Devices and methods for detecting target gases based on functionalized high surface area nanomaterials

Abstract

A sensing element for detecting a target gas or gases is disclosed. The element includes a pair of interdigitated electrodes, with a gap between each electrode in the pair. A high surface area metal oxide nanomaterial decorated with a noble metal is deposited on interdigitated electrode pair to bridge the gap. The surface area of the metal oxide nanomaterial is greater than 100m2 / g and the size of the metal oxide nanomaterial is within the range of 100-200nm. The resistance across the pair of electrodes varies when the metal oxide nanomaterial binds with the target gas or gasses. From this sensing element, sensor chips and sensor modules may be constructed to detect multiple gases.

Description

DEVICES AND METHODS FOR DETECTING TARGET GASES AT VERY LOW CONCENTRATIONS BASED ON FUNCTIONALIZED HIGH SURFACE AREA NANOMATERIALS 1.0 CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to provisional application 63 / 618639 filed on January 8, 2024, the entire contents of which are incorporated herein by reference. 2.0 FIELD OF THE INVENTION

[0002] Embodiments of the invention relate to the selective identification and accurate measurement of a target gas or a group of gases, within a complex air mixture where multiple gases are present and ambient environmental conditions vary (most importantly the humidity), and in particular, to the sensing of a gas by specially formulated and fabricated nanomaterials deposited on electrodes at the surface of a MEMS device integrating microheaters. 3.0 BACKGROUND

[0003] Many industries and urban communities already require detection and measurement of gases within target systems (“What is in the air?”). This information is used to control equipment (initiate action, monitor impact, implement feedback loop), detect exceptional events (e.g., leaks), make safety-related decisions, impact future-looking policies or operating procedures, assign offset values to some form of compensating mechanism, etc.

[0004] Global warming is an existential challenge to humanity that will drive large scale behavioral changes supported by technology and engineering. Examples include accelerating the transition to a sustainable and clean energy infrastructure while reducing the impact of the existing fossil-fuel infrastructure, creating an effective system of carbon offset, feeding a growing world population (eight billion people in 2022, expected to exceed ten billion by 2080) without making the global warming problem worse, remediating air quality problems and the related health consequences, etc.

[0005] Technology and engineering solutions to these problems tend to require accurate detection and measurement of gases at a very low concentration level (ppb), by itself a difficult problem, and the deployment of many gas sensors in the field, or the inclusion of gas sensors within each installed system, which compounds the difficulty.

[0006] Existing gas sensing equipment is often inadequate to respond to the challenge. Good performing solutions, where they exist, use expensive technology (e.g., spectrometry- based techniques) and are too big, and not energy efficient enough for practical deployment inthe field in sufficient numbers. Lower cost solutions, such as semiconductor metal oxide (MOx) technology, which exhibits a resistance change to gas exposure, cannot typically achieve the required selectivity nor a low enough detection limit, and need significant heating of the material to activate sensing and to offset the impact of humidity, which affects materials’ useful life and causes power consumption incompatible with many applications. In addition, while relatively small, typical MOx sensors are still not well suited to systems with a tight form factor or requiring a high level of integration.

[0007] What is needed is a modular sensor architecture that can address these shortcomings. 4.0 SUMMARY

[0008] A modular sensor architecture based on the combination of semiconductor technology with functionalized high surface area nanomaterials and filtering techniques to achieve a high level of gas sensing performance across multiple vectors (sensitivity, selectivity, accuracy, response time) while enabling components that are scalable (detect a single gas or multiple gases) and well suited to building a cost-effective, small form factor, standalone gas sensor or for integration into the electronics of complex equipment and systems (including those requiring low power).

[0009] According to one aspect, high surface area MOx nanomaterials, functionalized with metal atom clusters to increase sensitivity to the target gas or group of gases, are deposited on a specially designed MEMS platform with electrodes patterned on top of a substrate integrating microheaters, to complete a circuit whose electrical properties are modified when exposed, possibly through a filter specially designed to eliminate false response to cross- sensitive compounds, to an air mixture containing the target gas or gases, with the change in resistance detectable and measurable by signal-processing electronics, typically under the control of a microprocessor executing algorithms that translate the measured quantities into gas concentration values. 5.0 BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention can be better understood with reference to the following figures. The components within the figures are not necessarily to scale, emphasis instead being placed on clearly illustrating example aspects of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views and / or embodiments. Furthermore, various features of different disclosed embodiments can be combined to form additionalembodiments, which are part of this disclosure. It will be understood that certain components and details may not appear in the figures to assist in more clearly describing the invention.

[0011] FIG.1A illustrates the electronic sensitization mechanisms by Fermi-level control of functionalized MOx.

[0012] FIG.1B illustrates the chemical sensitization mechanisms by spillover of functionalized MOx.

[0013] FIG.2A illustrates a non-metal functionalized traditional MOx material.

[0014] FIG.2B illustrates a non-metal functionalized improved MOx material with high surface area.

[0015] FIG.2C illustrates a metal functionalized improved MOx material with high surface area.

[0016] FIG.3A1 is a micrograph image of high surface area non-metal functionalized nanomaterial at 30,000 magnification.

[0017] FIG.3A2 is a micrograph image of high surface area non-metal functionalized nanomaterial at 120,000 magnification.

[0018] FIG.3B1 is a micrograph image of high surface area metal functionalized nanomaterial at 30,000 magnification.

[0019] FIG.3B2 is a micrograph image of high surface area metal functionalized nanomaterial at 120,000 magnification.

[0020] FIG.4 illustrates a microheater structure that may be used with the embodiments herein.

[0021] FIG.5 illustrates a cluster of four sensing elements with microheaters.

[0022] FIG.6 illustrates a simplified nanomaterials development and testing process.

[0023] FIG.7A illustrates the precursor reagents dissolved in solution within a sealed tube under stirring.

[0024] FIG.7B illustrates the solution being heated by microwaves to above the boiling point.

[0025] FIG.7C illustrates the synthesis of nanocrystals under pressure.

[0026] FIG.8 illustrates a cross-section of a sensing element.

[0027] FIG.9A illustrates the sensor chip with a plurality of sensing elements.

[0028] FIG.9B illustrates the packaged sensor chip with a plurality of sensing elements.

[0029] FIG.10 illustrates a complete standalone sensor module.

[0030] FIG.11 is a chart showing some, non-limiting, sensor integration options.

[0031] FIG.12 is a cross-section of a sensor SIP implementation with two sensor chips.

[0032] FIG.13 is a schematic of the testing system used to test the present sensors.

