WO2024164049A1

WO2024164049A1 - Co2 gas sensor

Info

Publication number: WO2024164049A1
Application number: PCT/AU2024/050080
Authority: WO
Inventors: Zahra SHAHRBABAKI; Sina Naficy; Fariba Dehghani; Syamak FARAJIKHAH; Farshad Oveissi
Original assignee: University of Sydney
Current assignee: University of Sydney
Priority date: 2023-02-08
Filing date: 2024-02-07
Publication date: 2024-08-15
Anticipated expiration: 2025-08-08

Abstract

A CO₂ sensing copolymer and its preparation are described. Devices containing said CO₂ sensing copolymer and their fabrication are also described. The CO₂ sensing copolymer is based on the incorporation of at least two monomers which have differing pK_aH values. The devices are flexible, measure a wide range of CO₂ concentrations under various levels of humidity and temperature, and show excellent sensitivity towards CO₂ in the presence of interferents. Further devices are described that can simultaneously detect and measure CO₂ and ammonia concentrations.

Description

C0₂ gas sensor

Field of the disclosure

[0001] The present disclosure relates to copolymers having amine functionality and a device for sensing CO2 using said copolymers. The present disclosure also relates to methods of producing the copolymers and methods of fabricating a CO2 gas sensor device. The device is flexible and exhibits excellent selectivity towards CO2.

Background of the disclosure

[0002] CO2 detection plays a crucial role in different fields such as air-quality monitoring, fire detection, green-house gas monitoring, health, marine and environmental studies, and food supply chains.

[0003] Conventionally, different methods including fluorescence, gas chromatography, infrared spectrometry, photoacoustic spectroscopy, and Severinghaus type electrodes have been used for CO2 measurement.

[0004] Although traditional CO2 detection methods are highly selective and sensitive, they usually suffer from relatively high cost, bulkiness, high power consumption, and vulnerability to electromagnetic interference. In addition, these techniques require materials (e.g., metal oxide, glass, silicone) which are not compatible with growing interests in new applications such as packaged food. Metal oxide gas sensors are the predominant type of gas sensors that are currently on the market and most of them operate at high temperatures (300-700 °C), which is undesirable due to the demand for high voltage (10-100 V) and subsequently short lifetime.

[0005] Therefore, low-power, flexible, and lightweight CO2 sensors have been developed based on carbon nanotubes (CNTs), aluminium oxide, graphene, and polyethyleneimine (PEI). These materials have been employed in the fabrication of CO2 sensors operating at low temperatures (25-100 °C) and consuming milliwatts of power with a detection range of 10-4000 ppm. However, it would be desirable to increase the detection range to higher concentrations, matching the CO2 concentrations typically found in packaged food.

[0006] For these reasons, the interest in flexible, compact, low-cost, low-power, and lightweight CO2 sensors has recently increased. [0007] Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.

Summary of the disclosure

[0008] In one aspect the present disclosure provides a copolymer comprising a backbone functionalised with a plurality of amine functional groups and wherein the plurality of amine functional groups possess at least two different pK_an values.

[0009] In embodiments, at least one pKaH value is about 7 or higher and at least one other pKaH value is about 5 or lower.

[0010] In embodiments, at least one pKaH value is from about 7 to about 8.

[0011] In embodiments, at least one pKaH value is from about 4 to about 5.

[0012] In embodiments, the copolymer comprises the structure of Formula (I):

Formula (I) wherein:

Ri is selected from H, C1-6 alkyl, C3-6 cycloalkyl, C1-6 haloalkyl, C1-6 alkoxy, Ce- aryl, C3-6 heterocycle and Ce- heteroaryl;

A1 is an amine group of formula -N(R2)2,

R2 is independently selected from H, C1-6 alkyl, C3-6 cycloalkyl, C1-6 haloalkyl, C1-6 alkoxy, Ce- aryl, C3-6 heterocycle and Ce- heteroaryl; or, the R₂ groups in -N(R₂)₂ are connected to form a C3-6 heterocycle ring;

Xi is selected from an ester or an amide;

Li is selected from C1-6 alkyl; n denotes the number of repeat units in the copolymer; and wherein the copolymer contains at least two different A1 groups.

[0013] In embodiments, the copolymer comprises the structure of Formula (II):

Formula (II) wherein:

R1 and R₂ are independently selected from H, C1-6 alkyl, C3-6 cycloalkyl, C1-6 haloalkyl, C1-6 alkoxy, Ce- aryl, C3-6 heterocycle and Ce- heteroaryl;

A1 is an amine group of formula -N(Rs)₂ and A₂ is an amine group of formula -N(R4)₂; wherein,

R3 and R4 are independently selected from H, C1-6 alkyl, C3-6 cycloalkyl, C1-6 haloalkyl, C1-6 alkoxy, Ce- aryl, C3-6 heterocycle and Ce- heteroaryl; or, the R3 or R4 groups in one or both of -N(Rs)₂ or -N(R4)₂ are connected to form a C3- 6 heterocycle ring;

Xi and X₂ are independently an ester or an amide;

Li and L₂ are independently C1-6 alkyl; and n and m denote the number of repeat units in the copolymer. [0014] In embodiments, n and m are independently from about 10 to about 2000, or from about 10 to about 1000.

[0015] In embodiments, Ri and R2 are independently selected from H and C1-6 alkyl.

[0016] In embodiments, one or both of R1 and R2 are methyl.

[0017] In embodiments, R3 and R4 are independently selected from H, C1-6 alkyl, C3-6 cycloalkyl, Ce- aryl, and C3-6 heterocycle; or, both R3 or R4 groups in -N(Rs)2 or -N(R4)2 connect to form a C3-6 heterocycle ring.

[0018] In embodiments, R3 and R4 are independently selected from H and C1-6 alkyl; or, both R3 or R4 groups in -N(Rs)2 or -N(R4)2 connect to form a C3-6 heterocycle ring.

[0019] In embodiments, the plurality of amine functional groups are tertiary amine functional groups.

[0020] In embodiments, A1 is -N(CHs)2.

[0021] In embodiments, A2 is:

[0022] In embodiments, Xi is an amide and X2 is an ester.

[0023] In embodiments, Li is -CH2CH2CH2- and L2 is -CH2CH2-.

[0024] In embodiments, the copolymer comprises the structure of Formula (III):

Formula (III) wherein, x and y denote the number of the repeat units in the polymer.

