Method for preparing rice smoldering sensor by using COFs material and application of rice smoldering sensor in early warning of rice smoldering

Abstract

The invention discloses a method for preparing a rice smoldering sensor by using a COFs material and application of the rice smoldering sensor in rice smoldering early warning, and relates to the technical field of early fire early warning of rice in granaries. According to the COFs material prepared through reaction of melamine, terephthalaldehyde and dimethyl sulfoxide, organic molecules of the COFs material are connected through strong covalent bonds, and the COFs material has rich crystalline porous structures, nanoscale channels and spaces and a large specific surface area. Then adding the COFs material into a sodium alginate aqueous solution, uniformly stirring to obtain a mixed system, and sequentially soaking cut filter paper into the mixed system and a calcium chloride solution to prepare COFs coating filter paper; and finally, soaking in a mixed solution containing hydrochloric acid and aniline to obtain the smoldering sensor. According to the method, the rice smoldering super-early warning effect is achieved by detecting the concentration change of the main gas product furfural during rice smoldering, super-early fire early warning can be conducted, high stability is achieved, and the early warning cost is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of early warning of rice fire in granaries, and in particular to a method for preparing a rice smoldering sensor by using COFs material and an application of the sensor in rice smoldering warning. Background Art

[0002] Rice is one of the most important food crops for human beings, and the safety of rice during storage is of vital importance. However, there are many food losses caused by fire accidents every year. This is because rice is a combustible material, and the heat generated when the piled rice contacts the external heat source or becomes moldy is not easy to dissipate. When the rice is piled up, if its internal temperature continues to increase and reaches its ignition point, smoldering will occur. The energy and oxygen content required for smoldering are usually low. Even without an external heat source, the smoldering process can be self-sustaining when the heat released by the burning of rice can maintain its own pyrolysis reaction and oxidize the solid phase. Therefore, once the rice smoldering begins, it is difficult to extinguish. Generally speaking, the longer the smoldering lasts, the more difficult it is to extinguish. This long-term burning state makes smoldering a safety hazard that is difficult to ignore. Since smoldering is a slow burning and smokeless form, smoldering is highly concealed and difficult to detect, which is very dangerous.

[0003] Covalent organic framework (COFs) materials are a kind of crystalline porous materials formed by the reversible polymerization of light elements through covalent bonds under thermodynamic control. Compared with traditional materials, COFs have the characteristics of low density, good chemical stability, high specific surface area and regular pore structure. With these advantages, COFs materials have received extensive attention from scientists in various fields. Generally, COFs is a highly efficient adsorbent that can be used to capture gas molecules. Its adsorption process may involve a variety of adsorption mechanisms, including pore size effect, hydrogen bond, π-π bond, etc. Its adsorption efficiency mainly depends on the pore size, specific surface area and functional group characteristics of COFs materials.

[0004] The present invention mainly studies the pyrolysis products of rice combustion, and combines the excellent gas adsorption performance of three-dimensional covalent organic framework COF materials to prepare sensors for early warning of rice smoldering in granaries, builds a small-scale rice storage granary smoldering model, uses rice smoldering sensors to adsorb the volatile gas furfural produced during the rice process, and achieves early warning of rice smoldering through the color reaction of furfural and aniline hydrochloride, revealing the mechanism of warning rice smoldering through color reaction. This will effectively reduce the large amount of food loss caused by rice smoldering to a certain extent. Summary of the invention

[0005] The purpose of the present invention is to propose a method for preparing a rice smoldering sensor using COFs material, and the prepared sensor can be used for early warning of rice smoldering.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for preparing a rice smoldering sensor using COFs material comprises the following steps: firstly preparing a covalent organic framework material (COFs material) by reacting melamine, terephthalaldehyde and dimethyl sulfoxide, then adding the COFs material into a sodium alginate aqueous solution and stirring evenly to obtain a mixed system, and then soaking the cut filter paper in the mixed system and a calcium chloride solution in sequence to obtain a COFs coated filter paper; and finally soaking the COFs coated filter paper in a mixed solution containing hydrochloric acid and aniline to obtain a rice smoldering sensor (RSS).

