening leaving an analyte-sensitive area of ​​the front surface free, and wherein the sensor element has at least one contact element for electrical contacting; - an internal electrolyte accommodated within the housing, which is in contact with a medium surrounding the housing via an electrochemical transfer arranged in a housing wall; and - a reference electrode immersed in the internal electrolyte, wherein a flexible printed circuit board is arranged within the housing, which comprises at least one first conductor track which is connected to the at least one contact element of the sensor element, and at least one second conductor track which serves as a potential derivation of the reference electrode.

[0012] The accommodation of both the lead of at least one contact element of the sensor element and the potential lead of the reference electrode in a flexible printed circuit board makes it possible to dispense with the previously generally existing dichotomy between an interior sensor space, through which connecting wires of the sensor element are routed, and a sensor intermediate space containing an internal electrolyte, for example a reference electrode space containing the reference electrode immersed in a reference electrolyte. The flexible printed circuit board can be located directly in the conductive reference electrolyte; a medium-tight separation of a space in which the connecting wires are routed from a space containing the internal electrolyte is therefore no longer necessary. This simplifies the production of the sensor, as the complex inner tube assembly for forming an interior space that is medium-tightly separated from the internal electrolyte is no longer necessary.In addition, eliminating the inner tube creates more space inside the housing. This makes it possible to accommodate a flexible printed circuit board within the housing, even if its length significantly exceeds the axial length of the housing. This simplifies the production of various sensor types with different housing lengths, as different printed circuit board variants of different lengths no longer need to be kept in stock to connect the contact elements of the sensor element to the sensor circuit.

[0013] If several contact elements of the sensor element are present, all required connecting lines can be implemented as conductor tracks of the flexible printed circuit board.

[0014] A plastic film, such as a polyimide film, can be used as the base material for the flexible circuit board. The conductor tracks serving as connection lines for the sensor element and / or the conductor track serving as a potential derivation for the reference electrode can run within the electrically insulating base material or be covered by an additional plastic coating so that they are not in contact with the internal electrolyte.

[0015] The internal electrolyte can be contained in a first chamber formed within the housing, wherein the housing has a second chamber which is sealed off from the first chamber and in which a sensor circuit, in particular an electronic one, is accommodated, and wherein the first chamber is sealed off from the second chamber, in particular by means of at least one seal through which the flexible printed circuit board is passed. The seal can in particular be formed from an elastic material. The first chamber can be formed in a front-side, i.e. sensor element-side, region of the housing. The second chamber can be formed in a rear-side, i.e. connection-side, region of the housing.

[0016] The seal can, for example, comprise two or more sealing elements that are clamped against each other by being supported on the inner wall of the sensor housing. The flexible circuit board can be guided between the sealing elements from the first chamber into the second chamber.

[0017] The first and second conductor tracks can be electrically connected to the sensor circuit, wherein the sensor circuit is designed to detect one or more electrical measured variables via the conductor tracks and to process them into one or more, possibly digital, measurement signals and to output them to a higher-level unit connected to the sensor.

[0018] The sensor circuit can be arranged on a circuit board arranged within the second chamber, which is coupled to the flexible circuit board in such a way that the first and second conductor tracks are electrically connected to the sensor circuit. The circuit board comprising the sensor circuit can be designed, for example, as a rigid circuit board. The connection patterns of the flexible circuit board and the circuit board carrying the sensor circuit can be adapted to one another, thus further reducing manufacturing costs.

[0019] Common circuit board materials, e.g. based on phenol or epoxy resin, such as FR2, FR3, FR4, FR5, or polyimide, can be used as material for the rigid circuit board.

[0020] In another advantageous embodiment, the sensor circuit is arranged on a section of the flexible printed circuit board arranged within the second chamber.

[0021] In one embodiment, the housing may comprise a sensor body and an electronics housing part fixedly connected to the sensor body, wherein the sensor body has a sensor shaft which is at least partially tubular and a front section adjoining the sensor shaft at the front, which front section comprises the housing wall against which the front surface of the sensor element is pressed, and The rear end of the sensor shaft is closed by the electronics housing part.

[0022] The electronics housing part can be attached to the sensor shaft, particularly in the manner of a cap, and firmly connected to it, for example, by adhesive bonding. The sensor body can be made of glass or a plastic, such as PEEK, that is suitable for immersion in the measuring medium. The electronics housing part is not intended for immersion in the measuring medium and can therefore be made of any plastic.

