WO2025175342A1

WO2025175342A1 - Integrated sensing layer for improved co2 sensing

Info

Publication number: WO2025175342A1
Application number: PCT/AU2025/050135
Authority: WO
Inventors: Ronil RATH; Syamak FARAJIKHAH; Sina Naficy; Fariba Dehghani; Farshad Oveissi
Original assignee: University of Sydney
Current assignee: University of Sydney
Priority date: 2024-02-23
Filing date: 2025-02-19
Publication date: 2025-08-28
Anticipated expiration: 2026-08-23

Abstract

The present disclosure relates to a sensing material which may be applied in a chemiresistive carbon dioxide sensor, the sensing material comprising a nitrogen-containing polymer, and an amine having a molecular weight of less than about 1000 g/mol.

Description

Integrated sensing layer for improved CO2 sensing

Technical field

[0001] The present disclosure broadly relates to chemiresistive carbon dioxide sensors.

Background

[0002] Any discussion of the prior art throughout this specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0003] Carbon dioxide (CO2) is one of the most important greenhouse gases that regulates the planet’s temperature and also plays a fundamental role in human sustenance. CO2 gas is also relevant in various real-world applications ranging from food packaging to improve perishable food product shelf life (modified atmosphere packaging) to environmental monitoring of greenhouse gases, fire detection, medical diagnostics, and photosynthesis. Owing to the wide range of CO2 gas uses and broad concentration ranges, CO2 gas monitoring has become essential.

[0004] The previous generation of CO2 gas monitoring involved using analytical methods such as gas chromatography, infrared spectroscopy, and photoacoustic spectroscopy. While these methods provide reliable results with good repeatability, they are expensive, non-portable, and require high energy consumption and trained personnel to use. Chemiresistive CO2 sensors are an alternative and emerging technique due to their ease of fabrication and portability.

[0005] Among existing CO2 sensors developed based on chemiresistive principles, the most prominent ones are sensors that utilise metal oxide semiconductors (MOS). These sensors are favoured due to their convenient manufacturing process and their ability to exhibit rapid response and recovery times. Nonetheless, a significant drawback of these sensors is their inability to function effectively when subjected to high levels of relative humidity (RH) (above 60% RH). Additionally, they require elevated operating temperatures (ranging from 300 to 700 °C) and have a relatively large physical footprints. Another limitation is that many MOS-based CO2 sensors tend to detect other interfering gases, such as ammonia, in conjunction with the intended target, CO2. These limitations substantially restrict the potential applications of MOS- based sensors, particularly in scenarios where CO2 gas monitoring is required at room temperature within complex moist environments, such as in food packaging. [0006] An alternative approach involves the utilisation of CC -responsive polymers as the sensing material. In this methodology, the interaction with CO2 is contingent upon the functional groups present in the polymer. Among the functional groups sensitive to CO2 are polypyrrole, acrylamide, polyethyleneimine (PEI), and amine-functionalised polymers. However, polymeric CO2 sensors frequently display heightened sensitivity to ammonia, a characteristic that hampers their practicality in applications like intelligent food packaging.

[0007] The concentration of CO2 in packaging has been utilised to monitor the spoilage of meat. When the CO2 concentration exceeds 25%, the product is considered unsuitable for consumption. Given that protein-rich foods like meat inherently generate ammonia in the range of 1 to 5 ppm due to bacterial activity, the sensor system should possess the capability to discern CO2 selectively even in the presence of ammonia.

[0008] It would be desirable to provide a polymeric CO2 sensor material which functions at elevated humidity and/or exhibits low sensitivity to ammonia.

Summary

[0009] In a first aspect of the disclosure, there is provided a sensing material comprising: a nitrogen-containing polymer, and an amine having a molecular weight of less than about 1000 g/mol.

[0010] The following options may be used in conjunction with the first aspect of the disclosure, either individually or in any suitable combination.

[0011] The nitrogen-containing polymer may have a pKa of greater than about 2. The nitrogencontaining polymer may have a pK_a lower than that of ammonia.

[0012] The nitrogen may be located in the backbone of the polymer. The nitrogen may be located in a pendant group of the polymer.

[0013] The nitrogen-containing polymer may be selected from a polymer comprising an amine functional group, a polymer comprising an amide functional group, and a polymer comprising an aromatic nitrogen heterocycle. The nitrogen-containing polymer may be a polymer comprising an amine functional group.

[0014] The nitrogen-containing polymer may be a polymer of one or more monomers which comprise a nitrogen. The one or more monomers which comprise a nitrogen may be selected from the group consisting of aziridine, pyrrole, acrylamide, diamine, amine-containing acrylamide derivatives, and amine-containing acrylate derivatives. The one or more monomers which comprise a nitrogen may be amine-containing acrylamide derivatives and/or amine-containing acrylate derivatives. The amine-containing acrylamide derivative may be selected from the group consisting of /V-[3-(dimethylamino)propyl]methacrylamide, /\/-(3- aminopropyl)methacrylamide, /V-[3-(diethylamino)propyl]methacrylamide, /\/-[3- (disopropylamino)propyl]methacrylamide, and /\/-[3-

(methylethylamino)propyl]methacrylamide. The amine-containing acrylate derivative may be selected from the group consisting of 2-/V-morpholinoethyl methacrylate, 2- (dimethylamino)ethyl methacrylate, 2-(diethylamino)ethyl methacrylate, 2- (diisopropylamino)ethyl methacrylate, and 2-(methylethylamino)ethyl methacrylate.

[0015] The nitrogen-containing polymer may be selected from the group consisting of polypyrrole, polyacrylamide, polyethyleneimine, and poly(/V-[3- (dimethylamino)propyl]methacrylamide-co-2-/\/-morpholinoethyl methacrylate) (P(D-co-M)). The nitrogen-containing polymer may be P(D-co-M).

[0016] The nitrogen-containing polymer may have a number average molecular weight of between about 10 and about 120 kDa.

[0017] The amine may have a molecular weight of less than about 500 g/mol. The amine may have a pK_a greater than that of ammonia. The amine may be a primary, secondary or tertiary alkylamine. The amine may be selected from the group consisting of dimethylamine, diethylamine, diisopropylamine, dipropylamine, dibutylamine, methylethylamine, isopropylmethylamine, isopropylethylamine, triethylamine, diisopropylmethylamine, diisopropylethylamine, tripropylamine, tributylamine and triisopropylamine. The amine may be triethylamine or diethylamine. The amine may be interspersed amongst the nitrogencontaining polymer. Molecules of the amine may be dispersed throughout the nitrogencontaining polymer. The amine may be dissolved in the nitrogen-containing polymer.

[0018] The sensing material may be in the form of a layer having a thickness of between about 1 nm and about 20 nm.

[0019] In a second aspect of the disclosure, there is provided a sensor comprising the sensing material of the first aspect of the invention and a conductive substrate.

[0020] The following options may be used in conjunction with the second aspect of the disclosure, either individually or in any suitable combination. [0021] The conductive substrate may form a first electrode and a second electrode, which are not in contact with one another. The sensing material may be in contact with the first electrode and the second electrode.

[0022] The conductive substrate may be selected from the group consisting of carbon black, graphite, graphene, activated carbon, carbon nanotubes, carbon fibres, and copper foil.

[0023] The sensing material may be disposed on the conductive substrate. It may be in the form of a layer disposed on a surface of the conductive substrate. The layer may be an unbroken layer. It may be a continuous layer.

[0024] The sensor may further comprise a support material, wherein the conductive substrate is disposed on a surface of the support material. The support material may be selected from the group consisting of polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), glass, and paper. In this case, the sensor may comprise a support material forming a bottom layer, a conductive substrate forming a middle layer, and a sensing material forming a top layer.

[0025] In a third aspect of the disclosure, there is provided a method of preparing a sensor, the method comprising:

(i) depositing a conductive substrate on a surface of a support material;

(ii) depositing a nitrogen-containing polymer on the conductive substrate to form a sensing layer; and

(iii) depositing an amine having a molecular weight of less than about 1000 g/mol on the sensing layer to form an integrated sensing layer.

[0026] The following options may be used in conjunction with the third aspect of the disclosure, either individually or in any suitable combination.

[0027] The mass of the amine deposited in step (iii) may be least about 5 times the mass of the nitrogen-containing polymer deposited in step (ii). In the method of the third aspect of the disclosure, for every gram of nitrogen-containing polymer deposited in step (ii), at least about 50 mmol of amine is deposited in step (iii).