[0033] FIG.14 is a graph of the non-heated sensor chip response to 1.86 ppm formaldehyde (GS1-GS4, four electrodes).

[0034] FIG.15 is a graph of the non-heated sensor chip response to 1.86 ppm formaldehyde with cross-interferent 250 ppm acetone (GS1-GS4, four electrodes).

[0035] FIG.16 is a graph of the non-heated sensor chip response to 1.86 ppm formaldehyde with cross-interferent 50 ppm isopropyl alcohol (GS1-GS4, four electrodes).

[0036] FIG.17 is a graph depicting the response of the sensor chip to different formaldehyde exposures with a calculated limit of detection (LOD) of 90 PPB.

[0037] FIG.18 is a chart and graph depicting the response of the sensor chip to different formaldehyde exposures (> 1% signal change in < 2 minutes at 1.86 PPM). 6.0 DETAILED DESCRIPTION

[0038] Reference is made herein to some specific examples of the present invention, including any best modes contemplated by the inventor for carrying out the invention. Examples of these specific embodiments are illustrated in the accompanying figures. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described or illustrated embodiments. To the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

[0039] In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. Example embodiments of the present invention may be implemented without some or all these specific details. In other instances, process operations well known to persons of skill in the art have not been described in detail in order not to obscure unnecessarily the present invention. Various techniques and mechanisms of the present invention will sometimes be described in singular form for clarity. However, it should be noted that some embodiments include multiple iterations of a technique or multiple mechanisms unless noted otherwise. Similarly, various steps of the methods shown and described herein are not necessarily performed in the order indicated, or performed at all, in certain embodiments. Accordingly, some implementations of the methods discussed herein may include more or fewer steps than those shown or described. Further, the techniques and mechanisms of the present invention will sometimes describe a connection, relationship or communication between two or more entities. It should be noted that a connection or relationship between entities does not necessarily mean a direct, unimpeded connection, as a variety of other entities or processes mayreside or occur between any two entities. Consequently, an indicated connection does not necessarily mean a direct, unimpeded connection unless otherwise noted.

[0040] The following list of example features corresponds with the attached figures and is provided for ease of reference, where like reference numerals designate corresponding features throughout the specification and figures:

[0041] Low Surface Area MOx 5

[0042] High Surface Area MOx 10

[0043] Nanocrystallite MOx Structures 15

[0044] Metal Functionalization 20

[0045] MEMS Patterned Die (µhMEMS) 25

[0046] Temperature Sensor 30

[0047] Temperature Sensor Contacts 32

[0048] MicroHeater 35

[0049] MicroHeater Contacts 37

[0050] Electrodes 40

[0051] Gap 42

[0052] Nanomaterial Deposition Area 45

[0053] MATS 47

[0054] Sealed Tube 50

[0055] Precursor Reagents 55

[0056] Stirrer 60

[0057] Microwave Heat 65

[0058] Nanocrystals 70

[0059] Thin-Film Insulator 75

[0060] Sensing Element 80

[0061] Sensor Chip 85

[0062] Data / Power Pins 90

[0063] Package Lid 95

[0064] Membrane Filtered Exposure Window 100

[0065] Packaged Sensor Chip 105

[0066] Gas Exposure 107

[0067] Data / Power Lines 109

[0068] Data Acquisition Module 110

[0069] Control and Data Processing Module 115

[0070] Communication Module 120

[0071] Power Regulation and Management Module 125

[0072] Existing / Modified Application Electronics Options 127

[0073] Sensor Module 130

[0074] System In a Package (SIP) 135

[0075] Packaging 140

[0076] Package Substrate 145

[0077] Target Gas 1 200A

[0078] Target Gas 2 200B

[0079] Dry Air 205

[0080] Valves 206

[0081] Mass Flow Controllers 207

[0082] Temperature Controlled Bath 210

[0083] Manifold 215

[0084] Sensor Test Chamber 220

[0085] Exhaust 225

[0086] Processor 230

[0087] Temperature-Controlled Bubbler 235

[0088] Described herein is a modular sensor architecture combining functionalized high surface area MOx nanomaterials, a MEMS substrate with embedded microheaters and optional filters. A complete standalone sensing element is also described herein, as well as integration of the sensing element into a sensor ship or other larger electronic system within which the necessary data acquisition and processing functionality already exist or could be conveniently. The combination of one or more sensing elements into a sensor chip design that can be functionalized with one or more MATS nanomaterials, each targeting a specific gas of family of gases, followed by the combination of one or more sensor chips into a sensor module capable of detecting and measuring the concentration of one or more gases within a complex air mixture.

[0089] MOx gas sensing mechanisms have been studied since the 1980s, and the general framework relies on two fundamental models based on either electronic (Fermi-level control) or chemical (spillover mechanism) sensitization of materials by additives (surface functionalization with clusters of metal atoms), shown in FIGS.1A and 1B respectively. However, the either-or simplicity provided by these two models does not fully illustrate the complex interplay between materials reactivity and gas sensing performance nor the mutual influence of surface chemistry and electronic properties of the solid. Thanks to recently developed experimental methods (operando spectroscopy or in situ characterization) and to the availability of more powerful computational modelling techniques, it is becoming clear today that both chemical andelectronic contributions are fundamental to the sensitization effect and must be considered together to master the intricate relationship between materials structure and gas sensing performance.

[0090] Traditional gas sensors that use low surface area MOx 5 have poor stability and selectivity (see FIG.2A). High temperature (e.g., between 300-500 °C) is required to activate the low surface area MOx film layer to enable gas sensing through electron exchange at the activated MOx surface, but such a high level of heat presents severe power consumption challenges and degrades the MOx sensing layer over time through increased MOx material grain growth.

[0091] This invention is based instead on metal-functionalized, high surface area MOx nanomaterials (MATS). MATS are a class of microwave synthesized high-surface-area MOx nanomaterials, further functionalized by photochemical decoration with noble metal atom clusters to increase sensitivity and selectivity to a specific target gas or group of gases. MATS work at room temperature, through gas sensing mechanisms at the gas-material surface that combine both electronic and chemical sensitization occurring at noble metal-MOx interfaces, with a potential for increased sensing performance with moderate heating.

[0092] MATS are designed by combining the most recent understanding of gas sensing mechanisms with state-of-the art material synthesis methods and equipment, and with the careful selection of an optimized amount of metal atom cluster additives. MATS are formulated and deposited with a small volume dispensing instrument onto electrodes 40 that sit atop uniform microheaters 35, which reside within the nanomaterial deposition area 45, see FIGS.4 and 5. A pair of electrodes 40 are in an interdigitated configuration with a gap 42 between them. The MEMS patterned die (µhMEMS) 25 includes the substrate with microheater 35, microheater contacts 37, temperature sensor 30, and temperature sensor contacts 32. Although not shown in FIG.4, there is a thin-film insulator layer separating the microheater 35 from the electrode 40, see FIG.8.

