Ammonia colorimetric sensor for detecting freshness of meat as well as preparation method and application of ammonia colorimetric sensor

Abstract

The invention discloses an ammonia colorimetric sensor for detecting meat freshness. According to the sensor, cellulose aerogel is used as a sensing base material, the natural pigment hematoxylin is used as an indicator, and the natural pigment loaded bio-based ammonia colorimetric sensor is constructed by utilizing the characteristics of high porosity and large specific surface area of the cellulose aerogel and the characteristic that the hematoxylin is sensitive to the change of a pH value. And when the freshness of meat is reduced, ammonia gas and other substances can be released, so that the pH value of the environment is changed, and the color of the hematoxylin is changed, so that the putrefaction degree of food can be intuitively reflected. In order to improve the practicability of the sensor in a complex environment, methyl trimethoxy silane is used for carrying out hydrophobic modification treatment on the surface of the cellulose aerogel, and the treatment can effectively inhibit the interference of moisture on the cellulose aerogel. The invention provides a convenient, sensitive, rapid and efficient solution for detecting the freshness of the food.

Description

Technical Field

[0001] This invention belongs to the technical field of gas sensing, and relates to the hydrophobic modification of aerogels. Specifically, it relates to the preparation of sodium carboxymethyl cellulose doped composite cellulose aerogels and their surface hydrophobic modification, as well as the portable detection of low concentrations of ammonia gas released from the surface of spoiled food by colorimetry. Specifically, it is an ammonia colorimetric sensor that can be used to detect the freshness of meat, its preparation method, and its application. Background Technology

[0002] Since the beginning of the 21st century, with the continuous advancement of science and technology and people's ever-growing pursuit of a better life, the importance of food safety has been increasing. Spoiled food carries bacteria and harmful substances, and long-term consumption can lead to various diseases. Only by ensuring food safety can people eat with peace of mind, live healthier lives, and society develop stably. According to the Chinese Dietary Guidelines, adults are advised to consume 40-75g of meat and poultry per day. Therefore, monitoring the freshness of meat is essential.

[0003] During transportation, the nutrients in meat are decomposed by microorganisms in the environment, producing organic acids, neutral metabolites, hydrogen sulfide, ammonia, biogenic amines, and indole, among other things. The freshness of meat can be indirectly determined by detecting these volatile characteristic compounds. Currently, there are many methods for detecting meat freshness, mainly including near-infrared spectroscopy, electronic nose, electronic tongue, microbial detection, hyperspectral imaging technology, and the total volatile basic nitrogen (TVB-N) method. These methods can accurately assess the freshness of meat. However, their application in practical life is limited by the long testing time, complex sample pretreatment process (which cannot be non-destructive to the meat), high cost, and inability to determine freshness on-site.

[0004] From a food safety perspective, using gas sensors to monitor the freshness of meat in real time can effectively reduce food poisoning caused by consuming stale meat. Real-time data monitoring also allows consumers to intuitively understand the quality of the meat. Therefore, developing an efficient, intuitive, and portable gas sensor for food freshness monitoring is essential. Among various gas sensors, colorimetric gas sensors offer advantages such as low cost, portability, no need for complex pretreatment processes, high sensitivity, intuitive results, ease of operation, rapid response, and non-destructive testing capabilities. They have significant application value in the field of food freshness monitoring; therefore, this paper selects a colorimetric gas sensor to monitor food freshness. Summary of the Invention

[0005] The purpose of this invention is to provide a highly sensitive gas sensor capable of long-term, continuous monitoring of food spoilage levels under complex environments. Cellulose aerogel, a biomaterial with superior porosity, is selected as the matrix, and hematoxylin, a natural pigment, is used as the indicator. The surface of the cellulose aerogel is hydrophobically treated to construct a visual gas sensor for the rapid identification and quantitative detection of ammonia in spoiled food.