[0008] As a preferred technical solution of the present invention, in the preparation method:

[0009] When preparing COFs materials, the mass ratio of melamine to terephthalaldehyde was 0.63:1, the reaction temperature was 180°C, the reaction time was 72 hours, and the reaction conditions were nitrogen. After the reaction was completed, the obtained product was washed with acetone, tetrahydrofuran and dichloromethane in turn, and finally vacuum dried.

[0010] When preparing COFs coated filter paper, the mass ratio of COFs material to sodium alginate is 9:1, the concentration of sodium alginate aqueous solution is 5wt%, and it is stirred at room temperature for 24h to obtain a uniform mixed system. The filter paper is soaked in the mixed system and calcium chloride solution for 12h and 15-20min respectively, and needs to be dried in an oven at 40℃ after each soaking.

[0011] When preparing the rice smoldering sensor, the mixed solution consisted of hydrochloric acid (0.25 mL, 1.19 g / mL), aniline (1 mL) and ethanol (24 mL, 50% V / V), and the COFs coated filter paper was immersed in the mixed solution for 1 min.

[0012] The rice smoldering sensor prepared by the present invention can absorb the volatile gas furfural generated during the rice smoldering process, and achieve early warning of rice smoldering through the color reaction of furfural and aniline hydrochloride. First, a rice smoldering warning model device consisting of a rice smoldering sensor, a reaction device equipped with electric heating and thermocouple temperature measurement functions, and an RGB value display device is constructed; as the temperature of the rice in the reaction device increases, the smoldering sensor can absorb the furfural formed by the pyrolysis of the rice, causing a color change, and the RGB value of the color is identified and obtained through the RGB value display device, and the smoldering rice warning is achieved by presetting the RGB threshold.

[0013] In order to avoid the occurrence of granary fires, the present invention analyzes the pyrolysis characteristics of rice in the granary, and proposes a new granary smoldering early warning method based on the color reaction of furfural (the main product in the rice smoldering process) and aniline hydrochloride, as well as the excellent gas adsorption performance of three-dimensional covalent organic frameworks (COFs). A small-scale rice storage granary smoldering model was built, and the designed rice smoldering sensor (RSS) successfully detected the smoldering of rice in a small granary. RSS has good flexibility and ultra-sensitivity to rice smoldering. In addition, the early warning mechanism of RSS is also revealed. This work provides a new method for early warning of grain fires.

[0014] Compared with the prior art, the beneficial effects of the present invention are also shown in:

[0015] 1. The present invention uses dimethyl sulfoxide, terephthalaldehyde, and melamine as the main raw materials for synthesizing COFs materials. The organic molecules of the synthesized COFs materials are connected by strong covalent bonds, have high chemical stability and thermal stability, and have rich crystalline porous structures and nanoscale channels, spaces, and large specific surface areas, which provide prerequisites for the storage of gas molecules. At the same time, natural, renewable, and degradable filter paper is used as the main body, the COFs material is introduced, and its strong adsorption of gas is utilized to successfully prepare a sensor with rice smoldering warning function.

[0016] 2. The present invention achieves the rice smoldering super-warning effect by detecting the concentration change of furfural, the main gas product when rice is smoldering. It can not only provide super-early fire warning, but also has high stability and reduces the warning cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 (a) preparation process, (b) infrared spectrum, (c) nitrogen adsorption-desorption isotherm, (d) SEM morphology and (e-g) XPS spectra of the COFs material in Example 1.