[0028] The conductive substrate may form a first electrode and a second electrode, which are not in contact with one another. The integrated sensing layer may be in contact with the first electrode and the second electrode. It may bridge the first and second electrode. It may bridge a gap between the first and second electrode.

[0029] The conductive substrate may be selected from the group consisting of carbon black, graphite, graphene, activated carbon, carbon nanotubes, carbon fibres, and copper foil.

[0030] The support material may be selected from the group consisting of polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), glass, and paper.

[0031] The nitrogen-containing polymer may be as defined in respect of the first aspect of the disclosure. The amine may be as defined in respect of the first aspect of the disclosure.

[0032] In any one of steps (i)-(iii) said depositing may comprise printing, spin coating, dipcoating, or drop casting.

[0033] Step (iii) may be followed by a step of drying the sensor.

[0034] In steps (ii) and/or (iii) the nitrogen-containing polymer and/or the amine may be deposited in the form of a solution of the nitrogen-containing polymer and/or the amine. In this case the method may further comprise a step of evaporating the solution of the nitrogencontaining polymer and/or the amine following step (ii) and/or step (iii).

[0035] In a variant of the third aspect, there is provided a method of preparing a sensor, the method comprising:

(i) providing a conductive substrate;

[0036] The relevant options set out above for the third aspect may also be used in conjunction with this variant.

[0037] The step of providing a substrate may comprise depositing the conductive substrate on a surface of a support material. In this event, this variant is the same as the third aspect.

[0038] In a fourth aspect of the disclosure, there is provided a use of the sensor of the second aspect of the disclosure for determining the atmospheric concentration of CO2 [0039] In a fifth aspect of the disclosure, there is provided a method of determining a concentration of CO2 in an atmosphere, the method comprising:

(i) exposing the sensor of the second aspect of the disclosure to the atmosphere, and

(ii) measuring the resistance of the sensor.

Brief Description of the Drawings

[0040] Figure 1. Schematic illustration of the fabrication of an embodiment of the sensor of the present disclosure by 3D printing and spin coating,

[0041] Figure 2. XPS survey comparisons of C1s, O1s, and N1s of P(D-co-M)-diethylamine (DEA) (PDMD) (upper trace) and P(D-co-M) (lower trace), respectively.

[0042] Figure 3. Graph of the cross-selectivity of the PDMD-based sensor.

[0043] Figure 4. SEM images of the surface (top view) of the PDMD-coated sensor with a scale bar of 30 nm. (A) Image of the plastic substrate with carbon black electrodes without PDMD. (B) Image showing an even coating of PDMD solution on the surface of the sensor via spin coating.

[0044] Figure 5. Superimposed FTIR spectra between 550 cm'¹ and 4000 cm'¹ of DEA (diethylamine) (bottom), P(D-co-M) (middle), and PDMD (top). The peaks are identified by the shaded areas.

[0045] Figure 6. PDMD sensor mechanism. (A) P(D-co-M) protonation with bicarbonate ion over a wide pH range (4.4 - 7.3) and DEA protonation in the presence of bicarbonate ion and ammonium ion. . “+” indicates the protonated amine groups. (B) Change in DEA solution pH and resistance in the presence of CO2 gas. (C) Change in DEA solution pH and resistance in the presence of NH4OH solution. Open circle: Change in DEA solution pH. Open triangle: Change in DEA solution resistance.

[0046] Figure 7. Graphs illustrating PDMD sensor performance. (A) Sensor response to a wide range of CO2 concentrations (10³-10⁶ ppm). Test results have been triplicated, for some data points error bars are too small to be seen (B) PDMD, DEA and P(D-co-M) solid-state sensor response to NH3 gas (0.1 ppm to 25 ppm). (C) Sensor selectivity towards VOCs commonly associated with exhaled human breath. Test was conducted at room temperature (23 °C). (D) Sensor response time to 100% CO2 gas. (E) PDMD sensor recovery and reversibility in the presence and absence of humidified CO2 gas. (F) PDMD sensor response to 10⁶ ppm/100% CC>2 gas over a period of 35 days. ANOVA did not show any significant difference (p>0.05) for day 0 compared with days 7-35.

[0047] Figure 8. Comparison of the response of P(D-co-M), PDMD and PTEA (P(D-co-M)- TEA (triethylamine)) sensors in the presence of a range CO2 concentrations (0.1 - 100%).

[0048] Figure 9. Comparing the response time of PDMD and PTEA sensors in the presence of a range CO2 concentration (0.1 - 100%).

Description of embodiments

[0049] The inventors have found that incorporating a small amine, such as an amine having a molecular weight of less than about 1000 g/mol, into a sensing material composed of a nitrogen-containing polymer results in a sensor which functions well in high relative humidity and responds selectively to CO2 in the presence of ammonia as well as volatile organic compounds.

Sensing material

[0050] In a first aspect of the disclosure, there is provided a sensing material comprising: a nitrogen-containing polymer, and an amine having a molecular weight of less than about 1000 g/mol.

[0051] The ionic conductivity of the sensing material increases, and its resistance decreases, when it is exposed to carbon dioxide in the presence of moisture, i.e. water. Thus the sensing material is capable of detecting the presence of carbon dioxide.

[0052] The sensing material comprises a nitrogen-containing polymer. The nitrogencontaining polymer may be any polymer which becomes protonated in the presence of carbon dioxide and moisture, resulting in an increase in ionic conductivity and decrease in resistance. Accordingly, the nitrogen-containing polymer is a polymer which is capable of becoming protonated by HCOs- .

[0053] The nitrogen-containing polymer may have a pK_a greater than about 2, or a pK_a greater than about 3, 4, 5, 6, 7, 8, or greater than about 9. The nitrogen-containing polymer may have a pKa less than that of ammonia (which is typically given as about 9, or more exactly, about 9.25, however, it is understood that this may vary with the reaction conditions e.g. temperature). The nitrogen-containing polymer may have a pK_a between about 2 and about 9.25, or between about 2-3, 2-4, 2-5, 2-6, 2-7, 2-8, 2-9, 3-4, 3-5, 3-6, 3-7, 3-8, 3-9.25, 4-5, 4- 6, 4-7, 4-8, 4-9.25, 5-6, 5-7, 5-8, 5-9.25, 6-7, 6-8, 6-9.25, 7-8, 7-9, 7-9.25 or between about 8 and about 9.25. The nitrogen-containing polymer may have a pK_a between about 4 and about

7, or between about 4.4 and about 7.3. It may have a pKa of about 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9 or 9.25.

[0054] The nitrogen may be located in the backbone of the polymer, or may be located in a pendant group of the polymer. The nitrogen-containing polymer may be selected from a polymer comprising an amine functional group, a polymer comprising an amide functional group, and a polymer comprising an aromatic nitrogen heterocycle. The polymer may comprise one or more of an amine functional group, an amide functional group, and an aromatic nitrogen heterocycle. In this context, the functional group refers to a functional group which forms part of the repeating unit of the polymer, that is, it is contained in the monomer from which the polymer is formed, or is formed when the monomers from which the polymer is formed are joined together. The polymer may comprise an amine functional group. The polymer may comprise an amine functional and an amide functional group. The polymer may comprise two or more different amine functional groups. The amine functional group may be an acyclic amine group (e.g. dimethylamino) or may be a cyclic amine group (e.g. N-pyrrolidino or 3-pyrrolidino).

[0055] The nitrogen-containing polymer may be a polymer of one or more monomers which comprise a nitrogen. That is, the polymer may be composed of a single monomer, or it may be composed of two or more different monomers, at least one of which comprise a nitrogen, i.e. it may be a copolymer. Monomers which comprise a nitrogen include aziridine, pyrrole, acrylamide, diamine, amine-containing acrylamide derivatives, and amine-containing acrylate derivatives. An amine-containing acrylate or acrylamide derivative refers to a compound of general formula (I): wherein X is selected from O and NR⁵ and at least one of R¹, R² and R⁵ comprises an amine functional group. R⁵ may be H or alkyl or aryl, optionally having at least one amine substituent. At least one, optionally both, of R² and R⁵ may be H, or neither may be H.

