A sensor for detecting a chemical species and for mitigating drift

Abstract

The present invention relates to a sensor for detecting a concentration of chemical species and for mitigating drift, to a method for operating said sensor, to a computer program or integrated circuit for carrying out the method, and to a device comprising a hydrogen sensor for monitoring a hydrogen pressure or concentration.

Description

[0001] A SENSOR FOR DETECTING A CHEMICAL SPECIES AND FOR MITIGATING

[0002] DRIFT

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to a sensor for detecting a concentration of chemical species and for mitigating drift, to a method for operating said sensor, to a computer program or integrated circuit for carrying out the method, and to a device comprising a hydrogen sensor for monitoring a hydrogen pressure or concentration.

[0005] RELATED APPLICATIONS

[0006] The present application claims the benefit of priority from Dutch Patent Applications NL2035557, filed on August 4, 2023, in the name of Technische Universiteit Delft, Netherlands.

[0007] The entire contents of the above-referenced application and of all priority documents as referenced in any present or future Application Data Sheet filed herewith are hereby incorporated by reference for all purposes. BACKGROUND OF THE INVENTION

[0008] In a more generic perspective in an economy with hydrogen as a major energy carrier, the development of affordable, reliable, sensitive and selective hydrogen sensors is indispensable. Several types of hydrogen sensors are currently available, which exploit one or more of the following detection mechanisms: catalytic, electrochemical, mechanical, optical, acoustic, thermal conductivity, resistance and work function. Since hydrogen detection often takes place in an explosive environment, cf. for leak detection or for hydrogen-concentration measurements in gas streams, use of optical hydrogen sensors has a major advantage of being intrinsically safe due to the lack of electric leads in the sensing area.

[0009] These optical sensors rely on a metal hydride sensing material that gradually and reversibly absorbs a chemical species such as hydrogen when such chemical species is present in the vicinity of the sensor. In turn, the absorbed species changes the optical properties of the metal hydride, which can then be probed to determine the chemical species concentration / pressure. To do this, two major methods can be distinguished: intensity and frequency modulated sensors. In intensity modulated sensors, the optical transmission or reflectivity of the sensing material is probed, such as in a micro-mirror sensor. In frequency modulated sensors, such as fibre-Bragg gratings, and (localized) surface plasmon resonance sensors, the exposure of the sensor to a chemical species, such as hydrogen, causes a wavelength-shift of the response. While frequency modulated sensors have the substantial advantage that they are not sensitive to intensity fluctuations of the light sources, and are therefore more stable than intensity-modulated sensors, they contain more complex and expensive components. In particular, drifting intensity of the light source or detection efficiency compromise the accuracy and long-term stability of intensity -modulated sensors. Sensor drift is a phenomenon well-known to the skilled person. Drift can significantly influence the reliability and accuracy of sensor measurement data, resulting in errors or biases, and is therefore a clear disadvantage in this particular field of technology.

[0010] Typical prior art examples of sensors having drift are given in EP 3 385 702 Al, US2013 / 175437 Al, and EP 3 865 853 Al. EP 3 385 702 Al recites an optical gas sensing system for sensing hydrogen in a fluid is provided. The system comprises: an optical sensor comprising a fibre, the fibre including at an end portion a sensor element with a sensing layer configured to change its dielectric properties depending on a gas partial pressure at the sensing layer; at least one light source for coupling light with a first spectrum into the optical sensor; a dichroic element provided in the light path between the light source and the sensor element; a light detection unit comprising a light dispersion device and at least two photodetectors; wherein the light detection unit is configured for detecting light reflected by the dichroic element and light reflected or transmitted by the sensor element, after it has passed the light dispersion device, with the at least two photodetectors; and a control unit operatively coupled to the light detection unit. Further, a respective detection method for a gas in a fluid is provided, and electrical transformers employing the gas sensing system and method. US2013 / 175437 Al recites a hydrogen-sensing optical fibre provides rapid and reliable hydrogen detection and quantification, irrespective of temperature fluctuations. The hydrogensensing optical fibre does not exhibit significant irreversible hydrogen-induced attenuation losses after exposure to a hydrogen-rich atmosphere. EP 3 865 853 Al a method of validating a calibration of a sensor represented by a calibrated relationship between concentration of an analyte in a sample and measurements from a sensor of an optical property of a sensing substance, wherein the optical property of the sensing substance has a spectrum that varies with the concentration of the analyte in the sample, and the spectrum has an isosbestic wavelength (Xi) at which the optical property does not vary with concentration of the analyte, the method comprising making measurements of the optical property at three or more wavelengths of light while the sensing substance is exposed to the sample, determining whether the measurements of the optical property are inconsistent with the calibrated relationship, and outputting a warning signal in response to the measurements of the optical property being inconsistent with the calibrated relationship.

[0011] The present inventors developed a sensor, aiming at enhancing the performance of intensity -modulated sensors while preserving the simplicity and cost-advantages of an intensity-modulated sensor.

[0012] The present invention therefore relates to a sensor for detecting a chemical species and for mitigating drift and further aspects thereof, which overcomes one or more of the above disadvantages, without compromising functionality and performance.

[0013] SUMMARY OF THE INVENTION

[0014] It is an object of the invention to overcome one or more limitations of the sensors for detecting a concentration of chemical species and for mitigating drift of the prior art and at the very least to provide an alternative thereto. The present exemplary thin film metal hydride based optical hydrogen sensors provide an attractive option to sense hydrogen in a variety of conditions and have an attractive safety benefit over other methods of detection: They do not require the presence of electrical leads near the sensing area. These exemplary sensors rely on a change of the optical properties arising from a change in the hydrogenation of the metal hydride sensing layer in response to a different partial hydrogen pressure in the environment of the sensor. The response, depending on the hydrogen pressure, can be well within one second, such as within 0.1 second. Also the present thin-film device is very stable over time. Reference can be made to WO 2022 / 098230 Al of the same applicant for further details of certain aspects of sensor material and so on.

