Gas sensitive sensing electrode for detecting aldehyde flavoring substances in food and preparation method and application thereof

Abstract

The application discloses a kind of detection food aldehyde flavor substance perception gas sensitive sensing electrode and its preparation method and application, it is related to gas sensitive sensing and food detection technical field, to solve the technical problem of low sensitivity, poor selectivity and insufficient stability of existing gas sensitive electrode to food aldehyde flavor substance perception.The application includes: (S1) preparation Co 2+ @ZIF-8 precursor;(S2) pyrolysis conversion into nanocrystalline composite material;(S3) MXene-NH2, with carboxylated composite material, form complex by amidation reaction;(S4) the complex is heated first, then keep warm, finally annealing, obtain ternary composite sensitive material;(S5) ternary composite sensitive material is coated on interdigital electrode, after drying, it is obtained.The gas sensitive sensing electrode obtained by the application is used for high sensitivity, high selectivity perception of aldehyde flavor substance in food, and is suitable for food quality and safety monitoring and flavor evaluation analysis.

Description

Technical Field

[0001] This invention relates to the field of food detection technology. More specifically, this invention relates to a gas-sensitive sensing electrode for detecting aldehyde flavor compounds in food, its preparation method, and its application. Background Technology

[0002] Aldehydes are key components of food flavor and important markers of quality deterioration; rapid and sensitive detection of aldehydes is crucial for food quality monitoring. In fatty foods such as meat, aldehydes produced by lipid oxidation (such as hexanal and nonanal) are core indicators for evaluating the degree of oxidation and freshness. Therefore, developing sensing technologies capable of real-time and specific detection of these volatile aldehydes has become an urgent need in the field of intelligent food sensing.

[0003] Currently, sensing technologies for aldehyde gases mainly include quartz crystal microbalances (QCM), optical sensors, and electrochemical gas sensors. Among them, electrochemical gas sensors have attracted much attention due to their fast response and ease of integration; their performance hinges on the sensing material. While single metal oxide semiconductors (such as Co3O4) possess gas-sensing activity, they often operate at high temperatures and exhibit poor selectivity. Metal-organic frameworks (MOFs, such as ZIF-8) have high specific surface areas and tunable pore sizes, facilitating gas enrichment, but suffer from poor intrinsic conductivity. Two-dimensional materials like MXene exhibit excellent conductivity, but their catalytic activity against target gases is limited when used alone. Researchers have attempted to mechanically mix or simply composite these materials to construct composite materials. However, this physical composite approach results in weak interfacial connections between components, high contact resistance, and insufficient stability. Furthermore, the active components (such as metal oxides) are prone to agglomeration and deactivation, leading to suboptimal electron transport efficiency, catalytic activity, and long-term stability. This limits their high-performance detection of trace aldehydes in complex food systems.

[0004] Therefore, how to construct a sensitive material that combines efficient gas adsorption, excellent catalytic activity, and a fast and stable electron transport path through material design and preparation process innovation, so as to achieve high sensitivity, high selectivity and high stability in sensing key aldehydes in food, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] One object of the present invention is to solve at least the above-mentioned problems and to provide at least the advantages that will be described later.

[0006] Another objective of this invention is to provide a gas-sensitive electrode for sensing aldehyde flavor substances in food, its preparation method, and its application. This invention aims to systematically solve a series of technical challenges, from the construction of the microstructure of the sensitive material and the strengthening of interface engineering to the final optimization of performance.

[0007] To achieve these objectives and other advantages of the present invention, a method for preparing a gas-sensitive sensing electrode for detecting aldehyde flavor substances in food is provided, comprising the following steps: S1. Immerse ZIF-8 crystals in Co... 2+ In an alcoholic solution, the solvent was then slowly evaporated at 60-80°C to obtain Co. 2 + @ZIF-8 precursor; S2, Co 2+ The ZIF-8 precursor is placed in a tube furnace and heated to 380-420℃ at a rate of 1-3℃ / min under an inert atmosphere and held for 1-2 hours to pyrolyze it into a nitrogen-doped porous carbon-confined Co3O4 nanocrystalline composite material, wherein the size of the Co3O4 nanocrystals is 2-5nm. S3, Interfacial covalent modification and heterojunction assembly: (3a) Amination treatment of Ti3C2Tx MXene was performed to obtain surface-aminated MXene-NH2; (3b) The Co3O4 nanocrystalline composite material obtained in step S2 is subjected to surface carboxylation treatment; (3c) Disperse the MXene-NH2 and carboxylated Co3O4 nanocrystalline composite material in a solvent, add a condensing agent, and react at 40-60℃ for 4-8 hours. Covalently link the two through an amidation reaction to obtain a covalently linked composite. S4. Anneal the covalently linked composite obtained in step S3 in an argon mixed atmosphere containing 5-10% by volume hydrogen, heating it to 250-300℃ at 2-5℃ / min and holding it at that temperature for 0.5-1.5 hours to obtain a ternary composite sensitive material. S5. The ternary composite sensitive material obtained in step S4 is made into a slurry, coated onto the interdigital electrode, and dried to obtain a gas-sensitive sensing electrode.

[0008] Preferably, in the method for preparing the gas-sensitive sensing electrode for detecting aldehyde flavor substances in food, the amination treatment in step S3a includes the following steps: S3a1, Ti3C2T after intercalation stripping treatment x MXene, aminotrimethylenephosphonic acid, and glucose were dispersed in deionized water at a mass ratio of 1:(0.1-0.5):(0.5-2) and subjected to hydrothermal reaction at 150-180℃ for 6-12 hours. After the reaction, the mixture was centrifuged, washed, and dried to obtain solid product A. S3a2. Solid product A, tetraethylenepentamine and sodium triacetoxyborohydride are dispersed in N,N-dimethylformamide at a mass ratio of 1:(5-20):(0.5-3) and reacted at 40-60℃ for 24-48 hours. After the reaction, the solid is separated, washed and dried to obtain MXene-NH2.

[0009] Preferably, in the method for preparing the gas-sensitive sensing electrode for detecting aldehyde flavor substances in food, the surface carboxylation treatment in step S3b is as follows: the composite material obtained in step S2, conductive carbon black, and polytetrafluoroethylene binder are mixed at a mass ratio of 85:10:5 to prepare a working electrode; a platinum sheet is used as the counter electrode, and a saturated calomel electrode is used as the reference electrode; the electrode is anoly polarized in 0.5 M sodium sulfate electrolyte at a potential of +1.2V to +1.8V for 10-30 minutes.

[0010] Preferably, in the preparation method of the gas-sensitive sensing electrode for detecting aldehyde flavor substances in food, step S3c specifically comprises: mixing MXene-NH2 with the carboxylated composite material at a mass ratio of 1:1 to 1:3 to obtain a mixed powder; dispersing the mixed powder in N,N-dimethylformamide or N-methylpyrrolidone solvent to form a dispersion with a concentration of 1-5 mg / mL; subsequently adding the condensing agent 4-(4,6-dimethoxytriazine-2-yl)-4-methylmorpholine hydrochloride, and stirring at a constant temperature of 50°C for 6 hours under argon protection; after the reaction, centrifuging, washing, and drying to obtain a covalently linked composite; the amount of condensing agent added is 0.5-2% of the total mass of MXene-NH2 and the composite material.

[0011] Preferably, in the method for preparing the gas-sensitive sensing electrode for detecting aldehyde flavor substances in food, step 3a involves Ti3C2T x MXene is obtained through intercalation and stripping, specifically as follows: (1) Mix Ti3AlC2 powder with lithium fluoride and potassium fluoride in a mass ratio of 1:(1.5-2.5):(1.5-2.5), react at 550-650℃ for 2-4 hours under argon protection, cool and wash with water until neutral, and dry to obtain the first intermediate; (2) The first intermediate is heated to 800-1000℃ at a rate of not less than 100℃ / min under an argon atmosphere and held for 10-30 seconds to obtain the second intermediate; (3) The second intermediate is treated in an argon mixed atmosphere containing 5-10% ammonia by volume at 300-400℃ for 1-2 hours.

[0012] The present invention also provides a gas-sensitive sensing electrode obtained by the above-mentioned method for preparing a gas-sensitive sensing electrode for detecting aldehyde flavor substances in food.

[0013] The present invention also provides an application of the above-mentioned gas-sensitive sensing electrode in the detection of aldehydes in food.