[0006] To achieve the above objectives, the technical solution of this invention is as follows: an ammonia colorimetric sensor that can be used to detect the freshness of meat is prepared by the following method: a sodium carboxymethyl cellulose (CMC)-doped composite cellulose aerogel is surface modified by spraying. A hydrolyzed solution of methyltrimethoxysilane (MTMS) is uniformly sprayed at a distance of 15 cm from the aerogel, repeated 4 times with an interval of 5 min each time. After spraying, a hydrophobic silica layer is formed on the surface of the cellulose aerogel by a curing reaction, thus obtaining the hydrophobic composite cellulose aerogel MTMS-CNF-PVA-HEM-CMC-Na, i.e., M-CPHC.

[0007] The preparation method of the ammonia colorimetric sensor described above, which can be used to detect the freshness of meat, includes the following steps: 1) Preparation of MTMS hydrolysis solution: Take deionized water, anhydrous ethanol, and glacial acetic acid and stir them evenly. Add MTMS dropwise to the above mixed solution while stirring. After stirring, observe that the solution is clear and transparent, indicating that MTMS is hydrolyzed to produce silanol. 2) Surface hydrophobic modification by spraying: The MTMS hydrolysis solution was uniformly sprayed at a distance of 15 cm from the CPHC aerogel, and repeated 4 times with an interval of 5 min each time. This process was carried out in a fume hood. 3) Curing reaction: The CPHC aerogel coated with MTMS was placed at 25 ℃ and 50% relative humidity for 2 h to promote the polymerization of silanol to form a hydrophobic silica layer, and then cured to increase the degree of crosslinking, thus obtaining M-CPHC composite aerogel.

[0008] In the above-mentioned method for preparing an ammonia colorimetric sensor that can be used to detect the freshness of meat, step 3) involves curing the sensor in an oven at 50 ℃ for 1 h.

[0009] The preparation method of the ammonia colorimetric sensor that can be used to detect the freshness of meat, as described above, includes the following method for preparing sodium carboxymethyl cellulose doped composite cellulose CPHC aerogel: 1) Add polyvinyl alcohol powder to deionized water, stir at a constant temperature, and a clear and transparent polyvinyl alcohol solution is obtained; 2) Add 2% solids content nanocellulose dispersion, polyvinyl alcohol solution, and hematoxylin extract (HEM), and mix until completely homogeneous; 3) Add sodium carboxymethyl cellulose powder (CMC-Na) to the well-mixed solution and mix until completely homogeneous; 4) After the above solution has stood, freeze it in the freezer compartment of a refrigerator, and then dry it in a freeze dryer; 5) After freeze-drying, annealing is performed to obtain CNF-PVA-HEM-CMC-Na composite aerogel, namely CPHC aerogel.

[0010] In the above-mentioned method for preparing an ammonia colorimetric sensor that can be used to detect the freshness of meat, the percentage of sodium carboxymethyl cellulose powder (CMC-Na) in the total mass of cellulose and polyvinyl alcohol is 0-25 wt%.

[0011] In the above-mentioned method for preparing an ammonia colorimetric sensor that can be used to detect the freshness of meat, step 3) involves the following preparation method for the hematoxylin extract: 1) Weigh out the sappanwood and clean its surface, then dry it to constant weight, and then process it into small pieces of sappanwood. 2) Take sappanwood fragments, add deionized water, reflux extract at 70 ℃, filter while hot to obtain sappanwood pigment extract, and store in a sealed container away from light after being placed at room temperature.

[0012] In the above-mentioned method for preparing an ammonia colorimetric sensor that can be used to detect the freshness of meat, step 5) involves annealing in an oven at 80 ℃ for 4 h.

[0013] The above-mentioned ammonia colorimetric sensor, which can be used to detect the freshness of meat, is applied in the detection of meat freshness.

[0014] The above applications are described in the following ways: 1) Place the above-mentioned M-CPHC aerogel in a sealed gas test chamber, take 10 μL of ammonia water with a mass fraction of 28%, drop it into the gas evaporation chamber, wait for the solution to evaporate completely, and then take it out. Use the mobile smart software Color Grab to extract the R value and G value of the aerogel. Test the color change of M-CPHC aerogel to ammonia water of different concentrations according to this operation method. The concentration of ammonia gas is calculated using the ideal gas law pV=nRT. Plot the ammonia gas concentration on the x-axis and the R / G value on the y-axis. In the range of 0~800 ppm, ammonia gas and R / G value have a linear relationship. 2) Place the M-CPHC aerogel in a sealed container, place the ammonia gas to be tested in the sealed device, and take out the aerogel after 20 min. Extract the R and G values. The concentration of ammonia can be calculated based on the R / G value and the linear equation obtained in step 1).

