ORGANIC-INORGANIC SENSING MATERIAL

Description Technical Field

The present invention refers to a (chemical) sensor system unit for sensing an analyte concentration, like a C02 concentration in gas form, to a method of sensing and a method for manufacturing the sensor. Preferred embodiments refer to a sensor comprising a hybrid organic-inorganic material for sensing different analyte.

Technical Background - CQ2 detection

Detecting and controlling carbon dioxide (C02) concentrations attract a lot of attention, because a high performance chemical sensor may be applied to various industrial and biological applications like monitoring Heating, Ventilation and Air Conditioning (HVAC) to reduce energy consumption [1], monitoring patients for disease detection at early stages, capnography [2] and continuous monitoring of emissions in industrial combustion processes [3], For example, C02 is nowadays used in refrigerant industry for cooling computer cores and rooms. It is considered as a replacement gas for Freons (CFCs, HCFCs: hydroehlorofluorocarbons), which were considered as clean, non-toxic, odorless gases for many years. Today, however, it is known that these gases cause depletion of the atmospheric ozone layer [4], Using compressed C02 under high pressure provides an alternative here, as the global warming potential (GWP) of C02 is negligible compared to Freon gases. Therefore, a rapid detection of C02 is indispensable to permit leak detection in cooling systems by closing the valves in real time and prevent accidents. For these types of industrial applications, the C02 chemical sensors work as safety switches. In our daily life, the amount of exhaled C02 could be an indicator of occupancy in rooms kept under surveillance [5]. The C02 detector is used in security field and military situations to localize the presence of human activity, like to stop human rights violations due to human trafficking and to deliver emergency aid to the localized earthquake victims.

In the last 5 years, a lot of research activities were dedicated to find an efficient way to capture and store C02. C02 capture and storage (CCS) methods are an adequate solution to retard the global warming induced by greenhouse emissions. The European nations adopted CCS as an efficient strategy to tackle climate change [6]. For this application. C02 sensors are also needed to monitor the C02 geologic reservoirs in order to detect real time leaks occurring during the C02 injection or storage. There are only few existing devices to detect C02 in the desired environments, based on optical or resistive methods.

Technical Background - Conventional Devices

A conventional method to detect C02 is based on sensitive metal oxide layers [8]. These materials however inevitably show a cross sensitivity to other gases. According to literature, the development of a metal oxide sensor capable of detecting C02 below 2000 ppm also remains challenging [9]. The working principle of resistive metal oxide sensors is based on adsorbing oxygen on the metal oxide surface, which traps free electrons extracted from the conduction band of the used metal oxide sensing layer. An electron-depleted region is formed at the grain boundaries, which prevents -the electron flow between the electrodes, and consequently increases the oxide layer resistance. Most of the existing metal oxide based sensors require an operating temperature higher than 100°C in order to reach an acceptable sensitivity. Other researchers are still looking for efficient metal oxide materials, which are able to detect C02' at temperature less than 200°C, preferably less than 100°C.

The gas sensing method based on electrochemical method was investigated for C02 detection. The prepared sensing electrodes are sodium or Li based materials. An appropriate solid electrolyte is based on NASICON, YSZ or alumina. The measured electrical signal between a sensing electrode and a reference electrode is proportional to the C02 concentration. The reaction temperature is often situated between 200 - 500°C. Optical detection based on non-dispersive infrared (NDIR) method is one well-known way to detect gases [7]. As the C02 molecule vibrates and rotates at a well-defined frequency, at which the molecule loses its symmetry and absorbs light in the wavelength range between 4.1 and 4.4 μιτι, the NDIR method measures the optical signal in the infrared range and permits to identify the amount of detected C02. Although the NDIR method is accurate, makes fast measurements and has a good long-term stability, its high price, large device size and high power consumption are clear disadvantages. Moreover, its software and hardware necessitate a regular calibration and cleaning to maintain the full device performance.

An alternative approach of detecting C02 uses polymeric matrices with fluorescent dyes. Methods based on fluorescent dyes are simple, easy to use techniques and based on incorporating a pH indicator in an organic matrix. PH sensors are insensitive to electrical and electromagnetic fields and a reference sensor is unneeded. These types of sensors are mainly to detect C02 in liquid media. An increase of the C02 concentration in an aqueous solution induces a decrease of solutions pH solution, which is detected by measuring the fluorescence of the dyes. Several pH sensitive dyes were intensively studied such as: Phenol red (PR), bromothymol blue (B ^' l ^' B), cresol red (CR), 1 ,3-bis(dimethylamine)-2- propanol (OMP), DMEA, DMAH, MDEA, Triethanolamine (TEA). In a gas medium, the pH dyes should be incorporated to a mechanically stable matrix either by impregnation or grafting. A grafting can improve the leaching issues and increases the lifetime of pH dyes. Hydrophilicity of the matrix is an important parameter to allow diffusion of protons.

Until today, there is a lack of sensitive, reversible and stable chemical gas sensor. Currently, the best long-term stability is provided often by metal oxides and infrared spectroscopy based optical sensors. The main disadvantage of resistive and infrared methods is their high energy consumption. Therefore, there is a need for an improved approach.