[0014] Preferably, in the application described, the aldehydes include one or more of hexanal, octanal, nonanal, heptanal, (E)-2-octenal, and (E)-2-undecenal.

[0015] Preferably, in the aforementioned application, the gas-sensitive electrode is exposed to the headspace of the food being tested, and qualitative and quantitative detection of aldehydes is achieved by measuring changes in resistance.

[0016] The present invention has at least the following beneficial effects: 1. This invention constructs a ternary composite sensing material in steps, firstly by utilizing the porous structure of ZIF-8 to confine Co. 2+ The process involves pyrolysis to form uniformly dispersed Co3O4 nanocrystals, followed by interfacial covalent modification to firmly link the amino-modified MXene with the carboxylated composite material. Finally, annealing introduces oxygen vacancies. This method achieves precise microstructural control of the sensitive material, forming stable chemical bonds and heterojunction interfaces between components, significantly improving the material's electron transport efficiency and structural stability. The resulting electrode exhibits excellent sensitivity and selectivity in aldehyde gas detection, and its preparation process is controllable and reproducible, making it suitable for rapid detection of trace aldehydes in complex food systems.

[0017] 2. This invention specifically defines the amination treatment steps for MXene, introducing abundant amino functional groups onto the MXene surface through hydrothermal reaction and subsequent amination modification. This treatment not only enhances the surface activity of MXene but also provides sufficient reaction sites for the amidation reaction between it and the carboxylated composite material, thereby promoting the formation of a strong covalent bond between the two. The covalently bonded interface structure effectively reduces the contact resistance between the materials, improves electron transport efficiency, and enhances the mechanical stability and environmental interference resistance of the composite material, ensuring the stable performance of the sensor during long-term use.

[0018] 3. This invention employs an electrochemical anodic polarization method to perform surface carboxylation treatment on Co3O4 nanocrystalline composite materials, introducing carboxyl functional groups onto the material surface. This method is simple to operate, operates under mild conditions, and can achieve surface functionalization without damaging the main structure of the material. The introduction of carboxyl groups provides reactive groups for subsequent covalent bonding with amino-modified MXene, ensuring the chemical stability and uniformity of the composite interface. Furthermore, surface carboxyl groups may enhance the material's affinity for aldehyde molecules, further improving the sensor's adsorption capacity and response sensitivity.

[0019] 4. This invention clarifies the mass ratio, dispersion medium, type of condensing agent, and reaction conditions of MXene-NH2 and the carboxylated composite material. By optimizing the amidation reaction parameters, efficient and uniform covalent bonding between the two is achieved. This process ensures the chemical stability and structural consistency of the composite interface, avoiding common problems in physical mixing such as loose interfaces and component separation. The covalently bonded interface facilitates rapid electron transport between MXene and Co3O4, enhancing the overall conductivity and response speed of the material, while also improving the structural stability and service life of the composite material in complex gas environments.

[0020] 5. This invention specifies the intercalation stripping and post-processing technology for MXene. Through high-temperature fluoride salt etching, rapid thermal shock, and ammonia treatment, a structurally complete MXene material with abundant surface functional groups is obtained. This treatment not only effectively strips MXene layers and increases the specific surface area, but also introduces nitrogen doping and surface functionalization, enhancing its conductivity and chemical reactivity. The optimized MXene, as a conductive framework, can better recombine with ZIF-8 / Co3O4 to form a stable three-dimensional conductive network, providing an efficient path for gas molecule diffusion and electron transport, thereby improving the overall performance of the sensor.

[0021] 6. The gas-sensitive sensing electrode prepared using the method of this invention exhibits rapid response and recovery characteristics to hexanal at relatively low operating temperatures, while maintaining stable detection performance over a wide humidity range. This electrode combines the hydrophobic enrichment capacity of ZIF-8, the high catalytic activity of Co3O4, and the high conductivity of MXene; the synergistic effect of these three factors significantly improves the sensor's selectivity, sensitivity, and environmental adaptability to aldehydes. Its rapid dynamic response capability makes it suitable for real-time monitoring during food processing and storage, providing reliable technical support for food quality and safety. The prepared gas-sensitive sensing electrode, applied to the detection of aldehydes in food, expands its practical value in the fields of food safety and flavor evaluation. This electrode enables rapid and sensitive detection of trace aldehydes in the headspace of food, requiring no complex pretreatment, and is simple to operate with a rapid response. Its high selectivity effectively avoids interference from other volatile substances in food, making it suitable for quality monitoring and freshness evaluation of easily oxidized foods such as meat and oils, providing the food industry with an efficient and low-cost tool for aldehyde detection.

[0022] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0023] Figure 1 The image shows the scanning electron microscope (SEM) characterization results of the MXene / ZIF-8 / Co3O4 composite material in Example 3 of this invention.

[0024] Figure 2 The image shows the transmission electron microscope (TEM) characterization results of the MXene / ZIF-8 / Co3O4 composite material in Example 3 of this invention.

[0025] Figure 3 This is a standard curve fitting diagram of the response of the three composite materials MXene / ZIF-8 / Co3O4, MXene / ZIF-8 and MXene / Co3O4 to multiple concentrations of hexanal (200-2200ppb) in Example 4 of the present invention.

[0026] Figure 4 The standard curve fitting diagrams of the response of MXene / ZIF-8 / Co3O4 composite materials with different MXene ratios (10mg, 20mg, 30mg) to multiple concentrations of hexanal in Example 5 of the present invention show the effect of MXene content on gas-sensing performance and the optimal ratio.

[0027] Figure 5 This is a bar chart showing the response values ​​of the MXene / ZIF-8 / Co3O4 composite electrode (MCZ-20) in Example 6 of the present invention to 1400ppb hexanal in five repeatable tests.

[0028] Figure 6 The graph shows the response values ​​of the three composite materials MXene / ZIF-8 / Co3O4, MXene / ZIF-8 and MXene / Co3O4 to 1400ppb hexanal at different operating temperatures (60-140°C) in Example 7 of this invention.

[0029] Figure 7 The graph shows the response values ​​of MXene / ZIF-8 / Co3O4 composite materials with different MXene ratios to hexanal at different operating temperatures in Example 7 of this invention.

[0030] Figure 8 The figures show the baseline resistance (Ra) curves and the response value (ΔR) curves of the MXene / ZIF-8 / Co3O4 composite material sensor (MCZ-20) in Example 8 of this invention under different relative humidity (40%-90%).

[0031] Figure 9 This is the dynamic response-recovery curve of the MXene / ZIF-8 / Co3O4 composite material sensor (MCZ-20) in Example 9 of the present invention to a concentration of 1000 ppb of hexanal.

[0032] Figure 10This is a bar chart showing the percentage response of the MXene / ZIF-8 / Co3O4 composite material sensor (MCZ-20) in Example 10 of the present invention to various volatile substances at the same concentration (1000ppb).

[0033] Figure 11 This is a schematic diagram of the preparation process of the gas-sensitive sensing electrode for sensing aldehyde flavor substances in food according to the present invention. Detailed Implementation

[0034] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments, so that those skilled in the art can implement it based on the description.

[0035] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0036] It should be noted that, unless otherwise specified, the experimental methods described in the following implementation plan are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified.

[0037] In the description of this invention, the terms "lateral", "longitudinal", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", and "outer" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0038] Example 1 This embodiment provides a specific preparation method for the gas-sensitive sensing electrode for detecting aldehyde flavor substances in food, as described in this invention. It achieves the functionalized design and covalent interface assembly of ZIF-8, Co3O4, and MXene. Figure 11 As shown, the specific steps are as follows: S1, Preparation of Co 2+ @ZIF-8 precursor: Weigh 1.0 g of pre-synthesized and dried ZIF-8 crystals (with a regular dodecahedral morphology) obtained at room temperature, and immerse them in 50 mL of a 0.1 mol / L Co(NO3)2·6H2O ethanol solution. At room temperature (approximately 25°C), place the mixture in a shaker and shake at 100 rpm for 12 hours to ensure the Co... 2+Ions can diffuse sufficiently into the microporous channels (approximately 1.16 nm in diameter) of ZIF-8. Subsequently, the mixed suspension was transferred to a polytetrafluoroethylene evaporating dish and placed in a 70°C forced-air drying oven to slowly evaporate the solvent for approximately 48 hours. During this process, as the solvent decreases, Co... 2+ The ions are further confined within the ZIF-8 channels and undergo weak coordination with the nitrogen atoms on some of the 2-methylimidazolium ligands to form a uniformly loaded precursor, yielding a solid powder, denoted as Co. 2+ @ZIF-8 precursor.

