Composite sensing material, preparation thereof and application of composite sensing material in liquid food spoilage monitoring

Abstract

The invention discloses a composite sensing material as well as preparation and application thereof in liquid food spoilage monitoring. The material is of a metal oxide semiconductor / layered silicate mineral and wax integrated structure, and comprises a heterojunction sensitive layer formed by compounding metal oxide semiconductor nanoparticles and layered silicate minerals, and a wax super-hydrophobic protective layer coating the surface of the heterojunction sensitive layer. The preparation method comprises the following steps: dispersing the two raw materials into suspension liquid, coating the suspension liquid on an electrode to form the sensitive layer, and then coating and curing the wax material to form the protective layer. According to the material, the room-temperature gas-sensitive activity is enhanced through the heterojunction, meanwhile, the super-hydrophobic wax layer is used for resisting high humidity and liquid pollution, and function synergy is achieved. The obtained material is high in sensitivity and quick in response to decay mark gases such as ammonia gas and hydrogen sulfide at room temperature, and has excellent environmental interference resistance. The method is simple in process, low in cost, high in environmental adaptability, capable of being integrated in liquid food packaging, capable of achieving real-time in-situ monitoring of the decay process and wide in application prospect.

Description

Technical Field

[0001] This invention belongs to the field of composite material preparation and food quality and safety monitoring technology. It relates to a composite sensing material and its preparation method and its application in liquid food spoilage monitoring. In particular, it relates to a technology that synergistically achieves high-sensitivity gas sensing at room temperature and superhydrophobic protection by constructing a "metal oxide semiconductor / layered silicate mineral@wax" heterojunction structure. This provides an innovative solution to the problem of detecting early spoilage markers in real food packaging environments with high humidity and easy contamination. Background Technology

[0002] Milk and dairy products are an important source of nutrition in the human diet, rich in protein, calcium, and other nutrients. However, due to their high nutritional content, they are highly susceptible to spoilage during storage and transportation caused by microbial growth. Milk spoilage not only causes foodborne illnesses and endangers public health (approximately 600 million foodborne illnesses worldwide are related to spoiled food each year), but also results in enormous resource waste. Statistics show that about 20% of dairy products worldwide are discarded due to expired labels, even though some of these products are still edible. Furthermore, some products that have not yet reached their expiration date may have already spoiled. Traditional expiration date labels cannot accurately reflect the true freshness of milk.

[0003] Traditional methods for detecting milk spoilage mainly include microbial culture, high-performance liquid chromatography (HPLC), and gas chromatography-mass spectrometry (GC-MS). While these methods can accurately detect the degree of milk spoilage, they have significant limitations: they require complex sample pretreatment procedures, professional operators, and sophisticated laboratory equipment; the testing cycle is long; they cannot achieve rapid on-site testing or real-time online monitoring; and they are insufficient to meet the quality control needs of the entire food production, transportation, and sales chain.

[0004] Milk spoilage monitoring solutions based on gas sensing technology have become a research hotspot due to their advantages such as rapid response, low cost, and integrability. Among them, metal oxide semiconductor (such as SnO2, WO3, ZnO, etc.) gas sensors can specifically respond to characteristic volatile organic compounds and gases (such as ammonia NH3, hydrogen sulfide H2S, carbon dioxide CO2, etc.) produced during milk spoilage, and have broad application prospects. However, such sensors face two major technical bottlenecks in actual milk monitoring scenarios: First, they require high operating temperatures. Most metal oxide sensors need to be activated at temperatures above 200℃ to activate the surface reaction, resulting in high power consumption. Moreover, high-temperature environments are not suitable for flammable food packaging scenarios, posing safety hazards. Second, they have poor environmental adaptability. Milk packaging is usually in a high-humidity environment, and liquid milk leakage and splashing may occur. Liquid immersion and high humidity can severely degrade sensor performance, leading to signal drift, decreased sensitivity, and even permanent sensor failure.