[0056] In some embodiments, X may be NH or O, R¹ may be selected from CH3 and H, and R² may be -(CH2)_nNR³R⁴, wherein n may be between 1 and 6, and R³ and R⁴ may be independently selected from H and Ci-e alkyl, or may be joined to form a C2-6 cycloalkane, optionally comprising one or two additional heteroatoms selected from N and O in the ring. It will be understood that a C2 cycoloalkane must have at least one additional heteroatom in the ring. In a specific embodiment , X is NH or O, R¹ is selected from CH3 and H, and R² is - (CH2)_nNR³R⁴, wherein n is between 1 and 6, and R³ and R⁴ are independently selected from H and Ci-e alkyl, or are joined to form a C2-6 cycloalkane, optionally comprising one or two additional heteroatoms selected from N and O in the ring. For the avoidance of doubt, C2-6 cycloalkane refers to a cycloalkane having 2 to 6 carbon atoms in the ring. To illustrate, NR³R⁴ may form a morpholine ring, which is a C4 cycloalkyl comprising an additional heteroatom in the ring which is O.

[0057] The nitrogen-containing polymer may be a polymer of an amine-containing acrylamide derivative and/or an amine-containing acrylate derivative, or a polymer of aziridine. The nitrogen-containing polymer may be a polymer of aziridine. The nitrogen-containing polymer may be a polymer of an amine-containing acrylamide derivative and/or an amine-containing acrylate derivative. In some embodiments, the nitrogen-containing polymer may be a copolymer of two or more different amine-containing acrylamide derivatives, a copolymer of two or more different amine-containing acrylate derivatives, or a copolymer of two or more of a combination of different amine-containing acrylate derivatives and amine-containing acrylamide derivatives (e.g. a copolymer of an amine-containing acrylate derivative and an amine-containing acrylamide derivative). The amine-containing acrylamide derivative may be selected from the group consisting of /V-[3-(dimethylamino)propyl]methacrylamide, /\/-(3- aminopropyl)methacrylamide, /V-[3-(diethylamino)propyl]methacrylamide, /\/-[3- (disopropylamino)propyl]methacrylamide, and /\/-[3-

(methylethylamino)propyl]methacrylamide. The amine-containing acrylate derivative may be selected from the group consisting of 2-/V-morpholinoethyl methacrylate, 2- (dimethylamino)ethyl methacrylate, 2-(diethylamino)ethyl methacrylate, 2- (diisopropylamino)ethyl methacrylate, 2-(methylethylamino)ethyl methacrylate.

[0058] The nitrogen-containing polymer may be selected from the group consisting of polypyrrole, polyacrylamide, polyethyleneimine, and poly(/V-[3- (dimethylamino)propyl]methacrylamide-co-2-/\/-morpholinoethyl methacrylate) (P(D-co-M)). The nitrogen-containing polymer may be P(D-co-M) or polyethyleneimine. The nitrogencontaining polymer may be P(D-co-M). The nitrogen-containing polymer may be polyethyleneimine.

[0059] Where the nitrogen-containing polymer is P(D-co-M), the ratio of /\/-[3- (dimethylamino)propyl]methacrylamide (D) to 2-/V-morpholinoethyl methacrylate (M) may be greater than about 1 :1. The ratio of D to M may be between about 10:1 and about 1 :1 , or between about 10: 1 and about 2: 1 , between about 7:1 and about 3:1 , or about 5:1. It may be about 10:1 , 9:1 , 8:1 , 7:1 , 6:1 , 5:1 , 4:1 , 3:1 , 2:1. 3:2 or 1 :1.

[0060] The nitrogen containing polymer may have a number average molecular weight of between about 10 and about 120 kDa, or between about 10-20, 10-30, 10-40, 10-50, 10-60, 10-70, 10-80, 10-90, 10-100, 10-110, 20-30, 20-40, 20-50, 20-60, 20-70, 20-80, 20-90, 20- 100, 20-110, 20-120, 30-40, 30-50, 30-60, 30-70, 30-80, 30-90, 30-100, 30-110, 30-120, 40- 50, 40-60, 40-70, 40-80, 40-90, 40-100, 40-110, 50-60, 50-70, 50-80, 50-90, 50-100, 50-110, 50-120, 60-70, 60-80, 60-90, 60-100, 60-110, 60-120, 70-80, 70-90, 70-100, 70-100, 70-120, 80-90, 80-100, 80-110, 80-120, 90-100, 90-110, 90-120, 100-110, 100-120, or between about 110 and 120 kDa. The nitrogen containing polymer may have a molecular weight of between about 60 and about 100 kDa. The nitrogen containing polymer may have a molecular weight of between about 70 and about 90 kDa. The nitrogen containing polymer may have a molecular weight of about 40 kDa, or about 50, 60, 70, 80, 90, 100, 110 or about 120 kDa. The nitrogen containing polymer may have a molecular weight of about 80 kDa.

[0061] The sensing material further comprises an amine. Without wishing to be bound by theory, the inventors hypothesise that where the amine is more basic than ammonia, this may lessen the interaction of ammonia with the nitrogen-containing polymer, and reduce crossresponse.

[0062] The amine has a molecular weight of less than about 1000 g/mol. It may have a molecular weight of less than about 900, 800, 700, 600, 500, 400, 300, 200 or less than about 100 g/mol. The amine may have a molecular weight of between about 50 and about 1000 g/mol, or between about 50-100, 50-200, 50-300, 50-400, 50-500, 50-600, 50-700, 50-800, 50-900, 100-200, 100-300, 100-400, 100-500, 100-600, 100-700, 100-800, 100-900, 100- 1000, 200-300, 200-400, 200-500, 200-600, 200-700, 200-800, 200-900, 200-1000, 300-400, 300-500, 300-600, 300-700, 300-800, 300-900, 300-1000, 400-500, 400-600, 400-700, 400- 800, 400-900, 400-1000, 500-600, 500-700, 500-800, 500-900, 500-1000, 600-700, 600-800, 600-900, 600-1000, 700-800, 700-900, 700-1000, 800-900, 800-1000, or between about 900 and about 1000 g/mol. The amine may have a molecular weight of between about 50 and about 500 g/mol. The amine may have a molecular weight of between about 50 and about 200 g/mol. The amine may have a molecular weight of between about 50 and about 100 g/mol. It may have a molecular weight of about 50, 60, 70, 80, 90, 100, 120, 140, 160, 180, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950 or 1000 g/mol.

[0063] The amine may have a pKa greater than that of ammonia (which is typically given as about 9, or more exactly, about 9.25, however, it is understood that this may vary with the reaction conditions e.g. temperature). The amine may have a pK_a greater than about 9, or greater than about 9.25, 10, 12, 15, 20, 22, or greater than about 25. The amine may have a pK_a of between about 9.25 and about 30, or between about 9.25-15, 9.25-20, 9.25-25, 15-20, 15-25, 15-30, 20-25, 20-30, or between about 25 and about 30. The amine may have a pK_a of between about 9.25 and about 15. It may have a pK_a of about 9.25, 9.5, 9.75, 10, 10.5, 11 , 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5 or 20, or may have a pKa of more than 20.

[0064] The amine may be a primary, secondary or tertiary alkylamine. The amine may be a primary, secondary or tertiary C1.12 alkylamine. The amine may be a secondary or tertiary Ci- 12 alkylamine. The amine may be primary, secondary or tertiary C1.10 alkylamine. The amine may be a secondary or tertiary C1.10 alkylamine. The amine may be primary, secondary or tertiary C1.6 alkylamine. The amine may be a secondary or tertiary C1.10 alkylamine. The amine may be a secondary or tertiary C1.6 alkylamine. The amine may be selected from the group consisting of dimethylamine, diethylamine, diisopropylamine, dipropylamine, dibutylamine, methylethylamine, isopropylmethylamine, isopropylethylamine, triethylamine, diisopropylmethylamine, diisopropylethylamine, tripropylamine, tributylamine and triisopropylamine. The amine may be diethylamine or triethylamine. The amine may be diethylamine. The amine may be triethylamine.

[0065] In the sensing material, the amine and the nitrogen-containing polymer may form an integrated layer. That is, the amine and the nitrogen-containing polymer may not form separate layers, which may be, for instance, adjacent to one another. Instead, the amine may be interspersed amongst the nitrogen-containing polymer. The amine may be uniformly, or substantially uniformly, interspersed amongst the nitrogen-containing polymer.

[0066] The sensing material may be in the form of a film, or of a layer. The sensing material may have a thickness of between about 1 nm and about 20 nm, or between about 1-5, 1-10, 1-15, 5-10, 5-15, 5-20, 10-15, 10-20, or between about 15 nm and about 20 nm. The sensing material may have a thickness of between about 1 nm and about 10 nm. The sensing material may have a thickness of between about 5 nm and about 15 nm. The sensing material may have a thickness of between about 5 and about 10 nm. The sensing material may have a thickness of about 1 nm, or about 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, or about 20 nm. The sensing material may have a thickness of about 8 nm.