[0039] S2. Pyrolysis preparation of nitrogen-doped porous carbon-confined Co3O4 nanocrystalline composite materials: Approximately 0.5 g of the aforementioned Co²⁺@ZIF-8 precursor was evenly spread in an alumina ceramic boat and placed in the isothermal zone of a tube furnace (model GSL-1100X). First, high-purity argon gas (99.999% purity) was introduced at a flow rate of 300 sccm for 30 minutes to thoroughly purge air from the furnace tube. Subsequently, under continuous argon atmosphere protection, the temperature was increased using a programmed temperature control method: a heating rate of 2℃ / min, a target temperature of 400℃, and a holding time of 1.5 hours after reaching the target temperature. During this pyrolysis process, the organic framework of ZIF-8 carbonized, forming a nitrogen-doped carbon matrix (NC) with a rich microporous structure. Simultaneously, Co confined within the pores... 2+ Under the influence of heat and a carbon-reducing atmosphere, ions combine with residual oxygen and are transformed in situ and in a confined manner into Co3O4 nanocrystals. Due to the spatial confinement effect of the carbon matrix, the generated Co3O4 nanocrystals are uniform in size, with an average size of approximately 3 nm as determined by transmission electron microscopy, and are highly dispersed within the porous carbon network, effectively preventing aggregation. After the pyrolysis process, the mixture is naturally cooled to room temperature under argon protection, yielding a black, fluffy powder, which is the nitrogen-doped porous carbon-confined Co3O4 nanocrystal composite material, denoted as Co3O4@NC.

[0040] S3, Interfacial covalent modification and heterojunction assembly: This step aims to establish a robust covalent interface between MXene and Co3O4@NC through a chemical reaction.

[0041] (3a) Preparation of amino-modified MXene (MXene-NH2): First, a few-layer Ti3AlC2MXene dispersion was prepared. Specifically, 1g of Ti3AlC2MAX phase powder was thoroughly ground and mixed with 1.8g of LiF and 1.8g of KF, and placed in a tube furnace. The mixture was reacted at 600℃ for 3 hours under argon protection. After cooling, the mixture was repeatedly centrifuged and washed with deionized water until the pH of the supernatant was neutral. After freeze-drying, multilayer MXene was obtained. Subsequently, rapid thermal exfoliation was performed: the above product was heated to 900℃ at a rate >100℃ / min in argon and held for 15 seconds to obtain expanded MXene. Finally, it was treated at 350℃ for 1.5 hours in an argon atmosphere containing 8% ammonia. After cooling, it was ultrasonically dispersed in water, centrifuged to obtain the supernatant, and freeze-dried to obtain the few-layer Ti3C2T. x MXene powder.

[0042] Take Ti3C2T x 1.0 g of MXene powder, 0.3 g of aminotrimethylenephosphonic acid (ATMP), and 1.0 g of glucose were dispersed in 50 mL of deionized water and sonicated for 30 minutes to form a uniform dispersion. The dispersion was transferred to a 100 mL high-pressure reactor lined with polytetrafluoroethylene and subjected to hydrothermal reaction in an oven at 165 °C for 9 hours. After the reaction was completed, the mixture was allowed to cool naturally, and the precipitate was collected by centrifugation. The precipitate was washed three times alternately with deionized water and ethanol to remove unreacted substances, and then dried under vacuum at 60 °C overnight to obtain solid product A.

[0043] Subsequently, solid product A, 12 g of tetraethylenepentamine (TEPA), and 1.5 g of sodium triacetoxyborohydride (NaBH(OAc)3) were added to 100 mL of N,N-dimethylformamide (DMF), and the mixture was magnetically stirred in an oil bath at 50 °C for 36 hours. After the reaction was completed, the solid was obtained by centrifugation and washed thoroughly with DMF and ethanol successively. Finally, it was vacuum dried at 60 °C for 12 hours to obtain MXene with a surface rich in primary amino (-NH2) functional groups, denoted as MXene-NH2.

[0044] (3b) Surface carboxylation treatment of Co3O4@NC composite material: 100 mg of the Co3O4@NC composite material powder obtained in step S2 was thoroughly ground and mixed with 11.8 mg of conductive carbon black and 5.9 mg of polytetrafluoroethylene (PTFE) emulsion binder in an agate mortar. The mixture was pressed into a disc with a diameter of approximately 8 mm and a thickness of approximately 0.5 mm under a pressure of 10 MPa, and fixed on a specially designed electrode clamp as the working electrode. Electrochemical anodizing was performed using a standard three-electrode system: a large-area platinum sheet as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and a 0.5 mol / L Na2SO4 aqueous solution as the electrolyte. Under the control of an electrochemical workstation (CHI760E), a constant potential of +1.5 V relative to SCE was applied to the working electrode, and polarization was maintained for 20 minutes. During this process, electrochemical oxidation occurred on the surface of the composite material (especially the carbon matrix and the edges of Co3O4), generating oxygen-containing functional groups such as carboxyl groups (-COOH). After polarization, the working electrode was removed, the surface electrolyte was gently rinsed off with deionized water, the electrode material was carefully scraped off, and vacuum dried at 60°C to obtain a carboxyl-functionalized composite material, denoted as COOH-Co3O4@NC.

[0045] (3c) Preparation of covalently linked complexes by amidation reaction: Weigh 30 mg of MXene-NH2 and 60 mg of COOH-Co3O4@NC (mass ratio 1:2) and place them in a 50 mL round-bottom flask. Add 30 mL of anhydrous N,N-dimethylformamide (DMF) as a solvent and sonicate in an ice-water bath for 1 hour to form a homogeneous and stable dispersion. Under an argon atmosphere, add 0.9 mg (equivalent to 1% of the total mass) of the condensing agent 4-(4,6-dimethoxytriazine-2-yl)-4-methylmorpholine hydrochloride (DMTMM) to the dispersion. Transfer the reaction system to an oil bath at 50 °C and react at a constant temperature for 6 hours with magnetic stirring. During the reaction, the amino groups on the surface of MXene-NH2 and the carboxyl groups on the surface of COOH-Co3O4@NC undergo an amidation reaction catalyzed by the condensing agent to form a stable amide bond (-CO-NH-), thereby achieving covalent bonding between the two phases. After the reaction was complete, the mixture was transferred to a centrifuge tube and centrifuged at 8000 rpm for 10 minutes to collect the precipitate. The precipitate was then washed three times each with DMF, ethanol, and deionized water to completely remove unreacted reagents and byproducts. The final product was dried in a vacuum drying oven at 60°C for 12 hours to obtain a black powdery covalently linked complex, denoted as MXene-NH-CO-Co3O4@NC.

[0046] S4. Reduction annealing treatment to obtain ternary composite sensitive material: Approximately 50 mg of MXene-NH-CO-Co3O4@NC composite powder was uniformly placed in a ceramic boat and then placed in a tube furnace. High-purity argon gas (200 sccm) was first introduced for 10 minutes to purge the air, then the atmosphere was switched to a mixture of argon and hydrogen (8% H2 volume fraction, total flow rate 200 sccm) as the reducing atmosphere. The temperature was raised to 280°C at a programmed rate of 3°C / min and held at this temperature for 1 hour for annealing. This mild reducing annealing process has multiple functions: firstly, it repairs structural defects that may arise from the functionalization of MXene to some extent; secondly, it selectively introduces controllable oxygen vacancies at the interface between Co3O4 nanocrystals, MXene, and the carbon matrix, which can serve as highly efficient catalytic active sites; and thirdly, it further enhances the interfacial coupling and electronic interactions between the components. After the annealing process, the material was naturally cooled to room temperature under a mixed atmosphere, and the final ternary composite sensitive material was obtained, which is the core sensitive material of this invention, denoted as MCZ material.

[0047] S5. Preparation of gas-sensitive sensing electrode: Take 10 mg of the above MCZ material and place it in an agate mortar. Gradually add an appropriate amount of terpineol as an organic binder and grind thoroughly until a uniform, viscous slurry with good coating properties is formed. Using a precision screen printing machine, the slurry is uniformly coated onto a pre-cleaned alumina ceramic substrate (6 mm × 3 mm) with interdigitated gold electrodes (50 μm wide and 50 μm apart). The coated electrode sheet is placed in a drying oven at 120°C for 2 hours to cure, allowing the terpineol to completely evaporate and form a stable sensitive film. After cooling to room temperature, a sensing electrode suitable for gas-sensitive testing is obtained. The thickness of the sensitive film of this electrode can be controlled by the screen mesh count and the number of printing cycles; in this embodiment, the film thickness is approximately 20-30 μm.