[0015] The hydrophobic composite cellulose aerogel M-CPHC of the present invention has the advantages of easy preparation and high stability. Due to the presence of hematoxylin and the surface hydrophobic treatment, the modified material has good application in the detection of ammonia. Attached Figure Description

[0016] Figure 1 This is the standard curve for hematoxylin.

[0017] Figure 2 The color of the hematoxylin solution at different pH values.

[0018] Figure 3 The image shows the ultraviolet-visible spectrum of hematoxylin.

[0019] Figure 4 This describes the preparation process of composite cellulose aerogel.

[0020] Figure 5 For CPH(a), CPHC 10 (b) CPHC 15 (c) and CPHC 20 (d) SEM image of the aerogel.

[0021] Figure 6 For CPC 10 (a) and CPC 15 (b) Nitrogen adsorption-desorption curves of aerogel.

[0022] Figure 7 M-CPHC 15 and CPHC 15 Infrared spectrum of aerogel.

[0023] Figure 8 For CPHC 15 and M-CPHC 15 The water absorption effect of aerogel.

[0024] Figure 9 For CPHC 15 (a) and M-CPHC 15 (b) Photograph of the aerogel after water evaporation, (a1) is CPHC 15 Left view of the aerogel.

[0025] Figure 10 M-CPHC 15 and CPHC 15 XPS full spectrum (a) of aerogel, M-CPHC 15 Si 2p spectra of aerogel (b), M-CPHC 15 (c) and CPHC 15 (d) C1s spectrum of aerogel.

[0026] Figure 11 M-CPHC 15 Color changes of aerogel at different pH values ​​(a), color results from the mobile app Color Grab (b).

[0027] Figure 12 Used M-CPHC 15 Schematic diagram of the color restoration process of aerogel after 0 min (a), 10 min (b), 20 min (c), 30 min (d), and 40 min (e) at room temperature.

[0028] Figure 13 M-CPHC 15 Color changes of aerogels to ammonia at 0 ppm (a), 200 ppm (b), 400 ppm (c), 600 ppm (d) and 800 ppm (e).

[0029] Figure 14 This is a linear relationship curve between the R / G value and the ammonia concentration.

[0030] Figure 15 M-CPHC 15 Color changes of aerogel on pork after 0 h (a), 24 h (b), 72 h (c), 48 h (d) and 96 h (e).

[0031] Figure 16 This is a flowchart illustrating the operation of an ammonia colorimetric sensor.

[0032] Figure 17 M-CPHC 15 The response of aerogels to chicken (a), beef (b), pork (c), and fish (d) at different storage times is shown in Figure (a). For example, (a1), (a2), (a3), (a4), (a5), and (a6) show the color changes after 0 h, 6 h, 12 h, 24 h, 36 h, and 48 h, respectively. Detailed Implementation

[0033] Example 1: Extraction and content determination of hematoxylin

[0034] (a) such as Figure 1 As shown, the standard curve for hematoxylin is as follows: The surface of the sappanwood was cleaned, and then the cleaned sappanwood was placed in an oven at 60 ℃ and dried to constant weight. After removal, it was processed into small sappanwood fragments. 1 g of sappanwood fragments were weighed, 50 g of deionized water was added, and the mixture was refluxed at 70 ℃ for 50 min. The resulting solution was then filtered while hot to obtain the sappanwood pigment extract. The extract was cooled to room temperature and stored in a sealed container away from light. According to the existing standard curve of hematoxylin (sappanwood pigment can be obtained by oxidation of hematoxylin), the concentration of hematoxylin in the extract was 16.725 mg / L after 100-fold dilution using a UV-spectrum spectrophotometer. The corresponding concentration of hematoxylin in the sappanwood pigment was calculated to be 1.6725 g / L. The pigment content in the sappanwood pigment extract was replaced by the concentration of hematoxylin.

