Solid-state gas sensor and corresponding manufacturing method

Abstract

A solid-state gas sensor (10) includes a semiconductor chip (12), which in turn includes an element sensitive to the presence of one or more gaseous species. The sensor includes a support (13) on which the semiconductor chip is mounted. The support (13) has electrical connection pads (14A, 14B, 14C, 14D) connected to respective electrodes of the semiconductor chip and accessible from outside the sensor via respective electrical connection terminals (16A, 16B, 16C, 16D). The sensor includes a closure element (18) mounted on the support (13) to define a sensing chamber at the upper surface of the semiconductor chip (12). The closure element (18) has one or more through holes to allow gas exchange between the sensing chamber and the external environment. The sensor includes a selective membrane arranged internally in the closure element (18) between the upper surface of the semiconductor chip (12) and the through holes. The selective membrane includes cellulose nanocrystals.

Description

[0001] "Solid-state gas sensor and corresponding manufacturing method"

[0002] ★ ★ ★ ★

[0003] TEXT OF THE DESCRIPTION

[0004] Field of the invention

[0005] The present invention relates to solid-state gas sensors and related manufacturing methods.

[0006] Technological background

[0007] In the field of gas detectors, solid-state sensors are characterized by their low cost and high compactness, which allow scalable production of a wide palette of devices and their deployment in multiple applications. Specifically, the most widely used solid-state sensors include several types of sensors, including: semiconductor metal oxide-based sensors (also known as chemoresistive sensors or SMOx) , electrochemical sensors, catalytic sensors, pellistors, quartz microbalances, thermal conductivity sensors, optical sensors, and others.

[0008] In general, solid-state gas sensors suffer from low selectivity, that is, a poor ability to accurately distinguish one specific gas from others.

[0009] Various techniques have been tried in an attempt to increase the discrimination ability between different gases of solid-state sensors. For example, regarding SMOx sensors, some proposed approaches have been: (i) the use of different sensing materials, (ii) the modulation of the sensor operating temperature, (iii) the addition of noble metals with catalytic function, and (iv) the adoption of machine learning methods in combination with sensor arrays having different sensing materials .

[0010] Such techniques have some disadvantages: for example , they require the adoption of uncommon sensitive materials , the use of which may result in lower reliability due to limited experience in such contexts . Alternatively, software-based selectivity enhancement strategies require the implementation of more sophisticated data analysis methods to obtain reliable results , increasing the di f ficulty in device use .

[0011] As an example , document WO 2020 / 177884 Al describes a metal oxide resistive gas sensor that includes a support structure and a porous sensing layer arranged on or partially housed within the support . Electrodes are in electrical contact with the sensing layer, and a heater is in thermal contact with the sensing layer . The heater can be activated to heat the sensing layer to a target temperature to enable determination of the presence or concentration of a target gas such as hydrogen, based on a sensing signal provided through the electrodes . The sensing layer comprises a network of interconnected monocrystalline metal oxide nanoparticles , and a gas-selective coating . The coating comprises one or more amongst silicon oxide and silicon nitride and has a maximum thickness of 50 nm : it is thus an inorganic film applied directly to the sensing material ( the metal oxide ) through thermal or chemical vapor deposition procedures . The document WO 2020 / 177884 Al describes an alternative solution, in which a filter separated from the sensing layer ( i . e . , not in contact with it ) is used, employing polymers such as cellulose acetate . In this configuration, the selective filter has pores of a si ze that allows permeation only to gases with a kinetic diameter smaller than the pore si ze . The inert material of the filter has porosity up to 1 nm, with a preference for pore si zes of 0 . 3 nm or less . Polymers , and more speci fically fluoropolymers , are generally mentioned for this purpose .

[0012] A further approach is described in document CN 210639118 U, which mentions the use of a filter material having pores of about 0 . 3 pm to remove the influence of dust and particulate matter .

[0013] A further document of potential interest is the article "Selective and sensitive environmental gas sensors enabled by membrane overlayers" by Ji-Soo Jang et al . In this study, membranes based on graphene , metal organic frameworks (MOFs ) and metal oxides , particularly dichromium trioxide , are examined in detail . These materials are deposited directly on the sensing material . It follows that the ef fect of the operating temperature of the sensor can be a constraint : in particular, the use of graphene-based membranes and MOFs is only possible for sensors operating at room temperature , otherwise such membranes would be degraded by the operating temperature of the sensor . As for dichromium trioxide , this can withstand high operating temperatures but requires sophisticated deposition processes , which can result in increased costs and increased complexity of the manufacturing process .