[0048] The gas-sensitive sensing electrode obtained in this embodiment is used for the detection of aldehyde flavor substances in food. It has the characteristics of high sensitivity and high selectivity, and is suitable for food quality and safety monitoring and flavor evaluation analysis.

[0049] Example 2 Comparison of the preparation of binary composite materials MXene / ZIF-8 and MXene / Co3O4: (1) Preparation of MXene / ZIF-8 composite material (comparative material 1): Weigh 50 mg of the few-layer Ti3C2T prepared by the method in step S3a of Example 1. x MXene powder, along with 200 mg of pure ZIF-8 crystals (unsupported by Co) synthesized and dried at room temperature. 2+Both were placed in an agate mortar. 2 mL of anhydrous ethanol was added as a dispersion medium, and the mixture was manually ground for 30 minutes to ensure thorough and uniform physical mixing. The mixture was then dried at 60 °C to remove the ethanol. The dried powder was placed in a tube furnace and heated to 350 °C at 5 °C / min under an argon atmosphere and held for 1 hour to remove residual organic matter on the surface and enhance interfacial contact. After cooling, MXene / ZIF-8 control material 1 was obtained.

[0050] (2) Preparation of MXene / Co3O4 composite material (comparative material 2): Weigh 50 mg of the few-layer Ti3C2T prepared by the method in step S3a of Example 1. x MXene powder was placed together with 200 mg of commercially available cobalt tetroxide nanoparticles (average particle size approximately 20-50 nm) in an agate mortar. 2 mL of anhydrous ethanol was added, and the mixture was manually ground for 30 minutes to achieve a uniform physical mixture. After drying the mixture at 60 °C, it was placed in a tube furnace and heated to 300 °C at a rate of 5 °C / min under an argon atmosphere, held for 1 hour, and then cooled to obtain MXene / Co3O4 control material 2.

[0051] Example 3 Structural characterization of the MXene / ZIF-8 / / Co3O4 composite material prepared in Example 1: To verify the structure of the prepared material, the MXene / ZIF-8 / Co3O4 ternary composite material obtained in Example 1 was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

[0052] Figure 1 The following are the SEM characterization results of the MXene / ZIF-8 / Co3O4 ternary composite material: Figure 1 (a) and (e) are SEM images with a magnification of 2.00 kX and a scale bar of 5 μm; Figure 1 (b) is a SEM image with a magnification of 5.00 kX and a scale bar of 2 μm; Figure 1 (c) is a SEM image with a magnification of 20.00 KX and a scale bar of 500 nm; Figure 1 (d) is a SEM image with a magnification of 1.00 KX and a scale bar of 10 μm; Figure 1 (f) is a SEM image with a magnification of 10.00 KX and a scale bar of 1 μm; Figure 1 (g)-(i) are SEM images with a magnification of 20.00 KX and a scale bar of 200 nm; Figure 1 (a) Showing the overall packing morphology of the composite material; Figure 1 (b) Demonstrating the local dispersion state of the composite material; Figure 1(e) Local aggregation and porosity; Figure 1 (c) Show the surface morphology and microstructure of individual particles in the composite material; Figure 1 (d) Demonstrates the macroscopic packing and spatial distribution characteristics of composite materials; Figure 1 (f) Demonstrating the morphology and dispersibility of functional particles in the composite material; Figure 1 (g)-(i) show the interfacial bonding state and surface microstructure of the composite material.

[0053] Figure 2 TEM characterization results of the MXene / ZIF-8 / Co3O4 ternary composite material: Figure 2 (e) and (f) are TEM images with a magnification of 4000×; Figure 2 (a) and (c) are TEM images with a magnification of 5000×; Figure 2 (b) and (d) are TEM images with a magnification of 8000×; Figure 2 (h) is a TEM image with a magnification of 25k×; Figure 2 (g) and (i) are TEM images with a magnification of 40k×.

[0054] Figure 2 (a)-(c) show the overall morphology of the composite material and the dispersion state of each component; Figure 2 (d)-(f) show the interface bonding state between MXene and ZIF-8 / Co3O4; Figure 2 (g)-(h) show the lattice structure of Co3O4 and the layered features of MXene; Figure 2 (i) Verify the crystallinity of Co3O4 and the phase composition of the composite system.

[0055] like Figure 1 The SEM characterization results show that the prepared MXene / ZIF-8 / Co3O4 ternary composite material exhibits obvious multi-level composite structure characteristics. Figure 1 (d) As shown in the SEM image, the composite material exhibits a loosely packed state with no obvious large agglomerates. The spatial distribution at the macroscopic scale is uniform, indicating that the preparation process effectively avoids excessive agglomeration of the material, providing ample channels for the mass transfer process. Figure 1 (e) As shown in the SEM image, the composite material exhibits abundant pore structures (as indicated by the red dashed box area). These pores are formed by the stacking of component particles, further verifying the porous nature of the material and contributing to its increased specific surface area; Figure 1 (f) As shown in the SEM images, the functional particles exhibit a regular polyhedral morphology (approximately 0.5-1 μm in size) and are uniformly dispersed on the surface of the matrix material without obvious agglomeration, demonstrating that the components achieved synergistic dispersion during the preparation process; Figure 1As shown in the SEM images (g)-(i), the surface of the matrix material exhibits typical layered wrinkles, and a tight physical contact interface (without obvious gaps) is formed between the functional particles and the matrix. Furthermore, there is no damage or peeling on the particle surface, indicating that the interfacial bonding strength between the two is high, providing structural protection for the rapid transport of electrons / ions.

[0056] Under low magnification, the material exhibits a typical MXene layered stacking morphology, with a lamellar framework as the main structure. There are certain degrees of wrinkles and stacking gaps between the layers, which facilitates the diffusion and transport of gas molecules. A large number of relatively uniform polyhedral particles are observed evenly distributed on the surface and interlayer regions of the MXene sheets. Their morphology is highly consistent with the typical dodecahedral structure of ZIF-8, indicating that ZIF-8 has been successfully grown in situ on the MXene surface without significant agglomeration.

[0057] With increasing magnification, the ZIF-8 crystal boundaries became clearer, the crystal faces were intact, and the interface between the ZIF-8 crystal and the MXene substrate was tightly bonded. No obvious peeling or detachment was observed, indicating that a stable composite interface was formed between ZIF-8 and MXene through electrostatic interactions or surface functional group interactions. This "layered substrate-MOF crystal" structure not only effectively suppressed the recombination of MXene but also provided abundant anchoring sites for the subsequent loading and dispersion of metal oxides. Further observation of high-magnification SEM images revealed a large number of smaller nanoparticles distributed on the surface of the ZIF-8 crystal and at the edges of the MXene layers, with a surface roughness significantly higher than that of pure ZIF-8 crystals. These nanoparticles can be attributed to Co3O4 nanocrystals formed in situ after the heat treatment of ZIF-8. The introduction of Co3O4 transformed the originally smooth ZIF-8 surface into a porous, rough structure, thereby significantly increasing the specific surface area and the number of surface active sites, which is beneficial for the adsorption and surface reaction of target gas molecules.

[0058] like Figure 2 The TEM characterization results shown further validate the successful construction of the ternary composite structure. Figure 2 TEM images show that MXene retains its transparent two-dimensional sheet structure with relatively thin sheets and noticeable wrinkles in some areas, indicating that no severe structural damage occurred during the composite process. ZIF-8 derived structures are loaded onto the surface and edges of the MXene sheets, with uniform particle distribution, consistent with the SEM results.

[0059] In high-resolution TEM images, a tight contact interface is clearly observed between the MXene sheets and Co3O4 nanocrystals, with distinct lattice fringes appearing in localized areas, indicating the good crystallinity of Co3O4. This tight coupling between different phases facilitates the construction of continuous electron transport channels, reduces interfacial resistance, and provides a structural basis for rapid charge migration during gas-sensitive responses. Furthermore, the highly dispersed state of the Co3O4 nanocrystals effectively avoids the aggregation problem of metal oxides, enhancing the structural stability and reactivity of the composite material.

