Sensor for determining a measured value dependent on a concentration of reactive oxygen species

Abstract

Sensor for determining a measured variable correlated with a concentration of at least one analyte belonging to the class of reactive oxygen species in a measuring fluid, in particular a measuring liquid, comprising: - a sensor element intended for contact with the measuring fluid, which comprises an indicator substance, wherein the indicator substance is intended to be oxidized by the at least one analyte to an oxidized form of the indicator substance; - means for generating a current flow in the sensor element which causes a reduction of the oxidized form of the indicator substance and thereby a regeneration of the indicator substance; - an optical measuring sensor which is designed to detect a measuring radiation influenced by the oxidized form of the indicator substance and to generate a measuring signal, in particular an electrical one, based on the influenced measuring radiation; and - a sensor circuit which is connected to the optical measuring sensor for receiving the measuring signal and which is designed to determine a measured value of the measured quantity on the basis of the measuring signal.

Description

The invention relates to a sensor for determining a measurement variable correlated with a concentration of at least one reactive oxygen species in a measurement fluid.Reactive oxygen species (or abbreviated ROS), which are also referred to as "oxygen radicals", include, on the one hand, radicals such as the hyperoxide anion O 2-, the hydroxyl radical OH*, the peroxyl radical ROO* and the alkoxyl radical RO* of lipids, and also stable molecular oxidants such as hydrogen peroxide H 2 O 2, hydroperoxide ROOH, ozone O 3, the hypochlorite anion OCI - and also oxygen molecules in an excited state, such as singlet oxygen 1 O 2. The ROSs thus include radicals or unstable compounds which can form radicals in their environment.Both amperometric and optical sensors are known for determining the concentration of one or more ROS as analyte or analytes.However, the amperometric sensors known in the prior art on the one hand have a flow dependence and on the other hand do not have a high selectivity, i.e. frequently cross sensitivities to other analytes which are not negligible correspondingly occur. Accurate measurements of the concentration of ROS or measurement variables dependent thereon are therefore often only possible in disinfectants whose basic compositions, i.e. their ingredients, are known, so that cross sensitivities can be excluded or compensated by calculation.Optical sensors based on fluorescence quenching for detecting hydrogen peroxide have already been known for decades. One of the first sensors is known from Hermann E. Posch, Otto S. Wolfbins, Optical sensor for hydrogen peroxide, Microchimica Acta, Vol. 97, Issue 1, 41-50, 1989 and is based on a silicone spot which comprises a permeation-selective membrane, a catalyst for converting peroxides into oxygen and an oxygen-sensitive membrane. The ambient oxygen concentration must be taken into account arithmetically in the calculation of the peroxide concentration. A disadvantage of these sensors is the slow response times in the minute range and the only relatively small available measurement range of 0.1-10 mmol / l.Enzyme-based sensors with higher selectivity for peroxides are also known. However, these are not very temperature stable.A further development of optical sensors for determining concentrations of ROS aims at improving selectivity, for example by using dyes with selective receptor sites. Many of these optical ROS sensors have the disadvantage, on the other hand, that the optical change, on the basis of which the measured variable is determined, is either irreversible or only a few times reversible, since a chemical change of the indicator molecule takes place. For example, Christoph Staudinger, Sergy Borisov, Long-wavelength analyte-sensitive luminescent probes and optical (bio)sensors, Methods Appl. Fluorescence. 3 (2015) 1-73 discloses phosphorescent organic complexes which, when oxidized with a hypohalite with dye bonding, leads to an increase in phosphorescence in the near infrared range, but the initial state is only to be restored by a chemical reaction. This has the disadvantage that a sensor can only display a decreasing concentration of the analyte again when it is previously immersed in a regeneration solution which restores the initial state of the sensor by a chemical regeneration. If the sensor is therefore also intended to measure fluctuating concentration profiles, it must regularly be regenerated in this way. This method is time-consuming and energy-intensive and disadvantageous from a measurement standpoint, since no uninterrupted measurement is possible over longer periods of time. If such a sensor is to be used for the automated monitoring or control of a process, a replacement fitting is required which enables regular, automated removal of the sensor from the process and automated application of a regeneration solution to the sensor in order to restore the initial state. However, this is complicated and leads to regular interruptions.Color patches for detecting ROS in dosimeters or in food packaging are also known. The reactions underlying these color patches are also irreversible and therefore not suitable for a sensor which is intended to be used over a relatively long period of time in the process analysis for monitoring and / or controlling or regulating an industrial process.In H. Akbari Khorami, P. Wild, N. Djilali, Fiber optical sensors for hydrogen peroxide vapor, International Scholarly and Scientific Research & Innovation 9(10), 1145, 2015, the reduced form of Prussian Blue (other name: Berliner Blue or Turnbulls Reagent), also referred to as Prussian White (other name: Berliner White or Everitts salt), is described as an inorganic, chemically stable indicator substance for determining a concentration of hydrogen peroxide by means of reflection measurements. In this connection, attempts have been made to restore the original state by immersion in a regenerating solution containing a reducing agent. This is possible, but requires a mechanical dipping of the sensor into the regenerating solution and is associated with maintenance work. In addition, the reducing effect of the regeneration solution decreases with the lapse of time by consumption of the reducing agent, so that periodic replacement of the regeneration solution is required. A possible entrainment of measuring solution into the regenerating solution can cause incorrect measured values. A carry-over of regenerating solution into the measuring solution can also be harmful for the process to be monitored by means of the sensor.DE 10 2010 064 391 A1 and DE 10 2010 064 392 A1 each describe a method for determining an analyte content of a liquid sample by means of a bioanalyzer, wherein the bioanalyzer is designed as a flow measuring cell. In both publications, the sensor, in particular the sensor substrate, is cleaned with the aid of an electrochemical reaction. However, DE 10 2010 064 391 A1 and DE 10 2010 064 392 A1 do not disclose the combination of the features according to the applicable claim 1, in particular not a senor which contains an indicator substance which is designed to cause a color reaction in its oxidized form - as a reaction to the concentration of a reactive oxygen species as analyte - which color reaction can be evaluated as a measurement signal as a function of the analyte concentration. Furthermore, the regeneration of the sensor according to the invention takes place differently from in the publications DE 10 2010 064 391 A1 and DE 10 2010 064 392 A1, an agent located in the sensor according to the invention brings about a reduction and thus a regeneration of the oxidized substance-in the publications DE 10 2010 064 391 A1 and DE 10 2010 064 392 A1, the cleaning takes place with the aid of an oxidizing agent-technical agents for determining the concentration of the oxidizing agent are neither disclosed nor suggested in the aforementioned publications.Choi, S.; Chain, J.: Reusable biosensors via in situ electrochemical surface regeneration in microfluidic applications. In: Biosensors and Bioelectronics, Vol. 25, 2009, pages 527-531 (11) and Choi, S.; Chain, J.: A regenerative biosensing surface in microfluidics using electrochemical desorption of short-chain self-assembled monolayer. In: Microfluidics and Nanofluitics, Vol. 7, 2009, pages 819-827 each disclose a reusable biosensor. In these documents, the sensor is cleaned by means of an electrochemical reaction by applying a voltage to the carrier.It is therefore the object of the present invention to specify a sensor of the generic type for measuring a measured variable correlated with a concentration of reactive oxygen species, which overcomes the disadvantages specified above of the sensors known from the prior art.This object is achieved by a sensor according to claim 1.The sensor according to the invention for determining a measurement variable correlated with a concentration of at least one analyte belonging to the class of the reactive oxygen species in a measurement fluid, in particular a measurement liquid, comprises:a sensor element provided for contact with the measurement fluid, which sensor element comprises an indicator substance, wherein the indicator substance is intended to be oxidized to an oxidized form of the indicator substance by the at least one analyte;means for generating a current flow in the sensor element, which causes a reduction of the oxidized form of the indicator substance and thereby a regeneration of the indicator substance;an optical