Key parameters to enhance the antibacterial effect of graphene oxide in solution

Graphene oxide (GO) has lately become an interesting biomaterial due to its stunning properties and versatility, its claimed antimicrobial activity holds promise for potential health applications. Nonetheless, multiple reports investigating GO antibacterial activity lack rigor and uniformity on several aspects which are crucial when evaluating this effect. In this work, we highlight and address these parameters: morphology of the materials, exposure time, exposure methodology and concentration. We investigate the effect of GO and GO-based metallic composites observing these parameters on two pathogenic bacteria. Our nanomaterials have been characterized by means of SEM, EDX, DLS, FTIR and Raman spectroscopies. Escherichia coli and Salmonella Typhimurium suspended in saline solutions (no growth medium) have been exposed to GO (lateral size = 100 nm), silver nanoparticles, ceria nanoparticles, GO/silver and GO/ceria aqueous solutions for 0, 5, 15, 30, 60 and 90 minutes, before plating. Our experiments indicate that no prior exposure of the materials to bacteria (0 min) results in poor inactivation rates independently of concentration, while increasing times of interaction enhance inactivation. Moreover, our experiments show concentration-dependent results showing higher activity for concentrations of 100 μg mL^-1; and prove that 30 minutes of exposure are sufficient to deploy the antimicrobial effects of these materials. GO possesses the lowest inactivation rate, and the presence of silver and ceria nanoparticles in the GO surface boosts its antimicrobial effect. Thus, the enhancement of the antibacterial activity of graphene oxide relies on 30 minutes of interaction in water, concentration of 100 μg mL^-1, and its decoration by silver/ceria nanoparticles.