The objective of the present invention is to provide a concept for a sensor having a small size, low power consumption and high reliability for gas sensing under different condition, while maintaining the sensing accuracy of the above described concepts.

Summary

Embodiments of the present invention provide a sensor device (sensor system unit) for sensing an analyte concentration, like a C02 concentration. The sensor device comprises a hybrid sensing material, a transducer and a control unit. The hybrid sensing material comprises a mixture of at least amines and nanoparticles and has an electrical property. The electrical property is dependent on an analyte concentration, e.g., a C02 concentration in the surrounding. The transducer is configured to output an electrical sensor signal dependent on the electrical property of the hybrid sensing material. The control unit is configured to control / to drive the operation of the transducer and to receive the electrical sensor signal from the transducer, wherein the control unit preferably controls the transducer using an AC signal so as to perform an impedance measurement. Teachings disclosed herein are based on the finding that a hybrid sensing material configured to respond with a change of the material property, like an electrical property, on a present analyte/C02 concentration can be optimally monitored using an impedance measuring sensor. The impedance measuring sensor is typically driven by an alternating signal having a high voltage but a low current, such that the measuring itself minimally influences the hybrid sensing material and the determination of its electrical property.

According to another embodiment, the control unit of the impedance measuring circuit is also configured to receive the sensor signal and to evaluate same. Due to this concept, the control unit can adapt its control signal dependent on the sensor signal, such that the measuring is improved during operation.

According to another embodiment, the hybrid sensor material can also react with an optical property change on the recent analyte/C02 concentration. In this case the sensor device also comprises an optical sensor system, e.g., comprising a light emitting device as well as a light receiving device. The optical sensor system is focused on the hybrid sensor material, such that same can monitor the optical property. This optical sensor system can also be controlled using the above discussed control unit. This has the advantage that the analyte/C02 concentration can be determined in two ways so as to output information regarding the C02/analyte concentration being based on the sensor signal of the impedance measuring chip and the optical sensor.

According to another embodiment, the hybrid sensor material may be configured to react with an optical property change dependent on the current PH value in the surroundings.

Here, an optical sensor system may also be used to monitor the optical property of the hybrid sensor material.

According to another embodiment, the sensor device may comprise a micro pump configured to initiate a gas flow of a medium from the surroundings through the sensor device or to the hybrid sensor material, such that the medium passes the same.

Another embodiment refers to a method for manufacturing a sensor device. The method comprises the steps of providing an impedance measuring chip and a hybrid sensor material to same. After that the control unit is integrated into the manufacturing method.

According to an embodiment, the method may comprise the step of impregnation synthesis or covalent grafting to obtain the hybrid sensor materials. These methods are the preferred way to enable the production of high quality hybrid sensor materials.

Embodiments of the present invention will subsequently be discussed referring to the enclosed figures, wherein: Brief Description of the Drawing

Fig. 1 shows a schematic block diagram of a sensor device including a control unit according to a basic embodiment;

Figs, la- If illustrate schematic implementation details of the sensor device according to enhanced embodiments;

Fig. 2 shows a schematic flow chart for illustrating the manufacturing method; and

Figs. 3 and 4 show schematic diagrams illustrating the electrical impedance measurements according to further embodiments.

Detailed Description

Embodiments of the present invention will be discussed in detail below referring to the enclosed figures. Here, identical reference numerals are provided to objects having identical or similar functions so that the description thereof is mutually applicable and interchangeable.

Fig. 1 schematically discloses the integration of a sensor device 10, with a transducer and a hybrid sensing material, and a control unit 20 in a system unit. The control unit 20 is connected to the transducer, controls the operation of the transducer and, in operation, receives the output from the transducer. The transducer may be evaluated by the control unit 20 with regard to its impedance (behavior). Optionally, the system unit further comprises a display 21. The sensor device 10 with the hybrid sensing material 14 can be one of those presented below with respect to figures la, l b, Id- I f. Optionally, the sensor device comprises a heater and/or a temperature sensor (not shown). Fig. l a shows a sensor device 10 comprising a transducer 12 and a hybrid sensing material 14. The transducer 12 may, for example, be formed by two electrodes 12a and 12b facing each other, wherein the hybrid sensing material 14 is arranged in between. In other words this means that the hybrid sensing material 14 forms a dielectric layer having a dielectric constant, wherein the sensitive layer 14 is arranged on top of the electrode 12a.

The electrodes 12a and 12b may be made of any conducting materials (for example an inorganic conductor, an organic conductor or a mixed organic/inorganic conductor), preferably a metal or a conducting polymeric material, more preferably a metal selected from the group of Ni, Au, Ag, Pt, or combinations thereof.

