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Nanomaterials 2020, 10, 376 16 of 22 nanocomposite against E. coli and S. aureus [67]. The synthesized GO–Ag showed a dose-dependent antimicrobial effect and was stronger towards E. coli than towards S. aureus. Das et al. investigated the antibacterial activity of Ag-GO against E. coli and P. aeruginosa [12]. They found that P. aeruginosa was more sensitive than E. coli to Ag-GO. The investigation of the antibacterial activity of a GO–Ag hybrid by Mohammadnejad et al. showed a higher toxicity against E. coli than against S. aureus [68]. The results of the present work suggest that AgNPs play a pivotal role in the antimicrobial activity of GO/AgNP hybrids. The activity of AgNPs is dependent on several parameters, including those inherent to them such as size and shape. The size dependency has been examined in various studies [7,9,10,69–72]. The smaller the size, the higher the toxicity. Small-sized AgNPs have a larger surface area, resulting in more efficient cell–particle contact. Bare-silver nanoparticles are not stable in aqueous suspensions and have a tendency to aggregate, which limits their applications. GO plays an active role in the enhancement of the stability of the AgNPs, acting as a platform to prevent their agglomeration. The formation mechanism of the GO–AgNP hybrids seems to be through electrostatic interactions between the negatively charged oxygen-containing functional groups on the GO surface and the free silver ions, which are then reduced by the reducing agent, leading to the formation of AgNPs attached to the GO surface [11,18,21,22,67,73,74]. AgNPs have been widely investigated by numerous researchers [10,71,75–83], and most of the studies reported have been focused on their antimicrobial properties. Their mechanism of antimicrobial action, however, remains unclear and is still under debate. The possible mechanism includes the attachment of AgNPs to the microbial surface, which is thought to be mediated by electrostatic interaction between the negatively charged cell membrane of many microbes and positive surface charged nanoparticles. Once the nanoparticles have adhered to the microbial cells, they are able to penetrate into them, resulting in damage of their internal components. The cellular internalisation of AgNPs can also generate ROS and induce oxidative stress in bacterial cells due to released Ag+ ions. In the presence of dissolved oxygen (in aqueous solutions), the surface of silver nanoparticles is oxidized, and the AgNP oxidative dissolution leads to Ag+ ions. Both AgNPs and Ag+ ions damage microbial cells by interacting with sulfur-containing proteins (the thiol group of proteins, which are responsible for enzymatic activity) present in microbial membranes or inside these cells, and with phosphorus-containing compounds such as DNA. This chemical interaction can result in the inactivation of such proteins in the microbial cell wall. All these processes will finally lead to the death of microorganisms. The antimicrobial effect of GO–AgNP nanohybrids could be explained by the combined action of direct contact between the AgNPs and the microbial cells and the dissolution of Ag+ ions from the silver nanoparticles. There is an increasing number of patients carrying biomedical devices. Severe infectious diseases associated with their use cause great morbidity and mortality, especially in critically ill people [84]. These types of GO–AgNP nanohybrids have found applications in different areas, as was previously mentioned. Their antimicrobial properties make them appropriate for being used in the field of biomedicine, as well as in the preparation of polymer nanocomposites with antimicrobial properties. In previous work, we incorporated this new class of antimicrobial materials into poly(vinyl alcohol), a biocompatible and biodegradable polymer, to fabricate an antimicrobial polymer nanocomposite [85] that could be a potential wound-dressing material. The matter of cytotoxicity of nanomaterials is a particularly important aspect to consider in the applications of these materials in the biomedical field. The cytotoxicity of nanomaterials is governed by their physicochemical properties such as size, shape, surface charge, coating, and concentration [34,86]. AgNPs have been found to be toxic to several human cell lines, and their cytotoxicity occurs in a dose-, size-, and time-responsive manner (especially for those with sizes ≤10 nm) by creating ROS, oxidative stress, and DNA damage [87–89]. The smaller silver nanoparticles showed higher biological activity in comparison with the larger ones. However, the toxicity threshold for the same cell line was higher for small particles than for large ones. [90]. No cytotoxicity has been observed when AgNPs are coated with appropriate polymers at certain concentrations [91]. From the reported studies, it isPDF Image | Graphene Oxide–Silver Nanoparticle Nanohybrids
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