[0018] Figure 2 (a) the preparation process of the rice smoldering sensor (RSS) in Example 2, (b) the prepared COFs-coated filter paper, (c) the SEM image of the COFs-coated filter paper, (d) the SEM image of the untreated filter paper, (e) the folding-bending-rotation effect of the COFs-coated filter paper soaked in CaCl2 solution, and (f) the folding-bending-rotation effect of the COFs-coated filter paper not soaked in CaCl2 solution.

[0019] Figure 3 These are (a) the process of starch pyrolysis to produce furfural, (b) the process of cellulose hydrolysis to produce furfural, (c) the analysis of pyrolysis products of rice grains at 200°C, and (d) the analysis of pyrolysis products of rice grains at 280°C in Example 4.

[0020] Figure 4 In Example 4, (a) the infrared spectrum analysis of the RSS color development reaction, (b-e) the XPS spectra of the RSS color development reaction, and (f) the color development reaction mechanism of RSS.

[0021] Figure 5 These are (a) the color development effect diagram, (b) the absorbance curve, (c) the relationship curve between furfural concentration and color intensity, and (d) the color development time curve at different furfural concentrations in Example 4.

[0022] Figure 6 These are (a) a diagram of the rice smoldering experimental device, (b) a thermocouple distribution diagram, (c) a digital image of RSS color change, (d) a diagram of the change in temperature of the thermocouple measuring point with heating time, and (e) a diagram of the change in Y value with heating time in Example 5. DETAILED DESCRIPTION

[0023] The present invention is further described in detail below in conjunction with embodiments and drawings.

[0024] Example 1

[0025] Preparation of covalent organic framework materials (COFs):

[0026] The COFs material was synthesized by high-temperature polycondensation reaction without using a catalyst, using melamine, terephthalaldehyde and dimethyl sulfoxide, and reacting at 180°C for 72 hours under nitrogen protection.

[0027] Terephthalaldehyde (7.24 g), melamine (4.54 g) and dehydrated dimethyl sulfoxide (200 mL) were placed in a three-necked flask, heated to 180 ° C under nitrogen protection and stirred for 72 hours. After cooling, the mixture was filtered and then washed twice with acetone, tetrahydrofuran and dichloromethane. Finally, the yellow product obtained after washing was vacuum dried at 120 ° C for 12 hours to obtain COFs material. The structural formula and preparation route of COFs material are shown in Figure 1 As shown in a.

[0028] Infrared spectra of COFs materials Figure 1 As shown in b, at 3413cm -1 The characteristic peak at 1467 cm is generated by the stretching vibration of the melamine group -NH- -1 The absorption peak at 1190cm -1 The vibration band at 2870 cm-1 is related to the stretching of CN (-NH-) of secondary amine, which indicates that there are a large number of -NH- groups in the COFs framework, while the -1 and 1690cm -1There is no -CHO group at the place where -CHO and -NH2 groups are successfully condensed.

[0029] The nitrogen adsorption-desorption experiment was carried out on COFs materials at 77K. Figure 1 c is its nitrogen adsorption-desorption isotherm, which conforms to the characteristics of type II isotherm. This is because COFs material is a typical macroporous material, and its adsorption curve is characterized by a sharp increase in adsorption in the low pressure zone. This process is mainly monolayer adsorption. As the relative pressure continues to increase, the adsorption layer will gradually thicken, and the adsorption amount will increase rapidly when the saturated vapor pressure is reached. Figure 1 It can be observed in c that the BET specific surface area of ​​COFs is 182.1401m 2 / g. Its pore size is mainly distributed around 2nm, and during the adsorption process, the pore size of COFs is concentrated at 1.8nm; while during desorption, the pore size is concentrated at 2.17nm, such as Figure 1 As shown in the illustration in c.

[0030] The morphology of COFs was studied by scanning electron microscope (SEM) of Hitachi SU8010. Figure 1 d It can be clearly seen that many irregular spherical nanoparticles are randomly aggregated together, forming a three-dimensional network structure with numerous pores inside the COFs material, providing space for its adsorption of gas.