[0005] To improve the environmental stability of sensors, existing technologies mostly focus on the development of protective coatings. For example, Chinese invention patent CN116924698A discloses a protective coating that embeds hydrophobic silica nanoparticles in a polyurethane matrix. The process is relatively simple, but it relies on organic solvents, the matrix has insufficient long-term hydrolysis resistance, and it can only achieve passive protection, unable to achieve active sensing functions in conjunction with the coating. Chinese invention patent CN105541119B uses the sol-gel method to construct micro-nano rough structures to obtain superhydrophobicity, but its process is complex and sensitive to parameters, the coating has low mechanical strength, and its performance is easily degraded in high humidity environments. Chinese invention patent CN110078387A designs... The three-layer composite structure is prepared by dip coating, which requires extremely high consistency of raw materials and processes, making it difficult to guarantee performance uniformity. In addition, there is a contradiction between high light transmittance and high hydrophobicity, and performance verification in dynamic real environment is lacking. Chinese invention patent CN114409264B uses plasma enhanced chemical vapor deposition (PECVD) technology to prepare protective coatings. Although it can obtain coatings with strong adhesion and superior performance, the equipment and production costs are high, and it is severely limited by the shape and material of the substrate. It is difficult to integrate with semiconductor sensing technology and cannot achieve the integration of "sensing-protection".

[0006] Therefore, developing a sensing material that combines high sensitivity at room temperature, excellent resistance to moisture and contamination, and simple preparation process with low cost is of great significance for achieving real-time and reliable monitoring of milk spoilage. Summary of the Invention

[0007] To address the shortcomings of existing milk spoilage monitoring technologies in terms of room temperature sensitivity, adaptability to high humidity environments, resistance to liquid contamination, and cost of large-scale production, this invention aims to provide a composite sensing material that integrates high-sensitivity gas sensing with inherent superhydrophobic protection. This material, through a clever "metal oxide semiconductor / layered silicate mineral@wax" structural design, organically combines high room-temperature gas sensitivity with inherent superhydrophobic protection, effectively solving the technical bottlenecks of insufficient room-temperature sensitivity and poor environmental adaptability in existing sensors.

[0008] Another objective of this invention is to provide a method for preparing the above-mentioned composite sensing material, which is simple in process, mild in conditions, low in cost, and easy to scale up for production.

[0009] Another objective of this invention is to apply the aforementioned composite sensing material as a gas sensor in the monitoring of spoilage in liquid food, thereby achieving real-time, in-situ, and reliable detection of spoilage marker gases such as NH3 and H2S.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: 1. Composite sensing materials The core of the composite sensing material of this invention lies in the multifunctional integrated structure design of "metal oxide semiconductor / layered silicate mineral@wax". Through the synergistic effect of each component, it simultaneously achieves high-sensitivity sensing at room temperature and superhydrophobic protection. The specific composition is as follows: (1) Heterojunction Sensing Layer: Formed by a composite of metal oxide semiconductor nanoparticles and layered silicate minerals, serving as the sensing and structural framework. The metal oxide semiconductor nanoparticles are the sensing active component, selected from at least one of tin dioxide (SnO2), titanium dioxide (TiO2), tungsten oxide (WO3), zinc oxide (ZnO), and vanadium pentoxide (V2O5), possessing the ability to adsorb and respond to milk spoilage marker gases; the layered silicate minerals are selected from at least one of montmorillonite (MMT), kaolinite, attapulgite, and sepiolite, whose layered structure provides a stable supporting framework, while forming a tight heterojunction interface with the metal oxide semiconductor nanoparticles, significantly optimizing carrier separation and transport efficiency, and improving the material's adsorption capacity and response sensitivity to spoilage marker gases such as NH3 and H2S at room temperature.

[0011] The mass ratio of metal oxide semiconductor nanoparticles to layered silicate minerals is 1:1 to 1:8, preferably 1:2. This ratio range can ensure the stable formation of heterojunction structures, taking into account both sensing activity and structural strength.

[0012] (2) Superhydrophobic protective layer: A superhydrophobic layer with a hierarchical micro-nano structure is formed by coating the surface of the heterostructure with natural or synthetic wax materials. The wax materials are selected from at least one of carnauba wax, beeswax, and paraffin wax, which are widely available, inexpensive, and have good hydrophobicity and biocompatibility. The hierarchical micro-nano structure of the superhydrophobic protective layer endows the material with excellent superhydrophobic properties, with a water contact angle greater than 150° and a sliding angle less than 10°, which can effectively resist the wetting and splashing of liquid milk. At the same time, its porous structure ensures that the gas molecules of the putrefactive marker can diffuse freely to the sensing skeleton without affecting the sensing performance, thus achieving "protection-sensing" synergy.