[0060] Based on the combined SEM and TEM characterization results, the MXene / ZIF-8 / Co3O4 ternary composite material can be considered a successful construction of a hierarchical heterogeneous structure system with MXene as the conductive framework, ZIF-8 as the porous support structure, and Co3O4 as the active functional component. This structure not only combines the high conductivity of MXene, the pore enrichment capacity of ZIF-8, and the surface catalytic activity of Co3O4, but also achieves synergistic optimization of structure and performance through interfacial synergistic effects, providing an important microstructural basis for its high sensitivity, rapid response, and stable operation in hexanal gas detection.

[0061] Example 4 Gas-sensing performance testing of composite materials—response to different concentrations of hexanal: The MXene / ZIF-8 / Co3O4, MXene / ZIF-8, and MXene / Co3O4 materials prepared in Examples 1 and 2 were respectively mixed with an appropriate amount of terpineol to form slurries, which were then coated onto a ceramic tube substrate with interdigitated gold electrodes. After drying at 120°C for 2 hours, gas-sensitive sensing electrodes were formed. The electrodes were mounted on a CGS-8R gas-sensitive testing platform and connected to the testing circuit.

[0062] Under operating conditions of 100℃ and 60% relative humidity (RH), sensors made of three different materials were exposed to hexanal gas with concentration gradients of 200ppb, 600ppb, 1000ppb, 1400ppb, 1800ppb, and 2200ppb, respectively, and their resistance changes were recorded. The gas response value ΔR is defined as the ratio of the sensor's resistance in air (Ra) to its resistance in the gas being measured (Rg), i.e., ΔR = Ra / Rg.

[0063] Depend on Figure 3 It can be seen that all material systems exhibit a significant concentration-dependent response to hexanal, meaning that the sensor's response value continuously increases as the hexanal concentration gradually increases from a low level. This indicates that the prepared materials possess typical quantitative response characteristics to hexanal, and the adsorption-reaction-electron transfer process of hexanal on the material surface can be stably converted into an electrical signal, consistent with the working principle of conductivity-type aldehyde gas sensors.

[0064] The response curves show an overall linear upward trend, indicating that the interaction between the active sites on the material surface and hexanal molecules is highly reproducible. The expression for the multi-concentration fitting curve of the MXene / ZIF-8 material is y = 0.00276x + 0.77229, R0 2 =0.98024, the expression for the multi-concentration fitting curve of the MXene / Co3O4 material is y=0.00078x+0.85655, R 2 =0.98237, while the expression for the multi-concentration fitting curve of the ternary composite material MXene / ZIF-8 / Co3O4 is y=0.00827x+0.20426, R 2 =0.99187. Compared with the combination of the two materials, the ternary composite material has a higher response value to hexanal. The response value of the MXene / ZIF-8 / Co3O4 material electrode to hexanal at a concentration of 2200 ppb is about 3 times that of the MXene / ZIF-8 material electrode and about 10.6 times that of the MXene / Co3O4 material electrode.

[0065] The ultra-high sensitivity exhibited by the MXene / ZIF-8 / Co3O4 composite material can be attributed to the successful synergy and interface engineering among the three components. Compared to MXene / ZIF-8: While MXene / ZIF-8 possesses the conductive network of MXene and the porous adsorption properties of ZIF-8, it lacks the key catalytic active center of Co3O4, resulting in weak catalytic oxidation of hexanal and thus the lowest response. Compared to MXene / Co3O4: Although MXene / Co3O4 combines the conductivity of MXene and the catalytic activity of Co3O4, commercial Co3O4 particles are large, prone to aggregation, and only have physical contact with MXene, resulting in high interfacial resistance and poor accessibility to active sites.

[0066] Example 1 yielded uniformly sized, highly dispersed Co3O4 nanocrystals (2-5 nm) via ZIF-8 confined pyrolysis, and a covalent interface was constructed between Co3O4@NC and MXene through an amidation reaction. This structure combines: (a) the molecular enrichment effect of ZIF-8-derived porous carbon; (b) the highly efficient catalytic activity of nano-Co3O4; (c) the high-speed electron transport network of MXene; and (d) the low contact resistance and excellent stability brought by the covalent interface. These four factors work synergistically to achieve superior sensitivity to hexanal.

[0067] Example 5 Effects of different MXene ratios on gas-sensing performance: To investigate the effect of MXene content on the gas-sensing performance of ternary composite materials, three MXene / ZIF-8 / Co3O4 composite materials with different MXene loadings were prepared, and the three materials were fabricated into gas-sensing electrodes. Taking hexanal as an example, under the same test conditions (operating temperature 100℃, relative humidity 60%), the materials were exposed to hexanal gas at concentrations of 200 ppb, 600 ppb, 1000 ppb, 1400 ppb, 1800 ppb, and 2200 ppb, respectively, and the resistance changes were recorded and the response values ​​were calculated. The standard curve fitting results of the response of each material to different concentrations of hexanal are shown below. Figure 4 As shown.

[0068] The fitted curves show that all three composite materials exhibit good linear response to hexanal, with the response value increasing with increasing gas concentration, indicating that the materials possess the ability to quantitatively detect hexanal. Among them, the material with a ratio of 20 mg MXene + 50 mg ZIF-8 / Co3O4 exhibits the highest response slope and the best goodness of fit, with its standard curve equation being y = 0.06651x - 19.00425, R0. 2 =0.98417; The curve equation for 10mg MXene + 50mg ZIF-8 / Co3O4 is y = 0.00827x - 0.20426, R 2 =0.99187; The curve equation for 30mg MXene + 50mg ZIF-8 / Co3O4 is y = 0.04303x - 11.52034, R 2 =0.99383. This indicates that at a loading of 20 mg MXene, the material exhibits the highest sensitivity to hexanal and the widest linear response range.

[0069] The impact of MXene content on gas sensing performance is mainly attributed to its dual role in the composite system: on the one hand, MXene acts as a conductive framework, responsible for constructing continuous electron transport channels. Too low a content (e.g., 10 mg) leads to an incomplete conductive network, decreased electron transport efficiency, and a weaker response signal. On the other hand, too high a content (e.g., 30 mg) may cover part of the active surfaces of ZIF-8 and Co3O4, hindering effective contact between gas molecules and catalytic sites. It may also weaken the molecular sieving effect of ZIF-8 and the catalytic activity of Co3O4, thus resulting in a decrease in response value. A formulation of 20 mg MXene achieves the optimal balance between conductivity, accessibility of active sites, and interfacial synergistic effects, thus exhibiting the best overall gas sensing performance.

[0070] In summary, by systematically optimizing the mass ratio of MXene to ZIF-8 / Co3O4, the detection sensitivity and linear response characteristics of the composite material for aldehydes can be significantly controlled. The optimal ratio of 20 mg MXene + 50 mg ZIF-8 / Co3O4 provides important process parameter guidance for the practical application of this material in real-time monitoring of meat fat oxidation.

[0071] Example 6 Sensor repeatability and stability testing: To verify the reliability of the sensor, the MXene / ZIF-8 / Co3O4 material electrode (with a ratio of MCZ-20) prepared in Example 1 was selected and exposed to hexanal gas at a concentration of 1400 ppb five times consecutively under the conditions of 30°C and 60%RH, and the response value was recorded each time.

[0072] like Figure 5 As shown, the test results indicate that the composite electrode exhibits minimal fluctuations in its response signal across five repeated tests. In particular, the sensor with the 20mg MXene + 50mg ZIF-8 / Co3O4 ratio shows the smallest fluctuations in its five response values, demonstrating the best reproducibility. This indicates that the material formulation and preparation process possess high stability, ensuring the consistency and reliability of the sensor's output signal.

[0073] This excellent repeatability is mainly attributed to the stable microstructure of the MXene / ZIF-8 / Co3O4 ternary composite material itself. MXene, as a robust two-dimensional conductive substrate, effectively anchors the ZIF-8 and Co3O4 nanoparticles, preventing them from agglomerating or migrating during testing; at the same time, the stable heterojunction interface formed among the three ensures the reversibility and cycling stability of the gas-sensitive reaction.

[0074] Example 7 Operating temperature optimization: To determine the optimal operating temperature, the responses of different materials to 1400 ppb hexanal were tested at different temperatures. The test temperature range was 60°C to 140°C, in 20°C increments.