[0014] Within the scienti fic literature on the coupling of gas sensors and filters , the review "Highly selective gas sensing enabled by filters" by Jan van den Broek et al . provides an overview of materials that may be used as filters . This study considers in detail adsorptive filters such as activated carbon and zeolites filters , and it also examines catalytic filters , typically composed of metal oxides or noble metals . In addition, the study highlights the potential and challenges of selective membranes , focusing particularly on the properties and applicability of zeolites , MOFs , and silicon oxide . In this regard, MOFs and silicon oxide present the issues discussed earlier, while zeolites exhibit di f ficulties in producing defect- free membranes within a cost-ef fective framework .

[0015] The article "Organic Membranes for Selectivity Enhancement of Metal Oxide Gas Sensors" by Thorsten Graunke et al . describes the use of polymeric materials as a way to enhance the selectivity of metal oxide sensors . Various polymers are considered, including polyethylene (high and low density) , polypropylene , polymethylpentene , polyetherimide , polysul fone and others . Although many of these polymers result in increased selectivity toward hydrogen, a marked decrease in sensor signal was also observed with respect to all gases . This emphasi zes that while improvements in selectivity can be achieved, there may be trade-of fs on the overall sensitivity of the sensor .

[0016] Therefore , solid-state gas sensors that have high selectivity and solve one or more of the technical problems mentioned above are desirable .

[0017] Obj ect of the invention

[0018] The obj ect of the present invention is to provide an improved solid-state gas sensor that allows achieving higher selectivity than known gas sensors , solving one or more of the technical problems mentioned above .

[0019] Summary of the invention

[0020] In order to achieve the aforementioned obj ect , the present invention relates to a solid-state gas sensor comprising a semiconductor chip . The semiconductor chip in turn includes an element sensitive to the presence of one or more gas species . The sensor includes a support on which the semiconductor chip is mounted . The support has electrical connection pads connected to respective electrodes of the semiconductor chip and accessible from outside the gas sensor via respective electrical connection terminals . The sensor includes a closure element mounted on the support to define a sensing chamber of the gas sensor at an upper surface of the semiconductor chip . The closure element has one or more through holes to allow gas exchange between the sensing chamber and the external environment . The sensor includes a selective membrane arranged internally to the closure element between the upper surface of the semiconductor chip and the one or more through holes . The selective membrane includes cellulose nanocrystals .

[0021] As will become apparent in more detail from the description that follows , the use of a selective membrane that includes cellulose nanocrystals makes it possible to increase the selectivity of the sensor with respect to one or more speci fic gases without altering the operating mode of the solid-state sensor, thus enabling the use of a conventional semiconductor chip of any known type .

[0022] In addition, the present invention relates to a method of manufacturing a solid-state gas sensor . The method includes the following steps :

[0023] ( i ) providing a semiconductor chip comprising an element sensitive to the presence of one or more gaseous species ;

[0024] ( ii ) mounting the semiconductor chip on a support that has electrical connection pads accessible from outside the gas sensor via respective electrical connection terminals , and connecting respective electrodes of the semiconductor chip to the electrical connection pads ;

[0025] ( iii ) providing a closure element configured to be mounted on the support to define a sensing chamber of the gas sensor at an upper surface of the semiconductor chip, the closure element having one or more through holes to allow gas exchange between the sensing chamber and the external environment ;

[0026] ( iv) synthesi zing an aqueous solution comprising cellulose nanocrystals ;

[0027] (v) casting the aqueous solution in the closure element and letting the aqueous solution evaporate to form a selective membrane comprising cellulose nanocrystals arranged inside the closure element ; and

[0028] (vi ) mounting the closure element on the support .

[0029] Detailed description of the invention

[0030] Further features and advantages of the invention will be apparent from the following description with references to the annexed drawings , which are provided as non-limiting examples only, in which :

[0031] - Figures 1A, IB , 1C and ID illustrate di f ferent views of various components of a gas sensor with a measurement chamber according to the present invention;

[0032] Figure 2 is a diagram including two spectra illustrating the transmittance of cellulose nanocrystals as a function of the wavelength of incident radiation;

[0033] - Figure 3 is a diagram illustrating the electrical conductance of a solution of cellulose nanocrystals as a function of the volume of sodium hydroxide in the solution;

[0034] - Figure 4 is a diagram including four spectra obtained by photon correlation spectroscopy carried out on a solution of cellulose nanocrystals as a function of the average si ze of the nanocrystals in solution;

[0035] - Figure 5 is an image obtained by atomic force microscopy carried out on a dilute deposition of cellulose nanocrystals ;

[0036] - Figure 6 is a diagram showing the response pattern of three di f ferent sensors as a function of ethanol concentration;

[0037] - Figure 7 is a radar plot illustrating the response of three di f ferent sensors to six di f ferent gases at the same concentration;

[0038] Figure 8 is a radar plot illustrating the selectivity of three di f ferent sensors with respect to six di f ferent gases ;

[0039] - Figure 9 is a radar plot illustrating the barrier ef fect of three di f ferent sensors with respect to six di f ferent gases ;

[0040] - Figure 10 is a diagram illustrating the change in gas concentration on one side of a selective membrane during an experiment when the gas is introduced abruptly on the other side ;

[0041] - Figure 11 is a diagram illustrating the time dependence of total flux of gas permeated through the selective membrane in the experiment of Figure 10 ; and

[0042] Figure 12 is a diagram illustrating the correlation between the square of the thickness of a membrane and the time lag ( delay) of detection in the experiment of Figures 10 and 11 .

