CN111537479B - Preparation method of freshness indicating film based on ratio type fluorescent material - Google Patents
Preparation method of freshness indicating film based on ratio type fluorescent material Download PDFInfo
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- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
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Abstract
The invention belongs to the technical field of food packaging, and particularly relates to a preparation method of a freshness indicating film based on a ratio type fluorescent material; the method comprises the following specific steps: firstly, preparing a colloidal suspension for synthesizing a layered LEuH nano sheet and a colloidal suspension for synthesizing a layered LTbH nano sheet; tb is poured by sol-gel method 3+ /Eu 3+ Pouring the carrageenan inner film solution (inner film solution A) in a model filled with the PVDF film to form PVDF/indicating hydrogel, and finally performing supercritical drying to obtain the aerogel with the freshness indicating function. Compared with a natural anthocyanin intelligent indicating film, the material prepared by the invention has the characteristics of high stability and high sensitivity; simultaneously, the problem of insufficient mechanical property of the biodegradable film is solved; the indicator disclosed by the invention is safe, good in biocompatibility, biodegradable and strong in mechanical property, and has the function of monitoring the freshness of fresh meat and aquatic products in real time.
Description
Technical Field
The invention belongs to the technical field of food packaging, and particularly relates to a preparation method of a freshness indicating film based on a ratio type fluorescent material.
Background
The fresh food is aquatic products, meat, fruits and vegetables which are formed by planting, picking, culturing and catching and can be eaten by human beings without processing or primary processing. The fresh food is fresh and delicious, has the most nutritive value, and the demand is rapidly increased, and occupies a considerable share of the total amount of agricultural products and food. However, fresh food products are perishable, e.g. water containing a high proportion of protein will go through a period of stiffness after death, lasting minutes or hours, and it is difficult for the consumer to determine their freshness; meat with high fat and high water content is easily polluted by microorganisms and oxidized by lipid, and the freshness of the meat cannot be accurately expressed by the service life of the meat on a package. With the technological progress and the improvement of living standard, the requirements of people on nutrition, health and safety of foods are gradually improved. Therefore, intelligent packaging with monitoring, detecting, recording, tracking and communication functions becomes a research hotspot. The visual packaging film for food freshness is a hot spot direction, can provide related quality information through color change, and is simple, convenient and low in price. Currently, most green indicators, such as anthocyanins and chlorophylls, are used, which can change color according to changes in the surrounding chemical environment. For example, in Choi et al, a packaging film prepared by blending purple sweet potato anthocyanin and agar starch changes the color of the packaging film from red to green along with the decay of pork, so that the detection of the freshness of the pork is realized. However, natural anthocyanins are susceptible to environmental influences such as metal ions, illumination, temperature and the like, and the practical application value of the anthocyanins is reduced. Therefore, a green and safe high-performance indicator is urgently needed to be found to replace an anthocyanin indicator to obtain a novel visual intelligent packaging film.
Experiments show that Tb is in proper proportion 3+ /Eu 3+ The combination is a high performance indicator. Tb 3+ And Eu 3+ Named as trivalent terbium ion and europium ion, belongs to lanthanide metal ion. The two are combined into a perfect combination of a sensitizer and an activator 5 D 4 → 7 F J (J =6,5,4,3) transitions are often used as green light activators, the latter being due to 5 D 0 → 7 F J (J =2,1) transitions are often used as red light activators, while Tb 3+ Just can also serve as Eu 3+ When the two sensitizers are co-doped, the light color can be adjusted and controlled through energy transfer. Research shows that when the hydrogel containing europium hydroxide (LEuH) nano is placed in a closed environment of ammonia gas or ammonia water, the fluorescence intensity of the hydrogel is weakened along with the passage of time, and the fluorescence intensity is shown as the change of a photoluminescent image which can be distinguished by naked eyes, which indicates that the LEuH nano has the capability of monitoring the freshness of fresh meat and aquatic products in real time. Secondly, toxicological experiments demonstrated that, even at high doses (250 mg/kg/b.w), layered LEuH and LTbH (terbium hydroxide) nanorods were only slightly or even not toxic to 57BL/6 mice; chinese hamsters treated with the LEuH and LTbH nanorod solutions (12.5-125 mg/kg/b.w.) respectively did not undergo significant changes in cytogenetic toxicity compared to the control group, and the results of the study strongly suggest that LEuH and LTbH nanorods play an important role in vascular diseases. Thus Tb 3+ /Eu 3+ The layered hydroxide can be used as a freshness indicator to replace chemical synthesis indicators (such as curcumin, lemon yellow and grape skin red, the ADI values of which are respectively 0-1.0 mg/kg, 0-7.5 mg/kg and 0-2.5 mg/kg) with the limitation of the daily intake (ADI), and the ratio type indicator can show obvious photoluminescence image color conversion along with the increase of the putrefaction degree, but not only increase the fluorescence intensity by decreasingDetection sensitivity and accuracy. In addition, the layered hydroxide synthesized by lanthanide ions has the characteristics of low cost, easy synthesis, unique optical characteristics, biodegradability, good biocompatibility and the like, and accords with the modern theme of green environmental protection.