[0075] like Figure 6 , 7 As shown in the figures, a comprehensive analysis of the two charts reveals that the gas-sensitive responses of all tested sensors exhibit typical temperature dependence, with the response values ​​showing a "volcano-shaped" curve that first increases and then decreases with temperature. This phenomenon stems from the dynamic balance between temperature and processes such as adsorption, activation, and reactive desorption of gas molecules on the material surface. When the temperature is too low, gas molecules cannot obtain enough energy to overcome the reaction energy barrier, and react with chemically adsorbed oxygen (such as O2) on the material surface. 2- O -Effective redox reactions occur, resulting in a lower response. When the temperature is too high, physical adsorption of gas molecules becomes dominant, and their residence time on the material surface is too short, so they are desorbed before they can react, which also leads to a decrease in response value.

[0076] like Figure 6 As shown, the optimal operating temperature of the binary composite materials MXene / ZIF-8 and MXene / Co3O4 is around 120°C, while the optimal operating temperature of the ternary composite material (20mg MXene + 50mg ZIF-8 / Co3O4) is significantly lowered to around 100°C, and its peak response value at this temperature is much higher than that of the two binary materials. This result proves that the pn heterojunction successfully constructed between ZIF-8 and Co3O4 is the core of the improved sensitivity, and the introduction of MXene not only provides an efficient electron transport channel, but its unique surface chemical properties may also have a certain catalytic effect on the activation of gas molecules, together significantly reducing the activation energy required for the gas-sensitive reaction, thus allowing the optimal operating temperature to move to a lower range.

[0077] Further from Figure 7 The effect of MXene content on the temperature characteristics of the ternary composite material can be observed. The ratio of 20 mg MXene + 50 mg ZIF-8 / Co3O4 exhibits the highest response peak, while the response performance of the device decreases when the MXene content is increased to 30 mg. This indicates that there is an optimal MXene loading: an appropriate amount of MXene can form a complete conductive network and produce sufficient synergistic effect with the metal oxide; excessive MXene may partially cover or shield the active sites of ZIF-8 and Co3O4, weakening the dominant role of the heterojunction, thus leading to a decrease in gas-sensing performance.

[0078] In summary, through temperature characteristic optimization, the optimal operating temperature of the MXene / ZIF-8 / Co3O4 ternary composite material was determined to be around 100°C. This temperature not only ensures the sensor's high sensitivity to the target gas, but its relatively mild operating conditions also help reduce device power consumption, improve long-term operational stability, and extend service life, laying a solid foundation for its application in practical scenarios such as real-time monitoring of meat fat oxidation.

[0079] Example 8 Moisture resistance test: The effect of ambient humidity on the performance of the MXene / ZIF-8 / Co3O4 (MCZ-20) sensor prepared in Example 1 was evaluated. At an operating temperature of 100°C, the relative humidity (RH) in the test chamber was gradually increased from 40% to 90%, and the baseline resistance (Ra) of the sensor and its response (ΔR) to a fixed concentration (1400 ppb) of hexanal were measured at each humidity level.

[0080] like Figure 8 As shown, the test results indicate that ambient humidity has a significant impact on sensor performance. As the relative humidity increases from 40% to 90%, the sensor's baseline resistance (Ra) exhibits a clear monotonic increasing trend. This phenomenon can be attributed to the competitive adsorption and ionization mechanism of water molecules on the material surface. Specifically, water vapor molecules (H2O) preferentially occupy active sites on the material surface, sites that are originally used to adsorb oxygen molecules (O2) from the air and form reactive oxygen species (such as O2). 2- O - The adsorbed water molecules will further dissociate to form hydroxyl groups (-OH). - These hydroxyl groups act as electron donors, injecting electrons into the sensor's sensitive material, causing a decrease in its hole carrier concentration and thus an increase in baseline resistance.

[0081] Meanwhile, the data in the figure shows that the sensor's gas response value decreases with increasing humidity. This is mainly due to two factors: First, the occupation of surface active sites by water molecules directly reduces the number of active centers for effective reaction between hexanal molecules and the material; second, the water molecule layer that may form on the material surface physically hinders the diffusion of larger hexanal molecules into the active sites inside the material, thereby inhibiting the complete gas-sensing reaction process and resulting in a weakened response signal.

[0082] In summary, the MXene / ZIF-8 / Co3O4 ternary composite electrode exhibits both excellent gas-sensing performance and a certain degree of resistance to environmental humidity interference. This characteristic demonstrates that when applying this sensor to monitor meat fat oxidation in high-humidity environments such as the reheating process of stewed beef, it possesses a certain degree of resistance to humidity interference, although some measurement errors may still occur. Future research could focus on further hydrophobic modification of the composite material or the development of advanced signal processing algorithms to further improve the composite material's resistance to humidity interference and detection accuracy in complex environments, ensuring its practicality and reliability in the field of food safety monitoring.

[0083] Example 9 Dynamic response-recovery characteristic test: The dynamic response-recovery curve of the MXene / ZIF-8 / Co3O4 (MCZ-20) sensor prepared in Example 1 to 1000ppb hexanal was tested at an operating temperature of 100℃ and RH of 60%.

[0084] like Figure 9 As shown, the test results demonstrate that the sensor exhibits excellent rapid response and recovery capabilities. When the sensor is exposed to the target gas, its resistance value drops rapidly, and the response time ( T res The time required for the resistance change to reach 90% of its saturation value (defined as the time required) is only 29 seconds. After the target gas is removed, the sensor resistance can quickly recover to the initial baseline level, and its recovery time (…) is very short. T rec The resistance change (defined as the time required for the resistance to recover to 90% of its initial value) is 25 seconds. This rapid and highly reversible resistance change demonstrates the fast kinetics of the gas-sensitive reaction and the good balance between adsorption and desorption processes.

[0085] This outstanding dynamic performance is primarily attributed to the unique microstructural advantages of the MXene / ZIF-8 / Co3O4 composite material. The three-dimensional porous conductive network formed by MXene provides unobstructed channels for the rapid diffusion of gas molecules within the sensitive layer, significantly reducing the time required for gas molecules to reach and leave the active reaction sites. Furthermore, the extremely high conductivity of MXene itself enables the instantaneous conduction of electrical signal changes generated by the gas-sensitive reaction, avoiding response delays caused by charge accumulation. In addition, the uniform distribution of ZIF-8 and Co3O4 nanoparticles on the MXene sheets forms numerous active interfaces that are easily accessible to gas molecules, further accelerating the rate of surface redox reactions.

[0086] In summary, the rapid response and recovery characteristics of the MXene / ZIF-8 / Co3O4 composite material sensor enable it to perform real-time and dynamic tracking and monitoring of hexanal gas produced by fat oxidation during the stewing and reheating of meat, providing key technical support for timely and accurate reflection of changes in food quality.

[0087] Example 10 Selective testing: To evaluate the detection reliability of the sensor in complex gas environments, selective tests were conducted. Under the same conditions (100℃, 60%RH), the response of the MXene / ZIF-8 / Co3O4 (MCZ-20) sensor prepared in Example 1 to various gases with a concentration of 1000 ppb was tested, including: hexanal, octanal, nonanal, heptanal, (E)-2-octenal, (E)-2-undecenal, and 1-octen-3-ol.

[0088] like Figure 10 As shown, the test results indicate that the MXene / ZIF-8 / Co3O4 composite material exhibits significantly different response signals to the aforementioned gases. The percentage of response to aldehydes is higher than that to all other tested gases, demonstrating strong selectivity. This relatively low response to other volatile alcohols suggests that the gas-sensitive active sites or reaction pathways of this material possess a special affinity for or higher reactivity towards aldehyde molecules.

[0089] This excellent selectivity can be attributed to the ingenious composition and structural design of the composite material: First, Co3O4, as a metal oxide with abundant oxygen vacancies and variable valence states, has intrinsic activity for the catalytic oxidation of aldehyde groups (-CHO), and may preferentially catalyze the reaction of aldehydes on its surface; second, ZIF-8, as a porous metal-organic framework, may play a preliminary sieving role for gas molecules due to its regular pore structure, and its pore size is more matched with the dynamic diameter of hexanal molecules, thus facilitating the diffusion of hexanal molecules to internal active sites; finally, MXene, as a conductive substrate, not only ensures rapid electron transport, but its surface functional groups may also generate specific interactions with hexanal molecules, further enhancing the response signal.