[0023] The aforementioned seal, which seals the first electrolyte-filled chamber of the housing from the second chamber containing the sensor circuit, can be arranged in the rear region of the sensor shaft. The seal can comprise at least two, particularly elastic, plastic sealing elements that rest against the inner wall of the tubular sensor shaft and have two sealing surfaces that rest against each other with tension, between which the flexible circuit board is clamped, so that the circuit board is guided from the first chamber into the second chamber in a fluid-tight manner.The clamp seal formed in this way is particularly easy to handle during the manufacture of the sensor and allows a much more reliable sealing of the front, electrolyte-filled first chamber against the rear second chamber in which the sensor circuit is housed, compared to conventional sensors in which the inner tube described above and the potential derivation of the reference electrode must be guided through a sealing plug arranged in the sensor shaft.

[0024] The electronics housing part can comprise an interface for connection to a higher-level unit. In addition to a mechanical interface, e.g. a plug head that can be detachably connected to a complementary socket of a cable connected to the higher-level unit, the interface can comprise an electrical or electronic interface via which the sensor circuit is connected to the higher-level unit for the exchange of energy and data when the mechanical interface is connected to the complementary interface of the higher-level unit or the cable connected to the higher-level unit. For this purpose, the sensor circuit has circuit components that serve to provide signals at the interface. The interface can, for example, have galvanic contacts or be designed as a contactless capacitively or inductively coupling interface.

[0025] The reference electrode can comprise a silver wire that is at least partially chlorided and electrically connected to the second conductor track, for example, by a soldered connection. This silver wire is arranged within the first chamber of the housing and immersed in the internal electrolyte serving as the reference electrolyte. The internal electrolyte can be a 3 molar KCl solution. This solution is in electrolytic contact with the medium surrounding the housing via the electrochemical transfer arranged in the housing wall, which can be designed, for example, as a simple bore or as a porous plastic or ceramic diaphragm.

[0026] The electrically conductive connection between the reference electrode and the flexible printed circuit board can be arranged in a front-side, i.e. sensor element-side, area of ​​the flexible printed circuit board, preferably in the vicinity of the sensor element. By contacting the reference electrode with the flexible printed circuit board in the front end area of ​​the sensor, the requirement, as in sensors known from the prior art, for an additional potential derivation from the reference electrode to the sensor circuit arranged in the rear electronics housing part, which is guided over almost the entire length of the sensor, is eliminated. The front end area of ​​the sensor housing, in which the sensor element and the connection point between the reference electrode and the printed circuit board are arranged, can be filled with a potting compound, for example based on epoxy. This potting compound serves to improve the insulation of the connections, in particular soldered connections, between the contact elements orthe reference electrode and the first conductor tracks or the second conductor track of the flexible circuit board.

[0027] Instead of a wire soldered to the flexible circuit board, a layer system arranged on the circuit board can also serve as the reference electrode. In this embodiment, the reference electrode can comprise a silver layer arranged on the flexible circuit board, in electrical contact with the second conductor track, and at least partially covered by an AgCl layer.

[0028] To improve signal quality, the flexible circuit board can include an additional shielding layer.

[0029] The flexible printed circuit board can be equipped with a temperature sensor in the region of the sensor element, which is connected to a third conductor track of the flexible printed circuit board. The third conductor track can be connected to the sensor circuit, which in this embodiment is also designed to process signals provided by the temperature sensor. Like the potential derivation of the reference electrode and the connecting leads of the sensor element, the third conductor track can also be embedded in the electrically insulating base material of the printed circuit board or covered by an additional plastic coating so that it is not in electrical contact with the internal electrolyte.The temperature sensor and any other SMD components present on the flexible circuit board are preferably arranged, like the connection point of the reference electrode to the circuit board, in a front end area of ​​the housing near the sensor element, which may additionally contain a potting compound.

[0030] The sensor element can comprise an EIS structure. In particular, the sensor element can comprise an ion-sensitive field-effect transistor, which, in addition to the analyte-sensitive region arranged on the front surface serving as a gate, has a source terminal arranged on the front surface or on a rear surface of the sensor element facing away from the front surface and serving as a contact element, and a drain terminal arranged on the front surface or on the rear surface of the sensor element and serving as a further contact element. In this case, the flexible printed circuit board has two first conductor tracks, one of which is electrically connected to the source terminal and the other to the drain terminal.The flexible printed circuit board can be equipped with a temperature sensor arranged near the sensor element, in particular configured as an SMD component, which, as already mentioned, is conductively connected to a third conductor track of the flexible printed circuit board. The first two and the third conductor track are, as already described above, connected to a sensor circuit arranged in a second chamber that is fluid-tightly separated from the first chamber of the sensor housing, which contains the internal electrolyte. The sensor circuit serves to detect and further process signals provided via the contact elements of the sensor element and / or by the temperature sensor.