[0013] 2. Preparation method of composite sensing materials The preparation method of the present invention is simple, energy-efficient, requires no complex and expensive equipment, and is easy to scale up for production. Specifically, it includes the following steps: S1. Preparation of Composite Suspension: Metal oxide semiconductor nanoparticles and layered silicate minerals are added to a solvent according to a predetermined mass ratio. The mixture is first dispersed uniformly by magnetic stirring, and then further refined by ultrasonic treatment to form a non-agglomerated, uniformly dispersed composite suspension. The solvent is selected from at least one of anhydrous ethanol, an ethanol-water mixture, and deionized water, and can be flexibly chosen according to the solubility of the raw materials. The stirring speed is controlled at 500-700 rpm, the stirring time is 20-40 minutes, and the ultrasonic treatment time is 5-15 minutes to ensure that the two components are fully mixed and dispersed.

[0014] S2. Construction of the Heterojunction Sensitive Layer: The composite suspension prepared in step S1 is uniformly coated onto the surface of the interdigitated electrode using drop coating, spin coating, or spray coating processes. After coating, it is first dried in an oven at 50-70℃ for 1-3 hours to remove the solvent; then it is transferred to a muffle furnace and heat-treated in an air atmosphere at 350-400℃ for 0.5-2 hours to enhance the adhesion between the material and the electrode and the stability of the heterojunction structure, forming a sensitive layer with abundant active sites on the surface of the interdigitated electrode. The substrate of the interdigitated electrode can be alumina ceramic (rigid) or flexible PET to adapt to different packaging scenarios; the finger width and gap of the interdigitated electrode are both 80-120 μm to ensure the stability and sensitivity of the sensing signal.

[0015] S3. Preparation of superhydrophobic protective layer: The wax material is dissolved or melted to form a wax material. Solid wax can be dissolved in solvents such as toluene and isopropanol to form a wax solution, and low-melting-point paraffin can be directly heated and melted to form a wax liquid. The wax material is uniformly covered on the surface of the sensitive layer prepared in step S2 by spraying, spin coating or scraping process, and the thickness of the protective layer is controlled to be 10-50 μm. Then it is placed in an environment of 20-80℃ for 0.5-2 hours to cure, so that the wax material forms a stable hierarchical micro-nano structure, and the final composite sensing material is obtained.

[0016] 3. Applications of composite sensing materials The composite sensing material of the present invention exhibits excellent gas response performance at room temperature, with a response value of not less than 550% for NH3 at a concentration of 500 ppm and a response value of not less than 300% for H2S at a concentration of 500 ppm. It can also maintain a stable sensing signal in high humidity environments (relative humidity > 85%), and has strong resistance to liquid contamination. The baseline resistance and sensing performance can be restored within 1 minute after the liquid is wiped away.

[0017] The prepared composite sensing material, used as a sensitive layer and integrated with a signal readout circuit, can be encapsulated into a miniaturized sensor unit that can be attached to the inner wall of packaging or placed in the headspace. This unit can monitor changes in NH3 or H2S concentration in real time and is suitable for monitoring spoilage of liquid foods such as milk, yogurt, and juice. The material can be fabricated into rigid or flexible structures to adapt to different types of liquid food packaging. Furthermore, by adjusting the types and ratios of metal oxide semiconductors and layered silicate minerals, the sensing selectivity can be controlled, expanding its application to more liquid food freshness monitoring scenarios.

[0018] Compared with the prior art, the present invention has the following significant advantages: 1. Integrated Functional Innovation: A unified structure of "metal oxide semiconductor / layered silicate mineral@wax" is constructed, integrating high-efficiency room-temperature gas sensing and inherent superhydrophobic protection into a single material system. The heterojunction structure solves the problem of insufficient room-temperature activity in traditional metal oxide sensors. The in-situ constructed superhydrophobic wax layer fundamentally resists interference from high humidity environments and liquid wetting, achieving synergy between "active sensing" and "passive protection," breaking through the limitations of traditional single-function technologies.