[0043] In the accompanying figures , corresponding elements are indicated with the same reference numbers .

[0044] As anticipated, one or more embodiments relate to a solid-state gas sensor and a related manufacturing method . The obj ect of the invention is to increase the selectivity of the sensor with respect to one or more speci fic target gases , that is , to increase the ability of the sensor to discern between the target gas and other interfering gases . A sensor according to the invention can be used in situations where high accuracy in gas quanti fication is required, particularly for the detection of potentially explosive gases in the natural gas industry, in the renewable energy sector, and in the management of synthesis gases used in methane and ammonia production .

[0045] According to a speci fic and non-limiting example , the gas sensor of the invention may include a chemoresistive or SMOx gas sensor . The detection mechanism of SMOx gas sensors is based on the change in the degree of oxygen adsorption on the surface of a metal oxide film that is driven at a given working temperature . The change in the quantity of adsorbed oxygen molecules results from the interaction between the sensing film and the gas present in the atmosphere in which the sensor is located : an oxidi zing species temporarily increases the oxidation level of the surface , while a reducing species causes a corresponding reduction . This variation in the surface oxidation level of the sensing material ( SMOx ) changes its electronic transport properties and consequently its electrical resistance . By analyzing changes in the resistance of the sensitive material film, the gas concentration can be determined . The sensor is usually connected to an electronic board ( e . g . , via a printed circuit board, BOB ) . The resistance of the sensor is monitored via the electronic board, which also controls the power supply to the micro-heaters integrated on the sensor chip via two electrodes embedded on the sensor chip, and collects the resistance value of the sensing material via two electrodes also embedded on the sensor chip .

[0046] In this regard, one may refer to Figures 1A, IB, 1C and ID, which illustrate some components of a gas sensor 10 according to the invention, namely :

[0047] - a semiconductor chip 12 in which the solid-state gas sensor is implemented, which may comprise in a manner known per se ( again according to a non-limiting example , where the sensor is of the chemoresistive type ) one or more micro-heaters electrically connected between two respective electrodes ( to be powered thereby) and a film of SMOx sensing material , which is electrically connected between two respective electrodes in order to measure its resistance (Figure 1A) ;

[0048] - a support 13 (e.g., metallic) on which the chip 12 is mounted so that the electrodes (or pins) of chip 12 are in contact with respective electrical connection pads 14A, 14B, 14C, 14D of the support 13; the electrical connection pads of support 13 are in turn electrically connected with respective electrical terminals 16A, 16B, 16C, 16D of the sensor 10 so that it can be connected to, for example, a printed circuit board or PCB (Figure 1A) ;

[0049] - a closure element 18 (e.g., a cover or housing) , preferably cylindrical in shape and preferably made of polymeric or metallic material, which can be mounted on the support 13 of sensor 10 to define a sensing chamber at the upper surface of chip 12 (or, more generally, at the region of chip 12 that is sensitive to gases) ; closure element 18 may be compatible with a package of the TO-39 type or similar, or any other solid-state gas sensor package in which semiconductor chip 12 is housed; the closure element 18 has one or more through holes to allow gas exchange with the external environment (Figure IB: view of the top side of the closure element 18; Figure 1C: view of the bottom side of the closure element 18) ;

[0050] - sensor 10 as a whole, comprising the support 13 on which the chip 12 is mounted, and the closure element 18 mounted on the support 13.

[0051] According to the invention, the gas sensor 10 advantageously includes a membrane selective against one or more target gases, which is arranged internally to the sensing chamber defined by the closure element 18 when mounted on the support 13 (i.e., in the packaging of the sensor 10) such that interfering gas species cannot reach the surface of the chip 12. Specifically, and as will be discussed in more detail later, the membrane is produced using semipermeable materials obtained through low-cost and eco- friendly synthesis methods , with a focus on the industrial scalability of the product , resulting in an inexpensive and easily manufacturable gas sensor . In particular, the membrane is an organic membrane based on cellulose nanocrystals ("cellulose nano-crystals , " CNC ) , and the sensor manufacturing method is generally articulated in the following four macro-steps :

[0052] ( i ) synthesis of cellulose nanocrystals in solution starting from raw cellulosic materials ;

[0053] ( ii ) optional treatment of the cellulose nanocrystal solution with plastici zers and / or nanoparticles to improve permeability to speci fic gases ;

[0054] ( iii ) deposition of the cellulose nanocrystal solution in the protective closure element 18 of sensor 10 and formation of the selective membrane by evaporation of the solution; and

[0055] ( iv) assembly of the closure element 18 onto the support 13 and sealing of the j oints between the closure element 18 and the support 13 .