[0015] In a first aspect, the invention relates to a sensor (1) for detecting a concentration of chemical species and for mitigating drift, that is, for compensating drift or / or minimizing consequences of drift, comprising at least one electromagnetic radiation emitter (20), an electromagnetic input transmitter (15) for transmitting radiation of at least two electromagnetic frequencies ( / i, fi) from the at least one emitter of electromagnetic radiation to a chemical species sorbing material (12) at the same time, typically three- 220electromagnetic frequencies, in particular 23- 218electromagnetic frequencies, more in particular 24- 215electromagnetic frequencies, such as 25- 212electromagnetic frequencies, at least one electromagnetic output transmitter (16) for transmitting electromagnetic response from the chemical species sorbing material to an at least one electromagnetic radiation characteristics detector (22), wherein the sensor is configured such that radiation of a first electromagnetic frequency and radiation of a second electromagnetic frequency follow a trajectory from the electromagnetic emitter (20) to the chemical species sorbing material (12), and then to the at least one electromagnetic radiation characteristics detector (22), the chemical species sorbing material (12), that is, a material that is capable of absorbing, adsorbing, intercalating, or has any other interaction with the chemical species, and combinations thereof, wherein at least one electromagnetic characteristics of the sorbing material changes with an amount of chemical species sorbed, wherein the at least one electromagnetic characteristics is selected from an optical characteristics, and electrical characteristics, and from a magnetic characteristics, and wherein, as a function of chemical species concentration in the sorbing material, the at least one electromagnetic radiation characteristics for the first electromagnetic frequency is different from the at least one electromagnetic radiation characteristics of the second electromagnetic frequency, and a comparator (30), wherein the comparator is configured to compare the at least one electromagnetic characteristics in the first electromagnetic frequency range with the at least one electromagnetic characteristics in the second electromagnetic frequency range for the at least an amount of chemical species absorbed. It is noted that the present sensor may also be used as a detector, e.g. for establishing presence or absence (below a detection limit) of the chemical species. The present method is in particular suited for mitigating drift, and further aspects. In view of the present trajectory, the at least one emitter 20, the at least one transmitter 16, and the chemical species sorbing material 12, provide substantially the same physical environment to the at least two electromagnetic frequencies used. As such, a trajectory for a first electromagnetic frequency is substantially the same as for a second electromagnetic frequency, apart from possible minor differences, such as those caused by e.g. refraction. Based on the special characteristics of the present sorption material, having different responses for different frequencies to changes in sorpted chemical species, the present sensor is developed, as is the present method. Typically, the change is considered to relate to a noticeable change, or likewise, a detectable change.

[0016] In a second aspect the present invention relates to a method for operating a sensor according to the invention, comprising providing a first measurement mi,i of an at least one radiation characteristics C( / i,[c] i)ti of a first electromagnetic frequency fi at a first concentration of chemical species [c]i at a first point in time ti, and providing a first measurement m2,i of an at least one radiation characteristics C( / 2,[c] i) ti of a second electromagnetic frequency at said first concentration of chemical species [c]i at a first point in time ti, providing a second measurement mi, 2 of the at least one radiation characteristics C( / i, [c]2)ti of a first electromagnetic frequency fi at a second point in time t2, and providing a second actual measurement m2,2actof the at least one radiation characteristics C( / 2,[c]2)ti of a second electromagnetic frequency fi at a second point in time t2, resolving, based on an optional difference |mi,i- m2,i| between the first measurement mi,i and the second measurement m2,i, an according chemical species concentration difference Acyi = (second concentration of chemical species [c]2 / i - first concentration of chemical species [c]i / i|, based on the optional chemical species concentration difference Ac / i, resolving an expected value Ac / 2expfor the second measurement m2,2expof the at least one radiation characteristics C( / 2,[c]2)ti of a second electromagnetic frequency fi at a second point in time t2, resolving an optional measurement difference |m2,2exp- m2,2act| between the expected value ni2,2expand the actual value m2,2act, based on the size of measurement difference resolving a change, wherein the change is selected from a change in concentration of chemical species, sensor drift, measurement error, and a combination thereof, in particular wherein the sensor drift of the first frequency is correlated to the sensor drift of the second frequency, more in particular > 80% correlated, such as > 90% correlated. It may be considered that the present invention measures a response at a first point in time for at least two frequencies, the response reflecting a concentration of a first concentration of a chemical species, and at a later point in time, repeats the measurement in view of a second concentration of the chemical species, and so on. By cleverly comparing obtained responses, between frequencies, and between (sequential) measurements, it is possible to accurately determine the origin of an optional change is response characteristics. The present method may be repeated over and over, e.g. providing a multitude of measurements. Assumingly, the second concentration is different from the first, but if not, the measured difference will be close to zero, an optional remaining difference possibly caused by factors as device drift, and inherent limited measurement accuracy, leading to measurement errors, even if they might be close to zero or negligible. As the trajectories of the first and second frequency are substantially the same (see also above), it is considered that e.g. drift for the first frequency and for the second frequency is correlated, as is in fact shown in the experimental results. As such, a measurement difference can be (partly) attributed to factors a drift, sorpted concentration difference, and measurement error, with statistical accuracy. Such may also be repeated / used for some or all of the previous measurements. The first point in time, or any point in time, is typically considered to relate to substantially the same point is time, that is, either exactly the same point in time, such as by providing two or more frequencies by the present electromagnetic radiation emitter at the same point in time, or by switching between frequencies in a small switching time domain, such as within 300 msec, typically within 100 msec, such as within 1 msec.

[0017] Typically a calibration curve may be used. Also, the method can be performed for a multitude of frequencies. For instance, in the case of light, having frequencies ranging from about 30 PHz (Extreme UltraViolet) to 30 GHz (Far InfraRed), small frequency steps may be used; in terms of wavelength, wavelength steps of 2 nm are not much of a practical issue.

[0018] In a further aspect the present invention relates to a computer program, electronic circuit, or integrated circuit for carrying out the method of the present invention, in particular wherein the computer program is loaded on a processor or in a memory, in particular wherein the computer program comprising instructions for instruct the sensor to provide a first measurement mi,i of an at least one radiation characteristics C( / i, [c] i)u of a first electromagnetic frequency fi at a first concentration of chemical species [c]i at a first point in time ti, and to provide a first measurement m2,i of an at least one radiation characteristics C( / 2,[c]i)ti of a second electromagnetic frequency fz at said first concentration of chemical species [c]i at a first point in time ti, instruct the sensor to provide a second measurement mi, 2 of the at least one radiation characteristics C( / i, [c]2)ti of a first electromagnetic frequency fi at a second point in time t2, and to provide a second actual measurement mi ' of the at least one radiation characteristics C( / 2,[c]2)ti of a second electromagnetic frequency fz at a second point in time t2, resolving, based on an optional difference |mi,i- m2,i| between the first measurement mi,i and the second measurement m2,i, an according chemical species concentration difference Acyi = (second concentration of chemical species [c]2 / i - first concentration of chemical species [c]i / i|, based on the optional chemical species concentration difference Ac / i, to resolve an expected value Ac 2cxpfor the second measurement m2,2expof the at least one radiation characteristics C( / 2,[c]2)ti of a second electromagnetic frequency fz at a second point in time t2, resolving an optional measurement difference |m2,2exp- m2,2act| between the expected value ni2,2expand the actual value m2,2act, based on the size of measurement difference to resolve a change, wherein the change is selected from a change in concentration of chemical species, sensor drift, measurement error, and combinations thereof. Such may also be repeated / used for some or all of the previous measurements.

[0019] In a further aspect the present invention relates to a device comprising a hydrogen sensor according to the invention, for monitoring a hydrogen pressure or concentration, wherein the device is selected from an electro-magnetic transformer, a hydrogen storage device, and a battery. Inventors have demonstrated that an opposite response of different wavelengths to a change in hydrogen concentration can be used effectively to enhance the sensing performance and in particular mitigate drift. The fact that the optical transmission of a sensing material, such as a Ta-based sensing material, is reduced for blue / green light when it is exposed to hydrogen while in the same case the transmission increases for (near) infrared light can be used to differentiate between a changing hydrogen concentration and a drifting lamp: whereas the blue / green and infrared signal of a drifting light source is positively correlated, a change in hydrogen concentration causes two negatively correlated signals. While the presented approach is simple in nature, yet effective, as it can mitigate the drift on the signal by a factor of 5, the wavelength-dependent optical response allows for more advanced approaches to further improve the signal including incorporating temperature-effects and using artificial intelligence. An algorithm in combination with a sensing stack with the property of opposite responses to hydrogen for different wavelengths substantially enhances the performance of intensity -modulated hydrogen sensors while preserving the simplicity and cost advantages of an intensity-modulated sensor.