[0035] (ii) Color of hematoxylin solution at different pH values

[0036] Prepare phosphate buffer solutions with pH values ​​of 4, 5, 6, 7, 8, 9, 10, 11, and 12. Take 5.00 mL of each pH buffer solution and add 0.50 mL of hematoxylin solution to each, mix well, and observe the color of the solution. Figure 2 As shown, hematoxylin is pale yellow at pH 4, and the color gradually deepens with increasing pH. At pH 7, the color suddenly turns pale pink, and gradually deepens to dark red with further increases in pH. This demonstrates that the color change of hematoxylin solution is more pronounced in alkaline regions, making it suitable for detecting alkaline gases. Since hematoxylin exhibits different colors at different pH values, it suggests that hematoxylin has the potential to become a pH indicator.

[0037] Accurately pipette 10.00 mL of phosphate buffer solutions with pH values ​​of 4, 5, 6, 7, 8, 9, 10, 11, and 12, add 0.10 mL of hematoxylin solution to each, and perform UV-Vis spectral scanning using a UV-spectrum spectrophotometer. Figure 3 This is the ultraviolet-visible absorption spectrum of hematoxylin. The principle behind the color change of hematoxylin is as follows: Figure 3 As shown in the inset, when the pH is less than 7, hematoxylin oxidase has a characteristic peak at 447 nm, at which point the hydroxyl group remains unchanged. As the pH increases, the hydroxyl group is converted into an oxygen anion with a higher degree of conjugation, resulting in a red shift and hyperchromic effect in the absorption peak. The characteristic peak wavelength shifts from 447 nm to 538 nm, and the solution color gradually deepens to a deep red.

[0038] Example 2 Preparation of composite cellulose aerogel

[0039] (a) such as Figure 4 As shown, the preparation method is as follows: Composite cellulose aerogel (CPH) was prepared by freeze-drying. First, 8.0000 g of polyvinyl alcohol (PVA) powder was accurately weighed and added to 92.0 g of deionized water. After stirring at 90 °C for 1 h, a clear and transparent PVA solution was obtained. Then, 5.00 g of 2% solids cellulose nanoparticle dispersion (CNF), 0.50 g of PVA solution, and 2.00 mL of 1.6725 g / L hematoxylin extract (HEM) were added to a beaker and mixed until completely homogeneous. The mixture was then poured into a plastic mold. The mold was placed on a horizontal table and allowed to stand for 10 min until no air bubbles were observed. It was then frozen in the freezer for 12 h, followed by freezing in a vacuum freeze dryer for 24 h. Finally, the mold was annealed in an 80 °C oven for 4 h to obtain the composite cellulose aerogel (CPH).

[0040] (II) Sodium carboxymethyl cellulose doped composite cellulose aerogel CPHC n Preparation

[0041] Different proportions (percentage of the total mass of cellulose and polyvinyl alcohol) of sodium carboxymethyl cellulose powder (CMC-Na) were added to a solution of composite cellulose aerogel CPH. After mixing until completely homogeneous and allowing to stand, the mixture was freeze-dried to obtain sodium carboxymethyl cellulose doped composite cellulose aerogel CPHC, namely CPHC5 and CPHC. 10 CPHC 15 CPHC 20 and CPHC 25 Aerogel (subscripts indicate the mass fraction of CMC-Na). The specific amount of CMC-Na added is shown in Table 1.