[0056] Therefore , di f ferently from known solutions , the present invention provides an additional element ( the selective membrane ) suitable for increasing the selectivity of a conventional solid-state gas sensor, be it an SMOx-type sensor or another type of sensor using any other signal transduction method . The versatility in the applications of this device is due to the fact that the increase in selectivity stems from the gas transport properties of the membrane integrated in the actual device . The stated transport properties advantageously allow permeation of the gas of interest while providing an ef fective barrier to non-target gases ( interfering gases ) , consequently increasing the selectivity of the sensor without altering the data analysis protocols or the intrinsic electrical characteristics of the sensor itself .

[0057] The solution (or suspension) of cellulose nanocrystals is produced from cellulose in raw form through a process that can be articulated in one or two steps .

[0058] In the first case, raw cellulose in solution is subjected solely to mechanical stimulation to produce nanocrystals. For example, through ball milling or high- pressure homogenization, both cellulose nanocrystals and cellulose nanofibers are obtained in varying proportions depending on the fabrication method.

[0059] In the second case, the first process step is chemical and the second process step is mechanical. With this method, a more narrow and controlled size distribution of the nanocrystals can be achieved.

[0060] Specifically, initially, a cellulose functionalization reaction is carried out on the cellulose in its raw state that facilitates the disintegration into nanocrystals, such as an oxidation reaction. More specifically, in an aqueous solution the oxidation of a raw cellulose pulp ("kraft pulp") can be carried out, using sodium hypochlorite (NaClO - e.g., 14% active chloride) as the primary oxidant and 2,2, 6, 6- tetramethylpiperidin-l-oxyl (TEMPO) and sodium bromide (NaBr) as catalysts. The pH of the solution is kept alkaline (e.g., in the range of 10 to 11) by addition of sodium hydroxide (NaOH) . In this way, carboxyl groups are produced on the surface of the cellulose fibers. The production of carboxyl groups can be confirmed by analyzing the infrared transmittance spectra as exemplified in Figure 2, which illustrates a plot including two spectra S21 and S22 of the transmittance T (expressed in arbitrary units) of cellulose nanocrystals as a function of wavelength X (expressed in cim1) . The presence of peaks at X = 1620 cur1is indicative of the presence of carboxyl groups. The concentration of carboxyl groups can be determined by conductometric titration of the cellulose nanocrystal solution, as exemplified in Figure 3, which illustrates the electrical conductance G of the solution (expressed in mS) as a function of the volume of sodium hydroxide VNSOH (expressed in pl) . The experimental data in the graph of Figure 3 show an excess of H+ ions in the initial part, the ionization of COOH groups in the middle part, and an excess of OH~ ions in the final part. By processing these experimental data, it is possible to estimate the amount of carboxyl groups, which in the present example can be in the range of 1.4 pmol / mg to 1.5 pmol / mg (micromoles per milligram of nanocellulose) .

[0061] The chemical reaction is stopped by dilution by adding (deionized) water, e.g., after 3 hours. Then, the heterogeneous mixture obtained by oxidative treatment is washed with (deionized) water, e.g., six times, to remove excess reagents and waste by-products. Advantageously, the chemical process takes place in aqueous solution, reducing costs and the need for polluting solvents. It is also possible when at neutral pH to separate the supernatant from the solid fraction, so that catalysts can be reused to process additional raw cellulose.

[0062] Once a neutral pH supernatant is obtained, mechanical treatment (the second step in the process) can be carried out. Advantageously, the incorporation of carboxyl groups - which at neutral pH are negatively charged - establishes an electrostatic repulsion that mitigates the strength of the hydrogen bonds between the nanocellulose fibers. This phenomenon facilitates the subsequent process of mechanical separation of the cellulose nanocrystals. For example, the solution can be mechanically stimulated through ultrasonic probe-type homogenization. In one example, sonication can provide 4.5 W of power per milliliter of solution (more generally, between 1 W and 10 W of power per milliliter of solution) . The purpose of this mechanical process is to break the hydrogen bridge bonds that hold the cellulose fibers together, thereby releasing individual cellulose nanocrystals. Advantageously, such nanocrystals remain in solution without aggregation due to the electrostatic repulsion introduced by the carboxyl groups, which provide the nanoparticle with a Z potential on the order of -30 mV.