[0019] 2. Excellent environmental adaptability: The prepared composite sensing material has both excellent superhydrophobicity and room temperature sensing performance. It can still maintain a stable sensing signal in a high humidity environment with relative humidity >85%. It has self-cleaning and rapid recovery capabilities (performance recovery within 1 minute) after being splashed with liquid milk. It solves the key pain point that existing sensors have excellent laboratory performance but fail quickly in practical applications. It can be adapted to the complex environment of the entire milk storage and transportation chain.

[0020] 3. Low cost and environmental protection: The core raw materials (layered silicate minerals and natural waxes) are widely available and inexpensive; the preparation process does not require complex and expensive equipment such as high-temperature annealing and vacuum coating, and adopts coating and drying processes at room temperature and pressure, which has low energy consumption. Environmentally friendly reagents can be used as solvents, which is in line with the concept of green manufacturing and has great potential for large-scale production and industrial application.

[0021] 4. Precise and intelligent monitoring: It can dynamically track the entire process from freshness to spoilage by monitoring the concentration changes of characteristic gases such as NH3 and H2S in real time. It is more scientific and accurate than the traditional static "shelf life" label, and provides an effective technical tool for reducing food waste and ensuring consumer safety.

[0022] 5. Wide applicability: The sensing material can be made into rigid or flexible structures to adapt to different types of milk packaging. By adjusting the types and ratios of metal oxide semiconductors and layered silicate minerals, the sensing selectivity can be controlled. It can not only be used for milk spoilage monitoring, but also extended to the freshness monitoring of other liquid foods (such as yogurt, juice, and meat products). Attached Figure Description

[0023] Figure 1 The response curve of the composite sensing material of this invention to hydrogen sulfide; Figure 2 The response curve of the composite sensing material of this invention to ammonia gas; Figure 3 : Test diagram of the superhydrophobicity of the composite sensing material of this invention. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0025] Example 1: Preparation and Performance Testing of Tin Dioxide / Montmorillonite@Carnauba Wax Composite Sensing Material 1. Preparation of the sensitive layer suspension: Accurately weigh 0.2 g of tin dioxide (SnO2) nanoparticles and 0.4 g of calcium-based montmorillonite (MMT), and place them in 50 mL of anhydrous ethanol. At room temperature, first stir with a magnetic stirrer at 600 rpm for 30 minutes, then sonicate for 10 minutes to obtain a uniformly dispersed SnO2 / MMT composite suspension.

[0026] 2. Construction of the heterojunction sensitive layer: Gold interdigitated electrodes (100 μm in width and spacing) were fabricated on a clean alumina ceramic substrate (10 mm × 10 mm) using screen printing. 50 μL of the composite suspension was uniformly drop-coated onto the interdigitated electrode area using a microsyringe. The sample was then dried in a 60°C oven for 2 hours, followed by heat treatment in a muffle furnace at 350°C in air for 1 hour to enhance material adhesion and heterojunction stability, thus obtaining the SnO2 / MMT sensitive layer chip.

[0027] 3. Preparation and modification of the superhydrophobic wax solution: Weigh 0.5 g of carnauba wax particles and dissolve them in 20 mL of toluene. Stir magnetically in a 70°C water bath until completely clear to obtain a wax solution. Using an air spray gun (nozzle diameter 0.3 mm), spray approximately 2 mL of the wax solution evenly onto the surface of the aforementioned sensitive layer chip at a pressure of 0.15 MPa. The sample was then placed in an 80°C oven for curing for 1 hour. After natural cooling, the final composite sensing material, designated SM@Wax-1, was obtained.

[0028] 4. Performance Characterization: (1) Superhydrophobicity: Tested using a contact angle meter, such as Figure 3 As shown, SM@Wax-1 exhibits excellent superhydrophobicity with a water contact angle of 158° and a sliding angle of 7°. When 0.2 mL of liquid milk is dropped onto the material surface, the milk rolls in a spherical shape without wetting. After wiping away the milk, there are no residual stains on the material surface.

