| Chitosan(Chitosan,referred to as CS)is a kind of natural cationic polymers with good biocompatible and film-forming properties.Although chitosan with high molecular weight can form good film,it shows a poor spectrum of antibacterial ability.Nano-silver(AgNPs)is of antibacterial ability with a broad spectrum,but easy to reunite.Chitosan compounded AgNPs can stabilize AgNPs,enhance the antibacterial capacity of the natural biological materials chitosan,and then expand the applications.Therefore,this paper studied the AgNPs in situ synthesis within chitosan taking punicalagin as the reducing agent,and the antibacterial properties and mechanism of CS/AgNPs composites were also investigated.The film forming parameters of the composites were optimized by response to surface method and then the film was applied in strawberry preservation.The main findings were as follows:1.The CS/AgNPs sol was prepared by using punicalagin as a green reducing agent,and the characterization and antibacterial properties of CS/AgNPs were studied.The results showed that a narrow particle size,high silver particles and stable CS/AgNPs sol was produced at conditions mentioned as below.At room temperature,every 100 mL,pH 5 chitosan with concentration of 0.5%-1%,added 250 mL of silver nitrate with concentration of 0.6 mol/L,chelated for 30 min,and followed by adding 1% punicalagin solution 800 ?L,reduced for 6 h at 25 ?.The average particle size of Ag NPs nanoparticles in the composites was 8-11 nm detected by transmission electron microscopy(TEM).The crystal form was face-centered cubic reflected by the X-ray diffraction(XRD).And the Fourier transform infrared spectroscopy showed that AgNPs were present in the CS/AgNPs sol.The minimal inhibitory concentration of the CS/AgNPs sol on E.coli and S.aureus were both 0.096 ?L/mL,and the inhibitory rate was 95.5% with E.coli and 97.6% with S.aureus after a 24 h incubation.The SEM scanning and cell contents leakage experiments of the cultures after a 24 h incubation showed that CS/AgNPs sol can adsorbed to bacterial cells,disrupted the cell membrane,and increased cell permeability and then inhibited the cell growth.2.The antibacterial film prepared by the CS/AgNPs composites was researched.The effects of glycerol,ultrasonic time and drying temperature on the mechanical properties of the film were investigated.The surface roughness of the film was observed,and the acid and alkali resistances as well as the thermal stability of the film were explored.The CS/AgNPs sol prepared by using 1% chitosan as the initial material,and the film forming conditions were optimized.The results showed that among the three factors that affected the tensile strength and elongation at break,the drying time had the greatest influence on it and followed by the content of glycerol and the minimum influence of ultrasound time on it.The optimum preparation conditions of film were as follows: 5.5% glycerol was added to the initial film material(1% CS/AgNPs sol),being stirred for 10 min before ultrasonic for 60 min.The film was dried at a temperature of 43 ? for 4 h.The elongation at break(E)of the film was 21.71% and the strength(TS)was 18.17 MPa.The atomic force microscopy(AFM)and the scanning electron microscopy(SEM)both displayed a smooth surface of the film.The contact angle of the film was 45.96 ° with a swelling ratio of 2.25.It was curled in the dilute acid and swelled in the concentrated acid.The thermogravimetric analysis showed that the CS/Ag NPs film should be used at a temperature below 100 ?.3.The effects on strawberry storage were studied by packaging strawberry with the CS/AgNPs film.The results showed that this film could retard the decrease of the weight loss rate,the hardness,the soluble sugar,the titratable acid and the vitamin C loss with the storage time,and delay the increase of decay index and the respiration rate.When stored for 5 days,the weight loss rate was 3.48% lower than that of the control group and the decay index was 33.3% lower than that of the control group.The hardness was 42.33 g higher than that of the control group,and the soluble sugar was 2.2% higher than that of the control group with 0.36% increase of the titratable acid than that of control group.The respiration rate slowed down about 24% than that of the control group.The vitamin C was 26 mg/100 g higher than that of the control group.Strawberry encapsulated with CS/AgNPs could prolong the shelf life of strawberry,which provided a theoretical basis for prolonging the shelf life of strawberry.CS/AgNPs sol was synthesized in situ by using the green reducing agent punicalagin in this research.This sol displayed the good inhibitory effects on both E.coli and S.aureus.The mechanisms might be to disrupt the cell membrane and increase the cell permeability.The sol could be further made into an antibacterial film and showed the good properties on strawberry preservation. |
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