[0031] The invention is explained in more detail below with reference to the exemplary embodiment illustrated in the figures. Fig. 1 an overall view of an electrochemical sensor, partly in longitudinal section; Fig. 2 an enlarged longitudinal section of the front end section of the Fig. 1 shown sensor; Fig. 3 a detailed view of a Fig. 1 shown sensor containing a flexible circuit board with a sealing arrangement in a first position; Fig. 4 a detailed representation of the Fig. 3 shown circuit board with the sealing arrangement in a second position.

[0032] Fig. Figure 1 shows a schematic longitudinal section of an electrochemical sensor 1 for measuring the pH value of a measuring medium. The sensor shown here comprises an ion-sensitive field-effect transistor as the analyte-sensitive sensor element 2. Although the invention is described below using a pH sensor with a pH-sensitive ISFET as the sensor element, the invention is of course transferable to sensors with other sensor elements, in particular for ISFETs or ChemFETs or other sensor elements comprising EIS structures, as well as with ion- or analyte-sensitive membranes that provide a signal dependent on the concentration of an analyte in a measuring medium.

[0033] The sensor 1 comprises a housing 3, which is essentially formed from a sensor body 4 and an electronics housing part 5 firmly connected to the sensor body 4. The sensor body 4 can be made of a non-electrically conductive material, for example, glass or plastic. In the present example, the sensor body 4 is made of polyetheretherketone (PEEK). The sensor body 4 comprises a tubular sensor shaft, the front end section ( Fig. 2) is designed to be immersed in a measuring medium for measuring the pH value. The sensor shaft of the sensor body 4 is firmly connected to the back of the electronics housing part 5, which is placed on the back of the sensor shaft 4 like a cap and closes it. The electronics housing part 5 is made of a non-electrically conductive plastic.

[0034] As in Fig. 2, the sensor element 2 is pressed with its front surface 6 against a front wall 8 of the sensor body 4 via an elastic sealing element 7. The wall 8 has an opening 9 which leaves a pH-sensitive surface area of ​​the sensor element 2 free, so that this area comes into contact with the measuring medium when the sensor 1 is immersed in the latter.

[0035] The sensor element 2 is pressed against the elastic sealing element 7 by means of a pressure part 10 acting on its rear side facing away from the pH-sensitive surface area. The pressure part 10 can be made of an elastomer, for example, so that the sensor element is sandwiched between the elastic sealing element 7 and the pressure part 10. The pressure part 10 is supported against a counterpart 11 connected to the housing wall of the housing 3.

[0036] While the transistor gate forming the pH-sensitive surface area of ​​the sensor element 2 configured as an ISFET is in contact with the medium surrounding the housing 3, the source and drain of the ISFET are arranged in a region of the front surface 6 of the sensor element 2 that is sealed off from the surroundings of the housing 3 by means of the sealing element 7. Contact elements serving as source and drain connections can be arranged either, as in the example shown here, on the front surface 6 of the sensor element 2 or on a rear surface facing away from the front surface 6.

[0037] The contact elements are electrically connected by means of a solder connection to conductor tracks of a flexible printed circuit board 12, which runs axially through the sensor body 4. In the example shown here, the conductor tracks run within the flexible printed circuit board 12. They run over the entire length of the flexible printed circuit board 12 and are connected to a sensor circuit 13 arranged in the connection housing part 5. In the front end region of the sensor body 4, a reference electrode 14 is arranged on the flexible printed circuit board 12. This reference electrode is formed from a silver wire partially sheathed with PEEK. At one end, advantageously its sensor element end, the silver wire is conductively connected, for example by means of a solder connection, to another conductor track of the flexible printed circuit board, which is also connected to the sensor circuit 13.At its other end, the silver wire has an AgCl coating, which is immersed in an internal electrolyte 15 contained in the sensor body 4. In the present example, the internal electrolyte 15 comprises an aqueous 3 molar KCl solution. The internal electrolyte can also be a gel electrolyte, which can consist of a polymer with an embedded KCl solution. Via a microporous diaphragm 16, for example made of ceramic, arranged in the housing wall of the sensor body 4, the internal electrolyte 15 is in electrolytic contact with a medium surrounding the front end region of the sensor 1, so that charge transport can occur between the internal electrolyte 15 and the surrounding medium.

