WO2020249146A1

WO2020249146A1 - Sensor arrangement, method for production thereof and use of the sensor arrangement

Info

Publication number: WO2020249146A1
Application number: PCT/DE2020/000090
Authority: WO
Inventors: Stefanie HAMACHER; Alexey YAKUSHENKO; Saskia ROTH; Andreas Offenhäusser
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 2019-06-13
Filing date: 2020-05-05
Publication date: 2020-12-17
Also published as: DE102019004120A1; EP3983770A1

Abstract

The invention relates to a sensor arrangement, comprising a substrate, at least one contactable working electrode arranged on the substrate, at least one contactable counterelectrode arranged spaced apart from the working electrode, and a sponge-like matrix arranged on the working electrode and the counterelectrode opposite the substrate and connecting the working electrode and the counterelectrode to one another for exchange of molecules, the sponge-like matrix having pores which are filled with a liquid and the liquid comprising redox-active molecules which can diffuse freely between the working electrode and the counterelectrode. The invention also relates to a method for producing the sensor arrangement and to the use thereof.

Description

Sensor arrangement, method for its production and use

the sensor arrangement

The invention relates to a sensor arrangement and to a method for its manufacture and to the use of the sensor arrangement.

State of the art

A large number of temperature sensors which are based on the electrical properties or on the physical or chemical properties of various materials are known from the prior art. An overview of the temperature sensors, measuring ranges and the underlying physical effects can be found in the relevant works on measurement technology.

According to the underlying effect, they are divided into thermocouples (Seebeck effect), metal resistance thermometers, silicon elements, PTC thermistors, NTC thermistors (temperature dependence of the electrical resistance), photodiodes, photoconductors (temperature dependence of the forward voltage of the pn junction) Pyroelectric detectors (temperature radiation), expansion and gas thermometers (thermal expansion) and temperature measuring colors (temperature dependence of chemical reactions). There are temperature sensors that require an external power supply, as well as those that work without an external power supply. Temperature sensors come in a wide variety of sizes and shapes. The measured data can be recorded, read out remotely (remote readout), display a quantitative readout, indicate time and position information, require a line of sight, have high or low accuracy and cover a large or small temperature range. In short: temperature sensors are available in a very large number of designs. Many temperature sensors are based on the working principle of a thermistor. The resistance changes reproducibly with the temperature; two types: NTC & PTC are known here. The direct measurement of resistance is not suitable, which is why thermistors are built into advanced electrical bridge circuits. The temperature range of a thermistor is disadvantageously limited by the properties of the material and mostly shows a large non-linear dependence.

So-called resistance thermometers (RTDs, English resistance temperature detectors) and thermocouples have a large temperature range, but only a small linear dependency. This leads to a poor resolution. In addition, a thermocouple requires an accurate reference measurement. Furthermore, many of these thermometers are limited in their use and / or their field of application, since they are limited in their size and / or shape and are therefore not suitable for many applications.

Object of the invention

The object of the invention is to provide a new type of sensor arrangement. Furthermore, it is an object of the invention to provide a method for producing this sensor arrangement and its novel uses.

It is a further object of the invention to provide a sensor arrangement that is stable over the long term as a temperature sensor and can be used in a variety of ways, with high resolution and over a large temperature range, this sensor arrangement using a different physical effect than the temperature sensors from the prior art. A method for manufacturing the temperature sensor and uses of the temperature sensor are to be provided.

Solution of the task

The object is achieved with the sensor arrangement according to patent claim 1 and the method for producing and using the sensor arrangement according to the dependent claims. Advantageous refinements for this result from the patent claims that refer back to them. Description of the invention

The sensor arrangement according to the invention has the following features.

A substrate, at least one electrically contacted working electrode arranged on the substrate, an electrically contacted counter electrode arranged at a distance from the working electrode on the substrate, and a sponge-like matrix arranged on the working electrode and the counter electrode opposite the substrate, which exchange the working electrode and the counter electrode of molecules in the matrix, the sponge-like matrix having pores for this purpose that are filled with a liquid and the liquid comprises redox-active molecules that can freely diffuse between the working electrode and the counter electrode when a voltage or current is applied. For this purpose, the sponge-like matrix is also arranged between the electrodes on the substrate.

Usually a counter electrode that is larger than the working electrode is used. Depending on the area of application, several small (micro) working electrodes are arranged, in particular about 2 to 64 working electrode (s). As the number increases, the absolute value of the measured current increases.

The substrate, preferably a flexible substrate, can have a thickness between 1 .mu.m to 10 mm or bulk material and z. B. made of polyethylene theraphthalate (PET). The substrate is advantageously flexible and / or low-melting. This has the advantageous effect that it can be used in a large number of areas, since shape and size can be varied almost at will.

All common polymers that are used in printed electronics can also be used as substrates. These include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyimide (PI). Paper coated with one of the aforementioned polymers is also possible. The melting point of the substrates should be at least 100 ° C, even better at 130 ° C or higher.

In one embodiment of the invention, however, a silicon wafer can also be selected as the substrate. This advantageously causes the sensor to operate at higher temperatures (> 130 ° C) can be used as long as the decomposition temperature of an ionic liquid in the matrix is not reached. Normally the decomposition temperature of an ionic liquid is around 200-300 ° C.

At least one working electrode arranged in a punctiform manner is arranged on the substrate. The working electrode (s) can consist of a suitable, electrically contactable, highly conductive material. Precious metals are suitable, e.g. B. gold as the material of the working electrode. The working electrode is preferably contacted via a conductor track. This can also be made of a precious metal, e.g. B. consist of gold.