[0020] The present invention provides a solution to one or more of the above mentioned problems and overcomes drawbacks of the prior art.

[0021] DETAILED DESCRIPTION OF THE INVENTION

[0022] In an exemplary embodiment of the present sensor optical characteristics is selected from transmittance, from reflectance, from plasmon resonance (plasmons are considered to relate to collective oscillations of the free electron gas density, which, at optical frequencies, can couple with a photon to create a quasiparticle called a plasmon polariton), and from absorbance, in particular transmittance, and / or wherein the electrical characteristics is selected from dielectric constant, from density, from capacitance, from electrical resistance, from inductance, from conductance, from permittivity, from susceptibility, and from permeability, and wherein the magnetic characteristics is selected from magnetic flux density, from magnetic flux, from magnetic field strength.

[0023] In an exemplary embodiment the present sensor is configured for detecting a chemical species selected from CO, CO2, CH4, and hydrogen.

[0024] In an exemplary embodiment the present sensor comprises a frequency radiation reflector, such as an optical mirror (14), in particular a micro-mirror, and / or wherein the sensor comprises a catalyst (19), in particular a chemical species catalyst.

[0025] In an exemplary embodiment the present trajectory further comprises a trajectory part through the absorbing material to the frequency radiation reflector, and back to the electromagnetic output transmitter. That is, for instance a transmittance of the chemical species sorbing material may be characterized.

[0026] In an exemplary embodiment of the present sensor the electromagnetic output transmitter and electromagnetic input transmitter are the same.

[0027] In an exemplary embodiment of the present sensor the chemical species sorbing material comprising an alloy Mi.yAxBz, wherein y=x+z, and wherein B is optionally present, wherein the metal M is selected from Group V elements, and at least one alloying element A being selected from elements with a cubic unit cell and a rectangular cell, such as Pd, Ni, Pt, Ru, Rh, and combinations thereof, wherein hydrogenation of the sensing material is tuneable by changing an amount x of the at least one alloying element in the alloy Mi-yAxBz, wherein the at least one alloying element A is present in an amount of x=0.01-35 atom%; and wherein the at least one electromagnetic radiation characteristics detector 22 is a chemical species sorbing material read-out system 22, wherein the read-out system is selected from optical read-out systems, dielectric read-out systems, electro-magnetic read-out systems, such as an electrical current detector, an electrical voltage detector, an electrical resistance detector, and a transistor, and combinations thereof. The term “alloy” is used to describe a mixture of atoms. Therein a primary constituent is a metal. This primary constituent is first mentioned. The primary metal may be referred to as the base, the matrix, or the solvent. The secondary (and further) constituents are often called solutes. If there is a mixture of only two types of atoms it is called a binary alloy. If there are three types of atoms it is called a ternary alloy, etc. When referring to an alloy one typically uses the primary atom, e.g. a Mg-alloy comprises Mg as primary atom. In view of the primary atom forming the matrix, one can not permutate the order of elements of an alloy, as then the nature of the alloy would change, in this case from one metal to another. Put different, a Mg-alloy is not a Ca alloy, even if the Mg-alloy would comprise Ca as a further constituent.

[0028] In an exemplary embodiment of the present sensor the chemical species sorbing material (12) is provided as a thin layer, as a part of a thin layer, as nanoparticles, as microparticles, as a patterned nanosheet, as a nanowire, or a combination thereof.

[0029] In an exemplary embodiment of the present sensor the chemical species sorbing material is provided on a substrate (10), or on an adhesion layer (11) which is provided on the substrate (10), or is incorporated in an embedding material, such as a matrix material, or is incorporated in a fibre (15,16), or is deposited on a fibre (15,16), or a combination thereof. In the description and claims the term “on”, as in a first layer on a second layer, includes the first layer being directly on the second layer, the first layer being above the second layer, or the first layer being above the second layer with an intermediate layer in between or with several intermediate layers in between, wherein the first layer may be partly or fully provided on the second layer.

[0030] In an exemplary embodiment of the present sensor the metal M is selected from V, Nb, Ta, and alloys thereof, preferably Ta.

[0031] In an exemplary embodiment of the present sensor the chemical species sorbing material has a thickness or cross-section in the range of 1.5-2000 nm, preferably 5-200 nm, more preferably 10-100 nm.

[0032] In an exemplary embodiment of the present sensor the chemical species sorbing material further comprises at least one second alloying element Bz, wherein B is selected from metals, preferably from Pd, Ni, Pt, Ru, Rh, and combinations thereof, wherein hydrogenation of the sensing material is tuneable by changing an amount z of the at least one second alloying element in the alloy Mi-yAxBz, wherein the at least one alloying element B is present in an amount of 0.01-5 atom%, and / or further comprising (14) a capping layer between the chemical species sorbing material and a protective layer,

[0033] In an exemplary embodiment of the present sensor the capping layer preferably comprises one or more of Pd, Pt, Ag, Au, Ni, Cu, Ru, and Rh, preferably Pd and Au and optionally Cu.

[0034] In an exemplary embodiment of the present sensor the capping layer has a thickness in the range of 1.5-2000 nm, preferably 2-200 nm, more preferably 3-20 nm.

[0035] In an exemplary embodiment the present sensor further comprises (18) a protective layer provided on the sensing material, such as in the form of a layer, preferably wherein the protective layer has a thickness in the range of 5-2000 nm, preferably 10-200 nm, more preferably 10-80 nm, preferably wherein the protective layer comprises a polymer, such as polytetrafluoroethylene (PTFE), and / or polymethyl methacrylate (PMMA).

[0036] In an exemplary embodiment the present sensor comprises at least one intermediate adhesion layer (11,13).

[0037] In an exemplary embodiment of the present sensor the intermediate layer (11,13) each individually is selected from Ti, Cr, Au, or a combination thereof.

[0038] In an exemplary embodiment of the present sensor the intermediate layer (11,13) each individually has a thickness in the range of 1.5-400 nm, preferably 1.5-50 nm, more preferably 1.5-10 nm.

[0039] In an exemplary embodiment of the present sensor the chemical absorption of the sensing material is tuneable from 10'1Pa (0.001 mbar) - 108Pa (1,000,000 mbar), at a temperature of 301K.

[0040] In an exemplary embodiment of the present sensor a / the protective layer and a / the capping layer are combined.

[0041] In an exemplary embodiment of the present sensor a concentration of the at least one alloying element A in the optical sensing layer varies continuously from 0.01 atom% to a maximum atom %, wherein the maximum atom % A is in a range from 10-35 atom%.

[0042] In an exemplary embodiment the present sensor comprises at least one optical sensing layer, such as at least two layers, each layer comprising a sensing material, and / or comprising at least two sensing material domains, each domain comprising a different sensing material, wherein the domain has a size of 0.01-108pm2.

