Patent Description:
There are toxic gaseous substances which can cause health problems when a human body is exposed thereto even for a relatively short time. One of them is formaldehyde which is industrially used in a versatile manner (for example, in the production of plastics, in the construction industry, in the textile industry for crease-resistant and easy-care finishing, in the agricultural and food industries as a preserving agent, in cosmetics, body and mouth care products as a disinfectant and sterilant, etc.). Despite its high usability, short-term exposure to small formaldehyde doses (<NUM>-<NUM> ppm) is commonly accompanied by annoyance symptoms related to its inhalation in the upper respiratory tract. Exposure to higher formaldehyde doses have been reported to induce various manifestations including headache, drowsiness, and irritation to eye, nose, and throat. Much higher formaldehyde doses (<NUM> ppm) induce cytotoxic effects (cell and tissue damage). Moreover, formaldehyde has been classified as a human carcinogen because it causes nasopharyngeal cancer, pulmonary damage and probably leukemia at doses higher than <NUM> ppm.

Thus, formaldehyde concentrations which need to be detected are rather small - at the level of several tens of ppb. This makes it difficult to provide effective and rapid detection and measurement of formaldehyde in the air. Currently available portable formaldehyde sensors provide an insufficient limit of detection, which may lead to an erroneous reduction of an imminent health risk caused by formaldehyde exposure. Moreover, they demonstrate poor performance in terms of selective detection or classification of gas admixtures in the air.

Document <CIT> describes a method and a system of Pt and SnO<NUM> co-functionalized on single-walled carbon nanotubes (SWNTs) assembled on microelectrodes by electrochemical deposition where Pt nanoparticle's morphology, size, and density were tuned by controlling electrodeposition potential and time. The method and system to obtain the optimum condition for Pt decorated SnO<NUM>/SWNTs (Pt/SnO<NUM>/SWNTs) were performed and also correlate with its CO sensing performance. Light dependent sensing performance was examined with red, green and UV LED light under room temperature. With the assistance of the UV LED light illumination, the sensitivity of Pt/SnO<NUM>/SWNTs was further enhanced to <NUM> percent /ppmv to <NUM> ppmv of CO and the detection limit can push down to <NUM> pp mv.

Document <CIT> relates to a method of co-functionalizing single-walled carbon nanotubes for gas sensors, which includes the steps of: fabricating single-walled carbon nanotube interconnects; synthesizing tin oxide onto the single-walled carbon nanotube interconnects; and synthesizing metal nanoparticles onto the tin oxide coated single-walled carbon nanotube interconnects.

Document <CIT> aims to solve a plurality of problems existing in an existing VOCs (Volatile Organic Compounds) gas sensor, and provides a novel method for preparing a gas sensitive material based on a modified CNT (Carbon Nanotube)/metal nanoparticle (MNPs)/metal oxide nanoparticle (MONPs) hybrid material. The material can be applied to manufacturing of a low-energy-consumption high-sensitivity VOCs gas sensor. This document further provides a manufacturing method of the VOCs gas sensor based on a CNT/MNPs/MONPs gas sensitive material. By adopting the preparation method, the VOC gas sensor can be minimized, is high in consistency and super quick in response, and has extremely-low detection power consumption at the same time. Meanwhile, a preparation process flow is simple, and scale production can be realized. The gas sensor is structurally characterized by comprising a polyimide (PI) flexible substrate layer undergoing a hydrophobic treatment, a metal electrode layer and a CNT/MNPs/MONPs gas sensitive material layer in sequenced from bottom to top.

This summary is not intended to identify key features of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure.

The present invention is set out by the set of appended claims. In the following, parts of the description and drawing referring to examples, which are not covered by the claims are not presented as embodiments of the invention, but as illustrative examples useful for understanding the invention. In particular, the embodiments of the invention are determined by the appended claims.

It is an objective of the present disclosure to provide a technical solution that allows detection of gases (especially, formaldehyde) in the air.

The objective above is achieved by the features of the independent claims in the appended claims. Further embodiments and examples are apparent from the dependent claims, the detailed description and the accompanying drawings.

According to a first aspect, a gas sensing structure is provided. The gas sensing structure comprises a carbon nanotube (CNT) film, an n-type or p-type transition metal oxide (TMO) layer deposited on the CNT film, and noble metal nanoparticles deposited on the n-type or p-type TMO layer. This gas sensing structure is susceptible to changes in concentrations of different gases, i.e. gas vapors present in the air. More specifically, it may be used to detect changes in formaldehyde concentration in the air, starting from few parts per million (ppm) to few tens of parts per billion (ppb).

In one embodiment of the first aspect, the CNT film is made of single-walled CNTs. The single-walled CNTs may provide a substrate with good conductivity which is a framework or support for the thin TMO layer.