[0025] In embodiments, x and y are independently from about 10 to about 2000, or from about 10 to about 1000.

[0026] In embodiments, the number of repeat units x is from about 70 % to about 95 % of the total number of repeat units x+y.

[0027] In embodiments, the number of repeat units y is from about 5 % to about 30 % of the total number of repeat units x+y.

[0028] In embodiments, the number of repeat units x is from about 200 to about 1000.

[0029] In embodiments, the number of repeat units y is from about 10 to about 200.

[0030] In embodiments, the number average molar mass of the copolymer is about 50 to about 200 kDa.

[0031] In another aspect the present disclosure provides a method of producing a copolymer according to any one of the herein disclosed embodiments comprising the step of combining at least two monomers, initiator and solvent, said at least two monomers comprising amine functional groups having different pK_an values.

[0032] In another aspect the present disclosure provides a method of producing the copolymer of Formula (III), comprising the steps of: combining N-[3-(dimethylamino)propyl] methacrylamide (DMAPMAm), 2-N- morpholinoethyl methacrylate (MEMA), an initiator and a solvent; heating the mixture under an inert atmosphere.

[0033] In embodiments, the initiator is a radical initiator.

[0034] In embodiments, the radical initiator comprises one or more of azobisisobutyronitrile (Al BN), ammonium persulfate (APS), potassium persulfate (KPS), and 4,4'-azobis(4-cyanovaleric acid) (ACVA).

[0035] In embodiments, the solvent comprises one or more of ethanol, water, benzene, chloroform, dimethylformamide (DMF), tetrahydrofuran (THF), chlorobenzene, methyl-tert-butyl ether, and tert-butanol.

[0036] In embodiments the molar ratio of DMAPMAm to MEMA is from about 3:1 to about 1:3.

[0037] In embodiments, the molar ratio of DMAPMAm to MEMA is about 1:1.

[0038] In embodiments, heating the mixture is performed at above 20 °C, preferably between about 60°C and about 80°C.

[0039] In embodiments, heating the mixture is performed for at least 4 hours.

[0040] In another aspect the present disclosure provides a CO2 sensing device comprising one or more of the copolymers according to any one of the herein disclosed embodiments.

[0041] In another aspect the present disclosure provides a CO2 sensing device comprising: a substrate; at least two electrodes disposed on a surface of the substrate; and the copolymer according to any one of the herein disclosed embodiments in contact with the two electrodes.

[0042] In embodiments, the device is flexible. [0043] In embodiments, the copolymer is coated, at least in part, with a cationconducting solid electrolyte.

[0044] In embodiments, the cation-conducting solid electrolyte includes an alkali metal cation.

[0045] In embodiments, the cation-conducting solid electrolyte is a sodium modified sulphonated fluoropolymer.

[0046] In embodiments, the device further comprises a second sensor, said second sensor being sensitive to ammonia.

[0047] In embodiments, the second sensor comprises a cation-conducting solid electrolyte.

[0048] In another aspect the present disclosure provides a method of fabricating a CO2 sensing device comprising the steps of: applying at least two electrodes on a surface of a substrate; and applying the copolymer according to any one of the herein disclosed embodiments to contact the electrodes.

[0049] In embodiments, the method further comprises the step of coating the copolymer, at least in part, with a cation-conducting solid electrolyte.

[0050] In embodiments, the cation-conducting solid electrolyte includes an alkali metal salt.

[0051] In embodiments, the cation-conducting solid electrolyte is a sodium modified sulphonated fluoropolymer.

[0052] In embodiments, the method further comprises the step of drying the device.

[0053] In embodiments, the method further comprises the step of including a second sensor, said second sensor being sensitive to ammonia.

[0054] In embodiments, the second sensor comprises a cation-conducting solid electrolyte. [0055] In embodiments, the substrate comprises one or more of paper, polyethylene napthalate, polyethylene terephthalate, polyimide, polydimethylsiloxane, polycarbonate, silicone elastomer, and fabric, for example cotton fabric.

[0056] In embodiments, the substrate comprises polyethylene terephthalate.

[0057] In embodiments, the electrodes are selected from one or more of carbon black, siliver, gold, carbon nanotubes, graphene, and magnesium.

[0058] In embodiments, the copolymer is present in a concentration between about 1 wt.% to about 70 wt.%, based on the total weight of the device.

[0059] In another aspect the present disclosure provides a use of the CO2 sensing device according to any one of the herein disclosed embodiments in sensing, detecting or measuring CO2 in an environment comprising a food product.

[0060] In embodiments, the CO2 detection level of the CO2 sensing device is greater than about 10,000 ppm.

[0061] In embodiments, the response time of the CO2 sensing device is less than about 360 s.

[0062] In embodiments, the recovery time of the CO2 sensing device is less than about 840 s.

[0063] In embodiments, the sensing of CO2 is reversible.

[0064] In another aspect the present disclosure provides a method of sensing, or detecting, or measuring CO2 comprising exposing the CO2 sensing device according to any one of the herein disclosed embodiments, to an environment comprising CO2.

[0065] In embodiments, the environment comprising CO2 has a relative humidity at or above 70%, or 80%, or 90%.

[0066] In embodiments, the environment comprising CO2 further comprises ammonia.

[0067] In another aspect the present disclosure provides a method of sensing, or detecting, or measuring CO2 and ammonia comprising exposing the CO2 sensing device according to any one of the herein disclosed embodiments to an environment comprising CC^ and ammonia.

[0068] Advantages of the presently disclosed CO2 sensors include one or more of the following:

• they can measure high CO2 concentrations of relevance to packaged food;

• they can operate under conditions of high humidity;

• they are flexible;

• they can operate at ambient temperature;

• they have excellent cross-sensitivity to ammonia and sensitivity to CO2;

[0069] Any embodiment herein shall be taken to apply mutatis mutandis to any other embodiment unless specifically stated otherwise.

[0070] The present disclosure is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. Functionally-equivalent products, compositions and processes are clearly within the scope of the disclosure as described herein.