measuring sensor which is configured to record a measurement radiation influenced by the oxidized form of the indicator substance and to generate an, in particular electrical, measurement signal on the basis of the influenced measurement radiation; anda sensor circuit which is connected to the optical measuring sensor for receiving the measurement signal and which is designed to determine a measurement value of the measurement variable on the basis of the measurement signal.The measurement variable correlated with the concentration of the analyte in the measurement fluid can be a concentration of one or more of the reactive oxygen species mentioned at the beginning or a concentration of a substance in the measurement fluid, which is converted by a chemical reaction to form a reactive oxygen species and optionally further products. For example, this can be glucose, which is converted from glucose oxidase to hydrogen peroxide and gluconolactone, wherein the concentration of glucose in the measurement fluid correlates with the amount of hydrogen peroxide concentration formed on account of the conversion by the glucose oxidase.For example, the influenced measurement radiation detected by the optical measurement pickup can be a measurement radiation transmitted through the sensor element or reflected at or in the sensor element, the intensity of which is influenced, e.g. attenuated, by the oxidized form of the indicator substance due to an absorption. Alternatively, the influenced measurement radiation detected by the measurement pickup can be a luminescence radiation emitted by the oxidized form of the indicator substance or a luminescence radiation of a reference substance contained in the sensor element, which is influenced by the oxidized form of the indicator substance.The indicator substance and the oxidized form of the indicator substance can interact with the measurement radiation in different ways, so that a distinction can be made by means of the optical measurement pickup between a measurement radiation influenced by the indicator substance and a measurement radiation influenced by the oxidized form of the indicator substance, or that the measurement pickup can be designed such that it exclusively detects the measurement radiation influenced by the oxidized form of the indicator substance and converts it into a measurement signal. For example, the intensity of the influenced measurement radiation can be a measure of the amount of the oxidized form of the indicator substance formed in contact with the measurement fluid. This in turn depends on the concentration of the analyte in the measurement fluid, so that the intensity of the influenced measurement radiation is a measure for the concentration of the analyte and thus correlates directly with the measurement variable to be determined.The indicator substance can change its color, for example, by oxidation by the analyte to an oxidized form of the indicator substance, which corresponds to a change in its absorption behavior. The indicator substance can alternatively or additionally change its luminescence behavior, in particular its fluorescence or phosphorescence behavior, by oxidation, e.g. by changing a luminescence decay time, a phase angle or its luminescence spectrum, e.g. the shift of a luminescence wavelength or the change of the intensity of a luminescence radiation.The measuring transducer can be configured to generate the measurement signal in a corresponding manner to record an intensity and / or a phase angle shift and / or a decay time and / or a wavelength shift of the received measurement radiation.The disadvantages of the amperometric sensors known from the prior art, such as the flow dependence or the low selectivity with respect to the analyte, are avoided according to the invention by the use of an optical measuring sensor. On the other hand, the disadvantages of the previously known optical sensors for determining the concentration of ROS, such as the regular regeneration of the sensor by means of a chemical regeneration solution, are also avoided according to the invention in that the regeneration takes place by reducing the oxidized form of the indicator substance by means of a current flow effected in the sensor element. By reducing the oxidized form of the indicator substance by means of a current flow, the sensor can be brought very quickly, without additional chemicals and without longer interruptions, back into a state in which it can measure measured values of the measured variable correlated with a concentration of the at least one reactive oxygen species that is smaller than the previously recorded measured value.The sensor can advantageously be used for measuring the concentration of hydrogen peroxide as analyte.The indicator substance is advantageously an inorganic or organic substance which can be oxidized to a stable oxidized form, wherein the indicator substance and its oxidized form preferably differ in an optical property, so that the measurement radiation is converted in different ways by interaction with the indicator substance and the oxidized form of the indicator substance. Advantageously, the oxidized form of the indicator substance has a different color than the indicator substance. The indicator substance can be, for example, an electrochromic substance.The indicator substance can be, for example, a reduced form of Berliner Blue (also referred to as Prussian Blue or Turnbulls Reagent). The reduced form of Berliner Blue is also referred to as Berliner White (or Prussian White or Everitt's Salt). The indicator substance may also comprise an analog of Berliner Blue of the composition K n M1 m[ M2 o( CN) 6]p wherein K is selected from the group formed by Li, Na, K, Rb, Cs and NH 4 and wherein M1and M2are metals selected from the group formed by Fe, Co, Ni, Mn, Cd, Cu, Cr, V and Ru. M1 and M2 can be the same metal, such as e.g. in the case of Berliner Blue K 4 M 4[ Fe(CN) 6]3, or in the case of the co analogue of Berliner Blue K 2 Co 3[ Co(CN) 6]2 or different metals, such as e.g. in the case of KCr[Fe(CN) 6].The indicator substance may also comprise an electrochromic metal oxide. Examples of suitable compounds are electrochromic transition metal oxide compounds such as WO 3, MoO 3, IrO 2, NiO, Nb 2 O 5, RuO 2, BiVO 4. Also suitable are stable compounds of Cu(I) which can be oxidized to a corresponding Cu(II) compound, as well as ferrocene and ferrocene derivatives and ruthenium complexes.Suitable organic electrochromic indicator substances are electroactive conductive polymers, methylviologen, PANI, diquat dibromide, organometallic phthalocyanine compounds, for example ytterbium bisphthalocyanine), poly(3,4-ethylenedioxy)thiophene (PEDOT), polyfluorene, polyacetylene, polyphenylenevinylene, polyaniline, polypyrrole, polythiophene or poly-p-phenylene.Preferably, the indicator substance is thermally stable up to at least a temperature of 140° C. This is the case, for example, with inorganic indicator substances.The above-mentioned means for generating the current flow can comprise one or more transducer elements which are in contact with the indicator substance and are designed to convert mechanical, optical, thermal or chemical energy into a voltage or a current. This is advantageous if only little energy can be provided to the sensor for the regeneration of the indicator substance. The power generation by means of the converter elements can be initiated by an external pulse, for example by means of a pulse generator integrated into the sensor or by means of an external pulse generator. In particular, the pulse generator can comprise its own energy supply, so that it does not rely on energy of the sensor. The pulse generator can advantageously be connected to the sensor circuit or to a superordinate unit which is designed for communication with the sensor circuit. The pulse generator can be controlled by the sensor circuit or by the superordinate unit, advantageously in such a way that the times at which pulses are initiated are matched to the measurement value acquisition, in particular synchronized therewith.For example, the means for generating the current flow can comprise a plurality of piezoelectric crystals, in particular embedded in the sensor element, in contact with the indicator substance. The piezocrystals could comprise, for example, zinc oxide, lithium niobate or calinamines (iron oxide / zinc oxide), in particular in the form of nano-rods. In this embodiment, a pulse generator for generating the current flow can be a sound source or a pressure generator, e.g. a pneumatic system.In another example, the means for generating the current flow may comprise a Peltier element in contact with the indicator substance. In this embodiment, a heat source can be used as a pulse generator for generating current by means of the Peltier element, e.g. a resistance heater or a heat exchanger.The means for generating the current flow can comprise an electrochromic substance, by means of which an external light pulse is converted into current, which serves to reduce the oxidized form of the indicator substance. For example, the sensor element in this embodiment can comprise a Grätzel cell. The pulse generator for initiating the power generation can be a radiation source, e.g. a light source. The sensor element can thus comprise, for example, two electrochromic substances, one of the electrochromic substances serving as indicator substance which is oxidized by the analyte, and the second electrochromic substance serving to convert electromagnetic radiation, in particular light, into electrical current in order to bring about a current flow through the sensor element for regenerating the indicator substance of the