On the side of the working electrode opposite the substrate, the necessary redox processes take place by applying a voltage or a current via the contact or the conductor track. These electrode (s) are referred to below as the working electrode (s).

The counter electrode is also arranged on the substrate. The counter electrode is also contacted, e.g. B. via a conductor track. The counter electrode is preferably also made of a highly conductive material, e.g. B. from a precious metal such as gold. The counter electrode is spatially spaced from the working electrode on the substrate. The working electrode (s) and the counter electrode are planar, that is, they are arranged in the same plane on the substrate.

A voltage or a current is also applied to the counter electrode via the contact or the conductor track. This voltage on the counter electrode differs from the voltage applied to the working electrode. The voltage difference drives the cyclic redox reaction of the redox-active molecules on the working electrode and / or on the counter electrode.

For this purpose, a porous or sponge-like matrix is arranged on the working electrode and the counter electrode on the side of the working electrode (s) and the counter electrode opposite the substrate. The pores in the sponge-like matrix are filled with a liquid, in particular with an ionic liquid. The working electrode (s) are in spatial contact with the counter electrode via the sponge-like matrix. The sponge-like matrix connects the working electrode (s) with the counter electrode. The sponge-like matrix is also arranged on the substrate between the working electrodes) and the counter electrode.

The pores of the sponge-like matrix are filled with a liquid, in particular an ionic liquid, as mentioned. The liquid or the ionic liquid contains the redox-active molecules.

The sponge-like matrix itself, like the substrate, is not electrically conductive.

The (ionic) liquid in a passivated sensor arrangement or in a passivated sponge-like matrix has the advantageous effect that this sensor arrangement can be used as a temperature sensor with long-term stability without the temperature sensor showing a drift due to changes.

The melting point and the decomposition temperature of the long-term stable (ionic) liquid in the sponge-like matrix determine the measuring range of the temperature sensor.

An ionic liquid is a liquid as it is defined in the online edition of Römpp 4.0. Liquids consisting exclusively of ions (cations and anions) are called ionic liquids. In principle, ionic liquids are molten salts with low melting points. In general, not only those salt compounds that are liquid at ambient temperature are included, but also all salt compounds that melt below 100 ° C. In contrast to conventional inorganic salts, in ionic liquids the delocalization of the charge reduces the lattice energy and symmetry, which can lead to solidification points down to below -80 ° C

('https://roempp.thieme.de/roempp4.0/do/data/RD-09-02074, last accessed on

06/11/2019).

The properties of ionic liquids are advantageous: very low vapor pressure (non-volatility), thermal stability, non-flammable, electrical conductivity, electrochemical stability (towards oxidizing / reducing agents), high heat capacity, suitable for liquid-liquid phase separation (e.g. of a product), good dissolving properties, Especially for the redox-active molecules, wide range of variation in properties such as polarity, mixing behavior, viscosity (10-3000 mPas), density and melting point, large liquid areas, e.g. B. l-butyl-3-methylimidazolium tetrafluoridoborate is liquid from -49 ° C to + 400 ° C.

The advantage of using an ionic liquid is also the targeted design of certain parameters such as B. the melting point, the freezing point, the viscosity and other parameters when several of these liquids are used.

The melting points of ionic liquids (molten salts) are by definition below 100 ° C, which means that they are liquid at room temperature. The melting point of an ionic liquid for the sensor arrangement results in the lower temperature limit of the sensor, since the diffusion of redox molecules in a solid phase comes to a standstill.

The ionic liquids selected here are only shown as examples. These have a melting point of -18 ° C for 1-butyl-1-methylpyrrolidinium

bis (trifluoromethylsulfonyl) imide or -82 ° C for l-hexl-3-methyl-imidazolium tetrafluoroborate. A mixture of the two liquids results accordingly at a temperature between -18 ° C and -82 ° C, depending on the mixing ratio. This has the advantageous effect that the temperature range of a temperature sensor can be set exactly.

The decomposition temperature of an ionic liquid limits the temperature measurement with the sensor arrangement upwards. Most ionic liquids only decompose at 200 ° C or higher. At these temperatures, a used PET substrate would begin to melt and deformation would start at 130 ° C-150 ° C. A combination of an ionic liquid with a PET substrate is suitable for the sensor arrangement.

As mentioned, the temperature range of the temperature sensor can thus be advantageously designed by choosing different ionic liquids, which is also the subject of the invention.

Redox-active molecules or ions, which are dissolved in the (ionic) liquid of the sponge-like matrix, are deposited on the working electrode (s) and the counter cyclically oxidized and reduced by the voltages applied to the electrodes. The current is measured.

The redox-active molecules are either a) the ions of the (ionic) liquid itself. Then these ions of the (ionic) liquid themselves represent the redox-active molecules. B) the redox-active molecules, such as B. Quinones. These are reduced to hydroquinones, which are not ionic, and only the intermediate stage temporarily carries a negative charge, which, however, is not stable and therefore becomes quinone or hydroquinone depending on the applied potential.

Methylene blue as a redox-active molecule is also not really ionic. A reduction / oxidation of the heterocyclic ring takes place here. c) To cool a mixture of an ionic liquid and additional redox-active molecules. A mixture of an ionic liquid and redox-active molecules has the advantage that the ionic liquid itself is extremely stable and in it the redox-active molecules exert an exactly defined redox reaction at given voltages.

The pores of the spongy matrix have a size so that the redox-active molecules or ions can diffuse in the liquid-filled pores of the matrix. The redox-active molecules or ions diffuse in the pores from the working electrode in the direction of the counter electrode and vice versa.

It is therefore understood that the pores of the sponge-like matrix are selected and arranged depending on the size of the redox-active molecules used.