[0093] MATS are capable of sensing at room temperature, but the additional energy provided through limited heating (150-200 °C) from µhMEMS 25 further activates MATS without the MOx material degradation that occurs in traditional MOx gas sensors. In addition, the microheaters 35 turn µhMEMS 25 into an effective and low power platform to remove humidity effects.

[0094] FIGS.2A-2C compare previous materials to a preferred embodiment of MATS. Specifically, FIG.2A illustrates a MOx 5 with a non-metalized low surface area. FIG.2B illustrates a non-metal functionalized high surface area MOx 10. The smaller MOx nanocrystallite structures 15 with high surface area enhance sensitization. In FIG.2C, MATS 47includes the smaller nanocrystallite MOx structures 15 with high surface area and surface metal- functionalization 20 interfaces to further enhance sensitization.

[0095] FIG.3A1 and 3A2 show micrograph images at 30,000 and 120,000 magnifications of a practical embodiment of a high surface area MOx nanomaterial with the nanocrystallite structures clearly visible. FIG.3B1 and 3B2 show micrograph images at 30,000 and 120,000 magnifications of the same high surface area material after metal functionalization.

[0096] The innovative combination of electronic and chemical sensitization in the material synthesis of MATS delivers materials that are highly gas sensitive. MATS, used in concert with the µhMEMS platform, deliver enhanced material activation while the removal of humidity effects enables the development of relatively simple algorithms for the conversion of raw sensor data to accurate gas concentration values.

[0097] In all MOx gas sensors a process called reception transforms chemical information into a form of energy, and another process called transduction translates this energy into an analytically useful signal output. Gas reception takes place by reversible redox processes between reactive gases and the MOx surface. Changes in the electrical properties of the MOx material are translated into an electrical output as part of the transduction, which result in a measurable resistance change. Reception includes surface chemistry and formation of the space charge layer, while transduction includes charge transport in the sensing layer.

[0098] For analytically useful signals, changes of the surface charge must be translated into measurable electrical signals, which is practically done by measurement of DC resistance of the sensing layer. For this purpose, electrodes 40 are patterned on the µhMEMS 25 substrate and nanomaterial is deposited in a deposition area 45, as seen in FIG.5. This figure also illustrates a practical embodiment combining four basic µhMEMS sensing elements 80, thus demonstrating the scalability of the concept. In this way, different MATS can be combined within a modular sensor architecture making it possible to address more complex gas sensing problems (cross- sensitivity for example, where a given material will react to more than one gas analyte) and / or to use a single sensor to detect and measure multiple gases, in both cases helping the selectivity of sensors based on MATS + µhMEMS sensor architecture.

[0099] Sensing results depend on the way surface charge changes impact the electrical current flowing between the electrodes. There are several factors to consider, including (a) the structural and morphological properties of the sensing layer, (b) the electrical and chemical properties of the MOx material, (c) the size and shape of the MOx material, and (d) the geometry of the electrodes.

[0100] For these reasons, MATS are developed to work optimally with the geometries of the µhMEMS substrate and surface electrodes, and conversely the design of theµhMEMS substrate and of the electrodes is optimized while keeping in mind the type of sensing materials to be deposited and the equipment and techniques that will be used to accomplish the deposition. This mutual optimization process requires research and development in multiple disciplines by those skilled in the arts. FIG.6 provides a simplified view of the development flow for MATS. 6.1 MATS KEY FINDINGS AND MANUFACTURE

[0101] Improvement of MOx gas sensing performance involves material surface functionalization with additive clusters of metal atoms to change the reception and / or transduction mechanism. Sensitization corresponds directly to the strong interrelation of chemical and electronic effects. Thus, we must consider the mutual influence of chemical and electronic properties and processes instead of strictly separating electronic and chemical sensitization mechanisms as was done with previous models. The combination of chemical and electrical sensitization results in a highly powerful approach to improve gas sensing properties of functionalized MOx materials. MOx base materials may include, but are not limited to, zinc oxide (ZnO), tin oxide (SnO2), tungsten oxide (WO3), copper oxide (CuO), nickel oxide (NiO), iron oxide (Fe2O3), titanium dioxide (TiO2), indium oxide (In2O3), chromium oxide (Cr2O3), manganese oxide (Mn2O3), cobalt oxide (Co3O4), strontium oxide (SrO), vanadium oxide (V2O3), germanium dioxide (GeO2), niobium pentoxide (Nb2O5), molybdenum trioxide (MoO3), tantalum oxide (Ta2O5), lanthanum oxide (La2O3), cerium oxide (CeO2), and neodymium oxide (Nd2O3).

[0102] The main reasons for introducing clusters of noble metal atom additives onto the surface of MOx gas sensing materials are to increase their sensitivity, enhance their selectivity, and improve their stability as compared to pristine MOx materials. Better understanding of gas sensing mechanisms translates into better materials. Gas reception (i.e. how the chemical process takes place and translates into changes of the electronic properties) and transduction (i.e. how charge transport takes place within the gas sensing layer to transform those changes into sensor signals) are optimized by tuning properties of the MOx base material and the clusters of metal atom additives. Size and shape of the MOx base material, as well as metal atom additive concentration and coverage, significantly impact the transduction. The clusters of metal atom additives strongly influence the reception through a chemical and / or an electronic effect. Additionally, the size, concentration, and coverage of the clusters of metal atom additives on the MOx material impact effectiveness of the reception. MATS optimize size and shape of the MOx base material and clusters of metal atom additive coverage to enhance thetransduction process, while suitable clusters of metal atom additive size augments reception of the analyte gas.

[0103] A more efficient transduction is achieved by using a porous layer of high surface area MOx nanoparticles. The MOx material structure should allow the gas to interact with a maximum surface area and promote efficient transduction. Porous structures and layers increase accessibility of gases to the whole sensing layer while compact layers or large single crystalline structures (e.g. sputtered thin film commercialized gas sensors) have low surface area gas interaction such that changes in the space charge layer are restricted to a small part of the material.