sensor.The means for generating a current flow in the sensor element can comprise at least two electrodes, in particular configured as components of the sensor element, in contact with the indicator substance and a voltage source electrically conductively connected or connectable to the electrodes for applying a voltage between the electrodes. By means of the voltage source, a voltage can be applied to the electrodes, which causes a current flow through the sensor element to reduce the oxidized form of the indicator substance. The applied voltage is at least temporarily dimensioned in such a way that the oxidized form of the indicator substance is reduced by a current flowing between the electrodes and the indicator substance is thereby regenerated.In this embodiment, the sensor thus comprises:a sensor element provided for contact with the measurement fluid, said sensor element comprising an indicator substance and at least two electrodes in electrolytic contact with the indicator substance, the indicator substance being intended to be oxidized by the analyte to an oxidized form of the indicator substance;a voltage source connectable to the electrodes for applying a voltage between the electrodes;an optical measuring transducer which is configured to record an intensity of a measurement radiation influenced by the oxidized form of the indicator substance and to generate a measurement signal on the basis of the influenced measurement radiation, e.g. on the basis of an intensity, a phase angle or a decay time or a variable of the measurement radiation dependent thereon; anda sensor circuit which is connected to the optical measuring sensor for receiving the measurement signal and which is designed to determine a measurement value of the measurement variable on the basis of the measurement signal.The sensor may comprise a function generator connected to the voltage source, which serves to apply a predetermined voltage between the electrodes as a function of time. For example, the function generator can specify a DC voltage value which is constant over a predefined time duration or a sequence of a plurality of DC voltage pulses of predefined length. The sensor circuit may be configured to control the voltage source and / or the function generator. Alternatively, the sensor circuit and / or the function generator can be connected to a superordinate data processing unit for communication, wherein the superordinate data processing unit controls the function generator and the sensor circuit, in particular in order to coordinate, in particular synchronize, the acquisition of measurement values by means of the optical measurement pickup and the regeneration of the sensor by a current flow through the sensor element initiated by means of the voltage source.The electrodes may comprise at least one metal, for example copper, silver, platinum, gold, or carbon in an electrically conductive modification or a semiconductor material, for example silicon, germanium, gallium arsenide, indium tin oxide (ITO) or salts thereof, or a conductive polymer. Suitable conductive polymers are doped or non-doped, oxidized or non-oxidized conductive polymers, for example polypyrroles, cis- or trans-polyacetylenes, polyparaphenylene, polythiophene, polyparaphyenylenevinylene, polyaniline, poly(3,4-ethylenedioxythiophene:poly(styrenesulfonic acid), liquid-crystalline thiophene derivatives, poly(paraphenylene sulfide), poly(heptadiyne), poly(ethylenedioxythiophene) or Poly(isothianaphthen ). The conductive polymer further comprises polymers with conductive additives such as silver paste, CNT, SWCNT in a transparent polymer, e.g. TOPAS, or graphite.In an advantageous embodiment, the electrodes can be formed from a material which is transparent to the measurement radiation, which covers a wavelength range of the visible spectral range in one embodiment, for example indium tin oxide (ITO), fluorine tin oxide (FTO), aluminum zinc oxide (AZO), antimony tin oxide (ATO).In an advantageous embodiment, the sensor element comprises a membrane which contains the indicator substance. The indicator substance can be immobilized in a matrix and optionally encapsulated so that it cannot escape into the measurement liquid or can escape only slowly. In an advantageous embodiment, the matrix is formed from an inorganic material, but it can alternatively also comprise a polymer.The matrix with the indicator substance can form the membrane on its own. It is also possible for the membrane to be formed from a first layer formed by the matrix with the indicator substance and further layers. For example, at least one further layer can be provided which is permeable to the measurement medium and / or to the analyte and which obstructs or prevents the escape of the indicator substance into the measurement fluid. The membrane layer permeable for the analyte can be, for example, selectively permeable for the analyte, e.g. a reactive oxygen species, in particular hydrogen peroxide. The sensor element can comprise further protective or functional layers in addition to the membrane and the membrane layer permeable to the measurement fluid or the analyte.A first of the electrodes may cover at least a portion of an upper surface of the membrane and a second of the electrodes may cover at least a portion of the lower surface of the membrane. On their side facing away from the membrane, the electrodes can have further layers, for example in order to facilitate wetting by the measurement fluid.In alternative configurations, the electrodes can also be embedded in the membrane or contact the membrane in another manner.The first electrode can be transparent to the measurement radiation and the measurement radiation influenced by the oxidized indicator substance and impermeable to the analyte and / or the measurement fluid. This electrode is arranged on the side of the membrane facing away from the measurement fluid and allows measurement radiation to be radiated into the membrane arranged between the electrodes with the indicator substance.The optical measurement pickup can be configured, for example, to record the intensity of the measurement radiation transmitted through the oxidized form of the indicator substance. The measurement signal of the optical measurement pickup thus depends on the absorption of the measurement radiation by the indicator substance. In this case, the optical measurement pickup can have a radiation source which emits measurement radiation, wherein the measurement radiation is directed onto the membrane. Advantageously, the measurement radiation reaches the membrane through the transparent first electrode. The measurement pickup further has a radiation receiver to which the measurement radiation transmitted through the membrane with the indicator substance is directed. In this embodiment, the second electrode is also advantageously transparent to the measurement radiation, such that the measurement radiation transmitted through the membrane reaches the radiation receiver through the second electrode, wherein the transmitted measurement radiation can optionally be guided to the radiation receiver by means of a light guide.In an alternative configuration, the optical measurement pickup can be configured to record measurement radiation reflected or scattered at the membrane or a reflective layer of the sensor element or a luminescence radiation emitted by the oxidized indicator substance after excitation by the measurement radiation or a luminescence radiation of a reference substance influenced by the oxidized indicator substance. In this embodiment, the optical measuring transducer can comprise a radiation source that irradiates the measurement radiation into the membrane and a radiation receiver that is configured to output a measurement signal that is dependent on the received intensity, wherein the radiation source and the radiation receiver are each arranged in a sensor housing that is terminated at one end by the sensor element comprising the membrane. The measurement radiation can be guided from the radiation source via a light guide to the membrane and the reflected or scattered radiation or the luminescence radiation can be guided to the receiver by means of the light guide or an additional light guide.The sensor may comprise a housing in which the optical transducer and the sensor circuit are accommodated. The sensor element can close off the sensor housing on one side, in particular on a sensor end face intended for immersion in the measurement fluid. A geometry of the second electrode can determine a shape and / or properties of the sensor end face, for example in such a way that the shape of the sensor end face makes it difficult for gas bubbles or contaminants to accumulate.The second electrode covering at least a part of the underside of the membrane can be formed as a network structure from a conductive material, in particular a metal. In this way, the second electrode is on the one hand permeable to the measurement fluid, and on the other hand the network structure can be modified, e.g. coated with a hydrophilic polymer, in such a way that it is bubble-repellent and / or dirt-repellent. The network structure can also have a shape which makes it difficult or prevents the accumulation of gas bubbles.The sensor circuit can additionally be configured to perform amperometric measurements by means of the electrodes. For example, the sensor circuit can be configured to ascertain a current intensity of a current flowing between the first and the second electrode at a predefined voltage and to determine the measured variable or a further measured variable different from the measured variable on the basis of the ascertained current intensity. If the same