The sensitive layer 14 changes its electrical property, e.g., its dielectric behavior by modifying the C02 concentration 1 1 present in the surrounding environment. The transducer 12 transforms the received chemical information, here the changed dielectric constant, from the sensitive layer 14 into a measurable electrical signal. This may be based, for example, on the principle that the two electrodes 12a and 12b may form a capacitive sensor having the sensitive layer 14 in between which form the dielectric layer. Due to the change of the dielectric behavior of the sensitive layer 14, the capacitance or especially the impedance of the sensor device is changed so an electrical signal indicative for the impedance may be output by the transducer. The principle for determine the impedance will be discussed in detail referring to Fig. 3 and 4. Regarding the sensitive layer 14 it should be noted that the same is in contact to the C02 11 included by the surrounding. For example, the electrode 12a in figure la may be perforated or may have an opening to the surrounding for the C02 1 1. Electrodes 12a and

12b in figures la may have a comb-like shape and may be interdigitated. Fig. lb shows an alternative sensor device comprising a transducer 12 ^* and a hybrid sensing material 14 laterally arranged within a common layer. Again, the transducer 12' may, for example, be formed by two electrodes 12a' and 12b' positioned on a substrate 13, wherein the hybrid sensing material 14 is arranged in between. Electrodes 12a' and 12b' in figure lb may have a comb-like shape and may be interdigitated. Additionally, the hybrid sensing material may at least partially cover electrodes 12a' and 12b'.

The electrodes 12a' and 12b' may be made of any conducting materials (for example an inorganic conductor, an organic conductor or a mixed organic/inorganic conductor), preferably a metal or a conducting polymeric material, more preferably a metal selected from the group of Ni, Au, Ag, Pt, or combinations thereof.

In the case of figures l a and lb, the electrodes 12a, 12b resp. 12a', 12b' are connected to the control unit 20, which comprises a microcontroller 22 and an impedance measuring chip 23, as shown in figure l c. The impedance measuring chip 23 is connected to the respective electrodes 12a, 12b resp. 12a', 12b' or, in general to the transducer. Due to the connection the impedance measuring chip 23 can drive the electrodes 12a, 12b resp. 12a', 12b' (transducer) using a signal, e.g. an AC signal so as to performed the impedance measurement. The microcontroller 22 is connected to the impedance measuring chip 23. Optionally, the sensor device comprises a heater and/or a temperature sensor (not shown) and the microcontroller is connected to the heater and/or the temperature (if present).

Several approaches for driving the impedance measuring chip 23 are generally possible to determine the impedance (capacitance and/or resistance) variation: oscillators, charge based circuits or impedimetric spectroscopy, for example. For the oscillator method, the unknown capacitance of the transducer (e.g. 12, 12') is used to modify an oscillator circuit. The change in its resonance frequency is converted to the value of the capacitor's capacitance. In the charge based circuits, the capacitor of the transducer to be measured and the reference capacitor are at first step discharged. In the second step, the capacitor to be measured is charged. The last step permits to discharge the capacitor to be measured by connecting it to the reference capacitor. According to this approach, the impedance measuring chip 23 comprises an oscillator (circuitry) or another entity configured to output an alternating control signal (AC signal) to the transducer. Furthermore, the impedance measuring chip may be configured to determine the oscillating frequency of the transducer, where the oscillating frequency is indicative for the C02 concentration.

A further method is called Electrochemical impedance spectroscopy (EIS). EIS is a method that describes the response of an electrochemical cell to a small amplitude sinusoidal voltage signal (control signal having an AC portion) as function of frequency. The electrical impedance (Z) is defined as the ratio of the voltage across the device and the current flowing through it. The capacitance is derived mathematically from the capacitor impedance value. According to this approach the impedance measuring chip 23 is configured to drive the transducer using an AC control signal having a small amplitude and to determine the resulting voltage and current through the transducer to calculate the impedance (Z).

The actual tendency goes in the direction of integrating the read out electronics on a single chip. Each of the above capacitance measurements techniques can be miniaturized for a successful integration of signal processing circuitry with miniaturized capacitance sensors. Most of the existing cheap chips designed to measure capacitance of a chemical sensors look to the discharge times of RC networks. This means, that the entities 22, 23 and 10 may be implemented into one device, Fig. 3 shows a diagram of an electrical impedance measurement of the APTMS grafted on silicon dioxide nanoparticies at different relative humidity (RH = 20%, RH = 40%, RH = 60% and RH = 80%). In the wide y axis, the bars depict the amount of C02 injected into the gas measurement setup having C02 sensors. Fig. 4 shows the same diagram for a different measurement obtained in the case of amine impregnated on silicon dioxide nanoparticles, Figure Id shows an further embodiment. Here, the sensor device 10" comprises a field effect transistor with a pair of source/drain regions 12c, 12c' in a semiconductor substrate, a channel CH, a gate dielectric GD and a suspended gate electrode 12c", wherein the hybrid sensing material 14 is positioned between the channel CH and the gate electrode 12c". Due to the dielectric behavior of the sensitive layer 14 in the presence of C02, the source/drain current in the channel CH is changed so that an electrical signal responsive to the C02 concentration may be output by the transducer (i.e. field effect transistor 12c, 12c', 12c", GD, CH), Preferably, a gas inlet 15 is between the hybrid sensing material 14 and the gate dielectric CH. The gate electrode 12c" may be made of any conducting materials (for example an inorganic conductor, an organic conductor or a mixed organic/inorganic conductor), preferably a metal or a conducting polymeric material, more preferably a metal selected from the group of Ni, Au, Ag, Pt, or combinations thereof. Any semiconductor material may be used for the semiconductor substrate, e.g. Si, GaAs, and Ge.