[0043] In an exemplary embodiment of the present sensor the device is for use in combination with photons with a frequency in a range of 200-1200 nm.

[0044] In an exemplary embodiment of the present sensor the sensing material and capping layer exhibit optical interference at at least one given frequency.

[0045] In an exemplary embodiment of the present sensor in the chemical species sorbing material, a difference between the at least one radiation characteristics C( / i,[c]i) of a first electromagnetic frequency fi at a first concentration of chemical species [c]i and the at least one radiation characteristics C( / 2,[c]i) of a second electromagnetic frequency at said first concentration of chemical species [c] i, and a difference between the at least one radiation characteristics C( / i,[c]2) of a first electromagnetic frequency fi at a second concentration of chemical species [c]2 and the at least one radiation characteristics C( / 2,[c]2) of a second electromagnetic frequency range at said second concentration of chemical species [c]2 is not zero ([C( / i,[c]i)- C( / 2,[c]i)]-[C( / i,[c]2)- C( / 2,[c]2)] 0), in particular wherein said difference is not from [- 1 O’5, 1 O’5] * a value of the at least one electromagnetic radiation characteristic, in particular wherein the first concentration of chemical species [c] i, is at least 2 times larger or smaller than the second concentration of chemical species [c]2, in particular more than 102times larger or smaller, even more in particular 103times larger or smaller.

[0046] In an exemplary embodiment of the present sensor in the chemical species sorbing material, at a first frequency, the optical characteristics increases in magnitude as a function of amount of chemical species, and, at a second frequency, that optical characteristics decreases in magnitude as a function of amount of chemical species, wherein the first frequency is different from the second frequency.

[0047] In an exemplary embodiment the present sensor comprises an optical transmitter (16), such as an optical fibre, wherein the sensing layer is located at a tip of the optical transmitter, that is at a top end thereof, and / or wherein the sensing layer is located at a longitudinal side of the optical transmitter.

[0048] In an exemplary embodiment the present method measuring said difference is repeated at least two times at a time ti wherein ie [2,n], wherein n may be from 24-250, in particular from 26-240, more in particular from 28-230, even more in particular from 210-225, such as in particular from 215-220, in particular at regular intervals, such as intervals of 10-1000 seconds.

[0049] In an exemplary embodiment the present method in case of sensor drift, a sensor measurement value sensor and / or concentration is not or partially updated, and / or wherein resolving expected value for the second measurement m2,2expof the at least one radiation characteristics C( / 2,[c]2)ti of a second electromagnetic frequency fz at a second point in time t2 is based on an electromagnetic radiation characteristics intensity versus concentration calibration curve. Such may also be the case for some or all of the previous measurements.

[0050] The invention will hereafter be further elucidated through the following examples which are exemplary and explanatory of nature and are not intended to be considered limiting of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims.

[0051] FIGURES

[0052] Figure 1 a shows a schematic representation of the present device.

[0053] Figure 1 b shows a stack of layers.

[0054] Fig. 1c shows an exemplary prior art device.

[0055] Figure 2 shows wavelength-dependent optical transmission (a) and reflectivity (b).

[0056] Figures 3 a-c show wavelength-dependent optical transmission exposed to a constant hydrogen concentration and shows the data: uncorrected (a), algorithm-corrected (b) and by subtraction (c).

[0057] Figures 4 a-d and Figures 5 a-d demonstrate wavelength-dependent optical transmission exposed to alternating hydrogen concentrations: uncorrected and algorithm-corrected (figure 4) as well as uncorrected and by subtraction (figure 5).

[0058] Figures 6 illustrates how the method of operating a sensor of the invention works.

[0059] DETAILED DESCRIPTION OF FIGURES

[0060] In the figures:

[0061] I Sensor

[0062] 10 substrate

[0063] I I adhesion layer

[0064] 12 chemical species absorbing material (e.g. Ta-alloy)

[0065] 13 adhesion layer

[0066] 14 capping layer (mirror)

[0067] 15 electromagnetic input transmitter (multi-mode fibre)

[0068] 16 electromagnetic output transmitter (multi-mode fibre)

[0069] 17 fibre cladding

[0070] 18 protective layer

[0071] 19 catalyst layer (not shown)

[0072] 20 electromagnetic radiation emitter, e.g. lamp

[0073] 21 optional beam splitter

[0074] 22 electromagnetic radiation characteristics detector, e.g. CCD camera

[0075] 23 filter, e.g. frequency filter, such as high-pass filter and / or low-pass filter 30 comparator

[0076] Figure la shows an example of a schematic representation of an exemplary embodiment of the present device. The light, originating from a LED light source (20) that has maxima in both the green ( / 2) and near infra-red ( / I) spectrum, is coupled into the fibre and transported through the electromagnetic input transmitter (15) to the tip of the optical fibre sensor (see zoomed cutout on the right of fig. la). The light is partially reflected by the layer stack deposited on the tip of the fibre and then passed back through the fibre, the splitter (21) and the electromagnetic output transmitter (16). The signal is subsequently measured by e.g. a charge-coupled device (CCD) (22) where halve of the pixels are covered by a low- and high-pass wavelength filter to measure both the red and green intensity. Upon a change in pressure of a chemical species such as hydrogen, the hydrogenation of the sensing layer (12) (20-100 nm) changes, which in turn results in a change of the optical properties and thus the amount of light reflected by the layer stack and detected by a photodetector. The sensing layer is cleaved to the substrate by an adhesion layer that is often made of titanium. To prevent oxidation of the sensing layer and to catalyse the hydrogen adsorption a capping layer (14) (typically a Pd-based alloy) is used. The obtained green ( / 2) and near IR light ( / I) signals can be processed digitally or analogue to obtain the reading for the hydrogen concentration. In summary, figure la illustrates a sensor according to an exemplary embodiment of the present invention, comprising at least one electromagnetic radiation emitter (20), an electromagnetic input transmitter (15) for transmitting radiation of at least two electromagnetic frequencies ( / i, fi) from the at least one emitter of electromagnetic radiation to a chemical species sorbing material (12), at least one electromagnetic output transmitter (16) for transmitting electro-magnetic response from the chemical species sorbing material to an at least one electromagnetic radiation characteristics detector (22). The sensor is configured such that radiation of a first electromagnetic frequency and radiation of a second electromagnetic frequency follow a trajectory from the electromagnetic emitter (20) to the chemical species sorbing material (12), and then to the at least one electromagnetic radiation characteristics detector (22). The sensor also comprises a comparator (30), wherein the comparator is configured to compare the at least one electromagnetic characteristics in the first electromagnetic frequency range with the at least one electromagnetic characteristics in the second electromagnetic frequency range for the at least an amount of chemical species absorbed.

[0077] Figure 1 b demonstrates a stack of layers, comprising a substrate (10), an adhesion layer (11), a chemical species absorbing material (12), an adhesion layer (13), a capping layer (14), and a protective layer (18).