[0063] In various embodiments, the resulting nanocrystals are approximately cuboidal in shape, with dimensions of about 5 nm x 5 nm x 100-250 nm. The size of the nanocrystals can be confirmed by photon correlation spectroscopy carried out on the nanocrystals ' aqueous solution (exemplified in the graph of Figure 4) and / or by atomic force microscopy carried out on a dilute deposition of cellulose nanocrystals (exemplified in the image of Figure 5) . Specifically, the graph in Figure 4 illustrates four spectra obtained for four successive dilutions of a cellulose nanocrystal solution (e.g., spectrum S41 for 1 mg / ml, spectrum S42 for 0.5 mg / ml, spectrum S43 for 0.25 mg / ml, and spectrum S44 for 0.12 mg / ml) , which show an average size D of approximately 115140 nm (i.e., between 75 nm and 155 nm) . The image of Figure 5 is obtained by atomic force microscopy carried out on cellulose nanocrystals, and shows nanocrystals with length in the range of about 100 nm to 250 nm and width in the range of about 3 nm to 5 nm.

[0064] Optionally, the cellulose nanocrystal solution can be filtered to remove any macroscopic residues. For example, vacuum filtration can be performed using a cellulose acetate membrane having a pore size between about 5 pm and 10 pm as a filter.

[0065] Optionally, the cellulose nanocrystal solution can be concentrated to achieve a concentration suitable for fabricating a membrane of the desired thickness in the volume of the closure element (cap or casing) in which it will be deposited. For example, the solution can be heated in a water bath between about 40°C and about 80°C in a rotary evaporator with constant pumping (low vacuum or medium vacuum) until the concentration of nanocrystals reaches a target value, which for reasons of suspension stability may be about in the range of 1 mg / ml to 10 mg / ml.

[0066] In some embodiments, the previously described suspension of cellulose nanocrystals can be modified by introducing additional functional groups, replacing carboxyl groups on the surface of the nanocrystals.

[0067] Alternatively, or additionally, the chemical and physical properties of the suspension can be controlled by the addition of plasticizers or nanoparticles, with the aim of modulating its mechanical, morphological and gas transport properties, thereby altering its permeability and / or selectivity toward a specific gas. In various embodiments, the concentration of plasticizers or nanoparticles added to the cellulose nanocrystal solution can vary in the range of 0% to 50% of the total mass.

[0068] For example, in one or more embodiments, the nanocrystal solution can be added with polyethylene glycol (PEG) or polyethylene glycol containing amine groups (PEG-NH2 for short) .

[0069] According to a non-limiting example, PEG-NH2 (having a molecular weight of about 1500 g / mol) is dissolved in water in order to obtain an aqueous solution of PEG-NH2 having the same or similar concentration as the CNC solution (e.g., about 10 mg / ml) . The CNC solution is stirred vigorously, and the PEG-NH2 solution is slowly added to it until a desired weight-to-weight ratio between CNC and PEG-NH2 is obtained, such as 1:1 or 2:1. Once the desired ratio of CNC to PEG-NH2 is obtained, still maintaining vigorous stirring of the solution, the solution is brought to neutral pH by adding 0.1 molar hydrochloric acid (HC1) drop by drop and is then subjected to sonication (e.g., for 30 minutes) .

[0070] Regardless of whether the CNC solution has been added with plasticizers and / or nanoparticles of other types, the solution is deposited inside the protective closure element 18 of the sensor 10 (e.g., by drop casting inside the closure element 18 oriented as in Figure 1C) and let evaporate inside the closure element (e.g., at a temperature of 30°C to 90°C, such as 40°C) . During evaporation, the selective membrane selfassembles to form a dense film of cellulose nanocrystals.

[0071] In various embodiments, the final thickness 1 of the membrane (at the end of the evaporation process) can be in the range of 0.1 pm to 50 pm, such as between 5 pm and 20 pm.

[0072] In various embodiments, the membrane formed in the sensor closure element can be sub ected to heat treatment (e.g., between 70°C and 120°C) to promote the hardening process (hornif ication) which can be used to adjust the permeability of the membrane to specific gases.

[0073] The inventors observed by scanning electron microscopy that the films obtained from a solution of pure CNC (without additives) are substantially continuous, have no major defects or bubbles, and consist of multiple nanometer-thick layers aligned with each other and packed together, exhibiting a substantially smooth surface morphology. The films obtained from a solution of CNC plus PEG-NH2 exhibit essentially the same morphological characteristics, as well as the presence of aggregates of salt crystals on the membrane surface, due to the addition of hydrochloric acid for neutralizing the pH of the CNC+PEG solution. Such salt crystals appear to affect only the membrane surface, as they were not detected in the analyzed cross-sections, which instead resemble in all respects those of membranes obtained from a pure CNC solution. Thus, the surface morphology of CNC+PEG membranes is slightly less smooth than that of CNC-only membranes.

[0074] In addition, the inventors measured the water vapor transport capabilities ("water vapor transmission rate," WVTR) through a CNC membrane having a thickness of 10 pm, at a temperature of about 2111 °C and a pressure of about 5 mbar, obtaining a value of about 9.411.5 g / m2*day.