[0104] Optimized clusters of noble metal atom additive concentration must consider material reactivity and additive coverage affecting the maximum MOx surface area, either by chemical activation of the MOx surface or by forming heterojunctions. Clusters of metal atom additive size and coverage also greatly impact the sensitization effect. The amount of clusters of metal atom additive loading determines dispersion on the MOx surface and the reactivity. Clusters of metal atom additives situated close to MOx grain-grain contacts can affect charge transport (transduction), so smaller amounts of metal atom cluster additives are more efficient in controlling the functionalized MOx surface interface and material reactivity. Noble metals to be used for creating these clusters may include, but are not limited to, gold (Au), platinum (Pt), silver (Ag), rhodium (Rh), palladium (Pd), iridium (Ir), osmium (Os), rhenium (Re), and ruthenium (Ru).

[0105] Novel microwave-assisted material synthesis techniques are used to iterate rapidly on material variants. Specifically, precursor reagents 55 dissolved in solution within a sealed tube 50 (reaction vessel) under stirring 60 (FIG.7A) are heated by microwave 65 rapidly up to a temperature above the solution boiling point (FIG.7B) to synthesize high surface area nanocrystals 70 via an expedited, pressurized hydrothermal reaction (FIG.7C).

[0106] When microwaves pass through matter within a reaction vessel, dipole fields in the material and solvent attempt to realign with the oscillating electric field of the microwave. Molecules in the material and solvent cannot keep up with the oscillating field in microwaves, and the continual re-orientation of the molecules results in energy loss through molecular friction and dielectric loss. The amount of heat generated is dependent on the ability of the molecules to align themselves with the frequency of the electric field—this is known as the microwave dielectric heating effect. Microwaves operate at a frequency (2.45 GHz) lower than the energy needed to break a bond, but high enough that a molecule with a dipole releases heat through the rotational motion of the dielectric heating effect. Microwave reactors, equipped with pressure sealing technology and magnetic stirring, can provide uniform heating conditions,above reflux, to synthesize high surface area nanomaterial structures more homogeneous than structures achievable with non-stirred autoclave hydrothermal reactors. Additionally, microwave reactions are fast, with reaction times reduced by half for every increase of 10° C according to the Arrhenius equation. For example, a 24-hour hydrothermal reaction conducted at room temperature would take approximately 2 minutes at 120° C using a microwave reactor.

[0107] Microwave reactors directly apply energy to molecules rather than the reaction vessel and can volumetrically heat the mixture to the desired temperature within minutes. Nanomaterial synthetic recipes can then be devised and validated using an operator independent “push-button chemistry” user interface, which enables high throughput material development to obtain higher yields of uniform nanomaterial reaction products.

[0108] No more than a few tens of milligrams of MATS, functionalized through decoration with clusters of noble metal atoms, and precisely deposited onto each die of a µhMEMS wafer can fabricate thousands of chemiresistive gas sensors. Two fundamental aspects of the fabrication process include synthesis of the MATS and deposition on the µhMEMS platform. Traditional wet chemistry methods, including sol–gel decomposition of organometallic precursors and hydrothermal synthesis of colloidal solutions, can produce custom, noble metal- doped, or noble metal-decorated, metal oxide nanomaterials with high gas sensitivity. However, the lack of repeatability of both kinds of synthesis and deposition techniques, with this type of “material-dependent” approach, makes it impractical outside of the research lab.

[0109] A flame spray pyrolysis (FSP) method has also been used experimentally to synthesize and deposit noble metal-doped (but not decorated) metal oxide nanoparticles on a silicon wafer substrate. With this approach, a solution of noble metal and metal oxide precursors are rapidly nebulized, sintered, and sprayed on a masked wafer. A limited adjustment of material inputs, including noble metal and metal oxide precursor ratios, results in relatively low surface area (<100 m2 / g), spherical, noble metal-doped, metal oxide nanomaterials that require > 300 °C heating to achieve high gas sensitivity. However, extended > 300 °C heating of metal oxide nanomaterials results in nanomaterial grain growth that also degrades sensing performance and limits the useful life of the sensor (typically around 6 months).

[0110] Proper setup and use of FSP equipment does enable synthesis and deposition repeatability, but this “process-dependent” approach cannot be used to fabricate MATS, which are noble metal-decorated, metal oxide nanomaterials, with an even higher surface area (> 100 m2 / g), that can achieve superior gas sensitivity, without requiring the undesirable > 300 °C heating. Noble metal decoration of the nanomaterial surface (rather than doping throughout the material) results in a more reactive and gas sensitive material due to themuch higher number of noble metal-metal oxide interfaces at the material surface where the gas- material interactions take place.

[0111] In a preferred embodiment, the fabrication of MATS relies on a novel “material-dependent” approach. There are several ways to make metal oxide nanomaterials with high surface area using a “material-dependent” approach. One way uses hydrothermal reactions at a temperature above the boiling point within a pressurized autoclave. Typically, these reactions use metal precursors (metal nitrate, metal chloride, metal oxalate) dissolved in a solvent (typically water), with potentially one or two additives included as structure-directing agents. The main problem with scaling this approach in a repeatable manner is that nanomaterial crystal growth can take 1-2 days within non-stirred autoclaves.

[0112] The MATS novel fabrication method uses a microwave synthesizer, which enables the same type of hydrothermal reactions, but within a pressurized stirred vessel, and occurs in hours (instead of days). As shown in FIGS.7A-7C, the stirred tube 50 ensures uniformity of the high surface area crystallites 70 synthesized during the hydrothermal reaction under pressure, and the microwave energy 65 irradiating the entire solution provides rapid heating, with programmable control of pressure, temperature, and energy inputs, throughout the expedited synthesis process. Thus, uniform, highly gas-sensitive nanomaterials, with higher surface area, can be synthesized in a repeatable, customizable, scalable, and expedited manner. After the high-surface-area nanomaterials are fully processed (microwave-synthesized, washed, dried, sintered), the fabrication of MATS continues with a subsequent step to decorate the surface with clusters of noble metal atoms resulting in high sensitivity for the target gas. This decoration step differentiates MATS from other “one-pot synthesis” doped nanomaterials which have noble metals dispersed throughout the nanomaterial.

[0113] The noble metal-decoration is accomplished in two steps. First comes a photochemical reaction whereby UV-light shines through a solution of metal oxide nanoparticles, with addition of a specific amount of a precursor solution containing the noble metal. This reaction nucleates noble metal atoms on the metal oxide nanoparticle surface. The second step incorporates a strong reducing agent and an additional amount of noble metal precursor solution. This allows noble metal cations to covalently bond in clusters to already- nucleated noble metal atoms on the metal oxide nanoparticle surface.