[0071] Further aspects of the present disclosure and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief description of the drawings

[0072] Figure 1. a) Chemical structure of an example p(D-co-M) CO2 sensing copolymer. The letters a-l label the chemical environments for the ¹H-NMR spectra in Figure 1b. b) ¹H-NMR spectra of pMEMA (Top), p(D-co-M) (Middle) and pDMAPMAm (Bottom) acquired with CDCh as the solvent at 25 °C. c) FITR spectra of pMEMA (Top), p(D-co-M) (Middle) and pDMAPMAm (Bottom) d) Experimental titration curves obtained from 1 wt.% solution in water using 1M HCI. [0073] Figure 2. Schematic of a fabricated CO2 sensor along with the predominant protonation state of an example p(D-co-M) CO2 sensing copolymer at various pH values.

[0074] Figure 3. Relative change in resistance (shown by data points next to an arrow pointing left) and half-response time (time to reach 50% of this change, shown by data points next to an arrow pointing right) of the CO2 sensor to CO2 as a function of the concentration of the polymer solution.

[0075] Figure 4. Surface modification for sensing characteristics improvement, a) Layer-by-layer assemblies of the CO2 sensing copolymer with Nafion-Na for CO2 detection and b) Nafion-only sensor for ammonia detection, c) CC>2-selectivity of the fabricated sensors at a constant background of ammonia. Bottom trace, middle trace, and top trace represent the copolymer, Nation, and copolymer-Nafion-Na sensors, respectively, d) Reversible response of copolymer-Nafion-Na under alternate exposure to humidified CO2 and humidified N2, dotted line represents CO2 concentration (%) and the solid line represents R/Ro.

[0076] Figure 5. Relationship between relative resistance of CO2 sensor and humidity.

[0077] Figure 6. Electrical response of CO2 sensor to CO2. a) relative resistance and b) relative impedance of the sensor as a function of CO2 concentration.

[0078] Figure 7. KPFM (a&b), AFM topography images (c&d), and roughness profiles (e&f) (along the black lines) of the CO2 sensing copolymer film. Figures a, c, e show before and b, d, f show after exposure to humidity and CO2.

[0079] Figure 8. Pictorial representation of the gas sensing measurement setup. CO2 and N2 gas flow rates into the sensing chamber were controlled by installing two mass flow controllers. The sensing chamber includes the CO2 sensor, water, and a reference CO2 sensor with an upper limit of 100% to record the CO2 concentration. The measurement unit includes a precise source meter and a potentiostat for continuously recording the DC resistance and AC impedance of the sensor.

[0080] Figure 9. Plot showing a) the reversible response of CO2 sensor under alternate exposure to humidified CO2 and humidified N2. Dotted line represents CO2 concentration and the solid line represents R/Ro. b) Six cycles of bubbling and removal of CO₂.

[0081] Figure 10. Plot showing the effect of interfering gases on the response of CO2 sensor to CO2. a) Sensor’s response to different interfering gases at a CO2 concentration of 100 %. CC>2-selectivity of CO2 sensor at a constant background of b) ethanol, c) methanol, d) acetone, e) toluene, and f) ammonia.

[0082] Figure 11. Kimchi test as a food model, a) digital photograph of sensing chamber including kimchi, a reference CO2 sensor, a relative humidity-temperature sensor, and a CO2 sensor of the present disclosure, b) response of CO2 sensor to CO2 released by kimchi, trace going from top to bottom represents R/Ro measurement and trace going from bottom to top represents CO2 concentration (%) c) humidity (top line of data points) and temperature (bottom line of data points) profiles inside the sensing chamber during the test.

[0083] Figure 12. Graph showing the contact potential difference (CPD) between the tip and the polymer surface before and after exposure to CO2, respectively.

Detailed description of the embodiments

[0084] It will be understood that the disclosure described and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the disclosure.

Definitions

[0085] For purposes of interpreting this specification, terms used in the singular will also include the plural and vice versa.

[0086] As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

[0087] "About" as used herein when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass variations of ±20% or ±10%, in some instances ±5%, in some instances ±1%, and in some instances ±0.1% from the specified value, as such variations are appropriate to perform the disclosed methods.

[0088] Ranges: throughout this disclosure, various aspects of the disclosure can be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosure. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1 , 2, 2.7, 3, 4, 5, 5.3, and 6. This applies regardless of the breadth of the range.

[0089] Unless otherwise herein defined, the following terms will be understood to have the general meanings which follow.

[0090] The term “C1-6 alkyl” refers to optionally substituted straight chain or branched chain hydrocarbon groups having from 1 to 6 carbon atoms. Examples include methyl (Me), ethyl (Et), propyl (Pr), isopropyl (i-Pr), butyl (Bu), isobutyl (i-Bu), sec-butyl (s-Bu), tert-butyl (t-Bu), pentyl, neopentyl, hexyl and the like.

[0091] The term “C3-6 cycloalkyl” refers to non-aromatic cyclic groups having from 3 to 6 carbon atoms, including cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl. It will be understood that cycloalkyl groups may be saturated such as cyclohexyl or unsaturated such as cyclohexenyl.

[0092] The term “C1-6 haloalkyl” refers to a Ci ealkyl which is substituted with one or more halogens, such as for example, -CH2CF3, and -CF3.

[0093] The term “C1-6 alkoxy” refers to an alkyl group as defined above covalently bound via an O linkage containing 1 to 6 carbon atoms, such as methoxy, ethoxy, propoxy, isoproxy, butoxy, tert-butoxy and pentoxy.

[0094] The term “Ce- aryl” refers to a carbocyclic (non-heterocyclic) aromatic ring or mono-, bi- or tri-cyclic ring system containing 6 to 10 carbon atoms. Poly-cyclic ring systems may be referred to as “aryl” provided at least 1 of the rings within the system is aromatic. Examples of aryl groups include but are not limited to phenyl, biphenyl, naphthyl and tetrahydronaphthyl. 6-membered aryls such as phenyl are preferred. The term “alkylaryl” refers to Ci -ealkylaryl such as benzyl.

[0095] The term “C3-6 heterocycle” refers to a moiety obtained by removing a hydrogen atom from a ring atom of a heterocyclic compound which moiety has from 3 to 6 ring atoms (unless otherwise specified), of which 1 , 2, 3 or 4 are ring heteroatoms with each heteroatom being independently selected from O, S and N. Heterocyclic compounds include monocyclic and polycyclic (such as bicyclic) ring systems, such as fused, bridged and spirocyclic systems, provided at least one of the rings of the ring system contains at least one heteroatom.