In one embodiment of the first aspect, the n-type or p-type TMO layer comprises one of Mn<NUM>O<NUM>, TiO<NUM>, ZnO, SnO<NUM>, NiO, Fe<NUM>O<NUM>, and Cu<NUM>O. Each of these oxides may provide a high sensitivity of the gas sensing structure according to the first aspect.

In one embodiment of the first aspect, the noble metal nanoparticles are made of Au, Ag, Pt, Pd. Similarly, the nanoparticles of Au, Ag, Pt or Pd may increase the sensitivity of the gas sensing structure according to the first aspect.

According to the invention, the CNT film has a thickness of <NUM> to <NUM>. Such a thickness of the CNT film may enable its easier further processing (e.g., plasma treatment, TMO deposition, etc.). Moreover, it may assure the mechanical stability of the CNT film in particular and the gas sensing structure in general.

According to the invention, the n-type or p-type TMO layer has a thickness of <NUM> to <NUM>, and preferably <NUM> to <NUM>. Such a thickness of the TMO layer may provide a high response of the gas sensing structure according to the first aspect towards gas vapors.

According to a second aspect, a method for fabricating a gas sensing structure is provided. The method starts with the step of synthesizing a carbon nanotube (CNT) film. Then, the method proceeds to the steps of arranging the CNT film on a plate and depositing an n-type or p-type transition metal oxide (TMO) layer on the CNT film. After that, a next step is initiated, in which noble metal nanoparticles are deposited on the n-type or p-type TMO layer. The method ends up with the step of releasing the CNT film from the plate. By so doing, it is possible to provide a gas sensing structure that is susceptible to changes in concentrations of different gases, i.e. gas vapors present in the air. More specifically, such a gas sensing structure may be used to detect changes in formaldehyde concentration in the air, starting from few ppm to few tens of ppb. In one embodiment of the second aspect, the step of synthesizing the CNT film is performed by using an aerosol chemical vapor deposition technique, and the synthesized CNT film is comprised of single-walled CNTs. By using this technique, it is possible to obtain single-walled CNT films of high quality. In turn, the single-walled CNTs may provide a substrate with good conductivity which may serve as a framework or support for the thin TMO layer.

In one embodiment of the second aspect, the plate has an opening made therethrough, and the step of arranging the CNT film comprises arranging the CNT film such that the CNT film covers the opening on one side of the plate. The presence of the opening allows the CNT film to be treated from above and from below simultaneously. For example, the opening may enable uniform plasma treatment, TMO deposition and doping by noble metal nanoparticles, as well as allow dry processing of the CNT film. Furthermore, the opening may make it easier to release the ultrathin CNT film from the plate at the final step of the method according to the second aspect.

In one embodiment of the second aspect, the step of depositing the n-type or p-type TMO layer is performed by using an atomic layer deposition (ALD) technique. In this embodiment, the n-type or p-type TMO layer may comprise one of Mn<NUM>O<NUM>, TiO<NUM>, ZnO, SnO<NUM>, NiO, Fe<NUM>O<NUM>, and Cu<NUM>O, and the plate may be made of a material that withstands temperatures used in the ALD technique. The ALD technique allows one to obtain the thin and uniform TMO layer, thereby facilitating the miniaturization and high sensitivity of the gas sensing structure.

In one embodiment of the second aspect, the step of depositing the noble metal nanoparticles is performed by using an aerosol doping technique, and the noble metal nanoparticles are made of one of Au, Ag, Pt, Pd. The aerosol doping technique allows the nanoparticles to be distributed uniformly on the surface of the TMO oxide, with no nanoparticle agglomeration being formed.

In one embodiment of the second aspect, the noble metal nanoparticles are made of Au, and the aerosol doping technique is based on using HAuCl<NUM> as a dopant. HAuCl<NUM> is an Au containing compound which is relatively stable and soluble in various solvents (e.g., ethanol, acetonitrile, tetrahydrofuran, dimethylformamide, etc.).

In one embodiment of the second aspect, the method further comprises, before the step of depositing the TMO layer, the step of forming defect sites along sidewalls of CNTs of the CNT film by treating the CNT film in a low-frequency oxygen plasma. The presence of the defect sites may facilitate better deposition of the TMO oxide on the CNT film.