[0029] (2) Room temperature sensing performance: Tested at room temperature (25°C) and 50% relative humidity. For example... Figure 2 As shown, the response value to 500 ppm NH3 is 580%, the response time is 32 seconds, and the recovery time is 8 seconds. Figure 1 As shown, the response value to 500 ppm H2S is 350%, the response time is 30 seconds, and the recovery time is 6 seconds.

[0030] (3) Environmental adaptability: In a high-humidity environment with a relative humidity of 85%, the response value to 500 ppm NH3 remains above 560%, and the signal drift is less than 5% (please modify if inappropriate). 0.2 mL of liquid milk was dropped onto the sensor surface, left to stand for 10 seconds, and then gently wiped away with a lint-free cloth. Monitoring revealed that the sensor's baseline resistance and its response performance to NH3 completely recovered to their initial state within 1 minute, demonstrating excellent anti-contamination ability.

[0031] Example 2: Preparation and Performance Testing of Titanium Dioxide / Kaolin@Beeswax Composite Sensing Material This embodiment demonstrates the diversity of core material selection and the ability to selectively control sensing.

[0032] 1. Preparation of composite suspension: Weigh 0.25 g of titanium dioxide (TiO2) nanoparticles and 0.5 g of kaolin, disperse them in 50 mL of ethanol-water mixed solvent (ethanol to water volume ratio 3:1) at a mass ratio of 1:2, stir at 600 rpm for 30 minutes at room temperature, and sonicate for 10 minutes to form a uniform TiO2 / kaolin composite suspension.

[0033] 2. Construction of heterojunction sensitive layer: Using the same gold interdigitated electrode as in Example 1, the composite suspension was drop-coated onto the electrode surface, dried at 60°C for 2 hours, and then heat-treated at 400°C in air atmosphere for 1 hour to form a TiO2 / kaolin heterojunction sensitive layer.

[0034] 3. Preparation of the superhydrophobic protective layer: Weigh 0.5 g of beeswax and dissolve it in 20 mL of isopropanol. Stir in a 65℃ water bath until completely dissolved to prepare a beeswax solution. Using a spin coating process, spin coat the sensitive layer surface at 3000 rpm for 30 seconds to form a uniform wax film. Then, place it in a 70℃ oven for curing for 1 hour to obtain the composite sensing material, designated TK@BW-1.

[0035] 4. Performance Characterization: TK@BW-1 exhibits a water contact angle of 152° and a slip angle of 9°, demonstrating excellent superhydrophobicity. This material shows higher selectivity for H2S produced during the later stages of milk spoilage. Gas-sensing tests show a response value as high as 400% for 500 ppm H2S, while the response value for the same concentration of NH3 is 220%. This makes it suitable for scenarios requiring focused monitoring of sulfur-containing spoilage gases, proving that sensor selectivity can be controlled by adjusting the core material.

[0036] Example 3: Preparation and Performance Testing of Vanadium Pentoxide / Sepiolite@Paraffin Flexible Composite Sensing Material This embodiment demonstrates the fabrication capabilities and low-cost application scenarios of flexible sensors. 1. Preparation of composite slurry: Weigh 0.3 g of vanadium pentoxide (V2O5) nanowires and 0.3 g of sepiolite fiber, disperse them in 40 mL of deionized water at a mass ratio of 1:1, stir at 500 rpm for 40 minutes at room temperature, and sonicate for 15 minutes to form a uniform aqueous composite slurry.

[0037] 2. Construction of heterojunction sensitive layer: A flexible PET substrate with gold interdigitated electrodes is selected. The composite slurry is dropped onto the electrode surface by spin coating. The film is formed by spin coating at 2000 rpm for 60 seconds and drying in an oven at 80℃ for 2 hours. No high-temperature heat treatment is required (to adapt to the heat resistance of PET substrate) to form V2O5 / sepiolite sensitive layer.

[0038] 3. Preparation of superhydrophobic protective layer: Weigh 0.8 g of low melting point paraffin wax, heat to 70℃ to melt, and use a scraper to evenly coat the molten paraffin wax onto the surface of the sensitive layer to form a paraffin film with a thickness of about 50 μm. Let it solidify naturally at room temperature to obtain a flexible composite sensing material, numbered VS@PW-1.