Carrageenan is a linear sulfated polysaccharide derived from red seaweed and has wide applications in food packaging due to its good gelling, biocompatibility and biodegradable properties. However, their high hydrophilicity, poor mechanical properties and barrier properties limit their use. The polyvinylidene fluoride (PVDF) material with high mechanical property and good heat resistance can make up the defects of carrageenan, and the PVDF is a hydrophobic material, has good ultraviolet resistance, flexibility and mildew resistance, and is widely used as a membrane material. However, the method is limited to ultrafiltration treatment of sewage, protection of glass doors and windows of outdoor buildings and application of diaphragms in lithium batteries at present, and is rarely used for packaging food.
In the prior art, a patent of a carrageenin-chitosan blended composite film and a preparation method thereof (CN 110506785A) discloses an active packaging film for fresh-keeping application of cold fresh meat, which has the characteristic of strong fresh-keeping capability, but the mechanical property of a film formed by a tape casting drying method related to the technology is not high, and the freshness of the food quality cannot be monitored in real time. The patent "preparation process of an anthocyanin active intelligent packaging film" (CN201810667474. X) discloses an anthocyanin active packaging film which can be used for detecting food freshness, and the anthocyanin related to the technology is unstable and can generate color change, so that the result deviation of the visual packaging film is large. Therefore, the invention uses the formed PVDF film with strong mechanical property as the outer layer film to pour Tb by the sol-gel casting method 3+ /Eu 3+ The carrageenan inner layer membrane solution (inner layer membrane solution A) is poured in a model padded with a PVDF membrane to form PVDF/indicating hydrogel, and then the PVDF/indicating hydrogel is converted into aerogel to form a double-layer membrane based on a ratio type fluorescent material, so that the double-layer membrane has strong mechanical property and has the function of monitoring the freshness of fresh meat and aquatic products in real time.
Disclosure of Invention
Aiming at the problems of instability, low monitoring sensitivity, insufficient mechanical property and the like of a conventional biodegradable film of a common natural anthocyanin indicator in the existing research, a novel Tb is provided 3+ /Eu 3+ The ratio type fluorescent material is used as a freshness indicator, and Tb is poured by a sol-gel pouring method 3+ /Eu 3+ Pouring the carrageenan inner film solution (inner film solution A) in a model filled with the PVDF film to form PVDF/indicating hydrogel, and finally performing supercritical drying to obtain the aerogel with the freshness indicating function.