[0096] “Heteroaryl” is used herein to denote a heterocyclic group having aromatic character and embraces aromatic monocyclic ring systems and polycyclic (e.g. bicyclic) ring systems containing one or more aromatic rings. In polycyclic systems containing both aromatic and non-aromatic rings fused together, the group may be attached to another moiety by the aromatic ring or by a non-aromatic ring.

[0097] The present disclosure relates to CO2 sensing devices that comprise particular amine functionalised copolymers. The copolymers comprise at plurality of amine functional groups and wherein the plurality of amine functional groups possess at least two different pK_an values.

[0098] In embodiments, at least one pKaH value is about 7 or higher and at least one other pKaH value is about 5 or lower.

[0099] The at least one pKaH value about 7 or higher may be about 7.5 or higher, or about 8 or higher, or about 8.5 or higher, or about 9 or higher.

[0100] The at least one pKaH value about 5 or lower may be about 4.5 or lower, or about 4 or lower.

[0101] In embodiments, at least one pKaH value is from about 4 to about 5.

[0102] In embodiments, the amine functional groups are preferably tertiary amine functional groups. [0103] The skilled person, using available data on the pK_an of various amines, would be able to design and prepare a variety of copolymers containing amine functions having different pK_an values. For example, see Tshepelevish et al. Eur. J. Org. Chem., 2019: 6735-6748.

Method of producing the copolymer

[0104] The copolymers of the present disclosure may be produced by combining two or more polymerisable monomers each having amine functional groups of differing pKaH. For example, a first monomer may comprise amine functionality having a pKaH about 7 or higher and a second monomer may comprise amine functionality having a pKaH about 5 or lower.

[0105] The relative amount of first monomer to second monomer built into the copolymer will depend on the ratio of the monomers and their relative reactivity. The skilled person would be able to determine the relative amounts of comonomers necessary to produce a copolymer of particular composition.

[0106] The monomers may be polymerised by methods known in the art, for example via free radical polymerisation or addition polymerisation.

[0107] One or more initiators may be utilised and suitable initiators would be well known to the person skilled in the art. For example suitable radical initiators include azobisisobutyronitrile (AIBN), ammonium persulfate (APS), potassium persulfate (KPS), and 4,4'-azobis(4-cyanovaleric acid) (ACVA).

[0108] The polymerisation is typically performed in a suitable solvent, for example one or more of ethanol, water, benzene, chloroform, dimethylformamide (DMF), tetrahydrofuran (THF), chlorobenzene, methyl-tert-butyl ether, and tert-butanol.

[0109] The polymerisation is typically performed at above 20 °C, for example from about 60 °C to about 80 °C.

[0110] The time for polymerisation may be performed for at least 4 hours.

CO2 sensing devices

[0111] The CO2 sensing devices of the present disclosure may comprise: a) a substrate; b) at least two electrodes disposed on a surface of the substrate; and c) the copolymer according to the present disclosure in contact with the two electrodes.

[0112] The CO2 sensing devices of the present disclosure may be flexible.

[0113] The copolymer in a CO2 sensing device according to the present disclosure may be coated, at least in part, with a cation-conducting solid electrolyte.

[0114] The cation-conducting solid electrolyte may include an alkali metal salt. For example the salt may be a sodium salt. Other salts known in the art are contemplated.

[0115] The cation-conducting solid electrolyte may be a sodium modified sulphonated fluoropolymer. For example the sulphonated fluoropolymer may be Nation. Other sulphonated fluoropolymers known in the art are contemplated.

[0116] The person skilled in the art would be able to identify other suitable cationconducting solid electrolytes and sulphonated fluoropolymers.

[0117] The CO2 sensing devices of the present disclosure may comprise a second sensor, said second sensor being sensitive to ammonia.

[0118] The second sensor may comprise a cation-conducting solid electrolyte. Other materials for the second sensor are contemplated and the person skilled in the art would be able to identify suitable sensors for ammonia.

[0119] A suitable concentration of the copolymers described by the present disclosure in the CO2 sensing device may be between about 1 wt.% to about 70 wt.%, or between about 1 wt.% to about 60 wt.%, or between about 1 wt.% to about 50 wt.%, or between about 1 wt.% to about 40 wt.%, or between about 1 wt.% to about 30 wt.%, or between about 1 wt.% to about 20 wt.%, or between about 1 wt.% to about 10 wt.%,, based on the total weight of the device.

[0120] Suitable uses of the CO2 sensing devices of the present disclosure may include sensing, detecting or measuring CO2 in an environment comprising a food product. Other uses are contemplated. [0121] The CO2 sensing devices of the present disclosure may be able to detect CO2 at CO2 detection levels greater than 10,000 ppm.

[0122] The CO2 sensing devices of the present disclosure may have a response time of less than about 360 s.

[0123] The CO2 sensing devices of the present disclosure may have a recovery time of less than about 840 s.

[0124] The sensing of CO2 using the CO2 sensing devices of the present disclosure may be reversible.

Fabrication of CO2 sensing devices

[0125] A method of fabricating the CO2 sensing devices of the present disclosure may comprise the steps of: a) applying at least two electrodes on a surface of a substrate; and b) applying the copolymer according to the present disclosure to contact the two electrodes.

[0126] The method may further comprise the step of coating the copolymer, at least in part, with a cation-conducting solid electrolyte.

[0127] The cation-conducting solid electrolyte may include an alkali metal salt. For example the salt may be a sodium salt. Other salts known in the art are contemplated.

[0128] The cation-conducting solid electrolyte may be a sodium modified sulphonated fluoropolymer. For example the sulphonated fluoropolymer may be Nation. Other sulphonated fluoropolymers known in the art are contemplated.

[0129] The person skilled in the art would be able to determine other suitable cationconducting solid electrolytes and sulphonated fluoropolymers.

[0130] The method may further comprise the step of drying the device.

[0131] The method may further comprises the step of including a second sensor, said second sensor being sensitive to ammonia. Other materials for the second sensor are contemplated and the person skilled in the art would be able to determine suitable materials for sensing ammonia.

Substrates

[0132] The substrate may comprise one or more of paper, polyethylene napthalate, polyethylene terephthalate, polyimide, polydimethylsiloxane, polycarbonate, silicone elastomer, and fabric, for example cotton fabric. Other suitable substrates known in the art are contemplated.

[0133] Preferably, the substrate is polyethylene terephthalate.

Electrodes

[0134] The electrodes may be selected from one or more of carbon black, silver, gold, carbon nanotubes, graphene, and magnesium. Other suitable electrodes known in the art are contemplated.