Sensor [0067] In a second aspect of the disclosure there is provided a sensor comprising the sensing material of the first aspect of the disclosure and a conductive substrate.

[0068] Sensors based on chemiresistive materials are well-known in the art. In such sensors, a chemiresistive material is typically deposited on a conductive substrate, such that the conductive substrate forms two electrodes bridged by the chemiresistive material. The electrical resistance between the two electrodes may then be measured to determine the concentration of a target substance, in this case, carbon dioxide.

[0069] Thus the conductive substrate may form a first electrode and a second electrode, which are not in contact with one another, and the sensing material may be in contact with the first electrode and the second electrode. The first electrode and the second electrode may be interdigitated.

[0070] The sensing material may form a layer, or a film, on the conductive substrate. The thickness of the sensing material in the sensor of the second aspect of the disclosure may be between about 1 nm and about 20 nm, as described above in respect of the first aspect of the disclosure.

[0071] The conductive substrate may be any suitable conductive material, such as carbon black, graphite, graphene, activated carbon, carbon nanotubes, carbon fibres, and copper foil. The conductive substrate may be carbon black. The conductive substrate may have a conductivity at 20°C of greater than about 10³S/m, or of greater than about 2*10³, 5*10³, 10⁴, 5*10⁴, 10⁵, 5*10⁵, 10⁶, 5*10⁶, or 10⁷. It may have a conductivity of about 2*10³, 5*10³, 10⁴, 5*10⁴, 10⁵, 5*10⁵, 10⁶, 5*10⁶, 10⁷ or 5*10⁷.

[0072] The sensor may further comprise a support material, on which the conductive substrate is disposed. That is, the first electrode and second electrode composed of the conductive substrate may be disposed on a surface of the support material. The support material may be any suitable non-conductive material, for example polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), glass, or paper. The support material may be PET. The support may have an electrical resistivity at 20°C of at least about 10⁵, or at least about 10⁶’ 10⁷, 10⁸, 10⁹, 10¹⁰, 10¹¹ or 10¹².

[0073] Thus the sensor may comprise a support material forming a bottom layer, a conductive substrate forming a middle layer, and a sensing material forming a top layer. For the avoidance of doubt, as the conductive substrate may form a first electrode and a second electrode, which are not in contact with one another, and the sensing material may be in contact with the first electrode and the second electrode, some regions of the sensor may comprise only a support material forming a bottom layer and a sensing material forming a top layer.

Sensor fabrication

[0074] In a third aspect of the disclosure, there is provided a method of preparing a sensor, the method comprising:

(i) depositing a conductive substrate on a surface of a support material;

[0075] The nitrogen containing polymer may be as defined above in relation to the first aspect of the disclosure, and the amine may be as defined above in relation to the first aspect of the disclosure. That is, steps (ii) and (iii) of the method of the third aspect of the disclosure describe the preparation of the sensing material of the first aspect of the disclosure on a conductive substrate. That is, the 'integrated sensing layer' of the third aspect of the disclosure is the 'sensing material' of the first aspect of the disclosure. The method of the third aspect of the disclosure may describe the preparation of a sensor of the second aspect of the disclosure.

[0076] Thus also disclosed herein is a method of preparing the sensing material as described in the first aspect of the disclosure, the method comprising:

(i.a) depositing a nitrogen-containing polymer on a substrate to form a sensing layer; and

(ii.a) depositing an amine having a molecular weight of less than about 1000 g/mol on the sensing layer to form the sensing material. wherein nitrogen-containing polymer and the amine are as defined above in relation to the first aspect of the disclosure.

[0077] In respect of the preparation of the sensor, the conductive substrate may be deposited to form a first electrode and a second electrode, which are not in contact with one another. The first electrode and the second electrode may be interdigitated. The conductive substrate may be deposited by printing, spin coating, dip-coating, or drop casting. The conductive substrate may be deposited by printing. [0078] In respect of the preparation of the sensor, the support material may be any suitable non-conductive material, for example polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), glass, or paper. The support material may be PET. The conductive substrate may be any suitable conductive material, for example carbon black, graphite, graphene, activated carbon, carbon nanotubes, carbon fibres, and copper foil. The conductive substrate may be carbon black.

[0079] In respect of the preparation of the sensor or the sensing material, The nitrogen containing polymer and the amine may be deposited by printing, spin coating, dip-coating, or drop casting. The nitrogen containing polymer and the amine may be deposited by spin coating.

[0080] In respect of the preparation of the sensor or the sensing material, the nitrogen containing polymer and the amine may be deposited in the form of a solution of the nitrogencontaining polymer and/or a solution of the amine. Thus step (ii) may comprise depositing a solution of a nitrogen-containing polymer on the conductive substrate, and be followed by a step (ii.b) of evaporating the solution of the nitrogen-containing polymer to form a sensing layer. Similarly, step (iii) may comprise depositing a solution of an amine having a molecular weight of less than about 1000 g/mol on the sensing layer, and be followed by a step (iii.b) of evaporating the solution to form an integrated sensing layer. The solution of the nitrogencontaining polymer and the solution of the amine may comprise any suitable solvent, such as an alcohol, for example ethanol, methanol, or isopropanol. Evaporating the solution refers to evaporating the solvent, such that only the nitrogen-containing polymer and/or amine remains on the conductive substrate and/or sensing layer. This may be promoted by passing a stream of gas, e.g. air, over the solution. The gas may be a heated gas. The evaporating may be promoted by applying a partial vacuum. It may be promoted by heating. It may be promoted by a combination of any two or more of these.

[0081] The mass of the amine deposited in step (iii) of the method of preparing a sensor may be at least about 5 times the mass of the nitrogen-containing polymer deposited in step (ii) of the method of preparing a sensor (or, equivalently, steps (ii.a) and (i.a) respectively of the method of preparing a sensing material), or at least about 6, 7, 8, 9, 11 , 12, 13, 14, 15, 16, 17, 18, 19 or about 20 times the mass of the nitrogen-containing polymer deposited in step (ii). The mass of the amine deposited in step (iii) may be between about 5 and about 20 times the mass of the nitrogen-containing polymer deposited in step (ii), or between about 5-10, 5-15, 10-15, 10-20 or between about 15 and about 20 times the mass of the nitrogen-containing polymer deposited in step (ii). The mass of the amine deposited in step (ii.a) may be between about 5 and about 10 times the mass of the nitrogen-containing polymer deposited in step (i.a). The mass of the amine deposited in step (iii) may be between about 7 and about 11 times the mass of the nitrogen-containing polymer deposited in step (ii). The mass of the amine deposited in step (ii.a) may be about 5 times the mass of the nitrogen-containing polymer deposited in step (i.a), or about 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, or about 20 times the mass of the nitrogen-containing polymer deposited in step (i.a). The mass of the amine deposited in step (iii) may be about 9 times the mass of the nitrogen-containing polymer deposited in step (ii).

[0082] Alternatively, for every gram of nitrogen-containing polymer deposited in step (ii) of the method of preparing a sensor, at least about 50 mmol of amine may be deposited in step (iii) of the method of preparing a sensor (or, equivalently, steps (i.a) and (ii.a) respectively of the method of preparing a sensing material), or at least about 60, 70, 80, 90, 100, 110, 120, 130, 140, or at least about 150 mmol of amine may be deposited in step (iii). For every gram of nitrogen-containing polymer deposited in step (ii), between about 50 mmol and about 150 mmol of amine may be deposited in step (iii), or between about 50-70, 50-90, 50-110, 50-130, 70-90, 70-110, 70-130, 70-150, 90-110, 90-130, 90-150, 110-130, 110-150, or between about 130 mmol and about 150 mmol of amine may be deposited in step (iii). For every gram of nitrogen-containing polymer deposited in step (ii), between about 70 mmol and about 130 mmol of amine may be deposited in step (iii). For every gram of nitrogen-containing polymer deposited in step (ii), between about 70 mmol and about 110 mmol of amine may be deposited in step (iii). For every gram of nitrogen-containing polymer deposited in step (ii), between about 90 mmol and about 130 mmol of amine may be deposited in step (iii). For every gram of nitrogen-containing polymer deposited in step (ii), about 50 mmol of amine may be deposited in step (iii), or about 60, 70, 80, 90, 100, 110, 120, 130, 140, or about 150 mmol of amine may be deposited in step (iii). For every gram of nitrogen-containing polymer deposited in step (ii), about 90 mmol of amine may be deposited in step (iii). For every gram of nitrogen-containing polymer deposited in step (ii), about 120 mmol of amine may be deposited in step (iii).