[0075] As a further characterization of the membranes, the inventors performed thermogravimetric analyses of both CNC-only membranes and CNC + PEG-NH2 membranes. It was noted that some dried CNC-only membranes (i.e., at the end of the evaporation process) still have about 10% by weight of trapped moisture, begin to degrade at a temperature of about 185°C, have a peak degradation temperature of about 235°C and a final residual mass of about 27%. On the other hand, some dried CNC + PEG-NH2 membranes (i.e., at the end of the evaporation process) still have about 16.4% by weight of trapped moisture, have a first degradation temperature of about 245°C (due to the degradation of cellulose nanocrystals producing carbonaceous residues) , a second degradation temperature of about 395°C (due to the degradation of PEG-NH2 producing no residues) , and a final residual mass of about 17%.

[0076] It will be understood that the optimal ranges of membrane thickness depend on the specific application and may vary depending on the volume and concentration of the initial solution. The addition of such a membrane to an otherwise conventional solid-state gas sensor, and the possibility of varying composition and thickness of the membrane itsel f , can increase the upper limit of detectability of the sensor in addition to increasing its selectivity, allowing the detection of high gas concentrations with greater accuracy .

[0077] Advantageously, CNC-only membranes have an inherent selectivity toward hydrogen, while it is possible to vary their composition to make them more selective with respect to other analytes .

[0078] In one or more embodiments , an epoxy adhesive can be used to j oin the protective closure element to the packaging ( or support ) on which the sensor chip is mounted, making the packaging j oint air-tight .

[0079] In the remainder of this description, the experimental results observed by the inventors in some speci fic examples of chemoresistive sensors made according to various embodiments of the present invention, i . e . , comprising a cellulose nanocrystalbased membrane , will be illustrated . For this purpose, it is useful to mention that the comparison between the resistance measured during the introduction of the target gas ( at a given concentration) and the resistance measured in an atmosphere devoid of the gas under consideration allows the " sensor response" to be defined . Speci fically, for an n-type semiconductor sensor such as those considered here , the response is calculated by one of the following expressions , depending on whether the target gas is an oxidi zer or a where Resistancegasis the electrical resistance of the sensing material measured in the presence of the target gas and Resistanceairis the electrical resistance measured in the absence of the target gas.

[0080] Figure 6 is a graph showing, for illustrative purposes, the average response R (dimensionless) as a function of the concentration Cone (expressed in ppm) of ethanol (EtOH) for three sensors, all derived from the same silicon wafer and obtained by a single, common deposition of sensing material, all operated at the same working temperature, each of which is fitted with a protective closure element. The only difference between the three sensors lies in the membranes contained in their respective protective closure elements: the first (curve S61) has a pure nanocellulose membrane (CNC only) ; the second (curve S62) has a nanocellulose membrane and a plasticizer (CNC + PEG-NH2 in a 1:1 ratio at pH 7) ; and the third (curve S63) has no membrane at all. Analyzing the response of the sensors to the injection of ethanol, which has been identified as an interfering gas in the applications of greatest interest, shows an advantageous significant decrease in response in the range of 2.5 ppm to 50 ppm EtOH, which is particularly marked for the pure nanocellulose membrane compared to the sensor without a membrane.

[0081] Characterization of the detection properties of gas sensors can be performed through iterations of the process for a variety of gases and a variety of concentrations. Therefore, using a radar plot such as the one in Figure 7 facilitates the visualization of the response of the three sensors (the same as in Figure 6) to different gases (EtOH, CO, CH4, NO2, NH3, H2 ) at a given concentration (in this example, equal to 50 ppm) . Specifically, the curve S71 is for the sensor equipped with a pure nanocellulose membrane (CNC only) ; the curve S72 is for the sensor equipped with a CNC+PEG membrane, and the curve S73 is for the sensor without a selective membrane . The sensor without membrane shows predominant sensitivity to ethanol , while maintaining signi ficant , albeit minor, response to the other gases considered as well .

[0082] Furthermore , it is observed that the response to all interfering gases undergoes a noticeable decrease in the sensors with a membrane , while the response to hydrogen shows a more gradual decrease . Thus , defining hydrogen as the target gas and the remaining gases as interfering gases , the measure of theoretical selectivity is introduced . This value , which can be calculated according to the formula below, provides a useful index for assessing the degree of sensitivity of the sensor to the target gas in relation to the