[0031] The surface chemical structure and basic element composition of COFs were determined by XPS. Figure 1 As shown in e to g, it is found that there are three peaks on the XPS curve of COFs, which are 532ev (O1s), 399ev (N1s) and 287ev (C1s). The C1s spectrum of COFs can be fitted by the four peaks of 284.7eV, 286.3eV, 287.6eV and 289.1eV, which correspond to C=C or CC, CN, C=N and C=O ( Figure 1 f). Deconvolution of the N1s peak in COFs shows the presence of a CN=C bond in the triazine ring at 398.4 eV and a C-NH-C bond in the secondary amine at 399.6 eV ( Figure 1 g).

[0032] Example 2

[0033] Preparation of COFs-based Rice Smoldering Sensor (RSS):

[0034] The preparation process of rice smoldering sensor (RSS) is as follows Figure 2As shown in a. Sodium alginate (1 g) was placed in deionized water (19 mL) to prepare a 5 wt% sodium alginate aqueous solution. COFs (9 g) were first mixed with the sodium alginate aqueous solution, and then deionized water (130 mL) was added. The resulting mixture was stirred at room temperature for 24 h to obtain a uniform mixing system.

[0035] A filter paper strip (0.0156 g) with a width of 1 cm and a length of 4 cm was immersed in the above-mentioned mixed system for 12 h to obtain a filter paper coated with COFs. The filter paper was first dried in an oven at 40°C for 1 h, then immersed in a 0.2 mol / L CaCl2 solution for 20 min (to neutralize the negative charge of sodium alginate), and finally dried in an oven at 40°C for 1 h to obtain a filter paper coated with COFs.

[0036] The prepared COFs-coated filter paper was immersed in a mixture of hydrochloric acid (0.25 mL, 1.19 g / mL), aniline (1 mL) and ethanol (24 mL, 50% V / V) for 1 min to prepare a rice smoldering sensor (RSS).

[0037] exist Figure 2 In a, since sodium alginate has a high polarity, it can be tightly adsorbed on the surface of the high polarity filter paper. Therefore, sodium alginate was used as an adhesive and the COFs-coated filter paper was prepared by the immersion method. In order to further increase the interaction between COFs and the filter paper, the filter paper was further immersed in the CaCl2 solution. 2+ Ions exchange with sodium ions in sodium alginate. Ca ions with two positive charges 2+ Ions can act as cross-linking agents and form a cross-linked three-dimensional network structure with negatively charged alginate molecules through electrostatic adsorption. This enhances the stability of COFs test paper as a rice smoldering sensor. Figure 2 b. Compared with the filter paper of the same size, the surface of the filter paper coated with COFs is light yellow, proving that the COFs powder has successfully adhered to the filter paper. The COFs powder is evenly distributed on the surface of the filter paper without obvious cracks. The loading amount of COFs powder on the rice smoldering sensor is 0.0044 g / cm 2 . Figure 2 c and 2d are SEM images of COFs-coated filter paper and untreated filter paper, respectively. It can be found that the surface of the filter paper has been completely covered by spherical nanoparticles. COFs are evenly distributed on the surface of the filter paper, with only a few small holes. The COFs filter paper coated with CaCl2 has good bending, folding and rotation properties. When it is folded, bent and rotated, almost no COFs powder dripping is observed on the display platform, and no obvious cracks and protrusions are observed on the surface of the filter paper ( Figure 2e). For sensors that are not soaked in CaCl2 solution, COFs on the filter paper surface are easily detached when folded, bent, and rotated ( Figure 2 f). COFs powder can be better attached to the filter paper surface after being soaked in CaCl2 solution, which provides a good prerequisite for the design of RSS.