In order to achieve the purpose, the technical scheme adopted by the invention comprises four steps:
step one, synthesizing a colloidal suspension of a layered LEuH nano sheet and a colloidal suspension of a layered LTbH nano sheet:
(1) Preparation of NO 3 -LEuH precursor and NO 3 -LTbH precursor: taking a certain amount of Eu (NO) 3 ) 3 ·6H 2 O (europium nitrate hexahydrate) or Tb (NO) 3 ) 3 ·6H 2 O (terbium nitrate hexahydrate), HMT (hexamethylenetetramine) and NaNO 3 (sodium nitrate) is added into the aqueous solution, then the mixture is transferred into a polytetrafluoroethylene autoclave for reaction, and NO is obtained after the reaction through filtration, cleaning and drying 3 -LeuH precursor or NO 3 -an LTbH precursor;
(2) Preparing DS-LEuH and DS-LTbH: taking NO prepared in the step (1) 3 -LeuH precursor or NO 3 -LTbH precursor sample is dispersed in DS (sodium dodecyl sulfate) aqueous solution to obtain mass NO 3 -LeuH solution or NO 3 The LTbH solution is transferred to a polytetrafluoroethylene autoclave for heating reaction; centrifuging after reaction to obtain a precipitate, washing the precipitate with absolute ethyl alcohol, and drying to obtain DS-LEuH or DS-LTbH;
(3) Obtaining a colloidal suspension of lamellar LEuH nanoplates and a colloidal suspension of lamellar LTbH nanoplates: dispersing the DS-LEuH or DS-LTbH obtained in the step (2) into an Fm (formamide) solution, standing for a period of time, and stripping to obtain a colloidal suspension of a layered LEuH nanosheet or a colloidal suspension of a layered LTbH nanosheet;
step two, preparing an inner layer membrane solution A:
mixing the colloidal suspension of the layered LEuH nanosheets and the colloidal suspension of the layered LTbH nanosheets, which are prepared in the first step, to obtain a mixed solution A; slowly adding carrageenan powder into deionized water under continuous stirring, stirring at a certain temperature, adding the mixed solution A, stirring, adding a plasticizer, and continuously stirring to obtain an inner layer membrane solution A;
step three, preparing PVDF/indicating hydrogel:
placing the inner layer film solution A prepared in the step two at room temperature, cooling to a certain temperature, pouring the inner layer film solution A into a model padded with a PVDF film by adopting a sol-gel-casting method, and cooling to form PVDF/indicating hydrogel;
step four, preparing aerogel:
taking out the PVDF/indicating hydrogel prepared in the third step, immersing the PVDF/indicating hydrogel into ethanol solutions with different volume concentrations or sequentially immersing the PVDF/indicating hydrogel into the ethanol solutions with different volume concentrations, converting the PVDF/indicating hydrogel into alcohol gel after immersing, and then performing supercritical CO treatment on the alcohol gel 2 And drying under the condition to obtain the aerogel with a porous structure, namely the freshness indicating film based on the ratio type fluorescent material.
Preferably, the reaction is carried out in the autoclave in the step (1) at a temperature of 90 to 105 ℃ for 12 hours.
Preferably, the Eu (NO) in step (1) 3 ) 3 ·6H 2 O(Tb(NO 3 ) 3 ·6H 2 O)、HMT、NaNO 3 And deionized water in the following dosage ratio: 1-1.5mmol.
Preferably, the drying temperature in the step (1) is 40-45 ℃ and the time is 24h.
Preferably, the concentration of the sodium dodecyl sulfate aqueous solution in the step (2) is 0.2M; said NO 3 -the mass concentration of the LEuH solution is 0.1-0.2%; said NO 3 The mass concentration of the-LTbH solution is 0.1-0.2%.
Preferably, the temperature for the heating reaction in the autoclave in the step (2) is 90-95 ℃ and the time is 24 hours.
Preferably, the drying temperature in the step (2) is 50-55 ℃ and the time is 24h.
Preferably, the standing time in the step (3) is 48 to 52 hours.
Preferably, the stripping mass concentration of the colloidal suspension of the lamellar LEuH nanosheets in the step (3) is 0.2-1%; the stripping mass concentration of the colloidal suspension of the layered LTbH nanosheet is 0.2-1%.
Preferably, the volume of the colloidal suspension of the lamellar LEuH nanosheets and the colloidal suspension of the lamellar LTbH nanosheets in step two is 1.
Preferably, the stirring temperature in the second step under a certain temperature condition is 85-90 ℃, and the stirring time is 2-3 h; the time for adding the mixed solution A and stirring is 2-3 h; the time for adding the plasticizer and continuing stirring is 30min.
Preferably, the dosage ratio of the carrageenan powder, the deionized water, the mixed liquid A and the plasticizer in the second step is 2g.
Preferably, the cooling in the third step is carried out to a certain temperature of 35-40 ℃.
Preferably, the volume concentration of the ethanol solution in the fourth step is 10-50%; the sequential immersion is specifically sequential immersion from small concentration to large concentration; the soaking time is 40-60 min.
Preferably, the supercritical CO is used in step four 2 The temperature of drying under the conditions is 31.5 ℃, and the pressure is 7.4MPa.