[0083] In respect of the preparation of the sensor or the sensing material, in some embodiments, the amine may be a liquid and may be deposited neat. Step (iii) may be followed by a step (iii.b) of drying the sensor. This may comprise drying the sensor in air at room temperature, or at between about 20 and about 30 °C, or at about 25 °C. Drying in air refers to drying in ambient atmosphere and at ambient pressure, for example 900-110 hPa. The sensor may be dried for a period of between about 2 hours and about 24 hours, or between about 2-6, 2-12, 2-18, 6-12, 6-18, 6-24, 12-18, 12-14, or between about 18 hours and about 24 hours. The sensor may be dried for a period of between about 2 hours and about 18 hours. The sensor may be dried for a period of between about 4 hours and about 12 hours. The sensor may be dried for a period of about 2 hours, or about 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or about 24 hours. There may be a step of drying after any one or more step, optionally after each step. In a particular example, after printing the carbon black electrode, it is dried for between about 2 and about 24 hours. After spin-coating the P(D-co-M) solution it is allowed to air dry for about 5 minutes in the spin-coater itself to allow the remaining solvent in the machine to dry off and then after that DEA solution is spin coated and then further air dried overnight.

[0084] In respect of the preparation of the sensor or the sensing material, the integrated sensing layer, or sensing material, may have a thickness of between about 1 nm and about 20 nm, as described above in respect of the first aspect of the disclosure.

[0085] Also disclosed herein is a sensor prepared by the method of the third aspect of the disclosure.

Application in carbon dioxide sensing

[0086] In a fourth aspect of the disclosure, there is provided a use of the sensor of the second aspect of the disclosure, or a sensor prepared according to the method of the third aspect of the disclosure, for determining the atmospheric concentration of CO2.

[0087] In a fifth aspect of the disclosure, there is provided a method of determining the concentration of CO2 in an atmosphere, the method comprising:

(i) exposing the sensor of the second aspect of the disclosure, or a sensor prepared according to the method of the third aspect of the disclosure, to the atmosphere, and

(ii) measuring the resistance of the sensor.

[0088] As described above, the ionic conductivity of the sensing material increases, and its resistance decreases, when it is exposed to carbon dioxide in the presence of moisture, i.e. water. Thus the sensor may be used to determine the concentration of carbon dioxide in the atmosphere to which it is exposed.

[0089] The atmosphere may additionally or alternatively comprise ammonia. The ionic conductivity of the sensing material decreases, and its resistance increases, when it is exposed to ammonia, particularly ammonia at atmospheric concentrations of up to about 5 ppm. Thus the sensor is capable of distinguishing between the presence of carbon dioxide and the presence of ammonia. [0090] The method may comprise preparing a calibration curve, or calibration equation, relating resistance of the sensor to carbon dioxide concentration. It may therefore additionally comprise relating a measured resistance of the sensor to the calibration curve in order to determine a measured carbon dioxide concentration. In an embodiment, the method comprises preparing a calibration surface, or calibration equation, relating resistance of the sensor to both carbon dioxide and ammonia (on orthogonal axes). In this embodiment, the method may comprise relating the measured resistance to the calibration surface so as to determine both ammonia and carbon dioxide concentrations. Alternatively in this embodiment, the method may comprise independently determining an ammonia concentration and using the determined ammonia concentration and the calibration surface, or calibration equation, to determine a measured carbon dioxide concentration.

[0091] Some forms of the invention are set out below.

[0092] Form 1. A sensing material comprising:

• a nitrogen-containing polymer, and

• an amine having a molecular weight of less than about 1000 g/mol.

[0093] Form 2. The sensing material of form 1 , wherein the nitrogen-containing polymer has a pK_a of greater than about 2.

[0094] Form 3. The sensing material of form 1 or form 2, wherein the nitrogen-containing polymer has a pKa lower than that of ammonia.

[0095] Form 4. The sensing material of any one of forms 1 to 3, having nitrogen located in the backbone of the polymer.

[0096] Form 5. The sensing material of any one of forms 1 to 4, having nitrogen located in a pendant group of the polymer.

[0097] Form 6. The sensing material of any one of forms 1 to 5, wherein the nitrogencontaining polymer is selected from a polymer comprising an amine functional group, a polymer comprising an amide functional group, and a polymer comprising an aromatic nitrogen heterocycle.

[0098] Form 7. The sensing material of any one of forms 1 to 6, wherein the nitrogencontaining polymer is a polymer comprising an amine functional group.

[0099] Form 8. The sensing material of any one of forms 1 to 7, wherein the nitrogencontaining polymer is a polymer of one or more monomers which comprise a nitrogen. [00100] Form 9. The sensing material of form 8, wherein the one or more monomers which comprise a nitrogen are selected from the group consisting of aziridine, pyrrole, acrylamide, diamine, amine-containing acrylamide derivatives, and amine-containing acrylate derivatives.

[00101] Form 10. The sensing material of form 8 or form 9, wherein the one or more monomers which comprise a nitrogen are amine-containing acrylamide derivatives and/or amine-containing acrylate derivatives.

[00102] Form 11 . The sensing material of form 9 or form 10, wherein the amine-containing acrylamide derivative is selected from the group consisting of /\/-[3- (dimethylamino)propyl]methacrylamide, /V-(3-aminopropyl)methacrylamide, /\/-[3- (diethylamino)propyl]methacrylamide, /V-[3-(disopropylamino)propyl]methacrylamide, and /V- [3-(methylethylamino)propyl]methacrylamide.

[00103] Form 12. The sensing material of form 9 or form 10, wherein the amine-containing acrylate derivative is selected from the group consisting of 2-/V-morpholinoethyl methacrylate, 2-(dimethylamino)ethyl methacrylate, 2-(diethylamino)ethyl methacrylate, 2- (diisopropylamino)ethyl methacrylate, and 2-(methylethylamino)ethyl methacrylate.

[00104] Form 13. The sensing material of any one of forms 1 to 12, wherein the nitrogencontaining polymer is selected from the group consisting of polypyrrole, polyacrylamide, polyethyleneimine, and poly(/V-[3-(dimethylamino)propyl]methacrylamide-co-2-/\/- morpholinoethyl methacrylate) (P(D-co-M)).

[00105] Form 14. The sensing material of any one of forms 1 to 13, wherein the nitrogencontaining polymer is P(D-co-M).

[00106] Form 15. The sensing material of any one of forms 1 to 14, wherein the nitrogencontaining polymer has a number average molecular weight of between about 10 and about 120 kDa.

[00107] Form 16. The sensing material of any one of forms 1 to 15, wherein the amine has a molecular weight of less than about 500 g/mol.

[00108] Form 17. The sensing material of any one of forms 1 to 16, wherein the amine has a pKa greater than that of ammonia.

[00109] Form 18. The sensing material of any one of forms 1 to 17, wherein the amine is a primary, secondary or tertiary alkylamine. [00110] Form 19. The sensing material of any one of forms 1 to 18, wherein the amine is selected from the group consisting of dimethylamine, diethylamine, diisopropylamine, dipropylamine, dibutylamine, methylethylamine, isopropylmethylamine, isopropylethylamine, triethylamie, diisopropylmethylamine, diisopropylethylamine, tripropylamine, tributylamine and triisopropylamine.

[00111] Form 20. The sensing material of any one of forms 1 to 19, wherein the amine is triethylamine or diethylamine.

[00112] Form 21. The sensing material of any one of forms 1 to 20, wherein the amine is interspersed amongst the nitrogen-containing polymer.

[00113] Form 22. The sensing material of any one of forms 1 to 21 , in the form of a layer having a thickness of between about 1 nm and about 20 nm.

[00114] Form 23. A sensor comprising the sensing material of any one of forms 1 to 22 and a conductive substrate.

[00115] Form 24. The sensor of form 23, wherein the conductive substrate forms a first electrode and a second electrode, which are not in contact with one another, and the sensing material is in contact with the first electrode and the second electrode.

[00116] Form 25. The sensor of form 23 or form 24, wherein the conductive substrate is selected from the group consisting of carbon black, graphite, graphene, activated carbon, carbon nanotubes, carbon fibres, and copper foil.