measured variable is determined both by means of the optical measurement pickup of the sensor and amperometrically, the sensor circuit can additionally be configured to determine and evaluate deviations of the measured values determined by means of the two methods from one another. If necessary, the sensor circuit can be designed to output a warning in the event of a deviation exceeding a predefined threshold value.In one configuration, the sensor circuit can be configured to simulate a value of the measurement variable by applying a predefined voltage over a predefined period of time to the membrane, and to determine a measurement value of the simulated measurement variable by means of the optical measurement pickup, and to carry out calibration, verification or adjustment of the sensor on the basis of the determined measurement value. For this purpose, the sensor circuit can be connected to three electrodes which are in contact with the membrane and by means of which it can apply a controlled voltage to the membrane.The invention also relates to a measuring device comprising the sensor according to one of the embodiments described above and a superordinate unit connected to the sensor, in particular a measurement converter or control electronics or an energy supply, wherein the sensor and the superordinate unit are coupled to one another via a galvanically separated connection, in particular an inductive plug connector coupling and / or a radio connection, and wherein energy is transmitted unidirectionally from the superordinate unit to the sensor via the galvanically separated connection.The superordinate unit may comprise a data processing unit. The measuring device can furthermore be configured such that additionally data, in particular the measured variable, are transmitted bidirectionally between the sensor and the superordinate data processing unit via the galvanically separated connection.The sensor can have a housing which comprises at least the sensor circuit, the optical measurement pickup and the sensor element. In one possible embodiment, the sensor element can be firmly connected to the housing, so that it is not replaceable. In this case, it is advantageous if the indicator substance and a reference substance optionally comprised by the sensor element as well as the sensor matrix are formed from inorganic substances, so that they are chemically stable and the sensor thus has a correspondingly long operating duration. It is also possible, in particular if the sensor element comprises components which age during the service life of the sensor in a manner which impairs the functionality of the sensor, for the housing to comprise a replaceable cap which has the sensor element. In this case, a cap can be replaced after a certain operating duration with a cap having a new sensor element. In a further alternative embodiment, the sensor element itself can be designed to be replaceable.The invention is explained in more detail below with reference to the exemplary embodiments shown in the figures. The following are shown: FIG. 1 shows a sensor for measuring a concentration of a reactive oxygen species according to a first exemplary embodiment; FIG. 2 shows the sensor element of the sensor shown in FIG. 1; FIG. 3 shows a sensor for measuring a concentration of a reactive oxygen species according to a second exemplary embodiment; FIG. 4 shows a sensor for measuring a concentration of a reactive oxygen species according to a third exemplary embodiment; FIG. 5 shows a sensor for measuring a concentration of a reactive oxygen species according to a fourth exemplary embodiment; FIG. 6 shows a sensor for measuring a concentration of a reactive oxygen species according to a fifth exemplary embodiment; FIG. 7 shows a sensor element of the sensor shown in FIG. 6 ; FIG. 8 shows a sensor for measuring a concentration of ROS according to a seventh exemplary embodiment, which is designed to perform a self-calibration; FIG. 9 shows the sensor element of the sensor shown in FIG. 8; and FIG. 10 shows a sensor for measuring a concentration of ROS according to an eighth exemplary embodiment.FIG. 1 schematically shows a sensor 100 for measuring a concentration of hydrogen peroxide in a measurement fluid, in particular a measurement liquid, e.g. an aqueous disinfectant solution. The sensor 100 has a cylindrical sensor housing 9, which is closed on one end side by a sensor element 6. The sensor housing accommodates a sensor circuit 1 and an optical measuring pickup which has a radiation receiver 2 and a radiation source 3. A light-emitting diode or an arrangement of a plurality of light-emitting diodes is suitable as the radiation source 3, for example. The radiation receiver 2 may comprise one or more photodiodes or CCD elements. The radiation receiver 2 is configured to convert electromagnetic radiation into an electrical signal, e.g. a current or a voltage, the signal value of which depends on the intensity of the electromagnetic radiation. The sensor circuit 1 is configured to control the radiation source 3 for emitting measurement radiation and to receive signals of the radiation receiver 2 and process them as measurement signals.In the present example, the sensor 100 comprises a Y-shaped light guide 4, which on the one hand guides measurement radiation from the radiation source 3 to the sensor element 6 and on the other hand returns the measurement radiation converted in the sensor element 6 to the radiation receiver 2.Furthermore, the sensor 100 comprises a voltage source 8, which in the present example is likewise controllable by the sensor circuit 1.FIG. 2 shows the sensor element 6 of the sensor 100 in more detail. The sensor element 6 comprises a membrane 10 which comprises at least one layer formed from a matrix comprising an indicator substance. The matrix may be, for example, a polymer or an inorganic substance, e.g., a salt or a porous ceramic. The indicator substance can be immobilized in the matrix, e.g. in pores or in interstices of the polymer structure; it can also be chemically bonded to the matrix. In the present example, the matrix is formed from nafion in which Berliner white (also referred to as Prussian white or Everitts salt) is immobilized as indicator substance. In alternative configurations, the membrane can have further layers which can serve, for example, as barriers against the escape of the indicator substance from the membrane. Further functional layers can be provided, for example the membrane can comprise an opaque layer which prevents the entry of radiation from the environment, which is disruptive to the measurement, into the interior of the housing 9.In the present example, the sensor element 6 comprises a first electrode 11 which is arranged on the outer side of the membrane 10, i.e. facing away from the interior of the housing 9, and which at least partially covers the membrane 10. The sensor element 6 comprises a second electrode 12 which is arranged opposite the first electrode 11 on the side of the membrane 10 facing the interior of the housing 9 and at least partially covers the latter. The first electrode 11 is contacted by a first electrical line 13 which is connected to the voltage source 8. The second electrode 12 is contacted by a second electrical line 14, which is likewise connected to the voltage source 8. The voltage source 8 can thus apply a predefinable voltage between the electrodes 11 and 12, in particular predefined by the sensor circuit 1.In the present example, the first electrode 11 consists of transparent indium tin oxide (ITO) and has pores 15 via which the measurement liquid can reach the membrane 10. The first electrode may alternatively be formed of a variety of other materials, e.g., a metal, carbon, a conductive polymer, or a conductive composite material. In the present example, the second electrode 12 likewise consists of transparent ITO, so that measurement radiation from the light guide 4 can pass through the electrode 12 into the membrane 10 or measurement radiation converted in the membrane 10 can pass through the electrode 12 back into the light guide 4 and to the radiation detector 2.A measurement of the concentration of an analyte belonging to the class of ROS, in particular hydrogen peroxide, with the sensor 100 is now based on the following principle: the indicator substance is intended to be oxidized by the analyte, here hydrogen peroxide, to an oxidized form of the indicator substance. In the case of Berliner white as indicator substance, the following reaction takes place in which the colorless Berliner white is oxidized to the blue Berliner blue: H 2 O 2+ 2[ Fe II Fe II( CN) 6]2-+ 2 H +→2H 2 O+2 [Fe III Fe II( CN) 6]-.The Berliner blue formed can be detected optically, for example by means of an absorption or reflection measurement or, as in the present example, by means of a fluorescence measurement. For this purpose, the membrane 10 comprises a reference substance which is likewise immobilized in the matrix. The reference substance can be an organic or inorganic dye which is substantially chemically inert and in particular does not react with the analyte. The reference dye can also be excited by measurement radiation emitted by the radiation source 3 to produce a luminescence with a decay time in the range between 1 μs and 1000 μs. If in the matrix there is present in addition to the reference dye Berliner Blue, this influences the luminescence signal received by the radiation receiver. On the other hand, Berliner white does not influence the luminescence signal. Concentration differences can be determined in this way and detected by the radiation receiver 2, which outputs an electrical measurement signal representative of the hydrogen