[0135] Preferable, the electrode is carbon black.

Examples

Materials

[0136] /\/-[3-(dimethylamino)propyl] methacrylamide (DMAPMAm) (99%, Mw = 170.25 g mol^-1), 2-/V-morpholinoethyl methacrylate (MEMA) (95%, Mw = 199.25 g mol^-1), azobisisobutyronitrile (AIBN) (0.2 M in toluene, Mw = 164.21 g mol^-1), ethanol (> 99.5%, Mw = 46.07 g mol^-1), methanol (> 99.9%, Mw = 32.04 g mol^-1), acetone (> 99.9%, Mw = 58.08 g mol^-1), toluene (> 99.8%, Mw = 92.14 g mol^-1), ammonium hydroxide solution (30-33% NH₃ in H₂O, Mw = 35.05 g mol ¹), DMAc (> 99.9%, Mw = 87.12 g mol ¹), and CDCI₃ ( > 99.8%, Mw = 120.38 g mol ¹), and HCI (32 wt.% in H₂O, Mw = 36.46 g mol ¹) were purchased from Sigma-Aldrich (Australia) and used as received. The carbon black ink and the universal, transparent polyethylene terephthalate (PET) sheets were purchased from DYCOTEC (United Kingdom) and Amazon (Australia), respectively.

Analytical Methods

[0137] ¹H-NMR spectroscopy was performed on a Bruker 800 MHz spectrometer using CDCI₃ as the solvent and 5 mm NMR tubes at 25 °C. [0138] SEC measurement was performed on a LIFLC Shimadzu Prominence SEC system with two PhenogelTM columns (5 pm, 104 A, and 105 A) running in DMAc using BHT/LiBr at 0.05 wt.% as eluent at a flow rate of 1 ml min^-1 at 50 °C.

[0139] FTIR was carried out on a Thermo Scientific Nicolet 6700 spectrometer fitted with an ATR accessory (diamond crystal) at an angle of incidence of 90°. The data were collected at a resolution of 4 cm^-1 over the range of 450-4000 cm^-1 from the average of 32 scans.

[0140] To investigate the CO2 sensing of the p(D-co-M) copolymers, the fabricated sensors were exposed to different CO2 concentrations (1-100%) under high humidity levels (RH > 90%) at room temperature. The DC resistance was continuously measured with a precise source meter (KEITHLY 2450, Textronix, USA). The response of the sensor was expressed as R/Ro (relative resistance), where Ro is the initial resistance and R is the resistance of the sensors during the experiment. The AC impedance was evaluated using a potentiostat (ZIVE SP1 , WonATech, South Korea). The absolute impedance of the sensors (Z) was measured at a constant frequency of 1 kHz, and the response of the fabricated sensor was expressed as Z/Zo (relative impedance) where Zo is the initial and Z is the impedance of the sensors during the experiment.

[0141] AFM and KPFM using a Pt-lr coated tip were carried out on a Bruker Dimension ICON SPM to explore the surface morphology, roughness, and potential of the CO2 sensing film before and after exposure to CO2.

Example 1 : General synthesis of CO2 sensing polymers

[0142] DMAPMAm (10.87 mL, 0.06 mol) and MEMA (11.44 mL, 0.06 mol) with a molar ratio of 1:1 were mixed with AIBN (2.8 mL, 0.00055 mol) and ethanol (10 mL, 0.17 mol) as the initiator and solvent, respectively. The prepared mixtures were purged by N2 (45 mL.min^-1) for 20 minutes to eliminate any trapped oxygen. After degassing, they were incubated at 60 °C overnight to complete the free radical polymerisation under sealed conditions. To evaporate the remaining ethanol, the p(D-co-M) copolymers were transferred to glass Petri dishes and were then kept at 60 °C for 2 hr before being stored for further usage at room temperature. Figure 1a shows the chemical structure of p(D-co-M), with each different chemical environment labelled a-l. Example 2: Characterisation of CO2 sensing polymers

[0143] Comparison of ¹H NMR spectra of p(D-co-M) with the corresponding homopolymers indicated that the polymerisation was successful as both pDMAPMAm and pMEMA signals were observed in the ¹H NMR spectrum of p(D-co-M) (Figure 1 b). Labels a-l denote the assignment of each chemical environment from the structure in Figure 1a. The p(D-co-M) composition ratio was roughly estimated using the ratio between the methyl protons from the DMAPMAm segment at 2.27 ppm (f) and the methylene protons from the MEMA segment at 4.10 ppm (i). The ratio of DMAPMAm to MEMA segments in p(D-co-M) was 5. The number of repeating units of DMAPMAm and MEMA was determined using this ratio in combination with the Mn from size exclusion chromatography (SEC), as shown in Table 1.

Table 1. Molecular characterisations of pDMAPMAm, p(D-co-M), and pMEMA obtained from SEC and ¹H-NMR.

^c)obtained from SEC in DMAc; ^d)DMAPMAm; ^e)MEMA.

[0144] To further confirm the successful preparation of the copolymer, Fourier- transform infrared spectroscopy (FTIR) was performed. Figure 1c shows the FTIR spectra of pDMAPMAm, p(D-co-M), and pMEMA. The characteristic absorption bands at 1113 cm^-1 and 1723 cm^-1 were attributed to C-O-C and C=O, respectively, in the structure of the pMEMA polymer. Similarly, the characteristic peaks of pDMAPMAm polymer such as N-H (amide II), C=O (amide I), and N-H, appeared at 1525 cm^-1, 1629 cm^-1, and 3330 cm^-1, respectively. The observation of the distinctive peaks from DMAPMAm and MEMA in the FTIR spectra of the copolymer confirmed the existence of both monomers within the copolymer structure.

[0145] Since the basicity of the polymer has a significant effect on its response to CO2, it is important to know the pK_an values of a synthesised copolymer. Therefore, titration with 1 M HCI was performed on dilute aqueous solutions of polymer (1 wt.% in deionised water) to measure the pK_an values of the CO2 sensing copolymer of example 1. As shown in Figure 1 d, the pKaH values of CO2 sensing copolymer of example 1 were 7.3 and 4.4 which were attributed to the DMAPMAm and MEMA chains, respectively. Of note, these pKaH values were close to those obtained by titration of each homopolymer separately, 7.9 for pDMAPMAm and 4.1 for pMEMA. These observations confirmed the existence of both amine side groups with different pKaH values.