[00117] Form 26. The sensor of any one of forms 23 to 25, wherein the sensing material is in the form of a layer disposed on a surface of the conductive substrate.

[00118] Form 27. The sensor of any one of forms 23 to 26, further comprising a support material, wherein the conductive substrate is disposed on a surface of the support material.

[00119] Form 28. The sensor of forms 27, wherein the support material is selected from the group consisting of polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), glass, and paper.

[00120] Form 29. The sensor of form 27 or form 28, comprising a support material forming a bottom layer, a conductive substrate forming a middle layer, and a sensing material forming a top layer.

[00121] Form 30. A method of preparing a sensor, the method comprising: (i) depositing a conductive substrate on a surface of a support material;

[00122] Form 31. The method of form 30, wherein the mass of the amine deposited in step (iii) is least about 5 times the mass of the nitrogen-containing polymer deposited in step (ii).

[00123] Form 32. The method of form 30, wherein for every gram of nitrogen-containing polymer deposited in step (ii), at least about 50 mmol of amine is deposited in step (iii).

[00124] Form 33. The method of any one of forms 30 to 32, wherein the conductive substrate forms a first electrode and a second electrode, which are not in contact with one another, and the integrated sensing layer is in contact with the first electrode and the second electrode.

[00125] Form 34. The method of any one of forms 30 to 33, wherein the conductive substrate is selected from the group consisting of carbon black, graphite, graphene, activated carbon, carbon nanotubes, carbon fibres, and copper foil.

[00126] Form 35. The method of any one of forms 30 to 34, wherein the support material is selected from the group consisting of polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), glass, and paper.

[00127] Form 36. The method of any one offorms 30 to 35, wherein the nitrogen-containing polymer is as defined in any one of claims 2 to 15.

[00128] Form 37. The method of any one offorms 30 to 36, wherein the amine is as defined in any one of claims 16 to 17.

[00129] Form 38. The method of any one of forms 30 to 37, wherein in any one of steps (i)-(iii) said depositing comprises printing, spin coating, dip-coating, or drop casting.

[00130] Form 39. The method of any one of forms 30 to 38, wherein step (iii) is followed by a step of drying the sensor. [00131] Form 40. The method of any one of forms 30 to 39, wherein in steps (ii) and/or (iii) the nitrogen-containing polymer and/or the amine are deposited in the form of a solution of the nitrogen-containing polymer and/or the amine.

[00132] Form 41. The method of form 40, further comprising a step of evaporating the solution of the nitrogen-containing polymer and/or the amine following step (ii) and/or step (iii).

[00133] Form 42. Use of the sensor of any one of forms 23 to 29 for determining an atmospheric concentration of CO2.

[00134] Form 43. A method of determining a concentration of CO2 in an atmosphere, the method comprising:

(i) exposing the sensor of any one of forms 23 to 29 to the atmosphere, and

(ii) measuring the resistance of the sensor.

Definitions

[00135] The following are some definitions that may be helpful in understanding the description of the present disclosure. These are intended as general definitions and should in no way limit the scope of the present disclosure to those terms alone, but are put forth for a better understanding of the following description.

[00136] The term "atmosphere" should be taken to refer to a body of gas. It may be an open atmosphere or it may be an enclosed atmosphere. It may refer in particular to a body of gas in which the carbon dioxide concentration is to be, or has been, measured using the sensor and/or method of the invention.

[00137] Where reference is made herein to pK_a of a nitrogen-containing species or functional group, this should be taken to refer to the pK_a of the protonated form of that species or group. Thus for example, where reference is made to the pK_a of triethylamine, it relates to the equilibrium between triethylamine and triethylammonium ion.

[00138] As used herein, the term "continuous layer" refers to a layer without breaks, i.e. a layer in which it is possible to travel from any point on the layer to any other point on the layer without passing any point where there is no layer.

[00139] Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. The term "composed of" should be understood in the same manner as "comprising". Where "comprise", "composed of" and variations thereof are used herein, they should be taken to encompass the meaning "consisting of" or corresponding variation thereof, i.e. inclusion of the stated element and the exclusion of other element.

[00140] The terms "a" and "an" are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.

[00141] In the context of this specification the term "about" is understood to refer to a range of numbers that a person of skill in the art would consider equivalent to the recited value in the context of achieving the same function or result. The term "about" may refer to ±10% of the recited value.

[00142] The term "may" should be taken to encompass both positive and negative recitations unless the context indicates otherwise. For example, the phrase "A may be B" should be taken to indicate both the possibility that A is B and the possibility that A is not B.

[00143] Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of 1.0 to 5.0 is intended to include all sub-ranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 5.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 5.0, such as 2.1 to 4.5. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited herein is intended to include all higher numerical limitations subsumed therein.

[00144] Any description of prior art documents herein, or statements herein derived from or based on those documents, is not an admission that the documents or derived statements are part of the common general knowledge of the relevant art.

[00145] For the purposes of description, all documents referred to herein are hereby incorporated by reference in their entirety unless otherwise stated

Examples

[00146] The present disclosure is further described below by reference to the following nonlimiting examples.

Experimental Materials

[00147] Ammonium hydroxide solution (30-33% NH3 in water), ethanol (>99.5%), acetone (> 99.9%), methanol ( > 99.9%), toluene ( > 99.8%), diethylamine (DEA) (> 99.9%) were acquired from Sigma-Aldrich (Merck, Australia). For the synthesis of P(D-co-M), N-3- (dimethylamino)propyl methacrylamide (DMAPMAm) (99%), methoxyethyl methacrylate MEMA (95%), and 2,2’-azobis(2-methylpropionitrile) (AIBN) (0.2 M in toluene) were also purchased from Sigma-Aldrich (Merck, Australia). Carbon black conductive ink was purchased from Dycotec Materials Ltd, UK. Transparent polyethylene terephthalate (PET) sheets were procured from RS Components Pty Ltd, Australia.

P(D-co-M) synthesis

[00148] The synthesis of P(D-co-M) was carried out following the same steps as described in Advanced Materials Technologies 2023, 2201510. Briefly, 1 :1 molar ration of DMAPMAm (10.87 ml, 0.06 mol) and MEMA (11.4 ml, 0.06 mol) monomers were mixed with AIBN (2.8 ml, 5.5 x 10'⁴ mol) as the initiator and using ethanol (10 ml, 0.17 mol) as the solvent, respectively. Once the solution was prepared, it was degassed with N2 gas (flowrate 45 ml mim¹) to remove the trapped oxygen. For the free radical polymerisation, the solution was sealed and placed in an oven at 60 °C overnight and incubated.

PDMD sensor fabrication

[00149] To fabricate the solid-state P(D-co-M)-DEA (PDMD) sensor, carbon black conductive ink was deposited onto the transparent PET sheet to form a uniform pair of interdigitated electrodes (25 mm x 20 mm) using the EnvisionTEC 3D bioplotter (Germany). The conductive ink was deposited through a nozzle (250 pm), with optimised printing parameters being 21 °C, a pressure of 3.2 bar, and a printing speed of 10 mm S’¹. The printed electrodes were then placed in an oven at 60 °C to dry. Once the electrodes were dried, P(D-co-M) solution (1 wt% in ethanol, 50 pl) was spin-coated on top of the electrodes, leading to a uniform polymer coating covering the interdigitated electrodes only. MuTech micro coater (Argentina) was employed to spin coat the solution, and the parameters such as velocity, acceleration and time were optimised to 5000 rpm, 1000 rpm s^_1 and 20 s, respectively. After drying, DEA (50 pl) was spin-coated on top of the P(D-co-M) layer using the same spin-coating parameters (Figure 1). The prepared sensor was air-dried overnight, and the prototypes were used for further experiments. Moreover, XPS analysis was conducted to determine whether P(D-co-M) and DEA formed a multi-layer or an integrated layer system (Figure 2A).

Assessing PDMD sensor performance [00150] To evaluate the response of solid-state sensors to target gas at high humidity levels [> 80% RH], the sensor was sealed inside a sensor chamber (220 mL container) with 1.5 mL of water to humidify the sensor chamber and was placed in an oven at 37 °C for two hours. At the start of each sensor test, humidified N2 gas (N2 gas was bubbled through water) was purged into the chamber to remove residues of the gases and achieve a stable baseline resistance before the target gas (CO2) was introduced into the chamber. The sensor’s resistance was measured using a multimeter (2450 SourceMeter, Keithley). The flow rates of N2 and CO2 were controlled using the Omega FMA-2600A series mass flow controllers. A reference CO2 sensor (ExplorlR-W-100, USA) was placed inside the sensor chamber to provide real-time measurement of CO2 concentration in the chamber for validation of chemiresistive CO2 sensor. The sensor response was calculated using Equation 1 : 100 (1) where R_f denotes sensor resistance in the presence of target gas and R_o represents sensor resistance in the presence of humidified N₂ gas. All tests were triplicated, and results were reported with standard deviation.