The redox reaction can be initiated as follows:

At one electrode, e.g. B. the counter electrode, a voltage is applied in the case of a redox-active molecule with positive normal potential, which is more positive than this, z. B. +1 V, and at the same time 0 V on the working electrode. As a result, the counter electrode becomes the anode, which is positively charged and thus oxidizes the redox molecules. So that a stream can flow, the reduction of the redox-active molecules should take place at the same time on the working electrode (s), which then represents the cathode. As a result, two things happen: a depletion layer is created on the respective electrode due to the oxidation or reduction and non-oxidized or non-reduced molecules have to diffuse from the bulk layer. Furthermore, the just oxidized or reduced molecules diffuse from the respective electrode in the direction of the bulk material, which, in formal terms, leads to a diffusion from the counter electrode to the working electrode and vice versa.

In the case of a redox-active molecule with negative normal potential, - IV is applied to carry out the redox reaction.

Particularly suitable as redox-active molecules are methylene blue, ferrocene and derivatives thereof, hydroquinone and derivatives thereof, and hexacyanoferrate (II or III).

The sponge-like matrix is formed from, in particular, monomer acrylates. This advantageously has the effect that the pore size of the sponge-like matrix can be set in a defined manner over a wide range by the parameters during the polymerization. Other substances such as B. Silicates (by polycondensation of silica), plastics in general (PE, PP, PVC etc.), but especially polystyrene, are possible. The use of carbon nanotubes, which contain the (ionic) liquid and the redox molecules, is also conceivable.

The liquid in the pores comprises the redox-active molecules or ions. The voltages applied to the electrode (s) drive the cyclical, i.e. reversible, reduction and oxidation of the redox species at the electrode (s). As a result of the voltages applied to the counter electrode and the working electrode, the redox-active molecules or ions diffuse through the liquid in the pores and are then oxidized or reduced again at the electrodes.

The current generated by the redox reaction is measured by the temperature sensor as a function of the temperature. By measuring the current between the working electrode (s) and the counter electrode, conclusions can be drawn directly about the temperature in the liquid and thus that of the environment in which the measurement is made. This opens up the sensor arrangement in a particularly advantageous manner for long-term stable use as a temperature sensor in a wide temperature range that is predetermined by the choice of liquid. The temperature sensor, comprising the sensor arrangement according to the invention, measures the current as a function of the temperature or the viscosity and displays this advantageously.

As dissolved redox-active molecules, z. B. ferrocene and its derivatives, hydroquinone and its derivatives, methylene blue, hexacyanoferrate (II and III) are used.

For an existing system consisting of a sponge-like matrix plus (ionic) liquid, however, conclusions can also be drawn about the viscosity of the liquid in the pores. In one embodiment of the invention, the liquid itself can go through oxidation and reduction processes and deliver a corresponding measurable current or voltage between the working electrode (s) and the counter electrode. This advantageously has the effect that there is no need for an additional redox-active molecule in the liquid. For this purpose, an ionic liquid is preferably arranged in the pores. Ionic liquids (IL) are particularly suitable as liquids. It can e.g. B. 1-butyl-l-methylpyrrolidinium bis (trifluoromethylsulfonyl) imide, or strong eutectic solvents (English deep eutectic solvent; DES) can be used.

According to the invention, an ionic liquid can thus be replaced by a eutectic or by a eutectic solvent. Strongly eutectic solvents (DES = deep eutectic solvents) are a new class of ionic liquid analogues, which are very similar to ionic liquids in their properties. These solvents were first mentioned publicly in 2001 in a publication by AP Abbott (AP Abbott, G. Capper, DL Davies, HL Munro, RK Rasheed and V. Tambyrajah, Chem. Commun., 2001, 2010-2011.). In contrast to ionic liquids, which are made up of a clearly defined cation and anion, eutectic solvents are created by mixing Lewis and Bronsted acids and bases, which first creates anionic and cationic species. Through the targeted mixing of organic salts, usually a quaternary ammonium compound and a metal salt, a metal salt hydrate or a hydrogen bridge bond donor in certain molar ratios, compounds according to the invention can be obtained which have a much lower melting point than the pure substances.

By a mixture of, for example, choline chloride (quaternary ammonium compound, (2-hydroxyethyl) trimethylammonium chloride) with a hydrogen bond donor, e.g. Urea or glycerine, you get a DES with a melting point of 12 ° C or -40 ° C. The melting point of pure choline chloride, on the other hand, is over 300 ° C and that of urea is 133 ° C (A.P. Abbott, G.Capper, D.L. Davies, R.K. Rasheed,

V. Tambyrajah; Novel Solvent Properties of Choline Chloride / Urea Mixtures, Chemical Communications 2003, 70-71).

This behavior corresponds to the definition of a eutectic, in which there is a very specific composition of two substances that are not miscible with each other in the solid state, but completely miscible in the liquid state (Römpp:

https://roempp.thieme.de/roempp4.0/do/data/RD-05-02271; last accessed on

06/12/2019).

As eutectic solvents, for. B. a mixture of choline chloride and ethylene glycol (1: 2 or 1: 5 molar ratio or intermediate values) into consideration. The viscosity of a strongly eutectic solvent advantageously varies over a much wider range than that of an ionic liquid itself. Thus, one can choose a strongly eutectic solvent so that the viscosity is already closer to the desired values for printing during an inkjet printing process. These pressure methods of applying the liquid to the electrodes are also shown to be advantageous.

