Preparation Of Cuprous Oxide Composite Antibacterial Materials And Their Application In Food Packaging

Food packaging is an effective method to ensure food safety and quality.However,the non-biodegradable and non-renewable nature of traditional plastic packaging materials poses a great threat to the natural environment and human health.Therefore,new biodegradable packaging materials are of great interest to researchers.Chitosan(CS),a natural polysaccharide,is considered as one of the alternative materials to petroleum-based packaging materials because of its biodegradability,water solubility,biocompatibility and excellent film-forming ability.However,the poor mechanical properties,water barrier properties and anti-microbial properties of chitosan film greatly limit its practical application in food packaging.The introduction of other degradable materials and antimicrobial active substances as well as cross-linking agents can effectively enhance the overall performance of chitosan films.Polyvinyl alcohol(PVA)is a synthetic biodegradable polymer material with excellent film-forming ability,high toughness and chemical stability,which is often used to compound with chitosan to improve the mechanical properties and gas barrier performance of chitosan films.Cuprous oxide is a metal oxide antimicrobial material with abundant reserves and is an ideal antimicrobial additive for packaging materials because of its low toxicity,low price and simple preparation method.In this paper,with the aim of improving the performance of chitosan film as a packaging material,we designed and prepared a cuprous oxide based antimicrobial material as an antimicrobial additive,added ferulic acid with antimicrobial,antioxidant and UV-blocking functions as a cross-linking agent,and prepared a CS/PVA biodegradable antimicrobial multilayer film by layer-by-layer casting,and applied it to fruit preservation.The following are the main research contents and results of this thesis.(1)Cuprous oxide particles(Cu₂O@NCs)were prepared by in situ reduction under alkaline conditions using chitosan as the stabilizer and glucose as the reducing agent.The structure and morphology of the prepared Cu₂O@NCs were characterized by X-ray diffraction(XRD),which showed that the Cu₂O@NCs had a hematite crystal structure,and Fourier transform infrared spectroscopy(FT-IR),which showed the coordination between chitosan and cuprous oxide.The average particle size was about 300 nm,with rough surface and irregular spherical particles.The results of Cu ion release test showed that the Cu ion release rate of Cu₂O@NCs was significantly lower compared to that of unmodified Cu₂O.The antibacterial properties of Cu₂O@NCs were tested by the inhibition circle test,and the results showed that Cu₂O@NCs had good antibacterial effects on both Escherichia coli and Staphylococcus aureus.(2)LbL-PVA/CS/FA/Cu₂O@NCs composite multilayer membranes were prepared by blending Cu₂O@NCs into chitosan and polyvinyl alcohol membrane solution with ferulic acid as cross-linking agent by layer-by-layer casting method.The structure and morphology of the composite films were characterized by XRD,FT-IR and SEM,and the results showed that there were certain interactions(e.g.hydrogen bonding)between different film layers,but no covalent bonding,and the addition of FA and Cu₂O@NCs enhanced the interactions between the film components and made the structure of the composite films more dense.And when the addition of Cu₂O@NCs was 2%,the tensile strength of LbL-PVA/CS/FA/Cu₂O@NCs composite multilayer film reached 34.34 MPa,the elongation at break was 96.67%,the water vapor transmission rate was reduced to 0.16 g·mm·m-2h-lkPa-1,the water solubility was decreased to 19.90%,and the water contact angle was 86°.The light transmission of LbL-PVA/CS/FA/Cu₂O@NCs composite membrane was tested by UV-Vis spectrophotometer,and the results showed that the composite membrane exhibited excellent UV light barrier performance.the results of DPPH radical scavenging test showed that the composite membrane possessed excellent antioxidant activity.The bacterial inhibition performance of LbL-PVA/CS/FA/Cu₂O@NCs composite membrane was tested by plate colony counting method,and the results showed that the composite membrane achieved 72%and 66%inhibition of Escherichia coli and Staphylococcus aureus.In addition,the soil degradation test showed that the composite film had a weight loss rate of 41.87%after 28 days of burial in soil.The preservation test of cherry tomatoes showed that the prepared LbL-PVA/CS/FA/Cu₂O@NCs composite film had better freshness preservation effect on cherry tomatoes.(3)In order to further improve the stability of the cuprous oxide based antibacterial materials and reduce the release rate of copper ions,Cu₂O-Ag heterostructured composites were prepared by depositing nano-silver onto the Cu₂O surface using the thermal decomposition method,and Cu₂O-Ag was surface modified with tannic acid to synthesize the composite antibacterial agent(Cu₂O-Ag)@TA with core-shell structure.The structure and morphology of the powder samples were characterized by XRD,FT-IR,X-ray photoelectron spectroscopy(XPS),SEM,TEM,laser particle size analyzer and UV-vis diffuse reflectance spectroscopy(UV-vis DRS).The results of structural analysis showed that the silver nanoparticles were tightly bound to the cuprous oxide microspheres by charge transfer,forming a Cu₂O-Ag heterostructure,and the tannic acid was coated on the Cu₂O-Ag surface by chelation with the metal ions.The morphological characterization results showed that the silver nanoparticles were uniformly distributed on the surface of Cu₂O microspheres,and(Cu₂O-Ag)@TA was a core-shell structure with a tannic acid shell layer thickness of about 6 nm.In addition,(Cu₂O-Ag)@TA exhibited excellent dispersion stability and chemical stability,and could stably release Cu₂+within 14 days with enhanced reactive oxygen species release performance,thus achieving excellent antibacterial performance.(4)(Cu₂O-Ag)@TA was added to the LbL-PVA/CS/FA composite film matrix in a co-blended manner,and the XRD and FT-IR structural characterization illustrated that the crystal structure of(Cu₂O-Ag)@TA did not change and interacted with the composite film matrix(hydrogen bonding),which increased the crystallinity of the composite film.The morphological analysis shows that(Cu₂O-Ag)@TA is uniformly dispersed in the composite film matrix.In addition,the addition of(Cu₂O-Ag)@TA improved the mechanical properties,water resistance and antibacterial ability of the composite film.The results of the preservation test on cherry tomatoes showed that the LbL-PVA/CS/FA composite film doped with(Cu₂O-Ag)@TA effectively inhibited the moisture loss,microbial infection and nutrient loss of cherry tomatoes during storage,significantly prolonged its shelf life,and provided a feasible solution for a new biodegradable antibacterial packaging material.