Example 3: Determination of copolymer concentration for CO2 sensing

[0146] In order to design a chemiresistive CO2 sensor utilising the CC>2-responsive copolymer, it was necessary to determine the optimum polymer concentration accurately, and rapidly measure the concentration of CO2 with high sensitivity within the range of 1 and 100%. Various concentrations of the CO2 sensing copolymer including 1 , 10, 40, and 70 wt.% solutions were deposited between carbon black electrodes which were printed on PET substrates. Figure 2 shows schematically the fabricated CO2 sensor along with the predominant protonation state of an example p(D-co-M) CO2 sensing copolymer at various pH values. The sensors were then exposed to 3% CO2 and their DC electrical resistance was measured as the sensor response. The results in Figure 3 present the sensors' response (data points next to an arrow pointing left) and their half-response time (data points next to an arrow pointing right).

Example 4: Fabrication of CO2 sensors

[0147] A configuration of two carbon black electrodes (1 mm apart) were first printed on the PET substrates using a 3D-Bioplotter (EnvisionTEC, Germany). The carbon black ink was extruded through a 250 pm nozzle, and the printing parameters, including temperature, pressure, and speed of printing, were optimised at 21 °C, 3.2 bar, and 10 mm s’¹. After printing, the carbon black electrodes were dried at 37 °C for 6 hr for further use. Then, 50 pl of a solution of the copolymers of Example 1 (1-70 wt.% in ethanol) was drop-casted on the carbon black electrodes and was dried prior to the sensing measurements.

[0148] Surface coating of the CO2 sensor with sulphonated fluoropolymer (Nation) followed by surface modification with NaOH solution (Nation -Na), schematically shown in Figure 4a, reduced the sensor’s recovery time from 14 minutes to under 3 minutes upon alternate exposure to humidified CC^ and humidified N2 (Figure 4d). Nafion-Na- coated sensor also demonstrated selectivity towards CO2 in the presence of ammonia as an interfering gas (Figure 4c).

[0149] Surface coating with Nation alone (Figure 4b) made the sensor insensitive to CO2 but showed sensitivity to ammonia (Figure 4c). These results provide an opportunity for designing a flexible sensor array comprised of a Nation sensor and a copolymer-Nafion-Na sensor for simultaneous detection of CO2 and ammonia, sensitively and selectively.

Example 5: CO2 sensor evaluation

[0150] It was anticipated that CC>2-responsive polymers that possess tertiary amine groups may poorly interact with CO2 in their dried form, and the presence of water is advantageous for polymer protonation. Since the degree of protonation (DOP) and the resistance depend on the water content and the humidity level, it was important to determine the effect of this parameter on the CO2 response for the CO2 sensing copolymers. Therefore, the DC resistance of the fabricated CO2 sensor was measured as a function of relative humidity (RH) as shown in Figure 5.

[0151] By increasing the humidity, the relative resistance of the CO2 sensor decreased due to the absorption of water, which facilitated proton conduction. The largest reduction in resistance occurred at lower humidity levels, which reached 0.05 at 70%. For the higher humidity levels, the resistance decreased, albeit not as much as in the first section, until it plateaued for values above 76%. Thus, to eliminate the impact of humidity on the response of the sensor, all sensing experiments were conducted at RH > 90%. Considering that the relative humidity of the environment inside food packaging is -100%, the ability of the presently disclosed CO2 sensors to operate at high humidity levels is highly advantageous. [0152] In order to investigate the electrical response of the CO2 sensor to CO2, it was exposed to CO2 at concentrations ranging from 1 to 100%, and its DC resistance and AC impedance were monitored. The results in Figure 6a demonstrate that the relative resistance of the CO2 sensor significantly dropped from 1 to 0.12 by increasing the CO2 concentration inside the sensing chamber from 1 to 20%, which was attributed to the protonation of the amine groups. A reduction in the relative resistance was still observed for concentrations above 20%, before reaching 0.06 at 100% CO2.

[0153] Figure 6b shows the impedance profile of the sensor at 1 kHz. A decrease in the relative impedance of the sensor from 1 to 0.6 upon exposure to higher CO2 concentrations was consistent with the observed decrease in the relative resistance. The results in Figure 6 also underline that the copolymer of Example 1 is a suitable material for designing CO2 sensors, which are compatible with both AC and DC measurements. The systems based on AC can enable the fabrication of wireless sensors while those based on DC lead to simpler electronics.

[0154] To further investigate the surface electrical properties and surface charge of the CO2 sensor in response to CO2, the surface potential was measured by kelvin probe force microscopy (KPFM) (Figure 7a and 7b). The contact potential difference (CPD) between the tip and the polymer surface was -6.3 V and 0.04 V before and after exposure to CO2, respectively (Figure 12). Therefore, the surface potential was increased by + 6.34 V as a result of exposure to CO2. The reason for this observation is the protonation of the tertiary amine sites in the CO2 sensing copolymer, which accumulates positive charges on the polymer’s surface and increases its potential. To determine the surface morphology and roughness of the CO2 sensing copolymer film before and after exposure to CO2, atomic force microscopy (AFM) measurement was used. The results in Figure 7c&d demonstrate that the surface roughness of the CO2 sensing copolymer film decreased from 1327 nm to 684 nm after exposure to humidity and CO2.

[0155] To evaluate the response of the fabricated sensors to CO2, a gas-sensing measurement setup was designed (Figure 8). The prepared sensors were mounted in a sensing chamber with inlet and outlet ports for the target gas (either N2 or CO2 or a mixture of both) to flow inside the chamber. All measurements were carried out in a humid environment (RH > 90%). For this purpose, the measurement chamber containing water was maintained inside an oven at 37 °C overnight before each experiment. In addition, the gas flow was always humidified by passing through the water. CO2 and N2 gas flow rates into the chamber were controlled by installing two mass flow controllers (FMA-2600A Upstream Valve, OMEGA, USA). A reference CO2 sensor (ExplorlR-W-100, CO2 METER, USA) with an upper limit of 100% was used to record the CO2 concentration inside the measurement chamber during the sensing experiments.