By choosing an ionic liquid and / or a eutectic liquid with a corresponding melting point, the measurable temperature range of the temperature sensor can be selected and varied. Mixtures consisting of several ionic liquids can also be used. The liquids according to the invention in the sponge-like matrix, in particular in the case that an ionic liquid is used, advantageously have a very low, hardly measurable vapor pressure. They are thermally stable, which means that defined solutions can be prepared, the composition of which does not change chemically, even with strongly fluctuating temperatures. This means that the concentration of the redox molecules present remains stable, and the viscosity of the liquid only changes due to the temperature and not because some of the solution evaporates.

This advantageously has the effect that the sensor arrangement, as a temperature sensor, supplies measured values that are stable over the long term and that are reproducible over a wide range.

The sensor arrangement, in its most general form, thus comprises the substrate, at least one working electrode which is arranged on the substrate and which can be electrically contacted by a contact, and an electrically contactable counter-electrode on the substrate which is arranged at a planar distance from the working electrode. The distance between the working and the counter electrode can be 1 pm to 500 pm. On the working electrode and on the counter electrode, the sponge-like matrix is arranged opposite the substrate, which connects the working electrode and the counter electrode to one another for the exchange of redox-active molecules. The sponge-like matrix is also arranged on the substrate between the electrodes. For this purpose, the sponge-like matrix has pores which are filled with a liquid, the liquid containing the redox-active molecules which can diffuse from the working electrode to the counter-electrode and vice versa through the pores of the sponge-like matrix.

The sensor arrangement consisting of working electrode (s), counter electrode and sponge-like matrix with ionic liquid and redox-active molecules or ions is passivated by a suitable layer. Particularly suitable as a passivation layer are parylene C, photoresists such as e.g. B. SU8 and other insulating materials. The passivation layer advantageously prevents harmful influences such as (air) moisture and the sensor arrangement is permanently passivated.

A sensor array advantageously consists of a large number of (micro) electrodes as working electrode (s). This advantageously has the effect that the absolute value of the measured current is increased. About 2 to 64 or more working electrodes are advantageously arranged. As a rule, the dimensions of the working electrode are significantly smaller than the counter electrode. This has the advantageous effect that barrier layers, which prevent the diffusion of the redox-active molecules, can indeed occur on the working electrode. Due to the small size of the microelectrode (s), you don't get a planar depletion layer, but a hemispherical one. As a result, the area from which non-oxidized or non-reduced molecules can diffuse is much larger, which counteracts the depletion layer and leads to a constant current after the redox potential has been reached. If the width of the depletion layer doubles, the surface from which diffusion occurs is quadrupled because A = pr ² .

A method according to the invention for producing the sensor arrangement provides the following steps. a) A substrate is selected and at least one working electrode is arranged on it, b) a counter electrode is arranged on the substrate in a planar manner, i.e. in the same plane as the working electrode, with the working electrode and the counter electrode each being contacted by a contact, c) on the working electrode and on the counter electrode, a sponge-like matrix is arranged opposite the substrate, with pores which are filled with an (ionic) liquid, the liquid comprising redox-active molecules that are in the pores of the sponge-like matrix from the working electrode to Can diffuse against the electrode and vice versa.

The cyclic oxidation or reduction of the redox-active molecules on the electrodes takes place when a different voltage is applied to the working electrode and the counter electrode via the contacts or conductor tracks.

An electrically non-conductive substance is selected as the sponge-like matrix, also for the substrate.

For step c) of the procedure: In a particular embodiment of the invention, an ink is provided which comprises the starting constituents of the sponge-like matrix and / or the (ionic) liquid and / or the redox-active molecules together.

The ink is suitable for an ink jet printer and has a correspondingly low viscosity, that is to say the viscosity is 8-15, at most 21-22 centipoise, measured with a Brooksfield Cone / Plate Rheometer. The liquid to be examined is filled into a beaker of the measuring device so that the entire bottom is covered. The rotational movement of a spindle exerts a force on the surface of the liquid. The resistance of the liquid is measured and then converted into the shear force. The measuring device can then use the shear force to determine the viscosity.

The ink is printed onto the electrodes and the substrate between the electrodes by means of an ink jet printer.

The sponge-like matrix and its pores are made from the monomers, e.g. B. from acrylate monomers, which are components of the ink, formed. Immediately after printing, the working electrode (s), the counter electrode and the substrate between the electrodes are wetted with the ink. In this state, the sponge-like matrix is not yet formed.

It takes place z. B. UV irradiation and / or temperature sintering. This advantageously has the effect that the sponge-like matrix is formed from the monomers via a polymerization reaction with a photoinitiator. It will be advantageous to carry out a UV-controlled level polymerization reaction, which forms the sponge-like matrix on the electrodes and the substrate arranged in between.

For this purpose, an ink can be printed which contains the monomers and a photoinitiator to initiate a UV-driven polymerization reaction to form the sponge-like matrix.

The liquid for forming the spongy matrix can be arranged on the substrate and / or the electrodes not only by printing but also by spin coating or by drop casting. Other processes, such as lithography, are res conceivable and a person skilled in the art will select or carry out a suitable method for step c) of the method.

To fix the (ionic) liquid on the electrode (s) this is z. B. acrylate monomers, e.g. B. 1,6-hexanediol acrylate, trimethylolpropanthoxylate triacrylate, polyethylene glycol diacrylate (PEGDA) are added. When the photoinitiator is activated by UV light, these components are polymerized to form the matrix. In this context, UV matrix means that the liquid or ink arranged on the electrodes and the substrate forms the sponge-like matrix through UV radiation.

In this case, the (ionic) liquid or ink can be a UV-sinterable photoinitiator, such as. B. 2-Hydroxy-2-methyl-propiophenone be attached. This advantageously has the effect that, after UV radiation, the photoinitiator releases free radicals which lead to polymerisation of the monomer acrylates.