[0038] The housing 3 is cast in its front end region approximately up to the height of the counterpart 11 with a casting material, for example an epoxy resin, so that in particular the sensor element-side end of the reference electrode 14 connected to the flexible printed circuit board and its connection point with the conductor track serving as a potential discharge are insulated from the internal electrolyte 15.

[0039] The printed circuit board 12 thus serves to connect both the sensor element 2 and the reference electrode 14 to the sensor circuit. As stated above, the conventional division of the housing into an interior space containing the sensor element's connecting wires and a further space containing the internal electrolyte is omitted. This simplifies the production of the sensor 1 and also results in more space being available inside the sensor.

[0040] In the front-side encapsulated area of ​​the housing 3, preferably as close as possible to the sensor element 2, a temperature sensor 22 is additionally arranged on the flexible circuit board 12, which is connected to another conductor track of the flexible circuit board 12. The other conductor track, like the potential-dissipating conductor track of the reference electrode 14 and the conductor tracks connected to the sensor element 2, is connected to the sensor circuit 13, which detects and processes the signals provided by the temperature sensor 22.

[0041] A seal 17 is arranged in the tubular section of the sensor body 4 forming the sensor shaft, dividing the sensor housing into a first chamber 18 and a second chamber 19. The first chamber 18 is located in the sensor body 4 and contains the reference electrolyte 15. The second chamber 19 is formed by the rear end section of the sensor body 4 and the electronics housing part 5 plugged onto it. The sensor circuit 13 is arranged in this second chamber 19. In the example shown here, the seal 17 is formed by two elastic, abutting sealing elements 23, 24 that fill the cross-section of the sensor shaft and are supported against its inner wall, so that the printed circuit board 12, which is guided between the sealing elements 23, 24 from the first into the second chamber, is clamped between two abutting sealing surfaces of the sealing elements 23, 24.In this way, the second chamber 19 containing the sensor circuit 13 is sealed liquid-tight from the first electrolyte-filled chamber 18, so that no internal electrolyte 15 can penetrate into the second chamber 19.

[0042] The seal 17 is in the Fig. 3 and Fig. 4 in detail. In the example shown here, the sealing elements 23, 24 can be joined together to form a rotationally symmetrical, plug-like seal 17. A first sealing element 23 has a notch 25 extending axially with respect to the rotational symmetry axis of the seal 17, into which the second sealing element 24, as in Fig.3. The flexible printed circuit board 12 can be placed against a surface of the first sealing element 23 arranged within the notch 25 and, when the seal 17 is assembled, is pressed between this surface and an adjacent surface of the second sealing element 24. With their outer surfaces 26, 27, the sealing elements 23, 24 are supported against the inner wall of the tubular sensor shaft, thus sealing the first chamber 18 in a liquid-tight manner from the second chamber 19.

[0043] In addition, the area between the seal 17 and the end of the sensor body 3 open towards the rear of the sensor can be filled with a potting compound.

[0044] The flexible printed circuit board has a curved profile within the first chamber 18, so that the length of the flexible printed circuit board between the sensor element 2 and the seal 17 is greater than the distance between the seal 17 and the sensor element 2. This serves, on the one hand, to relieve the strain on the flexible printed circuit board 12. On the other hand, due to the space saved within the sensor body 4 as a result of omitting an additional inner tube, it is possible to manufacture various sensor types of different lengths using flexible printed circuit boards of one and the same length, wherein the printed circuit board 12 can be folded once or multiple times if the sensor body 4 is significantly shorter than the printed circuit board 12.

[0045] In the example shown here, the sensor circuit 13 is arranged on a rigid printed circuit board 20. The conductor tracks of the flexible printed circuit board 12 connected to the contact elements of the sensor element 2 and the reference electrode 14 are connected to corresponding terminals of the sensor circuit 13. To further simplify the production of the sensor 1, the connection patterns of the flexible printed circuit board 12 and the rigid printed circuit board 20 are coordinated.

[0046] The second chamber 19 can be filled with a potting compound, e.g., an epoxy resin. In particular, the sensor circuit 13 can be potted.

