| Metal-organic frameworks are the new type of porous materials with large specific surface area,good pore structure and high stability.However,most metal-organic frameworks are limited in their food application to the toxicity of the raw materials.Cyclodextrin metal-organic framework（CD-MOF）has the advantages of safety,non-toxicity,renewable environmental protection,large specific surface area and good biocompatibility,which has been widely studied as a carrier material.However,the low toxicity of raw materials limits the application of CD-MOF in food safety.In addition,CD-MOF has the problems of the long preparation time and easy disintegration in aqueous environments.Therefore,it is of great significance and expand its application in food safety to solve the problem that how to realize the rapid synthesis of the CD-MOF,enhance the antibacterial properties of CD-MOF in combination with antibacterial agents and solve the poor water stability of CD-MOF.The major content and results are summarized as follows:（1）The ultrasound-assisted rapid synthesis of CD-MOF has been reported and the process conditions were explored.It was found that its power,processing time and processing temperature would affect the morphology and structure of CD-MOF.The results showed that the optimal conditions were ultrasonic power of 540 W,ultrasonic processing time of 10 min and ultrasonic processing temperature of 60℃.The crystal obtained under the optimal condition was uniform in size and had a typical cubic morphology.The appearance and crystal properties of the obtained CD-MOF were characterized by scanning electron microscopy（SEM）,powder X-ray diffraction（PXRD）,Fourier infrared spectroscopy（FT-IR）,etc.It was found that the structure of the obtained CD-MOF had a crystallinity of 95.96±5.03%,and was similar to the CD-MOF prepared by the traditional method which had a cubic shape with an average size of 8.60±1.95μm.（2）The natural antibacterial product caffeic acid（CA）was used as the antibacterial agent for the loading behavior and mechanism of CD-MOF on CA were explored.CA was loaded on the obtained CD-MOF by dipping method to obtain the composite material with antibacterial effect（CA@CD-MOF）,the preparation process of CA@CD-MOF was optimized,and the effect of CD-MOF on caffeic acid was explored.The maximum load rate of caffeic acid could be reached at 19.63±2.53%when the reaction time was determined to be 720 min,the reaction temperature was 40℃,and the molar ratio ofγ-CD in CD-MOF to caffeic acid was 1:64.The appearance morphology,thermal stability and structural properties of CA@CD-MOF could be analyzed by SEM,PXRD,FT-IR,thermogravimetric analysis（TGA）,etc.The morphology and crystal structure of CA@CD-MOF were not obviously changed and caffeic acid mainly existed in the cavity of CD-MOF in amorphous form.The release of caffeic acid by CA@CD-MOF in water and phosphate buffered saline（PBS）environment showed a burst release of nearly 100%within 1 min.The slow-release function could be realized in ethanol environment,by fitting the release kinetics equation,the release process of caffeic acid in ethanol environment conformed to the Higuchi equation.The results of the antibacterial test showed that the MIC and MBC of CA@CD-MOF for Escherichia coli O157:H7（E.coli O157:H7）were both 25mg·m L-1,and the MIC for Staphylococcus aureus（S.aureus）was 25 mg·m L-1.Through molecular simulation to explore the loading mechanism of caffeic acid and CD-MOF,caffeic acid tended to be distributed first in the doubleγ-CD molecular pair（D-γ-CD）structure of CD-MOF,as the loading of caffeic acid increased,caffeic acid was gradually distributed into the large cavity inside the CD-MOF.