[0038] Example 3

[0039] Test method for the sensitivity of rice smoldering sensor:

[0040] Take 0.87mL of furfural and dilute it to 100mL with 50% (V / V) ethanol solution. Then take 0.1mL of the above solution and dilute it to 100mL with ethanol. Finally, prepare a 10ug / mL furfural standard solution at room temperature, mix different amounts of 10ug / mL furfural standard solution with ethanol to 25mL, and then add 1mL aniline and 0.25mL hydrochloric acid with a density of 1.125g / mL. Use the RGB value determined by the color software as a color reference, where R: 255; G: 192; B: 210. The brightness can be calculated according to the following formula. When the Y value of RSS is lower than the original value, it can be considered that RSS detects the smoldering of rice, and the corresponding time is recorded as the response time.

[0041] Y=0.299*R+0.587*G+0.114*B

[0042] Example 4

[0043] Early warning mechanism of the rice smoldering sensor:

[0044] 1. Rice pyrolysis products:

[0045] It is generally believed that the smoldering process is mainly divided into pyrolysis (the first stage) and redox reaction (the second stage). Therefore, in order to warn of the smoldering phenomenon of rice, it is necessary to analyze its pyrolysis products. In the actual rice storage process, due to insect pests and microorganisms, a large amount of oxygen in the granary will be consumed, and the granary is in an oxygen-deficient state. Therefore, the pyrolysis products of rice under low oxygen concentration are mainly analyzed. Studies have found that in the maximum weight loss stage of rice pyrolysis, a certain amount of 2-methylfuran, furfural and other substances are generated, of which furfural accounts for about 19.71% of the total amount of generated products. Jin et al. used gas chromatography-mass spectrometry (GC / MS) to study the products of oxidative decomposition of moldy and non-moldy rice at different temperatures. The study found that at 200°C, the gas phase products of rice were aldehydes, acids, sulfides, ketones, furans and other substances. The formation of aldehydes and acids is due to the breakage of the starch main chain. The process of starch pyrolysis to produce furfural is as follows: Figure 3 A. After cellulose hydrolysis, a certain amount of furfural will also be generated. The specific generation process is as follows Figure 3As shown in b. Starch and cellulose are hydrolyzed to produce 5-hydroxymethylfurfural, which is further decomposed to produce furfural.

[0046] The present invention uses GC / MS to test the pyrolysis products of rice grains at 200°C and 280°C. It is found that when the pyrolysis temperature reaches 200°C, the main gas phase products are aliphatic aldehydes and furan compounds, specifically hexanal, furfural and other substances ( Figure 3 c). When the temperature reaches 280°C, the content of fatty aldehydes decreases, and the main gas products are furan compounds such as furfural and 2,5-furandicarboxaldehyde ( Figure 3 d). Therefore, furan substances such as furfural will be produced during the smoldering process of rice, which provides a theoretical basis for the design of smoldering sensors for early warning of granary fires.

[0047] 2. Furfural color development mechanism

[0048] The reddening of the sensor is mainly achieved by the adsorption of furfural by COFs soaked in aniline hydrochloride. Figure 4 As shown in a, the color reaction of RSS was analyzed by FT-IR. -1 A broad infrared absorption peak was observed at 1536 cm -1 and 1467cm -1 The peak at 1044 cm corresponds to the stretching vibration of the C=C bond in the benzene ring, and the stretching vibration peak of the CO bond in the ether is at 1044 cm -1 Appeared nearby. At 724cm -1 and 690cm -1 The peak at 3200 cm corresponds to the stretching vibration of CH in olefins. For the cherry red compound produced by the reaction of furfural with aniline hydrochloride, -1 The stretching vibration peak of the CH bond in the benzene ring appeared at 1551 cm -1 and 1496cm -1 The infrared absorption peaks at 1149 cm -1 The peak at 729 cm corresponds to the stretching vibration of CO in ether. -1 and 685cm -1 The corresponding position is the stretching vibration of CH in olefin. In summary, the C=O characteristic peak of the aldehyde group in furfural almost completely disappeared, indicating that furfural did react with aniline hydrochloride solution. The FT-IR characteristic peak of the color reaction between furfural and aniline hydrochloride was identified. It proves that COFs can successfully adsorb furfural and make it react with aniline hydrochloride. Among them, Figure 4The peaks at 284.8 eV, 286 eV, 287.9 ​​eV and 291.6 eV in the C1s high-resolution XPS spectrum shown in b correspond to C=C or CC, CN or CO, C=N and π-π* bonds. From the high-resolution XPS spectrum of N1s, it is inferred that the peaks at 397 eV and 395.9 eV of the COFs dry powder adsorption furfural color reaction sample correspond to C=N and CN bonds, respectively ( Figure 4 c). The peak in the O1s high-resolution XPS spectrum of the sample is CO (529.5 eV) ( Figure 4 d).