In this case, the control unit 20 comprises a microcontroller 22 that is connected to the source, drain 12c, 12c' and gate electrodes 12c" of the field effect transistor to control the electric potentials applied thereto and reads out the output of the field effect transistor. Fig. le shows a further embodiment. Here, the sensor device 10"' is comparable to the devices of Figs, l a or l b, but further comprises a light emitting device LED (e.g. a light emitting diode or a laser), marked by the reference numeral 31, a light receiving device LRD (e.g. a photodiode), marked by the reference numeral 32. The hybrid sensing material 14 is positioned between the light receiving device 31 and the light emitting device LRD 32 so that the light beam emitted by the light emitting device LED 31 propagates through the hybrid sensing material 14 and is received by the light receiving device LRD 32. Due to the dielectric behavior of the sensitive layer 14 in the presence of C02, the optical transmission of the hybrid sensing material is changed so that an optical signal detected by the light receiving device LRD 32 changes.

In another embodiment, the light receiving device LRD 31 and the light emitting device LED 32 are positioned such that light emitted by the LED 32 is reflected by the hybrid sensitive layer 14 and received by the LRD. In these embodiments, an colorimetric pH indicator may be added to the hybrid sensing material 14 so that the LRD 32 also detects color change of the pH indicator. Then, the sensor device is used to measure C02 concentration and the pH in the surrounding. In Fig. le, the sensor device 10"' comprises first and second electrodes 12a" and 12b", as for the device of Fig. la and lb, in contact with the hybrid sensing material 14, the hybrid sensing material 14 being arranged in between the two electrodes 12a" and 12b".

In this alternative, the sensor unit 10"' comprises a microcontroller 22 and an impedance measuring chip 23, as for figures la to lb. The microcontroller 22 of the control unit 20 is connected to the light receiving device LED 31 and/or at least to the light emitting device LRD 32 to control the light emission of the LED 31 and reads out the output current of the LRD 32 As in the case of figures la and l b, the electrodes 12a", 12b" are connected to the impedance measuring chip 23 of the control unit, similar to figure lc. The detection of C02 is then based both on the electric (as in Figs, l a, lb and lc) and optical properties of the hybrid sensing material. The change in the refractive index can be monitored using reflection or transmission in the same time that the an AC electrical field is applied to the electrodes. This method combining electrical and optical measurements permits to have a stable reversible process of detected C02. Any adsorption of the C02 molecule on the amine functionalized sorbent induces a change in the dielectric constant and therefore the refractive index changes is detected optically.

In an alternative, the control unit does not comprise an impedance measuring chip 23 and the microcontroller 22 is connected to the electrodes 12a", 12b", the light receiving device LED 31 and the light emitting device LRD 32. The detection of C02 is then only based on the change in optical properties of the hybrid sensing material 14.

Figure If shows a further embodiment of the sensor device 10"". In this case, the sensor device 10"" according to figures la, lb, lc, Id and le further comprises a housing 25 with an first opening 26 and a second opening 27 and a micro pump 24. The sensing layer 14 is positioned between the first and second openings 26, 27. The micro pump may, for example, be positioned at the second opening 27. In operation, the micro pump 24 evacuates gas from the inside of the housing to create a gas flow from first opening 26 to the second opening 27. In a different operational mode, the micro pump 24 pumps gas into the housing 25 so as to create a gas flow from the second opening 27 to the first opening 26. When the sensor device 10 is placed in a system unit with a micro pump 24, as shown in figure If, the micro pump 24 is able to deliver gas to the sensor in order to control the response and the recovery time of the sensor. A regular flow of air on the sensor surface using a pump could bring a significant pulse to make the C02/amine chemical reaction fast and reversible.

In the case of the devices according to figures la - lc, If, the impedance (capacitance, resistance, ...) or dielectric constant is measured by the impedance measuring chip 23 connected to the respective electrodes. In the device according to figure le, the microcontroller or the impedance measuring chip is connected to the respective electrodes. In a chemisorption process, a chemical reaction takes place between the gas molecule and the absorber. The electrical field created inside and on the surface of the sensing material can influence the chemical forces between the gas molecule and the absorber. Therefore, the chemical reaction can be tuned by choosing the right electrical field strength. In our case, in the presence of gas molecules, an applied alternative voltage applies a force on the adsorbed gas molecules on the existing functional groups within the sensitive layer. Therefore , it is advantageous to measure the electrical property that changes with C02 concentration by a method with an alternative voltage applied to the respective electrodes of the transducer, i.e. electrodes 12a, 12b, 12a', 12b', 12a", 12b" in the case of the devices of Figs la, lb, lc, le and If.