[0042] Table 1. Composition of sodium carboxymethyl cellulose doped composite cellulose aerogel (CPHCn)

[0043] Figure 5 SEM images of composite cellulose aerogels with different CMC-Na addition amounts. Figure 5 In the diagram, (a), (b), (c), and (d) represent CPH, CPHC, and CPHC, respectively. 10 CPHC 15 and CPHC 20 SEM images of the aerogel. SEM characterization results show that the CPH cellulose aerogel is loose and porous, but the pores are relatively large, and the cross-linking effect is not uniform. With increasing CMC-Na content, the pores of the composite cellulose aerogel become denser and more uniform. The best structure was observed when the CMC-Na content was 15 wt%, forming an interwoven honeycomb network with uniform structure. When the CMC-Na content was 20 wt%, CPHC...20 The microstructure of the aerogel is disrupted, therefore CPHC is not considered. 20 Aerogel is used as a substrate for colorimetric sensors.

[0044] The preparation method is the same as in Examples (I) and (II), except that no hematoxylin is added. Figure 6 (a) and (b) are CPC respectively 10 and CPC 15 Nitrogen adsorption-desorption curves of the aerogel. CPC can be clearly seen in the figure. 15 Aerogels have a higher adsorption capacity than CPC. 10 Aerogels, both belonging to the type IV adsorption isotherm, are mesoporous materials. They possess abundant pore structure and a large specific surface area, along with good diffusion properties, which can improve the response and recovery speed of sensors. Therefore, choosing mesoporous materials as the substrate for colorimetric gas sensors is very suitable. Table 2 shows the CPC... 10 and CPC 15 The table of specific surface area, pore volume, and average pore size parameters of aerogels shows that CPC... 15 Aerogels exhibit excellent specific surface area, pore volume, and average pore size, therefore CPC was chosen for this study. 15 Aerogel is used as a substrate for colorimetric gas sensors.

[0045] Table 2. Nitrogen adsorption data analysis of sodium carboxymethyl cellulose doped composite cellulose aerogels

[0046] (III) Preparation of hydrophobic composite cellulose aerogel

[0047] Hydrophobic modification of the aerogel surface was performed by spraying. 10 mL of MTMS hydrolysis solution was placed in a spray bottle, and the surface of the aerogel was sprayed onto a sodium carboxymethyl cellulose (CPHC) composite cellulose aerogel. n The aerogel was uniformly sprayed at a distance of 15 cm, repeated four times with a 5-minute interval between each application, all within a fume hood. After spraying, the aerogel was placed in an environment with 25 °C and high relative humidity for 2 hours to promote the polymerization of silanol and the formation of a hydrophobic silica layer. It was then cured in a 50 °C oven for 1 hour to increase cross-linking. After curing, it was sealed and stored. This yielded a hydrophobic composite cellulose aerogel (M-CPHC). n Composite aerogel.

[0048] like Figure 7 As shown, Fourier transform infrared spectroscopy was used to characterize the main functional groups on the surface and inside of the aerogel. It can be seen that M-CPHC... 15 Aerogel interior, surface and CPHC 15 The infrared spectra of the aerogels are all within 3320 cm⁻¹.-1 A stretching vibration peak of -OH on cellulose appears at 2920 cm⁻¹. -1 and 1430 cm -1 The presence of stretching vibration peaks of CH bonds on the cellulose chain indicates that the hydrophobic modification of the cellulose aerogel surface using MTMS only alters the surface functional groups of the cellulose aerogel and does not destroy the basic chemical structure of cellulose.

[0049] After modification with MTMS, at 780 cm -1 A new absorption peak appeared, which is due to the silanization reaction between MTMS and cellulose, producing a stretching vibration peak of Si-O-Si / Si-C. This indicates the presence of silanol condensates on the surface of the modified aerogel, proving that the hydrophobic modification treatment was successful. Meanwhile, this characteristic peak did not appear inside the cellulose aerogel, proving that the surface modification was successful and did not affect the basic internal chemical structure.

[0050] To further verify the effect of MTMS hydrophobic modification, M-CPHC was applied to... 15 Aerogels and CPHC 15 The aerogel was placed in a sealed, humid environment, and its weight was recorded every hour. Figure 8 The figures show the water absorption of the two aerogels after different drying times. The data indicates that the hydrophobically modified aerogel has a significantly reduced water absorption, demonstrating a clear hydrophobic effect. It was also found that after natural drying, CPHC... 15 Aerogel edges curled up, M-CPHC 15 The aerogel remained unchanged, as Figure 9 As shown, in a humid environment, moisture affects the morphology of the aerogel and thus the stability of the sensor. Therefore, M-CPHC 15 Aerogels compared to CPHC 15 Aerogels are more stable.