[0049] From the above analysis, it can be seen that the C=O bond in furfural has reacted to form a C=N bond, which also shows that the aniline hydrochloride solution on the COFs test paper has indeed undergone a color reaction with furfural. Figure 4 e shows the content of C, N and O elements in the XPS spectra of COFs and COFs / furfural / aniline hydrochloride. It can be seen that the C and N content of COFs / furfural / aniline hydrochloride is significantly higher than that of COFs. The increase in the C and N content and the decrease in the O content are due to the color reaction between furfural and aniline hydrochloride. Therefore, the O in the C=O bond of furfural is replaced by the N in aniline hydrochloride to form a C=N bond, which also shows that the aniline hydrochloride solution on the COFs test paper did react with furfural. The color reaction mechanism of RSS is as follows Figure 4 f. First, hydrochloric acid reacts with aniline to form aniline hydrochloride. Second, the adsorbed furfural reacts with aniline hydrochloride, and the product exhibits a cherry red color. Therefore, this color reaction provides a basis for designing RSS for rice smoldering warning.

[0050] 3. Rice smoldering sensor detects concentration

[0051] RSS turns red quickly within 30 seconds, as shown in the color rendering diagram Figure 5 a. The color absorbance curve of the mixture of aniline hydrochloride and furfural is shown in Figure 5 As shown in b. It can be found that the greater the concentration of furfural, the greater the absorbance of the color development solution. As time increases, the absorbance will first increase and then gradually level off, and finally decrease slightly. This is due to the volatility of the furfural solution, and the absorbance will reach a maximum value at around 11 minutes. For a mixture with a furfural concentration of 2 μg / mL, the maximum absorbance is about 0.09. When the furfural concentration is 0.4 μg / mL, the maximum absorbance decreases to about 0.03. The effect of different furfural concentrations on the color development time can be seen in Figure 5d. It can be found that as the furfural concentration decreases, the color development time gradually increases. When the furfural concentration is 2, 1, and 0.5ug / mL, the color development time is less than 20s. When the furfural concentration is 0.1ug / mL, it takes about 20min for a reddish color to appear. The relationship curve between furfural concentration and color intensity was fitted, and it was found that it increased linearly with a slope of about 0.07764 and an intercept of 0.03, that is, the minimum color concentration of furfural is 0.03ug / mL ( Figure 5 c).

[0052] Example 5

[0053] Application of Rice Smoldering Sensor

[0054] The invention makes a small rice smoldering experimental device as follows Figure 6 As shown in a. The experimental device mainly consists of four parts: an electric heating device, a reaction device, a thermocouple temperature measuring device and an RGB value display device. This experiment uses a DF-101S thermal collector constant temperature heating magnetic stirrer to heat rice at a high temperature of 220°C using an oil bath method. The reactor is a three-necked flask with a volume of 500mL and a height of 17cm, in which the rice sample is placed. In order to detect the internal temperature of the rice, three K-type thermocouples (φ=1mm) are placed at equal distances in the vertical direction (L=2.5cm) in the center of the reactor. The first thermocouple (TC1) is placed 1cm above the bottom of the three-necked flask, the second thermocouple (TC2) is placed 3.5cm above the bottom, and the third thermocouple (TC3) is placed 6cm above the bottom to monitor changes in the internal temperature of the rice ( Figure 6 b). At the same time, observe the color change of the rice smoldering sensor at the opening of the reactor. Use the color software "Fcolor picker" near the reaction device to record the RGB value of the rice smoldering sensor color in real time. After the experiment, set the experimental balance to measure the mass change of the rice before and after smoldering.