The ratio type fluorescent material prepared by mixing the lamellar LEuH and LTbH suspension solution prepared in the step one in a certain ratio is sensitive to ammonia and biogenic amine substances and can be used as a freshness indicator of fresh food. Tb for leading green fluorescence in ammonia or biogenic amine gas environment 3+ The electrons on the material are firstly excited to be changed into an excited state and then relaxed to the lowest excited state through multi-phonon relaxation 5 D 4 Followed by partial electron relaxation from 5 D 4 (Tb 3+ ) Energy level to Eu 3+ (4f 6 ) Higher excitation level of the first and second electrodes, and then relaxes to 5 D 0 (Eu 3+ ) Energy level completes energy transfer work, and finally red fluorescence is generated 5 D 0 → 7 F 1,2,4 ). With the increasing of the putrefaction degree of putrefactive fresh meat and aquatic products, red fluorescence gradually becomes a dominant color, which shows that the photoluminescence image color of the indicating film is changed from dark green to light green, yellow, orange and red, the linear correlation between the film color and the freshness in the fresh-complete putrefaction range is realized, the determination of the freshness is accurately realized, and Eu in the indicator 3+ And Tb 3+ The combination is a perfect combination of the sensitizer and the activator, and the additional sensitizer (such as 2-trifluoroacetone) is not needed, so that the cost can be reduced.
The supercritical drying method can obtain high porosity (more than or equal to 95 percent) and low density (less than or equal to 0.086/cm) 3 ) The aerogel of (2) is beneficial to the contact of putrefactive gas and an indicating material, so that the detection accuracy is improved, and after 10 absorption-rinsing cycles, the aerogel still has excellent absorption capacity and shape retention, and the membrane can be recycled.
The invention has the advantages of
(1) The initial color of the double-layer indicating film prepared from the layered LEuH and LTbH is dark green, the color of the indicating film gradually changes into light green, yellow, orange and red along with the increase of the corruption degree, the freshness of fresh meat and aquatic products can be expressed in real time, and the 'service life' on the traditional package can be replaced.
(2) The LEuH/LTbH ratio type fluorescent indicator is not influenced by temperature and illumination, and has higher stability compared with indicators such as anthocyanin; the indicator is safe, good in biocompatibility and biodegradable, and the problems that the package pollutes the environment, toxic substances in the package migrate and the like are not needed to be worried about.
(3) The tensile strength of the outer PVDF film used by the invention is as high as 30.2MPa, the defect of the single carrageenan film performance (22.6 MPa) can be made up, and the PVDF film has low hydrophilicity (water absorption is only 0.04%) and antibacterial property, thereby playing a certain fresh-keeping role.
(4) The porosity of the aerogel obtained by supercritical drying reaches more than or equal to 95 percent, and the low density reaches less than or equal to 0.086/cm 3 ) Of the aerogel ofThe ratio type fluorescent indicator is contacted with the putrefactive gas generated in real time, so that the detection sensitivity and accuracy are improved; and after 10 absorption-rinsing cycles, the aerogel still has excellent absorption capacity and shape retention, and the membrane can be recycled.
Detailed description of the invention
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to specific embodiments below.
Example 1:
step one, synthesizing a colloidal suspension of a layered LEuH nano sheet and a colloidal suspension of a layered LTbH nano sheet:
(1) Preparation of NO 3 -LEuH precursor and NO 3 -LTbH precursor: adding 1mmol Eu (NO) 3 ) 3 ·6H 2 O or Tb (NO) 3 ) 3 ·6H 2 O，1mmol HMT，13mmol NaNO 3 Adding the mixture into 80mL of aqueous solution, and heating the mixture for 12 hours at 90 ℃ in a polytetrafluoroethylene high-pressure kettle; after filtration, washing and drying (40 ℃,24 h), NO is obtained 3 -LeuH precursor or NO 3 -an LTbH precursor;
(2) Preparing DS-LEuH and DS-LTbH: 0.2g of NO 3 -LeuH precursor or NO 3 The LTbH precursor sample was dispersed in 200mL of aqueous solution containing 0.2M DS and then heated in a Teflon autoclave at 90 ℃ for 24h. Centrifuging the anion exchange product, washing the precipitate with anhydrous ethanol for several times, and drying the final product at 50 deg.C for 24 hr to obtain DS-LEuH and DS-LTbH;
(3) Obtaining a colloidal suspension of lamellar LEuH nanoplates and a colloidal suspension of lamellar LTbH nanoplates: dispersing 0.1g of DS-LEuH or DS-LTbH sample into 50mL of Fm solution for 48h to obtain a stripped colloidal suspension of a layered LEuH nanosheet or a layered LTbH nanosheet;
step two, preparing an inner layer membrane solution A:
slowly adding 2g of carrageenan powder into 100mL of deionized water under continuous stirring, and stirring for 3h at 85 ℃; then adding 20mL of mixed solution A (DS-LEuH: DS-LTbH = 1;
step three, preparing PVDF/indicating hydrogel:
cooling the inner layer film solution A to 35 ℃ at room temperature, pouring the inner layer film solution A into a model padded with a PVDF film by adopting a sol-gel-casting method, and cooling to form PVDF/indicating hydrogel;
step four, preparing aerogel:
taking out PVDF/indicating hydrogel, sequentially soaking in a series of ethanol solutions (volume concentration of 10%,20%,30%,40%, 50%), respectively soaking for 1h, converting PVDF/indicating hydrogel into alcogel, and performing supercritical CO extraction 2 Drying under the condition (7.4 MPa,31.5 ℃) to obtain the aerogel with a porous structure, namely the freshness indicating film based on the ratio type fluorescent material.