According to a third aspect, a gas sensor is provided. The gas sensor comprises a substrate, a signal electrode structure, and the gas sensing structure according to the first aspect. The signal electrode structure is formed on the substrate and comprises at least two signal electrodes having a gap therebetween. The gas sensing structure according to the first aspect is arranged on the signal electrode structure such that the gas sensing structure bridges the gap between the at least two signal electrodes. Changes in a gaseous environment lead to changes in a measured resistance of the gas sensing structure. Given this, the gas sensor according to the third aspect represents a chemiresistive sensor. With such configuration, the gas sensor is able to detect changes in concentrations of different gases, i.e. gas vapors present in the air. More specifically, it may be used to detect changes in formaldehyde concentration in the air, starting from few ppm to few tens of ppb. Moreover, the gas sensor according to the third aspect is portable, stable, reversible, has a low power consumption, and their response time might be as good as <NUM>-<NUM>.

In one embodiment of the third aspect, the substrate is made of a dielectric or dielectric-coated material that withstands temperatures in a range from a room temperature up to <NUM>. This may allow the gas sensor according to the third aspect to operate in a wide temperature range.

In one embodiment of the third aspect, the gas sensor further comprises a heater and a thermoresistor that are both formed on the substrate for temperature control. This may allow one to carry out temperature calibration of the gas sensor according to the third aspect with respect to each gas present in the air.

In one embodiment of the third aspect, the gas sensor is configured to detect formaldehyde, ethanol or acetone in ambient air. This may make the gas sensor according to the third aspect more flexible in use. In particular, the gas sensor according to the third aspect may be trained to detect these and other gas vapors in the air.

According to a fourth aspect, a multi-gas sensing apparatus is provided. The apparatus comprises the gas sensor according to the third aspect, at least one processor and a memory. In this case, it is required that the signal electrode structure of the gas sensor comprises at least three pairs of the adjacent signal electrodes bridged by the gas sensing structure according to the first aspect. The memory is coupled to the at least one processor and stores processor-executable instructions. Being executed by the at least one processor, the processor-executable instructions cause the at least one processor to receive measurement signals from the gas sensor and use them to determine whether there is at least one of formaldehyde, ethanol and acetone in ambient air. With such configuration, the apparatus may allow selective determination of gas signals from the gas sensor included therein, thereby facilitating detection of different gases present in the air.

In one embodiment of the fourth aspect, the at least one processor is configured to perform said determining by using a pattern recognition algorithm. By using the pattern recognition algorithm, such, for example, as Linear Discriminant Analysis (LDA), Principal Component Analysis (PCA), etc., it is possible to provide selective detection or classification of different gas vapors in the air.

Other features and advantages of the present disclosure will be apparent upon reading the following detailed description and reviewing the accompanying drawings.

Various examples of the present disclosure are further described in more detail with reference to the accompanying drawings. However, the present disclosure may be embodied in many other forms and should not be construed as limited to any certain structure or function discussed in the following description. In contrast, these examples are provided to make the description of the present disclosure detailed and complete.

According to the detailed description, it will be apparent to the ones skilled in the art that the scope of the present disclosure encompasses any example thereof, which is disclosed herein, irrespective of whether this example is implemented independently or in concert with any other example of the present disclosure. For example, the structure, device, apparatus and/or method disclosed herein may be implemented in practice using any numbers of the examples provided herein. Furthermore, it should be understood that any example of the present disclosure may be implemented using one or more of the elements presented in the appended claims.

The word "exemplary" is used herein in the meaning of "used as an illustration". Unless otherwise stated, any example described herein as "exemplary" should not be construed as preferable or having an advantage over other examples.

In the examples disclosed herein, the term "sense" and its derivatives, such as "sensing", "sensor", etc., may refer to an action, operation or step intended to detect gases (especially, formaldehyde), i.e. gas vapors in the air and measure the concertation thereof. Correspondingly, the term "gas sensing" may be considered similar to "gas detection", the term "gas sensor" may be considered similar to the "gas detector", and so on.

Although the examples disclosed herein are mostly related to formaldehyde detection, this should not be construed as any limitation of the present disclosure - i.e. the examples disclosed herein may be similarly and equally used in relation to any other (toxic and/or nontoxic) gases and/or vapors of volatile organic and/or inorganic compounds (VOCs) present in the air. For example, the examples disclosed herein may be used to detect vapors of ethanol, isopropanol and/or acetone in the air.

The most popular method for formaldehyde detection and quantitation is a high-performance liquid chromatography (HPLC) technique which is, though, generally applied to analyze formaldehyde in liquid phase. The combination of the HPLC technique and a gas chromatography technique coupled with mass spectrometry or fluorescent analysis is characterized by high sensitivity and selectivity. There are also other methods for formaldehyde detection, such, for example, as ion chromatography and polarography. However, all these methods require a rather bulky detection apparatus that, at the same time, is rather expensive and has a high power consumption.