[0047] In addition to means for further processing the measurement signals, in particular for amplifying and digitizing them, the sensor circuit 13 includes a memory for storing sensor data and / or measured values. In the example shown here, the circuit also includes an inductive interface 21 integrated into a mechanical sensor plug head. This interface 21 can be connected to a complementary socket (not shown) to connect the sensor 1 to a higher-level unit, for example, a measuring transducer, a conventional computer, or a bus coupler. Data, in particular measured values, and energy can be transmitted via the inductive interface.

Claims

[1] Sensor (1) for detecting an analyte concentration with: - a housing (3); - a sensor element (2) arranged within the housing (3), which has a front surface (6) pressed against a housing wall (8) of the housing (3), wherein the housing wall (8) has an opening (9) leaving an analyte-sensitive region of the front surface (6) free, and wherein the sensor element (2) has at least one contact element for electrical contacting; - an internal electrolyte (15) accommodated within the housing (3), which is in contact with a medium surrounding the housing (3) via an electrochemical transfer (16) arranged in the housing wall; and - a reference electrode (14) immersed in the internal electrolyte (15), characterized bythat a flexible printed circuit board (12) is arranged within the housing (3), which comprises at least one first conductor track which is connected to the at least one contact element of the sensor element (2), and at least one second conductor track which serves as a potential derivation of the reference electrode (14). [2] Sensor (1) according to claim 1, wherein the internal electrolyte (15) is contained in a first chamber (18) formed within the housing (3), and wherein the housing (3) has a second chamber (19) which is sealed off from the first chamber (18) and in which a sensor circuit (13), in particular an electronic one, is accommodated, and wherein the first chamber (18) is sealed off from the second chamber (19), in particular by means of at least one seal (17) through which the flexible printed circuit board (12) is passed. [3] Sensor (1) according to claim 2, wherein the first and second conductor tracks are electrically conductively connected to the sensor circuit (13). [4] Sensor (1) according to claim 2 or 3, wherein the sensor circuit (13) is arranged on a printed circuit board (20) arranged within the second chamber (19), which is coupled to the flexible printed circuit board (12) in such a way that the first and the second conductor track are electrically conductively connected to the sensor circuit (13). [5] Sensor (1) according to one of claims 2 to 4, wherein the housing (3) comprises a sensor body (4) and an electronics housing part (5) fixedly connected to the sensor body (4), wherein the sensor body (4) has a sensor shaft which is at least partially tubular and an end section which adjoins the sensor shaft at the front and which comprises the housing wall (8) against which the front surface (6) of the sensor element (2) is pressed, and wherein the rear end of the sensor shaft is closed by the electronics housing part (5). [6] Sensor (1) according to claim 5, wherein a seal (17) sealing the first chamber (18) with respect to the second chamber (19) is arranged in the rear region of the sensor shaft, said seal comprising at least two plastic sealing elements (23, 24) tensioned against one another, between which the flexible printed circuit board (12) is clamped. [7] Sensor (1) according to claim 5 or 6, wherein the electronics housing part (5) comprises an interface (21) for connection to a higher-level unit. [8] Sensor (1) according to one of claims 1 to 7, wherein the reference electrode (14) comprises a silver wire connected to the second conductor track and at least partially chlorided. [9] Sensor (1) according to one of claims 1 to 8, wherein the reference electrode comprises a silver layer arranged on the flexible printed circuit board and in electrical contact with the second conductor track. [10] Sensor (1) according to one of claims 1 to 9, wherein the electrically conductive connection between the reference electrode (14) and the flexible printed circuit board (12) is arranged in a front region of the flexible printed circuit board (12), preferably in the vicinity of the sensor element (2). [11] Sensor (1) according to one of claims 1 to 10, wherein the flexible printed circuit board (12) comprises an additional shielding layer. [12] Sensor (1) according to one of claims 1 to 11, wherein the flexible printed circuit board (12) is equipped in the region of the sensor element (2) with a temperature sensor (22) which is connected to a third conductor track of the flexible printed circuit board (12). [13] Sensor (1) according to claim 12, wherein the third conductor track is connected to the sensor circuit (13). [14] Sensor (1) according to one of claims 1 to 13, wherein the sensor element (2) comprises an EIS structure. [15] Sensor (1) according to one of claims 1 to 14, wherein the sensor element (2) comprises an ion-sensitive field-effect transistor (ISFET) which, in addition to the analyte-sensitive region arranged on the front surface (6) serving as a gate, has a source terminal arranged on the front surface (6) or on a rear surface of the sensor element opposite the front surface (6) and serving as a contact element, and a drain terminal arranged on the front surface (6) or on the rear surface of the sensor element (2) and serving as a further contact element.

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