[0078] Figure 1c demonstrates a stack of layers, with reference to the present invention comprising a substrate 10, a chemical species absorbing material 12, a capping layer 14, a electromagnetic input transmitter 15, and a fibre cladding 17, and for EP 3 385 702 Al (left numerals) an optical fibre 15 and cladding 18, a dichroic element 35, and a sensing layer 22. The dichroic element is configured to reflect light in narrow first frequency, the thicker line, and transmitting a second frequency, the thinner line. The trajectories for the first and second parts of frequency SA are clearly not the same, the thinner line being longer than the thicker line. Light reflected by the sensing layer 22, being second spectral fraction SB, has a substantially similar as that of incoming light SA from the light source, but having different intensities. SB and DP relate to spectral fractions of the same frequency therefore.

[0079] Figure 2 shows wavelength-dependent optical transmission (a) and reflectivity (b) of a thin film with a 4 nm Ti adhesion layer, a 20 nm Tao.8sPdo.i2 sensing layer and a 30 nm Pdo.6Auo.4 capping layer when exposed stepwise to various concentrations of hydrogen in an Ar atmosphere at a constant pressure of 100 kPa (1 bar). The data is normalized to the optical transmission / reflectivity at CH2 = 0.001% / PH2 = 1 Pa.

[0080] Figures 3 a-c show wavelength-dependent optical transmission of a thin film with a 4 nm Ti adhesion layer, a 20 nm Tao.88Pdo.12 sensing layer and a 30 nm Pdo.eAuo.4 capping layer measured for about one week when exposed to a constant hydrogen concentration at room temperature. The data is normalized to the optical transmission / reflectivity at t = 0. Panel (a) shows uncorrected data, while panel (b) shows the data corrected with the algorithm. Panel (c) shows the subtraction of the optical response at = 935 nm from the one at A = 535 nm.

[0081] Figures 4 a-d demonstrate wavelength-dependent optical transmission of a thin film with a 4 nm Ti adhesion layer, a 20 nm Tao.ssPdo.n sensing layer and a 30 nm Pdo.eAuo.4 capping layer measured for about one week when exposed for 24 h to CH2 = 4%, and then alternating for 1 h to CH2 = 4% followed by one hour to CH2 = 0.005%. The data is normalized to the optical transmission at t = 0 h. Panel (a) shows uncorrected data, while panel (b) shows the data corrected with the algorithm. Panels (c) and (d) show a close-up of the first 40 h for the uncorrected and corrected data, respectively.

[0082] Figures 5a-d show optical transmission at = 935 nm subtracted from the one at = 535 nm of a thin film with a 4 nm Ti adhesion layer, a 20 nm Tao.8sPdo.i2 sensing layer and a 30 nm Pdo.eAuo.4 capping layer measured for about one week when exposed for 24 h to CH2 = 4%, during which at t = 12 h the optical fibers were deliberately moved. Subsequently, from t = 24 h onwards the sample was exposed for 1 h to CH2 = 4% followed by one hour to CH2 = 0.005%. The data is normalized to the optical transmission / reflectivity at t = 0 h. Panel (a) shows uncorrected data, while panel (b) shows the data corrected with the algorithm. Panel (c) and (d) show a close-up of the first 40 h for the uncorrected and corrected data, respectively.

[0083] Figures 6 a and b illustrate and explain how the method of operating a sensor as taught herein works. The following parameters are shown: ci: first concentration of chemical species at a first point in time (ti), C2: second concentration of chemical species at a second point in time (t2) fl first electromagnetic frequency: upper straight continuous line in figures 6 a-b, f2 second electromagnetic frequency: lower straight continuous line in figures 6-b. mi,i: first measurement related to ci and fl at ti, m2,i: first measurement related to ci and f2 at ti, mi, 2: second measurement related to C2 and fl at t2, m2, 2: second measurement related to C2 and f2 at t2.

[0084] Radiation characteristics are expressed as

[0085] C( / i,[c]i)ti, being radiation characteristics of a first electromagnetic frequency fi at a first concentration of chemical species [c] 1 at a first point in time ti,

[0086] C( / 2,[c]i)ti, being radiation characteristics of a second electromagnetic frequency fi at a first concentration of chemical species [c] 1 at a first point in time ti,

[0087] C( / i,[c]2)ti, being radiation characteristics of a first electromagnetic frequency fi at a second concentration of chemical species [c]2 at a second point in time t2,

[0088] C( / 2,[c]2)ti: being radiation characteristics of a second electromagnetic frequency fi at a second concentration of chemical species [c]2 at a second point in time t2.

[0089] Ac / i: is the delta or changed radiation characteristics for frequency fi between measurements and can be calculated as: second concentration of chemical species [c]2 / i - first concentration of chemical species [c]i / i.

[0090] Comparable to Acyi the Ac / 2 can be calculated for frequency fi as: second concentration of chemical species [c]2 / 2 - first concentration of chemical species [c] 1 / 2. For this second delta however an actual measured value (Ac / 2act) exists as well as an expected value (Ac / 2exp). Consequently, also the 1112.2 exists as an expected value (ni2,2exp) as well as an actual measured value (ni2,2act). Consequently, an optional measurement difference between the expected value ni2,2expand the actual value m2,2actcan be calculated as ni2,2exp- m2,2act. The method of operating a sensor as taught herein can now, based on figures 6 a and b, be further explained as a method, comprising providing a first measurement mi,i of an at least one radiation characteristics C( / i, [c] i)u of a first electromagnetic frequency fi at a first concentration of chemical species [c]i at a first point in time ti, and providing a first measurement m2,i of an at least one radiation characteristics C( / 2,[c]i)u of a second electromagnetic frequency at said first concentration of chemical species [c]i at a first point in time ti, providing a second measurement mi, 2 of the at least one radiation characteristics C( / i, [c]2)u of a first electromagnetic frequency fi at a second point in time t2, and providing a second actual measurement m2,2actof the at least one radiation characteristics C( / 2,[c]2)u of a second electromagnetic frequency fi at a second point in time t2, resolving, based on an optional difference |mi,i- m2,i| between the first measurement mi,i and the second measurement m2,i, an according chemical species concentration difference Acyi = (second concentration of chemical species [c]2 / i - first concentration of chemical species [c] 1 | / i, based on the optional chemical species concentration difference Ac / i, resolving an expected value Ac / 2expfor the second measurement m2,2expof the at least one radiation characteristics C( / 2,[c]2)u of a second electromagnetic frequency fi at a second point in time t2, resolving an optional measurement difference | ni2,2exp- m2,2act| between the expected value ni2,2expand the actual value m2,2act, based on the size of measurement difference resolving a change, wherein the change is selected from a change in concentration of chemical species, sensor drift, measurement error, and a combination thereof, in particular wherein the sensor drift of the first frequency is correlated to the sensor drift of the second frequency, more in particular > 80% correlated, such as > 90% correlated.

[0091] EXPERIMENTAL

[0092] 1.1. Sample preparation

[0093] The Tai-yPdy samples are composed of a 4 nm titanium adhesion layer, a 20 nm Tai- yPdy layer and a 30 nm cap layer to catalyze the hydrogen dissociation and recombination reaction and prevent the film from oxidation. In this particular example, the thin film sample was composed of a 4 nm titanium adhesion layer, a 20 nm Tao.8sPdo.12 sensing layer and a 30 nm Pdo.eAuo.4 capping layer to catalyze the hydrogen dissociation and recombination reaction and prevent the film from oxidation (nominal thicknesses), was deposited on 10 * 10 mm2 quartz substrates were used with a thickness of 0.5 mm and surface roughness <0.4 nm (Mateck GmbH, iilich, Germany) using magnetron sputtering.