Given this and due to the high danger of formaldehyde intoxication and its health risk, various small-scale formaldehyde sensors based on the use of sensing materials as receptors or transducers have been developed in recent years. Most of these sensors are conductometric ones and allow for real-time measurement and day-to-day use. As the sensing materials, these sensors may use metal oxides with nanostructured morphologies, such as nanoparticles, nanotubes, nanofibers, nanowires, nanorods, in order to detect the vapors of the VOCs (including formaldehyde) in the air. However, the existing small-scale formaldehyde sensors still suffer from an insufficiently low detection limit, high power consumption and slow response, as well as demonstrate poor performance in terms of selective detection or classification of gas admixtures in the air.

The examples disclosed herein provide a technical solution that allows mitigating or even eliminating the above-sounded drawbacks peculiar to the prior art. In particular, the technical solution disclosed herein enables detection of gases (especially, formaldehyde) in the air by using an ultrathin gas sensing structure comprising a carbon nanotube (CNT) film, an n-type or p-type transition metal oxide (TMO) layer deposited on the CNT film, and noble metal nanoparticles deposited on the TMO layer. Such a gas sensing structure is characterized by high sensitivity. For example, it is susceptible to changes of formaldehyde concentration in the air at levels starting from few ppm to few tens of ppb. Moreover, a gas sensor based on the gas sensing structure is portable, stable, reversible, has a low power consumption, and its response time may be as good as <NUM>-<NUM>. To provide the possibility of detecting different gases in the air, a multi-sensor approach based on a pattern recognition algorithm is also disclosed.

<FIG> shows a schematic cross-sectional view of a CNT <NUM> included in the CNT film of the gas sensing structure in accordance with one exemplary example. Those skilled in the art should understand that the CNT film may comprise multiple CNTs like the CNT <NUM>, and the only one CNT <NUM> is shown and described here for the sake of simplicity. As shown in <FIG>, the CNT <NUM> is covered by an n-type or p-type TMO layer <NUM>. The TMO layer <NUM> is in turn covered by noble metal nanoparticles <NUM>. It should be noted that the CNT <NUM>, the TMO layer <NUM> and the nanoparticles <NUM> are shown significantly enlarged and without observing the ratio of sizes therebetween in <FIG> for convenience of description. In real practice, the CNT size is of few nm, the whole CNT film has a thickness of <NUM> to <NUM>, and the TMO layer <NUM> has a thickness of <NUM> to <NUM> (more preferably <NUM> to <NUM>), thereby providing the ultrathin gas sensing structure. The CNT <NUM> may be a single-walled CNT (SWCNT), whereupon the CNT film is a SWCNT film. The n-type or p-type TMO layer <NUM> may be selected from the group of TMOs comprising Mn<NUM>O<NUM>, TiO<NUM>, ZnO, SnO<NUM>, NiO, Fe<NUM>O<NUM>, and Cu<NUM>O, depending on particular applications. For example, ZnO is currently one of the most promising oxides with the highest sensitivity to formaldehyde, for which reason it should preferably be used in cases where formaldehyde detection is required. The sensitivity of ZnO may be improved by doping it with proper materials, such as Mn, Sn or CdO. The rest of the TMOs may also be doped with proper materials to provide a desired conductivity type (i.e. p-type or n-type), depending on particular applications. The noble metal nanoparticles <NUM> may be made of Au, Ag, Pt, Pd. The appearance of the nanoparticles <NUM> on the TMO layer <NUM> increases the response of the whole gas sensing structure.

<FIG> shows a flowchart of a method <NUM> for fabricating the gas sensing structure in accordance with one exemplary example. The method <NUM> is performed as described below.

Step S1: A CNT film <NUM> is synthesized. The CNT film <NUM> comprises multiple CNTs <NUM>. The step S1 may be performed by using an aerosol chemical vapor deposition technique which allows obtaining a randomly oriented network of SWCNTs. However, the present disclosure is not limited to this deposition technique, and the CNT film <NUM> may be produced by using other methods for CNT synthesis, such, for example, as arc discharge, laser ablation, high-pressure carbon monoxide disproportionation, liquid electrolysis, chemical vapor deposition method, etc..

Step S2: The synthesized CNT film <NUM> is arranged on a plate <NUM> to implement a free-standing architecture. In other words, the plate <NUM> is used as a support for the ultrathin (and, therefore, damageable) CNT film <NUM>. As shown in <FIG>, the plate <NUM> has an opening <NUM> which may improve further treatment of the CNT film <NUM>. The CNT film <NUM> should have a size slightly bigger than the opening <NUM> to cover the opening <NUM> on one (in <FIG> - upper) side of the plate <NUM> and to have enough space to be attached to the plate <NUM>. At the same time, the opening <NUM> is an optional feature of the plate <NUM>, i.e. the opening-free plate <NUM> may alternatively be used in the method <NUM> with no significant loss in the quality of the fabricated gas sensing structure.

