Abstract
The coupling reactions of polyethylene glycol (PEG) with two different nano-carbonaceous materials, graphene oxide (GO) and expanded graphene oxide (EGO), were achieved by amide bond formations. These reactions yielded PEGylated graphene oxides, GO-PEG and EGO-PEG. Whilst presence of the newly formed amide links (NH-CO) were confirmed by FTIR stretches observed at 1732 cm−1 and 1712 cm−1, the associated Raman D- and G-bands resonated at 1311/1318 cm−1 and 1584/1595 cm−1 had shown the carbonaceous structures in both PEGylated products remain unchanged. Whilst SEM images revealed the nano-sheet structures in all the GO derivatives (GO/EGO and GO-PEG/EGO-PEG), TEM images clearly showed the nano-structures of both GO-PEG and EGO-PEG had undergone significant morphological changes from their starting materials after the PEGylated processes. The successful PEGylations were also indicated by the change of pH values measured in the starting GO/EGO (pH 2.6–3.3) and the PEGylated GO-PEG/EGO-PEG (pH 6.6–6.9) products. Initial antifungal activities of selective metallic nanomaterials (ZnO and Cu) and the four GO derivatives were screened against Candida albicans using the in vitro cut-well method. Whilst the haemocytometer count indicated GO-PEG and copper nanoparticles (CuNPs) exhibited the best antifungal effects, the corresponding SEM images showed C. albicans had, respectively, undergone extensive shrinkage and porosity deformations. Synergistic antifungal effects all GO derivatives in various ratio of CuNPs combinations were determined by assessing C. albicans viabilities using broth dilution assays. The best synergistic effects were observed when a 30:70 ratio of GO/GO-PEG combined with CuNPs, where MIC50 185–225 μm/mL were recorded. Moreover, the decreased antifungal activities observed in EGO and EGO-PEG may be explained by their poor colloidal stability with increasing nanoparticle concentrations.
Highlights
Research in nanomaterials is rapidly growing, especially due to their potential for biomedical and healthcare applications
FTIR spectra of the resulting Graphene oxide (GO)-polyethylene glycol (PEG) and expanded graphene oxide (EGO)-PEG (Figure 2) confirmed the newly formed amide bonds (1732 and 1712 cm−1 NH-CO IR stretches) in both PEGylated products. Raman spectra of both PEGylated products GO-PEG and EGO-PEG (Figure 3b,d) confirmed that their carbonaceous structures remained unchanged after isolations from the coupling syntheses
Whilst scanning electron microscopy (SEM) images showed the changes in the layered-structures in both PEGyalted GO and EGO, the corresponding transmission electron microscopy (TEM) identified the directional and morphological changes of the lattice structures in both GO-PEG and EGO-PEG from their starting materials (Figures 4 and 5)
Summary
Research in nanomaterials is rapidly growing, especially due to their potential for biomedical and healthcare applications. Expanded graphene oxide (EGO) with a specific surface area of 116 m2/g and median mesoporous pore diameter of 220 Å was provided by Castilla-La Mancha University (Madrid, Spain). For the PEGylation of GO and EGO (Figure 1), 4arm-PEG5K-NH2 was used as the PEG coupling reagent, while 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hypochlorite (EDC HCl) was employed as the carboxyl activating regent to enable amide bond formation during the chemical conjugation. Both chemicals were purchased from Sigma-Aldrich (Madrid, Spain). Both GO-PEG and EGO-PEG were isolated as black solids
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