The above mentioned properties, especially with regard to the response and recovery times, the accuracy, particularly for sensing at room temperature and the energy demand, result from the material used as sensitive layer 14. As discussed above, the material for the sensitive layer 14 is a so-called hybrid (organic-inorganic) material comprising nanoparticles and amines, e.g. in the shape of a polymer containing functional amine groups. Alternately, the sensitive layer 14 may comprise further polymers to achieve a homogenous stable mixture.

Regarding Fig. 1 it should be noted that, the sensor device 10 may optionally comprises a heater and/or a temperature sensor (not shown). In this case the microcontroller 22 is connected to the impedance measuring chip 23 and to the optional heater and/or temperature sensor. Optionally, the control unit 20 discussed in context of Figs.1 and lc can further comprise a display 21 as user interface.

The developed hybrid material combining organic and inorganic material will be discussed below in detail. It should be noted here that discussion below belongs to the preferred embodiments, wherein other implementations may also be possible.

The layer 14 responds to a small change in the concentration of carbon dioxide 1 1 in its vicinity by changing its electrical properties, impedance, capacitance, resistance, dielectric constant. As, for example, a capacitance variation takes place as a consequence of a change in the C02 concentration, a simple electronic circuit can be used to evaluate the sensor signal. Amine based solid sorbents are one way to detect C02 in gas phase. Several polymers containing amino groups (see Table) were tested as sensitive layer to detect C02. The used polymer layer comprises two or more amine based polymers which can contain primary and/or secondary and/tertiary amines.

Polymer having amine groups

3-aminopropyitrimethoxysilane ( APT S)

3-Aminopropyi)triethoxysi!ane (APTES)

N-[3-(trimethoxysilyl) propyl]-ethylenediamine (AEAPTS)

Polypropyleneimine (PPI)

mono (di)ethanolamine (MEA (DEA))

Polyethyleneimine (PEI)

2-diethylamino-ethanol (DEEA)

1 ,4 diaminobutane (DAB)

1 ,3-propanediamine (DIAP)

^" 2, 2-dTmethyl- 1 , 3-propaned]amine (DMPDAY ^~

1 - piperazineethanol (HEP)

2- diisopropylamino-ethanol (DIPAE)

N1 -methyl- 1 , 3-PropanediamTne (MAPA) ^{~ ~}

"Poiya iylamine (PAA) ^{~ ' ~ ~}

Tetraethylenepentamine (TE A)

The reaction mechanism of polymer containing primary amino groups with C02 is based on the reversible formation of carbonate or bio carbonate products, as illustrated below. The chemical reactions of C02 with primary, secondary and tertiary amino groups in the presence or absent of water.

Reaction of C02 with primary amino groups:

2R1NH2 + C02 = [R 1NC02 J - + [R1NH 3 ]+ carbamaie

2 R1NH2 + C02 + H20 ^~ [R1N H3 ]+ + [HC03]- bicarbonate Reaction of C02 with secondary amino groups:

R ₂ R ₁ NH + C0 ₂ + H ₂ 0 ≠ [R ₂ R _t NH] ⁺ + [HC0 _S ] ^~ Reaction of C02 with tertiary amino groups:

R _U N + C0 ₂ + H _z O ≠ [R- _j NHV + \HC0 ₃ Y

The reaction takes place by an interaction between the adsorbate molecule C02 and the functional group on the adsorbents surface. A physisorption interaction occurs due to van der Waals forces, which are rather weak and ensure a fast desorption process. A chemisorption process is related to a chemical interaction, which is more stable and under these conditions irreversible requiring a high temperature desorption step.

Preferably, the nanoparticles may be selected from Si02, A1203, ΊΊ02, Zr02, and the like, and combinations thereof. The nanoparticles diameter may be in the range from 1 nm to 1000 nm, preferably in the range from 10 nm to 900 nm, preferably in the range of 50 nm to 750 nm, preferably in the range of 100 nm to 500 nm, preferably in the range of 150 nm to 450 nm. Preferably, in the hybrid sensing material, the ratio between the total weight of nanoparticles and the total weight of polymer-based amines is in the range from 1 :5 to 5: 1.

The inorganic nanoparticles can be porous or non-porous. The porous nanoparticles materials have large surface area, high surface-to-volume ratios and different pore sizes.

The large surface areas and pore size distribution are a favorable parameters for ligand adsorption in and on sorbent. The porosity increases the ligand concentration and the gas rate diffusion. Several synthesis methods for synthesis of nanoparticles materials exist. The preferred methods for synthesizing of porous and non-porous materials should be fast and inexpensive.

In general, functional ization of inorganic sorbents, e.g. nanoparticles, considered as carriers or supports in the matrix, might be obtained following two approaches: physical impregnation or covalent grafting of the desired functional groups, ligands, on the sorbents surface and inside the pores.