Assessing the cross-selectivity of PDMD-based sensor

[00151] The cross selectivity of the PDMD sensor was assessed in the presence of isl and a mixture of N₂with VOCs such as acetone, toluene, and ethanol (50 ppm). For the given test, three identical sensors were prepared and its response to 10⁶ ppm C02was assessed under the two conditions. In the presence of N₂ background, the triplicated PDMD-response to CO2 was -81.33%, while as the triplicated response to the gas mixture (N2 /OCS) was -85.6% as shown in Figure 3, indicating the presence of interfering VOCs, has a minimal impact on the sensor’s ability to detect CO2gas. To further validate the sensor’s cross-selectivity a t-test was conducted, and the result was not statistically significant (ns) as p > 0.05, indicating the presence of interfering VOCs, had a minimal impact on the sensor’s ability to detect CO2 gas.

Characterisation

XPS analysis

[00152] An XPS analysis was carried out to study whether the sensing materials formed a multilayer system or an integrated sensing layer on top of the electrodes. The surface chemistry of these samples was analysed using X-Ray Photoelectron Spectroscopy (K-alpha* XPS, Thermo Fisher Scientific). The X-ray gun spot size was set to 400 pm, and the flood gun was turned on. For each sample, XPS survey included C1s, O1s, N1s (ten times each). The XPS experiment was constructed, and data was analysed using the connected software (Advantage Data System, Thermo Fisher Scientific). XPS survey of P(D-co-M) and PDMD was conducted, and the relevant elemental peaks were compared.

[00153] During sensor fabrication, P(D-co-M) solution is spin-coated on top of the electrodes which is followed by a layer of DEA. To determine whether the sensing system forms a multilayer or an integrated layer, an XPS analysis was conducted. In this study, the XPS survey of P(D-co-M) and PDMD were compared, specifically the 01s peak. Since DEA does not have any elemental oxygen present in its polymer chain, if the system formed a multilayer, the response of oxygen would be minimised. However, as seen in Figure 2A-B, PDMD layer does show a response to 01s, with the ratio being 0.59 [P(D-co-M):PDMD], indicating that the sensing material forms an integrated layer instead of a multilayer. This drop in elemental oxygen is due to P(D-co-M) being a larger polymer, with a molecular weight of 80,000 Da, compared to DEA with a molecular weight of 73.14 g/mol. As a result, DEA sit in the polymer matrix of P(D-co-M), rather than forming a multi-layer on top. Furthermore, the PDMD layer also showed an increase in elemental carbon which can be attributed to the methyl groups of DEA present in the matrix of P(D-co-M). Calculation and comparison of binding energy of the XPS survey are shown in Table 1 below.

[00154] Table 1.

SEM analysis

[00155] An SEM analysis was performed to determine the surface topography. In this study, the topography of the sensor surface with the printed carbon black ink, was compared with the PDMD -coated sensor as shown in Figure 4A-B. The SEM analysis revealed that an even coating of PDMD solution was achieved via spin-coating as seen in Figure 4B.

FTIR analysis

[00156] FTIR analysis was carried out to understand whether DEA was retained when preparing PDMD samples. P(D-co-M) shows clear peaks indicating the presence of N - H, above 3000 cm'¹ and between 1600 and 1250 cm'¹, C = O, around 1600 cm'¹, and C - O - C below 1100 cm'¹. The spectrum of DEA, however, was very noisy, and only N - H induced peaks were recognisable. PDMD, which for this test was composed of half P(D-co-M) and half DEA in volume, showed a similar spectrum to P(D-co-M), albeit being less intense. No shifts or peaks movements were observed. See Figure 5.

Results and discussion

Sensing mechanism

[00157] Water soluble gases such as CO2 and NH3 dissociate in the presence of water/humidity to form acidic and basic charged species, respectively. The PDMD sensor response is affected by the pH change induced by these species. CO2 on dissociation produces carbonic acid ions (HCO3 ) that are acidic, whereas NH3 dissociation forms ammonium ions (NH4⁺) that are basic in nature.

[00158] P(D-co-M) interaction with HCO3 ions occurs because of the amine functional groups. As this polymer has a broad pKa value (7.3 to 4.4), the protonation of amine groups depend on the pH of their immediate environment. In the absence of CO2 gas, the amine groups are neutral, whereas when the pH falls below 7.3 due to the presence of the HCO3 groups, most of the N(CHS)2 are protonated to NH(CHs)2⁺, as shown in Figure 6A. The continuous exposure to HCO3 , results in further protonation [N(CH2)2⁺] of the amine group as pH approaches 4.4. At this pH, both the amine sites are protonated (Figure 6A), which increases the ionic conductivity of the sensing material, which in turn reduces the sensor’s resistance.

[00159] DEA interaction with the HCO3 ions also depends on the state of the amine groups. Unlike P(D-co-M), DEA does not have a broad pK_a and is a strong base with a pK_a of 10.64. The protonation of the amine groups also depend on the pH of their immediate environment. In the absence of CO2, the amine functional groups are neutral, and as soon as HCO3 ions are introduced into the surrounding environment, the NH^Hs are protonated to NH2(C2Hs)2⁺ (Figure 6A) as shown in the following reaction.

(C₂H₅)₂NH + HCO (C₂H₅)2NH₂ ⁺ • CO3²- (2)

[00160] This interaction between DEA and CO2 increases the ionic conductivity of the sensor, leading to a decrease in sensor resistance. To further validate this behaviour, CC^ gas (1 mimin'¹) was bubbled into DEA solution (10 mL) and the change in solution pH and resistance was monitored. Over time, it was observed that the pH of DEA approached 10.65, which is close to the pKa of DEA (10.64), indicating that all the DEA ions in the solution interacted with the HCOs- as shown in Figure 6B, while the solution resistance progressively dropped with the increase in CO2 concentration. [00161] In contrast, when sensor was exposed to NH3 gas, the pH of the immediate environment surrounding the sensor was above 7, as ammonium ions have a pK_a of 9.25 and hence are basic in nature. Since DEA has a higher pK_a (10.64) than NH4⁺, in the presence of both ions, DEA would protonate the ammonium ions, as shown in the following reaction.

(C₂H₅)2NH₂ ⁺ + NH₃ (C₂H₅)₂NH + NH₄ ⁺ (3)

[00162] To further validate this interaction and mechanism, ammonium hydroxide solution (NH4OH) was added dropwise into DEA solution (10 mL) and the change in solution pH, and resistance was monitored. Initially, at lower concentrations of NH₄OH solution, DEA showed an increase in solution resistance due to DEA protonating the ammonium ions. However, as the concentration of ammonium ions increases, the number of DEA functional groups that can protonate these ions were not sufficient. As a result, a drop in solution resistance was observed, while the pH of the solution increased by elevating NH₄OH concentration, as shown in Figure 6C.

Characterisation summary

[00163] Overall, the characterisation tests demonstrate that the PDMD sensing material does not form a multi-layered structure, instead it forms a uniform film on top of the electrode as shown in the SEM images. Furthermore, the XPS analysis PDMD layer showed a drop in the amount of elemental oxygen that could be detected when compared to the P(D-co-M) layer and an increase in the amount of elemental carbon that could be detected in the PDMD layer. This could be attributed to the presence of DEA in the matrix of P(D-co-M). The FTIR analysis showed no major changes in the peaks that could be detected in P(D-co-M) and PDMD, respectively. Given that DEA is volatile, the FTIR analysis proved to be inconclusive in determining the presence of DEA on top of the P(D-co-M) sensing layer.

Assessing PDMD sensor performance

[00164] To assess the sensor's response to CO₂, the sensor was subjected to a broad concentration range (10³-10⁶ ppm). This concentration was precisely measured utilising a reference CO₂ gas sensor incorporated within the sensor chamber (Figure 4). The sensor’s response to CO₂ gas depends upon the interaction between the tertiary amine groups and the dissociated carbonic acid ions (HCO3 ). As CO₂ concentration increased from 1 % to 100%, the normalised sensor resistance (see Equation 1) dropped from -3.8% to -83.5%, due to the higher ionic conductivity between the tertiary amine functional group and HCOs- ions. The sensor’s response was compared with P(D-co-M) and DEA solid-state sensors response to CO₂ gas, to verify whether the addition of DEA to P(D-co-M) affected the sensor’s sensitivity towards CO2 gas. Overall, the PDMD sensor exhibits a response towards a broad concentration range of CO2 gas, similar to the pristine P(D-co-M) sensor, in Figure 7A.