[0083] The selectivity of the three sensors considered here as examples can therefore also be visuali zed more clearly using a radar plot , such as the one exempli fied in Figure 8 . Here in particular, the curve S 81 illustrates the theoretical selectivity toward hydrogen for the sensor equipped with a pure nanocellulose membrane ( CNC-only) ; the curve S 82 illustrates the theoretical selectivity toward hydrogen for the sensor equipped with a CNC+PEG membrane ; and the curve S 83 illustrates the theoretical selectivity toward hydrogen for the sensor without a selective membrane . The sensor equipped with a CNC-only membrane shows the highest selectivity with respect to all interfering gases , exhibiting particular selectivity for hydrogen in the presence of nitrogen dioxide and methane . The sensor equipped with membrane with CNC+PEG is at an intermediate level between the other two ; in fact , it exhibits higher selectivity than the sensor without a membrane , particularly with respect to ammonia, but also a greater response than the sensor equipped with a membrane with only CNC . It is therefore possible to advantageously modulate both the level of selectivity and the gas species toward which one is most selective depending on the concentration and chemical properties of the plastici zer used .

[0084] To discriminate the intrinsic sensitivity of the sensor toward a speci fic gas from the selective ef fect of the membrane , it is useful to analyze an additional metric . By placing a sensor without a membrane in the measuring chamber and using it as a reference , the barrier ef fect of the membrane against a speci fic gas can be quanti fied using the formula below :

[0085] The barrier ef fect describes the extent to which a given membrane actually stops a given gas , regardless of the inherent selectivity of the solid-state sensor . The radar graph in Figure 9 exempli fies the barrier ef fect of the same three sensors as in the previous graphs ; speci fically, the curve S 91 illustrates the barrier ef fect of the sensor equipped with a pure nanocellulose membrane ( CNC only) ; the curve S 92 illustrates the barrier ef fect o f the sensor equipped with a CNC+PEG membrane , and the curve S 93 illustrates the barrier ef fect of the sensor without a selective membrane . Ethanol is the gas whose passage is most hindered by the nanocellulose membrane , followed by nitrogen dioxide .

[0086] Further experimental results related to a quantitative analysis of the permeability of CNC-only- containing membranes to selected target gases , including helium, hydrogen, nitrogen, and oxygen, are discussed in the remainder of this description . Permeability to these gases was determined using a measurement apparatus equipped with a mass spectrometer as a detector, capable of monitoring transient changes in gas concentration in an inert flow. From the results of this measurement, the specific values of permeability and diffusivity of a given gas with respect to the tested membrane emerge. With reference to nitrogen and oxygen, the signals detected by the detector were practically zero, coinciding with the background noise, indicating a permeability for these gases below the values detectable by the instrument. In contrast, for helium and hydrogen, diffusivity and permeability values could be calculated using a signal-concentration calibration scale to determine the concentration of the gas over time.

[0087] In particular, the maximum membrane permeability was calculated by knowing the concentration, membrane thickness, area, and pressure difference between the two sides of the membrane. The measurement consists of recording the change in concentration of the gas under test on the side of the membrane through which an inert gas is passed, as a function of time. The pressure of the gas under test is instantaneously varied on one side of the membrane while on the other side, a logistic-like curve results as a response, as shown in the diagram in Figure 10. That Figure exemplifies the trend of gas concentration Cone (expressed in ppm) in the low- pressure side as a function of time (expressed in seconds) after a sudden helium injection with a pressure difference of about 1 bar between the two sides of a given membrane (the horizontal line represents the average concentration under steady-state conditions, which is about 110 ppm) .

[0088] Membrane diffusivity was calculated by applying the time lag method: time integration of the concentration signal Cone from the spectrometer (exemplified in Figure 10) allows calculating the total gas flux Q(t) through the membrane, which under steady-state conditions takes on the following linear expression (with a = c / 1, where c is the gas concentration at the side where the target gas passes, 1 is the membrane thickness, D is the diffusivity of the membrane, 6 is the total time lag of the sensor given by the sum of the time lag 6mem due to the membrane and the dead time of the system 0o) :

[0089] The intercept of the linear fit of the linear part of the curve Q(t) with the x-axis is used to determine the overall time lag of the measurement system 0, which is directly proportional to diffusivity and inversely proportional to the square of the membrane thickness (minus any dead time 6o of the instrument) . The trend of the curve Q(t) and the linear fit of its part corresponding to the steady-state condition are exemplified in the graph of Figure 11: in this example, a CNC membrane having an area equal to 960 mm2and a thickness equal to 12 pm subjected to the abrupt introduction of helium at a pressure of 1 bar is considered. To compensate for the dead time 6o due to the specific experimental configuration of the instrument, the relationship between the overall time lag and the thickness of different membranes having the same composition and area but different thickness was considered, specifically by plotting the time lag 6 (expressed in seconds) as a function of the square of the membrane thickness 1 (expressed in pm2) , as exemplified in the graph of Figure 12. The slope of the curve of Figure 12 allows the permeability coefficient of the membrane to be determined, while the offset of the same curve (i.e., the intercept with the y-axis) allows the dead time 6o of the system to be determined. Therefore, from the overall time lag measurements it is possible to determine the diffusivity values D after estimating the dead time 6o of the system, using the following expressions:

[0090] The experimental di f fusivity and permeability values obtained for a CNC membrane with a thickness of 12 pm and an area of 960 mm2are given in the following table :

[0091] Of course , subj ect to the principles of the invention, the details of construction and embodiments may vary widely from what is described and illustrated purely by way o f example , without departing from the scope of the present invention as defined in the appended claims .