[0055] The specific experimental steps of the rice smoldering color warning experiment are as follows: first, the three working ends of the thermocouple are placed vertically and equidistantly inside the three-necked flask and fixed. Then, 250g of rice sample is weighed and placed in the three-necked flask, and it is placed in an electric heating device filled with silicone oil and heated by oil bath method. The temperature is set to 220℃. At the same time, the prepared COFs test paper soaked in aniline hydrochloride is placed at the opening of the three-necked flask. The color change of the COFs test paper and the RGB value when the red color visible to the naked eye appears are recorded in real time using the color software "F Color Picker". After the color development starts, a new test paper is placed every 10 minutes, and the RGB value and the corresponding thermocouple temperature of each test paper are recorded. After the experiment is over (heating for 2 hours), the mass change of the rice is recorded.

[0056] Figure 6As shown in a and b, a rice smoldering early warning system consisting of RSS and color software was built. The color change of RSS is recorded by the color software "F Color Picker" and converted into Y value (brightness) through calculation. Once the Y value of RSS begins to decrease, it means that the rice begins to smolder. The early warning system will immediately send an early warning signal to the relevant departments. The digital image of the color change of RSS is shown in Figure 6 c. As the heating time of rice increases, the red color on the rice smoldering sensor continues to deepen. The RSS at 120 min shows an obvious brick red. Generally, the internal temperature of rice can reflect the stage of rice pyrolysis, thus roughly identifying whether rice is smoldering. The temperature change of the thermocouple measuring point with time is shown in Figure 6 As shown in Figure d, it can be divided into two stages: dehydration and pyrolysis. In the dehydration stage (0-40min), the internal temperature of the rice was low at the initial stage, about 18℃. After about 40min, TC2 and TC3 reached their maximum values, 94.7 and 82.1℃, respectively. That is, the rice began to pyrolyze at 40min (pyrolysis stage). In the pyrolysis stage, the rice at the bottom began to pyrolyze. The combined effect of the released heat and the external heat source caused the bottom temperature (TC1) to rise sharply, while the rice in the middle and upper parts was mainly in the stage of water volatilization. Therefore, the RSS at the opening of the three-necked flask began to appear red at 40min and the color gradually deepened. That is, the furfural produced by the smoldering of rice was adsorbed by COFs and reacted with aniline hydrochloride. At the same time, with the transfer of heat, more rice began to smolder, producing more volatile furfural. TC2 and TC3 remained basically unchanged in the pyrolysis stage, which was because the smoldering of rice entered a stable period. Since the pyrolysis reaction plays a leading role, the heat released by the oxidation reaction is reduced, resulting in a decrease in the heat transferred to these two places, and the absorbed heat and the dissipated heat are almost balanced. At the same time, it can be observed that there is a slight carbonization phenomenon on the surface of the rice at the outer wall of the beaker, and a large amount of smoke appears. The carbonization is mainly due to the high temperature at the wall of the cup and the maximum heat exchange. The mass loss of rice after the smoldering test is about 12%. This is mainly due to the removal of moisture inside the rice and the smoldering effect of the rice. The Y value of pure RSS is 251.4 ( Figure 6 e), when the rice starts to smolder (40 min), the internal TC1 reaches about 98°C, the RSS immediately turns light red, and the Y component increases from 251.4 to 217. As the heating time increases, the conversion value (brightness) of the Y component gradually decreases. All of these prove that the RSS designed in the present invention has an ultra-high sensitivity for warning the smoldering of rice.