[00165] The sensors response in the presence of NH₃ gas was evaluated. At lower concentrations of NH₃ (0.1 to 5 ppm), the PDMD sensor differentiated between CO2 and NH₃ gas via the change in resistivity (increased in the presence of NH₃ and decreased in the presence of CO2). This effect was attributed to the higher pK_a of DEA (10.64) compared to NH₃ (9.25). Therefore, at lower NH₃ concentrations, DEA protonates NH₃, which reduces the ionic conductivity of the sensor, hence resulting in an increase in resistivity, as shown in Figure 7B. At higher concentrations of NH₃ (> 5 ppm), the DEA functional groups cannot protonate NH₃ ions quickly enough due to the high ion mobility and solubility of the gas, and a drop in the resistance of the sensor was observed. This effect was also observed in DEA, which exhibits two different electrical responses for CO2 and NH₃. On the other hand, the solid-state P(D-co-M) sensor cannot differentiate between NH₃, and CO2 gas as in the presence of both gases; the sensor shows a drop in resistance (Figure 7A-B).

[00166] The sensor's selectivity was evaluated in the presence of VOCs including alcohol, an aromatic hydrocarbon, and ketones. These VOCs are commonly employed for the evaluation of sensor performance tailored for packaging applications. Exposure of the sensors to 50 ppm concentration of each VOC was performed. The sensors labeled as PDMD and DEA displayed a rise in resistance upon exposure to these VOCs. Conversely, when subjected to acetone, ethanol, and methanol, the P(D-co-M) sensor demonstrated a reduction in resistance, albeit of a lesser magnitude than the response induced by CO2, as illustrated in Figure 7C. Notably, the PDMD sensor exhibited modest sensitivity and enhanced selectivity towards these VOCs in comparison to the unmodified P(D-co-M) sensor.

[00167] Subsequently, the response time of the PDMD sensor was evaluated and compared to that of the DEA and P(D-co-M) sensors. As displayed in Figure 7D, the PDMD and P(D-co- M) sensors exhibited a faster response time in contrast to the DEA sensor. This disparity in response time can be attributed to the presence of tertiary amine functional groups. Following this analysis, the reversibility and recovery time of the PDMD sensor were investigated through seven cycles of alternating exposure to humidified CO2 gas and its absence. The outcomes that are depicted in Figure 7E demonstrate that during CO2 gas exposure, the sensor exhibited a rapid decline in resistance. Conversely, in the absence of CO2, the sensor reclaimed its original resistance within an average duration of 60 seconds. The prompt recovery of the PDMD sensor can be attributed to the accelerated desorption of CO2 gas from the amine functional groups within PDMD. This stands in contrast to the P(D-co-M) sensor, which necessitated 14 minutes (equivalent to 840 seconds) for recovery. [00168] Finally, the sensor's long-term stability and its response to C02was assessed. In this study three identical PDMD sensors were prepared, stored under the same conditions, and tested in the presence of 100% (10⁶ ppm) CO2 gas over 35 days, once every 7 days, starting from day 0, and the results are reported with standard deviation. Figure 7F shows the average sensor response to CO2 gas (10⁶ ppm). A one-way ANOVA analysis was performed by comparing the sensor response on day 0 to days 7-35. The differences in the mean were not statistically significant (ns) as p > 0.05, indicating that the sensor could provide reliable results for long term CO2 gas monitoring. Due to the fast response and recovery times, along with selectivity and long-term stability, the designed PDMD sensor has the potential to be used for selective CO2 gas monitoring in complex wet environment such as smart food packaging and breath analysis.

[00169] The sensor disclosed herein has demonstrated the ability to function at ambient temperature and within elevated humidity conditions surpassing 80% RH, while additionally offering a wider detection range spanning from 10³ to 10⁶ ppm. Moreover, the sensor has displayed potential in surmounting certain constraints associated with extant iterations of CO2 sensors, thus underscoring its viability for potential applications including CO2 detection in food packaging and other prospective domains.

PTEA sensor

[00170] A sensor composed of P(D-co-M) doped with triethylamine (TEA) (PTEA sensor) was prepared according to the same method as described above for the PDMD sensor. The sensor response across a range of CO2 concentrations and over time was assessed in the same manner as for the PDMD sensor. The PTEA sensor shows an improved response time compared to the PDMD sensor. Results are shown in Figures 8 and 9.

Industrial applicability

[00171] The present disclosure is useful in the field of CO2 sensing, such as in food packaging or breath analysis.

[00172] Those skilled in the art will appreciate that the disclosure described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the disclosure includes all such variations and modifications. The disclosure also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of an two or more of said steps, features, compositions and compounds.

Claims

CLAIMS:

1. A sensing material comprising: a nitrogen-containing polymer, and an amine having a molecular weight of less than about 1000 g/mol.

2. The sensing material of claim 1, wherein the nitrogen-containing polymer has a pK_a lower than that of ammonia.

3. The sensing material of claim 1 or claim 2, having nitrogen located in the backbone of the nitrogen-containing polymer and/or in a pendant group of the nitrogen-containing polymer.

4. The sensing material of any one of claims 1 to 3, wherein the nitrogen-containing polymer is a polymer comprising an amine functional group.

5. The sensing material of any one of claims 1 to 3, wherein the nitrogen-containing polymer is selected from the group consisting of 2-/V-morpholinoethyl methacrylate, 2- (dimethylamino)ethyl methacrylate, 2-(diethylamino)ethyl methacrylate, 2- (diisopropylamino)ethyl methacrylate, 2-(methylethylamino)ethyl methacrylate, polypyrrole, polyacrylamide, polyethyleneimine, and poly(/V-[3-(dimethylamino)propyl]methacrylamide-co- 2-/V-morpholinoethyl methacrylate) (P(D-co-M)).

6. The sensing material of any one of claims 1 to 5, wherein the amine has a pK_a greater than that of ammonia.

7. The sensing material of any one of claims 1 to 6, wherein the amine is a primary, secondary or tertiary alkylamine.

8. The sensing material of any one of claims 1 to 7, wherein the amine is selected from the group consisting of dimethylamine, diethylamine, diisopropylamine, dipropylamine, dibutylamine, methylethylamine, isopropylmethylamine, isopropylethylamine, triethylamine, diisopropylmethylamine, diisopropylethylamine, tripropylamine, tributylamine and triisopropylamine.

9. The sensing material of any one of claims 1 to 8, in the form of a layer having a thickness of between about 1 nm and about 20 nm.

10. A sensor comprising the sensing material of any one of claims 1 to 9 and a conductive substrate.

11. The sensor of claim 10, wherein the conductive substrate forms a first electrode and a second electrode, which are not in contact with one another, and the sensing material is in contact with the first electrode and the second electrode.

12. The sensor of claim 10 or claim 11 , wherein the conductive substrate is selected from the group consisting of carbon black, graphite, graphene, activated carbon, carbon nanotubes, carbon fibres, and copper foil.

13. The sensor of any one of claims 10 to 12, wherein the sensing material is in the form of a layer disposed on a surface of the conductive substrate.

14. The sensor of any one of claims 10 to 13, further comprising a support material, wherein the conductive substrate is disposed on a surface of the support material.

15. The sensor of claim 14, wherein the support material is selected from the group consisting of polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), glass, and paper.

16. A method of preparing a sensor, the method comprising:

(i) depositing a conductive substrate on a surface of a support material;

17. The method of claim 16, wherein the conductive substrate forms a first electrode and a second electrode, which are not in contact with one another, and the integrated sensing layer is in contact with the first electrode and the second electrode.

18. The method of claim 16 or claim 17, wherein the conductive substrate is selected from the group consisting of carbon black, graphite, graphene, activated carbon, carbon nanotubes, carbon fibres, and copper foil.

19. The method of any one of claims 16 to 18, wherein the support material is selected from the group consisting of polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), glass, and paper.

20. A method of determining a concentration of CO2 in an atmosphere, the method comprising:

(i) exposing the sensor of any one of claims 10 to 15 to the atmosphere, and

(ii) measuring the resistance of the sensor.