[0090] In summary, the high selectivity of the MXene / ZIF-8 / Co3O4 composite material for aldehydes enables it to accurately and specifically capture changes in the concentration of aldehydes—a key marker of fat oxidation—amidst the complex volatile background generated during the reheating of stewed beef. This characteristic significantly reduces the interference of other coexisting volatile substances (such as other alcohols) on the detection results, providing an important material basis for accurately evaluating the degree of fat oxidation in meat based on specific marker concentrations, and ensuring the specificity and reliability of the detection.

[0091] Example 11 Application in the detection of aldehydes in food: The MXene / ZIF-8 / Co3O4 (MCZ-20) gas-sensitive electrode prepared in Example 1 of this invention is integrated into a portable detection device for the detection of aldehydes in actual food samples. Taking stewed and reheated beef as an example, 5g of sample is placed in a 20mL headspace vial, sealed, and equilibrated in a 60℃ water bath for 30 minutes. Subsequently, the gas inlet of the device is connected to the headspace via a polytetrafluoroethylene tube, allowing headspace gas to flow through the sensor surface at a flow rate of 100mL / min for 30 seconds. The sensor resistance change is measured and recorded in real time by the built-in circuit of the device. Based on the pre-established standard curve (as obtained in Examples 4 and 5), qualitative and quantitative analysis of aldehydes such as hexanal in the headspace can be achieved, thereby rapidly assessing the degree of fat oxidation and freshness of meat.

[0092] Overall performance comparison: The main performance indicators of Example 1 of the present invention (MXene / ZIF-8 / Co3O4, MCZ-20) are compared with those of the comparative materials (MXene / ZIF-8, MXene / Co3O4) and common single metal oxide sensors (SnO2 sensors) in the prior art. The results are summarized in Table 1.

[0093] Table 1 Performance Comparison Results As shown in Table 1, the MXene / ZIF-8 / Co3O4 ternary composite gas-sensitive electrode provided by the present invention is significantly superior to the comparative materials and common single metal oxide sensors in the prior art in terms of sensitivity, response / recovery speed, optimal operating temperature, moisture resistance and selectivity. It has excellent comprehensive performance and is especially suitable for complex and high-humidity food detection environments.

[0094] Based on the above technical solutions, the core innovation of this application lies in the ternary composite structure, interface engineering design, and adaptability to food scenarios. Therefore, there are still various alternative or extended implementation methods. As long as the basic principle of this invention, "synergistic enhancement of aldehyde detection performance based on ZIF-8 enrichment + Co3O4 catalysis + MXene conductive framework," is not changed, they all fall within the protection scope of this application. For example, in terms of MOF selection, ZIF-8 can be replaced by materials with similar hydrophobicity and pore size characteristics, such as ZIF-67, ZIF-L, and UiO-66-NH2, whose microporous structure can also effectively screen for medium molecular weight aldehydes such as hexanal. If it is desired to further improve the adsorption capacity, materials such as MOF-74 and MIL-101 with adjustable pore size or hierarchical pore structure can also be considered to enhance the enrichment efficiency under headspace conditions. In the metal oxide part, Co3O4 can be replaced with transition metal oxides such as MnO2, NiO, CuO, and ZnCo2O4, which have comparable catalytic performance. By utilizing their active centers with oxygen vacancies or reversible valence states, the catalytic oxidation ability of aldehydes can be improved, and similar electron release and gas-sensitive response effects can still be achieved.

[0095] In the conductive substrate portion, MXene (Ti3C2T) x While possessing an excellent layered conductive structure, if practical preparation conditions are limited, highly conductive carbon materials such as graphene, reduced graphene oxide (rGO), carbon nanotubes (CNTs), and nitrogen-doped carbon (NC) can be used as alternatives to serve as a high-speed electron transport network, ensuring that electrons generated by catalytic oxidation can be efficiently output to the electrode. In practical interface construction methods, the composite method of ZIF-8 / Co3O4 and MXene is not limited to electrostatic self-assembly; solvothermal methods, layer-by-layer self-assembly, in-situ growth, and spray drying can also be employed. As long as a stable interfacial bond can be formed, enabling the three to constitute a continuous "adsorption-catalysis-conductivity" pathway, the technical effects of this invention can be achieved.

[0096] In terms of applications, this technology is not only applicable to hexanal detection, but can also be extended to other aldehydes formed during fat oxidation (such as octanal and nonanal), as well as key flavor compounds such as volatile sulfides and ketones in heated meat. Furthermore, the materials of this invention are also suitable for industrial food storage processes, shelf-life monitoring of meat products, and quality assessment in cold chain transportation. In addition, the headspace sampling method proposed in this application can be fine-tuned according to specific food types, such as changing the equilibrium temperature, the volume ratio of the enclosed space, or using photothermal heating to accelerate volatilization, thereby expanding the applicability of this technology in various food systems.

[0097] In summary, various alternative materials, composite methods, and application scenarios can be flexibly adjusted without deviating from the core design concept of this invention, providing sufficient scalability for the large-scale application and engineering promotion of this technology in the field of food testing.

[0098] Example 12: Application of MXene / ZIF-8 / Co3O4 composite gas-sensitive electrode for aldehyde detection in meat products and smart food cover This embodiment uses the MXene / ZIF-8 / Co3O4 (MCZ-20) gas-sensitive electrode prepared in Example 1 for the rapid detection of aldehydes in meat products. The detection of aldehyde content reflects the degree of fat oxidation in meat. It is also suitable for reheating meat dishes, assisting in determining the number of reheating cycles and the degree of reheating. It is applicable to various common meats such as chicken, beef, lamb, and pork. This embodiment provides two optional detection device implementation methods: Method 1 is a portable detection device based on a three-electrode system (suitable for laboratory or on-site sampling); Method 2 is a convenient home detection device integrated into a smart food cover (suitable for everyday household reheating scenarios).

[0099] Method 1: Portable detection device based on a three-electrode system 1. Setup of the detection device Using the gas-sensitive sensing electrode prepared in Example 1 as the working electrode, the Ag / AgCl electrode as the reference electrode, and the platinum wire electrode as the counter electrode, a three-electrode system was assembled and connected to an electrochemical workstation and signal acquisition module to construct a portable detection device. This device is compact, can be powered by a power bank or button battery, and is suitable for on-site detection in laboratories, restaurants, or homes.

[0100] 2. Sample preparation and testing procedures Four common meats—fresh chicken, beef, lamb, and pork—were selected and prepared into standard meat dish samples. Different reheating conditions were set for each meat sample (reheating once, twice, and three times). Conventional household microwave reheating was used, with a reheating time of 2 minutes and a reheating power of 700 W. Three parallel samples were prepared for each reheating condition.

[0101] Each meat reheated sample was placed in a sealed detection container. After the sample stably released aldehyde gas, the sensing electrode of the above three-electrode system was inserted into the sealed container to ensure that the electrode sensitive layer was in full contact with the gas released from the sample. The electrode resistance change signal was collected in real time through an electrochemical workstation, and the electrode response time and resistance change difference were recorded.

[0102] 3. Test Results This gas-sensitive electrode exhibits excellent response performance to characteristic aldehydes such as hexanal, octanal, and nonanal released during the reheating process of four types of meat samples. It demonstrates rapid response (response time 29 seconds) and high sensitivity (response value 18.4 for 2200 ppb hexanal). With increasing reheating cycles, the aldehyde content released by each meat sample shows a significant upward trend, and the electrode response value increases accordingly, effectively distinguishing samples reheated once, twice, and three times. While the aldehyde release varies slightly among different meats due to differences in fat content and oxidation characteristics, the electrode consistently detects all aldehydes with a regular pattern, demonstrating good compatibility with various meats including chicken, beef, lamb, and pork.

[0103] Method 2: Convenient home detection device integrated into smart food covers 1. Intelligent food cover structure and integration The smart food cover is dome-shaped and made of food-grade heat-resistant material (such as polypropylene or silicone). Its bottom edge has a flexible sealing ring, which can form a sealed or semi-sealed space with the food container (such as a plate or bowl), or, when covered, seal tightly against the table or countertop where the container is placed, ensuring the container and its contents are completely enclosed within the sealed space. A miniature portable detection device is installed on the top or side wall of the inner wall of the food cover. This device contains: (1) Gas-sensitive sensing electrode (using MCZ-20 material prepared in Example 1, which can be used alone as a two-electrode system or integrated as a micro reference / counter electrode); (2) A miniature ceramic heating element and a temperature sensor can heat the electrode and maintain it at 100±5℃ (i.e., the optimal operating temperature determined by this invention). (3) A rechargeable micro button battery (capacity ≥ 50mAh) to power heating and data acquisition; (4) Near Field Communication (NFC) unit, used to receive trigger signals from the mobile terminal and transmit detection data back; The overall package size does not exceed 40mm×40mm×15mm.