peroxide concentration in the measurement fluid to the sensor circuit 1. The reference dye can be immobilized in the matrix together with the indicator substance or in a further layer of the membrane.The sensor circuit 1 is configured to determine a current measurement value of the hydrogen peroxide concentration from the measurement signal of the radiation receiver 2 on the basis of an empirically determined assignment rule which assigns measurement signals of the radiation receiver 2 to measurement values of the hydrogen peroxide concentration. The sensor circuit 1 can comprise a display or another display element, via which it can output the determined measured value. Alternatively, it can also have a communication interface that can be connected to a superordinate data processing unit, e.g. a computer, a measurement transmitter, a portable operating device, e.g. a smartphone or tablet PC, or a programmable logic controller in a wired or wireless manner, via which communication interface it can output the determined measured value to the superordinate data processing unit. The higher-level data processing unit can also supply the sensor with energy. The coupling between the data processing unit and the sensor can have a galvanically separated, in particular an inductive, interface.The oxidation of Berliner white to Berliner blue is not readily reversible. In order to reduce the Berliner Blue formed by interaction with the measurement fluid back to Berliner White and thus to put the sensor into a state in which a new measurement value can be determined, a DC voltage suitable for electrochemical reduction of Berliner Blue is applied between the first electrode 11 and the second electrode 12. The voltage required for cathodic reduction of Berliner Blue to Berliner White depends on the respective electrode and matrix material. It is preferably between 0.7 V and -0.7 V, more preferably 0.5 and -0.5 V for oxidation and reduction based on a platinum reference electrode. It can be applied for a predetermined time period t, which in the present example is less than 5 min, preferably less than 1 min, even more preferably less than 30 s, but is at least 0.1 s. In alternative embodiments, it is also possible to apply a voltage between the first and the second electrode, which voltage varies as a function of time, for example in the form of a few short-term DC voltage pulses or in the form of one or more successive voltage ramps, in which the voltage is increased linearly from zero to a predefined value of the specified range and is then lowered again. A voltage profile adapted to the configuration of the sensor element 6, i.e. its materials, in particular the indicator material and layer structure, can be determined in preliminary experiments.In an alternative embodiment to the given exemplary embodiment, the sensor can also have a third electrode serving as a reference electrode in addition to the two electrodes 11 and 12. The reference electrode can be, for example, a platinum wire or a platinum wire coated with a conductive, sparingly soluble salt. In this embodiment, the voltage source 8 comprises a 3-electrode circuit which is designed to apply a predetermined voltage as a function of time, in particular a voltage pulse or a voltage which is constant for a predetermined period of time, between one of the electrodes 11 and 12, which serves as a working electrode, and the reference electrode.The sensor circuit 1 and / or a superordinate unit connected to the sensor circuit 1 for communication can be connected to the voltage source 8 in order to control the latter. In the present example, the sensor circuit 1 comprises a function generator which is designed to control the voltage source 8 in such a way that the voltage source 8 applies a voltage predetermined as a function of time to the electrodes 11, 12. The function can be selected by an operator of the sensor and be predefined for the sensor circuit 1.In a very simple embodiment, the function generator can be dispensed with by the sensor circuit for controlling the voltage source actuating only one switch which selectively connects the voltage source which outputs a constant DC voltage to the electrodes or disconnects them from them. In this case, the sensor circuit can close the switch for a predetermined period of time, for example 30 s, in order to apply a DC voltage pulse to the electrodes.In the present example, after the detection of each measured value, the sensor circuit 1 controls the voltage source 8 in such a way that the voltage source applies a DC voltage pulse of 30 s duration to the electrodes 11, 12 so that after the detection of each measured value, the membrane 10 is regenerated, i.e. an oxidized form of the indicator substance present in the membrane 10 is electrochemically reduced.If large changes in the concentration in the measurement fluid are not to be expected, or the concentration of hydrogen peroxide exclusively increases due to the process, regeneration of the membrane 10 does not have to take place after the acquisition of each measurement value; instead, regenerations at relatively long time intervals are sufficient in these cases. The frequency of such regeneration by applying a voltage to the electrodes 11, 12 can be preset to the sensor circuit 1 by means of an input of an operator or by a signal of the higher-order data processing unit possibly connected to the sensor circuit 1 in an advantageous embodiment of the sensor 100.In an alternative embodiment, the sensor element 6 or the membrane 10 can contain, in addition to a first indicator substance, e.g. Berliner white, a further indicator substance, e.g. an analogue of Berliner white or of Berliner blue having the structure A n M m[ Fe(CN) 6] xH 2 O, where A is an alkali metal and M is a transition metal, e.g. cobalt, nickel, manganese, cadmium, copper, chromium, vanadium or ruthenium. In this way, in the case of measurements with greater transverse sensitivity, a confidence value can be calculated by means of a comparison measurement. In this case, the radiation receiver acquires both the intensity of a measurement radiation converted by the first indicator substance and the intensity of a measurement radiation converted by the further indicator substance. Based on the two measurement signals of the radiation receiver representing the respective intensities, the sensor circuit can ascertain two measurement values and compare them with one another. If the two measured values deviate too much, the sensor circuit 1 can output this, for example in the form of an error or warning message. The two measurements do not have to be made permanently in parallel, but can be activated by the operator as desired by an input to the sensor circuit 1 or to a superordinate unit connected to the sensor circuit.In the present exemplary embodiment, the housing 9 is formed in one piece, i.e. the sensor element 6 is not replaceable. This is advantageous if the sensor element maintains its full functionality over a long period of time, for example if the components of the sensor element, in particular the matrix, the indicator substance and / or the reference dye, are not subject to any substantial aging, which can lead to an impairment of the sensor functionality. This is possible, for example, if the sensor element is formed from inorganic substances. If the sensor element comprises substances which are subject to aging, e.g. an organic matrix, an organic indicator substance or an organic reference dye, the sensor element can be designed to be replaceable or the housing can have a replaceable cap which comprises the sensor element.FIG. 3 schematically shows a second exemplary embodiment of a sensor 200 for determining a concentration of a reactive oxygen species. The sensor 200 is substantially constructed analogously to the sensor 100 shown in FIG. 1, but differs in the manner in which measurement radiation is guided to the sensor element or the measurement radiation influenced by the oxidized form of the indicator substance is fed back from the sensor element. Identical components of the sensors 100 and 200 are denoted by the same reference numerals in FIGS. 1 and 3. Like the sensor 100 illustrated in FIG. 1, the sensor 200 comprises a sensor circuit 1, a radiation source 3, a radiation receiver 2 and a sensor element 6, which are configured in the same way as the corresponding elements of the sensor 100. In addition, the sensor 200 comprises a semi-transmissive mirror 5, through which measurement radiation of the radiation source 3 reaches a light guide 4 and is guided to the sensor element 6. Measurement radiation influenced in the sensor element 6 by the oxidized form of the indicator substance is guided back through the light guide 4 onto the semi-transparent mirror and is directed by the latter onto a radiation detector 2. The acquisition of measured values and the regeneration of the indicator substance take place in the same way in the sensor 200 shown in FIG. 3 as in the sensor 100 shown in FIG. 1.FIG. 4 schematically shows a further exemplary embodiment of a sensor 300 in which the measurement value determination is based on an absorption measurement. Components of the sensor 300 which are configured identically to those of the sensor 100 described with reference to FIG. 1 are identified by the same reference numerals as the corresponding components of the sensor 100. Like the sensor 100 of the first exemplary embodiment, the sensor 300 comprises a sensor circuit 1, a radiation source 3 and a radiation receiver 4.The sensor 300 has a housing 9, which forms a cuvette 16 designed as a depression. If the end face of the housing 9 surrounding the cuvette 16 is immersed in a measurement fluid, this