Antimicrobial activity of silver loaded MnO_2 nanomaterials with different crystal phases against Escherichia coli

Silver-loaded MnO_2 nanomaterials（Ag/MnO_2）,including Ag/α-MnO_2,Ag/β-MnO_2,Ag/γ-MnO_2and Ag/δ-MnO_2 nanorods,were prepared with hydrothermal and impregnation methods.The bactericidal activities of four types of Ag/MnO_2 nanomaterials against Escherichia coli were investigated and an inactivation mechanism involving Ag~＋ and reactive oxygen species（ROS）was also proposed.The bactericidal activities of Ag/MnO_2 depended on the MnO_2 crystal phase.Among these nanomaterials,Ag/β~-MnO_2 showed the highest bactericidal activity.There was a 6-log decrease in E.coli survival number after treatment with Ag/β~-MnO_2 for120 min.The results of 5,5-dimethyl-l-pyrroline-N-oxide spin-trapping measurements by electron spin resonance indicate OH and O_2~- formation with addition of Ag/β-MnO_2,Ag/γ-MnO_2 or Ag/δ-MnO_2.The strongest peak of OH appeared for Ag/β-MnO_2,while no OH or ·O_2~-signal was found over Ag/α-MnO_2.Through analysis of electron spin resonance（ESR） and Ag＋elution results,it could be deduced that the toxicity of Ag~＋ eluted from Ag/MnO_2 nanomaterials and ROS played the main roles during the bactericidal process.Silver showed the highest dispersion on the surface of β-MnO_2,which promoted ROS formation and the increase of bactericidal activity.Experimental results also indicated that Ag/MnO_2 induced the production of intracellular ROS and disruption of the cell wall and cell membrane.