[0114] The described MATS synthesis process can be used to make MATS consisting of any of the above listed MOx materials decorated with any of the above clusters of noble metal atoms. One specific example for a formaldehyde sensor consists of nucleating clusters of platinum (Pt) atoms onto high surface area tin oxide (SnO2). This general approach for synthesizing high surface area metal oxide nanomaterials with clusters of noble metal atomsdecorated on the surface can be applied for making materials capable of sensing other target gases besides formaldehyde. The high-surface-area, metal oxide nanomaterials provide higher energy sites for effectively building strongly adhered islands of noble metal atom clusters (thus, decorating the surface). 6.2 CONSTRUCTION OF SENSING ELEMENTS AND INTEGRATION OF SENSING ELEMENTS INTO MORE COMPLEX STRUCTURES

[0115] FIG.8 shows a cross-section of a preferred embodiment of the sensing element 80 (MATS + µhMEMS). A basic sensing element is a single pair of interdigitated electrodes (IDEs) 40 separated by a gap (several microns) that is bridged by deposition of a MATS nanomaterial (100-200nm) 47. The MATS is deposited and annealed to ensure good contact with the surface of the IDEs 40. The combination of the IDEs 49 and MATS 47 creates a conducting element whose resistance will vary when exposed to a specific gas or family of gases. An optional integrated microheater 35 is separated from the electrode layer by a thin-film insulator 75. The design is optimized through thermal diffusion analysis to enable quick heating of MATS 47 deposited directly above the insulator 75.

[0116] As previously mentioned, a preferred embodiment of the core technology will optimize the geometry of the µhMEMS platform, including design of the electrodes and the gaps between electrodes, to best fit the deposition and annealing techniques to be used for the MATS. The deposition process itself will be feasible with cost-effective and readily available liquid handling and dispensing technology to achieve reliable, cost-effective manufacturability in high volume. The built-in modularity of a sensor architecture based on this technology includes a preferred embodiment of a packaged sensor chip (see FIGS.9A and 9B) which will integrate multiple µhMEMS sensing locations on the same MEMS substrate together with the necessary connectivity to provide biasing to the individual sensing locations and to access the DC resistance of the sensing layer at each location. A preferred embodiment follows an array pattern, although other structural arrangements are possible.

[0117] FIGS.9A and 9B show a preferred embodiment of the sensor chip 85 housed in a package forming a packaged sensor chip 105. A sensor chip is generally a N x P array of sensing elements 80 (with N >= 1 and P >= 1), built on a substrate together with the necessary interconnect to independently bias each individual sensing element in order to measure the variation of its resistance, after deposition of one or more MATS (only one MATS per sensing element) and exposure to an air mixture.

[0118] In this embodiment is a 3X3 MEMS sensor chip 85 that includes a 3 x 3 array of sensing elements 80 with individual control, through data / power pins 90 of themicroheater at each location and a temperature readout pin at each location, in addition to the pin used to bias the MATS material (Vbias), which is also the pin used to measure changes in resistance (Isense). Separate ground pins are provided for the microheaters / temperature sensors and for the electrode network of the sensing locations.

[0119] As shown in FIG.9B, a preferred embodiment of the MEMS sensor chip 85 is packaged forming the packaged sensor chip 105. The lid 95 of the packaged sensor chip 105 includes an exposure window 100 to allow exposure of the MATS to the ambient air. The exposure window 100 may be a cutout or a porous filter membrane specifically fabricated to allow the target gas or gases to diffuse through, while potential undesirable cross-sensitive components of the air mixture are adsorbed and kept out (e.g., through use of a selective Metal- Organic Framework (MOF) functionalized membrane). The latter construction can be used to eliminate false positives in applications where undesirable cross-sensitive components are well- identified (e.g., distinguishing between oxidizing gases ozone (O3) and nitrogen dioxide (NO2)). As detailed below, the packaged sensor chip 105 may be packaged with other modules to have a completely integrated system.

[0120] The sensor chip 85 may use a semiconductor substrate in which is embedded a single microheater serving all sensing elements and a single temperature sensor and it is fabricated using MEMS process technology. Alternatively, the substrate may have several microheaters, one per sensing element or group of sensing elements, and a temperature sensor for each microheater, and it is fabricated using MEMS process technology.

[0121] In one embodiment, a single MATS nanomaterial is deposited on all sensing elements of the array thus allowing for redundancy (for example to increase sensor manufacturing yield) and / or for averaging the measured variation of resistance across several sensing elements when exposed to the target gas. Alternatively, different MATS nanomaterials may be deposited on different sensing elements to detect different gases or family of gases with the same sensor. The heterogenous MAT deposition may be helpful in resolving false positives in cases when a specific nanomaterial is cross-sensitive to more than one gas in a complex air mixture. The sensor ship may be a bare die. The die may also be package with an opening in the lid to allow the ambient air mixture to reach the sensor surface. The opening may also have a filter membrane (e.g. MOF membrane) to allow only certain components of the ambient air mixture to reach the sensor surface.

[0122] FIG.10 shows the simplified block diagram of a preferred embodiment of a standalone gas sensor chip 85. A mixed signals Data Acquisition Module 110 provides biasing to the sensing locations within the sensor chip 85 and measures changes in resistance values when exposed to gas analytes (practically, the module measures current changes for a knownbias voltage). The biasing and data acquisition is via data / power lines 109. Because the bias voltage is another variable potentially affecting sensing performance of a MATS, a preferred embodiment provides the flexibility to separately bias each sensing location (or perhaps, groups of locations) in cases where sensing locations are functionalized with different MATS. The raw gas sensing analog information is converted to a digital format before being passed on to a microcontroller in the Control and Data Processing Module 115. The microcontroller runs algorithms on the sensing data (one or multiple algorithms, depending on how many different MATS are used in the chosen implementation of the sensor chip) to generate gas concentration values. In turn, the gas information is made available outside the gas sensor via a Communication Module 120 implementing one or more of a variety of possible wired or wireless protocols. Each of the modules may be connected to a power regulation and management module 125. All these modules may be implemented in a sensor module 130, discussed in more detail below.

[0123] A sensor module based on a MATS + µhMEMS sensor chip 85 is suitable for multiple forms of integration as is illustrated in FIG.11.. The area bounded by line 127 (existing / modified application electronics options) identifies options that can be used with existing (or modified) application electronics, with enhanced firmware, to accommodate connection of MATS + µhMEMS gas sensing and addition of gas sensing functionality to the application.

[0124] A sensor module is one (or more) sensor chip(s), in either bare die or packaged form, together with the necessary electronics to convert analog raw sensor data into information usable by the end-user application, such as for instance gas concentration values or the crossing of an alert threshold. The sensor module can be packaged into a suitable protective enclosure to create a standalone sensor product, or it can be used without enclosure, as a component into a larger appliance (e.g. a device measuring ambient air quality). The enclosure of the sensor module and / or that of the appliance must be designed to allow ambient air to reach the sensor chip(s), and their corresponding sensing elements.