[0094] The deposition was performed in 0.3 Pa of Ar by magnetron sputtering in an ultra- high vacuum chamber (AIA Int.) with a base pressure of 10-6Pa. During deposition, the substrates were rotated to enhance the homogeneity. The deposition rates of the layers were 0.10 nm s’1(125 W DC) for Ta, 0.05 nm s’1(100W DC) for Ti, 0.13 nm s’1(50 W DC) for Pd and 0.11 nm s-1(25 W DC) for Au. All targets have a diameter of 5.08 mm (2 inch) and a purity of at least 99.9% (Mateck GmbH, iilich, Germany). Before the deposition, the Ta target was pre-sputtered for 120 min to avoid possible contamination from the tantalum oxide and nitride layers present at the surface of the target.

[0095] The Tai-yPdy samples are composed of a 4 nm titanium adhesion layer, a 40 nm Tai- yPdy layer and a 10 nm cap layer to catalyze the hydrogen dissociation and recombination reaction and prevent the film from oxidation.1.2. Structural measurements

[0096] The sample was checked for layer thickness and quality with X-ray Reflectometry (XRR) and X-ray Diffraction (XRD) using a Bruker D8 Discover (Cu-Ka X = 0.1542 nm) equipped with a LYNXEYE XE detector (Bruker AXS GmbH, Karlsruhe, Germany). XRD measurements were performed with a Gobel mirror and a 0.6 mm fixed slit on the primary and two 0.6 mm slits on the secondary side with the detector operated in 0D mode. Differently, for the XRR measurements three 0.1 mm slits were used. The data were fitted with GenX3 to obtain estimates for the layer thickness, roughness and density of the thin films.

[0097] 1.3. Optical measurements

[0098] The optical measurements under hydrogen were performed in a flow setup. The setup consists of a small custom-made pressure cell with quartz windows to allow optical transmission. For the transmission measurements, one fiber was placed below and one above the cell. For the reflection measurements, two fibers were mounted under an angle of 30°. In both cases, one fiber transports the light from the Ocean Optics HL-2000-FSHA halogen light source (Ocean Insights Orlando, FL, United States of America). The intensity of the light from the fiber can be adjusted with a Thorlabs Cage System Iris Diaphragm with a diameter of 0.8-20 mm (Thorlabs, Inc, Newton, New Jersey, United States of America), after which it is focused on the sample with lenses. The reflected or transmitted light was coupled into the other optical fiber connected to an Ocean Optics Maya2000 Pro Series Spectrometer (Ocean Insights Orlando, FL, United States of America). To set the flow and hydrogen concentration, three flow-controllers were used with a maximum flow of 0-100 s.c.c.m., 0-200 s.c.c.m. and 0-250 s.c.c.m. (Thermal Mass GF Series, Brooks Instrument, Hatfield, PA, United States of America). One of these flow-controllers was connected to the Ar gas network (5N purity), the other two to either gas mixtures of 0.1% H2 and 4% H2 in Ar and 100% H2 ( CH2ICH2 < 2%, Linde Gas Benelux BV, Dieren, the Netherlands). The set-up was located inside a lab of which the temperature was 19.5 °C that fluctuated by ± 0.5 °C.

[0099] 2. Optical Response of the Sensing material

[0100] Figure 2 shows that an opposite response to hydrogen is realized for a Tao.88Pdo.12 sensing layer, both in transmission and reflection. For a thin film with a 4 nm titanium adhesion layer, a 20 nm Tao.88Pdo.12 sensing layer and a 30 nm Pdo.eAuo.4 capping layer, the film was exposed stepwise to different hydrogen concentrations in argon using a flow set-up while measuring the optical transmission and reflection (under 30°) with a halogen light source and spectrometer at room temperature. Figure 2 demonstrates that the optical transmission decreases monotonously for blue and green light (X = 450, 500 nm) with increasing hydrogen concentration by about 6% between CH2 = 0.001% and 100%. Differently, the optical transmission increases monotonically for near infra-red light by up to 18% between CH2 = 0.001% and 100%, while a substantial decrease is seen in reflectivity. Similar responses, yet in opposite direction are found for reflection.

[0101] Figure 3(a) shows that a drift is visible in the signal, including some spikes: over a week time, a maximum change in transmission of about 0.4%. Especially for the green light, this is about 10% of the entire signal change when the hydrogen concentration would be increased from 0.001% to 100% and would thus result in a substantial measurement error. This highlights the large effect drift can have on the sensor's reading. Most importantly, Figure 3(a) shows that the signal of the different wavelengths are strongly correlated, following the same spikes in the data. Indeed, the correlation between the various green / blue light wavelengths on the one hand, and the infra-red light varies between 0.9 and 0.98, being the highest between A = 535 and 895 nm light. An algorithm was considered that averages every 150 seconds the optical response and then checks whether the change in optical response of both the X = 535 and 895 nm channel is positive or negative. For example, if both changes are positive or negative, then the algorithm identifies this as drift and does not update the reading. If the two signals show a positive and a negative change, or vice versa, then the algorithm considers it as a change in hydrogen concentration and updates the response. The time period (or periods) should be chosen long enough to ensure that the changes in the signal are not dominated by noise.

[0102] Figure 3(b) shows the corrected response by the algorithm and illustrates that the algorithm can mitigate the drift of the signal by up to a factor of 5.6. This is further detailed in Tables 1 and 2 that report that for e.g. X = 535 nm light, the algorithm reduces the maximum deviation from 0.39% to 0.07%, i.e. an improvement by a factor of 5.6.

[0103] Another approach was to subtract the response at = 935 nm from the one at = 535 nm. Figure 3(c) shows the result: compared to the individual responses the drift is reduced, by up to a factor of 1.5 using this 'difference method'.

[0104] In order to see how both responses are affected when hydrogen pressures are changed or when deliberately the optical fibers were moved connected to both the light source and the spectrometer, a new measurement was performed. First, the sample was left for 24 h at CH2 = 4% while at t = 12 h, the optical fibers were moved. Subsequently, from t = 24 h onwards, the sample was exposed for 1 h to CH2 = 4% followed by one hour at CH2 = 0.005%, a procedure which was repeated for 4 days. The corresponding results can be found in Fig. 4 for the correlation-based approach and in Fig. 5 for the approaches based on taking the difference in the signal between two wavelengths. Fig. 4 shows that the algorithm can minimize the drift, also between the pressure step. In addition, it filters the abrupt movement of the fiber at t = 12 h completely, reducing the overall drift from 0.40% to 0.06% for = 535 nm and from 0.15% to 0.05% for = 935 nm. For the subsequent steps that were applied after t = 24 h a reduction is seen in the maximum deviation (lowest - highest value recorded of the CH2 = 4% level) from 0.64% to 0.3% with respect to just considering = 535 nm and from 0.6% to 0.5% when only k = 935 nm would be considered. Similar improvements are observed for the CH2 = 0.005% level. For the difference based algorithm, it was observed that it cannot completely remove the change in signal due to the abrupt movement of the fiber at t = 12 h as this change is of a different magnitude for the two wavelengths. However, the algorithm does reduce the overall drift substantially by a factor of 2 during the first 24 h. When alternating pressure steps are applied, the difference approach is very effective: it reduces the overall drift during the period 24 < t < 130 h to 0.28%, i.e. by a factor 2.0, significantly more than the factor 1.2 for the correlation based mechanism at = 935 nm.

