Antimicrobial Properties of Graphene Oxide Nanosheets: Why Size Matters

TLDR

Graphene oxide is a promising antimicrobial surface material, yet the link between its physicochemical properties and activity remains unclear. This study examined how the size of graphene oxide sheets influences their antimicrobial effect against *Escherichia coli*. GO sheets with average areas from 0.01 to 0.65 μm² were fabricated and tested for activity both in suspension and as surface coatings. Smaller sheets (0.01 μm²) yielded a four‑fold increase in coating‑based killing due to oxidative damage, whereas larger sheets (0.65 μm²) more effectively entrapped bacteria in suspension, a reversible effect that allowed recovery after sonication; these results guide design of size‑optimized graphene antimicrobial surfaces.

Abstract

Graphene oxide (GO) is a promising material for the development of antimicrobial surfaces due to its contact-based antimicrobial activity. However, the relationship between GO physicochemical properties and its antimicrobial activity has yet to be elucidated. In this study, we investigated the size-dependency of GO antimicrobial activity using the Gram-negative bacteria Escherichia coli. GO suspensions of average sheet area ranging from 0.01 to 0.65 μm2 were produced and their antimicrobial activity evaluated in cell suspensions or as a model GO surface coating. The antimicrobial activity of GO surface coatings increased 4-fold when GO sheet area decreased from 0.65 to 0.01 μm2. The higher antimicrobial effect of smaller GO sheets is attributed to oxidative mechanisms associated with the higher defect density of smaller sheets. In contrast, in suspension assays, GO interacted with bacteria in a cell entrapment mechanism; in this case, the antimicrobial effect of GO increased with increasing sheet area, with apparent complete inactivation observed for the 0.65 μm2 sheets after a 3 h exposure. However, cell inactivation by GO entrapment was reversible and all initially viable cells could be recovered when separated from GO sheets by sonication. These findings provide useful guidelines for future development of graphene-based antimicrobial surface coatings, where smaller sheet sizes can increase the antimicrobial activity of the material. Our study further emphasizes the importance of an accurate assessment of the antimicrobial effect of nanomaterials when used for antimicrobial surface design.