Bellow, different methods for producing hybrid sensor materials will be discussed The wet impregnation method can be used method to incorporate the desired ligands having the chosen functionalities into a sorbent. On one side, the desired sorbent, called also carrier or support, or many of them, are dissolved into a liquid solvent preferably a volatile solvent. On the other side the ligands are also dissolved into a liquid solvent preferably a volatile solvent. Then, both the carrier and the ligand solution are physically mixed and stirred for a while. It is advised to choose solvents having high dissolving ability of the precursor and the ligand but a weak interaction with them. Subsequently, the excess solvent is removed by evaporation at atmospheric pressure or by using rotary vapor at adequate pressure adapted to the used solvent. In contrary to air drying under defined pressure, the hybrid suspension is first frozen and then subsequent freeze drying led to a high loading of organic material on the inorganic matrix.

Another production method is called the grafting method. Grafting describes the formation of covalent bonds between the carriers and the ligands. Grafting can be divided into different types: (a): "grafting from" process: in this case monomers react with the surface functional groups of the matrix and the ligands are then formed in situ by living polymerization. This process allows the grafting of higher amounts of polymers than the (b) "grafting onto" process. The latter is defined as the reaction of polymers containing functional groups with the surface functional groups of the matrix. This allows a control of the grafted polymer chain length by controlling the grafting rate parameter.

In the case of amine functionalized inorganic sorbents, the functionalization is mostly done by silanization, where the desired functional groups can be anchored and immobilized using different type of organic polymers. If amine groups are desired on the sorbents surface, aminosilane are considered as a promising candidate. Silanization is defined as method of covering the matrix surface with organo-functional alkoxysilane molecules.

Below, the method for manufacturing the sensor 10- 10" " of Figs, la to le will be discussed in detail taking reference to the flow chart of Fig. 2. Fig. 2 shows the method for manufacturing 100 comprising the two basic steps 102 of providing the transducer and 104 for providing the sensitive layer.

The transducer 12 may be provided on a substrate (glass or any other suitable material) or may comprise a substrate. After providing the transducer (step 102) carbon dioxide- sensitive material is provided, preferably on top of the transducer 12 or in between the respective electrodes, see for example Fig. 2 (cf. step 104). The step 104 is based on the assumption that the hybrid sensing material is available. The material - resulting either from impregnation or grafting - can be the hybrid organic/inorganic material under the form of powder. In order to obtain a solid layer (coating), for example to be used in the devices as shown in figures la to le, a coating procedure can be used. The coating process can be performed by different techniques. If the functional materials show agglomeration and big particles size, a grinding step can be included and adapted to the transducer size. The powder material containing the functional ized nanoparticles is dissolved in appropriate solvent with the appropriate amount to reach the desired viscosity (step 106). Generally, the solvent with the functionalized nanoparticles is prepared in a way to flow on substrate surfaces having different topographies. Multi-coating process is another possible method to realize stacks and thick layers. The suitable solvent is selected so that a good dispersion of the nanoparticles is obtained. The solution's viscosity is another parameter influencing the required layer thickness. If the prepared nanoparticles show a weak adhesion to the substrate, a polymeric adhesion layer can be integrated between the devices and the layer having the sensing materials. It serves as an interconnecting layer between the substrate and the layer containing nanoparticles. Two possibilities are envisaged. The first possibility is that the functionalized nanoparticles are embedded in the polymeric adhesion layer. The second possibility is that the sensing layer having functionalized nanoparticles is on the top of the polymeric adhesion layer. This method ensures the mechanical stability of the layer containing functionalized nanoparticles, of course dependent on the nanoparticles size. The polymeric adhesion layer is preferably is neutral which means it does not show any interaction with the gas to be sensed. The providing of the sensitive layer 14 may be done by using deposition methods such as drop in, spin coating, spray coating, dip coating, doctor blade or other known deposition methods to coat an element like a transducer. Some of these methods require that the hybrid sensitive material, or in more detail, the nanoparticles and the amines are dispersed in an appropriate solvent before same can be provided. Therefore, the method 100 comprises an step 106 of dispersing the hybrid sensitive material in a solvent.

In order to immobilize the hybrid sensitive material, e.g., on the transducer, the manufacturing method 100 comprises a step of drying 108 subsequent to the step 104 of providing the hybrid sensitive material.

The method for manufacturing will be discussed in detail below, wherein the features discussed below are just design variants of the basic method 100 discussed above, i.e., optional features. Here, inorganic nanoparticles are used with, for example, surface-exposed -OH groups which facilitate the functionalization with amine based polymers. Therefore, the amine groups can be strongly attached to the nanoparticles surface via covalent or ionic bonding, depending on the nature of the polymer containing the amino-functionalized extremity. A stable immobilization is desired to ensure thermal stability of the amino-groups on the surface of the nanoparticles at temperatures below the thermal decomposition temperatures of the used polymer. The nanoparticles (like Si02, A1203, Ti02, Zr02,...) are dispersed in an appropriate solvent (e.g. water, ethanol, methanol, etc.) and mixed with one or several polymer-based amino-groups under vigorous stirring for a necessary time to get a homogeneous mixture. To achieve a bonding between the nanoparticles and the amino-groups, the stirred solution undergoes a drying step (e.g. under vacuum) for some hours. The resulting material is powdered and contains amine groups impregnated or grafted on the surface of the nanoparticles , The ratio of applied nanoparticles to amino-polymer determines the density of amine groups on the nanoparticles surface. This ratio is adjustable depending on the desired application. The nanoparticles and the polymeric chains can be selected from various commercially available materials. To coat the transducer, it is necessary to dilute the prepared amine functionalized nanoparticles in an appropriate solvent (e.g. water, ethanol, methanol, etc.) and then to dispense it on the electrodes and dry it under appropriate condition and at temperatures, for example, between 20°C to 200°C for some time (e.g. 5 minutes - 5 h).