enters the cuvette 16. Cuvette 16 has opposing side walls. One of the side walls is closed by a sensor element 6 containing an indicator substance, which can otherwise be configured in the same way as the sensor element 6 shown in FIG. 2.The other side wall of the cuvette 16 comprises a window 17 opposite the sensor element 6, which is formed from a material that is transparent to the measurement radiation emitted by the radiation source 3. The radiation source 3 is arranged in the end-face end region of the housing 9 behind the window 17, so that in the present example the radiation receiver 2 and the radiation source 3 are arranged on different sides of the sensor element 6. The measurement radiation emitted by the radiation source 3 thus initially passes through the window 17, the cuvette 16 and the sensor element 6. The converted measurement radiation exits from the sensor element 6 into the light guide 4, which guides the converted measurement radiation to the radiation receiver 2. This is configured to output an electrical signal dependent on the received intensity of the converted measurement radiation to the sensor circuit 1. The sensor circuit uses an empirically determined assignment rule, which assigns measurement signals of the radiation receiver 2 to values of the measurement variable, to determine a measurement value of the concentration of the analyte in the measurement fluid.Like the sensor 100 of the first exemplary embodiment, the sensor 300 comprises a voltage source 8 which can be controlled by the sensor circuit 1 or a superordinate unit connected to the sensor circuit 1 and is connected to electrodes of the sensor element 6 via electrical conductors 13, 14. The voltage source and the electrodes serve, as described with reference to the first exemplary embodiment, for regenerating the indicator substance by electrochemically reducing the oxidized form of the indicator substance.FIG. 5 schematically shows a further exemplary embodiment of a sensor 400, in which the measurement value determination is based on an absorption measurement. Components of the sensor 300 which are configured identically to those of the sensor 100 described with reference to FIG. 1 are identified by the same reference numerals as the corresponding components of the sensor 100. The sensor 400 comprises a sensor circuit 1, a radiation source 3 which is configured to emit measurement radiation, and a radiation receiver 2. furthermore, the sensor 400 comprises a sensor element 6 which comprises an indicator substance and which can be configured like the sensor element 6 illustrated in FIG. 2. the sensor element 6 comprises in particular two electrodes which are contacted via electrical lines 13, 14 and are connected to a voltage source 8. As described in detail above with reference to the first exemplary embodiment, the voltage source 8 can be controlled by the sensor circuit 1 or a superordinate unit connected to the sensor circuit 1 in order to carry out a regeneration of the indicator substance.In the present exemplary embodiment, the housing 9 has a lateral opening 20, via which the sensor element 6 can be acted upon from the side with a measurement liquid containing the analyte. Unlike the sensor 300 described above with reference to FIG. 4, the radiation source 3 and the radiation receiver 2 are not arranged on different sides of the sensor element 6. Rather, the sensor 400 shown here comprises a first light guide 4, which serves to guide the measurement radiation emitted by the radiation source 3 to the sensor element 6, and a second light guide 18, which guides the measurement radiation influenced after passage through the sensor element 6 on account of absorption by the indicator substance to the radiation receiver 2. Thus, in the exemplary embodiment shown here, the radiation receiver 2 can be arranged in the vicinity of the radiation source 3, in particular on the same side of the sensor element 6 as the radiation source 3.FIG. 6 schematically illustrates a further exemplary embodiment of a sensor 500, in which the sensor is regenerated by generating a current which is generated by an external pulse acting on the sensor element. Like the sensor 100 of the first exemplary embodiment, the sensor 500 comprises a housing 9 with a sensor circuit 1 arranged therein, a radiation source 3, a radiation receiver 2 and a sensor element 6. measurement radiation of the radiation source 3 is guided through a semi-transparent mirror 5 to the sensor element 6, measurement radiation reflected back from the sensor element 6 and influenced by the oxidized form of an indicator substance contained in the sensor element is directed via the semi-transparent mirror 5 to the radiation receiver 2. The radiation source 3 can comprise a light emitting diode or another light source for emitting measurement radiation of a wavelength or a wavelength range, the radiation receiver 2 having one or more photodiodes or a CCD element. The radiation receiver 2 can also be designed as a miniature spectrometer which is designed to record a spectrum of the measurement radiation influenced by interaction with the indicator substance contained in the sensor element 6.The sensor element 6 is shown in detail in FIG. 7. In the present example, it comprises a plurality of layers. A first layer is the membrane 10 formed from a matrix having an indicator substance embedded therein. In addition to the indicator substance, the matrix contains a reference substance which can be excited to luminescence by the measurement radiation. On its side facing away from the interior of the housing 9, the sensor element 6 has a protective coating 18 which is permeable to the measurement fluid or at least the analyte. For example, the protective coating 18 may have pores 19. An opaque layer 20 is arranged between the protective coating 18 and the membrane 10, which prevents radiation from the outside from entering the interior of the housing, but which is at least permeable to the analyte. On the side facing the housing, the membrane 10 has a piezoelectric layer 21, the function of which is explained further below.In the present exemplary embodiment, the membrane contains, in addition to an indicator substance that can be oxidised by ROS, a reference substance that can be excited to luminescence by the measurement radiation. The use of such a reference substance together with the indicator substance allows, as mentioned above, a determination of the analyte concentration, for example by means of phase angle measurement. Alternatively, the intensity of the luminescent radiation emitted by the oxidized indicator substance or a decay time of the luminescence intensity (in these cases also without reference substance) can also be determined for determining the analyte concentration.In the exemplary embodiment shown here, the indicator substance is regenerated by an external stimulus which causes a current flow through the sensor element 6 or the membrane 10 and thus causes an electrochemical reduction of the oxidized form of the indicator substance (here: Berliner Blue). In the present example, the piezoelectric layer 21 is used for this purpose. this layer 21 comprises a multiplicity of nano-rods formed from zinc oxide. These nano-rods are designed to convert pressure into electrical energy. The layer 21 can be produced by spraying zinc oxide onto the surface of the membrane 10 and then heating to about 90° C. During the heat treatment at 90° C., the nano-rods grow and form a layer adhering to the membrane.In addition, the sensor 500 comprises a pressure source 7, e.g. in the form of a sound source. The pressure source 7 can alternatively also be configured to output a pneumatically generated pressure. In an alternative configuration of the sensor element, piezoelectric nano-rods can be directly embedded in the membrane 10. For regenerating the indicator substance, the sensor circuit 1 or a superordinate unit connected to the sensor circuit 1 can be configured to control the pressure source 7 for generating a pressure pulse which causes a current flow in the sensor element 6 via the membrane 10, which is dimensioned to electrochemically reduce the oxidized form of the indicator substance contained in the membrane.Although the determination of the measured value on the basis of a measurement of a phase angle shift between a luminescence radiation of the indicator substance and a luminescence radiation of a reference substance is described in this exemplary embodiment in combination with the regeneration of the indicator substance by a pressure pulse using piezoelectric transducers, this type of determination of the measured value can of course also be used in combinations with all other methods and devices described here for regenerating the indicator substance, in particular with the regeneration by means of electrodes which are in contact with the membrane containing the indicator substance and to which a voltage can be applied as a function of time, as described in the examples according to FIGS. 1 to 5.FIG. 8 schematically illustrates a sensor 600 which can perform a self-checking operation, in particular a self-verification operation or a self-calibration operation. The sensor 600 comprises a sensor housing 9, a sensor circuit 1, a voltage source 28 controllable by the sensor circuit 1, a radiation source 3 controllable by the sensor circuit 1, which is configured to emit measurement radiation and a radiation receiver 2. the sensor 600 further comprises a sensor element 6, which comprises an indicator substance, which can be oxidized from a ROS analyte, for example hydrogen peroxide, to an oxidized form, wherein the oxidized form of the indicator substance has a