[0156] The reversibility of the response of the CO2 sensor to CO2 was monitored for six cycles of alternate exposure to humidified CO2 and humidified N2 at room temperature. As shown in Figure 9a, the relative resistance of the sensor rapidly declined from 1 to 0.06 in 6 minutes upon exposure to CO2, while it levelled off after removing CO2 from the chamber. By switching the gas to N2, the resistance increased to its initial value within 14 minutes. The longer recovery time of 14 minutes compared with the response time of 6 minutes indicates that the CO2 desorption is slower than its adsorption on tertiary amine sites. The successive CO2 and N2 bubbling for six cycles showed the reproducibility of the CO2 sensor (Figure 9b).

[0157] To investigate the effect of interfering gases on the response of the CO2 sensorto CO2, the sensor was exposed to a broad range of functional chemical groups (i.e. , alcohol, ketone, aromatic hydrocarbon, and amine) typically found in food packaging environments. Therefore, the selectivity tests were performed with a variety of confounding species including ethanol, methanol, acetone, toluene, and ammonia vapours. The presence of ethanol, methanol, acetone, and toluene in food packaging environments is attributed to the use of these solvents in multilayer packaging materials or printing and laminating processes, which can change the taste or cause unexpected health problems in humans.

[0158] The selectivity testing was carried out with two different experimental setups. In the first method, 50 pl of each selected volatile organic compound was injected into the system (5 minutes between each injection) at a constant background of 100% CO2 (Figure 10a). In the second method, the sensor was first exposed to the individual interfering gases and then to CO2 with a constant background of that interferer gas (Figure 10b-f). Testing under these two conditions allow for determining any effects on the response of the sensor when both CO2 and interferer are present. The sensor revealed a distinguishable response to CO2 in the presence of all the interferers tested. However, there is an apparent cross-sensitivity for ammonia, as can be seen in Figure 10a and Figure 10f. The reason for this observation is the high solubility of ammonia in water which results in its high reactivity.

Example 6: CO2 sensor for use in in situ food monitoring

[0159] An experiment was set up to assess the feasibility of using the designed sensor for the detection of food spoilage or CO2 release during the fermentation of live bacteria. To this end, a model system was used in which the CO2 released during the fermentation of kimchi was measured. Kimchi contains live bacteria and produces CO2 at room temperature due to continuous lactic acid fermentation. Therefore, it is a good candidate to test the presently disclosed CO2 sensor.

[0160] To test the performance of the sensor in this real-world scenario, the CO2 sensor was mounted in a food container (1 L) with 200 g kimchi. A commercial reference CO2 sensor and a relative humidity-temperature sensor were also added to the setup to monitor the CO2 concentration, humidity, and temperature during the experiment (Figure 11a). As can be seen in Figure 11b, by fermentation of kimchi, the concentration of CO2 inside the container increased over time, and consequently, the relative resistance of the sensor fell gradually. Of note, the humidity and temperature changes inside the sensing chamber were negligible during the test (Figure 11c).

Claims

1. A copolymer comprising a backbone functionalised with a plurality of amine functional groups and wherein the plurality of amine functional groups possess at least two different pKaH values.

2. The copolymer according to claim 1 , wherein at least one pKaH value is about 7 or higher and at least one other pKaH value is about 5 or lower.

3. The copolymer according to claim 1 or claim 2, wherein at least one pKaH value is from about 7 to about 8.

4. The copolymer according to any one of claims 1 to 3, wherein at least one pKaH value is from about 4 to about 5.

5. The copolymer according to any one of claims 1 to 4 comprising the structure of Formula (I):

Formula (I) wherein:

A1 is an amine group of formula -N(R2)2,

R2 is independently selected from H, C1-6 alkyl, C3-6 cycloalkyl, C1-6 haloalkyl, C1-6 alkoxy, Ce- aryl, C3-6 heterocycle and Ce- heteroaryl; or, the R2 groups in -N(R2)2 are connected to form a C3-6 heterocycle ring;

Xi is selected from an ester or an amide; Li is selected from C1-6 alkyl; n denotes the number of repeat units in the copolymer; and the copolymer contains at least two different Ai groups.

6. A copolymer according to any one of claims 1 to 5 comprising the structure of Formula (II):

Formula (II) wherein:

Ri and R₂ are independently selected from H, C1-6 alkyl, C3-6 cycloalkyl, C1-6 haloalkyl, C1-6 alkoxy, Ce- aryl, C3-6 heterocycle and Ce- heteroaryl;

A1 is an amine group of formula -N(Rs)2 and A₂ is an amine group of formula -N(R4)₂; wherein,

R3 and R4 are independently selected from H, C1-6 alkyl, C3-6 cycloalkyl, C1-6 haloalkyl, C1-6 alkoxy, Ce- aryl, C3-6 heterocycle and Ce- heteroaryl; or, the R3 or R4 groups in one or both of -N(Rs)2 or -N(R4)2 are connected to form a C3- 6 heterocycle ring;

Xi and X₂ are independently an ester or an amide;

Li and L₂ are independently C1-6 alkyl; and n and m denote the number of repeat units in the copolymer.

7. The copolymer according to claim 6, wherein n and m are independently from about 10 to about 2000, or from about 10 to about 1000.

8. The copolymer according to claim 6 or claim 7, wherein Ri and R2 are independently selected from H and C1-6 alkyl.

9. The copolymer according to any one of claims 6 to 8, wherein one or both of R1 and R2 are methyl.

10. The copolymer according to any one of claims 6 to 9, wherein R3 and R4 are independently selected from H, C1-6 alkyl, C3-6 cycloalkyl, Ce- aryl, and C3-6 heterocycle; or, both R3 or R4 groups in -N(Rs)2 or -N(R4)2 connect to form a C3-6 heterocycle ring.

11. The copolymer according to any one of claims 6 to 10, wherein R3 and R4 are independently selected from H and C1-6 alkyl; or, both R3 or R4 groups in -N(Rs)2 or -N(R4)2 connect to form a C3-6 heterocycle ring.

12. The copolymer according to any one of claims 1 to 11 , wherein the plurality of amine functional groups are tertiary amine functional groups.

13. The copolymer according to any one of claims 6 to 12, wherein A1 is -N(CHs)2.

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14. The copolymer according to any one of claims 6 to 13, wherein A2 is: O

15. The copolymer according to any one of claims 6 to 14, wherein Xi is an amide and X2 is an ester.

16. The copolymer according to any one of claims 6 to 15, wherein Li is - CH2CH2CH2- and l_₂ is -CH2CH2-.

17. A copolymer according to any one of claims 1 to 16 comprising the structure of Formula (III):

18. The copolymer according to claim 17, wherein x and y are independently from about 10 to about 2000, or from about 10 to about 1000.