Examples - Manufacturing 1

The grafting synthesis of amine group on Si02 nanoparticles was accomplished via a reflux synthesis method. First the desired matrix (Si02) was weighed into a 100 mL 3- neck round-bottomed flask. A solvent was added to the nanoparticles and the mixture was stirred for a while. Subsequently the ligand (APTMS) for example was added to the mixture and the mixture with the ligand was heated under reflux for some hours under a through-flow of gas. Next, the residual solvent was removed by evaporation and the remaining product hybrid organic -inorganic material is obtained under the form of a solid powder.

Examples - Manufacturing 2 The impregnation synthesis of amine group on Si02 nanoparticles as follow. First the desired matrix was weighed, the appropriate solvent was added and the matrix mixture was stirred for some minutes. The desired ligand (having primary amines and /or secondary amines) was mixed with a solvent and stirred for some minutes. Next, the ligand solution was added to the matrix solution dropwise with whereby the stirring was continued for some minutes. Subsequently, the solvent was removed and the solid called hybrid organic- inorganic material was dried so as to obtain a solid powder.

Examples - Manufacturing of a coating

Then the obtained hybrid organic -inorganic material, either from impregnation synthesis or grafting synthesis, was diluted in appropriate solvent to obtain an homogenous solution with the desired viscosity. A pair of interdigitated gold microelectrodes are formed on a glass substrate so as to form a chip. Each chip consists of pair of interdigitated electrodes, a heater and a temperature sensor. The chip was glued and gold wire bonded to a designed printed circuit board (PCB). Next, the transducer with the interdigitated electrodes were coated with a thin film by spin coating or drop coating and dried to evaporate the remaining solvent and obtain a dry layer on the top of the electrodes. Now the transducers coated with a layer having a sensing material are ready for electrical measurements.

Electrical impedance spectroscopy measurements were performed using a SOLAT ON 1260 A gain-phase frequency analyzer controlled by a PC, allowing automated data collection. Up to ten chips can be measured in the same gas environment. The electrical properties such as permittivity, impedance and conductivity provide valuable information for assessing the sensitivity of the sensitive layer towards a specific analyte. The impedance measurements can be accomplished at different frequencies (1 MHz-1 Hz). The chips can be heated to different temperatures (from 25 °C to 200 °C). In this case, the capacitance was recorded at 40 kHz, 0 V DC voltage and 300 mV AC peak-to-peak.

The gas stream consisting of a mixture of synthetic air, C02 and water vapor is introduced and distributed radially into the circular chamber. The whole chamber and pipe lines were heated to an appropriate temperature to avoid water condensation on the pipes walls.

In figure 3, the level of relative humidity (RH) was changed between 20, 40, 60 and 80%. The temperature of the chamber was kept at a constant value of 25 °C whereas the chips were heated to an operation temperature of around 60 °C by applying a voltage to the heater. The change in the resistance of the temperature sensors is correlated to the chip temperature, which is supposed the same as the sensitive layer. The amount of C02, which is introduced into the measurement chamber, was varied from 400 to 2000 ppm by a step of 400 ppm. The observed change in capacitance is correlated to the presence of different amounts of C02 at a certain relative humidity. The response curves reveal a correlation between the sensor capacitance, CQ2 amount and RH level in the surrounding environment. The reference capacitance (measured at 400 ppm C02) increases by increasing RH from 20 to 80%. The capacitance change depending on relative humidity and C02 concentration and the operation temperature. Increasing the C02 concentration above 400 ppm leads to a decrease in the capacitance. Each C02 concentration from 400 to 2000 ppm has its own fingerprint regarding capacitance values. The sensing behavior of the C02 sensor monitored under different concentrations of C02 and RH levels confirms the reversibility of the hybrid sensitive layer. Therefore, the developed hybrid nanomaterial shows great potential for various applications requiring C02 detection, including indoor air quality control.