Step S3 (optional): The CNT film <NUM> is treated in a low-frequency oxygen plasma to form defect sites along sidewalls of the CNTs <NUM> constituting the CNT film <NUM>. In this case, it is advantageous to have the opening <NUM> in the plate <NUM> because it may provide better results of the plasma treatment of the CNT film <NUM>. This is because the opening <NUM> allows the CNT film <NUM> to be treated from above and from below simultaneously (as shown in <FIG>, it is achieved by arranging the plate <NUM> with the CNT film <NUM> between two plate electrodes <NUM>, <NUM> fed by a plasma generator <NUM>). The defectiveness of the CNTs <NUM> increases as plasma exposure time increases. In the meantime, this time should not be too long in order not to destroy the structure of the CNT film <NUM>. As noted above, the step S3 is optional and may be omitted with no significant loss in the quality of the fabricated gas sensing structure.

Step S4: The n-type or p-type TMO layer <NUM> is deposited on the CNT film <NUM>, i.e. on each CNT <NUM> of the CNT film <NUM>. The step S4 may be performed by using an atomic layer deposition (ALD) technique. In this case, it is required to have the plate <NUM> made of a material that withstands temperatures used in the ALD technique, such, for example, as aluminum. It should also be noted that the defect sites formed by treating the CNT film <NUM> in the low-frequency oxygen plasma in the optional step S3 may provide better results of the ALD technique (i.e. better TMO deposition). During the ALD technique, the TMO layer <NUM> is grown on the CNT film <NUM> by exposing its surface to alternate gaseous species, which are typically referred to as precursors and reactants. In <FIG>, precursor molecules <NUM> are schematically shown as open circles.

Step S5: The noble metal nanoparticles <NUM> (shown as solid circles in <FIG>) are deposited on the n-type or p-type TMO layer <NUM>. The step S5 may be performed by using an aerosol doping technique. If the gold nanoparticles <NUM> are used, the aerosol doping technique may be based on using HAuCl<NUM> as a major dopant.

Step S6: The CNT film <NUM> is released from the plate <NUM>. It is the last step of the method <NUM>. The released CNT film <NUM> having the TMO layer <NUM> and the nanoparticles <NUM> deposited thereon represents the gas sensing structure itself. It should be noted that the presence of the opening <NUM> in the plate <NUM> may facilitate said releasing. Thus, the plate <NUM> is used only for the fabrication of the gas sensing structure. After that, the released CNT film <NUM> is used in a gas sensor design, as will be discussed below in detail.

<FIG> shows a schematic top view of a gas sensor <NUM> in accordance with one exemplary example. The gas sensor <NUM> comprises a substrate <NUM>, a signal electrode structure <NUM>, and a gas sensing structure <NUM>. The gas sensing structure <NUM> is obtained by using the method <NUM>, i.e. it is the CNT film <NUM> that has undergone the step S1-S6 (among which the step S3 is optional) of the method <NUM>. Again, the constructive elements of the gas sensor <NUM> are shown significantly enlarged and without observing the ratio of sizes therebetween in <FIG> for convenience of description. The substrate <NUM> may be made of a dielectric or dielectric-coated material that withstands temperatures in a range from a room temperature up to <NUM>. For example, the substrate <NUM> may be made as an oxidized Si substrate. The signal electrode structure <NUM> is formed (for example, by means of metal sputtering) on the substrate <NUM> and comprises a set of spaced signal electrodes, with each two adjacent signal electrodes having a gap therebetween. The gas sensing structure <NUM> is arranged on the signal electrode structure <NUM> such that the gas sensing structure <NUM> bridges the gaps between the signal electrodes. Given this, each pair of the adjacent signal electrodes bridged by the gas sensing structure <NUM> implements a chemiresistive sensor whose electrical resistance changes in response to changes in the nearby chemical environment (for example, changes of a gas concentration in the air). There are <NUM> such pairs of the adjacent signal electrodes, i.e. <NUM> chemiresistive sensors, included in the gas sensor <NUM> shown in <FIG>. However, it should be noted that the number and shape of the signal electrodes constituting the signal electrode structure <NUM> may vary depending on particular applications. In one other exemplary example, the gas sensor <NUM> may be implemented as a single-gap chemiresistive sensor, in which the signal electrode structure <NUM> comprises only two signal electrodes and the gas sensing structure <NUM> bridges the gap therebetween.