characteristic absorption and / or a characteristic luminescence behavior, which can be detected by means of the radiation source 3 and the radiation receiver. The indicator substance can be, for example, Berliner white, which is oxidized to Berliner blue by hydrogen peroxide.The sensor 600 comprises a Y-shaped light guide 4, which connects the radiation source 3 and the radiation receiver 2 to the sensor element 6, so that radiation emitted by the radiation source is guided via the light guide 4 to the sensor element 6 and measurement radiation influenced by the indicator substance of the sensor element 6 is guided via the light guide 4 to the radiation receiver. In the present example, the sensor element 6 is applied as a layer structure directly on the exit surface of the light guide 4.The sensor element 6 is shown in detail in FIG. 9. It has a membrane 10 which is formed from an inorganic matrix, which can consist, for example, of a salt which is transparent to the measurement radiation and the influenced measurement radiation, such as potassium chloride or lithium oxide, and the indicator substance contained in the matrix, here Berliner white.On its side intended for contact with the measurement fluid, the membrane is covered with a protective layer 24. The protective layer 14 is selective for a particular ROS analyte, here hydrogen peroxide, in the present example.Two electrodes 11, 12 are embedded in the membrane 10, which electrodes can be connected to the voltage source 28 via electrical lines 13, 14. The electrodes 11, 12 consist of metal in the present example, but they can also consist of a material transparent to the measurement radiation or the influenced measurement radiation, as in the example described above. In the example described here, the electrodes are configured as metal structures embedded in the matrix, e.g. as metal wires or as planar metal structures which have one or more openings through which the measurement radiation can penetrate into the membrane 10. The lines 13, 14 can be carried with the light guide 4, e.g. wound around it or applied to it as a coating.The sensor 600 can be designed for regenerating the indicator substance quite analogously according to the method described in connection with the first exemplary embodiment. Additionally, the sensor 600 may perform self-verification and / or self-calibration.For this purpose, the sensor circuit 1 can control the voltage source 28 in such a way that, after a regeneration of the indicator substance, it applies a predefined voltage between the electrodes 11, 12 which brings about a targeted oxidation of the indicator substance to its oxidized form. The voltage is dimensioned such that it simulates a specific analyte concentration, i.e., a specific H 2 O 2- concentration here. The colour change of the indicator substance occurring in this case is detected by the sensor circuit 1 by means of an absorption or fluorescence measurement with the radiation source 3 and the radiation receiver 2. The measured value of the analyte concentration determined on the basis of the associated measurement signal of the radiation receiver can be compared with the simulated concentration by the sensor circuit 1 for the verification or calibration of the sensor 600. Based on this comparison, a drift correction can optionally be carried out.In an alternative embodiment of the present exemplary embodiment described with reference to FIGS. 8 and 9, the sensor can also comprise a third electrode, which serves as a reference electrode. The reference electrode can be configured, for example, as a platinum wire or as a platinum wire with a coating comprising a sparingly soluble, electrically conductive salt. The reference electrode is also in contact with the membrane 10, for example it may be embedded therein. In this case, the voltage source may include a 3-electrode circuit configured to apply a predetermined voltage between the reference electrode and an electrode serving as a working electrode of the two electrodes 11 and 12.FIG. 10 schematically illustrates a further exemplary embodiment of a sensor 700 which is configured to determine an analyte concentration based on a reflection measurement. The sensor 700 comprises a housing 9 in which a sensor circuit 1, a radiation source 3, a radiation receiver 2, a sensor element 6 and a light guide 4 which guides radiation from the radiation source 3 to the sensor element 6 and guides radiation from the sensor element 6 back to the radiation receiver 2. The sensor element 6 has a membrane 10 which comprises a matrix and an indicator substance immobilized in the matrix, e.g. Berliner Blue as indicator substance for H 2 O 2 as analyte. The membrane 10 is covered on its side facing the radiation source 3 with a first electrode 12 which is transparent to the measurement radiation through which measurement radiation can pass into the membrane 10. On its opposite side, the membrane 10 is covered with a second electrode 11, likewise transparent to the measurement radiation. Both electrodes 11, 12 can be designed as indium tin oxide layers. Above the second electrode 11 a further layer 25 is arranged which is configured to reflect the measurement radiation of the radiation source 3 transmitted through the membrane 10 and the electrodes 11, 12. The further layer 25 can be a metal mirror, for example. The measurement radiation reflected at the further layer 25 is radiated back to the light guide 4 through the further layers of the sensor element 6 and is conducted from the latter to the radiation receiver 2.A measuring fluid can be applied to the membrane 10 via a lateral opening 20 of the housing 9. As described above, the indicator substance is oxidized by an analyte contained in the measurement fluid, here hydrogen peroxide, to a colored oxidized form, here Berliner Blue. The radiation intensity received by the radiation receiver 2 can therefore be evaluated by the sensor circuit 1 in a manner completely analogous to that described above for the absorption measurement on the basis of the measurement signals of the radiation receiver in order to determine values of the measurement variable.A series of examples for producing a sensor element or a membrane for a sensor for measuring a concentration of one or more reactive oxygen species are given below.In general, the individual components can be applied in layers according to the invention. However, this is not necessary, rather a plurality of membrane structures is conceivable. Thus, layer structures, matrix structures, interpenetrating polymer networks composed of two or more purely inorganic or purely organic materials or composed of a combination of organic and inorganic materials can be applied to a transparent substrate or directly to an end face (exit face) of a light guide. All methods known to the skilled worker, such as dipcoating, spraycoating, doctor coating, spincoating, CVD (chemical vapor deposition), CVP (physical vapor deposition), laminating, printing, electro-spinning, electrodeposition, are conceivable here.Example 1:Polyallylamine and indicator such as cobalt tetraaminophthalocyanine and polyacrylic acid are applied alternately as layer stacks to a substrate by doctoring in a plurality of layers:The layer sequence may appear as follows:1) Polyallylamine hydrochloride + Indicator2) Polyacrylic acid3) Polyallylamine Hydrochloride + Indicator4) Polyacrylic acidAfter coating the layers, an electric voltage is applied to electrodeposit the dye polymer polycobalt tetraaminophthalocyanine. The indicator substance is colored tan in its oxidized form and pink in the reduced state. Alternatively, Berliner white / Berliner blue without electrodeposition can also be used as indicator substance.Example 2:The membrane may also comprise a matrix of polyacrylic acid or nafion with microspheres of polyallylamine embedded therein, in which the indicator substance, e.g. Berliner white, is encapsulated. Polyacrylic acid or nafion is very well suited as an organic matrix because it well absorbs water and thus also an aqueous measurement liquid, thus ensuring a short response time of the sensor.Example 3: Example 3:A mixture of tetraethylosiloxane with Berliner white is dispersed in glycerol / 1M potassium chloride / acetic acid solution and the mixture is applied to a quartz glass disc (8 mm in diameter) serving as substrate and cured for 2 days.Example 4: Example 4:500 mg of Nafion and 5 mg of Berliner white are dispersed in a solution of ethanol / water (1:1) and dropped onto a 5 μm thick teflon film.Example 5: Example 5:A solution of Berliner white, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) / polyaniline / tungsten oxide is applied to an indium tin oxide substrate and bonded to a further, porous indium tin oxide substrate. The indium tin oxide substrates serve as electrodes for applying a voltage across the membrane formed of poly(2-acrylamido-2-methyl-1-propanesulfonic acid / polyaniline / tungsten oxide / Berliner white. Tungsten oxide is used to convert light into electrical energy, so that a reduction of the Berliner blue to Berliner white can be triggered by a light pulse.Example 6:A solution of Berliner blue and a histamine-containing polymer is applied to an indium tin oxide substrate and bonded to a further porous indium tin oxide substrate.Example 7:A solution of Berliner blue, graphite and nafion is applied to an indium tin oxide substrate and bonded to a further porous indium tin oxide layer.Example 8:A primer, for example siloxane, is applied to an indium tin oxide substrate and, after a short waiting time of about 30 min, a solution of Berliner Blue, Nafion (dissolved in ethanol water 1:1) is applied by drops and, after a short waiting