Sol-gel preparation of Ag-doped MgO nanoparticles with high efficiency for bacterial inactivation

Herein, Ag-doped magnesium oxide (MgO) nanoparticles were prepared by a citric acid-assist sol-gel method. It is evidenced that the size of MgO particles decreases after Ag doping and a small amount of Ag is doped into MgO crystal. The bacterial inactivation of as-prepared Ag-doped MgO against Escherichia coli (E. coli) suggests that Ag doping can greatly enhance the antibacterial activity of MgO nanoparticles and 1% Ag-doped MgO inactivates effectively 7-log bacterial cells within 20 min. The releases of metal ions (Ag+ and Mg2+) from Ag-doped MgO are at a very low level, which would not play the leading role in bacterial inactivation. The mechanism for the improvement of antibacterial activity of Ag-doped MgO is concluded as three aspects. Firstly, Ag doping can inhibit the grain growth of MgO nanoparticles, resulting in smaller size of MgO particles. Secondly, when Ag+ is doped into MgO matrix, more oxygen vacancies will be generated to keep an overall neutral charge. Thirdly, Ag-doped MgO has a relatively low electron-transfer resistance, which can accelerate the electron transfer within MgO crystal, in favour of the single-electron reduction of adsorbed oxygen. All these would substantially enhance ROS production and the contact interaction between bacterial cells and nanoparticles. Therefore, Ag doping can markedly promote the antibacterial activity of MgO nanoparticles.