As also shown in <FIG>, the gas sensor <NUM> further comprises a heater <NUM> and a thermoresistor <NUM> that are both formed on the substrate <NUM> for temperature adjustment and control, respectively. Each of the heater <NUM> and the thermoresistor <NUM> is shaped as a meander but is not limited to this shape. The operation of both the thermoresistor <NUM> and the heater <NUM> is controlled by control electronics (not shown in <FIG>).

<FIG> schematically shows a multi-gas sensing apparatus <NUM> in accordance with one exemplary example. The apparatus <NUM> is intended to distinguish between different gases present in the air. To do this, the apparatus <NUM> comprises a processor <NUM>, a memory <NUM>, and the gas sensor <NUM>. The memory <NUM> stores processor-executable instructions <NUM> which cause the operation of the processor <NUM>. It should be noted that the number, arrangement and interconnection of the constructive elements constituting the apparatus <NUM>, which are shown in <FIG>, are not intended to be any limitation of the present disclosure, but merely used to provide a general idea of how the constructive elements may be implemented within the apparatus <NUM>. For example, the apparatus <NUM> may further comprise different signal processing means, such, for example, as amplifiers, comparators, etc., as well as different display means. Moreover, if the apparatus <NUM> is implemented as an individual device, the apparatus <NUM> may further comprise a transceiving means configured to communicate the results of its operation to a remote monitoring center.

The processor <NUM> may be implemented as a central processing unit (CPU), general-purpose processor, single-purpose processor, microcontroller, microprocessor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), digital signal processor (DSP), complex programmable logic device, or the like. It is worth noting that the processor <NUM> may be implemented as any combination of the aforesaid. As an example, the processor <NUM> may be a combination of two or more CPUs, general-purpose processors, etc..

The memory <NUM> may be implemented as a nonvolatile or volatile memory used in modern electronic computing machines. As an example, the nonvolatile memory may include Read-Only Memory (ROM), ferroelectric Random-Access Memory (RAM), Programmable ROM (PROM), Electrically Erasable PROM (EEPROM), solid state drive (SSD), flash memory, magnetic disk storage (such as hard drives and magnetic tapes), optical disc storage (such as CD, DVD and Blu-ray discs), etc. As for the volatile memory, examples thereof include Dynamic RAM, Synchronous DRAM (SDRAM), Double Data Rate SDRAM (DDR SDRAM), Static RAM, etc..

The processor-executable instructions <NUM> stored in the memory <NUM> may be configured as a computer executable code causing the processor <NUM> to perform certain operations. The computer executable code for carrying out operations or operations for the examples may be written in any combination of one or more programming languages, such as Java, C, C++, Python, or the like. In some examples, the computer executable code may be in the form of a high-level language or in a pre-compiled form, and be generated by an interpreter (also pre-stored in the memory <NUM>) on the fly.

Being caused by the processor-executable instructions <NUM>, the processor <NUM> is configured to receive measurement signals from the gas sensor <NUM> and, based on the measurement signals, detect the presence of different gases, for example, formaldehyde, ethanol, and/or acetone, in the air. Those skilled in the art should understand that these three gases are indicated as one nonrestrictive example. In general, there should be at least three chemiresistive sensors (i.e. at least three pairs of the adjacent signal electrodes bridged by the gas sensing structure <NUM>) in the gas sensor <NUM> to provide the possibility of selectively detecting and distinguishing between formaldehyde, ethanol, and acetone in the air. Moreover, in one exemplary example, the processor <NUM> may perform said detecting by using a pattern recognition algorithm, such, for example, as Linear Discriminant Analysis (LDA), Principal Component Analysis (PCA), etc..

By using the method <NUM>, several gas sensing structures <NUM> were fabricated as follows.

The CNT film <NUM> in each gas sensing structure <NUM> was represented by a SWCNT film synthesized by the aerosol (floating catalyst) chemical vapor deposition technique in the step S1 of the method <NUM>. SWCNTs were collected on a filter as a randomly oriented network of SWCNTs. The collection time was tuned to prepare the SWCNT film with a transmittance of <NUM>% at a wavelength of <NUM>. The SWCNT film was then transferred, in the step S2 of the method <NUM>, from the filter to the aluminum plate <NUM> with the opening <NUM> of <NUM>-<NUM> in diameter to implement the free-standing architecture. Next, the SWCNT film was treated, in the step S3 of the method <NUM>, in the low-frequency oxygen plasma generated by using low-pressure plasma system PICO from DIENER ELECTRONIC GmbH (under the following operational conditions: power - <NUM> W, oxygen pressure - <NUM> mbar, frequency - <NUM>) to facilitate the formation of defect sites along sidewalls of the SWCNTs. For different SWCNT films, the plasma exposure time was set to be one of <NUM>, <NUM>, <NUM>, and <NUM> in order not to destroy the structure of the SWCNT film. Most of the SWCNT films was treated in the low-frequency oxygen plasma during <NUM>.