[0039] 4. Performance Characterization: VS@PW-1 exhibits excellent flexibility; its sensing performance and superhydrophobic properties remain largely unchanged even when the bending radius is reduced to 5 mm. It has a water contact angle of 162° and a thick paraffin layer, providing extremely strong resistance to liquid penetration. At room temperature, its response to 500 ppm NH3 is 550%, with a response time of approximately 40 seconds. Although the response speed is relatively slow, the preparation process is extremely simple and inexpensive, requiring no complex equipment. This makes it suitable for single-use smart milk packaging scenarios where high response speed is not a primary concern and large-scale deployment is required.

[0040] The composite sensing material and its preparation method provided by this invention are simple, low-cost, and reliable, making them highly suitable for mass production. Sensors integrating this material can be easily embedded or attached to the inner wall or inner lid of liquid food packaging such as milk cartons, yogurt cups, and juice bottles, forming an intelligent packaging system. This system can monitor changes in the headspace gas inside the packaging in real time and transmit spoilage risk information to consumers or supply chain managers wirelessly or via wired means. It has significant practical application value for improving food safety, reducing food waste, and promoting the intelligent upgrading of packaging.

Claims

1. A composite sensing material, characterized in that, include: A heterojunction sensitive layer formed by metal oxide semiconductor nanoparticles and layered silicate minerals, and a superhydrophobic wax coating covering the surface of the heterojunction sensitive layer.

2. The composite sensing material of claim 1, wherein, The metal oxide semiconductor nanoparticles are selected from at least one of tin dioxide, titanium dioxide, tungsten oxide, zinc oxide, and vanadium pentoxide; the layered silicate minerals are selected from at least one of montmorillonite, kaolinite, attapulgite, and sepiolite; the mass ratio of the metal oxide semiconductor nanoparticles to the layered silicate minerals is 1:1 to 1:

8.

3. The composite sensing material of claim 1, wherein, The wax used in the superhydrophobic wax coating is selected from at least one of carnauba wax, beeswax, and paraffin wax.

4. The composite sensing material of claim 1, wherein, The composite sensing material has a hierarchical micro / nano structure, a water contact angle greater than 150°, and a sliding angle less than 10°.

5. A method for preparing the composite sensing material according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1. Metal oxide semiconductor nanoparticles are mixed with layered silicate minerals in a solvent to form a uniform composite suspension; S2. The composite suspension is coated onto the surface of the interdigitated electrode, and after drying and / or heat treatment, a heterojunction sensitive layer is formed on the surface of the interdigitated electrode. S3. The wax material is dissolved or melted to form a wax, which is then coated onto the surface of the heterojunction sensitive layer. After curing, the superhydrophobic wax coating is formed, thus obtaining the composite sensing material.

6. The production method according to claim 4, characterized by, The solvent in step S1 is selected from at least one of anhydrous ethanol, ethanol-water mixed solvent, and deionized water; the mixing process includes magnetic stirring and ultrasonic treatment, wherein the magnetic stirring speed is 500-700 rpm, the stirring time is 20-40 minutes, and the ultrasonic treatment time is 5-15 minutes.

7. The preparation method according to claim 4, characterized in that, In step S2, the coating method is drop coating, spin coating or spray coating; the drying temperature is 50-70℃ and the drying time is 1-3 hours; the heat treatment temperature is 350-400℃ and the heat treatment time is 0.5-2 hours.

8. The production method according to claim 4, characterized by, In step S3, the coating method is spraying, spin coating, or scraping; the curing temperature is 20-80℃, the curing time is 0.5-2 hours, and the thickness of the superhydrophobic protective layer is 10-50 μm.

9. The application of the composite sensing material according to any one of claims 1-4 in the monitoring of spoilage of liquid food.

10. Use according to claim 9, characterized in that, The liquid food is milk, yogurt, or juice. The composite sensing material is used to detect ammonia and / or hydrogen sulfide produced during the spoilage of the liquid food. At room temperature, the response value to ammonia at a concentration of 500 ppm is not less than 550%, and the response value to hydrogen sulfide at a concentration of 500 ppm is not less than 300%.

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