（3）Simultaneous loading of the two antibacterial agents by CD-MOF was further achieved by using the widely studied silver nanoparticles（Ag NPs）as the antibacterial agent to improve the synergistic antibacterial performance of the composite.The hydroxide ions in the CD-MOF structure were used to reduce silver nitrate to prepare Ag NPs and the CD-MOF that could simultaneously load CA and Ag NPs（CA@Ag@CD-MOF）was constructed by impregnation method.The preparation process of CA@Ag@CD-MOF was optimized and proved to be safe and non-toxic by cytotoxicity test.The maximum load rate of caffeic acid and Ag NPs could be reached at 19.63±2.53%and 4.97±0.84%respectively when the silver nitrate reaction concentration was 7.5 m M and the silver nitrate reaction time was 12 h,the molar ratio ofγ-CD in CD-MOF to caffeic acid was determined to be 1:64,caffeic acid reaction time was 15 h as the best preparation conditions.The Energy Dispersive Spectrum（EDS）,SEM,PXRD,FT-IR,TGA,etc.were used to analyze the appearance,morphology,thermal stability and structural properties of CA@Ag@CD-MOF.PXRD results showed that the characteristic peaks of silver element were appearing at 38.12°,44.28°and 64.43°,indicating the successful preparation of nano-silver.EDS results showed that the distribution of silver and potassium in CA@Ag@CD-MOF was consistent,indicating that the prepared nano-silver was evently distributed in CD-MOF.CA@Ag@CD-MOF had no obvious changed in morphology and crystal structure and caffeic acid mainly existed in the cavity of CD-MOF in amorphous form.CA@Ag@CD-MOF released caffeic acid and silver ions in a water environment as a burst release within 1 min,and almost no release of silver ions in an ethanol environment.The results of the antibacterial test showed that the MIC and MBC of CA@Ag@CD-MOF for E.coli O157:H7 were also 250μg·m L-1,while the MIC and MBC for S.aureus were both 500μg·m L-1.The cytotoxicity test showed that the prepared MOF crystals are non-toxic at 1000μg·m L-1.（4）In order to improve the water stability of CA@Ag@CD-MOF for sustained release applications,the hydrophobic material polydimethylsiloxane（PDMS）was chosen as the matrix to prepare the CA@Ag@CD-MOF/PDMS hybrid membrane by physical blending method.The preparation process of the hybrid membrane was optimized and the slow-release antibacterial ability of the hybrid membrane was explored.The amount of CA@Ag@CD-MOF was optimized based on the mechanical properties of the membrane,and the amount of 15 wt%CA@Ag@CD-MOF was the best preparation condition（CA@Ag@CD-MOF/PDMS-15）.Compared with pure PDMS,CA@Ag@CD-MOF/PDMS-15 had an 18%increase in elongation at break and a nearly 100%increase in elastic modulus.SEM,XRD,FT-IR,etc.were used to analyze the appearance,morphology and structural properties of the membrane.The XRD results showed that the characteristic peaks attributable to CD-MOF appeared at6.9°and 16.7°.The characteristic peaks at 38.1°,44.3°and 64.4°are attributable to the appearance of the single substance of silver,indicating that CA@Ag@CD-MOF was successfully incorporated and its crystal structure remained basically unchanged after being incorporated into the PDMS matrix.The water contact angle results showed that the hydrophobicity of the membrane was greater than 90°,indicating that the membrane had good hydrophobicity due to the introduction of the PDMS matrix.The swelling results showed that the swelling degree of the membrane in water under the optimal preparation conditions was about 41.8%,indicating that the good hydrophilicity of CD-MOF endowed the membrane with the characteristics of swelling in an aqueous environment.The membrane could achieve a slow-release effect in an aqueous environment.68%of caffeic acid could be released in 24 h.The release process of caffeic acid conformed to the first-order kinetic equation;66%of silver ions could be released in 24 h.The release process of silver ions was most consistent with the Higuchi equation.After the membrane was immersed in different solvent environments（water,ethanol,toluene,and acetonitrile）for 48 h,the release results showed that caffeic acid and silver ions were mainly released in water,and the SEM results showed that the cross-section of the membrane after immersion in water had more voids and holes.The antibacterial kinetic curve of E.coli O157:H7 and S.aureus showed that the membrane had a good slow-release antibacterial effect and could completely kill E.coli O157:H7in a shorter time than S.aureus. |
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