After that, ZnO was deposited, as the TMO layer <NUM>, on each SWCNT film in the step S4 of the method <NUM> by using the ALD technique. The deposition of ZnO was performed in a cross-flow R-<NUM> standard reactor (Picosun, Finland) under N<NUM> pressure of about <NUM>-<NUM> mbar. The deposition of ZnO was carried out with a diethyl zinc (DEZ) precursor at <NUM> and with water as a reactant. A duty cycle was <NUM> DEZ followed by <NUM> of N<NUM> purging (<NUM> sccm), the reactant (H<NUM>O) was purged for <NUM> followed by <NUM> of N<NUM> purging (<NUM> sccm). N<NUM> purging was used to displace the precursor or reactant from a reactor chamber because the growth reaction (i.e. ZnO deposition) is a self-terminated cycle process which includes the alternation of exposure to precursor and exposure to reactant, thereby leading to the stepwise growth of TMO depending on a number of cycles. The number of cycles varied from <NUM> to <NUM>.

Further, the gold nanoparticles <NUM> were deposited on the TMO layer <NUM> made of ZnO in the step S5 of the method <NUM> by using the aerosol doping technique based on HAuCl<NUM> as the major dopant. A common nebulizer was used to produce an aerosol using about <NUM>-<NUM> of HAuCl<NUM> solution (HAuCl<NUM>*<NUM><NUM>O). Each SWCNT film with the ZnO layer <NUM> covered with aerosol particles was then subjected to thermal treatment, which favors the formation of the gold nanoparticles <NUM> with no their severe agglomeration due to the operational conditions. The appearance of the gold nanoparticles <NUM> on the surface of the ZnO layer <NUM> increases the sensitivity of the gas sensing structure <NUM>.

Finally, the SWCNT film was released from the aluminum plate <NUM> in the step S6 of the method <NUM>. Thus, there were several gas sensing structures <NUM> fabricated as described above. These gas sensing structures <NUM> were then used to fabricate several gas sensors <NUM>.

<FIG> show scanning electron microscope (SEM) images of the gas sensor design and the fabricated gas sensing structure <NUM>, respectively. In particular, <FIG> shows the arrangement of the fabricated (transparent) gas sensing structure <NUM> on the signal electrode structure <NUM>. As can be seen, the gas sensing structure <NUM> covers a part of the signal electrode structure <NUM>. The signal electrode structure <NUM> was formed on the oxidized Si substrate <NUM> of the gas sensor <NUM> and comprised <NUM> sputtered Pt strip electrodes (with a <NUM> Ti seed layer). An electrode height was about <NUM>, and a distance between the adjacent electrodes was about <NUM>. Thus, each two adjacent Pt electrodes implement a single chemiresistive sensor. Although not shown in <FIG>, there also were the meander Pt heater <NUM> and the meander Pt thermoresistor <NUM> formed on the front side of the substrate <NUM> for temperature adjustment and control. The operation of the heater <NUM> and the thermoresistor <NUM> was controlled by a custom-made electronic unit with an accuracy of about <NUM>. The size of the whole gas sensor <NUM> was <NUM> × <NUM><NUM>. According to the SEM image shown in <FIG>, it was calculated that each SWCNT in the structure of the SWCNT film had a thickness of about <NUM>-<NUM>. It should also be noted that an Energy-dispersive X-ray spectroscopy (EDX) technique was used to confirm Au presence (i.e. the nanoparticles <NUM>) on the surface of the fabricated SWCNT film.

<FIG> shows a block diagram of an experimental setup <NUM> for gas sensing tests in accordance with one exemplary example. In particular, the experimental setup <NUM> was used to carry out the gas sensing tests on the gas sensors <NUM> comprising the above-described fabricated gas sensing structures <NUM>. The gas sensors <NUM> were arranged as a sensor array on a Si/SiO<NUM> wafer, 5x5 mm<NUM>, wired into a ceramic card (not shown in <FIG>) installed inside a gas-tight chamber <NUM>. As an analyte source (i.e. test gases), a calibration gas mixture (the analyte, <NUM> ppm or <NUM> ppm, mixed with synthetic air) in a gas vessel <NUM> was used, and a background atmosphere was synthetic air from a gas vessel <NUM>. The gas vessel <NUM> is connected to the chamber <NUM> via a reducer valve <NUM>, a mass flowmeter <NUM> and an optional gas pressure valve <NUM>. Similarly, the gas vessel <NUM> is connected to the chamber <NUM> via a reducer valve <NUM>, a mass flowmeter <NUM> and an optional gas pressure valve <NUM>. The reducer valves <NUM> and <NUM> were used to control a gas pressure of the calibration gas mixture and the synthetic air coming from the gas vessels <NUM> and <NUM>, respectively. The mass flowmeters <NUM> and <NUM> were used to control a mass flow rate of the calibration gas mixture and the synthetic air, respectively. The operation of the mass flowmeters <NUM> and <NUM> was controlled by a PC <NUM> using LabVIEW software. The exhaust gases were released via an exhaust <NUM> to the environment. The experimental setup <NUM> further comprised a humidity sensor <NUM> (in particular, Testo <NUM> sensor) for monitoring a humidity level inside the chamber <NUM>, and a signal acquisition unit <NUM> for processing measurement signals from the gas sensors <NUM> and providing the processing results to the PC <NUM> for their display and analysis.