time, Berliner Blue is again applied by drops in an aqueous 1M KCI solution. A porous indium tin oxide substrate is then bonded at the edges with an acrylate adhesive and provided with a primer in the middle. The substrate so treated is adhered to the coated indium tin oxide substrate. Alternatively, an indium tin oxide substrate with holes or porous structures on the media side can also be filled with Berliner Blue and Nafion. In this case, the bonding and the application of a primer with a small pore size of less than 200 μm would be omitted.Example 9:Prussian blue is sputtered onto a microporous indium tin oxide substrate.Example 10:The sensor arrangement of this example is suitable for calibration, adjustment or verification of the sensor by simulating an analyte concentration (cf. exemplary embodiment according to FIGS. 8 and 9 ). This allows calibration, adjustment or verification in the process without the need to remove the sensor from the process plant.For this purpose, a 3-electrode arrangement in the layer sequence ITO+Monolithische layer Berliner Blue / KCl-Pt / ITO is used in the membrane. The ITO electrode arranged closer to the housing interior can be connected as a working electrode and the other electrode permeable to the measurement fluid or at least the analyte can be connected as a counter electrode, while the Pt-KCI layer serves as a reference electrode. As an alternative to potassium chloride, an ion- or electron-conducting substance which is sparingly soluble or insoluble in water, such as Al-Li alloys, graphite, Li cobalt oxide or Li vanadium oxide, can be used. By defined application of an oxidative positive or reducing negative voltage between the reference electrode and the working electrode, individual concentrations of specific analytes can be simulated. For example, an optical measurement value check can be carried out by the sensor itself, which makes external calibration of the sensor superfluous. Thus, by applying a voltage of 0.7 V to -0.7 V, more preferably 0.5 V to -0.5 V, with respect to the KCI-Pt reference electrode, zero point calibration of H 2 O 2 can be performed. The simulation value can also serve for adjustment based on empirical values.Example 11:In this example, the sensor element can be constructed as described with reference to Example 10, wherein glucose oxidase is additionally introduced into the Nafion matrix. In the presence of glucose in the measurement fluid, hydrogen peroxide is formed by reaction with the glucose oxidase present in the matrix, which hydrogen peroxide causes oxidation of the Berliner white present in the matrix to Berliner blue. As described above, an absorption or luminescence measurement can be used to determine a concentration of H 2 O 2 in the Nafion matrix, which in turn is a measure of the glucose concentration of the measurement fluid. The sensor circuit can be configured accordingly to determine a measurement value of the glucose concentration in the measurement fluid from a measurement signal of the radiation receiver according to an, for example, empierically determined, assignment rule.

Claims

Sensor for determining a measurement variable correlated with a concentration of at least one analyte belonging to the class of the reactive oxygen species in a measurement fluid, in particular a measurement liquid, comprising: - a sensor element provided for contact with the measurement fluid, which sensor element comprises an indicator substance, wherein the indicator substance is intended to be oxidized by the at least one analyte to an oxidized form of the indicator substance; - means for generating a current flow in the sensor element, which current flow causes a reduction of the oxidized form of the indicator substance and thereby a regeneration of the indicator substance; - an optical measurement pickup, which is designed to detect a measurement radiation influenced by the oxidized form of the indicator substance and to generate a measurement signal, in particular an electrical measurement signal, on the basis of the influenced measurement radiation; a sensor circuit which is connected to the optical measuring sensor for receiving the measurement signal and which is designed to determine a measurement value of the measurement variable on the basis of the measurement signal.Sensor according to Claim 1, wherein the measurement pickup is designed to generate the measurement signal in order to record an intensity and / or a phase angle shift and / or a decay time and / or a wavelength shift of the received measurement radiation.The sensor of claim 1 or 2, wherein the analyte is hydrogen peroxide.The sensor of any one of claims 1 to 3, wherein the indicator substance comprises an organic or inorganic electrochromic substance.The sensor of claim 4, wherein the indicator substance has the composition K n M1 m[ M2 o( CN) 6]p wherein K is selected from the group formed by Li, Na, K, Rb, Cs and NH 4 and wherein M1 and M2 are selected from the group of metals formed by Fe, Co, Ni, Mn, Cd, Cu, Cr, V and Ru; and wherein M1 and M2 may be the same or different metals from each other.The sensor of any one of claims 1 to 5, wherein the indicator substance is thermally stable up to at least a temperature of 140°C.The sensor according to any one of claims 1 to 6, wherein the means for generating the current flow comprise one or more transducer elements in contact with the indicator substance, which are configured to convert mechanical, optical or thermal energy into a voltage or a current.Sensor according to one of Claims 1 to 7, wherein the means for generating the current flow comprise a multiplicity of piezoelectric crystals, in particular embedded in the sensor element, which are in contact with the indicator substance.The sensor of any of claims 1 to 7, wherein the means for generating the current flow comprises a Peltier element in contact with the indicator substance.A sensor according to any one of claims 1 to 7, wherein the means for generating the current flow comprise an electrochromic substance by means of which an external light pulse is converted into current.Sensor according to one of Claims 1 to 6, wherein the means for generating a current flow in the sensor element comprise at least two electrodes, in particular designed as components of the sensor element, which are in contact with the indicator substance, and a voltage source which is or can be electrically conductively connected to the electrodes for applying a voltage between the electrodes.A sensor according to claim 11, wherein the sensor comprises a function generator connected to the voltage source, which function is to apply a predetermined voltage between the electrodes as a function of time.The sensor according to claim 11 or 12, wherein the electrodes comprise at least one metal, for example copper, silver, platinum, gold or carbon, or a semiconductor material, for example ITO or LiWoO 3, or a conductive polymer.The sensor of any of claims 11 to 13, wherein the sensor element comprises a membrane containing the indicator substance, and wherein a first of the electrodes covers at least a part of an upper side of the membrane and a second of the electrodes covers at least a part of the lower side of the membrane.The sensor of claim 14, wherein the first electrode is transparent to the measurement radiation and the converted or affected measurement radiation and is opaque to the analyte and / or the measurement fluid.Sensor according to claim 14 or 15, wherein the second electrode, for example having a network structure, is permeable at least to the analyte and / or to the measurement fluid.The sensor according to any one of claims 11 to 16, wherein the sensor element closes the sensor on a sensor end face and wherein a geometry of the second electrode determines a shape and / or properties of the sensor end face, for example in such a way that the shape of the sensor end face makes it difficult for gas bubbles or contaminants to accumulate.Sensor according to one of Claims 11 to 17, wherein the sensor circuit is designed to determine a current intensity of a current flowing between the first and the second electrode at a predefined voltage and to determine the measurement variable or a further measurement variable different from the measurement variable on the basis of the determined current intensity.Sensor according to one of Claims 11 to 18, wherein the sensor circuit is designed to simulate a value of the measurement variable by applying a predefined voltage over a predefined period of time, in particular by means of a 3-electrode circuit, to the membrane and to determine a measured value of the simulated measurement variable by means of the optical measurement pickup, and to carry out calibration, verification or adjustment of the sensor on the basis of the determined measured value.Measuring device comprising the sensor according to one of claims 1 to 19, and a superordinate unit connected to the sensor, in particular a measurement converter or control electronics or an energy supply, wherein the sensor and the superordinate unit are coupled to one another via a galvanically separated connection, in particular an inductive plug connector coupling and / or a radio connection, and wherein energy is transmitted unidirectionally from the superordinate unit to the sensor via the galvanically separated connection.Measuring device according to claim 20, wherein the superordinate unit comprises a data processing unit, and wherein additionally data, in particular the measurement variable, is transmitted bidirectionally between the sensor and the superordinate data processing unit via the galvanically isolated connection.

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