In this way, the sponge-like matrix can advantageously be produced by UV radiation, according to the general equation

! iv

R - R 2 R ^'

with R-R = photoinitiator, ^ = radical

It is possible, instead of a UV-sensitive photoinitiator, a metastable molecule, e.g. B. dibenzoyl peroxide, 2,2'-azobis (isobutylnitirl) (abbr .: AIBN) or inorganic peroxides such as peroxodisulfates to be used. In the case of dinezoyl peroxide, the 0-0 bond can of the peroxide are split both photolytically and thermolytically, all other radical initiators mentioned are split thermolytically.

In the case of the silicates or silicic acid mentioned above, the polymerization is initiated by changing the pH value. To adjust the viscosity and surface tension of the (ionic) liquid or

In addition, low-viscosity solvents such as propylene carbonate or gamma-nonalactone can be added to the liquid, in particular an ionic liquid, or these can also be part of the ink.

This has the particularly advantageous effect that an (ionic) liquid comprising all components for a printable ink is provided which represent the starting products for the sponge-like matrix of the sensor arrangement.

An ink according to the invention for printing on the electrode (s) and / or the substrate in accordance with step c) of the production method then comprises in particular:

1. One or more polymerizable monomers, in particular acrylate monomers, to form a sponge-like matrix,

2. At least one photoinitiator,

3. A solvent that lowers the viscosity of the monomers and also of the (ionic or possibly the eutectic) liquid itself in the liquid so that the ink can be printed by an inkjet printer, 4. At least one type of redox-active molecule, provided that the solvent according to 3. does not itself carry out this reaction in the sponge-like matrix to be formed.

This ink is placed on the working electrode (s) and / or counter electrode and the substrate by an ink jet printing process.

The viscosity of an ionic liquid is more critical because it is usually higher. But the viscosity of the ionic liquid is increased by adding monomers, the photoinitiative tor, the redexo mediator (if available) and the low-viscosity solvent advantageously lowered.

In a further embodiment of the invention, after the pressure has been applied to the electrode (s) and the substrate, the liquid is fixed to form the spongy matrix. For this purpose, a sintering process by increasing the temperature and / or in particular a UV polymerization reaction is advantageously carried out so that the sponge-like matrix can be formed by polymerization of the monomers in the ink.

The electrode surfaces and the substrate between the two electrodes should be activated before the (ionic) liquid or ink is applied, e.g. B. by an oxygen or an argon plasma.

At least partially printed sensor arrangements for temperature sensors are thus advantageously produced. Partially printed temperature sensors combine various advantages, such as, in particular, fast and inexpensive production and a large measuring range as well as a variable shape and size of the layers arranged.

Alternatively, the sensor system can also be applied to a silicon wafer. In particular, an S1O2 substrate is selected for this purpose. This has the advantageous effect that the sensor arrangement can be used as a temperature sensor at significantly higher temperatures (above 130 ° C), since z. B. a PET substrate starts to deform at this temperature and would melt a little later.

Both substrate classes are to be regarded as equivalent for different temperatures.

During step d) the substrate, the electrodes and the sponge-like matrix are passivated.

Wireless communication can be enabled for the sensor arrangement by adding a digital output.

The sensor arrangement is advantageously used as a temperature sensor. However, it can also be used for other physico-chemical parameters. The measuring principle of the developed sensor arrangement as a temperature sensor is based on a change in the viscosity of the (ionic) liquid when the temperature changes. An increase in the temperature of the liquid in the sponge-like matrix has a direct impact on the viscosity of the liquid, which is thereby reduced. By contrast, a decrease in temperature increases the viscosity of the liquid in the pores of the spongy matrix. In most cases, an exponential dependence of the two parameters can be observed.

A change in viscosity thus leads to a change in the diffusion speed of the dissolved redox-active molecules or ions within the (ionic) liquid and thus to a measurable change in the current between the working electrode and the counter electrode. The change in the current is measured as a function of the temperature of the liquid, and thus of the temperature of the environment in which the measurements take place.

This is a different effect than with the temperature sensors from the prior art.

The sensor arrangement advantageously has an ammeter or a voltmeter that measures the current or the voltage between the working electrode (s) and the counter electrode, which occurs when a voltage is applied to the working electrode and the counter electrode, which the redox-active molecules to the Forcing electrodes to undergo a reversible redox reaction.

The above dependency is described by the Stokes-Einstein equation:

With D = diffusion coefficient, k _ß = Boltzmann constant, T = temperature, h = viscosity, r = radius of the spherical particles.

Forming gives: With

k _ß = Boltzmann constant, r = radius of the spherical redox molecule, h = viscosity of the liquid, T = temperature, D = diffusion constant, Ki = self-defined constant

The Cottrell equation describes the time course of the current after a potential jump in chronoamperometry and relates the measured current to the diffusion constant:

The expression that emerges from the Stokes-Einstein relationship is now substituted for D with

n = number of electrons transferred during the redox reaction, F = Faraday constant, A = electrode surface, c = concentration of redox molecules, t = time = 0.1 s = steady state, D = diffusion constant, K2 = self-defined constant; I = measured current The Arrhenius equation describes the behavior of the viscosity of Newtonian liquids with temperature

h is now replaced by Arrhenius' expression in the above equation:

h = viscosity of the liquid, h = material constant, EA = activation energy (also a kind of material constant), R = universal gas constant, T = temperature

All unchangeable quantities were summarized in the constants Ki and K ₂ and play no further role in the relationship between current and temperature. In order to simplify it further, the constants Ki, K ₂ and hoo can be combined to form K ₃ , so that the current only depends on the root expression.