By using the experimental setup <NUM>, formaldehyde, ethanol and acetone analytes were tested. A constant flow mode with a flowrate of air/analyte (i.e. gas vapors of interest) mixed with air to be <NUM> or <NUM> sccm was used. Volatile organic vapors (formaldehyde, ethanol, isopropanol and acetone) were mixed with air to provide a desired concentration, <NUM> ppb, <NUM> ppb, <NUM> ppb and <NUM>. <NUM> ppm, using the mass flowmeters <NUM> and <NUM>. The exposure of the gas sensors <NUM> to each analyte was done by several pulses to check reproducibility and other relevant parameters. The gas sensors <NUM> were tested in a wide temperature range, i.e. from a room temperature up to <NUM>. The sensor response was calculated as follows: S = (Rb - Rg) * <NUM>%/Rg, where Rb is the baseline resistance of the gas sensor <NUM> in synthetic air, and Rg is its resistance at the analyte exposure (in case of the resistance decrease). In case of the resistance increase, the following expression was used: (Rg - Rb) * <NUM>%/Rb.

<FIG> show responses of one of the fabricated gas sensors <NUM> to formaldehyde (HCOH) with concentrations <NUM> ppb, <NUM> ppb and <NUM> ppb in the calibration gas mixture with air at temperatures <NUM>, <NUM>, <NUM> and <NUM>. In particular, each response is represented by a dependence of the sensor resistance in MOhm on time in min. One can easily find a rather pronounced change in the sensor resistance at the appearance of HCOH in the air (which corresponds to the pulses on the dependences shown in <FIG>). Moreover, the responses are rather reproducible - they show similar values when the pulses are repeated. The response time is in a range of <NUM>-<NUM>, achieving <NUM> (given a pipeline length between the place of the reducer valve <NUM> and the chamber <NUM>, as well as a flow rate). As temperature decreases, the response values increase, highly pronounced at <NUM>. The calculated response follows the concentration dependence. The concentration increase yields a greater response - the maximum response value is equal to <NUM>% at <NUM> to HCOH with concentration <NUM> ppb, and the mean response value is about <NUM>%.

<FIG> show a response-concentration dependence and a limit of detection, respectively, of the gas sensor <NUM>, which were obtained by using the dependences shown in <FIG>. The limit of detection was evaluated as an intersection of concentration trend lines with the line corresponding to <NUM>% of the response (which is considered to be an acceptable value used in the industry to judge whether a material is good for sensor fabrication). As a result, it was found that the limit of detection is about <NUM> and <NUM> ppb for <NUM> and <NUM>, respectively, and <NUM> and <NUM> ppb for <NUM> and <NUM>, respectively. In <FIG>, the response was calculated as (Rg - Rb)/Rb.

Although the response of each chemiresistive sensor (i.e. each pair of the adjacent signal electrodes bridged by the gas sensing structure <NUM>) included in the gas sensor <NUM> is not selective to an analyte type, it is possible to evaluate the analyte type by analyzing the measurement signals obtained from at least three chemiresistive sensors (i.e. at least three pairs of the adjacent signal electrodes bridged by the gas sensing structure <NUM>). As noted above, such an analysis may be made by using the pattern recognition algorithm. In particular, the LDA was used to classify the analytes present in the calibration gas mixture with air. The measurement signals were normalized prior to the classification.

Claim 1:
A gas sensing structure (<NUM>) comprising:
a carbon nanotube, CNT, film (<NUM>);
an n-type or p-type transition metal oxide, TMO, layer (<NUM>, <NUM>) deposited on the CNT film (<NUM>); and
noble metal nanoparticles (<NUM>) deposited on the n-type or p-type TMO layer (<NUM>, <NUM>),
characterized in that the CNT film (<NUM>) has a thickness of <NUM> to <NUM>, and
in that the n-type or p-type TMO layer (<NUM>, <NUM>) has a thickness of <NUM> to <NUM>.