[0051] Figure 10 M-CPHC 15 and CPHC 15 XPS full spectrum and fine spectrum analysis of the aerogel. Full spectrum (a) shows that the composite cellulose aerogel exhibits a Si 2p peak after MTMS hydrophobic modification, which is related to M-CPHC. 15 Fine 2p spectral analysis (b) of the aerogel Si revealed that M-CPHC 15 The presence of Si-O bonds on the aerogel surface indicates successful MTMS grafting, forming a hydrophobic silicon-oxygen network on the surface; [the text abruptly ends here, likely due to an incomplete sentence or missing information.] 15 and CPHC 15 Fine C1s spectra of the aerogel (c) and (d) show that the CO bond content decreased after hydrophobic treatment with MTMS, which is due to CPHC.15 The hydroxyl groups on the aerogel surface were replaced by silanes, and Si-OC bonds appeared. This indicates that the mechanism of methyltrimethoxysilane-modified cellulose aerogels involves the formation of covalent bonds, namely Si-OC bonds, through a condensation reaction between the hydrolyzed Si-OH groups and the -OH groups on the cellulose surface. This grafts methyltrimethoxysilane molecules onto the cellulose aerogel surface, simultaneously forming a Si-O-Si network structure. This network structure can form a uniform protective film on the cellulose aerogel surface, improving the hydrophobicity and stability of the aerogel.

[0052] Example 3: Recognition of ammonia by cellulose aerogel loaded with hematoxylin.

[0053] To verify that the cellulose aerogel loaded with hematoxylin still has an indicative function, the following experiment was designed: Phosphate buffer solutions with pH values ​​of 4, 5, 6, 7, 8, 9, 10, 11, and 12 were prepared and placed in spray bottles respectively. The solutions were then used to treat M-CPHC. 15 The surface of the aerogel is sprayed with a coating, and the color of the aerogel changes as follows: Figure 11 As shown, the color gradually changes from yellow to deep pink as the pH value increases, and M-CPHC 15 The aerogel exhibited a gradient change within an alkaline range, and colorimetric analysis was performed using the Color Grab mobile app. The results indicate that hematoxylin, once loaded onto the cellulose aerogel, still changes with pH and retains its indicative function.

[0054] A piece of M-CPHC that turns pink under alkaline conditions 15 The aerogel was placed in a normal environment, and its color was observed every 10 minutes. The color change results of the aerogel were as follows: Figure 12 As shown, (a) represents the initial state, and (b), (c), (d), and (e) represent the color changes after 10 min, 20 min, 30 min, and 40 min, respectively. It can be seen that M-CPHC... 15 The aerogel's color gradually changed from pink to orange and then to yellow, indicating that M-CPHC 15 Aerogels have reversible separation properties and can be reused.

[0055] To test the sensitivity of the ammonia colorimetric sensor to ammonia gas, the following experiment was designed: M-CPHC... 15 The aerogel was placed in a sealed gas chamber. 10 μL of ammonia solution (28% by mass) was added dropwise to the evaporation chamber (18 L volume). After the solution had completely evaporated, the aerogel was removed. The R value (red value) and G value (green value) of the aerogel were extracted using the Color Grab mobile app. M-CPHC was then tested using this method. 15The color change of aerogel with 20 μL, 30 μL and 40 μL volumes of ammonia water.

[0056] The concentration of ammonia was calculated using the ideal gas law pV=nRT, corresponding to concentrations of 200, 400, 600, and 800 ppm. The color changes are as follows: Figure 13 As shown, with the increase of ammonia concentration, M-CPHC 15 The aerogel's color changed from yellow to orange and then to pink, exhibiting a gradient effect. A linear relationship curve was plotted with ammonia concentration on the x-axis and R / G value on the y-axis. Figure 14 The linear relationship between the R / G value and ammonia concentration was plotted, revealing a linear relationship between ammonia and the R / G value within the range of 200–800 ppm. This indicates that quantitative detection of ammonia can be achieved within this range. (Based on the formula LOD = 3) / k The detection limit of the sensor was calculated to be 26.76 ppm.

