Citation:

Baek, S., S. Joo, C. Su, AND M. Toborek. Antibacterial effects of graphene- and carbon-nanotube-based nanohybrids on Escherichia coli: implications for treating multidrug-resistant bacteria. JOURNAL OF ENVIRONMENTAL MANAGEMENT. Elsevier Science Ltd, New York, NY, 247:214-223, (2019). https://doi.org/10.1016/j.jenvman.2019.06.077

Impact/Purpose:

Carbon nanotubes (CNTs), graphene oxide (GO), TiO2, and ZnO are among the most widely used nanoparticles in numerous products such as batteries, paints, pharmaceuticals, and cosmetics. These nanoparticles have been indicated to have antibacterial effect on bacteria because they cause either bacterial membrane damage or ROS generation. While previous studies had focused on the toxicity of singular nanomaterial, we assessed the toxicity effect of synthesized nanohybrids on E. coli cells. In this study, multi-walled CNTs and GO were conjugated with TiO2 and ZnO nanoparticles, respectively, and the toxicity mechanism of nanohybrids on E. coli was explored. Among the four types of nanohybrids, ZnO-conjugated nanohybrids 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, via 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 of E. coli 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. The dominant toxicity mechanism was found to be ROS generation, but nanohybrids attachment to the cell membrane also contributed to the overall toxicity to bacterial cells. The results can be used by OCSPP in evaluating health risks of nanomaterials.

Description:

Some nanomaterials including Fe0, Ag0, 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 coli (DH5α, a multidrug-resistant coliform bacterium). Graphene-oxide (GO)-based nanohybrids (ZnO–GO and TiO2–GO) induced increasing dispersion compared to carbon-nanotube (CNT)-based nanohybrids (ZnO–CNT and TiO2–CNT). Among the four types of nanohybrids, ZnO-conjugated nanohybrids 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, via 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 of E. coli 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.

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