[0104] 2. Usage and Workflow Users place the dish to be reheated (such as stewed beef, braised pork, fried food, etc.) in a container, cover it with a smart food cover, ensuring the cover seals tightly against the container's edge (or place the container on a table or countertop, cover it with the food cover, ensuring the sealing ring fits snugly against the table). During the natural cooling or heat preservation process after reheating (or with active micro-heating by the user), aldehyde flavor compounds produced by fat oxidation accumulate in the headspace inside the food cover. The user brings an NFC-enabled mobile terminal (such as a smartphone) close to the sensing area on the food cover, triggering the NFC unit to automatically activate the gas-sensitive sensing electrode for resistance signal acquisition. The acquired signal is transmitted via NFC to an application (APP) installed on the mobile terminal. The APP has a pre-established quantitative relationship model and grading thresholds (based on the standard curves obtained in Examples 4 and 5 and the selective data from Example 10), which calculates and outputs in real time the dish's "reheated flavor level" (e.g., slight, obvious, severe) or "fat oxidation degree level," displaying it visually on the phone screen. The entire detection process takes no more than 90 seconds.

[0105] 3. Technical Effects Non-destructive testing: No contact or damage to the food is required, and the food is safe to eat after testing.

[0106] Optimal operating temperature guarantee: The built-in miniature heating element ensures that the sensor always operates near the optimal temperature of 100°C, guaranteeing high sensitivity and fast response.

[0107] Low power consumption and convenience: The battery consumes less than 10mAh per test and can support hundreds of tests; NFC triggering, zero power consumption in standby mode.

[0108] Moisture resistance: Based on Example 8, it maintains a stable response within a relative humidity range of 40-90%, adapting to the high humidity environment inside the vegetable cover.

[0109] Versatility across multiple meats: It has been verified that it can stably detect chicken, beef, mutton, pork, etc., and distinguish between different reheating times.

[0110] Scope of application and expansion of the two methods Method 1 (three electrodes + electrochemical workstation) is suitable for scenarios requiring precise quantification, data recording, or scientific analysis, such as food quality inspection and laboratory research; Method 2 (smart food cover + NFC) is suitable for everyday rapid testing scenarios such as homes and restaurants. Both methods are based on the same core sensitive material and have the same detection principle, allowing users to choose or combine them according to their actual needs.

[0111] The technical solution of this embodiment can be further extended to areas such as food quality monitoring during food delivery, safety reminders for reheating leftover food in collective canteens, and auxiliary evaluation of dietary flavors for the elderly or people with special dietary needs. Integrating gas-sensitive sensing electrodes into food covers, food coverings, or portable detection pens of different sizes and shapes all fall within the protection scope of this invention.

[0112] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention. Applications, modifications, and variations of the invention will be readily apparent to those skilled in the art.

[0113] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A method for preparing a gas-sensitive sensing electrode for detecting aldehyde flavor compounds in food, characterized in that, Includes the following steps: S1, ZIF-8 crystals were immersed in Co 2+ alcohol solution, followed by slow solvent evaporation at 60-80 °C to obtain Co 2+ @ZIF-8 precursors; S2, Co 2+ The ZIF-8 precursor is placed in a tube furnace and heated to 380-420℃ at a rate of 1-3℃ / min under an inert atmosphere and held for 1-2 hours to pyrolyze it into a nitrogen-doped porous carbon-confined Co3O4 nanocrystalline composite material, wherein the size of the Co3O4 nanocrystals is 2-5nm. S3, Interfacial covalent modification and heterojunction assembly: (3a) Amination treatment of Ti3C2Tx MXene was performed to obtain surface-aminated MXene-NH2; (3b) The Co3O4 nanocrystalline composite material obtained in step S2 is subjected to surface carboxylation treatment; (3c) Disperse the MXene-NH2 and carboxylated Co3O4 nanocrystalline composite material in a solvent, add a condensing agent, and react at 40-60℃ for 4-8 hours. Covalently link the two through an amidation reaction to obtain a covalently linked composite. S4. Anneal the covalently linked composite obtained in step S3 in an argon mixed atmosphere containing 5-10% by volume hydrogen, heating it to 250-300℃ at 2-5℃ / min and holding it at that temperature for 0.5-1.5 hours to obtain a ternary composite sensitive material. S5. The ternary composite sensitive material obtained in step S4 is made into a slurry, coated onto the interdigital electrode, and dried to obtain a gas-sensitive sensing electrode.

2. The method for preparing the gas-sensitive sensing electrode for detecting aldehyde flavor substances in food as described in claim 1, characterized in that, The amination treatment in step S3a includes the following steps: S3a1, Ti3C2T after intercalation stripping treatment x MXene, aminotrimethylenephosphonic acid, and glucose were dispersed in deionized water at a mass ratio of 1:(0.1-0.5):(0.5-2) and subjected to hydrothermal reaction at 150-180℃ for 6-12 hours. After the reaction, the mixture was centrifuged, washed, and dried to obtain solid product A. S3a2. Solid product A, tetraethylenepentamine and sodium triacetoxyborohydride are dispersed in N,N-dimethylformamide at a mass ratio of 1:(5-20):(0.5-3) and reacted at 40-60℃ for 24-48 hours. After the reaction, the solid is separated, washed and dried to obtain MXene-NH2.

3. The method for preparing the gas-sensitive sensing electrode for detecting aldehyde flavor substances in food as described in claim 1, characterized in that, The surface carboxylation treatment in step S3b is as follows: the composite material obtained in step S2, conductive carbon black and polytetrafluoroethylene binder are mixed at a mass ratio of 85:10:5 to prepare a working electrode; a platinum sheet is used as the counter electrode and a saturated calomel electrode is used as the reference electrode, and anodic polarization is performed in 0.5 M sodium sulfate electrolyte at a potential of +1.2V to +1.8V for 10-30 minutes.

4. The method for preparing the gas-sensitive sensing electrode for detecting aldehyde flavor substances in food as described in claim 1, characterized in that, Step S3c specifically involves: mixing MXene-NH2 with the carboxylated composite material at a mass ratio of 1:1 to 1:3 to obtain a mixed powder; dispersing the mixed powder in N,N-dimethylformamide or N-methylpyrrolidone solvent to form a dispersion with a concentration of 1-5 mg / mL; subsequently adding the condensing agent 4-(4,6-dimethoxytriazine-2-yl)-4-methylmorpholine hydrochloride, and stirring at a constant temperature of 50°C for 6 hours under argon protection; after the reaction is completed, centrifuging, washing, and drying are performed to obtain the covalently linked composite material; the amount of condensing agent added is 0.5-2% of the total mass of MXene-NH2 and the composite material.

5. The method for preparing the gas-sensitive sensing electrode for detecting aldehyde flavor substances in food as described in claim 1, characterized in that, In step 3a, Ti3C2T x MXene is obtained through intercalation and stripping, specifically as follows: (1) Mix Ti3AlC2 powder with lithium fluoride and potassium fluoride in a mass ratio of 1:(1.5-2.5):(1.5-2.5), react at 550-650℃ for 2-4 hours under argon protection, cool and wash with water until neutral, and dry to obtain the first intermediate; (2) The first intermediate is heated to 800-1000℃ at a rate of not less than 100℃ / min under an argon atmosphere and held for 10-30 seconds to obtain the second intermediate; (3) The second intermediate is treated in an argon mixed atmosphere containing 5-10% ammonia by volume at 300-400℃ for 1-2 hours.

6. The gas-sensitive sensing electrode obtained by the preparation method of the gas-sensitive sensing electrode for detecting aldehyde flavor substances in food as described in any one of claims 1-5.

7. The application of the gas-sensitive sensing electrode as described in claim 6 in the detection of aldehydes in food.

8. The application as described in claim 7, characterized in that, Aldehydes include one or more of hexanal, octanal, nonanal, heptanal, (E)-2-octenal, and (E)-2-undecenal.

9. The application as described in claim 8, characterized in that, By exposing a gas-sensitive electrode to the headspace of the food being tested, qualitative and quantitative detection of aldehydes can be achieved by measuring changes in resistance.

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