[0057] In a practical application of the ammonia colorimetric sensor, a piece of fresh pork and a piece of M-CPHC were placed in a glass dish. 15 The aerogel was sealed with plastic wrap, and color samples were taken every 24 hours. Results are as follows: Figure 15 As shown, Figure (a) is the initial state, and Figures (b), (c), (d), and (e) are the M-CPHC values ​​after 24 h, 48 h, 72 h, and 96 h, respectively. 15 Aerogel color change. Results showed that the aerogel turned orange after 24 hours. Using the Color Grab mobile app, the R value was 252 and the G value was 200, calculating an ammonia concentration of 34.50 ppm. This high ammonia content indicates that pork left sealed at room temperature for 24 hours is not recommended for consumption. When using the ammonia colorimetric sensor of this invention to monitor pork freshness, caution should be exercised when the aerogel turns orange, and consumption should be avoided when the color gradually changes towards pink.

[0058] Figure 16 This is a flowchart illustrating the operation of an ammonia colorimetric sensor. It utilizes M-CPHC. 15 Aerogel monitoring of meat: After observing color changes in the aerogel, the Color Grab mobile app was used to capture the color, read the R and G values, and calculate the ammonia concentration at this point using a working curve to determine the food's freshness. M-CPHC... 15 When aerogels were placed next to pork that had been left at room temperature for 12 h, 24 h, 36 h, and 48 h, respectively, the M-CPHC showed a decrease in freshness due to the decline in pork freshness. 15The color of the aerogel also changed, and the time it took for the aerogel to completely change color was recorded. The results are shown in Table 3. Within 24 hours, the color of M-CPHC15 aerogel did not completely change within 10 minutes. As the degree of pork spoilage increased, the color of M-CPHC15 aerogel... 15 The time required for the entire aerogel area to completely change color also gradually shortened. It took 7 minutes to completely change color at 36 h and 5 minutes at 48 h. This indicates that the time required for the entire aerogel area to change color can be used as an evaluation index of pork freshness. Furthermore, judging meat freshness based on time improves the convenience for consumers.

[0059] Table 3. M-CPHC in pork at different degrees of spoilage 15 Time required for all areas of the aerogel to change color

[0060] To verify that this ammonia sensor can be used with various types of meat and has universal applicability, 2.5 g of chicken, beef, pork, and fish were respectively compared with M-CPHC. 15 The aerogels were sealed together and stored. Changes in M-CPHC were observed over time. 15 Color changes in aerogels. Color changes are as follows... Figure 17 As shown, figures (a), (b), (c), and (d) represent M-CPHC. 15 The response of aerogels to chicken, beef, pork, and fish is shown in Figure (a) as an example. (a1) represents the initial state, and (a2), (a3), (a4), (a5), and (a6) represent the color changes after 6 h, 12 h, 24 h, 36 h, and 48 h, respectively. Over time, the ammonia emitted by different meats due to decreased freshness can affect the M-CPHC. 15 Aerogel color change, M-CPHC 15 The aerogel's color gradually changes from yellow to deep pink, which indicates that M-CPHC 15 The aerogel responded to all four types of meat, and this ammonia sensor can be applied to a variety of meats.

Claims

1. An ammonia colorimetric sensor for detecting the freshness of meat, characterized in that, The preparation method is as follows: the surface of sodium carboxymethyl cellulose doped composite cellulose CPHC aerogel is modified by spraying. The hydrolyzed solution of methyltrimethoxysilane (MTMS) is uniformly sprayed at a distance of 15 cm from the aerogel, and repeated 4 times with an interval of 5 min each time. After the spraying is completed, a hydrophobic silica layer is formed on the surface of the cellulose aerogel by curing reaction, and the hydrophobic composite cellulose aerogel MTMS-CNF-PVA-HEM-CMC-Na, i.e., M-CPHC, is obtained.