Mechanical property test of the double-layer indicating film:
the mechanical properties of the film mainly include tensile strength and elongation at break. The membrane samples were placed in an environment of 20 ℃ and 60% RH and equilibrated for 48h before testing. The film is cut into strips of 100mm multiplied by 20mm and fixed on a microcomputer controlled electronic universal tester, the original stretching distance is set to be 50mm, the stretching speed is 50mm/min, and each sample is tested for 3 times. The Tensile Strength (TS), the elongation at break (EB) and the Young modulus (EM) are respectively calculated by formulas (a), (b) and (c);
wherein TS is tensile strength; f is the maximum load; s is the initial cross-sectional area of the film sample; EB is elongation at break; Δ l is at film breakSum of length difference l 0 Is the initial test length of the film. As a result, as shown in Table 1, the average maximum load was 60400N, and the average membrane area was 2000mm 2 The average value of elongation at break is 121.2 percent, and the average value of tensile strength is 30.2MPa, which shows that the double-layer film has good mechanical properties.
Table 1 mechanical testing of indicator films based on ratiometric fluorescent materials prepared in example 1
Note: samples 1-5 refer to 5 replicates.
Example 2:
step one, synthesizing a colloidal suspension of a layered LEuH nano sheet and a colloidal suspension of a layered LTbH nano sheet:
(1) Preparation of NO 3 -LEuH precursor and NO 3 -LTbH precursor: 1.2mmol of Eu (NO) 3 ) 3 ·6H 2 O or Tb (NO) 3 ) 3 ·6H 2 O，1.2mmol HMT，15mmol NaNO 3 Added to 96mL of an aqueous solution and heated in a polytetrafluoroethylene autoclave at 90 ℃ for 12 hours. After filtration, washing and drying (42 ℃,24 h), NO is obtained 3 -LeuH precursor or NO 3 -an LTbH precursor;
(2) Preparing DS-LEuH and DS-LTbH: 0.25g of NO 3 -LeuH precursor or NO 3 The sample of LTbH precursor was dispersed in 250mL of aqueous solution containing 0.2M DS and then heated in a Teflon autoclave at 95 ℃ for 24h. Centrifuging the anion exchange product, washing the precipitate with anhydrous ethanol for several times, and drying the final product at 52 deg.C for 24 hr to obtain DS-LEuH and DS-LTbH;
(3) Obtaining a colloidal suspension of lamellar LEuH nanoplates and a colloidal suspension of lamellar LTbH nanoplates: dispersing 0.12g of DS-LEuH or DS-LTbH sample into 60mL of Fm solution for 48h to obtain a stripped colloidal suspension of a layered LEuH nanosheet or a layered LTbH nanosheet;
step two, preparing an inner layer membrane solution A:
slowly adding 2g of carrageenan powder into 100mL of deionized water under continuous stirring, and stirring for 3h at 85 ℃; then, 20mL of mixed solution A (DS-LEuH: DS-LTbH = 1;
step three, preparing PVDF/indicating hydrogel:
placing the inner layer membrane solution A at room temperature, cooling to 40 ℃, pouring the inner layer membrane solution A into a model padded with a PVDF membrane by adopting a sol-gel-casting method, and cooling to form PVDF/indicating hydrogel;
step four, preparing the aerogel:
taking out PVDF/indicating hydrogel, sequentially soaking in a series of ethanol solutions (volume concentration of 10%,20%,30%,40%, 50%), respectively soaking for 1 hr to convert PVDF/indicating hydrogel into alcogel, and subjecting to supercritical CO 2 And drying under the condition (7.4 MPa,31.5 ℃) to obtain the aerogel with a porous structure, namely the freshness indicating film based on the ratio type fluorescent material.
