Why does nitrogen-doped graphene oxide lose the antibacterial activity?

Graphene and its derivatives attract extensive research interests in the biomedicine field due to their outstanding physiochemical properties. Lots of studies have reported that graphene materials exhibit antibacterial activities. However, antibacterial mechanisms of graphene materials still remain controversial and need further investigation. Herein, graphene oxide (GO) with and without nitrogen-doping were fabricated on the titanium surface by cathodic electrophoretic deposition and antibacterial activities were systematically investigated. Results showed that GO on the titanium surface presented antibacterial activity, while nitrogen-doped GO lost the antibacterial activity. The reason is that antibacterial mechanisms for the GO-metal system contain two steps. First, electron transfer occurs from bacterium’s cell membrane to GO surface which destroys the bacterial respiratory chain; subsequently, electrons on GO surface induce the production of reactive oxygen species (ROS) that damage the membrane structure and eventually lead to bacterial death. For nitrogen-doped GO, nitrogen atoms denote electrons into GO leading to n-type doping. Nitrogen-doped GO as an electron donor cuts off the electron transfer from the cell membrane to GO and subsequently inhibits the production of ROS. This is why nitrogen-doped GO exhibits no antibacterial activity. This work confirms the antibacterial mechanisms for the GO-metal system with a synergistic effect of non-oxidative electron transfer and ROS mediated oxidative stress.