In an integrated device, the transducer having the two interdigitated electrodes coated with the sensing material is connected to impedance measuring chip designed for impedance measurements. An on board frequency generator is an integral part of the impedance measuring chip. The impedance measuring chip applies a small AC voltage excitation to the interdigitated electrodes with a specific frequency. The AC voltage amplitude is adjustable. At a given temperature and in the presence of gas molecules, the applied alternative voltage applies a force on the adsorbed gas molecule on the existing functional groups within the sensitive layer. A DC bias can be added if necessary. The impedance measuring chip is connected to microcontroller for saving the data and sending the desired parameters to a display. For the present sensor devices, the preferred parameters for operation are the following: a) operating temperature between -30°C and 200°C.

b) frequency between IHz and IMHz.

c) applied AC and/or DC bias voltage, ranging between -30V and 30V In the case of a reaction between the amine groups and the C02, the AC frequency used is preferably lower than IMHz in order to allow the gas molecules to be caught and to be easily repealed by the amine groups alternatively if measured in a dynamic mode, where different concentration of C02 gas and carrier gas are cycled. This relationship between the resent C02 concentration and the measured electrical sensor signal will be discussed referring to Fig. 4. Fig. 4 shows for example the sensor response - for example capacitance C in pF - to a C02 concentration at 60°C for different relative humidity levels RH. The hybrid sensing material comprising polymer having primary 62650

amines and Si02 nanoparticles having a diameter of 300 ran is coated between the gold microelectrodes. The prepared solution (comprising solvent with amine based polymer and Si02 nanoparticles) is dispensed on gold interdigitated electrodes of the transducer on glass substrate by spin coating technique. Although spin coating was used in the example of figure 4, any other deposition technique can be used.

The sensor was heated to 60 °C by applying a voltage to the integrated heater on the transducer. The capacitance value is measured under the defined gas stream having different concentrations of C02. The sensor sensitivity is evaluated by recording change in film capacitance with respect to base line in this case (RH I CO2):(20% I SOOppm), see Fig. 4.

C02 fluxes of different concentrations were introduced in a step-wise way in the chamber ("dynamic mode"). For each relative humidity level, the response of the sensor for C02 concentrations in the range 500 - 3000 ppm was traced. At high relative humidity levels (RH = 60% and RH = 80%) a slight drift of the base line capacitance could be observed.

The response time is calculated from the capacitance decrease when introducing the synthetic air/ C02 mixture at a desired relative humidity inside the chamber. The recovery time of the capacitive sensors was determined by cutting off the C02 flow (500 ppm) and introducing only synthetic air at a desired relative humidity concentration. The response and recovery times were determined at t90 to be less than 2min. t90 describes the time needed for the signal to attain 90% of the difference between two states.

The response curves reveal a correlation between the sensor capacitance, C02 amount and RH level in the surrounding environment. The capacitance value increases by increasing RH from 20 to 80% at a constant C02 concentration. The delta capacitance depending on relative humidity is of lpf/10% RH. Increasing the C02 concentration above 500 ppm leads to a decrease in the capacitance. Each C02 concentration from 500 to 3000 ppm has its own fingerprint regarding capacitance values. The sensing behavior of the C02 sensor monitored under different concentrations of C02 and RH levels confirms the reversibility of the hybrid sensitive layer. Therefore, the developed hybrid nanomaterial shows great potential for various applications requiring C02 detection, including indoor air quality control. With regard to the embodiments of Figs, la to If it should be noted that the sensor device 10 may optionally comprise additional elements like a heater or a temperature sensor (not shown). The heater and/or temperature sensor may be positioned laterally with respect to the transducer or vertically below the transducer or vertically above the transducer. The relative positions are not limited as long as the heater and temperature sensor function in the desired way. Both enable to operate the C02 sensor 10 within the proper temperature range. The sensing temperature is preferably between, i.e. -30°C to 200°C. The temperature range plays a role with regard to the power consumption as well as with regard to the response and recovery times.

The above characterized C02 sensor 10 requires the electrical power to detect C02 (cf. reference numeral 11), for example, in the range between 500 ppm and 10,000 ppm. The transducer power consumption mainly results from the heater 16 used to reach the desired working temperature. Increasing the sensor operating temperature from room temperature to 60°C increases the power consumption to some mW. Therefore, the C02 sensor 10 can target mainly applications going from indoor air control to automotive applications.

The above mentioned properties, especially with regard to the response and recovery times, the accuracy, particularly for sensing at room temperature and the energy demand, result from the material used as sensitive layer 14. As discussed above, the material for the sensitive layer 14 is a so-called hybrid (organic-inorganic) material comprising nanoparticles and amines, e.g. in the shape of a polymer containing functional amine groups.

Some aspects have also been described in context of an apparatus and it is clear that these aspects also represent a description of the corresponding method, wherein a block or a device corresponds to a method step or feature of a method step. Analogously, aspects described in context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. Some or all of the method steps may be executed by (or using) a hardware apparatus, like, for example, a microprocessor, a programmable computer or an electronic circuit. In some embodiments, some, one or more of the most important method steps may be executed by such an apparatus. In some embodiments, an application specific integrated circuit (AC) or programmable logic device (for example a field programmable gate array) may be used to perform some or ail of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may be incorporated with a microprocessor in order to perform one of the methods described herein. Generally, the methods are preferably performed by any hardware apparatus.

The above described embodiments are merely illustrative for the principles of the present invention. It is understood that modifications and variations of the arrangement and the details described herein will be apparent to a person skilled in the art. It is the intent thereto to be limited only by the scope of the impending patent claims and not by specific details presented by way of description and explanation of the embodiments herein.

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