Study On Preparation And Properties Of Silver Nanocomposites

With the improvement of people’s living standards,health has become the focus of people’s attention.Meanwhile,the existence and mutation of some pathogenic microorganisms are threatening human health.Therefore,the development of antibacterial,bactericidal materials has become one of the focus of researches.As a kind of inorganic antibacterial agent,silver nanoparticles have attracted much attention due to their good antibacterial properties,wide antibacterial spectrum and no antibiotic resistance.However,silver nanoparticles are easy to agglomerate due to high activity,although the organic surface modification of silver nanoparticles can inhibit the aggregation of silver nanoparticles,which will also affect their antibacterial properties.At the same time,lots of silver ions released from silver antibacterial materials will also pose a great risk to human health and ecological environment.Therefore,it is of great scientific significance and application value to obtain silver nanoparticles with high antibacterial properties and to control the release of silver ions effectively.In this dissertation,two aspects of silver based nano antibacterial materials were prepared to solve the above problems:first,study on the preparation and properties of silver nanoparticles with synergistic antibacterial effect;second,study on the preparation and properties of silver nanoparticles without capping agents.The main research contents are as follows:（1）Chitosan stabilized silver nanocomposites colloids were prepared by chemical reducing method using chitosan as a stabilizing agent.The solubility of chitosan was utilized to control the precipitation and redispersion of silver nanoparticles,to remove the impurities ions.It was found that the concentration of chitosan shows great influence on particle size of silver nanoparticles.At moderate concentration of chitosan,the particle size of silver nanoparticles could be controlled in the range of 1～5 nm.And if the concentration was too low or high,silver nanoparticles would grow up.Moreover,the concentration of chitosan can also affect the purification process and long-term stability of nano silver sol.105 mg/L silver/chitosan nanocomposites（silver 5 mg/L）could totally inactivate ca.10⁷ CFU/mL E.coli within 120 min.Morover,after purification,the concentration of nitrate and sodium ions dereased from 55.51 mg/L and 118.74 mg/L to 0.30 mg/L and 0.28 mg/L,respectively,and it has no effect on the antibacterial property of sol.（2）Silver nanoparticles supported on silica spheres/mesoporous silica were prepared in basic ethanol using silica spheres/mesoporous silica as carrier.In the reduction reaction,none organic surfactant was used,hence silver nanoparticles were not capped by organic surface modifiers.Surprisingly,silver ions could be reduced by ethanol in the presence of sodium hydroxide.And the final product of the reaction depends on the mole ratio of silver nitrate and sodium hydroxide,when[NaOH]/[AgN03]≤1,the final products were mixture of silver and silver oxide,otherwise,the product was silver.Additionally,the concentration of silver nitrate and the amount of silica spheres/mesoporous silica had crucial influence on the morphology of silver nanoparticles.The antibacterial results suggested that the antibacterial activities of uncapped silver nanocomposites were considerably poor when performed in nutrient broth.The MIC（Minimum inhibitory concentration）values of SiO₂-Ag and mSiO₂-Ag were 200 mg/L and 160 mg/L,respectively.While the bacterial inactivation plots demonstrated that 25 mg/L SiO₂-Ag and mSiO₂-Ag nanocomposites（silver 1.5 mg/L）could entirely inactivate ca.10⁷ CFU/mL E.coli within 80 and 120 min,respectively,in PBS solution.The investigation on the antibacterial mechanism revealed that the release of silver ions rather than ROS production is the main mechanism for the antibacterial activity of silver nanoparticles.（3）The reduction of 4-nitrophenol by excess amount of sodium borohydride was employed as model reaction,uncapped mSiO₂-Ag nanocomposite was used as catalyst.It was found that mSiO₂-Ag nanocomposites exhibit very excellent catalytic performance,the apparent reaction rate constant was calculated to be 0.02422 s-1.When the mass of catalyst was taken into consideration,the rate constant was 1211 s-1 g-1.The effects of temperature,the amount of catalyst,the amount of NaBH4 and surface modifiers were also investigated.It was found that the reduction reaction becomes much faster if the temperature,the amount of catalyst and sodium borohydride increase.The reusability demonstrated mSiO₂-Ag nanocomposites are of good stability.The activation energy was calculated to be 35.07 kJ/mol.Unfortunately,surface modifiers would hinder the diffusion of reactant molecules towards catalyst,consequently lead to the decrease of reaction rate,suggesting the catalytic reduction reaction occurs on the surface of mSiO₂-Ag nanocomposites catalyst.（4）Nano magnesium oxide was used as carrier and catalyst to prepare MgO-Ag nanocomposites in ethanol.Nano MgO was nanoplates with particles size of about 30 nm while the particle size of silver nanoparticles was in the range of 3～7 nm.Possible catalytic reduction of silver ions mechanism was proposed:first,silver ions exchange with magnesium ions to produce silver oxide;second,silver oxide was reduced by ethanol with nano-MgO as catalyst;third,the formed silver nanoparticles adsorbed on the surface MgO nanoplates.As-prepared MgO-Ag nanocomposites showed very excellent antibacterial activity.The MIC value of as-prepared nanocomposites was 20 mg/L,which is much lower than those of MgO and Ag nanoparticles components.While 25 mg/L MgO-Ag nanocomposites（silver 1.25 mg/L）could entirely inactivate ca.10⁷ CFU/mL E.coli within 100 min.Additionally,the antibacterial studies suggested that MgO and Ag nanoparticles show a synergistic effect on inhibition of bacterial growth.Moreover,the antibacterial mechanisms of MgO-Ag nanocomposites were ROS production and contact interaction between nanocomposites and bacterial cells.