Claims

CLAIMS1. Solid-state gas sensor (10) , comprising:- a semiconductor chip (12) comprising an element sensitive to the presence of one or more gaseous species;- a support (13) on which the semiconductor chip (12) is mounted, the support (13) having electrical connection pads (14A, 14B, 14C, 14D) connected to respective electrodes of the semiconductor chip (12) and accessible from the outside of the gas sensor (10) via respective electrical connection terminals (16A, 16B, 16C, 16D) ;- a closure element (18) mounted on the support (13) to define a sensing chamber of the gas sensor (10) at an upper surface of the semiconductor chip (12) , the closure element (18) having one or more through holes to allow gas exchange between the sensing chamber and the external environment; and- a selective membrane arranged internally to the closure element (18) between the upper surface of the semiconductor chip (12) and the one or more through holes, the selective membrane comprising cellulose nanocrystals .

2. Gas sensor (10) according to claim 1 of chemoresistive type, wherein the semiconductor chip (12) comprises :- at least one micro-heater electrically connected between a first electrode and a second electrode, and- a layer of sensing material electrically connected between a third electrode and a fourth electrode, the layer of sensing material being arranged on the upper surface of the semiconductor chip (12) and thermally coupled to the at least one micro-heater.

3. Gas sensor (10) according to claim 1 or claim 2, wherein the cellulose nanocrystals have transverse dimensions in the range of about 3 nm to about 5 nm and longitudinal dimension in the range of about 100 nm to about 250 nm.

4. Gas sensor (10) according to any of the previous claims, wherein:- the selective membrane comprises a plasticizing additive and / or nanoparticles;- preferably the plasticizing additive and / or the nanoparticles are present in a concentration between 0% and 50% by mass;- preferably the plasticizer is PEG-NH2.

5. Gas sensor (10) according to any of the previous claims, wherein the selective membrane has a thickness between about 0.1 pm and about 50 pm, preferably equal to about 10 pm.

6. A method of manufacturing a solid-state gas sensor (10) , the method comprising: i) providing a semiconductor chip (12) comprising an element sensitive to the presence of one or more gaseous species; ii) mounting the semiconductor chip (12) on a support (13) that has electrical connection pads (14A, 14B, 14C, 14D) accessible from the outside of the gas sensor (10) via respective electrical connection terminals (16A, 16B, 16C, 16D) , and connecting respective electrodes of the semiconductor chip (12) to the electrical connection pads (14A, 14B, 14C, 14D) ; iii) providing a closure element (18) configured to be mounted on the support (13) to define a sensing chamber of the gas sensor (10) at an upper surface ofthe semiconductor chip ( 12 ) , the closure element ( 18 ) having one or more through holes to allow gas exchange between the sensing chamber and the external environment ; iv) synthesi zing an aqueous solution comprising cellulose nanocrystals ; v) casting the aqueous solution in the closure element ( 18 ) and letting the aqueous solution evaporate to form a selective membrane comprising cellulose nanocrystals arranged internally to the closure element ( 18 ) ; and vi ) mounting the closure element ( 18 ) on the support ( 13 ) .7 . Method according to claim 6 , wherein step ( iv) of synthesi zing the aqueous solution comprising cellulose nanocrystals comprises :- forming an aqueous solution of raw cellulose pulp containing cellulose fibers ;- introducing electrostatic charges on at least part of the surface of the cellulose fibers via a chemical functionali zation process ; and- mechanically stimulating the aqueous solution to separate the cellulose nanocrystals .8 . Method according to claim 6 or claim 7 , wherein the cellulose nanocrystals have transverse dimensions in the range of about 3 nm to about 5 nm and longitudinal dimension in the range of about 100 nm to about 250 nm .

9. Method according to any of claims 6 to 8 , wherein step ( iv) of synthesi zing the aqueous solution comprising cellulose nanocrystals comprises adding a plastici zing additive and / or nanoparticles to the aqueous solution, preferably in a concentration between0% and 50% by mass, wherein preferably the plasticizing additive is PEG-NH2.

10. Method according to any of claims 6 to 9, wherein the selective membrane has a thickness between about 0.1 pm and about 50 pm, preferably equal to about 10 pm.

11. Method according to any of claims 6 to 10, comprising the step of thermally treating the selective membrane, preferably at a temperature between 70°C and 120°C, to promote hornif ication of the selective membrane .

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