Preparation Of Silver/Chitosan Complexes And Evaluation Of Their Antioxidant And Antimicrobial Activity In Vitro

Chitosan is a polycationic natural polysaccharide with excellent properties including biocompatibility and biodegradability, non-toxicity, absorption and film-forming properties. In recent years, chitosan has been paid more and more attention due to its antioxidant and antimicrobial activities. Furthermore, chemical modifications of chitosan have been attempted in an effort to increase the biological activity and widen its application.Herein, silver/chitosan complexes were prepared using chitosan as starting material and characterized. The antioxidant and antimicrobial activities in vitro were evaluated with respect to food quality control and fruit and vegetable fungal diseases prevention application. Results are as follows:(1) In a heated alkaline solution, the silver/chitosan nanoparticle with small particle size was obtained by reducing silver nitrate with chitosan under optimized reaction conditions, in which chitosan was used as both the reducing and stabilizing agent. The silver/chitosan complex was characterized and determined by UV-visible spectroscopy, Fourier transform infrared spectroscopy, dynamic light scattering measurement and transmission electron microscopy.(2) Antioxidant acticity of the prepared silver/chitosan complex was evaluated according to DPPH (2,2-diphenyl-1-picrylhydrazyl) free radical scavenging ability, potassium ferricyanide reducing power and hydroxyl radical scavenging ability. The results demonstrated that the antioxidant acticity of silver/chitosan complex was significantly better than unmodified chitosan. At a concentration of14mg/mL, DPPH scavenging capacity of our silver/chitosan complex reached78.6%, which was much higher than that of natural chitosan (8.3%). The reduction power of silver/chitosan complex was twice higher than that of chitosan. At a concentration of10mg/mL, hydroxyl radical scavenging ability of silver/chitosan complex was83.1%and significantly higher than that of chitosan (57.2%).(3) The antibacterial activity of silver/chitosan complex was determined against Siaphylococcus aureus, Salmonella cho/eraesuis, Escherichia coli and Bacillus subtilis, using unmodified chitosan as control. The minimal inhibitory concentration (MIC) of silver/chitosan against S. aureus, S. choleraesuis, E. coli and B. subtilis were0.064,0.064,0.032and0.032mg/mL, respectively; the minimum bactericidal concentration (MBC) of silver/chitosan against these tested strains were0.256,0.256,0.128,0.128mg/mL, respectively. And the antibacterial activity of silver/chitosan complex was significantly better than that of chitosan. The damaged bacterial morphologies of both Gram-positive (G+) S. aureus and Gram-negative (G) S. choleraesuis upon treating with silver/chitosan were further studied by transmission electron microscopy. TEM examination showed that there were serious damages on walls as well as protoplasts of the treated bacteria, and the damage of Gram-positive (G+) bacteria was more serious than that of Gram-negative (G-) bacteria.(4) The silver/chitosan complex also exhibitd a good antifungal activity on pathogenic fungi which were isolated and identified from blueberry. And silver/chitosan complex showed a good inhibition on mycelial growth, conidial germination and germ tube elongation. The inhibitory indexes of mycelial growth for silver/chitosan and chitosan were70.6%and11.7%, respectively, at0.4mg/mL. At a concentration of0.20mg/mL, conidial germination and germ tube elongation were entirely suppressed by silver/chitosan, while a higher concentration (2.0mg/mL) of chitosan was needed to get the same effect. Besides, silver/chitosan significantly reduced postharvest decay of blueberry. The decay of blueberry decreased with the increasing of silver/chitosan concentration. The decay percentage decreased to10%at a concentration of1.6mg/mL.