[0105] In conclusion, it was shown that an opposite response of different wavelengths to a change in hydrogen concentration can be used effectively to enhance the sensing performance and in particular reduce drift. Indeed, the fact that the optical transmission of a Tabased sensing material is reduced for blue / green light when it is exposed to hydrogen while in the same case the transmission increases for (near) infrared light can be used to differentiate between a changing hydrogen concentration and a drifting lamp: whereas the blue / green and infrared signal of a drifting light source is positively correlated, a change in hydrogen concentration causes two negatively correlated signals. While the presented approach is simple in nature, yet effective, as it can reduce the drift on the signal by a factor of 5, the wavelength-dependent optical response allows for more advanced approaches to further improve the signal including incorporating temperature-effects and using artificial intelligence. The algorithm in combination with a sensing stack with the property of opposite responses to hydrogen for different wavelengths substantially enhances the performance of intensity -modulated hydrogen sensors while preserving the simplicity and cost advantages of an intensity- modulated sensor. For experimental results reference is also made to an article submitted for publication, titled “Algorithm to suppress drift for micro-mirror and other intensity modulated hydrogen sensors”, which article and its content is incorporated by reference.

Claims

CLAIMS1. A sensor (1) for detecting a concentration of chemical species and for mitigating drift, comprising-at least one electromagnetic radiation emitter (20), an electromagnetic input transmitter (15) for transmitting radiation of at least two electromagnetic frequencies ( / i, fi) from the at least one emitter of electromagnetic radiation to a chemical species sorbing material (12) at the same point in time,-at least one electromagnetic output transmitter (16) for transmitting electromagnetic response from the chemical species sorbing material to an at least one electromagnetic radiation characteristics detector (22),-wherein the sensor is configured such that radiation of a first electromagnetic frequency and radiation of a second electromagnetic frequency follow a same trajectory from the electromagnetic emitter (20) to the chemical species sorbing material (12), and then to the at least one electromagnetic radiation characteristics detector (22),-the chemical species sorbing material (12), wherein at least one electromagnetic characteristics of the sorbing material changes with an amount of chemical species sorbed, wherein the at least one electromagnetic characteristics is selected from an optical characteristics, and electrical characteristics, and from a magnetic characteristics, and wherein, as a function of chemical species concentration in the sorbing material, the at least one electromagnetic radiation characteristics for the first electromagnetic frequency is different from the at least one electromagnetic radiation characteristics of the second electromagnetic frequency, and-a comparator (30), configured for receiving input from the at least one electromagnetic radiation characteristics detector (22), wherein the comparator is configured to compare the at least one electromagnetic characteristics in the first electromagnetic frequency range with the at least one electromagnetic characteristics in the second electromagnetic frequency range for the at least an amount of chemical species absorbed.

2. The sensor according to claim 1, wherein optical characteristics is selected from transmittance, from reflectance, from plasmon resonance, and from absorbance, in particular transmittance, and / or wherein the electrical characteristics is selected from dielectric constant, from density, from capacitance, from electrical resistance, from inductance, from conductance, from permittivity, from susceptibility, and from permeability, and wherein the magnetic characteristics is selected from magnetic flux density, from magnetic flux, from magnetic field strength.

3. The sensor according to any of claims 1-2, wherein the sensor is configured for detecting a chemical species selected from CO, CO2, CH4, and hydrogen.

4. The sensor according to any of claims 1-3, wherein the sensor comprises a frequencyradiation reflector, such as an optical mirror (14), in particular a micro-mirror, and / or wherein the sensor comprises a catalyst (19), in particular a chemical species catalyst, and / or wherein the trajectory further comprises a trajectory part through the absorbing material to the frequency radiation reflector, and back to the electromagnetic output transmitter, and / or wherein the electromagnetic output transmitter and electromagnetic input transmitter are the same.

5. The sensor according to any of claims 1-4, wherein the chemical species sorbing material comprising an alloy Mi.yAxBz, wherein y=x+z, and wherein B is optionally present, wherein M is a metal selected from Group V elements, and at least one alloying element A being selected from elements with a cubic unit cell and a rectangular cell, such as Pd, Ni, Pt, Ru, Rh, and combinations thereof, wherein hydrogenation of the sensing material is tuneable by changing an amount x of the at least one alloying element in the alloy Mi-yAxBz, wherein the at least one alloying element A is present in an amount of x=0.01-35 atom%; and wherein the at least one electromagnetic radiation characteristics detector (22) is a chemical species sorbing material read-out system (22), wherein the read-out system is selected from optical read-out systems, dielectric read-out systems, electro-magnetic read-out systems, such as an electrical current detector, an electrical voltage detector, an electrical resistance detector, and a transistor, and combinations thereof.

6. The sensor according to any of claims 1-5, wherein the chemical species sorbing material (12) is provided as a thin layer, as a part of a thin layer, as nanoparticles, as microparticles, as a patterned nanosheet, as a nanowire, or a combination thereof, and / or wherein the sensor comprises at least one of a substrate (10), an adhesion layer (11), an embedding material, a capping layer, a protective layer, and a fibre (15,16), wherein the chemical species sorbing material is provided on the substrate (10), or wherein the chemical species sorbing material is provided on the adhesion layer (11) which is provided on the substrate (10), or wherein the chemical species sorbing material is incorporated in the embedding material, such as a matrix material, or wherein the chemical species sorbing material is incorporated in the fibre (15,16), or wherein the chemical species sorbing material is deposited on the fibre (15,16), or a combination thereof, and / or wherein M is a metal selected from V, Nb, Ta, and alloys thereof, preferably Ta, and / or wherein the chemical species sorbing material has a thickness or cross-section in the range of 1.5-2000 nm, preferably 5-200 nm, more preferably 10-100 nm, and / or wherein the chemical species sorbing material further comprises at least one second alloying element Bz, wherein B is selected from metals, preferably from Pd, Ni, Pt, Ru, Rh, and combinations thereof, wherein hydrogenation of the sensing material is tuneable by changing an amount z of the at least one second alloying element in the alloy Mi.yAxBz, wherein the at least one alloying element B is present in an amount of 0.01-5 atom%, and / or further comprising (14) a capping layer between the chemical species sorbing material and a protective layer, wherein the capping layer preferably comprises one or more of Pd, Pt, Ag, Au, Ni, Cu, Ru,and Rh, preferably Pd and Au and optionally Cu, and / or wherein the capping layer has a thickness in the range of 1.5-2000 nm, preferably 2-200 nm, more preferably 3-20 nm, and / or further comprising (18) a protective layer provided on the sensing material, such as in the form of a layer, preferably wherein the protective layer has a thickness in the range of 5- 2000 nm, preferably 10-200 nm, more preferably 10-80 nm, preferably wherein the protective layer comprises a polymer, such as polytetrafluoroethylene (PTFE), or polymethyl methacrylate (PMMA), and / or comprising at least one intermediate adhesion layer (11,13), and / or wherein the intermediate layer (11,13) each individually is selected from Ti, Cr, Au, or a combination thereof, and / or wherein the intermediate layer (11,13) each individually has a thickness in the range of 1.5- 400 nm, preferably 1.5-50 nm, more preferably 1.5-10 nm, and / or wherein the chemical absorption of the sensing material is tuneable from 10'1Pa (0.001 mbar) - 108Pa (1,000,000 mbar), at a temperature of 301K, and / or wherein a / the protective layer and a / the capping layer are combined, and / or comprising at least one optical sensing layer (12), such as at least two layers, each layer comprising a sensing material, and / or wherein a concentration of the at least one alloying element A in the optical sensing layer varies continuously from 0.01 atom% to a maximum atom %, wherein the maximum atom % A is in a range from 10-35 atom%, and / or comprising at least two sensing material domains, each domain comprising a different sensing material, wherein the domain has a size of 0.01-108pm2, and / or wherein the device is for use in combination with photons with a frequency in a range of 200-1200 nm, and / or wherein the sensing material and capping layer exhibit optical interference at at least one given frequency.