[0125] The sensor module may be manufactured as a Printed Circuit Board (PCB) on which reside one or more sensor chip(s), the circuitry to convert the analog raw sensor data into a digital format, a processor to run algorithms converting the raw sensor data into gas concentration values or some other higher-level information, and the necessary ancillary circuits to provide power, timing and interconnect. The sensor module maybe implemented as a single chip. In cases where the end user application already has the necessary electronics to directly use and transform the analog raw sensor data from the sensor chip(s), the sensor module is a virtual concept combining one or more sensor chip(s) with the native hardware and software ofthe end-user application. In another preferred implementation, the form factor of the sensor module is reduced by combining most of the module mixed-signals electronics surrounding the sensor chip(s) into a System-On-a-Chip (SoC).

[0126] In another preferred implementation, the sensor module 130 form factor is still further reduced by stacking the sensor chips 85 in die form with the SoC, also in die form, into a single package to create a System-In-a-Package (SIP) as illustrated in FIG.12. Specifically, a SIP 135 includes stacking one or more sensor chips 85 on top of a System on a Chip (SoC) 130 gathering the rest of the necessary functionality to create a complete standalone multi-gas sensor, which shows a (much simplified) conceptual view of two sensor chips 85 stacked with a SoC in a single package. A SoC is an integrated circuit (IC) that combines all the components of an electronic system onto a single chip. Those skilled in the art will understand that modern semiconductor packaging technology offers multiple options to implement such a configuration. One such option, shown in FIG.12 includes packaging 140 and package substrate 145 with an exposure window 100.

[0127] The sensor chip 85 also makes it possible, depending on the chosen level of integration and the needs and built-in capabilities of the system-level application, for a portion of the gas sensor functionality to be delivered by electronics already built into the larger system for other purposes. For example, if the system in which gas sensing capability is to be incorporated already has a microprocessor-based control unit and the ability to connect additional peripheral functionality, then it may be possible to directly add the sensor chip 85 and a Data Acquisition module on the hardware side and to modify the software to incorporate a routine specific to the added gas sensing functionality. Such an embodiment is preferred in cases where for reasons of cost, or perhaps due to the constraints of a particular form factor, use of a standalone gas sensor is not desirable. The table in FIG.11 identifies on a line-item basis what portion of the gas sensing hardware / firmware can potentially be absorbed by system electronics. 6.3 NON-LIMITING TARGET GASES

[0128] A few examples of gas sensing applications that become possible or are greatly enhanced by using preferred embodiments of the MATS + µhMEMS technology are highlighted below.

[0129] Formaldehyde (CH2O) is another hazardous substance which has many industrial applications, among others: in car manufacturing, explosives, plastic, resins, chemicals, and other artificial materials. Dissolved in water (formalin), it is also used as a disinfectant in industry and in medicine. In addition to being an irritant that can cause difficulty breathing and can trigger or aggravate asthma symptoms, it is a potential cancer hazard in casesof long exposure by inhalation, even to very low concentrations. It is one of the most common indoor air pollutants with exposure in a residential environment typically coming from treated wood products (plywood or particle board) or paints, varnishes, and floor finishes. Protecting factory personnel as well as individual homes from the hazards of formaldehyde is a current priority across industries, but formaldehyde is a Volatile Organic Compound (VOC) that is difficult to detect and measure by itself and can also be difficult to differentiate from other VOCs. MATS + µhMEMS sensor chips, in particular with multiple sensing locations functionalized with different MATS, are good candidates to create selective, reliable, low concentration formaldehyde sensors.

[0130] Methane (CH4) is a potent greenhouse gas estimated to be 80 times more effective at trapping heat in the atmosphere than carbon dioxide (CO2). Methane is much less persistent in the atmosphere (one decade versus centuries) than carbon dioxide, which means that the earth will respond quickly to a reduction in methane emission, leading to slower near- term warming, something that is very desirable to avert a climate catastrophe. The plumes from “super-emitters,” releasing methane in the atmosphere at very high rates, can be detected from space with imaging spectrometers installed on satellites or the International Space Station. However, only very large emissions can be seen from space, and while it is also possible to equip airplanes with spectrometers, this only enables the additional detection of emissions that are about one order of magnitude (ten times) smaller, leaving the problem of a myriad of even smaller leaks within oil and gas refinery sites and within the transport and distribution networks out of reach. The normal amount of methane in the atmosphere is approximately 2 ppm (parts per million). Small instruments incorporating MATS + µhMEMS sensor chips and capable of methane detection in the 2 to 500 ppm range could enable building dense networks of fixed sensing stations within a facility (or along a pipeline) as well as outfitting mobile sensing stations on automated vehicles following a prescribed route (possibly stopping at regular intervals), or even drones flying on a set pattern (possibly hovering on station as needed). Conceivably, MATS + µhMEMS sensors could also be integrated into wearable devices carried by facility personnel, providing a mechanism to detect and map potential leaks while enhancing employee safety (500 ppm is well below the critical safety alert level for methane).

[0131] In the search for clean energy solutions to reduce carbon dioxide emissions, ammonia (NH3) will play a key role in a future climate-neutral economy by enabling emission-free power generation, heavy transport (maritime in particular), heating, and industrial processes. Ammonia, already a widely traded chemical on the world stage, is a compound consisting of three parts hydrogen and one part nitrogen, which releases zero carbon dioxide emissions when combusted in a thermal power plant. In addition, ammonia contains 18%hydrogen by weight and is much easier to liquefy than hydrogen, which makes it a good candidate to solve the hydrogen transportation and storage problem, thus helping to enable hydrogen itself as a clean source of energy. However, ammonia is a toxic product, in both liquid and gas forms, and can cause serious harm to humans. In gas form, ammonia vapors must be controlled through process enclosure and the use of ventilation. Detection of ammonia concentration levels exceeding safe limits can be done with instruments incorporating MATS + µhMEMS sensor chips which can generate alerts or automatically start equipment used for ventilation and maintenance of a safe level of gas. Food and beverage companies use many tons of ammonia for refrigeration, flash freezing, and bulk storage, all running the risk of gas releases. Industrial farms (swine farms in particular) also have a need for ammonia gas sensors capable of low ppm detection to monitor and periodically trigger the cleaning and ventilation of facilities housing large numbers of animals. 6.4 PRELIMINARY EXPERIMENTAL RESULTS -FORMALDEHYDE SENSOR

[0132] The test system shown in FIG.13 is a custom vapor mixing and delivery station, built into a fume hood. The system includes target gas sources (200A, 200B) and a dry air source 205 provided by an air compressor dried by refrigeration and through moisture traps. The air and gas are dispensed by valves 206, measured by mass flow controllers 207, temperature controlled by bath 210, mixed in manifold 215, and finally introduced into sensor test chamber 220. Humidity is also delivered by bubbling air through a temperature-controlled bubbler 235 and is measured independently with a humidity sensor in the test chamber. The gas mixture is exhausted 225 from the test chamber 220. The valves 206 and mass flow sensors 207 are controlled by processor 230, which also controls the temperature of the gas mixture introduced to the sensor test chamber 220 and the humidity from the bubbler 235. The test system can mix chemicals and deliver these mixtures to a temperature–controlled sensor test chamber 220 which has a typical relative humidity (0-80% ± 2%) and temperature (10 – 35°C ± 0.5° C) operating range of the system.