If one plots I against T and substitutes the value from the literature for an exemplary ionic liquid for EA / R, it becomes apparent that the course of the theoretical curve is similar to that of the measured curves. In order to adapt the theoretical curve to the curve actually measured, one must also take into account all the parameters that were summarized in the constants Ki and K ₂ or K ₃ . That depends on the exact measuring arrangement, since the electrode size, the radius of the redox molecule, the concentration used and so on also play a role.

It goes without saying that the temperature sensor is equipped with appropriate software for this purpose, which performs the above-mentioned transformations and, from this, the calculation of the temperature from the current measured value obtained.

For the voltage- or current-driven redox reaction of the ions or the redox-active molecules at both electrodes, knowledge of the standard potential of the ions or the redox-active molecules is helpful, but not absolutely necessary, since most redox reactions between a potential of +1 V, for example to -1 V. If this clamping If voltages are applied to the working electrode and / or the counter electrode, the cyclic redox reaction will take place regularly, and provided that the voltage on the working electrode alternates.

For a redox-active molecule with positive normal potential, this means: if a positive voltage is applied to the counter electrode that is more positive than the normal potential of the redox-active molecules and the working electrode is not energized, this drives the oxidation of the redox-active molecule into its oxidized form on the counter electrode. Then a positive voltage is applied to the working electrode (s), which is more positive than the normal potential of the redox-active molecules and the counter electrode is not applied with voltage. This drives the reduction of the redox-active molecule into its reduced form at the counter electrode.

In the case of a redox-active molecule, a more negative normal potential must be applied.

It goes without saying that when a voltage is applied to the counter electrode and / or to the working electrode, the redox-active molecules are reversibly reduced and oxidized at the counter electrode and the working electrode.

As already described above, diffusion occurs with each of the two pulses. The oxidation or reduction at the respective electrodes creates a so-called depletion layer, which causes a concentration gradient, whereby diffusion occurs, every system tries to compensate for a gradient, here by diffusion. That is, the just oxidized molecules diffuse from the electrode surface in the direction of the bulk material and non-oxidized molecules diffuse in the direction of the electrode surface. This happens faster at a higher temperature than at a lower temperature, which is shown by an increase in the current when heating up or a decrease in the current when cooling down.

The oxidation / reduction of a molecule is in the nanosecond range, while the measurement time in this case is 0.1 s. This means that the measurement time is significantly longer than the oxidation / reduction processes, which leads to pronounced diffusion phenomena.

The counter-pulse reverses the previous processes, whereby the previously created depletion layers on the respective electrode are broken down again, but this is based on the (back) diffusion of the oxidized or reduced molecules from the Bulk layer. All this means that it theoretically happens that a molecule actually diffuses from the working to the counter electrode or vice versa, but that this does not necessarily have to happen.

The reason for the counter pulse, in which the current is also not measured, is to make the sensor more durable and to make the measurements more reproducible, as you set almost the same state every time.

Various measurement techniques, including voltammetry and amperometry, are suitable for using the sensor arrangement as a temperature sensor. A person skilled in the art will select a suitable measurement method.

For amperometry: A voltage pulse with a defined voltage and a defined period of time is applied to the working electrode. In application example 1, -1 V was applied to the working electrode for 0.1 s and the current was measured at the same time. A corresponding counter pulse of +1 V was then applied to the counter electrode. The current was not measured here. As a result, oxidation and reduction are reversible.

For voltammetry: The potential at the working electrode is changed linearly over a defined voltage range, depending on the redox mediator used. Starting from an initial potential Uo, the voltage is increased linearly until a reversal potential is reached and the voltage decreases again until Uo is reached. The rate of change in potential can be varied, it was always 0.05 V / s for the present patent application. In response to the change in voltage, the change in current is measured, which occurs due to oxidation and reduction processes at the electrodes.

The temperature of the liquid in the sensor arrangement is deduced from the change in the current. The amplitude of the oxidation peak and the reduction peak increases with increasing temperature and decreases with decreasing temperature.

It goes without saying that the temperature sensor is calibrated.

Embodiments In the following, the invention is explained in more detail on the basis of exemplary embodiments and the enclosed figures, without this being intended to restrict the invention.

Show it:

Figure la-h: production of the sensor arrangement

Figure 2: Polymerization reaction

Figure 3a-b: Amperometric measurements

Unless stated otherwise, identical reference symbols denote identical components or layers.

First exemplary embodiment:

A low-melting substrate 1 made of polyethylene theraphthalate (PET) is selected. This ses has a thickness of 125 μm. It's flexible.

On this substrate 1, a layer 2 of first 5 tun Ti and then 100 nm Au is arranged. Both layers are vapor-deposited onto the substrate 1 in the clean room.

This layer system of titanium and gold layer 2 is then structured by lithography or by laser ablation, see FIG. La. The working electrodes 2a-c and the counter-electrode 2d are formed. The remaining portion of the titanium and gold layer 2 in the figure lb left in the picture is irrelevant. The working electrode (s) 2a-c and the counter electrode 2d are each provided with a conductor track 6 as an electrical contact. In this way, voltage can be applied to the electrodes after the temperature sensor has been manufactured.

Either the microelectrodes 2a-c are formed directly as working electrodes during structuring or these are produced in a further step by printing SU-8 ink 3 on the working electrode 2. That is, a large part of the gold layer 2 from the area from which the working electrode (s) are formed is passivated with the SU-8 ink 3 and it only small windows 2a-c remain, in which the gold is exposed on the surface, see Figure 1b.

