Graphene oxide (GO) has attracted great interest in many different areas, as a delivery vehicle for antibacterial agents and has shown high potential. Although silver nanoparticles (AgNPs) have strong antibacterial effect, the biological application of AgNPs is often hindered by their aggregation and low stability. In this study, we developed an approach of polyoxyethylene bis (amine) (PEG) directed AgNPs grown on GO, then we combined the two materials to prepare a series of functionalized GO bearing different size AgNPs, and studied the size effects of AgNPs on growth inhibition of Escherichia coli (E.coli) and Staphylococcus aureus (S.aureus). We evaluated the antibacterial effect of GO@PEG@AgNPs on E.coli and 16 loaded on a greater number of GO, the higher will be the load efficiency, so that smaller-sized AgNPs show stronger antibacterial activity.The time-and concentration-dependent antibacterial effects of the four GO@PEG@AgNPs composites were evaluated by OD 600 . As shown in Fig 4B, before treatment with the four composites, the OD 600 value of E. coli was 0.77 and that of S. aureus 0.85. Furthermore, after treatment with 10 µg/mL GO@PEG@AgNPs composites for 4 h, the OD 600 of E. coli decreased to 0.2 (Fig 4Ba), indicating almost no E. coli survival, whereas the OD 600 of S. aureus was 0.4 (Fig 4Bb), indicating that the cytotoxicity of GO@PEG@Ag composites were greater to E. coli than to S. aureus. We also observed the same trend in concentration dependence (Fig 3S) of toxicity to E. coli and S. aureus treated with the four composites. As shown in Fig 4A, for each composite, the number of surviving E. coli was smaller than that of surviving S. aureus. Moreover, in the group treated with 10 nm GO@PEG@AgNPs, nearly no E. coli bacterial colony on the LB agar plate was formed, and only a few E. coli cells remained viable in the treated culture, markedly lower than the number of viable S. aureus cells. This result may be because of the greater thickness of gram-positive than gram-negative bacteria cell walls.
Ca 2+ and Mg 2+ can decrease the antibacterial activity of GO@PEG@AgNPsTo investigate the effect of 10 nm GO@PEG@AgNPs on the bacterial surface, various metallic ions such as Zn 2+ , Cu 2+ , K + , Na + , Ca 2+ , and Mg 2+ on bacterial suspensions were investigated. The results revealed that the OD600 of E.coli clearly increased when Ca 2+ and Mg 2+ were added to the bacterial suspensions, indicating that the antimicrobial activity of the nanoparticles decreased (Fig 5). Antibacterial Fig 4. Viable bacteria remaining in the LB-agar plates of E. coli and S.aureus after being treated with four types GO@PEG@AgNPs respectively (A) (a and f are control groups, b and g are 80 nm, c and h are 50 nm, d and i are 30 nm, e and j are 10 nm), Effect of time antibacterial activity (B) of four types GO@PEG@AgNPs represented by OD600 for (a) E. coli and (b) S. aureus.