[0057] In summary, the present invention first reveals the rice smoldering products through GC / MS, and finds that furfural is one of the main pyrolysis products in the rice smoldering process, which provides a theoretical basis for the design of rice smoldering sensors and the realization of ultra-early warning of rice smoldering. Then, based on the excellent gas adsorption performance, hydrogen bonding and electrostatic interaction of COFs, a rice smoldering sensor (RSS) composed of COFs, filter paper and aniline hydrochloride is designed. The designed RSS not only has good flexibility (bending, folding and rotation), but also has ultra-high sensitivity. Its minimum monitoring concentration of furfural produced during rice smoldering is 0.03ug / mL, and the response time of RSS is as low as 14s. At the same time, the working mechanism of RSS is revealed, that is, the color reaction between aniline hydrochloride and furfural. Finally, a rice smoldering early warning system composed of RSS and color software is built. The color change of RSS is recorded by color software and converted into Y value (brightness) by calculation. By judging whether the Y value becomes smaller, the early warning of rice smoldering can be achieved efficiently and quickly.

[0058] The above contents are merely examples and explanations of the concept of the present invention. The technicians in this technical field may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the concept of the invention or exceed the scope defined by the claims, they should all fall within the protection scope of the present invention.

Claims

1. A method for preparing a rice smoldering sensor using COFs material, characterized in that: Firstly, covalent organic framework material (COFs material) was prepared by reacting melamine, terephthalaldehyde and dimethyl sulfoxide, and then added into sodium alginate aqueous solution and stirred evenly to obtain a mixed system. The cut filter paper was immersed in the mixed system and calcium chloride solution in turn to prepare COFs coated filter paper; finally, the COFs coated filter paper was immersed in a mixed solution containing hydrochloric acid and aniline to prepare a rice smoldering sensor (RSS).

2. The method according to claim 1, characterized in that When preparing COFs material, the mass ratio of melamine to terephthalaldehyde used was 0.63:1, the reaction temperature used was 180°C, the reaction time was 72h, and the reaction condition was nitrogen.

3. The method according to claim 1, characterized in that When preparing COFs materials, after the reaction is completed, the obtained product is washed with acetone, tetrahydrofuran and dichloromethane in sequence, and finally dried in vacuum.

4. The method according to claim 1, characterized in that When preparing the COFs coated filter paper, the mass ratio between the COFs material and the sodium alginate used was 9:1, the concentration of the sodium alginate aqueous solution was 5 wt %, and it was stirred at room temperature for 24 h to obtain a uniform mixed system.

5. The method according to claim 1, characterized in that When preparing COFs coated filter paper, the filter paper is soaked in the mixed system and calcium chloride solution for 12 hours and 15 to 20 minutes respectively, and needs to be dried in an oven at 40°C after each soaking.

6. The method according to claim 1, characterized in that When preparing the rice smoldering sensor, the mixed solution consisted of hydrochloric acid (0.25 mL, 1.19 g / mL), aniline (1 mL) and ethanol (24 mL, 50% V / V), and the COFs coated filter paper was immersed in the mixed solution for 1 min.

7. The rice smoldering sensor prepared by the method according to any one of claims 1 to 6, characterized in that: It can absorb furfural, a volatile gas produced during the smoldering of rice, and achieve early warning of rice smoldering through the color reaction of furfural and aniline hydrochloride.

8. The application of the rice smoldering sensor in rice smoldering early warning as claimed in claim 7, characterized in that: Firstly, a rice smoldering early warning model device was constructed, which consisted of a rice smoldering sensor, a reaction device equipped with electric heating and thermocouple temperature measurement functions, and an RGB value display device. As the temperature of rice in the reaction device increased, the smoldering sensor could adsorb furfural formed by the pyrolysis of rice, causing color changes. The RGB value of the color was identified and obtained through the RGB value display device, and the early warning of rice smoldering was achieved by presetting the RGB threshold.

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