These layers are isolated by passivation with SU-8 3. A mixture of SU8 2010 and g-butyrolactone (1: 1) is applied by printing. The reference number 3 in Figure lb corresponds to the reference number 3 in Figure lg.

Then the layer system is for 5 min. sintered at 95 ° C, then irradiated with UV for 20 s and finally sintered again for 5 min at 95 ° C.

Then a mixture of ionic liquid 4 is used to form the

arranged spongy matrix (Figure lc). This mixture contains all the components, i.e. the two ionic liquids, the acrylate monomers, the photoinitiator and the methylene blue

All components of the ionic liquid 4 for forming the sponge-like matrix are combined in a printable ink. The ink comprises an ionic liquid, acrylate monomers and photoinitiator, g-nonalactone and the redox molecule, here e.g. B. Methylene Blue.

The ink is printed or applied by spin coating, drop casting or lithography.

As the ink for the ink jet printing method, the following were specifically used: a. 1-butyl-l-methylpyrrolidinium bis (trifluoromethylsulfonyl) imide as the first ionic liquid, l-hexl-3-methyl-imidazolium tetrafluoroborate as the second ionic liquid, gamma-nonalactone as a low-viscosity solvent, 1,6-hexadiol acrylate as the first monomer, trimethylolproprylatanthoxylate as the second monomer and 2-hydroxy-methyl-propiophenone as the photoinitiator. These components are mixed in the following ratio: 40 wt%: 10 wt%: 33.4 wt%: 5.5 wt%: 10.5 wt%: 0.6 wt%. In this mixture, methylene blue is dissolved as a redox-active molecule in a concentration of 40 mM. A saturated solution is created. Undissolved methylene blue is filtered off. b. The amperometric data shown in FIGS. 3 a and 3 b have been measured with a sensor with this composition.

Alternatively, instead of methylene blue, the following is also suitable: c. a 0.2 M solution of ferrocene dimethanol

During the sintering or fixing of the ink, the ionic liquid is irradiated with UV light for a few seconds. Different irradiation times will result in a more closely-meshed or even more wide-meshed structure of the polymer matrix, since either fewer or more radicals are initiated and the chain length can thereby be varied. The sponge-like matrix is thus formed from the ionic liquid or ink 4 (FIG. 1c).

For acrylate monomers, the reaction is shown as follows, see FIG. 2. FIG. 2 includes its own reference symbols:

UV irradiation turns photoinitiator 1 into two radicals 2a and 2b through homolytic bond cleavage, both of which attack a double bond of an acrylate unit of a monomer (3 or 5) in an initiation reaction and transfer the radical to the monomer. Chain growth then takes place in that the monomer radical again attacks another monomer or an already formed acrylate oligomer 4a, b, 5a-c. This creates a network of different long and branched polymer chains that connect through so-called termination reactions.

In order to evaporate the volatile components, the sensor is heated to 125 ° C for 30-60 minutes after the polymerization reaction.

Then the passivation layer 5, which can consist of SU-8, parylene C or water-based passivations, is applied and this is sintered depending on the material (FIG. 1d).

Reference number 6 shows the conductor tracks for connection to a measuring device. The sensor is either connected directly to the measuring device (not shown) via the conductor tracks 6, consisting of gold. To facilitate and improve contacting, is used with A connector 7 is glued to silver glue, to which the cables 8a and b of the potentiostat / measuring device are clamped, see Figures le and f.

FIG. 1g shows a cross section along the working electrode 2b, the section being made according to the dotted line in FIG. Further passivation layers 3 and 5 and the ionic liquid 4 complete the sensor arrangement.

FIG. 1h shows the sponge-like matrix 88 after the polymerization reaction. It is shown that the sponge-like matrix 88 has pores 84 in which the ionic liquid is present. The ionic liquid has freely diffusible redox-active molecules 89. Depending on the viscosity and thus the temperature of the liquid, these diffuse from the working electrode to the counter electrode and can reversibly enter into a redox reaction.

The temperature can be measured as a function of the current by means of voltammetric or amperometric measurements, see FIG. 3.

These measurements show the reproducibility. The temperature is increased or decreased by 10 ° C every three minutes and always varied from -20 ° C to 60 ° C. The plots contain seven heating (h = heating) and cooling cycles (c = cooling). It can be seen that the values for heating and cooling are very similar (see heating only and cooling only) but differ somewhat in comparison. Cooling seems to be more linear than heating.

It is shown that the sensor arrangement according to the invention supplies reproducible values of the current as a function of the temperature. This enables it to be used as a temperature sensor.

Further embodiments:

Further exemplary embodiments for the ink according to Table 1

The various components are not to be combined line by line, but rather freely combined with one another. For the mixing ratios, the person skilled in the art can orientate himself on the exemplary embodiments shown. Use of a pure ionic liquid as a transport medium and without further redox-active species in the sponge-like matrix:

As embodiment 1, but: 1-ethylimidazolium nitrate is mixed with the acrylate monomers and the photoinitiator in the following ratio: 80 wt% 1-ethylimidazolium nitrate: 12.6 wt% 1,6-hexanediol acrylate: 6.7 wt% trimethylolpropane methoxylate triacrylate : 0.7 wt% 2-hydroxy-2-methylproiophenone.

As shown in exemplary embodiment 1, this mixture is applied to the sensor arrangement and the sponge-like matrix is formed by UV irradiation. After connection to a measuring device, a voltage of 0.5 V is applied, which leads to the reduction of the nitrate anion to NO _x . This process cannot be reversed, but with the very short measurement times and the large number of molecules present in comparison, this guarantees a sufficiently long service life for the temperature sensor. Since the NO _x can escape as a gas, there are also no deposits on the cathode. Since the 1-ethylimidazolium nitrate has a melting point of about 10 ° C, the temperature was only measured from 10 ° C to 60 ° C in 10 ° C steps. The spongy matrix is also passivated in this case.

