Antibacterial effects of graphene- and carbon-nanotube-based nanohybrids on Escherichia coli: Implications for treating multidrug-resistant bacteria

Some nanomaterials including Fe-0, Ag-0, and ZnO are well known for their antibacterial effects. However, very few studies have examined antibacterial effects of nanohybrids. Given that metal oxides, mainly ZnO and TiO2, are known to increase mobility, surface area, and photocatalysis when combined with carbon-based nanomaterials, ZnO- and TiO2-conjugated carbon nanotube and graphene oxide nanohybrids were investigated for their antibacterial effects on Escherichia colt (DH5a, a multidrug-resistant coliform bacterium). Graphene-oxide (GO) based nanohybrids (ZnO-GO and TiO2-GO) induced increased dispersion compared to carbon-nanotube (CNT)based nanohybrids (ZnO-CNT and TiO2-CNT). Among the four types of nanohybrids, ZnO-conjugated nano hybrids exhibited a higher antibacterial property, resulting in the antibacterial effect (measured with growth inhibition of cells) in the order ZnO-GO > ZnO-CNT > TiO2-GO > TiO2-CNT. Among four possible antibacterial mechanisms (generation of reactive oxygen species (ROS), physicochemical characteristics, the steric effect, and release of metal ions), a primary mechanism ROS generation was identified; whereas, physicochemical characteristics and the steric effect were part of contributing mechanisms. The increasing dispersion of TiO2/ZnO on GO may have contributed to the antibacterial effects due to increasing surface areas. Similarly, significant damages to E. colt cell membranes were found by the GO sheet with its sharp edges. Our results suggest that applying GO-based ZnO or TiO2 could be an effective antibacterial method, especially for the treatment of multidrug-resistant bacteria in the water.