And (3) testing the sensitivity of the indicating film to ammonia gas:
80mL of aqueous ammonia (8 mmol/L) was put into a 500mL Erlenmeyer flask, and a membrane (20 mm. Times.20 mm) was suspended at a liquid level of 10mm, and image information was recorded every 5min for 60min using a camera under fluorescent conditions. As the reaction time increased, the color of the freshness indicating film gradually changed from dark green to light green, yellow, orange, red under the fluorescent conditions, indicating that ammonia gas causes a change in the color of the freshness indicating film. Then, the freshness indicating film which is subjected to the ammonia gas sensitivity test is placed on a bottle mouth filled with an acidic solution, and under the action of acidic steam, the color of the freshness indicating film is gradually changed from red to original dark green, so that the freshness indicating film can be repeatedly used. And when fresh meat and aquatic products are rotten, ammonia gas and biogenic amines (cadaverine, putrescine, trimethylamine and the like) are generated, and the substances are proved to be used as indicator variables of the rotten deterioration of the meat and aquatic products. In the putrefaction process of the fresh meat and the aquatic products, the freshness indicating film gradually changes from the initial dark green to the light green, then to the yellow and the orange, and finally to the red, so the freshness indicating film has the potential of indicating the freshness of the fresh meat and the aquatic products in real time.
Example 3:
step one, synthesizing a colloidal suspension of layered LeuH nano-sheets and a colloidal suspension of layered LTbH nano-sheets:
(1) Preparation of NO 3 -LEuH precursor and NO 3 -LTbH precursor: 1.5mmol of Eu (NO) 3 ) 3 ·6H 2 O or Tb (NO) 3 ) 3 ·6H 2 O，1.5mmol HMT，19mmol NaNO 3 Added to 120mL of an aqueous solution, and heated at 105 ℃ in a polytetrafluoroethylene autoclave for 12 hours. After filtration, washing and drying (45 ℃,24 h), NO is obtained 3 -LeuH precursor or NO 3 -an LTbH precursor;
(2) Preparing DS-LEuH and DS-LTbH: 0.3g of NO 3 -LeuH precursor or NO 3 -an LTbH precursor sample was dispersed in 200mL of aqueous solution containing 0.2M DS, then heated in a teflon autoclave at 95 ℃ for 24h; centrifuging the anion exchange product, washing the precipitate with anhydrous ethanol for several times, and drying the final product at 55 deg.C for 24h to obtain DS-LEuH and DS-LTbH;
(3) Obtaining a colloidal suspension of lamellar LEuH nanoplates and a colloidal suspension of lamellar LTbH nanoplates: dispersing 0.15g of DS-LEuH or DS-LTbH sample into 65mL of Fm solution for 52h to obtain a stripped colloidal suspension of a layered LEuH nanosheet or a layered LTbH nanosheet;
step two, preparing an inner layer membrane solution A:
slowly adding 2g of carrageenan powder into 100mL of deionized water under continuous stirring, and stirring for 3h at 90 ℃; then adding 20mL of mixed solution A (DS-LEuH: DS-LTbH = 1;
step three, preparing PVDF/indicating hydrogel:
cooling the inner layer film solution A to 40 ℃ at room temperature, pouring the inner layer film solution A into a model padded with a PVDF film by adopting a sol-gel-casting method, and cooling to form PVDF/indicating hydrogel;
step four, preparing the aerogel:
taking out PVDF/indicating hydrogel, sequentially soaking in a series of ethanol solutions (volume concentration of 10%,20%,30%,40%, 50%), respectively soaking for 1 hr to convert PVDF/indicating hydrogel into alcogel, and subjecting to supercritical CO 2 And drying under the condition (7.4 MPa,31.5 ℃) to obtain the aerogel with a porous structure, namely the freshness indicating film based on the ratio type fluorescent material.