2. The method for preparing an ammonia colorimetric sensor for detecting meat freshness as described in claim 1, characterized in that, Includes the following steps: 1) Preparation of MTMS hydrolysis solution: Take deionized water, anhydrous ethanol, and glacial acetic acid and stir them evenly. Add MTMS dropwise to the above mixed solution while stirring. After stirring, observe that the solution is clear and transparent, indicating that MTMS is hydrolyzed to produce silanol. 2) Surface hydrophobic modification by spraying: The MTMS hydrolysis solution was uniformly sprayed at a distance of 15 cm from the CPHC aerogel, and repeated 4 times with an interval of 5 min each time. This process was carried out in a fume hood. 3) Curing reaction: The CPHC aerogel coated with MTMS was placed at 25 ℃ and 50% relative humidity for 2 h to promote the polymerization of silanol to form a hydrophobic silica layer, and then cured to increase the degree of crosslinking, thus obtaining M-CPHC composite aerogel.

3. The method for preparing an ammonia colorimetric sensor for detecting meat freshness according to claim 2, characterized in that, In step 3), the curing is performed in a 50°C oven for 1 hour.

4. The method for preparing an ammonia colorimetric sensor for detecting meat freshness according to claim 2, characterized in that, The preparation method of sodium carboxymethyl cellulose doped composite cellulose CPHC aerogel is as follows: 1) Add polyvinyl alcohol powder to deionized water, stir at a constant temperature, and a clear and transparent polyvinyl alcohol solution is obtained; 2) Add 2% solids content nanocellulose dispersion, polyvinyl alcohol solution, and hematoxylin extract (HEM), and mix until completely homogeneous; 3) Add sodium carboxymethyl cellulose powder (CMC-Na) to the well-mixed solution and mix until completely homogeneous; 4) After the above solution has stood, freeze it in the freezer compartment of a refrigerator, and then dry it in a freeze dryer; 5) After freeze-drying, annealing is performed to obtain CNF-PVA-HEM-CMC-Na composite aerogel, namely CPHC aerogel.

5. The method for preparing an ammonia colorimetric sensor for detecting meat freshness according to claim 4, characterized in that, The percentage of sodium carboxymethyl cellulose (CMC-Na) powder in the total mass of cellulose and polyvinyl alcohol is 0-25 wt%.

6. The method for preparing an ammonia colorimetric sensor for detecting meat freshness according to claim 4, characterized in that, In step 3), the preparation method of the hematoxylin extract is as follows: 1) Weigh out the sappanwood and clean its surface, then dry it to constant weight, and then process it into small pieces of sappanwood. 2) Take sappanwood fragments, add deionized water, reflux extract at 70 ℃, filter while hot to obtain sappanwood pigment extract, and store in a sealed container away from light after being placed at room temperature.

7. The method for preparing an ammonia colorimetric sensor for detecting meat freshness according to claim 4, characterized in that, In step 5), the annealing treatment is performed in an oven at 80 ℃ for 4 h.

8. The application of the ammonia colorimetric sensor of claim 1 for detecting meat freshness in the detection of meat freshness.

9. The application according to claim 8, characterized in that, The method is as follows: 1) Place the M-CPHC aerogel as described in claim 1 into a sealed gas test chamber, take 10 μL of ammonia water with a mass fraction of 28%, drop it into the gas evaporation chamber, wait for the solution to evaporate completely, and then take it out. Use the mobile smart software Color Grab to extract the R value and G value of the aerogel. Test the color change of M-CPHC aerogel to ammonia water of different concentrations according to this operation method. The concentration of ammonia gas is calculated using the ideal gas law pV=nRT. With the ammonia gas concentration as the abscissa and the R / G value as the ordinate, the ammonia gas concentration and the R / G value are linearly related in the range of 0~800 ppm. 2) Place the M-CPHC aerogel in a sealed container, place the ammonia gas to be tested in the sealed device, and take out the aerogel after 20 min. Extract the R and G values. The concentration of ammonia can be calculated based on the R / G value and the linear equation obtained in step 1).

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