Use of a strongly eutectic solvent instead of an ionic liquid as a transport medium for the redox-active species through the sponge-like matrix:

As in embodiment 1, however: there are choline chloride and ethylene glycol

(ChCLEthGly) mixed in a 1: 5 molar ratio and dissolved methylene blue (40 mM). Undissolved methylene blue was removed by filtration. The acrylate monomers and the photoinitiator were then mixed in. The composition thus consists of 80 wt% ChCLEthGly: 12.6 wt% 1,6-hexanediol acrylate: 6.7 wt% trimethylolpropane ethoxylate triacrylate: 0.7 wt% 2-hydroxy-2-methylproiophenone.

This mixture is applied to the sensor arrangement as in exemplary embodiment 1 and the sponge-like matrix is formed by UV irradiation. After connecting to a measuring device, a voltage of 1 V is applied, which leads to reversible redox reactions. The temperature was varied from -20 ° C - 60 ° C in 10 ° C steps. These explanations show that there is a wide selection of different liquids, ionic liquids, eutectic liquids, which the person skilled in the art will select in the manner mentioned in order to arrive at a sensor arrangement or a temperature sensor according to the invention. It is conceivable that the sensor arrangement, as described, can also be used for measuring other physico-chemical parameters.

Claims

Patent claims

1. Sensor arrangement comprising a substrate and at least one electrically contacted working electrode arranged on the substrate, a contacted counterelectrode on the substrate spaced apart from the working electrode, and a sponge-like matrix arranged on the working electrode and the counterelectrode opposite the substrate, which the working electrode and the counter electrode connects to one another for the exchange of molecules in the matrix, the sponge-like matrix having pores which are filled with a liquid and the liquid comprises redox-active molecules that can carry out a redox reaction when a voltage or a current is applied.

2. Sensor arrangement according to claim 1,

characterized in that

the sponge-like matrix is also arranged on the substrate between the electrodes.

3. Sensor arrangement according to previous claims 1 to 2,

characterized in that

the arrangement comprising the substrate, the or the working electrode (s) and the counter electrode is passivated.

4. Sensor arrangement according to one of the preceding claims,

characterized in that

the working electrode (s) and the counter electrode are electrically contacted via conductor tracks.

5. Sensor arrangement according to one of the preceding claims,

marked by

an ammeter or voltmeter that measures the current or voltage between the working electrode (s) and the counter electrode, which occurs when a voltage is applied to the working electrode and counter electrode that exceeds the drives redox-active molecules on the electrodes to a reversible, cyclic redox reaction.

6. Temperature sensor with a sensor arrangement according to one of the preceding claims, characterized in that

this measures the current generated by the redox reactions and determines the temperature.

7. A method for producing a sensor arrangement according to one of the preceding claims,

characterized by the steps: a) at least one working electrode is arranged on a substrate, b) a counter-electrode is arranged on the substrate in a planar manner to the working electrode, the working electrode and the counter-electrode being contacted, c) on the working electrode and the counter-electrode being opposite to the substrate a sponge-like matrix is arranged with pores which are filled with a liquid, the liquid having redox-active molecules which can be oxidized and reduced on the working electrode and / or on the counter electrode if a different voltage is present on the working electrode and the counter electrode is created via the contacts.

8. The method of claim 7

characterized in that

the sponge-like matrix is also placed on the substrate between the electrodes.

9. The method according to any one of the preceding claims 7 to 8,

marked by

Step d) with a passivation of the substrate, the electrodes and the sponge-like matrix.

10. The method according to any one of the preceding claims 7 to 9,

characterized in that

for step c) a liquid containing acrylate monomers is arranged on the electrodes and / or the substrate, the liquid having a photoinitiator for initiating a UV-driven polymerization reaction to form the sponge-like matrix.

11. The method according to any one of the preceding claims 7 to 10,

characterized in that

UV irradiation and / or sintering is carried out for the polymerization reaction.

12. Use of the sensor according to one of the preceding claims 1 to 6 as a temperature sensor.

13. Use according to the previous claim 12,

wherein a change in temperature of the liquid in the sponge-like matrix causes a change in the measured current by causing a change in the viscosity of the liquid, which leads to a change in the diffusion of the redox-active molecules in the pores of the sponge-like matrix.

14. Use according to one of the preceding claims 12 to 13,

in which

The following applies to a redox-active molecule with a positive normal potential: A positive voltage is applied to the counter electrode that is more positive than the normal potential of the redox-active molecules and the working electrode is not subjected to voltage, which causes the oxidation of the redox-active molecule in its oxidized form the counter electrode drives and then a positive voltage is applied to the working electrode, which is more positive than the normal potential of the redox-active molecules and the counter-electrode is not charged with voltage, which drives the reduction of the redox-active molecule to its reduced form on the counter electrode.

15. Use according to one of the preceding claims 12 to 13,

characterized in that in which

The following applies to a redox-active molecule with a negative normal potential: A negative voltage is applied to the counter electrode, which is more negative than the normal potential of the redox-active molecules and the working electrode is not applied with voltage, which reduces the redox-active molecule to its reduced form drives on the counter electrode and then a negative voltage is applied to the working electrode, which is more negative than the normal potential of the redox-active molecules and the counter-electrode is not applied with voltage, which drives the oxidation of the redox-active molecule in its oxidized form on the counter electrode.