Freshness indicating film of ratiometric fluorescent material test for salmon quality:
the cleaned salmon is placed in a PET preservation box, an interlayer is prepared on the inner side of a surface inspection window on the PET preservation box, and meanwhile, the prepared double-layer film is cut into a square with the size of 30mm multiplied by 30mm and placed in the interlayer. The obtained salmon was refrigerated at 4 ℃ for 10 days, and the color of the film was recorded every 1 day under a fluorescent state using a camera while the content of TVB-N (volatile basic nitrogen) in the salmon was measured. After the data are collated, the rancidity degree of the salmon is related to the color of the film.
Specifically, the initial value of the salmon TVB-N is 8mg/100g, the color of the indicating film is dark green, the TVB-N value is continuously increased along with the increase of the storage time, the freshness indicating film is yellow at the 5d, the salmon TVB-N content is 19.28mg/100g, and the salmon TVB-N is in a sub-fresh state; by 8d, the TVB-N content of the salmon is more than 30mg/100g, which indicates that the salmon begins to putrefy and cannot be eaten, and the freshness indicating film is dark orange; at 10d, the salmon TVB-N content was 33.85mg/100g, indicating that the salmon had completely spoiled, at which time the freshness-indicating film was deep red.
The TVB-N content of the salmon is measured by referring to the national standard GB 5009.228-2016 < determination of volatile basic nitrogen in food >; the national standard GB2733-2015 fresh and frozen animal aquatic products stipulates that the TVB-N content of fresh seawater fishes is less than or equal to 30mg/100g.
Description of the invention: the above embodiments are only used to illustrate the present invention and do not limit the technical solutions described in the present invention; thus, although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted; all such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and within the scope of the following claims.
Claims (7)
1. A preparation method of a freshness indicating film based on a ratio type fluorescent material is characterized by comprising the following specific steps:
step one, preparing a colloidal suspension of a layered LEuH nano sheet and a colloidal suspension of a layered LTbH nano sheet;
step two, mixing the colloidal suspension of the layered LEuH nanosheets and the colloidal suspension of the layered LTbH nanosheets, which are prepared in the step one, to obtain a mixed solution A; slowly adding carrageenan powder into deionized water under continuous stirring, stirring at the temperature of 85-90 ℃, then adding the mixed solution A, stirring, adding a plasticizer, and continuously stirring to obtain an inner layer membrane solution A; wherein the plasticizer is glycerol;
step three, cooling the inner layer film solution A prepared in the step two to the temperature of 35-40 ℃ at room temperature, pouring the inner layer film solution A into a model with a PVDF film by adopting a sol-gel pouring method, and cooling to form PVDF/indicating hydrogel;
step four, taking out the PVDF/indication hydrogel prepared in the step three, immersing the PVDF/indication hydrogel into ethanol solutions with different volume concentrations or sequentially immersing the PVDF/indication hydrogel into the ethanol solutions with different volume concentrations, converting the PVDF/indication hydrogel into alcohol gel after immersing, and then performing supercritical CO (carbon monoxide) treatment on the alcohol gel 2 Drying under the condition to obtain the aerogel with the porous structure, namely the freshness indicating film based on the ratio type fluorescent material.
2. The preparation method of the ratio-based fluorescent freshness indicating film according to claim 1, wherein the volume of the colloidal suspension of the layered LEuH nanosheets and the colloidal suspension of the layered LTbH nanosheets in step two is 1 to 3 to 5.
3. The method for preparing the ratio-based fluorescent material freshness indicating film according to claim 1, wherein in the second step, the stirring is carried out at 85 to 90 ℃ for 2 to 3 hours; the time for adding the mixed solution A and stirring is 2 to 3 hours; the time for adding the plasticizer and continuing stirring is 30min.
4. The method for preparing a ratio-based fluorescent freshness indicating film according to claim 1, wherein the amount ratio of the carrageenan powder, the deionized water, the mixed solution A and the plasticizer in step two is 2g:100mL:20mL:2mL.
5. The method for preparing a ratio-type fluorescent material-based freshness indicating film according to claim 1, wherein the volume concentration of the ethanol solution in step four is 10% -50%; the sequential immersion is specifically sequential immersion from small concentration to large concentration; the soaking time is 40 to 60min.
6. The method for preparing a ratio-based fluorescent freshness-indicating film according to claim 1, wherein the supercritical CO is obtained in step four 2 The drying temperature under the conditions is 31.5 ℃ and the pressure is 7.4MPa.
7. The freshness indicating film prepared by the method according to any one of claims 1 to 6 is used for visual detection of freshness of fresh meat and aquatic products.
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