Antibacterial Activity of Reduced Graphene Oxide

The increasing biological use of graphene-based materials has prompted research inquiries on their effects on microorganisms. The work herein reported different types of microbiological activity of reduced graphene oxide (RGO). At relatively high concentrations (200 and 400 μg/mL), RGO exhibited antibacterial activity on the model bacterium Escherichia coli, while at lower concentrations (10 and 50 μg/mL), interestingly, no antibacterial effect was observed. Instead, an increase in the viable population after exposure at lower concentrations was observed, verified by colony counting and fluorescence microscopy. Further investigation ruled out the possibility of nutrient release from RGO being responsible for this growth-enhancing effect, whereby a comparable number of viable cells were found in the particle-free RGO leachate systems relative to the control. A before and after exposure X-ray photoelectron spectroscopy (XPS) analysis of the RGO detected less presence of C-C bond on the particle surface, suggesting the ability of the bacterium for the use of the carbon-based materials for growth. This potential RGO-cell interaction is further supported by the observed emergence of C-N bond on the particle surface, the nitrogen moieties most likely of bacterial (cell envelope) origins. Although still an early evidence, such RGO-cell interactions could explain the viable cell increase observed at the lower concentration RGO systems. The present study highlights the concentration-dependent microbiological effects of RGO, clarifying the contradicting reports on the growth enhancing versus antibacterial effect of graphene-based materials. The knowledge is important not only for the antibacterial formulation of carbon-based materials but also when assessing their environmental impact.