[0133] The sensor test chamber 220 is made from stainless steel and is coated to reduce interactions with the incoming chemical mixtures. The test system can mix up several target gases (0 - 100% of source).

[0134] The experimental MATS sensors were measured using a 2-point measurement via a datalogger. To measure resistors, the datalogger drives a fixed current and measures the voltage across the resistor. The datalogger automatically selects the appropriate current depending on resistance range.

[0135] To test the MATS sensor sensitivity to formaldehyde, a glass bubbler is filled with paraformaldehyde powder and molecular sieves to allow air to pick up and deliver it to the sensors. It is assumed that the paraformaldehyde depolymerizes to form formaldehyde in the headspace. Based on the Material Safety Data Sheets (MSDS) value of the saturated vapor pressure (Psat) of paraformaldehyde (https: / / fscimage.fishersci.com / msds / 96373.htm, 1.2mm Hg at 25°C), it can be estimated that at a dilution of 1cc / min in 1000cc / min humidified air, the formaldehyde concentration is approximately 1.86 ppm (using Equation 1 below). Conc. (ppm) = (Psat / 760 Torr) * (chemical flow / total flow) * 1,000,000 (1)

[0136] Experimental MATS sensors chips were held in paraformaldehyde at 25°C (Psat ~ 1 Torr) for 1.86 ppm formaldehyde testing (FIG.14) and 1.86 ppm formaldehyde with high concentration interferents, 250 ppm acetone (FIG.15) and 50 ppm IPA (FIG.16). The interferents were held at 5°C to produce lower vapor concentration. Most tests were carried out using 50% background humidity. To calculate the concentrations of the interferents (IPA and acetone) equation 1 was used with the following Psat values, respectively: 12.0 and 90.9 Torr.

[0137] Referencing FIGS.14-16, graphs are presented showing the sensitivity of sensor chips with four electrodes (GS1, GS2, GS3, GS4) to formaldehyde only (FIG.14) and formaldehyde plus high concentrations of problematic indoor cross-interferents acetone (FIG. 15) and isopropyl alcohol (FIG.16). Formaldehyde testing (1.86 ppm) done together with either cross-interferent acetone (250 ppm) or isopropyl alcohol (50 ppm) shows the excellent sensor selectivity even when presented with unnaturally high indoor levels of both cross-interferent gases acetone and isopropyl alcohol. For formaldehyde exposure only, the sensor chip response to different formaldehyde exposures is shown with calculated Limit of Detection (LOD = 90 ppb for GS1-GS4 at 1.86 ppm) without any heating (FIG.17). Additionally, the sensor chip response to different formaldehyde exposures shows > 1% signal change in < 2 minutes at 1.86 ppm without any heating (FIG.18). Overall, the sensor chip selectivity, sensitivity, and response time for formaldehyde is exceptional for a functionalized metal oxide material (MATS) tested without any material heating.

[0138] While the tests conducted demonstrate a calculated LOD for the MATS sensor of 90 ppb formaldehyde, these tests were done without microheaters. Directly heating the sensor will further activate the materials to enhance gas sensitivity, boil off fluctuations from surface bound water vapor associated with humidity, and clear the sensor throughout the duration of exposure. With use of microheaters in thermal contact with the MATS sensor, a future LOD < 30 ppb formaldehyde is expected.

[0139] Tests showed that the MATS sensors are a marked improvement over other commercial formaldehyde gas sensors that use either thin film metal oxides orelectrochemical sensing mechanisms. Specifically, the MATS sensors presented no cross- sensitivity to high ppm levels of troublesome VOCs (ethanol, isopropyl alcohol, acetone). Even without microheating to further activate the materials or clear the sensor, the MATS sensors could still detect trace amounts of formaldehyde (1.86 ppm) within an environment that included simultaneous exposure to other VOCs at much higher concentration including ethanol (22 ppm), isopropyl alcohol (50 ppm), and acetone (250 ppm).

[0140] The MATS sensors were also shown to remain insensitive to even higher single gas exposures of ethanol (110 ppm), isopropyl alcohol (80 ppm), or acetone (600 ppm), which indicates exceptionally high selectivity for only formaldehyde.

[0141] Although exemplary embodiments and applications of the invention have been described herein including as described above and shown in the included example figures, there is no intention that the invention be limited to these exemplary embodiments and applications or to the way the exemplary embodiments and applications operate or are described herein. Indeed, many variations and modifications to the exemplary embodiments are possible as would be apparent to a person of ordinary skill in the art. The invention may include any device, structure, method, or functionality, as long as the resulting device, system or method falls within the scope of one of the claims that are allowed by the patent office based on this or any related patent application.

Claims

0 CLAIMS 1. A sensing element (80) for detecting a target gas or gases, the sensing element comprises: a pair of interdigitated electrodes (40), with a gap (42) between each electrode in the pair; a high surface area metal oxide nanomaterial decorated with a noble metal (47) deposited on interdigitated electrode pair (40), the deposition bridges the gap (42), wherein: the surface area of the metal oxide nanomaterial (47) is greater than 100m2 / g; the size of the metal oxide nanomaterial (47) is within the range of 100-200nm; and the resistance across the pair of electrodes (40) varies when the metal oxide nanomaterial (47) binds with the target gas or gasses.

2. The sensing element of claim 1, further comprising a microheater (35) in thermal contact with the pair of electrodes (40).

3. The sensing element of claim 2, further comprising a temperature sensor (30) configured to detect the temperature of the pair of electrodes (40).

4. The sensing element of claim 3, further comprising an insulator (75) separating the micro- heater (35) from the pair of electrodes (40).