7. The sensor according to any of claims 1-6, wherein, in the chemical species sorbing material, a difference between the at least one radiation characteristics C( / i,[c]i) of a first electromagnetic frequency fi at a first concentration of chemical species [c]i and the at least one radiation characteristics C( / 2,[c]i) of a second electromagnetic frequency at said first concentration of chemical species [c] i, and a difference between the at least one radiation characteristics C( / i,[c]2) of a first electromagnetic frequency fi at a second concentration of chemical species [c]2 and the at least one radiation characteristics C( / 2,[c]2) of a second electromagnetic frequency range at said second concentration of chemical species [c]2 is not zero ([C( / i,[c]i)- C( / 2,[c]i)]-[C( / i,[c]2)- C( / 2,[c]2)] 0), in particular wherein said difference is not from [- 10’5, 10’5] * a value of the at least one electromagnetic radiation characteristic, in particular wherein the first concentration of chemical species [c] i, is at least 2 times larger or smaller than the second concentration of chemical species [c]2, in particular more than 102times larger or smaller, even more in particular 103times larger or smaller.

8. The sensor according to any of claims 1-7, wherein, in the chemical species sorbing material, at a first frequency, the optical characteristics increases in magnitude as a function of amount of chemical species, and, at a second frequency, that optical characteristics decreases in magnitude as a function of amount of chemical species, wherein the first frequency is different from the second frequency.

9. The sensor according to any of claims 1-8, comprising an optical transmitter (16), such as an optical fibre, wherein the sensing layer (12) is located at a tip of the optical transmitter and / or wherein the sensing layer (12) is located at a longitudinal side of the optical transmitter.

10. A method for operating a sensor according to any of claims 1-9, comprising providing a first measurement mi,i of an at least one radiation characteristics C( / i, [c] i)u of a first electromagnetic frequency fi at a first concentration of chemical species [c]i at a first point in time ti, and providing a first measurement m2,i of an at least one radiation characteristics C( / 2, [c] i) ti of a second electromagnetic frequency at said first concentration of chemical species [c]i at the first point in time ti, providing a second measurement mi,2 of the at least one radiation characteristics C( / i,[c]2)ti of a first electromagnetic frequency fi at a second point in time t2, and providing a second actual measurement m2,2actof the at least one radiation characteristics C( / 2,[c]2)ti of a second electromagnetic frequency fi at the second point in time t2, resolving, based on an optional difference |mi,i- m2,i| between the first measurement mi,i and the second measurement m2,i, an according chemical species concentration difference Acyi = (second concentration of chemical species [c]2 / i - first concentration of chemical species [c]i l / b based on the optional chemical species concentration difference Ac / i, resolving an expected value Ac / 2expfor the second measurement ni2,2expof the at least one radiation characteristics C( / 2,[c]2)ti of a second electromagnetic frequency fi at a second point in time t2, resolving an optional measurement difference | ni2,2exp- m2,2act| between the expected value m2,2expand the actual value ni2,2act, based on the size of measurement difference resolving a change, wherein the change is selected from a change in concentration of chemical species, sensor drift, measurement error, and a combination thereof, in particular wherein the sensor drift of the first frequency is correlated to the sensor drift of the second frequency, more in particular > 80% correlated, such as > 90% correlated.

11. The method according to claim 10, wherein measuring said difference is repeated at least two times at a time ti wherein ie[2,n], in particular at regular intervals, such as intervals of 10-1000 seconds.

12. The method according to any of claims 10-11, wherein, in case of sensor drift, a sensor measurement value sensor and / or concentration is not or partially updated, and / or wherein resolving expected value for the second measurement m2,2expof the at least oneradiation characteristics C( / 2,[c]2)ti of a second electromagnetic frequency fz at a second point in time t2 is based on an electromagnetic radiation characteristics intensity versus concentration calibration curve.

13. A computer program, electronic circuit, or integrated circuit for carrying out the method of any of claims 10-12, in particular wherein the computer program is loaded on a processor or in a memory, in particular wherein the computer program comprising instructions for instruct the sensor to provide a first measurement mi,i of an at least one radiation characteristics C( / i, [c] i)u of a first electromagnetic frequency fi at a first concentration of chemical species [c]i at a first point in time ti, and to provide a first measurement m2,i of an at least one radiation characteristics C( / 2,[c]i)ti of a second electromagnetic frequency fz at said first concentration of chemical species [c]i at the first point in time ti, instruct the sensor to provide a second measurement mi, 2 of the at least one radiation characteristics C( / i, [c]2)ti of a first electromagnetic frequency fi at a second point in time t2, and to provide a second actual measurement ni2,2actof the at least one radiation characteristics C( / 2,[c]2)ti of a second electromagnetic frequency fz at the second point in time t2, resolving, based on an optional difference |mi,i- m2,i| between the first measurement mi,i and the second measurement m2,i, an according chemical species concentration difference Acyi = (second concentration of chemical species [c]2 / i - first concentration of chemical species [c]i / i|, based on the optional chemical species concentration difference Acyi = Ac / 2exp, to resolve an expected value for the second measurement ni2,2expof the at least one radiation characteristics C( / 2,[c]2)ti of a second electromagnetic frequency fz at a second point in time t2, resolving an optional measurement difference | ni2,2exp- m2,2act| between the expected value m2,2expand the actual value ni2,2act, based on the size of measurement difference to resolve a change, wherein the change is selected from a change in concentration of chemical species, sensor drift, and measurement error.

14. A device comprising a sensor according to any of claims 1-9, wherein the sensor is a hydrogen sensor, for monitoring a hydrogen pressure or concentration, wherein the device is selected from an electro-magnetic transformer, a hydrogen storage device, and a battery.

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