19. The copolymer according to claim 17 or claim 18, wherein the number of repeat units x is from about 70 % to about 95 % of the total number of repeat units x+y.

20. The copolymer according to any one of claims 17 to 19, wherein the number of repeat units y is from about 5 % to about 30 % of the total number of repeat units x+y.

21. The copolymer according to any one of claims 17 to 20, wherein the number of repeat units of x is from about 200 to about 1000.

22. The copolymer according to any one of claims 17 to 21 , wherein the number of repeat units y is from about 10 to about 200.

23. The copolymer according to any one of claims 1 to 22, wherein the number average molar mass of the copolymer is about 50 to about 200 kDa.

24. A method of producing the copolymer according to any one of claims 1 to 23 comprising the step of combining at least two monomers, initiator and solvent, said at least two monomers comprising amine functional groups having different pK_an values.

25. A method of producing the copolymer according to any one of claims 17 to 23, comprising the steps of: a) combining /V-[3-(dimethylamino)propyl] methacrylamide (DMAPMAm), 2- /V-morpholinoethyl methacrylate (MEMA), an initiator and a solvent; b) heating the mixture under an inert atmosphere.

26. The method according to claim 24 or claim 25, wherein the initiator is a radical initiator.

27. The method according to claim 26, wherein the radical initiator comprises one or more of azobisisobutyronitrile (AIBN), ammonium persulfate (APS), potassium persulfate (KPS), and 4,4'-azobis(4-cyanovaleric acid) (AC A).

28. The method according to any one of claims 24 to 27, wherein the solvent comprises one or more of ethanol, water, benzene, chloroform, dimethylformamide (DMF), tetrahydrofuran (THF), chlorobenzene, methyl-tert- butyl ether, and tert-butanol.

29. The method according to any one of claims 25 to 28, wherein the molar ratio of DMAPMAm to MEMA is from about 3:1 to about 1 :3.

30. The method according to any one of claims 25 to 29, wherein the molar ratio of DMAPMAm to MEMA is about 1:1.

31. The method according to any one of claims 25 to 30, wherein heating the mixture is performed at above 20 °C, preferably between about 60°C and about 80°C.

32. The method according to any one of claims 25 to 31 , wherein heating the mixture is performed for at least 4 hours.

33. A CO2 sensing device comprising: a) a substrate; b) at least two electrodes disposed on a surface of the substrate; and c) the copolymer according to any one of claims 1 to 23 in contact with the two electrodes.

34. The CO2 sensing device according to claim 33, wherein the device is flexible.

35. The CO2 sensing device according to claim 33 or 34, wherein the copolymer is coated, at least in part, with a cation-conducting solid electrolyte.

36. The CO2 sensing device according to claim 35, wherein the cation-conducting solid electrolyte includes an alkali metal salt.

37. The CO2 sensing device according to claim 35 or 36, wherein the cationcontaining solid electrolyte is a sodium modified sulphonated fluoropolymer.

38. The CO2 sensing device according to any one of claims 33 to 37, wherein the device further comprises a second sensor, said second sensor being sensitive to ammonia.

39. The CO2 sensing device according to claim 38, wherein the second sensor comprises a cation-conducting solid electrolyte.

40. A method of fabricating a CO2 sensing device comprising the steps of: a) applying at least two electrodes to the surface of a substrate; and b) applying the copolymer according to any one of claims 1 to 23 to contact the electrodes.

41. The method according to claim 40, wherein the method further comprises the step of coating the copolymer, at least in part, with salt of a cation-containing solid electrolyte.

42. The method according to claim 41, wherein the cation-containing solid electrolyte includes an alkali metal salt.

43. The method according to claim 41 or 42, wherein the cation-containing solid electrolyte is a sodium modified sulphonated fluoropolymer.

44. The method according to any one of claims 40 to 43, wherein the method further comprises the step of drying the device.

45. The method according to any one of claims 40 to 44, wherein the method further comprises the step of including a second sensor, said second sensor being sensitive to ammonia.

46. The method according to claim 45, wherein the second sensor comprises a cation-conducting solid electrolyte.

47. The CO2 sensing device according to any one of claims 33 to 39 or the method according to any one of claims 40 to 46, wherein the substrate comprises one or more of paper, polyethylene napthalate, polyethylene terephthalate, polyimide, polydimethylsiloxane, polycarbonate, silicone elastomer, and fabric, for example cotton fabric.

48. The CO2 sensing device or the method according to claim 47, wherein the substrate comprises polyethylene terephthalate.

49. The CO2 sensing device according to any one of claims 33 to 39, 47 or 48, or the method according to any one of claims 40 to 48, wherein the electrodes are selected from one or more of carbon black, silver, gold, carbon nanotubes, graphene, and magnesium.

50. The CO2 sensing device according to any one of claims 33 to 39 or 47 to 49, or the method according to any one of claims 40 to 49, wherein the copolymer is present in a concentration between about 1 wt.% to about 70 wt.%, based on the total weight of the device.

51. Use of the CO2 sensing device according to any one of claims 33 to 39 or 47 to 50 in sensing, detecting or measuring CO2 in an environment comprising a food product.

52. The CO2 sensing device according to any one of claims 33 to 39 or 47 to 51 , wherein the CO2 detection level is greater than about 10,000 ppm.

53. The CO2 sensing device according to any one of claims 33 to 39 or 47 to 52, wherein the response time of the device is less than about 360 s.

54. The CO2 sensing device according to any one of claims 33 to 39 or 47 to 53, wherein the recovery time of the device is less than about 840 s.

55. The CO2 sensing device according to any one of claims 33 to 39 or 47 to 54, wherein the sensing of CO2 is reversible.

56. A method of sensing, or detecting, or measuring CO2 comprising exposing the CO2 sensing device according to any one of claims 33 to 39 or 47 to 55, to an environment comprising CO2.

57. The method according to claim 56, wherein the environment comprising CO2 has a relative humidity at or above 90%.

58. The method according to claim 56 or claim 57, wherein the environment comprising CO2 further comprises ammonia.

59. A method of sensing, or detecting, or measuring CO2 and ammonia comprising exposing the CO2 sensing device according to any one of claims 38 or 39 or 47 to 55, to an environment comprising CO2 and ammonia.