5. The sensing element of claim 1, further comprising an array of pairs of interdigitated electrodes (40) with a gap (42) between each in the array, wherein the pair of interdigitated electrodes (40) is part of the array of interdigitated electrodes.

6. The sensing element of claim 5, further comprising a microheater (35) in thermal contact with the array of electrodes (40).

7. The sensing element of claim 6, further comprising a temperature sensor (30) configured to detect the temperature of the array of electrodes (40).

8. The sensing element of claim 7, further comprising an insulator (75) separating the micro- heater (35) from the array of electrodes (40).

9. The sensing element of claim 1, wherein the metal oxide material is selected from a group consisting of: zinc oxide (ZnO), tin oxide (SnO2), tungsten oxide (WO3), copper oxide (CuO), nickel oxide (NiO), iron oxide (Fe2O3), titanium dioxide (TiO2), indium oxide (In2O3), chromium oxide (Cr2O3), manganese oxide (Mn2O3), cobalt oxide (Co3O4), strontium oxide (SrO), vanadium oxide (V2O3), germanium dioxide (GeO2), niobium pentoxide (Nb2O5), molybdenum trioxide (MoO3), tantalum oxide (Ta2O5), lanthanum oxide (La2O3), cerium oxide (CeO2), and neodymium oxide (Nd2O3).

10. The sensing element of claim 1, wherein the noble metal is selected from a group consisting of: gold (Au), platinum (Pt), silver (Ag), rhodium (Rh), palladium (Pd), iridium (Ir), osmium (Os), rhenium (Re), and ruthenium (Ru).

11. The sensing element of claim 1, wherein the target gas is methane, ammonia or formaldehyde.

12. The sensing element of claim 1 wherein the target gas is formaldehyde and the limit of detection of formaldehyde is less than 90ppb.

13. The sensing element of claim 12, further comprising a microheater (35) in thermal contact with the pair of electrodes (40), wherein the limit of detection of formaldehyde is less than 30ppb.

14. A sensor chip (85), comprising an array of sensing elements (80) of claim 1.

15. The sensor chip of claim 14, wherein the array of sensing elements (80) comprise a MEMS patterned die (25).

16. The sensor chip of claim 14, wherein each sensing element (80) in the array comprises a microheater (35) in thermal contact with the electrodes (40).

17. The sensor chip of claim 16, further comprising a temperature sensor (30) configured to detect the temperature of the array of sensing elements (80).

18. The sensor chip of claim 14, further comprising at least two high surface area metal oxide nanomaterials decorated with a noble metal, each of which is formulated to target a different target gas.

19. The sensor chip of claim 14, further comprising a package lid (95) with an exposure window (100), wherein the window (100) is configured to expose the metal oxide nanomaterial (47) to the target gas or gases from outside of the package lid (95).

20. The sensor chip of claim 19, wherein: the exposure window (100) comprises a membrane; and the target gas can permeate the membrane.

21. The sensor chip of claim 14, wherein the metal oxide material is selected from a group consisting of: zinc oxide (ZnO), tin oxide (SnO2), tungsten oxide (WO3), copper oxide (CuO), nickel oxide (NiO), iron oxide (Fe2O3), titanium dioxide (TiO2), indium oxide (In2O3), chromium oxide (Cr2O3), manganese oxide (Mn2O3), cobalt oxide (Co3O4), strontium oxide (SrO), vanadium oxide (V2O3), germanium dioxide (GeO2), niobium pentoxide (Nb2O5), molybdenum trioxide (MoO3), tantalum oxide (Ta2O5), lanthanum oxide (La2O3), cerium oxide (CeO2), and neodymium oxide (Nd2O3).

22. The sensor chip of claim 14, wherein the noble metal is selected from a group consisting of: gold (Au), platinum (Pt), silver (Ag), rhodium (Rh), palladium (Pd), iridium (Ir), osmium (Os), rhenium (Re), and ruthenium (Ru).

23. The sensor chip of claim 14, wherein the target gas is methane, ammonia or formaldehyde.

24. The sensor chip of claim 14, wherein the target gas is formaldehyde and the limit of detection of formaldehyde is less than 90ppb.

25. A sensor module (130) comprising: one or more sensor chips (85) of claim 14; a processor (230) connected to the one or more sensor chips (85) to measure the resistance across the electrodes (40); a substrate (145) supporting the one or more sensor chips (85) and processor (230); andpackaging (140) enclosing the one or more sensor chips (85), the processor (230) and substrate (145), wherein the packaging (140) comprises a target gas exposure window (100) configured to expose the metal oxide nanomaterial (47) to the target gas or gases from outside of the packaging (140).

26. The sensor module of claim 25, wherein the processor (230) individually biases each electrode.

27. The sensor module of claim 25, wherein the processor (230) comprises a power regulation module (125), a data acquisition module (110), a data processing module (115) and a communication module (120).

28. The sensor module of claim 25, comprising: a microheater (35) in thermal contact with the one or more sensor chips (85); a temperature sensor (30) configured to detect the temperature of the electrodes (40); and the processor (230) is connected to the microheater (35) and the temperature sensor (30), wherein the processor (230) is configured to heat the electrodes (40) to a predetermined temperature based on temperature detected by the temperature sensor (30).

29. The sensor module of claim 28, wherein the predetermined temperature is between 150°C and 200 °C.

30. The sensor module of claim 25, further comprising at least two high surface area metal oxide nanomaterials decorated with a noble metal, each of which is formulated to target a different target gas.

31. The sensor module of claim 25, wherein the metal oxide material is selected from a group consisting of: zinc oxide (ZnO), tin oxide (SnO2), tungsten oxide (WO3), copper oxide (CuO), nickel oxide (NiO), iron oxide (Fe2O3), titanium dioxide (TiO2), indium oxide (In2O3), chromium oxide (Cr2O3), manganese oxide (Mn2O3), cobalt oxide (Co3O4), strontium oxide (SrO), vanadium oxide (V2O3), germanium dioxide (GeO2), niobium pentoxide (Nb2O5), molybdenum trioxide (MoO3), tantalum oxide (Ta2O5), lanthanum oxide (La2O3), cerium oxide (CeO2), and neodymium oxide (Nd2O3).

32. The sensor module of claim 25, wherein the noble metal is selected from a group consisting of: gold (Au), platinum (Pt), silver (Ag), rhodium (Rh), palladium (Pd), iridium (Ir), osmium (Os), rhenium (Re), and ruthenium (Ru).

33. The sensor module of claim 25, wherein the target gas is methane, ammonia or formaldehyde.

34. The sensor module of claim 25, wherein the target gas is formaldehyde and the limit of detection of formaldehyde is less than 90ppb.

Images